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Gympie Regional Flood Mitigation Study Mitigation Options Report

Reference: 235960

Prepared for: Gympie Regional Council

Revision: 3

29 July 2013

Project 235960 File 235960_Gympie Flood Mitigations Options_FINAL_REV3.docx 29 July 2013 Revision 3

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Aurecon Australia Pty Ltd ABN 54 005 139 873 11/102 Wises Road Maroochydore QLD 4558 PO Box 5196 Maroochydore QLD 4558 Australia T F E W

+61 7 5443 4055 +61 7 5443 7597 [email protected] aurecongroup.com

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Document control

Report title Mitigation Options Report

Document ID Project number 235960

File path 235960_Gympie Flood Mitigations Options_FINAL_REV3.docx

Client Gympie Regional Council Client contact

Rev Date Revision details/status Prepared by

Author Verifier Approver

0 4 July 2013 Draft for Client Review C Smyth A Charteris C Russell

1 18 July 2013 Draft with Client Comments C Smyth A Charteris C Russell

2 19 July 2013 Final Draft with Client Comments C Smyth A Charteris T Graham C Russell

3 29 July 2013 Final Report C Smyth A Charteris T Graham C Russell

Current Revision 3

Approval

Author signature

Approver signature

Name Allan Charteris Name Chris Russell

Title Associate, Water Services Title Water Services Leader, Brisbane

Project 235960 File 235960_Gympie Flood Mitigations Options_FINAL_REV3.docx 29 July 2013 Revision 3

Gympie Regional Flood Mitigation Study

Date 29 July 2013 Reference 235960 Revision 3

Aurecon Australia Pty Ltd ABN 54 005 139 873 11/102 Wises Road Maroochydore QLD 4558 PO Box 5196 Maroochydore QLD 4558 Australia T F E W

+61 7 5443 4055 +61 7 5443 7597 [email protected] aurecongroup.com

Project 235960 File 235960_Gympie Flood Mitigations Options_FINAL_REV3.docx 29 July 2013 Revision 3 Page i

Executive summary

Introduction The communities, residents and businesses of the Gympie Regional Council area have a deserved reputation for flood resilience, and the community’s ability to quickly recover is envied by many other less prepared and/or flood aware communities. Nevertheless, flooding is extremely disruptive for those directly affected as well as those nearby, who may not be directly flooded, but are affected by power outages, evacuation procedures or isolation. The economic burden of this is expressed as losses to the business community, through closures during periods of flood preparation and recovery, loss of access to vendors, supplier and staff, and delays waiting for major transport routes to re-open.

Gympie Regional Council have embarked on a process to gain a better understanding of the real cost of flooding to the community, and to identify options and opportunities to improve flood resilience. This study examines flood conditions across the Region, and evaluates a number of strategies to provide improved flood protection and/or reduce exposure to flood risks. Flood mitigation options considered include:

Levees – to annex the affected community from the floodplain

Floodways – creating preferred alternate routes for flood waters

Detention basins – capturing and reducing flood peaks

Buy-back/relocation – removing the affected properties from the floodplain

Flood warning – reducing impacts by improving warning time and flood level accuracy

Waterway modification – increasing conveyance efficiency in waterways by removing bottlenecks

Improving road networks and/or bridges – where flooding brings community isolation

The study has been undertaken considering flood issues across the region, and has completed assessments at the following locations, grouped broadly in to three districts:

Western district – Flood affected communities to the west of the Mary River:

− Goomeri

− Kilkivan

− Widgee

− Wooloolga

− Kandanga

− Imbil

Eastern district – Flood affected communities on the Cooloola coast:

− Cooloola Cove

− Rainbow Beach

− Tin Can Bay

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Gympie – Flood affected suburbs within Gympie

− Mary Street (CBD)

− Commissioner Gully

− Two Mile

− Mount Pleasant

− One Mile

− Monkland and Glanmire

− East Deep Creek

− Southside

The study has drawn on available data from Gympie Regional Council and the Gympie Regional Chamber of Commerce, together with flood data derived from the GHD 2012 Gympie Regional Flood Study as well as a range of public domain resources. Key data has included topography, flood extents, flood damage estimates, and numerous previous flood investigations. In addition, community consultation undertaken throughout the region has gathered first-hand information regarding the social and economic impacts of flooding on businesses. These data have informed the mitigation assessment and the cost benefit analysis.

To ensure that the Cost Benefit Analysis (CBA) findings are defensible in the broader government domain, Aurecon has used the Rapid Appraisal Method (RAM) for Floodplain Management framework as detailed in Rapid Appraisal Method (RAM) for Floodplain Management (Victoria Department of Natural Resources and Environment) for the CBA economic appraisal work. The RAM is a nationally recognised approach and provides a comprehensive overview of the types of flood damage impacts for assessment. Further, the assessments have been undertaken in consideration of the Queensland Government’s recently released Flood Resilience funding framework.

Screening assessment At a screening level, flood risks and mitigation option benefits for all the flood affected communities listed above have been assessed. The outcome of this process was the identification of a range of site specific flood resilience recommendations at each location and the selection of three flood affected areas likely to yield the most economically attractive cost-benefit from the implementation of flood mitigation works.

Detailed options assessment In consultation with the Project Steering Committee (PSC), three areas were selected for further detailed assessment to determine the cost-benefit of improved flood resilience. The locations selected were:

Mary Street precinct

One Mile precinct

Monkland/Glanmire areas

The development of a series of conceptual flood mitigation options at each of these locations was undertaken and each option was subject to cost-benefit analysis assessing capital and lifetime costs relative to the improved flood resilience offered.

Mary Street precinct The screening exercise identified two broad options that would reduce flood risks to the CBD and Mary Street area: relocation and strategic levees. At the agreed target immunity level of 20 year ARI, the objective is to offer flood relief for up to 90 businesses in the CBD.

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A range of levee locations were considered: within Nelson Reserve, raising the Bruce Highway, and within the Touch Football fields adjacent to Albert Park oval. All the levees have a crest height at least 8 m above ground level within the parks, and a footprint of at least 50 m wide (at side slopes of 3H:1V). Each levee requires a spillway to allow the controlled flooding of the area behind during periods of flooding in excess of the levee immunity level, and pumps to address concurrent local catchment flooding. Backflow prevention will be required to ensure the potential for flooding via the stormwater system is addressed.

There are a number of critical engineering design issues relating to levees protecting Mary Street that need to be understood and resolved. These are summarised as:

Coincident flooding – major local rainfall within the Mary Street catchment coinciding with flooding in the Mary River

Geotechnical considerations – Nelson Reserve, the Bruce Highway and Albert Park Touch Football fields are all, to some degree, constructed on landfill areas requiring consideration of the geotechnical capacity to support a major levee structure

Flood infiltration through mines – the area was heavily mined during the gold rush period, and the potential underground linkages to adjacent shafts is a significant risk

Residual flood risk – this relates to the selection of a design immunity and height of the levee. At an immunity level of 20 year ARI, the expectation of overtopping is quite high. As such, each levee option requires a spillway and other infrastructure to allow the controlled flooding of the area behind the levee.

The assessment process has identified a preferred alignment of the levee through the Albert Park Touch Football fields, at a crest level of 22 m on the Kidd Bridge gauge (approximately 59 m AHD) offering around 30 year ARI flood protection. This offers a significant improvement in flood resilience for businesses in the CBD up to Smithfield Street. This option has been assessed at a cost of $22.7 million, and a BCR of 1.1 over the projected lifetime of the asset (at least 50 years).

One Mile precinct The One Mile precinct includes over 50 industrial properties, and over half of these are directly affected by 100 year ARI event flooding. Moreover, at the southern end of the precinct adjacent to Deep Creek, there are several sites that become inundated at the 10 year ARI flood level (corresponding to 18.79 m on the Kidd Bridge gauge). Improved resilience, typically, can be offered to such pockets of flood risk through the use of strategic levees (or buyback) targeting the flood risk at a commensurate level.

Three options have been developed to offer improved resilience in the precinct, drawing on combinations of strategic levees and voluntary buyback. A key difficulty for the properties at the southern end of the precinct is the steepness of the land adjacent to Deep Creek. This prohibits the use of earthen levees as there is no viable footprint available. For this southern section, a block wall levee is proposed, around 2.2 m high, offering improved flood immunity, but due to practical limitations, only to the 10 year ARI flood level.

None of the options assessed offer an economically viable solution.

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Monkland/Glanmire precinct Flooding in excess of the 5 year ARI flood level (around 16 m at Kidd Bridge) results in closures of the Bruce Highway (Inglewood Bridge) and Tin Can Bay Road (Pengellys Bridge). This leads to the isolation of Monkland and Glanmire, and is considered to represent a significant economic impact to the local businesses and the broader community, as this area represents the major industrial precinct in Gympie providing employment for a large number of residents across around 200 businesses.

Isolation of these areas due to flooding results in lost production. Gympie Regional Council has previously estimated the economic losses resulting from such isolation to be in the order of $750,000 per day. Recognising that flood isolation can last a number of days, the economic damage of isolation can be significant. Further, there are around 500 residential properties in the Monkland/Glanmire precinct. This population is isolated from the main parts of town, with limited access to retailers and emergency services during flooded conditions.

The ultimate “solution” to the access issues at Monkland is the future Bruce Highway Gympie Bypass. This section of road construction is programmed to occur beyond the 10 year horizon, and is part of the broader regional road upgrade strategy. The new highway is expected to offer 100 year ARI flood immunity for traffic, and concept planning has identified interchanges at Noosa Road/Penny Road and Gympie Connection Road. Accordingly, the highway offers a high quality 100 year ARI flood free alternative route for all traffic travelling between Gympie and Monkland. Further, once integrated into broader road networks north and south, flooding of the Mary River will no longer result in closures of the Bruce Highway, and all Gympie businesses on the east of the Mary River will benefit from continuous connection to suppliers, retailers and employees during flood events.

A range of alternate access options were assessed to provide an interim access solution until the Bruce Highway bypass is completed. From these a targeted and cost effective road network improvement plan has been identified to offer interim connectivity until the Bruce Highway bypass is in place along with internal road network improvements consistent with the long term regional traffic management objectives.

The proposed works comprise the construction of a short connecting road, extending the existing Penny Road to Hyland Rd, and completing an alternative road access circuit, offering up to 20 year ARI flood free access between Gympie and Monkland/Glanmire. The works also include an upgrade to Drummond Drive, which is a steeply graded gravel access that includes a weight limited bridge (2t) across the Mary Valley Rattler railway line. Drummond Drive currently offers up to 20 year ARI flood immune access between Monkland and Glanmire for smaller vehicles only. Improvements to Drummond Drive could offer all-vehicle connectivity between Monkland and Glanmire up to the 100 year ARI flood level and is considered an integral part of a broader long term 100 year ARI flood free access solution associated with the future Bruce Highway bypass. Without such works, the future flood free benefits enjoyed by Monkland businesses resulting from the future Bruce Highway bypass would not be available to Glanmire businesses.

The cost of these works has been assessed at $4.4 million with a BCR of 1.8.

Recommendations Key recommendations from the screening level assessment include:

Flood levee for Goomeri – Council should undertake investigations to assess the feasibility of a levee on the southern side of town to reduce the risk of flooding due to breakout flows from Nangur Creek

Relocation of the Kandanga Bowls Club – consideration of the relocation of the Bowls Club, buildings and greens, away from this highly at-risk floodplain area next to Kandanga Creek

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Raising of Coondoo Creek bridge on Tin Can Bay Road – Council should engage with TMR to investigate the potential for significantly reducing the isolation risks for some 4000 properties in the Cooloola coast area

Raising River Road at Centro Way – Council should engage with TMR to further investigate statewide benefits associated with improved passage for commercial vehicles travelling south along the Bruce Highway. These improvements, via River Road and Pengellys Bridge, would allow southbound passage some six to ten hours earlier than is available via the (still flooded) Bruce Highway Inglewood Bridge.

Improved connectivity between Gympie and Southside – to address isolation risks for over 400 residential lots in Southside, via a 100 year ARI flood free crossing of the Mary River at Gympie. Council should engage with TMR to understand where this issue rests in the context of state-wide transport planning, and determine the need to develop a flood management plan to support Southside residents during times of isolation.

Voluntary Buyback – consideration should be given to the voluntary buy back of residences and businesses exposed to frequent and significant above floor flood inundation. The flood return period for which such voluntary buy back should be triggered needs to be considered within the context of resilience of the property and its land use. Nevertheless, as a minimum, it is recommended that Council consider buyback for businesses and residences exposed to above floor flooding at the 5 year ARI level.

Finally, the screening assessment and consultation process acknowledged a broader and region-wide issue of flood resilience on the district’s rural roads, and that a generally poor level of flood immunity across the rural transport network has a significant impact in recovery in rural areas. These rural State Roads require improvement to offer rural areas a ready means to recover following natural disasters.

In addition to the specific recommendations above, there are a range of generic flood resilience actions that can be implemented by Council to reduce exposure to flood risks. These include:

Planning Controls – through the planning scheme, its overlays and associated controls, Council can roll out prudent forward looking floodplain management, putting in place transparent mechanisms to minimise the risk of inappropriate development in at-risk flood areas

Drainage Maintenance – an ongoing commitment to the maintenance of Council drainage infrastructure is required, to ensure existing drainage and flooding issues do not escalate as a result of declining infrastructure condition

The detailed cost-benefit assessment has identified two opportunities to offer a significant improvement in flood resilience for the Gympie business community. It is recommended Council pursue funding for these preferred strategies in the CBD and Monkland/Glanmire via the Queensland Government’s Disaster Mitigation and Resilience program:

Gympie CBD Flood Levee – flood levee located in the Albert Park Touch Football fields, offering protection up to 22 m on the Kidd Bridge gauge

Monkland Glanmire Flood Access Improvements – construction of a new road link from Penny Road to Hyland Road and improvement works for Drummond Drive

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Contents 1 Introduction 4

1.1 Background 4 1.2 Project scope and objectives 4 1.3 Project oversight 5 1.4 Source data 6 1.5 Limitations 6

2 Flood history 7

2.1 Gympie 7 2.2 Regional centres 9 2.3 Key flood issues affecting resilience 9 2.4 Climate change 9

3 Initial screening 12

3.1 Introduction 12 3.2 General considerations 13 3.3 Western district 13 3.4 Eastern district 26 3.5 Gympie 28 3.6 Priority areas 45

4 Economic analysis framework 48 4.1 The rapid assessment method 48 4.2 Data sources 50 4.3 Other considerations 51

5 Mitigation options assessment 53

5.1 Mitigation options 53 5.2 Mary Street (CBD) 59 5.3 Monkland and Glanmire 67 5.4 One Mile 75

6 Cost benefit analysis and Business Case 79

6.1 Introduction 79 6.2 Mary Street (CBD) 79 6.3 One Mile 81 6.4 Monkland and Glanmire 83 6.5 Sensitivity analysis – effect of economic life 84


6.6 Conclusions 86 7 Funding mechanisms 87

7.1 Recommendations 88 8 Summary and recommendations 90

8.1 At the screening level 90 8.2 Project recommendations 91

9 References 92

Appendices Appendix A

Consultation report Appendix B

Flood maps Appendix C

Gympie CBD mines

Figures Figure 1 Flood levels at Kidd Bridge 8 Figure 2 January 2013 flooding in Goomeri 15 Figure 3 Indicative Levee alignment to address breakout flow flooding from Nangur Creek 16 Figure 4 Mayor Ron Dyne inspects flood damage at Kilkivan Showgrounds (Gympie Times) 18 Figure 5 Gympie-Woolooga Road crossing of Widgee Creek 20 Figure 6 January 2013 flooding in Woolooga 21 Figure 7 January 2013 flooding at the Kandanga Bowls Club 23 Figure 8 Flooding at Coondoo Creek 27 Figure 9 Flooding in Mary Street, January 2013 (Source: Gympie Times) 29 Figure 10 Flooding at Centro January 2013 (Source: Gympie Times) 36 Figure 11 Topography showing key choke points (circled) downstream from Gympie 42 Figure 12 Mary River cross sections 43 Figure 13 Mary River Long section showing effect of Fishermans Pocket works 43 Figure 14 Indicative Levee locations to protect the Gympie CBD (Indicative Only – not to scale) 60 Figure 15 Alternative routes providing access to Monkland/Glanmire 68 Figure 16 Mary Valley Railway at Gilldora 69 Figure 17 Conceptual levee options at One Mile 76 Figure 18 Mary Street CBD damage curve comparison between the Base Case and Flood mitigation

options 80 Figure 19 One Mile damage curve comparison between base case and flood mitigation options 82


Tables Table 1 Projected climate change influences for South-East Queensland (Source: DERM 2009) 10 Table 2 Mary Street CBD Business property buyback cost by ARI (aggregated costs) 30 Table 3 Summary of number of lots to potentially benefit from flood mitigation works 45 Table 4 Key generic CBA modelling assumptions 49 Table 5 Flood Mitigation Options Comparison 54 Table 6 Cost Estimates for Road Options 72 Table 7 Mary Street CBD mitigation options and costs in 2013 prices over a 20 year assessment

period 79 Table 8 CBA results at 7% nominal discount rate over a 20 year assessment period 80 Table 9 One Mile mitigation options and costs in 2013 prices over a 20 year assessment period 81 Table 10 CBA results at 7% discount rate over a 20 year assessment period 82 Table 11 Monkland and Glanmire mitigation options and costs in 2013 prices over a 20 year

assessment period 83 Table 12 CBA results at 7% discount rate over a 20 year assessment period 84 Table 13 CBA results at 7% nominal discount rate over a 50 year assessment period 85 Table 14 CBA results at 7% discount rate over a 50 year assessment period 85 Table 15 Funding sources and their applicability 88


1.1 Background Gympie and its surrounding regions have a long recognised history of flooding from both riverine flood events emanating from the Mary River and also from local runoff from tributary streams such as Deep Creek. Since records of flood levels at Gympie began being collected in the 1870s, at least 16 flood events have reached Major Flood classification level at the Kidd Bridge gauge. The most significant flood events to have been recorded at the Gympie flood warning gauge include February 1893, February 1999, March 1955, February 1992 and April 1989.

On several occasions during the period of available record (around 140 years) there have been sequences of repeated Moderate to Major Flood events that have resulted in disruption to rural life and inundation of urban areas. The period since January 2011 is an example of this behaviour with five flood events being experienced over at two year period that have resulted in impacts within the CBD of Gympie. In addition to the inundation of a business or residence, a key impact of flooding is isolation and the severing of transport and evacuation routes within the flood affected areas. The Kidd Bridge for example has a deck level of around 10.0 m (46.83 m AHD) and an estimated flood capacity of 720 m3/s, which is much less than a Moderate Flood Classification of 12.0 m Gauge Height.

The impact of flooding on Gympie and the region is significant. The 1999 floods were reported to have directly affected over 130 businesses in Gympie, with costs associated with flood preparation (moving stock out), clean up, repair, restocking and loss of trade. Furthermore, while flooding may last only a few days, the follow-on impacts can last several weeks and in the case of the rural communities can impact production for many months, sometimes years. Such commercial impacts can reduce business confidence and result in reputational damage that lasts for many years. Improving resilience, by reducing the frequency with which businesses and the community are exposed to flooding, and reducing recovery times, is a fundamental means to address such devastating impacts of flooding.

1.2 Project scope and objectives Gympie Regional Council is seeking to identify the real cost of flood events to the Gympie region community. A Flood Mitigation Assessment has been undertaken to identify the most appropriate mechanisms to minimise the economic, environmental, social and reputational impacts of flood events to the town of Gympie and other nominated townships within the region.

Aurecon has undertaken this study drawing on a methodology to support Gympie Regional Council’s goals for the preparation of a Business Case “that shall be adequate as the primary support for Council’s submissions for funding to both the Federal and Queensland Governments”. To ensure that the Cost Benefit Analysis (CBA) findings and report are defensible in the broader government domain, Aurecon has used the Rapid Appraisal Method (RAM) for Floodplain Management framework as

1 Introduction


detailed in Rapid Appraisal Method (RAM) for Floodplain Management (Victoria Department of Natural Resources and Environment) for the CBA economic appraisal work. The RAM is a nationally recognised approach and provides a comprehensive overview of the types of flood damage impacts for assessment.

A staged approach to the study has been undertaken, as outlined below:

Phase 1: Initial screening: Flood risks and mitigation benefits The 100 year ARI flood risks, potential options and the derived community and business benefits have been assessed at a screening level. This provided a mechanism to compare the number of lots affected with the benefits, to rank and compare identified floodplain management initiatives across the region. The objective of this phase was to identify for further detailed assessment up to three flood affected communities that are likely to have the most economically attractive cost-benefit resulting from the implementation of flood mitigation works

Phase 2: Cost benefit analysis For the three flood affected communities selected as an outcome of Phase 1, Aurecon has assessed in further detail the flood risks, and identified feasible mitigation options. In turn these options have been assessed to determine costs and benefits. A lifecycle economic evaluation has provided benefit-cost ratios for the preferred options, and includes capex, opex and risk considerations.

Phase 3: Business case development The findings from the Initial Study Phase and Cost Benefit Analysis has provided the base data for the preparation of concise and targeted business case reports, supporting those options that have a clear benefit to the Gympie region.

As the Business Case reports will represent the primary support for funding submissions to the State and Federal Governments, it is important that potential funding sources to be targeted are identified. To that end, the latest revisions to the funding mechanisms have been reviewed to ensure the options proposed meet the eligibility criteria for State support.

1.3 Project oversight A Project Steering Committee was established to provide oversight and guidance to the study. Recognising the practical difficulties in gathering an extended group together to attend workshops and review project documentation, the PSC be formed from a small group of senior and well-qualified representatives, allowing decision making to be made quickly and ensuring buy-in at the highest level. The Project Steering Committee comprised:

Cr Ron Dyne, Mayor, Gympie Regional Council

Mr Bernard Smith, CEO, Gympie Regional Council

Mr Bob Fredman, Director of Engineering Services, Gympie Regional Council

Mr Garry Davidson, Gympie Chamber of Commerce

The PSC provided valuable knowledge and experience relating to flood behaviour and the history of flooding throughout the region, as well as community and business actions and response during times of flood. This input was central to the successful outcomes of the study.


1.4 Source data The study has drawn on available data from Gympie Regional Council and the Gympie Regional Chamber of Commerce, together with flood data derived from the GHD 2012 Gympie Regional Flood Study as well as a range of public domain resources. Key data has included:

Topographic information for each study area

Flood level observations

100 year ARI flood levels throughout the Gympie region (as GIS data from GHD 2012)

Flood frequency and rating curves for the flood gauge at Kidd Bridge

Cadastre and planning scheme zones

Aerial photography

QRA flood envelopes for historical events and/or design events

Flood damages estimates for recent floods

Previous relevant Flood Studies

Flood models

Relevant information from recent flood reconstruction projects

1.5 Limitations This study has relied on the accuracy of data provided. The data and information used within the study have been assessed for consistency with industry standards and other independent sources, however, no independent verification of these data has taken place. Accordingly, the accuracy of the findings of the study is limited to the accuracy assigned to the various input data.


2.1 Gympie Gympie and its surrounding regions have a long recognised history of flooding from both riverine flood events emanating from the Mary River and also local runoff from tributary streams such as Deep Creek and One Mile Creek. Since records of flood levels at Gympie began being collected in the 1870s, at least 16 flood events have reached Major Flood classification level at the Kidd Bridge gauge (FWS 040426). The most significant flood events to have been recorded at the Gympie flood warning gauge include February 1893, February 1999, March 1955, February 1992, January 2013, April 1989 and January 2013.

The maximum recorded historical peak event occurred in February 1893 with a level of 25.45 m Gauge Height (62.38 m AHD) and a corresponding flow of 11,500 m3/s. The second largest flood on record occurred in February 1999 and had a peak level of 21.95 m at Kidd Bridge (58.88 m AHD). The January 2011 and 2013 flood events peaked just below 20 m at Kidd Bridge (57 m AHD).

The GHD report suggests a 1% AEP at Gympie based upon flood frequency analysis is 11,161 m3/s. This equates to a level of 25.22 m at Kidd Bridge (62.15 m AHD).

Inundation of Nelson Reserve and Bruce Highway: February 2013 (Source: Gympie Times)

2 Flood history


However, in the last few years Gympie and the surrounding regions have been impacted by a series of large flood events that have resulted in significant disruption of the Central Business District (CBD) and isolation of a number of communities. In particular the floods resulting from Ex-Tropical Cyclone Oswald in January 2013 which reached 19.98 m Gauge Height on the Kidd Bridge and the January 2011 flood which reached 19.95 m Gauge Height were the two largest of five recent flood events to impact the region.

Floods of this magnitude resulted in 80 businesses in the CBD being flooded with over 50 homes threatened. These events also resulted in power cuts to over 900 businesses and residences across the region and disrupted water supplies and sewerage services to many communities. Such wider impacts affect the entire community, with secondary and tertiary impacts to businesses and residences well beyond the flooded zone. This can include reputational damage to districts subject to flooding, reduction in property values, and a range of stress related disorders.

Figure 1 below provides a graphical illustration of the statistical flood levels in Gympie in Gauge and Australian Height Datums, and includes a series of points representing the 50 highest flood events on record.

Figure 1 Flood levels at Kidd Bridge


2.2 Regional centres Regional centres of the area include towns such as Imbil, Kilkivan, Woolooga, Widgee and Goomeri. These centres were adversely impacted by the floods of January 2011 and January 2013 in particular, with more than 80 council roads and most state roads cut or closed during these events. The floods also resulted in disruptions to water supplies and sewerage treatment services for many of the impacted centres.

The recovery process for some of these centres has been quite long, and many of these communities were still recovering from the January 2011 flood event when Ex-Tropical Cyclone Oswald resulted in further damage in January 2013. Recovery projects such as the reconstruction of the Bellthorpe Range Road and Blackbutt Range Road are examples of the disruptions to some of these regional transport routes, ultimately affecting these regional centres within the Gympie Regional Council district. Further, closures and flood damage to crucial roads within the region (Burnett Highway, Wide Bay Highway, etc) has had a significant impact on the region’s own ability to deliver timely recovery for rural towns and agricultural areas.

2.3 Key flood issues affecting resilience The Gympie community has become accustomed to dealing with flood impacts, with warning and response systems established to cater for these occurrences. Eighty percent of floods at Gympie occur in the December to April period, associated with summer rain and/or ex-tropical cyclone rain depressions. The Bureau of Meteorology seeks to provide predictions of flood heights whenever Mary River flood levels are expected to exceed 12 m on the gauge boards adjacent to the Kidd Bridge. The objective is to provide between 21 to 27 hours warning for residents and businesses in Gympie to assist in preparations to evacuate or make safe.

The performance of the warning system for the tributary streams is less effective as these responses are more akin to flash flooding and so the available response time is much less than that of the riverine flooding.

The Gympie community has developed an awareness of flood impacts and have adapted accordingly by having well practiced preparation plans that enable the community to know what to do and how to do it effectively during the onset of a flood event. For example, in January 2011, Gympie CBD businesses started to evacuate three days prior to the peak of the flood, saving a lot of stock and reducing damage bills. Well prepared flood aware communities are able to be much more effective in reducing damage to their goods and possessions than communities that are not prepared.

The resilience of essential infrastructure is vital to the recovery process. Sensible planning with respect to essential services such as electricity supply, water supply, sewerage and telecommunications as well as emergency response facilities such as police stations and hospitals is critical in assisting a speedy recovery. The integrity of these services can be maintained or their recovery promoted by locating them in flood-free locations, by siting them above flood levels or by flood-proofing them.

Flooding will continue to occur in the Mary River catchment, and good planning, preparation and all means to recover rapidly following the event will improve the community’s resilience to flooding.

2.4 Climate change The degree to which the projected impacts of climate change affect a site depends on its location, particularly latitude, and proximity to the coast. A summary of projected changes for South-East Queensland has been prepared based on information available through the Queensland Department of Environment and Heritage Protection (DEHP, formerly DERM) and is presented in Table 1 below.


Table 1 Projected climate change influences for South-East Queensland (Source: DERM 2009)

Extreme Weather Events Projected Changes

Drought A significant increase in the number of ‘exceptionally hot’ years is predicted. These are projected to increase in frequency from an average of approximately one every 22 years to an average of one every 1.7 years by the period 2010 to 2040

It is projected for slight increases in the frequency of exceptionally low soil moisture years, from an average of one every 16.5 years (1900 to 2007) to an average of one every 12.6 years (2010 to 2040)

Hot Days More frequent hot days (temperature above 35oC) are virtually certain and more frequent warm spells/heatwaves over most land areas are very likely. The number of hot days per year above 35oC as projected for three representative stations with good historical records for 2030 (mid emissions scenario) and 2050 and 2070 (low and high emissions scenarios) are:

Warm Nights Minimum temperatures will continue to increase more rapidly than maximum

temperatures throughout most of Queensland with up to a 50% increase in warm nights by 2080 to 2099 under a medium emissions scenario

Extreme Rainfall Projections indicate an increase in two-hour, 24-hour and 72-hour extreme rainfall events for large areas of south-east Queensland, despite a projected decrease in rainfall across most of Queensland, the projected increase in rainfall intensity could result in more flooding events

Fire Danger The most dangerous fire conditions occur with low relative humidity and high temperature and wind speed after periods of low rainfall (which raises the drought factor). The Forest Fire Danger Index is commonly lower for Queensland, with fewer high risk days than in other states, due to higher levels of relative humidity

In South-East Queensland, decreases in relative humidity combined with projections of increased temperature, an increase in the number of hot days and less frequent rainfall events, are likely to increase the number of high Forest Fire Danger Index days

Tropical Cyclones Evidence from the BoM shows there is a greater tendency for tropical cyclones in Queensland to track further southward in La Niña than in El Niño dominant decades (ENSO). The projected southward shifts in the primary regions of cyclone development through the coming century are expected to lead to an increase in the number of severe tropical cyclones in South-East Queensland by between 22% and 56% by 2050

Rainfall associated with tropical cyclones (within 300 km) is likely to increase by approximately 20% by 2100

Severe Thunderstorms Current climate change models do not have fine enough resolution to project small-scale events such as thunderstorms, although these would reflect the general trend in increasing intensity as projected for cyclones

East coast low pressure systems which mostly occur during the winter months in South-East Queensland, develop rapidly into intense storm systems which result in short warning time. The long term average for these is about 2.5 per year, since 1960 this average has increased to 3.7 (Granger and Hayne, 2001)


Extreme Weather Events Projected Changes

Storm Surge The height of storm surge on the Queensland east coast is expected to increase with rising sea level and changes in tropical cyclone behaviour. Projections show significant rises in storm surge independent of the mean sea level rise, especially for the South-East Queensland coast

Based on the literature review, and the above summary in Table 1, a number of key environmental climate change risks affecting communities within the Gympie region have been identified, including:

Increased storm and rainfall intensity

Changes to cyclone behaviour

Increased occurrence and length of heatwaves

From a purely floodplain management perspective, the likely consequences of the climate change on flood impacts in the Gympie region suggest that it is only going worsen the effects of flooding due to the increase in the number and intensity of severe storm events.


3.1 Introduction As noted in previous sections, there are numerous locations throughout the Gympie Regional Council district that are affected by flooding. A preliminary screening of nominated areas has been undertaken to provide an understanding of local flood issues and identify potential options that may be available to improve flood resilience in those areas.

The screening assessment draws on available flood information provided by GRC in the form of 100 year ARI flood extents determined as part of recent works by the Queensland Reconstruction Authority (QRA), specific flood investigations undertaken (by others) on behalf of Council. Where a representation of the 100 year ARI flood extent is not available, the extent of floodplain, defined by QRA, has been used as a proxy. An assessment of flood behaviour has been made drawing on this data, together with anecdotal observations and photography collected during recent flood events.

Information from Council’s planning scheme relating to landuse types has been compared with the flood information, to identify the number of lots potentially at risk, and with understanding of the flood behaviour, what mitigations options may be suitable at each location to reduce the flood risks.

Key data from the screening assessment has been consolidated and summarised in a series of tables, graphics and maps to allow comparison and ranking of the flood risks and potential benefits of identified mitigation options. At this screening level, a full CBA is not possible. Rather, the data has been provided in a consistent format to provide a relative scale risk-benefit ratio, taking into consideration key characteristics including the number of businesses likely to be at risk of above-floor flooding, magnitude of costs of implementing the mitigation option (where practical), and the number of businesses that may benefit.

The following sections present the findings of the assessment, grouped broadly in to three districts:

Western district – Flood affected communities to the west of the Mary River:

− Goomeri

− Kilkivan

− Widgee

− Wooloolga

− Kandanga

− Imbil

Eastern district – Flood affected communities to the east of the Mary River:

− Cooloola Cove

− Rainbow Beach

− Tin Can Bay

3 Initial screening


Gympie – Flood affected suburbs within Gympie

− Mary Street (CBD)

− Commissioner Gully

− Two Mile

− Mount Pleasant

− One Mile

− Monkland and Glanmire

− East Deep Creek

− Southside

3.2 General considerations The screening assessment has reviewed flooding issues at various locations in Gympie and across the region. Specific options to improve flood immunity and flood resilience have been identified. In addition to the specific options, there are a range of generic flood resilience actions that can be implemented by Council to reduce exposure to flood risks. These include:


Drainage Maintenance – an ongoing commitment to maintenance of drainage infrastructure is required, to ensure existing drainage and flooding issues do not escalate as a result of declining infrastructure condition

Voluntary Buyback – Consideration should be given to the voluntary buyback of residences and businesses exposed to frequent and significant above floor flood inundation. The flood return period for which such voluntary buyback should be triggered needs to be considered within the context of resilience of the business and its land use. Nevertheless, as a minimum, it is recommended that Council consider buyback for businesses and residences exposed to above floor flooding at the 5 year ARI level.

3.3 Western district

3.3.1 Introduction The rural western districts of Gympie Regional Council have experienced significant flooding over the past few years, affecting roads, flooding towns and impacting the rural and agricultural community. Our consultation process has confirmed that the community’s key concerns relating to flooding include:

Loss of topsoil from prime agricultural land, reducing the feed available for stock and land available for cropping

Road conditions that prevented landholders from getting their cattle to market, particularly after the 2013 Australia Day flood. When cattle were able to be sent to market the price per kilogram had dropped due to increased numbers of cattle from drought-affected western Queensland. Limited road access also increased people’s feelings of isolation.

Lost crops due to be harvested as flooding hit, significantly reducing landholders income

Somewhat limited communication in relation to rising flood levels across all river systems, rather than just those that affect Gympie

The length of time taken to complete repairs to farm infrastructure such as fencing and irrigation systems. In most cases repairs from the 2011 flooding had just been completed when the 2013 Australia Day flood hit. The need to rectify flood damage means that farm improvements cannot be undertaken.

Minimal funding support provided by the Queensland State and Australian Federal Governments to assist with recovery, especially given the importance of agriculture in the regional economy

Increased disease and infections affecting cattle and crops


Increased traffic on local roads due to sightseers and people using back roads to return home; damaged fencing means it is far more likely that cattle are on these roads

Beyond these regional and widespread issues, there are a number of specific flood related issues in the rural towns as discussed in the following sections.

3.3.2 Goomeri

Introduction Goomeri is a rural town located around 75 km west of Gympie, home to Australia’s only Pumpkin Festival (held in May each year). The town is surrounded by beef cattle and dairy grazing country, and lies at the crossroads of the Wide Bay, Bunya and Burnett Highways.

Flood behaviour, and resulting issues/risks Nangur Creek flows from the south and is crossed by the Burnett Highway about 1 km from the Goomeri town centre. Before reaching the town, Nangur Creek turns west and connects with Barretts Creek. The north side of town borders the floodplain of Chippendall Creek, which flows from the east, parallel with the Wide Bay Highway. The confluence of Chippendall Creek with Nangur Creek occurs approximately 2 km north of Goomeri.

Each of the three creek systems can flood independently of the others, and result in closures to the Burnett, Wide Bay or Bunya Highways, which are all major traffic routes. These closures can last for a few days during major events, but it is recognised that damage to these roads caused by flooding can significantly hinder recovery efforts for many weeks following the event, increasing travel times and restricting the size of vehicles that can safely use the roads. This impacts both the town and the rural properties and businesses that rely on the road network for the distribution of good and services, and the provision of flood recovery services.

Direct flood risks in Goomeri arise from inundation and flooding from the surrounding creeks. Flood flows in Chippendall Creek can impact residential and commercial areas on the northern side of town. During major flooding of Nangur Creek, floodwaters overtop the creek bank to the south of the town, and the local topography directs these breakout flows northwards through the bowls club, football reserve and swimming pool area towards the main parts of town. East and west parts of town are isolated due to these fast flowing floodwaters (See Figure 2, January 2013).


Figure 2 January 2013 flooding in Goomeri

Type and number of flood affected properties Flood assessments have identified approximately five residential, eight commercial/industrial and five rural residential properties on the northern side of town (primarily on the north side of Jones Street) that are partially affected by 100 year ARI flooding in Chippendall Creek (inundation does not coincide with the footprint of buildings). However, there are several lots midway along the south side of Llewellyn Street zoned as rural, where aerial photography indicates that some of these have residential buildings located in close proximity to Chippendall Creek and potentially subject to above floor inundation.

Flood flows in Nangur Creek that overtop and split from the primary flowpath impact up to ten residential properties and around twenty commercial/industrial properties within the town. An additional four to five industrial zoned lots located to the south of the town and east of the Burnett Highway are affected by 100 year ARI flooding, but inspection of flood extents with aerial photography indicates there are no major buildings directly affected.

Conceptual options to address flood risks Whilst rare, the behaviour of overtopping flood waters from Nangur Creek is of concern, as a breach of the creek bank can occur quickly and result in significant flows through the heart of the town. Protection from such flood behaviour, up to the 100 year ARI flood event in Nangur Creek may be available through the installation of a levee on the southern part of town that prevents such flows from propagating northwards. Based on a review of the available flood and topographic information, a 400 m to 500 m long levee may be sufficient, running generally east-west and located between the creek and the football reserve. Topography and nominated 100 year flood levels indicate a maximum levee crest elevation up to 2 m above existing ground levels. An indicative alignment of the levee is illustrated in Figure 3 below.


Figure 3 Indicative Levee alignment to address breakout flow flooding from Nangur Creek

At the 100 year ARI flood level, flooding in Chippendall Creek directly affects the buildings of only a few properties located at the fringe of the flood inundation extent. Flood levels in this area should be reviewed and the location and type of future development in the flood fringe be managed through planning scheme controls.

Number who may benefit The levee option as described above offers flood relief and benefit (up to the 100 year ARI flood level) for up to twenty commercial/industrial properties in Goomeri, and ten residential properties. There would also be benefits to community infrastructure (including local roads and parks) for which the flood risks are significantly reduced. Further, the potential for east-west isolation of the town during such flooding would be significantly reduced.

Challenges/difficulties/issues The levee is relatively low, and faces few engineering challenges in construction or maintenance. The issue in design will be the interface of the levee with the Burnett Highway and the connection to high ground on the eastern side of the highway that provides a robust and aesthetically acceptable solution. Nevertheless, it must be recognised that such a levee solution does not make the town flood free, and a greater than 100 year ARI flood could occur in the Nangur Creek catchment, overtopping the levee and flooding the town. At a 100 year ARI planning level, these residual risks are low, but must be taken into consideration. A levee option, coupled with improved food warning in the Nangur


Creek catchment would be a practical method to minimise these risks. A preliminary cost estimate for the levee capital works for the levee and highway interface is in the range $750,000 to $1,000,000.

Flood levels in Nangur and Chippendall Creeks have been derived from 1D steady state numerical modelling using the HEC-RAS software and with limited representation of hydraulic structures (GHD 2012). At an overview level, this is appropriate, and offers an industry accepted screening level assessment of flood conditions. However, the flood behaviour in the creeks around Goomeri is complex with breakout flows from Nangur Creek being a primary mechanism for flooding of the town. Greater confidence in flood levels for setting planning controls and for the design of a flood levee would be offered utilising a dynamic 2D flood modelling approach.

Recommendations The potential flood resiliency benefits offered by a levee on Nangur Creek appear to warrant further investigation. As a first step, it is recommended Council commission a flood investigation of the Nangur and Chippendall Creek systems, utilising 2D dynamic flood modelling methods, with model calibration to the January 2013 flood event. The objective of the investigation would be to better define flood behaviour throughout the town for planning purposes, and provide design guidance on the location and geometric characteristics of a flood levee to the south of the town. A detailed and comprehensive costing and cost-benefit of the levee could then be undertaken.

3.3.3 Kilkivan

Introduction Kilkivan is a rural town located around 50 km west of Gympie, on the Wide Bay Highway and is an important hub for the surrounding beef cattle and dairy grazing properties. Kilkivan hosts a number of major annual events including the Kilkivan Great Horse Ride (held each April – this event routinely draws close to 1,000 horses and riders), the Kilkivan Campdraft (held in September each year) and the Kilkivan Beef Festival (held in late September). All these events, to some degree, utilise the showgrounds north of the town on the floodplain of the Wide Bay Creek.

Flood behaviour, and resulting issues/risks The town is set back from the Wide Bay Creek, and major flooding has limited direct impact on residential and commercial properties in the town. One Mile Creek, through the eastern part of town, and drainage channels parallel with Mudlo Road are affected by local rainfall, but there appear to be few, if any, directly affected residential or commercial buildings.

A key impact of flooding of Wide Bay Creek, is the propagation of overbank flows on the southern side of the creek affecting the showgrounds and rural residential properties. Estimates of the 100 year ARI flood indicate flood depths on the floodplain in excess of 1.5 m, and anecdotal observations suggest quite high velocities of heavily debris laden flows resulting in significant damage to community infrastructure.


Figure 4 Mayor Ron Dyne inspects flood damage at Kilkivan Showgrounds (Gympie Times)

The surrounding topography results in virtually all Wide Bay Creek overbank flows to be directed through the showgrounds, as there is higher ground on the northern side of the creek which prohibits the propagation of flood flows in this area. The large expanses of floodplain upstream of the showgrounds means that, during future flood events, the infrastructure necessary to support the showgrounds operations (ie fences, stock holding areas) will continue to be exposed to flood debris damage.

Type and number of flood affected properties During a major flood event, the showgrounds arena and associated buildings are directly affected by fast flowing debris laden floodwaters, to depths in excess of 1.5 m. Downstream, a significant portion of three rural residential properties that back onto Wide Bay Creek are also affected, however, the inundation does not coincide with the footprint of buildings on those lots.

Farther downstream (east), there are several lots that back onto the creek that are zoned as rural, and aerial photography indicates that some of these properties have residential buildings potentially subject to above floor inundation. As these properties are located near the downstream boundary of the flood modelling (GHD 2012), it is not clear if the inundation shown in this area is a true reflection of 100 year ARI flood behaviour, or an artefact of the model formulation.

Conceptual options to address flood risks At the 100 year ARI flood level, flooding in Wide Bay Creek directly affects the buildings of a number of rural properties located at the downstream end of the flood mapping area. Flood levels in this area should be reviewed and the location and type of future development in the flood fringe be managed through planning scheme controls.

In the showgrounds area, improved resiliency could be offered by reducing the exposure of showgrounds infrastructure to flood debris damage. For example, permanent fencing could be re-aligned with the flood flow direction to minimise debris impact damage, but fencing that is required transverse to the direction of flood flows could be temporary, erected during events, or removed based on flood warning advice. Strategically placed flood defences, mounds or levees could also be designed to redirect flood debris around permanent infrastructure, and these may be enhanced when coupled with dedicated bypass floodways, free of structures and trees. Finally, relocation of the showgrounds to a less active part of the floodplain may be worthy of consideration.


Number who may benefit There are few residential or commercial properties directly affected by flooding. Accordingly, the key benefits of flood mitigation works relate to the improved resiliency of the showgrounds. This would benefit the wider community, with reduced costs during flood recovery periods and enabling the showgrounds to be available for events a short time after flooding has receded.

Challenges/difficulties/issues The type and size of such flood defence at the showgrounds would need to be designed recognising that most of the Wide Bay Creek overbank flows are directed through the site. Any reduction in conveyance capacity caused by flood defence works may result in increased flood levels upstream, and/or increased scour in adjacent areas where flow in concentrated. As noted above, such flood defences may need to be coupled with the bypass floodways.

Recommendations It is recommended that Council and the community give consideration to implementation of a range of flood defence options to improve the resilience of the showgrounds site to future flood damage risk.

There appear to be a number of rural zoned lots at the north-eastern end of the study area that have residential properties within the flood inundation extent. Flood levels in this area should be reviewed and the location and type of future development in the flood fringe in these and other areas be managed through appropriate planning scheme controls.

3.3.4 Widgee

Introduction Widgee is a small rural community located about 25 km west of Gympie in the valley of Widgee Creek. The rural residential hub lies primarily along the Gympie-Woolooga Road and Power Road. The countryside is rural pastureland used for beef cattle and dairy grazing. The community hosts the Bushman’s Carnival each May (a part of the professional rodeo circuit).

Flood behaviour, and resulting issues/risks Flooding in the Widgee community is governed by Widgee Creek, Station Creek and Little Widgee Creek. Widgee Creek flows from the south-west through town and enters Station Creek, which subsequently flows into the Mary River downstream of Gympie. The Gympie-Woolooga Road crosses Widgee Creek at a significant bridge crossing. Little Widgee Creek crosses the Gympie-Woolooga Road and joins Station Creek about 700 m downstream of the Widgee Creek confluence.

Widgee, Station and Little Widgee Creeks can cause flooding in Widgee, either independently or together. Available flood mapping shows the 100 year ARI flood breaks out from the creek channels and inundates significant portions of the floodplain. Road access is cut in two locations on the Gympie-Woolooga Road (at Widgee and Little Widgee Creek crossings) plus at the intersection with Power Road. Numerous rural residential properties are flooded, although typically this is the lower parts of the blocks away from habitable dwellings.


Figure 5 Gympie-Woolooga Road crossing of Widgee Creek

The main impact resulting from flooding up to the 100 year ARI event is loss of access via Gympie-Woolooga and Power Roads. This would only be an issue at or near the flood peaks, and given the relative sizes of the catchments (Little Widgee Creek 33 km2, Widgee Creek 109 km2), would not be expected to close these roads for extended periods (ie a matter of hours for Little Widgee; 6 to 12 hours for Widgee Creek).

Type and number of flood affected properties Flood assessments have identified some 78 rural residential, nine rural and one Community Use properties are directly affected (ie inundated or partly inundated) by the 100 year ARI flood event. A further 166 rural residential, 42 rural, 17 industrial, three community use and one commercial properties are indirectly impacted by the 100 year ARI flood by having their access cut.

Conceptual options to address flood risks Given the primary impact is the loss of access via the flooding of the Gympie-Woolooga Road, an upgrade to the Widgee Creek road bridge and the little Widgee Creek culvert are the appropriate options. This would involve some additional waterway area combined with a raising of the road gradeline to be above the design flood level.

Number who may benefit This option does not reduce flood risk to particular properties, rather benefits the 300 or so properties that currently lose access during major flooding. Therefore the benefit is across the whole local community.

Challenges/difficulties/issues The Gympie-Woolooga Road is a Main Road and therefore owned and managed by the Queensland Department of Transport and Main Roads (TMR). As such Council has no control over this road and any upgrade would need to be pursued by TMR. No cost estimate has been prepared at this stage, but typically road and culvert upgrades are relatively expensive.


Recommendations Council should consult with TMR regarding any current or future plans to upgrade the Gympie-Woolooga Road and discuss whether an upgrade to the road is possible and what funding mechanisms might be appropriate.

3.3.5 Woolooga

Introduction Woolooga is a small rural community on Wide Bay Creek, located approximately 35 km west of Gympie. It offers a range of services to the local surrounding rural community.

Flood behaviour, and resulting issues/risks The January 2011 and January 2013 floods resulted in significant inundation of the town, with numerous buildings and residences flooded well above floor level.

Figure 6 January 2013 flooding in Woolooga

Major flooding is associated with catchment derived flows in nearby Wide Bay Creek, and can be exacerbated by local catchment flows discharging through the town. Reports of flood conditions in January 2013 suggest that rainfall in the local catchment was the primary source of flooding (Gympie Times article 1738879). However, high water levels in Wide Bay Creek can back up into this local catchment, and it is considered that these combined influences resulted in the observed flooding of January 2013.

Type and number of flood affected properties Design flood level information is not currently available for Woolooga, and potential flood inundation areas are derived from an estimate of the floodplain extent (prepared by QRA). Nevertheless, the floodplain extent and photography from January 2013 compare well, and the floodplain extent is considered a reasonable representation of the observed flood conditions.

It is understood that at least eight commercial and 26 residential zoned lots were directly affected by January 2013 flooding. This included the community hall, local pub and shops.

Conceptual options to address flood risks The local catchment drains about 256 Ha of rural grazing lands, at general catchment slopes of 1 in 20. The drainage line for this catchment generally follows Bauple-Woolooga Road and passes through the middle of Woolooga prior to discharging to Wide Bay Creek. Flood flows generated in this catchment may affect rural and rural residential properties at the southern end of Chamberlain Street, as well as residential properties in the middle of town. Increased detention capacity of the wetlands


south of Fraser Street may offer relief from local catchment flooding for approximately four flood affected properties farther down this local catchment drainage line.

At estimated 100 year ARI flood levels, the Wide Bay Creek floodplain and flood extents back up through the local catchment and flood the town. A 450 m long levee, within the footprint of the now disused railway line, and up to 3 m high may offer the town relief from up to 100 year ARI flooding from Wide Bay Creek. A backflow prevention device (flap gated structure) will be required, accommodating local catchment flows at the confluence with Wide Bay Creek. A preliminary cost estimate for such a levee is $1.3 million.

Number who may benefit Increased detention capacity of the wetlands south of Fraser Street may offer improved relief from local catchment flooding for approximately four flood affected properties downstream of the wetlands.

A levee, providing protection up to 100 year ARI flood flows backing up from Wide Bay Creek into the local drainage line and inundating properties will benefit up to eight commercial and 26 residential properties. Should these major works be considered, then improved flood detention in the local catchment may be prudent to ensure that flood conditions during minor events are not exacerbated in the town due to the levee.

Challenges/difficulties/issues As noted earlier, design 100 year ARI flood levels in Woolooga have not been determined, and design of any flood infrastructure would require such flood behaviour to be quantified.

A key design challenge for the levee is crossing the local drainage path – preserving local drainage conveyance while prevent backflow up the local catchment. Flap gated backflow devices are considered appropriate, but will require maintenance and protection from vandalism. Further, these devices can be jammed open by debris, allowing backflow to occur. Given the location, in a rural floodplain setting, the potential for clogging by debris exists and careful design will be required. Backflow devices can also interfere with fauna migration up and down creeks, and the need to cater for this will need to be considered within the context of this local creek system.

Recommendations The lack of design 100 year ARI flood information at Woolooga is a concern from emergency management and town planning perspectives. It is recommended that Council commission a flood investigation of this section of Wide Bay Creek, utilising appropriate flood modelling methods, with model calibration to the January 2013 flood event. The objective of the investigation would be to better define flood behaviour throughout the town for planning purposes, and provide design guidance on the location and geometric characteristics of a flood levee to the west of the town. A detailed and comprehensive costing and cost-benefit of the levee could then be undertaken.

3.3.6 Kandanga

Introduction Kandanga is a rural community on Kandanga Creek, a tributary to the Mary River. It is located approximately 25 km south of Gympie and is a key service hub for the local Mary River Valley community. All properties in the town are zoned “Village” and there is no differentiation between residential and commercial properties in the planning scheme information provide to the study team.


Flood behaviour, and resulting issues/risks Kandanga can be affected by local catchment flooding of Kandanga Creek, and this is often exacerbated by concurrent flooding in the Mary River. During such conditions, the Mary Valley Highway can be closed and Kandanga Creek can further isolate the town closing the Kandanga-Amamoor Road to the north and Kandanga Creek Road to the west.

Kandanga has a well-documented history of flooding, with the Bowls club displaying flood photography from 1974, 1989, 1992, 1999 and the 2013 flood events. In all cases there was significant inundation and damage to the facilities and buildings.

Type and number of flood affected properties Design flood information is not currently available for Kandanga, and potential flood inundation areas are derived from an estimate of the floodplain extent (prepared by QRA). Nevertheless, the floodplain extent and photography from January 2013 compare well, and the floodplain extent is considered a reasonable representation of the observed flood conditions.

The flood extent estimate indicates up to 25 properties are directly affected with part or all of the property inundated. A few properties at the south-west of town on Main Street and Summerville Road are located on the lower floodplain, and may be affected during much more frequent events, however, most of these properties are high set, with some inherent flood resilience.

The Bowls Club is located next to Kandanga Creek just upstream from the railway and road crossing. At this location, the floodplain is relatively narrow and flood velocities are quite high, as evidenced in flood photography on display (Note the significant flood gradient as floodwaters pass between the club house and caravan).

Figure 7 January 2013 flooding at the Kandanga Bowls Club

During larger floods, a key risk is that the bowls club building will be swept away, potential impacting and severely damaging the Kandanga-Amamoor Road and historic Rattler bridge.


Conceptual options to address flood risks The presence of community assets such as bowls greens, carparking, swimming pools and football fields within the floodplain is an accepted practice, which balances land use with flood conveyance and storage needs. However, buildings and other facilities in the floodplain, that by their presence may adversely influence flood behaviour, is considered inappropriate. The Bowls Club buildings appear to be located in the main overbank flowpath, and together with the narrow floodplain, their presence is far from ideal, and may be exacerbating flood scour in the creek channel. Levees or floodplain filling to offer flood resilience to the building is considered inappropriate, as this section of the floodplain is relatively narrow and further reductions in floodplain storage or conveyance would have significant impacts elsewhere. Relocation of the Bowls Club building away from this high energy flood conveyance area is suggested, assuming a nearby location for the building is available in order to maintain access to the existing greens. Should a nearby location not be available, it may be prudent to consider relocation of the Bowls Club, buildings and greens, away from this highly at-risk floodplain area.

Number who may benefit Voluntary buyback, land swap or relocation would benefit up to eight properties on the lower floodplain that appear to be exposed to frequent flooding.

The Bowls Club is a recognised hub of the community, and is located in a flood prone area of high hazard (high velocity). Relocation of the Bowls Club buildings would reduce exposure to this hazard and potential damages to the road and rail bridges across Kandanga Creek. Relocation may also result in a minor improvement to flood conditions in other parts of Kandanga.

Challenges/difficulties/issues As always, there are equity issues with buyback, and challenges in sourcing funding to carry out such schemes. Relocation of the Bowls Club building faces the challenge of findings a suitable location such as not to lose the identity of this important community asset.

Recommendations The lack of design 100 year ARI flood information at Kandanga is a concern from emergency management and town planning perspectives. It is recommended that Council commission a flood investigation of this section of Kandanga Creek, utilising appropriate 2D flood modelling methods, with model calibration to the January 2013 flood event. The objective of the investigation would be to better define flood behaviour throughout the town for planning purposes.

Assuming a nearby location is not available for the relocation of the Bowls Club building, it would be prudent to consider relocation of the Bowls Club, buildings and greens, away from this highly at-risk floodplain area next to Kandanga Creek.

3.3.7 Imbil

Introduction Imbil is the largest town in the lower Mary Valley, located about 10 km downstream (north-east) of Borumba Dam and 30 km south of Gympie. Hynes Sawmill is a significant local industrial site located just to the west of Imbil. Cattle and timber are the main elements of the local economy.


Flood behaviour, and resulting issues/risks Imbil is located on the southern bank of Yabba Creek, a major tributary of the Mary River. Borumba Dam is located in the headwaters of Yabba Creek. Fortunately the creek channel is still quite incised in the immediate vicinity of Imbil, hence when the creek floods, the floodwaters remain essentially contained within the creek and its immediate confines. Just downstream (east) of Imbil, the flood flows spill over on a broader floodplain.

The 100 year ARI flood event results in relatively little significant impact to properties. Whilst numerous properties are shown as inundated, the habitable dwellings typically are above the 100 year ARI level. The exception to this looks to be a handful of dwellings on Yabba Creek Road overlooking the creek. The 100 year ARI flood also cuts the main Yabba Creek Road just west of town, and cuts the Kandanga-Imbil Road at the creek crossing.

Type and number of flood affected properties The GHD flood study indicates that the 100 year ARI flood directly impacts (ie inundates) part or all of 13 commercial, 16 residential and 12 rural residential properties. As noted above however, in most cases the inundation does not affect the habitable dwelling on the property. There are a handful of houses that appear likely to be inundated on Yabba Creek Road overlooking the creek.

The 100 year ARI flood indirectly impacts (ie loss of access/isolation) up to 20 commercial and over 250 residential or rural residential properties.

Conceptual options to address flood risks Given the small number of dwellings directly affected, one immediate practical option would be a voluntary buyback of those properties with dwellings at risk that were not flood resilient (or capable of being made flood resilient). Such an option carries risk in relation to equity/public perception issues, plus residual risk for those not impacted.

A second option is improved flood warning for Imbil. The Bureau of Meteorology operates a flood warning system for the Mary River catchment however the flood warnings issued are focussed on Gympie and Maryborough only at this stage. This option involves an upgrade to the system so that warnings of predicted river gauge height could be issued for Imbil to assist local residents with preparation and if necessary evacuation before the onset of a flood. This option would involve a significant cost for the additional real-time gauges and other equipment involved plus a significant community awareness program would be required.

A third option is to upgrade the Yabba Creek crossing of the Kandanga-Imbil Road, with resulting improvement in access during flooding.

Number who may benefit For the buyback option, the total number of properties involved is five commercial and five residential.

For the improved flood warning, some 65 properties would be able to directly benefit from this scheme.

Challenges/difficulties/issues Property buyback can be difficult politically, but would be best explored with impacted residents one-on-one to gauge support. The dwellings involved should be inspected carefully for opportunities to improve resiliency.

As Imbil is located relatively high in the catchment, upgrading the flood warning system will realistically only enable warning times in the vicinity of a few hours. By comparison, the existing warning systems try to offer Gympie and Maryborough warning times of 21 to 27 hours and 18 to 24 hours respectively.


Warning times of only a few hours may not be worth the investment, but the idea does warrant further investigation.

As with all road upgrade options, capital cost of an upgrade to the Kandanga-Imbil Road would be significant. The upgrade may also worsen flooding for downstream properties.

Recommendations The properties with habitable dwellings directly impacted should be inspected to determine their level of resilience and also to discuss with owners the possibility of buyback.

A flood warning system specifically for Imbil should be investigated with BoM.

3.4 Eastern district Introduction This district is centred around the towns of Cooloola Cove, Rainbow Beach and Tin Can Bay.

Cooloola Cove is a recently developed (late 1980s) township of about 2000 on the south-west shoreline of Tin Can Bay inlet.

Rainbow Beach is a coastal village located between Tin Can Bay and the Pacific Ocean, with a population of about 1000. The village is a popular tourist site with fishing, four-wheel driving, camping, bushwalking and various other activities on offer.

The town of Tin Can Bay is located on the west side of the bay, with a population of about 1900. The predominant activity is boating and fishing.

Flood behaviour, and resulting issues/risks The area is subjected to flooding from two main mechanisms, being freshwater flooding from the local catchments that feed Tin Can Bay inlet, or storm tide inundation, or both. Numerous small creeks/flowpaths traverse the relatively low lying land surrounding Tin Can Bay.

For freshwater flooding, there is minimal flooding of properties in the townships up to the 100 year ARI flood event. However there are significant issues with access, with the main access roads (being Tin Can Bay Road and a number of other connectors) being cut during freshwater flooding. A particular issue is the Coondoo Creek bridge crossing along Tin Can Bay Road – this crossing is inundated and up to 3 m underwater in major flooding. Anecdotally this bridge can be cut and out of service for two to three days per flood event. There are several other flood prone crossings along the route to Tin Can Bay and Rainbow Beach including Carland Creek, Ross Creek, Searys Creek and other minor crossings. These can also be cut by flooding, but typically for relatively short periods (several hours), and often coinciding with road closures at Coondoo Creek. Accordingly, the greatest improvement in flood resilience for the Tin Can Bay, Cooloola Cove and Rainbow Beach communities would be offered by reducing the frequency and period of closure of the Coondoo Creek crossing.


Figure 8 Flooding at Coondoo Creek

Regarding storm tide inundation, this results in minimal property impacts to both Cooloola Cove and Rainbow Beach, but does affect some properties in Tin Can Bay township (at the 100 year ARI event level). This type of inundation moves in to affected areas like an extreme high tide, with resulting elevated levels lasting for many hours before receding. It can also flow back up through any stormwater systems potentially causing flooding of streets and houses even if the frontal dune isn’t overtopped. Overall inundation times can therefore be for many hours.

Type and number of flood affected properties In total there are around 4000 properties at risk of isolation during flooding because of the vulnerability of the access roads to flooding. This number is made up of:

Commercial 66

Industrial 74

Residential 3718

Rural Residential 68

The actual number of properties at risk of storm tide inundation is approximately 20, but with numerous others borderline. These are predominantly residential properties.

Conceptual options to address flood risks Crossing upgrades are the obvious solution to the access problem. A significant upgrade (ie approximately a 3 m lift in grade line) to the Tin Can Bay Road bridge crossing Coondoo Creek would be required to give an appropriate level of immunity to the road. Preliminary estimates for this level of upgrade are around $3 to 5 million.

The most appropriate option for the properties at risk of storm tide inundation in Tin Can Bay township would be some temporary (demountable) barriers – such barriers could be stored at site and erected based on a storm tide warning and prior to any flooding. These could be then removed once the threat had passed. Various proprietary products are available for such purposes.


Number who may benefit As noted above, if the road crossings were upgraded, the some 4000 properties would benefit from having flood free access up to the 100 year ARI event for flooding in Coondoo Creek and adjacent catchments.

It is estimated about 20 residential properties could benefit directly from some temporary storm tide barriers in Tin Can Bay township.

Challenges/difficulties/issues The environment adjacent to the Tin Can Bay access roads is sensitive and in some areas pristine (Cooloola National Park). Therefore a road access upgrade would need to be undertaken using a high level of environmental management, with associated costs. Significant environmental constraints exist with these options. It is also noted that Tin Can Bay Road is a State controlled Main Road, hence Council does not control this asset.

The main issue relating to the use of temporary barriers to prevent storm tide inundation is the risk involved in their proper use. To be effective, those installing the barriers need to be well-drilled in their erection and operation during the event. Furthermore the barriers need to be well-maintained as there will usually be little warning time available between becoming aware of the specific storm tide threat and the onset of inundation. There exists a residual risk of collapse during an event, with resulting rapid inundation.

Recommendations Further investigation is required into the feasibility and costs of the bridge upgrades nominated. As noted above, Tin Can Bay Road is owned by TMR, and the responsibility for undertaking such upgrade works would rest with the State.

Further investigation is required into the use of demountable barriers for Tin Can Bay. These types of solutions can be cost-effective provided the residual risks are understood and well-managed.

3.5 Gympie

3.5.1 Introduction Gympie has a recognised history of flooding from both riverine flood events emanating from the Mary River and also from local runoff from tributary streams such as Deep Creek. Since records of flood levels at Gympie began being collected in the 1870s, at least 16 flood events have reached Major Flood classification level at the Kidd Bridge gauge (greater than 17 m gauge datum). The Kidd Bridge gauge provides the reference level for flooding in Gympie, and gauge zero is equivalent to 36.93 m AHD (Australian Height Datum).

What is most striking about the flooding in Gympie is the vertical range of flood levels in the Mary River at Kidd Bridge (refer Figure 1). Inundation of the City Centre starts to occur at around 16.0 m (52.93 m AHD), or around the 5 year ARI flood level. The 100 year ARI flood level is some 9.2 m higher than this with the highest recorded flood in Gympie occurring in February 1893 at 25.45 m above gauge datum (62.38 m AHD).

The screening assessment investigated flood issues through Gympie, splitting the city into eight districts or precincts, and flood levels in each of the precincts were derived relative to the statistical flood levels at Kidd Bridge. That is, coincident extreme flooding in, for example, Deep Creek and the Mary River has not been assessed, and flooding in those precincts that border Deep Creek has been assessed in consideration of backwater effects from the Mary River.


3.5.2 Mary Street (CBD)

Introduction Gympie has a deserved reputation for flood resilience, and the community’s ability to quickly recover is envied by many other less prepared and/or flood aware communities. Nevertheless, flooding of the City Centre is extremely disruptive for the businesses that are directly affected as well as those nearby, who may not be directly flooded, but are affected by power outages and evacuation procedures.

Flood behaviour, and resulting issues/risks Floodwaters from the Mary River enter the city centre as flood levels exceed around 16 m at Kidd Bridge. This flood behaviour is a backwater effect, with the CBD area providing offline flood storage for the Mary River. The relatively low levels of immunity that currently exist, together with the significant vertical range in flood levels can result in significant above flood level inundation.

The Bureau of Meteorology broadcasts flood warnings with warning times generally in excess of 24 hours. Regular warning updates provide the community with more precise projections of flood levels and this information guides evacuation needs.

The January 2013 flooding peaked at 19.98 m on the Kidd Bridge gauge, resulting in flood depths at the lower end of Mary Street in excess of 2.5 m, and resulting in inundation of over 100 properties, both business and residential. This flooding lasted for a number of days, and followed a flood of similar magnitude occurring in January 2011.

Figure 9 Flooding in Mary Street, January 2013 (Source: Gympie Times)


Type and number of flood affected properties At the 100 year ARI flood level, some 5 m higher than the January 2013 flood, up to 180 businesses are directly affected, with many others closed due to power outages and closure of the city centre. Around 20 residential properties may be flooded above floor level with many others affected by flooding due to restricted access and/or loss of services.

The 1999 flood (21.95 m at Kidd Bridge) was reported to have inundated around 130 businesses in the city, and around 80 within the CBD. The 2013 flood was around 2 m lower than the 1999 event, and directly affected a similar number of businesses.

Conceptual options to address flood risks Flood mitigation in the city centre is challenging. The vertical range of flood waters and the density of urbanisation results in many properties being affected, and land available for the installation of structural flood mitigation is limited. Levees restricting flood inundation offer a potential solution. Such options have been progressed for further assessment.

Another option explored to reduce damage costs to existing businesses and residences from flood events within the Mary Street CBD precinct is the buyback of properties within the flood zone. This option has been evaluated below by aggregating the buyback cost of properties likely to be subject to above floor flooding for 5 to 100 year events.

The valuation methodology used flood mapping of the current ARI flood event periods across the Mary St CBD area to identify the flood affected properties eligible for buyback. Each identified property and their relative position and propensity for inundation were then detailed for 5 to 100 year ARI flood event levels. These properties were then matched using the plot-lot identifier with their RP Data ownership profile. Properties which have had ownership transfer and the price at which the transfer took place have been limited to the past 5 years of sales data. Property price details were then allocated and rationalised over the size of the plot area to determine a property value rate ($/m2).

The land value per square metre was then used to extrapolate values for adjacent properties which have not had recent transactions. The aggregated buyback costs were determined for the 5 to 100 year ARI flood events which are published in Table 2 with the number of properties affected also reported.

Table 2 Mary Street CBD Business property buyback cost by ARI (aggregated costs)

ARI Flood Event (years)

No. of properties for Buyback

Buyback Cost – lower estimate ($millions)

Buyback Cost – upper estimate ($millions)

5 5 $1.8 $3.0

10 36 $16.5 $28.2

20 56 $29.4 $50.1

50 90 $47.9 $81.7

100 109 $60.3 $102.7 The number of affected properties described in Table 2 is not equal to the number of businesses within the buyback zones since there is more than one business on some properties.

The results in Table 2 estimate that the buyback of the five most at-risk properties located within the 5 year ARI flood affected area will cost between $1.8 and $3 million. It is further estimated that the 10 and 20 year ARI affected properties will cost between $16.5 and $50.1 million as 36 and 56 properties are inundated respectively. These are similar magnitude to costs for some of the levee options. The buyback cost for the 50 and 100 year ARI options are an order of magnitude higher with the estimate


cost between $47.9 and $102.7 million. The number of properties inundated during the 50 and 100 year ARI flood events is only 19 properties different since the Mary Street flood basin is very steep.

This method of valuation has been conducted at a high level and uses a simplified approach that does not account for the type of business and relocation costs; and therefore can be considered as the minimal cost for a buyback solution. The benefit of the reduced residual flood risk generated from property buybacks in flood zones has not been priced into the buyback cost, but it should be recognised that the improvement in flood resilience for resumed properties extends across the full spectrum of flooding, and is not limited to a particular flood immunity of a levee.

RP Data can be considered as the appropriate data source for conduct the valuations using the most recent sales and property data. In conducting the analysis, RP Data recommends a Forecast Standard Deviation (FDS) of 26% for property valuation estimates from the Gympie sales data which has been applied for this analysis. This volatility is evident in a recent property valuation survey completed by the Department of Natural Resources and Mines Queensland (DNRM) which has indicated that at 1 October 2012 properties within the Gympie township have decreased 10% during 2012 in value across most areas with a 20% and 23% drop in industrial and commercial properties respectively. This volatility is primarily attributed to broader ongoing economic uncertainty particularly in the retail and services sector, but nevertheless highlights that property values fluctuate with economic conditions.

City-wide flood mitigation options (dam upstream or channel works downstream) are discussed separately in Sections 3.5.10 and 3.5.11).

Number who may benefit The number who may directly benefit from flood mitigation works will depend on the level of flood immunity that can be offered. At a 100 year ARI flood immunity level, up to 180 businesses and 30 residences could receive direct benefit, and at the 20 year ARI level, around 100 businesses and 10 residences directly benefit. Notwithstanding the inconvenience during times of flood, the key benefit of mitigation works for both the business community and residences is the reduced frequency of flooding, and the commensurate reduction in flood repair and recovery cost.

Moreover, improved flood resilience in the heart of the community is considered to offer wider benefits across the district, reducing the impact of flooding at the secondary and tertiary level, and allowing day-to-day activities to go on and promoting faster recovery.

Challenges/difficulties/issues There are a number of critical engineering design issues relating to levees to be understood and resolved. These are summarised as:

Coincident Flooding – major local rainfall within the Mary Street catchment coinciding with flooding in the Mary River. The local rainfall will be trapped behind the levee, stored and/or pumped over the levee crest.

Geotechnical Considerations – Nelson Reserve, the Bruce Highway and Albert Park are all, to some degree, constructed on landfill areas. In addition to specific investigations regarding geotechnical capacity of the underlying material, there may be the need to increase the design of the levee core to provide additional shear strength.

Flood infiltration through Mines – the area was heavily mined during the gold rush period, and there are dozens of mine pits are identified by DEHP (formerly DERM) within the Mary Street precinct. While many of these have been filled and/or capped, the potential underground linkages of adjacent shafts remains a significant risk.


Residual Flood Risk – this relates to the selection of a design immunity and height of the levee. Unless design to the probable maximum flood (PMF) level, the potential for the levee to be overtopped remains. A spillway is required to manage overtopping, but the perception of flood “protection” offered by the levee may result in a decline in flood awareness and preparedness, which can significantly influence flood damage costs, evacuation efficiency and overall community mindset. Accordingly, an acceptable level of residual flood risk is a key consideration.

A key concern with flood mitigation actions, and particularly levees, is the potential for such infrastructure to result in increased flooding in other locations, caused by the displacement of flood storage. At the 20 year ARI flood level, Mary Street area offers around 650,000 m3 of flood storage. Should this flood storage be lost due to flood mitigation, there may be increases in flood levels along the Gympie Reach of the Mary River. Preliminary estimates indicate an increase in flood levels in adjacent areas of up to 100 mm, and numerical modelling would be required to confirm this (it is expected that modelling would indicate a much lower impact, as the flood storage area in the City Centre is wholly offline from the Mary River channel and floodplain).

Relocation of affected properties to an area outside the flood extent offers an option where the risk of inundation is eliminated. Buyback or land swap and relocation presents challenges in terms of equity. Moreover, it should be recognised that the loss of the historic city centre through relocation may result in significant social impacts and loss of identity. Further, identification of a suitable alternate site will present its own challenges. Ultimately, a key stumbling block for buyback or land swap is identifying a funding source that can be used for such activities, with costs potentially running into the hundreds of millions of dollars. The total flood mitigation funding pool from Queensland Government in 2013 is $40 million.

The Gympie city centre has some particular challenges in relation to mine voids from disused mining activities. Such voids can cause settlement or subsidence to overlying ground, with resulting foundation risks. The locations of such voids needs to be mapped and understood when considering mitigation options with foundation issues (eg levees). Similar foundation constraints may exist in old landfill sites as well (ie Nelson Reserve).

Moreover, the potential for inundation via floodwaters moving through connected mineshafts is of unknown magnitude but considerable consequence. The Department of Natural Resources and Mines (DNRM) confirmed that there is a likelihood that mines as far away as the Monkland to be connected to mines in the Mary Street precinct (pers comms Hillary Kjorstad, DNRM). This was evidenced during a dewatering exercise, when mine vents that usually breathe out changed to a sucking habit. While most of the known mines in Gympie may be capped, the nature and integrity of such capping is unknown, and it is possible that floodwaters could enter the interconnected system of mineshafts, with failure of a cap resulting in flooding within the protected area. Appendix C includes maps provided by DNRM showing the location of known mines and mine shafts in the Gympie CBD area. Anecdotal evidence suggests that there exist additional mines for which the location and depth is unknown.

Recommendations There are a number of businesses located in the most flood prone area bounded by Monkland Street, Jaycee Way and River Road. These properties become inundated to depths of up to 1 m during the 5 year ARI event (16.05 m Kidd Bridge, 52.98 m AHD). Council should consider voluntary buy back for properties at such a high level of flood risk exposure.

Levees offer a targeted benefit, and may be suitable to provide improved flood resilience in the city centre. However, it is unlikely that a viable 100 year ARI flood mitigation option exists, due to the size of structure required (height ~12.5 m, and footprint ~75 m wide). Moreover, a smaller levee may offer a significant reduction in exposure to more regular flood events, which may prove to be cost effective


given the high degree of exposure for much of the city centre. A detailed examination of levee options at a reduced flood level is recommended.

3.5.3 Commissioner Gully

Introduction The Commissioner Gully area covers a precinct north of the CBD between Channon Street and Pinewood Avenue at the northern end of the city. The precinct includes three steep sided gullies that intersect the Bruce Highway and all are affected by flooding from the Mary River. The northern end of the precinct can be affected by both backwater flooding from the Mary River and locally generated flows in Two Mile Creek.

Flood behaviour, and resulting issues/risks Backwater flooding from the Mary River can extend up to 1 km from river centre line back into the urban area, and 100 year ARI flood depths at the Bruce Highway can be in excess of 4 m. Bruce Highway closures in this precinct begin at around the 5 year ARI flood level (16.05 m Kidd Bridge gauge).

Type and number of flood affected properties At the 100 year ARI flood level, up to 80 commercial properties are affected, and such flooding directly impacts more than 150 residential properties. The sewage treatment plant is located at the western end of Pine Street, and is partially within the 100 year ARI flood envelope. The northern industrial precinct is also affected (around 25 individual properties).

Conceptual options to address flood risks The depth of inundation and the steepness of the gullies render flood mitigation extremely problematic. While theoretically possible to offer flood resilience via strategic local levees protecting individual or groups of properties, available space and topographic constraints indicate such options would be cost prohibitive. A combination of resumption or voluntary buyback, raising of individual properties, and structural flood walls may offer improved resilience, and would need to be investigated on a site by site basis.

Broad scale levees, for example a levee along the alignment of Brewer Road, may offer protection to around 10 businesses, and 30 or more residences. At the 20 year ARI flood level this structure would be over 10 m high, with a footprint of 65 m. At this low immunity level, provision for overtopping would be required. Costs would be similar to levees protecting the CBD, but with considerably fewer beneficiaries. A cost benefit analysis is unlikely to identify such options as cost-effective.

Number who may benefit As noted above, site-by-site strategic levee, building or floor pad raising, and buyback may benefit up to 30 or more commercial properties, and could be applied to a further 100 (or more) residential properties. Broadscale options, while potentially benefiting a larger number of at-risk properties, are unlikely to be cost effective.

Challenges/difficulties/issues Flooding occurs in three separate gullies and the depth of flood inundation requires very large flood levees in order to offer any real change in flood resilience.

In common with other flood affected locations, buyback and/or relocation of affected properties to an area outside the flood extent offers an option where the risk of inundation is eliminated. Buyback or land swap and relocation presents challenges in terms of equity and identify at what level of flood risk buyback or relocation is warranted – some businesses, by their nature, are quite flood resilient and


provided they have sufficient flood warning times and support, can readily continue in their flood prone location.

Recommendations Site-by-site flood resilience works offers a real opportunity for each business to identify its own risk appetite, and act accordingly. As much of the flooding in this precinct is due to backwater, loss of floodplain storage due to filling or site specific protection measures will have limited impact on flood levels elsewhere. Council should develop a strategy to assist commercial and residential property owners in selecting site-by-site flood mitigation options consistent with a desired level of flood immunity.

There are a number of businesses (particularly those between Jane and Pine Streets) that are subject to inundation at the 5 year ARI level. Voluntary buyback for these highly at-risk properties should be considered.

3.5.4 Two Mile

Introduction Two Mile is a precinct at the northern end of town and includes the Bruce Highway corridor. It is mostly rural with some commercial areas on east side of highway.

Flood behaviour, and resulting issues/risks This precinct is affected by local flooding from Two Mile Creek, but flood levels and flood behaviour is dominated by backwater effects arising from flooding of the Mary River. Such backwater flooding closes the Bruce Highway and at extreme flood conditions, requires flood preparation actions at a number of commercial properties.

Type and number of flood affected properties Up to ten commercial properties are partially inundated at the 100 year ARI flood level. However, there does not appear to be any above floor flooding associated with the 20 year ARI event conditions. Closure of the Bruce Highway happens between the 5 and 10 year ARI flood event levels, and during the 20 year ARI event there is up to 4 m of floodwater over the lowest sections of the Bruce Highway.

Conceptual options to address flood risks Raising of the Bruce Highway or protection via a levee to west would improve the resilience of this infrastructure. Such works would also afford improved flood protection to properties on the east of the highway and not affect local flood behaviour in Two Mile Creek. However, it is unlikely TMR would support such works (in principle or via shared funding), as a commitment to 100 year ARI flood free transport network has been made via the Cooroy to Curra Bruce Highway upgrade.

Given the few number of properties affected, voluntary buyback would appear feasible but the flood risk exposure for these properties in much less than in other areas.

Number who may benefit Additional works to raise or protect the Bruce Highway from closure due to flooding would benefit a wide cross section of the community, both local and farther afield, that utilise this vital transport corridor.

Voluntary buyback could resolve up to 100 year ARI flood exposure issues for up to ten commercial properties.


Challenges/difficulties/issues The priority or need for these options is significantly lower than in other areas, where many more properties are exposed to flood risks at a much higher frequency. The limited benefit offered by either option would not stack up economically.

Further, a levee offering improved flood resilience for this section of the Bruce Highway is an isolated benefit, as the highway is closed in several other locations during a Mary River flood within Gympie and farther north (eg at the Wide Bay Highway and Tiaro) reinforcing the need to consider a region wide solution, such as that offered by Cooroy-Curra upgrade.

The businesses potentially affected by flooding are located away from the centre of Gympie, and it is understood there is informal flood response planning and procedures to prepare for forecast flooding. Such local scale resilience is encouraged, and should be recognised within the broader framework of emergency management.

Recommendations Businesses that are located within the 100 year ARI flood extent in this precinct need be aware of the flood risks to which they are exposed, and, as necessary, Council should assist these businesses in preparing flood response plans to ensure there is integration with Community wide emergency response activities.

3.5.5 Mount Pleasant

Introduction The Mount Pleasant precinct represents an area south of the CBD to Deep Creek. The key flood risk area is a gully south of Gympie Centro at the intersection of the Bruce Highway with Centro Way and River Road.

Flood behaviour, and resulting issues/risks Flood waters from the Mary River back up into the gully, and close the Bruce Highway and River/Graham Road at around the 5 year ARI flood level. The lower carparks of the Centro were inundated in January 2013. Commercial areas around Hyne Street are inundated during the 100 year ARI event, but are flood free under the 20 year ARI event. Further along, properties on the southern side of the Bruce Highway are zoned commercial, but development of these areas has not taken place. Most of the commercial properties on the north side of the Bruce Highway are above 100 year ARI flood level.

At the 5 year ARI flood level, Pengellys Bridge over Deep Creek on Tin Can Bay Road is open while the Bruce Highway crossing of Deep Creek at Inglewood Bridge is closed. This impacts on the connectivity between the Gympie CBD and locations farther south.

Type and number of flood affected properties At the 100 year ARI flood level, around 20 commercial and industrial businesses are directly affected by flooding, with many others impacted to some degree due to isolation. Around 100 residences may be directly impacted by partial inundation. At the 20 year ARI flood level, these numbers are much lower, with only a handful of properties around Fox Street, Hilton Lane and Tweed Lane affected by inundation. Floor level data is not available to allow identification of the exact number of properties inundated.


Figure 10 Flooding at Centro January 2013 (Source: Gympie Times)

Conceptual options to address flood risks Review of aerial photography and flood mapping information indicates there are few properties at risk of flooding during the 20 year ARI flood event. Site-by-site options (levees, buyback, house raising) are considered appropriate to address the property flood issues identified.

With the Bruce Highway cut at Centro Way and at Inglewood Bridge during a 5 year ARI event, transport connectivity between the CBD and locations further south are severely compromised. During major flood events, large transport vehicles must wait until these sections open before proceeding, but an alternative transport route across Deep Creek (via Pengells Bridge) is available at the 5 year ARI flood level and if able to be accessed means that recovery efforts for the CBD and other areas can be initiated earlier (estimated by Council to offer relief earlier by as much as 6 to 10 hours).

The key elements of creating this alternate B-Double route include the raising of Centro Way/River Road by around 2 to 2.5 m and improving a bridge on Graham Street that crosses the railway.

Number who may benefit Flood resilience is about offering improved opportunities for communities to recover quicker after natural disasters. The alternate flood route offers this with minimal works, benefiting the Mary Street CBD and the Mount Pleasant Precinct. Accordingly, this option is considered to offer benefit to at least 200 businesses that have cleaned up after flooding, but cannot provide access for suppliers due to continued closure of the Bruce Highway at Deep Creek.

Site-by-site flood resilience works may offer improved resilience to a limited number of businesses and residents.

Challenges/difficulties/issues There are no major challenges that may prohibit these transport upgrades. Council have estimated the cost of these works at around $3.3 million.


Recommendations Site-by-site flood resilience works offers an opportunity to improve flood resilience. The major flooding in this precinct is due to backwater, and the loss of floodplain storage due to filling or site specific protection measures will have limited impact on flood levels elsewhere. Council should develop a strategy to assist commercial and residential property owners in selecting site-by-site flood mitigation options consistent with a desired level of flood immunity.

The commercial benefits of transport upgrade in Centro Way and Graham Street should be further assessed, to understand the real benefit of offering southbound Bruce Highway B-Double traffic access across Deep Creek and destinations farther south some six to ten hours earlier than is available via Inglewood Bridge.

3.5.6 One Mile

Introduction The One Mile precinct covers an area east of the Rattler line in Gympie. It hosts residential, commercial and industrial areas, and includes Tin Can Bay Road, which is the primary link from the south to the eastern districts of the Council area.

Flood behaviour, and resulting issues/risks The One Mile precinct is affected by backwater flooding from Deep Creek (and Mary River). It has large flood storage areas utilised as playing fields, but flooding can affect the major transport corridors as well as many commercial and residential properties. Tin Can Bay Road is cut by the 5 year ARI flood event, but alternate routes are available for passenger and small truck traffic from the north. Southerly traffic cannot traverse Deep Creek, and the flooding of Tin Can Bay road in this area is considered secondary to other major isolation issues (Deep Creek, Coondoo Creek).

Type and number of flood affected properties At the 100 year ARI flood level, backwater flooding inundates around 35 industrial properties and in excess of 200 residential properties. In the southern part of the precinct, around Victoria Road and Graham Street, several industrial properties are inundated at the 10 year ARI flood level. Accordingly, these areas flood earlier than other parts of town.

Conceptual options to address flood risks The at-risk commercial areas sit as pockets within the broader precinct and lend themselves to strategic targeted levee options. A ring levee around the industrial properties in the south could be coupled with a levee protecting vulnerable properties in the north, either as a levee parallel with Wises Road, or within the playing fields at the back of selected properties.

Some adjacent industrial properties not able to be included in the flood mitigation option would be subject to voluntary buyback and rezoning to a flood consistent category.

Number who may benefit Around 20 industrial properties would benefit from the mitigation strategy, and in the case of the Wises Road option, a further 20 residential properties would also benefit.

Challenges/difficulties/issues A key challenge is again the vertical range of flooding, meaning large structures are required. The topography near those at-risk properties in the south adjacent to Deep Creek is very steep, and there is insufficient footprint to support an earthen levee form.


Further, as many of these properties are exposed to flood risks at an early level, buyback, rather than protection, may be significantly more cost effective, as there is no residual risk associated with buyback.

Recommendations Examination of levee and buyback options for flood affected businesses in One Mile is recommended.

3.5.7 Monkland and Glanmire

Introduction Monkland and Glanmire are the major industrial precincts in Gympie, providing employment for a large number of residents in Gympie across approximately 200 businesses.

Flood behaviour, and resulting issues/risks Flooding in excess of the 5 year ARI flood level (16.05 m gauge datum) results in closure of both main access routes (Bruce Highway and Tin Can Bay Road) and while much of the area remains flood free, the businesses are isolated from staff, vendors and suppliers.

Isolation of these areas results in lost production, and Gympie Regional Council has estimated the economic losses resulting from such isolation of order $750,000 per day. Recognising that flood isolation can last a number of days, the economic damage of isolation can be significant.

Further, there are around 500 residential properties in the Monkland/Glanmire precinct. This population is isolated from the main parts of town, with limited access to retailers and emergency services during flood events.

Type and number of flood affected properties Around 200 businesses in Monkland and Glanmire are isolated from their staff, retailers and suppliers. There are around 15 businesses that are directly affected by 20 year ARI flooding, but typically this relates to only minor inundation of part of a site.

Around 100 residential properties are directly affected by flooding, most of which can be attributed to backwater affects from the Mary River into Deep Creek. No floor level information is available to accurately document the number of above floor flooded properties, however, inspection of flood mapping indicates that around 25 properties potentially experience above floor flooding during the 20 year ARI flood event.

Conceptual options to address flood risks Strategic levees bordering sections of Deep Creek could offer flood relief to at-risk residential properties in Monkland, however, site specific options may be more appropriate, including house raising and voluntary buyback.

The isolation of the area from the rest of town is a key economic and community safety consideration. Improved access via a more flood resilient bridge across Deep Creek or utilising alternate routes offers significant benefits maintaining desirable levels of connectivity.

During flood events, when the Bruce Highway is inundated, Monkland and Glanmire are connected via Drummond Drive. This steeply graded gravel access includes a weight limited bridge across The Rattler track and currently offers around 20 year ARI flood immune access to smaller vehicles. Improvements to Drummond Drive could offer all-vehicle connectivity between Monkland and Glanmire.


Number who may benefit As noted above, improved access will directly benefit around 200 businesses and up to 500 residential properties. Moreover, the economic benefit would likely extend beyond this precinct, as there are residents cut off from employers located north of Deep Creek.

Challenges/difficulties/issues Eventually, the Bruce Highway bypass will provide up to 100 year ARI flood free access from Gympie to Monkland/Glanmire with interchanges at Gympie Connection Road and Penny Road/Noosa Road. These works are programmed at a beyond 10 year horizon. Gympie to Monkland access is restricted at the 5 year ARI flood level, and statistically speaking, there is an 86% chance of a 5 year ARI (or greater) event occurring sometime in the next 10 years. Accordingly, an interim solution providing an adequate level of flood resilience needs to be set within this context.

Recommendations Connectivity with 100 year ARI immunity between Monkland and Glanmire can be offered through upgrading Drummond Drive, and together with the Bruce Highway offers 100 year ARI flood free access throughout. This is a key infrastructure upgrade consistent with long term regional planning. Additional improvements to Noosa Road, Penny Road and Hall Road may also be required to meet increased traffic needs once the Bruce Highway bypass is in place.

An interim solution that offers connectivity between Gympie and Monkland/Glanmire during times of flood is also identified as a key opportunity to improve resilience.

3.5.8 East Deep Creek

Introduction East Deep Creek is a largely rural and rural residential precinct, east of the railway line. The key transport corridor of Tin Can Bay Road crosses this precinct. The Bruce Highway upgrade Cooroy to Curra route is planned to cross this precinct, just east of the railway line.

Flood behaviour, and resulting issues/risks This precinct is affected by local flooding from Deep Creek, but like many other areas in Gympie, flood levels and flood behaviour is dominated by backwater effects arising from flooding of the Mary River. Such backwater flooding closes many roads including Tin Can Bay Road and numerous local roads.

Flood warning times for local events within Deep Creek are poor, and the accuracy of such local flood warnings is also limited due to a limited number of recording stations within the catchment.

Type and number of flood affected properties Up to 24 rural residential properties are partially inundated at the 100 year ARI flood level. However, there does not appear to be many (perhaps one or two along Ascot Road) potentially affected by above floor flooding. At a 20 year ARI flood level, Tin Can Bay Road is closed at a number of locations, in particular at the intersection with Lockhart Road and again at Randwick Road. Alternate flood free access routes are available.

Conceptual options to address flood risks Given the few number of properties affected, voluntary buyback would appear feasible for properties regularly at risk of above floor flooding. However, in a wider context, the flood risk exposure for these properties in much less than in other areas. Site-by-site improvements may yield better results (eg house raising).

Improved flood warning for local Deep Creek events will enhance the flood preparedness (and hence resiliency) of this community and users of the Tin Can Bay Road.


Number who may benefit Additional works to raise or protect Tin Can Bay Road from flooding would benefit a wide cross-section of the community, both local and further afield, that utilise this vital transport corridor. However, alternate routes are available, such that investment costs may not yield value for money, particularly in consideration of the future Bruce Highway upgrade through this precinct.

Voluntary buyback may be appropriate at a selected number of locations.

Challenges/difficulties/issues The priority or need for these options is significantly lower than in other areas, where many more properties are exposed to flood risks at a much higher frequency.

Recommendations Improved flood warning for local Deep Creek events will enhance the flood preparedness (and hence resiliency) of this community and users of the Tin Can Bay Road. Council should engage with the Bureau of Meteorology to identify the feasibility of warning upgrades for this catchment.

3.5.9 Southside

Introduction The Southside precinct comprises all suburbs and localities west of the Mary River. It is mostly residential (around 4500 lots), but includes small pockets of land zoned commercial or industrial.

Flood behaviour, and resulting issues/risks The Kidd Bridge and Normanby Bridge become closed due to flooding which isolates Southside from the central parts of Gympie. Accordingly, Gympie businesses are impacted by reduced access to staff who are residents in Southside. The nearest access downstream (Wide Bay Highway) may also be closed due to the same flood event, and access via crossing upstream (if open) is constrained by the closure of the Inglewood and Pengellys Bridges across Deep Creek. These closures may last for several days during a major flood event.

Type and number of flood affected properties There are up to 150 residential and three commercial lots partially or fully inundated by 100 year ARI flooding of the Mary River. The majority of residential lots are not inundated at the 20 year ARI flood level with only a few identified with significant flooding during the recent January 2013 flood event.

While there are impacts to businesses that cannot access staff, a significant concern is the extended period of isolation from Gympie, and the associated health and emergency services.

Conceptual options to address flood risks Both the Kidd and Mary Valley Highway bridges are TMR controlled roads. A new, upgraded or suitable alternate access is needed to afford the Southside community access to vital services during periods of flooding.

Number who may benefit Improved transport infrastructure, linking Southside and Gympie during times of flood will benefit the entire Southside community and the wider business community within Gympie who may be reliant on staff who reside in this precinct.


Challenges/difficulties/issues The cost of a new or improved bridge over the Mary River offering 100 year ARI flood free access will be expensive. Further, a new or upgraded crossing of the Mary River is a major undertaking, and needs to be planned and integrated in the wider transport network.

Recommendations In areas subject to flooding, Council should continue to utilise robust planning controls to restrict inappropriate development.

A review of the TMR planning indicates no major upgrade works to provide a 100 year ARI flood free crossing of the Mary River at Gympie. Council should engage with TMR to understand where this issue rests in the context of state-wide transport planning, and determine the need to develop a flood management plan to support Southside residents during times of isolation.

3.5.10 Major upstream flood mitigation works A highly effective type of major flood mitigation works is a large dam located upstream of the area subject to inundation. An example of this is Wivenhoe Dam on the Brisbane River, which is a dual-purpose dam (water storage and flood mitigation).

For Gympie, the Traveston Dam provides a good case study of the potential benefits of a major flood mitigation project and also highlights many of the issues that come with such a project.

The dam was to be located on the Mary River, 27 km upstream of Gympie. The proposed Stage 1 of the dam would have had a full supply level (FSL) of EL 71 m AHD, with an impounded volume of 153,000 ML inundating 3039 Ha. The main channel of the Mary River would have been inundated for 36.5 km. The dam wall proposed was predominantly roller compacted concrete, with a wall height of about 25 m above ground level. The spillway included operable gates to manage flood events.

The dam as proposed would have provided significant beneficial flood mitigation to areas downstream, particularly in Gympie. For example, the dam could have reduced the area flooded by the February 1999 flood by 30%. This would have saved all houses and about 100 businesses from inundation, significantly reducing the impact of this flood. For the remaining 30 businesses, flood levels could have been reduced by up to 4 m, although the time of inundation would have increased from 58 to 72 hours.

For the nominated design flood, assessments undertaken at the time forecast a flood height reduction in Gympie of 3.5 to 4m, with resulting benefits relating to lower direct damage.

The dam however had many negative impacts, including:

Upstream flooding at Kandanga and Imbil

Significant environmental impacts

Significant social impacts

Stage 1 of the project also carried a significant cost, some $1.19 billion of construction costs.

The project was stopped by the Federal Government due to environmental concerns. This option is considered unfeasible and it is recommended that major flood mitigation works like Traveston Dam not be progressed for further consideration due to cost and lack of appetite for such projects.


3.5.11 Major downstream flood mitigation works The most effective type of major downstream flood mitigation works can be modifications to the river or creek channel, particularly to open up restricted reaches. One such scheme identified is to remove the restriction or choke in the Mary River channel around Fisherman’s Pocket. The restricted reach of the Mary River is located between 6 and 15 km downstream of the Kidd Bridge, illustrated in Figure 11 below.

Figure 11 Topography showing key choke points (circled) downstream from Gympie

Cross-sections have been extracted from the digital terrain data, at locations illustrated in Figure 11, and are shown in Figure 12 for comparison. At the 100 year ARI level, the conveyance area available for flood flow is around 7500 m2, but at these choke points, significantly less conveyance area is available (11 km – 4500 m2, 13 km – 6800 m2, 15 km – 4000 m2). To open up these chokes at Fisherman’s Pocket would involve the excavation and removal of a significant amount of material – some 55 Million cubic metres of material would be required to be removed, a large proportion of which would be rock.


Figure 12 Mary River cross sections

Whilst this scheme has not been the subject of detailed hydraulic analysis, preliminary estimates suggest a flood level reduction could be achieved at Kidd Bridge of a maximum of 1.5 m at the 100 year ARI level (see Figure 13), and is likely to yield lesser benefit at lower flood levels. It should be noted that there would be a significant worsening of flooding downstream from the works, as the effect of the current restriction is to act like a detention basin, with temporary storage upstream and lower flows and flood levels downstream.

Figure 13 Mary River Long section showing effect of Fishermans Pocket works


There would also be very significant environmental impacts with such a scheme, with the Traveston Dam case study providing some indications. These include:

Terrestrial Flora/Fauna Impacts

− Traveston Dam proposal was refused by the Federal Government on the basis of impacts to Threatened Species protected under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)

− Endangered RE’s and Essential Habitat located in riparian areas near the identified ‘chokes’

− Several threatened plants listed under the EPBC Act may be present in the project area

− Offsets will be required for removal or impacts to EVNT species under Nature Conservation Act 1992 (NC Act) and EPBC Act

− Potential impact to riparian vegetation which is used as habitat by protected fauna

Conservation Areas/Heritage Areas

− Mary River is the largest catchment draining into the Great Sandy Strait RAMSAR Wetland and World Heritage Area. The works may result in potential impacts to downstream marine flora/fauna through changes to the flow regime, ie as a result of a dam

− Brooyar State Forest located in close proximity to Fisherman’s Pocket/Glastonbury Creek

Aquatic flora

− Two protected aquatic plant species are recognised to occur in the Mary River (Vallisneria nana and Aponogeton elongatus)

Aquatic fauna

− Threatened species identified within the Mary River catchment include the Mary River Cod (Maccullochella peelii mariensis), Lungfish (Neoceratodus forsteri) and the Mary River Turtle (Elusor macrurus). The Mary River catchment is identified as having one of the highest levels of endemic turtle species in Australia.

− Aquatic fauna may be impacted by change of flow regime and water quality (eg. increased flow rate and reduced areas of ‘calm water’ resulting in impacts to fish movement)

Construction Impacts

− Management of sediment and erosion control, impact of turbidity and noise effects associated with blasting.

− Disposal and transportation of excavated materials

− Requirement for access and works in State land (eg Brooyar State Forest)

− Need to establish access agreements through private land

Native Title – Notification required for works on State land (eg State forest, watercourses)

Cultural Heritage – history of previous occupation near watercourses and potential for impact to protected sites

Further, the change in geomorphology of the river may lead to undesirable outcomes, including increased erosion throughout the Gympie reaches of the Mary River due to increased flood flow velocities.

The cost of such a scheme would be also very significant – even applying a modest rate for excavation and removal of material (and noting that rock blasting would likely be required for much of it), the cost would run in to the hundreds of millions of dollars. At a very generous rate of $10/m3, this would cost some $550 million to complete (but more likely three to five times this cost).


The approvals process for such works will be onerous, and would include:

Referral likely to be required under the EPBC Act due to presence of protected species with the Mary River. Potential for the activities to be considered as a ‘Controlled Action’, requiring approval under the EPBC Act

Riverine protection permit (Water Act 2000)

Interfering with water in a Watercourse (Water Act 2000)

Quarry material allocation (Water Act 2000)

Waterway Barrier Works (Fisheries Act 1994)

ERA16 (Blasting)

Vegetation clearing approvals (Vegetation Management Act 1999 and NC Act)

Allocation of quarry material

With only 1.5 m of flood relief at Gympie, clearly such a scheme is not viable on any measure and is not worthy of any further consideration.

3.6 Priority areas

3.6.1 Selection of priority areas The screening process has identified a range of opportunities to improve flood resilience across the Gympie Regional Council area. Table 3 below summaries the findings and was presented for discussion with the Project Steering Committee. In consultation, three locations were identified for further detailed cost-benefit assessment, involving conceptual engineering design, pricing, flood damage assessment and determination of cost benefit ratios.

Table 3 Summary of number of lots to potentially benefit from flood mitigation works

Precinct Tota

l in

Stud

y Pr

ecin

ct

Num

ber d

irect

ly

affe

cted

by

Q10

0 Fl

oodi

ng

Num

ber a

ffec

ted

by

Isol

atio

n Di

srup

tion

Leve

e

Floo

dway

Impr

oved

Dr

aina

ge

Dete

ntio

n

Relo

catio

n Bu

y-ba

ck

Impr

oved

Flo

od

war

ning

Wat

erw

ay

mod

ifica

tion

Impr

oved

acc

ess

Commissioner Gully 831 317 514 215 195Cooloola Cove 1756 189 1567 150 150 1733East Deep Creek 208 169 39 35 10 25Glanmire 118 52 66 12 12 60Goomeri 463 100 363 56 56 56 411 11 411Kandanga 120 25 8 5Imbil 392 65 327 16 318 283Kilkivan 406 61 345 8 19 19 324 2Mary Street (CBD) 503 226 277 213 213Monkland 722 348 374 218 120 369Mount Pleasant 524 182 342 149 149One Mile 733 319 414 271 136Rainbow Beach 1377 345 1032 328 328 1324Southside 2676 531 2145 303 150 1824Tin Can Bay 1532 118 1414 99 99 1467Two Mile 82 51 31 10 10Widgee 317 88 229 60 262 262Woolooga 118 74 44 34 28 56 28Gympie Study Areas 6397 2195 4202 803 985 271 3340Cooloola Study Areas 4665 652 4013 427 150 427 150 4524

Number of lots potentially to benefit from :


The three areas selected for further assessment to improved flood resilience were:

Mary Street precinct

One Mile precinct

Monkland/Glanmire areas

There were a number of other flood resilience recommendations that should be placed on a priority list for further investigation:

Flood levee for Goomeri

Relocation of the Kandanga Bowls Club

Raising of Coondoo Creek bridge on Tin Can Bay Road

Raising River Road at Centro Way to allow southbound Bruce Highway commercial traffic access across Deep Creek and destinations farther south some six to ten hours earlier than is available via Inglewood Bridge.

Improved connectivity between Gympie and Southside – ie 100 year ARI flood immune bridge

Voluntary buy back for properties exposed to 5 year ARI flooding

Finally, the screening assessment and consultation process acknowledges a broader and region-wide issue of flood resilience on the districts rural roads, and that a generally poor level of flood immunity across the rural transport network has a significant impact in recovery in rural areas. These rural State Roads require improvement to offer rural areas a ready means to recover following natural disasters.

3.6.2 Target immunity The Queensland State Planning Policy 1/03 “Mitigating the Adverse Impacts of Flood, Bushfire and Landslide” (2003) is the relevant planning policy for Local Government in Queensland. This policy uses the concept of a Defined Flood Event (DFE) to create a delineated flood zone for use in the Council Planning Scheme.

The Queensland State Government position is that generally the ARI 1 in 100 year (or 1% AEP) flood is the appropriate event for determining the natural hazard management area (flood). The Government however also recognises that it may be appropriate to adopt a different DFE depending on local circumstances. If adopting a lower standard, the onus is on the Council to demonstrate to the State that the lower DFE is appropriate to the circumstances.

It is noted that SPP 1/03 is currently under review although any new approach is not anticipated to significantly differ from the current policy.

For new development, the typical approach is to ensure properties have at least 100 year ARI immunity. This is also a level of immunity that Insurers feel is appropriate. However when it comes to mitigation works to improve flood risk for existing impacted development, the approach regarding immunity needs to be more pragmatic, targeted more to what can be achieved rather than a specific flood immunity. Certainly targeting a 100 year ARI immunity for mitigation works should be considered the starting point, but as noted above, if this is not feasible within reason, then the target immunity should focus on what is achievable.


As an example of this, Cairns Regional Council constructed a flood levee in the Lake Placid/ Kamerunga area just north of Cairns City a few years back. The immunity of the levee is approximately 38 year ARI. The reason for the lesser immunity is that 100 year ARI standard was not practical or financially feasible, hence the outcome of targeting a lesser immunity to a still achieve a strong economic benefit within reasonable cost constraints. There are many other examples of such outcomes.

Therefore for the Gympie context, it is a reasonable approach to consider achieving a 100 year ARI immunity outcome for a flood mitigation option, but to be prepared to relax this if constraints make the 100 year ARI immunity impractical.

In consideration of the above, and in consultation with the Project Steering Committee, a target immunity of 20 year ARI was agreed where 100 year ARI immunity was not feasible.


4.1 The rapid assessment method The methodology to undertake the assessment has been based on the Rapid Assessment Method (RAM) which is primarily used to evaluate and determine the economic benefits for flood plain management and mitigation measures options for urban and rural settings.

For this Gympie assessment, the evaluation technique is Cost Benefit Analysis (CBA) that uses locally sourced and timely survey results and other input data to monetise direct damage to the Gympie economy by creating Average Annual Damage (AAD) curves over a range of flood events. Specifically the CBA has been confined to direct tangible damage data that represents the economic losses experienced by affected Gympie businesses and industries. This data comprises the following:

Impact of the flood event on damages to building structures

Contents of buildings

Flood preparation, evacuation, clean-up and reinstatement

Loss of business (turnover)

The above economic items are the major components of business and industry losses when affected by flooding and therefore are sufficient to assess the scale of economic damage impact from flood events of various magnitudes in Gympie.

There are also other local and regional indirect tangible damages including losses from disruption of normal economic and social activities that arise as a consequence of the physical impact of flood events (ie costs associated with emergency response, community support, employment, disruption to transport, services infrastructure and broader flow-on impacts through the regional economy). These indirect damages have not been modelled in the CBA, but the RAM identifies from other studies that such damages can be in the order of 30% of the value of direct tangible damages.

CBA models have been individually developed for each of the proposed mitigation options by the locations using common assumptions to ensure consistency in approach and results. The assumptions that are generic to the CBA modelling are as follows:

4 Economic analysis framework


Table 4 Key generic CBA modelling assumptions

Item Assumption

Mitigation measure costs The CBA models the cash flow for the cost of mitigation activities namely capital costs and recurrent costs. Capital costs comprise the design and construction costs that are incurred at the beginning of an investment. Recurrent costs are the annual on-going costs required to operate, maintain the capital asset. An annualised average recurrent cost has been adopted for mitigation measures assessed in the CBAs

Assessment period Benefits are projected across a 20 year assessment period post construction in accord with current guidelines for Queensland Government’s Disaster Mitigation and Resilience funding program

Discount rate A discount rate equivalent to the prescribed nominal rate of 7% has been adopted in the CBA in accord with current guidelines for Queensland Government’s Disaster Mitigation and Resilience funding program (incorporating Royalties for the Regions, Local Government Floods Response Subsidy and Natural Disaster Resilience Program)

Prices All cash flows are in current 2013 (ie non escalated) prices

Average annual damage (AAD) AAD is the reduction in annual average damages that can be realised through flood mitigation measures and is calculated as:

Reduction in AAD = AAD without project – AAD with project

The calculation of an AAD estimate requires potential damage costs for a range of flood events. The estimate requires:

The size of flood event where damage to property begins

Potential damage for the design event

Potential damages caused from the probable maximum flood

Specific damage costs inputting into the AAD estimate for this CBA is comprised of the following data:

Impact of the flood event on damages to building structures

Contents of buildings

Reinstatement

Loss of business (turnover)

Sensitivity test incorporating residual value

A mitigation measure comprising the construction of a levee has an effective life of 50 years and beyond which is far in excess of the 20 year assessment period prescribed in the guidelines for Queensland Government’s Disaster Mitigation and Resilience funding program

As a sensitivity test, the damage cost benefits stream for levee measures in the CBA has been extended to 50 years post construction to reflect the effective life of the levee asset

Investment decision outputs The following CBA decision outputs have been calculated to assist decision making:

Benefit Cost Ratio (BCR) Ratio of the total present value of benefits over the present value of costs. The higher the BCR, the better the result when ranking mitigation options

Net Present Value (NPV): Calculation of net value (ie total present value of benefits minus total present value of costs) over the assessment period. The higher the NPV, the better the result when ranking mitigation options

Present value of benefits: Total value of discounted benefits over the assessment period


Item Assumption

Present value of costs: Total value of discounted costs over the assessment period

The AAD from flooding is the most used indicator used to measure the level of potential flood damages and is expressed in units of dollars per year over the assessment period. It expresses the costs of flood damage as a uniform annual amount based on the potential damages inflicted by a range of flood magnitudes. The flood damage costs for a range of flood events are plotted on a flood damage curve. The CBA determines the net economic benefit of the cost of a mitigation measure for a flood event level when assessed against the reduction in AAD (ie AAD without project (base case) – AAD with project) resulting from the measure.

The net difference between the damage profiles for the base case against the flood mitigation measure can then be determined for the range of flood events at the Q1, Q5, Q20, Q50 and Q100 ARI levels.

An uplift factor of 15% has been applied to quantify the additional economic damage to properties above the Q20 ARI flood level for the flood mitigation measures. This uplift factor has been adopted for higher level flood events to account for the lack-of-preparedness caused by the increased complacency by business owners due to the installation of flood proofing mechanisms.

These monetised benefits are then annualised in the CBA over the economic life period of the flood mitigation solutions (ie 20 years for this CBA) to determine their aggregated Present Value (PV). The aggregated PV Cost is then compared to the aggregated PV Benefits over the 20 year assessment period to determine if the CBA has a net economic benefit and to guide decision making in the ranking of the mitigation measures.

4.2 Data sources The assessment has used the following steps to estimate direct tangible flood damages costs used in the AAD and later in the CBA:

Survey of individual businesses for damage costs

Identification of other data sources to fill data gaps to estimate damage costs

Adoption of an average damage costs for “like” businesses

Application of average damage costs to all businesses in-scope of relevant flood events

The estimation of direct tangible flood damages has used a number of data sources as the basis for a robust assessment of economic loss for the base case and subsequent flood mitigation measures. Primary and timely information on the economic impact of flooding was initially gathered through a combination of an online survey and interviews with local business owners identified by Gympie Regional Council. In total, 40 business owners from across the Council area either completed the online survey or participated in an interview. The interviews gathered similar information to the survey and allowed business owners to expand on their responses and provide a broader understanding of flooding impacts and the damage costs inflicted on businesses. The major data items that were relevant to the estimation of damage costs are as follows:

Location and type of business

Annual turnover

Number of employees

Depth of flooding


Trading days and income lost because of flooding

Costs involving flood preparation, evacuation, clean-up and reinstatement

Other sources of data included the Gympie Regional Council economic profile, the Queensland Government Office of Economic and Statistical Research (OESR), the Australian Bureau of Statistics (ABS), GIS mapping software and RP Data. These sources were used to derive the factors to calculate local economic averages and to supplement consultation results, as well as to verify the input and output data to ensure the assessment is plausible and suitable for this level of analysis. Previous damage estimates derived from recent regional flood events have also been used as an order-of-magnitude sanity check of the results.

The process for determining the AAD estimates and damage curves involved using damage cost data for business, industry and residential properties to be affected by the mitigation measure which were then overlaid across the flood modelling Graphical Information System (GIS) outputs. This process ensured that every property within each study area and flood event could be appropriately identified and modelled, as they were defined by a unique plot-lot identifier. A number of factors such as premises occupancy rates, plot-lot number multipliers and damage costs were then applied to the averages to uniquely model each property affected by flooding. Actual business data has been used where available across the study areas.

4.3 Other considerations As the data collection exercise has centred on consultation with a sample of Gympie businesses and industries, an inherent level of uncertainty lies within the economic assessment. It has been established that a high correlation exists between the input data and assessment outputs with regional averages published by external sources to this study which provide comfort to the accuracy and appropriateness of the results.

There are some matters that should be taken into account when considering the results of the CBAs namely:

The adopted 20 year assessment period will understate the full benefits that will accrue to the community for mitigation measures involving the construction of levees. With proper periodic and routine maintenance, a levee has an effective life of 50 years and beyond. A sensitivity test has been incorporated in the CBA that extends the benefit stream to 50 years following construction for levee options.

Such factors as capital appreciation and reduced insurance premiums of properties with better flood immunity are also benefits that have not been monetised and would serve to improve the investment outputs from the CBA

Another area of under assessment is the exclusion damage to Council and State controlled infrastructure which have not been monetised. Bridges and sections of roads can be washed away during flood events and most published estimates of flood damages to roads are dominated by these items. Some damages to road pavement and bridge foundation do not become evident until considerably after the flood event including weakening of pavement as subgrade wets up and the rapid potholing and seal loss following seal cracking. Similarly there are economic costs of many flow-on effects from the closure of major roads which in turn cause indirect effects by interrupting the local economy. These factors may result in a flood mitigation measures being under assessed.

The estimates of damage costs used in the CBA modelling are local and direct in their nature. There will also be other economic impacts at a regional level that cannot be monetised because of limited data availability and the problems in apportioning out a subset of effects and consequences to a geographic area or time period.


A final risk to the economic assessment remains in the effectiveness of the flood mitigation options and this has not been quantified in this assessment. The assessment has primarily modelled the effects of a riverine flood event and therefore localised flooding has not been assessed. Also each of the options has been assessed within each study area as mutually exclusive. The study areas may not be mutually exclusive.

Finally, the framework that has been adopted for the assessment is an economic framework and identifies the net community impacts of flood mitigation measures. The framework does not distinguish or differentiate between the potential sources of funding to secure the mitigation measures. A separate financial assessment of funding sources including debt to equity modelling and possible rates uplift based on improved land valuations would be required if Council required any analysis of this position.


5.1 Mitigation options Mitigation Options, or Floodplain Management Measures, can be grouped into a number of principal categories:

Structural flood mitigation works (eg levees, channel improvements, detention basins, etc)

Buyback

Land use planning controls

Development and building controls

Flood emergency measures

The first two options relate to partial or complete elimination of the flood risk, while the others relate to the management of existing, future and residual risks.

Clearly the preferred approach to sound floodplain management is via land use planning and development and building controls, with flood emergency measures applied as appropriate. However, many parts of Australia were developed before there was a good understanding of flood risk, hence many developed areas already exist in flood prone areas. In many of these flood prone areas, structural flood mitigation works can offer partial to significant reduction to the flood risk.

A brief discussion on the various types of mitigation measures available and the advantages/ disadvantages of each is provided below.

5 Mitigation options assessment


Table 5 Flood Mitigation Options Comparison

Category Option Description Advantages/Disadvantages

Structural Flood Mitigation Measures

Levees – permanent

A physical barrier, typically an earthen embankment, built to keep flood waters from entering the protected area.

Unless designed for a PMF event, all levees will ultimately be overtopped.

Flood Emergency Plans are needed to deal with the residual risk associated with any levee.

Advantages

Protect area within the levee up to design flood level

Cost effective

Disadvantages

Risk of levee overtopping or failing with resulting significant impacts

Reduction in flood awareness and preparedness due to a false perception of being “flood free”

Equity issues – properties not protected by levee

Dealing with local drainage inside levee

Significant footprint, land requirements

Visual and physical barrier – land impacts

Needs to be well maintained to be effective – therefore is an on-going liability requiring a commitment from the asset manager in terms of funding and maintenance obligations

May increase flood levels elsewhere

Levees – temporary or demountable

A physical barrier, erected before the flood waters arrive, designed to keep flood waters from entering the protected area.

These can include sandbags, and various proprietary demountable barriers.

Flood Emergency Plans are needed to deal with the residual risk associated with any levee.

Similar for permanent levees, plus

Advantages

Are temporary, so only in place for event and then removed

Minimal footprint

Disadvantages

Need a well-drilled team of people to install and monitor during event

Need to be kept handy to the site, not stored remotely



Backflow prevention devices

Devices such as flap gates/valves that are added to a piped stormwater system to prevent water flowing back up the pipe and causing flooding.

Advantages

Cost effective

Disadvantages

Only suitable for limited applications and dictated by local topography

Devices vulnerable to debris which impacts their effectiveness

Channel and drainage improvements

Includes widening/deepening/realigning or clearing the creek/river channel to increase flow carrying capacity.

Also includes upgrading the drainage system with improved overland and underground (pipe) drainage.

Advantages

Can be very cost effective in appropriate situations

Disadvantages

Underground drainage improvements expensive

Environmental impacts

Worsening downstream flooding

Crossing structure upgrades

Improvement to existing bridge/culvert structures by adding more waterway area, usually by adding extra spans to bridges, adding extra culverts or modifying the inlet and outlet. Targeted at structures with a large head-loss across them, usually because the existing waterway area is too small.

Advantages

Can achieve significant flood level reductions upstream of the structure

Disadvantages

Increase in flood levels downstream of structure

Generally expensive

Bypass floodways Redirection of a portion of floodwaters away from areas at risk using an alternative flow path. Often used in conjunction with levees.

Advantages

Can be effective if used with levees

Disadvantages

Constrained by topography

Constrained by land availability

Worsening flooding elsewhere



Detention basins A small dam that provides temporary storage for floodwaters. Advantages

Reduce peak flooding downstream

Land can be used for sporting fields, etc

Disadvantages

Require a significant amount of land

Most effective when used in the upper reaches of catchments

Can worsen flooding downstream if timings of sub-catchment peaks coincide

Dams Impoundment on the main River or Creek. Even if full, dams can significantly reduce downstream flood discharges.

Advantages

Can be extremely effective at reducing downstream flooding.

Disadvantages

Significant cost (capital and on-going)

Requires significant amount of land

Environmental impacts

Social impacts

Land Use Planning Controls

Zoning Aimed at guiding inappropriate future development away from high risk areas of the floodplain.

Division of flood-prone land into appropriate land uses via zoning.

Advantages

Most effective option to prevent development on flood prone land

Integral part of local Planning Scheme

Disadvantages

Does not help with existing flood prone development

Voluntary Purchase/Buyback

Aimed at removing existing high hazard development from the floodplain.

Can be voluntary or resumption type process.

Advantages

Removes the problem in existing flood prone development



Disadvantages

Can be politically difficult to sell

Perceived and real equity issues

Social impact – affordable housing issues

If voluntary, can take time and be left with pockets of resistance

Development and Building Controls

Strategic site planning

Working with the flooding constraints with a site. Considers such factors as siting of buildings in lower hazard areas, consideration of evacuation routes, etc.

Advantages

Minimise flood risk

Disadvantages

Residual flood risk may still be significant

Flood-proofing buildings

Design and construction of buildings with appropriate water-resistant materials.

Advantages

Structural damage to building minimised if flooded

Disadvantages

Occupants still suffer the social impact of flooding

Minimum floor levels

Set habitable floor levels to a minimum defined floor level based on a design flood level plus freeboard.

Advantages

Can be varied for commercial development depending on risk appetite

Disadvantages

Flooding may still cause impact through access difficulties and other issues

May still flood at higher flood elevations

House raising Physical lifting of house to above some defined flood level. Only suitable for lower hazard areas.

Advantages

Can be cost effective

Disadvantages

Not possible for certain types of house construction (eg slab-on-ground, masonry)


Category Option Description Advantages/Disadvantages Potential for isolation during flood event

Flood emergency measures

Emergency Response Planning

The plan which sets out how a flood emergency will be managed when it occurs including the responsibilities of key agencies, etc. An essential requirement to deal with flood prone areas.

Advantages

Clear responsibilities will ensure effective response

Disadvantages

None

Flood forecasting and warning

Forecasting is the process of predicting the severity of flooding at a particular location. Warning is the process of alerting occupants of flood-prone areas of the immediacy and severity of flood risk and persuading those at risk to take action to prevent losses.

Advantages

Advance notice of flood enables effective response

Disadvantages

None

Flood preparedness

Affected community and agency awareness of what to do and how to do it during the onset of a flood.

Covers everything from training agency staff in evacuation, rescue, etc to having local residents aware of securing property in case of evacuation. Community engagement is a critical part of this.

Advantages

More effective response

Less flood damage and social impact

Disadvantages

None

Flood response The operations that may be initiated to reduce the hazard of an actual flood. This includes everything from road control, evacuation, rescue and providing information and advice to communities affected by flooding.

Advantages

Reduces the consequences of flooding

Disadvantages

None

Flood recovery Clean-up, welfare, restoration of services and other forms of assistance provided by volunteers and agencies in the aftermath of a flood.

Advantages

Faster recovery

Disadvantages

None


It should be noted that with levees, unless the levee is designed to provide protection up to and including the Probable Maximum Flood (PMF), there exists a residual risk of overtopping during a flood event. Also, any levee can fail during a flood event even if not overtopped (eg by a piping failure). Therefore levees represent a significant commitment, not just for their construction, but also their on-going maintenance.

The maintenance commitment can include inspections, repairs, vegetation maintenance, annual inspections, maintenance and testing of the various levee structure components, and drilling of staff and emergency management team in procedures to be applied during an event. A levee above a height of 4m becomes a referable structure under the ANCOLD designation, and carries with it specific design requirements. Conditions relating to monitoring and maintenance, including annual inspections, surveys and detailed five yearly geotechnical condition assessments also apply.

Strong community support is essential for levee solutions due to their impact (visual, land severance, access, etc), and the fact that community awareness/preparedness of the risks is needed for effective management. There is also a potential social/equity issue if there are property owners not protected by the levee who feel they should be.

The risk of complacency is a real issue with any flood mitigation works. Community awareness programs are a key part of floodplain management and must constantly remind people living on flood prone land of the flood risks, particularly where there is a false perception of being flood free as a result of the mitigation actions. It is essential that any flood mitigation applied to Gympie ensures the residual flood risks are well communicated and understood by the community.

5.2 Mary Street (CBD) As introduced in previous sections, protecting the CBD from flooding is challenging, due primarily to the significant vertical variation in flood levels and the relatively low return period at which flooding in the CBD commences (around 5 year ARI). At the 100 year ARI flood level, 25.22 m on the Kidd Bridge gauge (62.15 m AHD), up to 180 commercial and about 30 residential properties are directly affect by flooding. At the 20 year ARI flood level, about 74 CBD zoned properties in Mary Street are inundated, and due to dual occupancy etc this affects up to 86 individual businesses. At the 20 year ARI level (20.71 m Kidd Bridge, 57.64 m AHD), the lowest point on the Bruce Highway is about 4.7 m under water.

The screening exercise identified two broad options that would reduce flood risks to the CBD and Mary Street area: relocation and strategic levees. At a target immunity level of 20 year ARI, the objective is to offer flood relief for up to 90 businesses in the CBD.

Relocation of affected properties to an area outside the flood extent offers a “safe” solution, where the risk of inundation is eliminated. A land swap scheme may allow the CBD to relocate and receive the desired level of flood immunity. However, it should be recognised that the loss of the historic city centre through relocation may result in significant social impacts and loss of identity.

A number of different levee locations were considered, as illustrated Figure 14. All the levees have a crest height at least 8 m above ground level within the parks, and a footprint up to 50 m wide (at side slopes of 3H:1V). At an immunity level of 20 year ARI, the expectation of overtopping is quite high. As such, the levee requires a spillway to allow the controlled flooding of the area behind during greater than 20 year ARI flooding, and pumps to address concurrent local catchment flooding. Backflow prevention will be required to ensure the potential for flooding via the stormwater system is addressed. All levees offer a similar degree of protection, offering up to 20 year ARI flood immunity for around 85 to 90 businesses.


Figure 14 Indicative Levee locations to protect the Gympie CBD (Indicative Only – not to scale)

A northern levee alignment (not shown) was identified that attempts to preserve as much as possible of Nelson Reserve, but in doing so, sacrifices much of Gympie Memorial Park under its footprint. The north-east portion of Memorial park is required as a detention basin, offering storage of locally generated runoff, and doubles as the location of the spillway and stilling basin. Permanent but demountable flood barriers would span Young St and Monkland St, tying in the levee to higher ground but requiring minimal change to the existing road geometries. A levee in this location would create a disconnect between Nelson Reserve and the CBD, and the loss of Gympie Memorial Park. These are considered to be significant social and community impacts that detract from this option’s desirability. Further, the proximity of the stilling/detention basin to Reef Street is not ideal, offering limited temporary locally generated flood storage above the design level (and therefore requiring several large pumps to cater for local flows).


Option A – Nelson Reserve levee alignment connects the high point at the Bruce Highway intersection with Monkland Street with River Road near the swimming pool (it is understood that GRC is currently considering relocation of the pool). The landside levee slope could be designed at an incline of 1V:6H (rather than a design slope of 1V:3H) offering informal seating in front of the Sound Shell. In doing so, the levee could be constructed to create an amphitheatre effect around the Sound Shell. The spillway and stilling basin would be located at the northern end of the levee, away from the amphitheatre, and permanent demountable flood barriers would be required at the road interfaces. Nelson Reserve and Memorial Park provide storage for locally generated flood flows, and pumps would be required should coincident flooding occur.

A similar degree of flood resilience may be offered by raising the Bruce Highway (Option B), using the road embankment as a levee. There would be some increase in footprint either side of the road to allow for the 4.5 m increase in elevation. The River Road exit (Bruce Highway northbound) would be lost and this overpass would be filled, which is likely to result in the need to add an additional right turn lane at Monkland Street and upgrades to roads in Jaycee Way and Young Street to cater for increased traffic volumes (it may be possible to retain this exit by redesigning the River Road tunnel under the new Bruce Highway in order to house flood gates, but this option has not been costed). The preferred position for the spillway is near the middle of the embankment, integrated with stilling/detention basin and pump well. This option results in minor losses to Nelson Reserve and to the Touch Football fields. A new/revised entry/exit to Albert Park will be required (left in, left out). Construction of this option will have significant impact to Bruce Highway traffic, requiring alternate routes and bypasses during construction.

Another alignment places a levee within the Touch Football fields riverside of the Bruce Highway (Option C). The levee would result in the loss of both of the Touch Football fields and relocation of this facility would be required. The River Road exit from the Bruce Highway could be retained, however, it is likely that a new/revised entry/exit to Albert Park would be required (access via the River Road exit would be lost). The spillway and stilling/detention basin would be located in the upper Touch Football field. Due to filling in the Touch Football fields, geotechnical and structural investigations would be required to determine its exact location, and the manner in which it would interface with River Terrace. A further opportunity exists at this location, due to the available footprint, where a higher levee crest could be accommodated, offering protection up to around 22 m at Kidd Bridge (around 59 m AHD). This higher crest would offer protection up to the 30 year ARI level, and extend the protected area in the CBD up to Smithfield Street.

The Memorial Park alignment is unattractive due to its location, length and limited flexibility in design, and in consultation with the PSC has not been considered further. Costs for spillways, detention basins, pumps etc for all options are similar, so the volume of earthworks in the levee becomes a primary cost driver. The Nelson Reserve alignment is the shortest levee and may offer opportunities to be integrated with the new Sound Shell. Raising the Bruce Highway has the least impact on the recreational reserves, but will result in considerable traffic management problems during construction (requiring major roadworks to provide temporary diversion of Bruce Highway Traffic) and is the most expensive. The riverside options result in the loss of the Touch Football fields, but have no impact within Nelson Reserve or Memorial Park. A provisional cost of $1.2 million has been included in the costings to assist in the relocation of the Touch Footfall facilities.

It should be recognised that a levee would result in lost floodplain storage, and that such losses may result in increased flood levels elsewhere. The loss of floodplain storage due to the proposed levee options has been estimated to result in a maximum increase in flood levels elsewhere along the Gympie reach of the Mary River of up to 100 mm (detailed modelling would be required to confirm this, but would be expected to show much lower flood impacts). Within the context of 20 year ARI flood levels across Gympie, a 100 mm increase in flood levels may result in increased impacts to, perhaps, a dozen properties. Such impacts could be readily addressed on a case by case basis.


As noted previously, unless a levee solution is designed to provide protection up to and including the Probable Maximum Flood (PMF), there exists a residual risk of overtopping during a large flood event. The benchmark flood immunity target of the 20 year ARI design event has been selected by the Project Steering Committee as a pragmatic target offering relief to those areas that regularly experience flooding. Nevertheless, it must be recognised that floods higher than the 20 year ARI flood level can occur and that this level has been exceeded on nine occasions in Gympie’s recorded history and twice in the last two decades (February 1992 and February 1999). Statistically speaking, there is about a 1 in 4 chance of a flood event at or greater than the 20 year ARI flood occurring anytime within a given six year period. And, over the projected structural (and maintained) lifetime of the flood mitigation infrastructure (50 years), there is better than 90% chance of a flood greater than the 20 year ARI design event. Importantly, the perception of flood “protection” offered by the levee may result in a decline in flood awareness and preparedness, which can significantly influence flood damage costs, evacuation efficiency and overall community mindset to a flood emergency scenario which is still very likely to occur.

Option A – Nelson Reserve Levee across Nelson Reserve, 8 m high, 280 m long, approximately 59,000 m3 earthworks

Concrete spillway 80 wide, drop structure, stilling basin and connections to underground drainage

Demountable flood barriers at Young Street and Monkland Street, and associated roadworks

Backflow prevention on the stormwater system

Pumps – to address concurrent flooding in Gympie City catchment, assuming concurrence of 20 year ARI flood in Mary River with 100 year ARI rainfall in Mary Street catchment for a long duration coinciding with loss of drainage to the Mary River

Drainage bypass – to address locally generated flows in excess of pump and/or temporary storage capacity, likely to be integrated with raising of River Road near Swimming Pool

Isolation of sewerage systems connected transverse to the levee

Geotechnical improvements to provide foundation support for levee in landfill area

Geophysical assessments and mine rehabilitation to eliminate backflow flooding potential

Selected buyback and redevelopment of sites within the works area

Local drainage improvements, revegetation, landscaping etc

Design, procurement, contractor and contingency fees

Cost Estimate $23.3 million

Option B – Bruce Highway Raise Bruce Highway by 4.5 m, 400 m of works, approximately 65,000 m3 earthworks








Revised traffic entry/exit to Albert Park


Additional traffic lanes at Monkland St intersection, to compensate for loss of northbound Bruce Highway – River Road exit

Reconstruction of highway, pavements, lighting, signalling, etc

Traffic management during construction – alternate routes and bypasses



Option C – Albert Park Touch Football fields, 20 year ARI Levee alignment riverside of Bruce Highway, up to 11 m high, 440 m long, approximately 58,500 m3

earthworks




Drainage bypass – to address locally generated flows in excess of pump and/or temporary storage capacity





Relocation of Touch Football facilities


Revised entry/exit to Albert Park



Option D – Albert Park Touch Football fields, 30 year ARI Levee alignment riverside of Bruce Highway, up to 12.5 m high, 440 m long, approximately

95,000 m3 earthworks








Relocation of Touch Football facilities


Revised entry/exit to Albert Park




Mary Street - Option A

Item Infrastructure CAPEX1.0 Levee1.1 Earthworks $2,948,6501.2 Spillway $731,0001.3 Stilling basin $172,0002.0 Backflow prevention $321,2003.0 Bypass pipes (Southern) $1,030,0004.0 Geotechnical investigations4.1 Geophysical survey and geotechnical investigations $350,0004.2 Provisional cost of improvement works $2,000,0005.0 Local drainage $500,0006.0 Buyback $2,400,0007.0 Demolition $90,0008.0 Carparking $2,912,8329.0 Landscaping and rehabilitation $450,000

10.0 Isolation of sewerage systems $25,00011.0 Pumps $110,00012.0 Bypass pipes (Northern) $980,00013.0 Demountable floodwalls $80,969

Subtotal $15,101,651

Contractor Preliminaries/Profit 26% $2,253,329Engineering / Design 7% $1,057,116


Risk & Contingency 15% $2,761,814Procurement & Construction Management inclusive of Owners Costs 12% $2,082,598

Total $23,256,508


Mary Street - Option B

Item Infrastructure CAPEX1.0 Levee1.1 Earthworks $3,243,4001.2 Spillway $731,0001.3 Stilling basin $247,0002.0 Backflow prevention $321,2004.0 Geotechnical investigations4.1 Geophysical survey and geotechnical investigations $350,0004.2 Provisional cost of improvement works $2,000,0005.0 Local drainage $500,0006.0 Buyback $2,400,0007.0 Landscaping and rehabilitation $450,0008.0 Roadworks (inc Bruce Highway and Monkland St) $6,450,0009.0 Isolation of sewerage systems $25,000

10.0 Pumps $250,000





Total $27,056,310


Mary Street - Option C

Item Infrastructure CAPEX1.0 Levee1.1 Earthworks $2,925,3001.2 Spillway $731,0001.3 Stilling basin $172,0002.0 Backflow prevention $321,2003.0 Geotechnical investigations3.1 Geophysical survey and geotechnical investigations $350,0003.2 Provisional cost of improvement works $2,000,0004.0 Local drainage $500,0005.0 Buyback $3,500,0006.0 Landscaping and rehabilitation $450,0007.0 Relocation of touch football fields $1,200,0008.0 Isolation of sewerage systems $25,0009.0 Pumps $250,000


Contractor Preliminaries/Profit 26% $1,813,370Engineering / Design 7% $874,965



Total $19,183,555


5.3 Monkland and Glanmire Flooding in excess of the 5 year ARI flood level (around 16 m at Kidd Bridge) results in closures of the Bruce Highway (Inglewood Bridge) and Tin Can Bay Road (Pengellys Bridge). The isolation of Monkland and Glanmire is considered to represent a significant economic impact to the local businesses and the broader community.

The Bruce Highway bypass will provide up to 100 year ARI flood free access from Gympie to Monkland/Glanmire with interchanges at Gympie Connection Road and Penny Road/Noosa Road. These works are programmed at a beyond 10 year horizon. A range of road improvement options are identified to offer interim connectivity and/or road network improvements consistent with the long term regional traffic management objectives, and are illustrated in Figure 15 below.

An “around the hills” option has previously been proposed by Council (Route A+B), and this has been re-examined within the current flood immunity context. The route is some 32 km and includes sections that require widening, pavement improvements, intersection upgrades and two sections of new road within existing road reserves.

Mary Street - Option D

Item Infrastructure CAPEX1.0 Levee1.1 Earthworks $4,750,0001.2 Spillway $877,2001.3 Stilling basin $172,0002.0 Backflow prevention $321,2003.0 Geotechnical investigations3.1 Geophysical survey and geotechnical investigations $350,0003.2 Provisional cost of improvement works $2,000,0004.0 Local drainage $500,0005.0 Buyback $3,500,0006.0 Landscaping and rehabilitation $450,0007.0 Relocation of touch football fields $1,200,0008.0 Isolation of sewerage systems $25,0009.0 Pumps $250,000

10.0 Demountable floodwalls $113,158





Total $22,686,231


Two additional options are identified, essentially using existing routes, but upgrading roads and bridges to offer increased connectivity during periods of up to 20 year ARI event flooding. Route E follows Mount Pleasant Road and Graham Street to a new crossing of Deep Creek just east of the railway bridge, joining with Union Street to Inglewood Road and Tin Can Bay Road. Route F upgrades the existing Tin Can Bay Road by raising critical crossings around 4.5 m to provide up to 20 year ARI connectivity across Deep Creek and other waterways. Route E and Route F are common from the Tin Can Bay Road intersection with Araluen Terrace south to the Monkland industrial area.

The ultimate “solution” to the access issues at Monkland is the future Bruce Highway Gympie Bypass (route BH). According to the TMR website, this section of road construction is programmed to occur beyond the 10 year horizon, and is part of the broader regional road upgrade strategy. The new highway would offer 100 year ARI flood immunity to traffic, and concept planning has identified interchanges at Noosa Road/Penny Road and Gympie Connection Road. Accordingly, the highway offers a high quality 100 year ARI flood free alternative route for all traffic travelling between Gympie and Monkland. Further, once integrated into broader road networks north and south, flooding of the Mary River will no longer result in closures of the Bruce Highway, and Monkland businesses will benefit from continuous connection to suppliers, retailers and employees during flood events.

Figure 15 Alternative routes providing access to Monkland/Glanmire


Route section D, Drummond Drive, is a steeply graded gravel access that includes a weight limited bridge across the railway line. It currently offers up to 20 year ARI flood immune access between Monkland and Glanmire for smaller vehicles only. Improvements to Drummond Drive could offer all-vehicle connectivity between Monkland and Glanmire and is considered an integral part of a broader long term 100 year ARI flood free access solution associated with the future Bruce Highway bypass. Without such works, the future flood free benefits enjoyed by Monkland businesses will not be available to Glanmire businesses. A simple box design for the bridge crossing is proposed, similar to that constructed for the Mary Valley Railway at Gilldora (see Figure 16 below).

Figure 16 Mary Valley Railway at Gilldora

According to the TMR website, initial planning for the future Bruce Highway interchange at Noosa Road/Penny Road offers north facing ramps only (ie north bound on ramp, south bound off ramp). This satisfies flood connectivity needs within Gympie, and for connection farther north, but for business reliant on connections with southerly destinations the traffic interface is via the existing Bruce Highway (from about the Caltex Station) and Smith Street or Hall Road. During flood events, the Bruce Highway between Monkland/Glanmire and the Caltex station is flooded at Six Mile Creek, so south connections would need to be made by traversing north along the bypass to the Gympie Connection Road exit, and circling back along the highway. This adds about 15 km to the trip, which would not be required were south facing lanes included. TMR have confirmed (pers comm. Doug Waas) that this concept planning is far from complete, and a future revised option to include south facing ramps may be considered. Accordingly, it is recommended Council engage with TMR to pursue such an option as considered appropriate to meet the traffic needs of the Monkland/Glanmire area. It is anticipated that Council may be required to contribute to the cost of these additional ramps.


Route A + B Enterprise Road

− Widening of single lane to 7 m seal (735 m)

− Upgrade unsealed road to 7 m seal (2150 m)

Old Goomboorian Road



− Upgrade intersection

East Deep Creek Road


− Upgrade intersection

Lynch Road


− Construction of new Road (1060 m)

Hyland Road to Penny Road




Route C No works proposed – may be trafficable at Tin Can Bay Road (Deep Creek) up to 10 year ARI flood

event

Route D Drummond Drive

− Re-profile road so it is above the 63.5 m 100 year ARI level

This will require an approximate 3.5 m lift over 120 m

Suitable culverts to maintain the flow of flood waters are to be provided (the road will not act as a levee bank)

Widen this section of Drummond Drive to 10 m and asphalt seal

− 10m seal width will to maintain consistency with the northern and southern extents

Pavement to be designed to TMR/Gympie Council standards

Access to property is to be maintained with minimal resumptions to allow for earthworks

Drainage, signs and lines are to be considered, street lighting is not required

− Replace existing to match the new road formation and B-Double loadings

Structure similar to Mary Valley, Rattler crossing at Gilldora

PUP on existing bridge to be considered


Route E Mount Pleasant Road

− No Works

River Road

− Signalised intersection

− Property resumption (x1)

Union Street


− Bridge (Deep Creek)


− Property resumption (x2)

Inglewood Road

− Localised widening (50 m)

− Bridge (over rail)

Route F Tin Can Bay Road – Red Hill Road to Inglewood Road

− Levee bank (protecting from One Mile flooding)

− Bridge (Deep Creek)

− Fill Section to raise road level (826 m)

− New pavement (826 m)

− Drainage

Route E and F common part Tin Can Bay Road – Nicholas Christopher Drive to Langton Road

− Fill Section to raise road level (450 m)

− New pavement (450 m)

− Drainage

Bruce Highway Interchange with Penny Road / Hall Road

− Additional south facing ramps (x2)

Cost estimates for the various route upgrades are provided in Table 6 below.


Table 6 Cost Estimates for Road Options

Length (m) Cost per m Section cost Total cost

Route A

Enterprise Road – Old Goomboorian Road

Widening of single lane to 7m seal 735 $510 $374,850

Upgrade of unsealed road to 7m seal 2150 $1,300 $2,795,000

$3,169,850

Old Goomboorian Road – Lynch Road



Upgrade intersection 1 $200,000 $200,000

$2,210,500

Total Cost $5,380,350

Route B

East Deep Creek Road – Lynch Road


Upgrade intersection 1 $200,000 $200,000

$972,650

Lynch Road


Construction of new road 1060 $1,500 $1,590,000

$3,449,000

Hyland Road

Widening of single lane to 7m seal 2022 $510 $1,031,220

Construction of new road 1200 $1,500 $1,800,000

Upgrade of unsealed road to 7m seal 725 $1,300 $942,500

$3,773,720


Route C

No works proposed

Route D

Drummond Drive

Upgrade unsealed road to 7m seal 260 $1,300 $338,000

Bridge 28 $44,000 $1,232,000

Fill section to raise road level 140 $1,055 $147,700

$1,717,700




Route E

Mount Pleasant Road

No works

River Road

Specialised intersection 1 $400,000 $400,000

Property resumption 1 $500,000 $500,000

$900,000

Union Street

Construction of new road 280 $1,500 $420,000

Bridge (Deep Creek) 180 $72,000 $12,960,000


Property resumption 2 $500,000 $1,000,000

$14,543,200

Inglewood Road

Localised widening 50 $1,500 $75,000

Bridge (rail) 65 $44,000 $2,860,000

$2,935,000

Tin Can Bay Road – Nicholas Christopher Drive to Langton Road

Fill section to raise road level 450 $550 $247,500

New pavement 450 $1,300 $585,000

Culverts 40 $13,894 $555,760

$1,388,260

Total cost $19,766,460

Route F

Tin Can Bay Road – Red Hill Road to Ingle Wood Road

Levee bank 495 $8,320 $4,118,400

Bridge (Deep Creek) 180 $72,000 $12,960,000


New pavement 826 $1,300 $1,073,800

Culverts 40 $13,894 $55,760

$19,434,840

Tin Can Bay Road – Nicholas Christopher Drive to Langton Road


New pavement 450 $1,300 $585,000

Culverts 40 $13,894 $555,760

$1,388,260




Route G

Bruce Highway Interchange with Penny Road

South facing ramps 2 $7,500,000 $15,000,000

$15,000,000

Total Cost $15,000,000 Note – the above costs are base costs, and require escalation associated with:

Contractor Preliminaries/Profit 26%

Engineering/Design 7%

Risk and Contingency 15%

Procurement and Construction Management inclusive of Owners Costs 12%

The key objective in assessing isolation problems at Monkland/Glanmire is the prevention of loss of production at businesses within this precinct, which is due primarily to the loss of access to employees and isolation from other production critical services. A minimal solution would involve the construction of a new rural road between Hyland Road and Penny Road, spanning the gap in the existing road network (nominated as A/B Optimal), and improving the Drummond Drive access. These works are defined as:

Glanmire Monkland Access – minimal Penny Road to Hyland Road – construction of one kilometre of new rural road within existing road

reserve east of Hyland Road joining to Penny Road. It is proposed to construct a 7 m wide gravel road to provide an alternate route to businesses during times of flooding. Design considerations include:

− Gravel pavement design 7 m width

− Minimal cross drainage considerations

− Environmental management

Drummond Drive – Upgrade Drummond Drive to 100 year ARI level and increase its capacity to a B Double standard. Design considerations include:

− Re-profile road so it is above the 63.5 m 100 year ARI level

This will require an approximate 3.5 m lift over 120 m

Suitable culverts to maintain the flow of flood waters are to be provided (the road will not act as a levee bank)

Widen this section of Drummond Drive to 10 m and asphalt seal

− 10 m seal width will to maintain consistency with the northern and southern extents

Pavement to be designed to TMR/Gympie Council standards

Access to property is to be maintained with minimal resumptions to allow for earthworks

Drainage, signs and lines are to be considered, street lighting is not required

− Replace existing to match the new road formation and B-Double loadings

Structure similar to Mary Valley, Rattler crossing at Gilldora

Public Utilities on existing bridge to be considered



Route A/B Optimal

Hyland Road/Penny Road

Gravel pavement design 7m width

Minimal cross drainage considerations

Environmental management

1000 $1,050 $1,050,000

$1,050,000

Drummond Drive

Upgrade unsealed road to 7m seal 260 $1,300 $338,000

Bridge 28 $44,000 $1,232,000

Fill section to raise road level 140 $1,055 $147,700

$1,717,700


Contractor Preliminaries/Profit 26% $719,602

Engineering/Design 7% $193,739

Risk and Contingency 15% $415,155

Procurement and Construction Management inc Owners Costs 12% $332,124

TOTAL $4,428,320

5.4 One Mile The One Mile precinct includes over 50 industrial properties, and over half of these are directly affected by 100 year ARI flooding. Moreover, at the southern end of the precinct adjacent to Deep Creek, there are several sites that become inundated at the 10 year ARI flood level (corresponding to 18.79 m Kidd Bridge gauge). Improved resilience, typically, can be offered to such pockets of flood risk through the use of strategic levees (or buyback) targeting the flood risk at a commensurate level.

Three options have been developed to offer improved resilience in the precinct drawing on combinations of strategic levees and voluntary buyback. These target the at-risk commercial properties at the southern end of the precinct, around Victoria Road/Graham Street, and the businesses on Tin Can Bay Road near the intersection with George Street. A key difficulty for the properties at the southern end of the precinct is the steepness of the land adjacent to Deep Creek. This prohibits the use of earthen levees as there is no viable footprint available. For this southern section, a block wall levee is proposed, around 2.2 m high, offering improved flood immunity, but due to practical limitations, only to the 10 year ARI flood level. An additional 2m in height would be required to offer 20 year ARI immunity, and this is not considered feasible or practical.


Figure 17 Conceptual levee options at One Mile

Option A: Levee north and levee south Levee around Tin Can Bay Road businesses, up to 6 m high, 400 m long, approximately 35,000 m3

earthworks

Demountable flood barriers at Tin Can Bay Road, and roadworks on Cootharaba Road to tie into levee



Levee around Victoria Road/Graham Street businesses, up to 3 m high, 600 m long, approximately 28,000 m3 earthworks and including a 180 m section of 2.2 m high structural flood wall

Demountable flood barriers at Tin Can Bay Road (x2)

Buyback of up to three similarly at-risk properties outside the levee




Option B: Wises Road levee and levee south Levee south of Wises Road, up to 7 m high, 420 m long, approximately 43,350 m3 earthworks

Demountable flood barriers at Tin Can Bay Road

Spillway 80 wide, drop structure, stilling basin and connections to underground drainage

Backflow management (flap gated culverts)



Levee around Victoria Road/Graham Street businesses, up to 3 m high, 600 m long, approximately 28,000 m3 earthworks and including a 180 m section of 2.2 m high structural flood wall

Demountable flood barriers at Tin Can Bay Road (x2)

Buyback of up to three similarly at-risk properties outside the levee



Option C: Buyback of flood affected businesses Buyback, demolition and conversion to parkland of up to 20 properties

Preliminary cost estimates for these options are provided below.

One Mile - Option A (South levee + North levee)

Item Infrastructure CAPEX1.0 North Levee $1,755,0001.1 Drainage $500,0002.0 South Levee $1,560,0002.1 Drainage $500,0002.2 Demolition $54,0002.3 Buyback $1,200,000

Subtotal $5,569,000


Subtotal $7,094,770

Risk & Contingency 15% $1,064,216Procurement & Construction Management inclusive of Owners Costs 12% $804,593

Total $8,963,578


One Mile - Option B (South levee + Wises Rd levee)

Item Infrastructure CAPEX1.0 South Levee $1,560,0001.1 Drainage $500,0001.2 Demolition $54,0001.3 Buyback $1,200,0001.4 Rehabilitation $108,0002.0 Wises Rd Levee $2,167,5002.1 Spillway and stilling basin $67,2002.2 Drainage $529,600

Subtotal $6,186,300


Subtotal $7,915,779

Risk & Contingency 15% $1,187,367Procurement & Construction Management inclusive of Owners Costs 12% $897,929

Total $10,001,074

One Mile - Option C (Buyback of all nominated properties)

Item Infrastructure CAPEX1.0 North buyback1.1 Buyback $1,600,0001.2 Demolition $72,0001.3 Rehabilitation $144,0002.0 South buyback2.1 Buyback $6,400,0002.2 Demolition $288,0002.3 Rehabilitation $576,000

Subtotal $9,080,000

Contractor Preliminaries/Profit 26% $280,800Engineering / Design 7% $635,600

Subtotal $9,996,400


Total $12,619,156


6.1 Introduction The CBA has been undertaken for a suite of flood mitigation measures to inform Council’s ranking of the measures and to assist with decision making. Each of the proposed mitigation measures have been outlined previously in Section 5. Specifically this CBA process in the Gympie situation has determined the net economic benefits resulting from implementing the flood mitigation measures by estimating the reduction in direct tangible damage costs experienced by businesses and industries inundated by flood water. It is noted that the CBA is limited by the assumptions that have been adopted in the modelling and that there are other intangible damages and broader regional and local flow-on economic impacts not included in the CBA that would serve to improve the modelling results.

This section will discuss the results of the CBA modelling which has been conducted in accordance with the framework set out in Section 4. Following the scanning of flood affected localities in the Gympie region, three precincts have been selected by council for CBA modelling namely:

Mary Street (CBD)

One Mile

Monkland and Glanmire

6.2 Mary Street (CBD)

6.2.1 Flood mitigation options cost The following Table 7 describes the initial capital cost and the recurrent costs of each levee option over the 20 year assessment period.

Table 7 Mary Street CBD mitigation options and costs in 2013 prices over a 20 year assessment period

Options A B C D

Description Levee Option A Levee Option B Levee Option C Levee Option D

Capital Costs $23,256,508 $27,056,310 $19,183,555 $22,686,231

Recurrent Costs (annual) $40,000 $40,000 $40,000 $40,000 Table 7 describes that Option C is the lowest cost option.

6 Cost benefit analysis and Business Case


6.2.2 The cost of flooding The AAD curve for the Q2 to Q100 ARI flood levels for the flood mitigation options against the base case for the Mary Street area is presented in the figure below.

It is observed in Figure 18 that a step change at Q20 occurs for the levee options A, B and C and at Q30 for levee option D. This step change demonstrates how the damage cost jumps above the base case I (ie existing damage costs experienced) due to the flood waters topping the levee. The benefits for each option can be observed by the area between each AAD curve and the base case line up to when the levee is breached. The benefit result is penalised by the increased damage beyond a Q20 or Q30 flood, depending upon the levee option.

Figure 18 Mary Street CBD damage curve comparison between the Base Case and Flood mitigation options

6.2.3 CBA results The following Table 8 describes the CBA results for the four levee options over the 20 year assessment period.

Table 8 CBA results at 7% nominal discount rate over a 20 year assessment period

Options A B C D


PV Total Benefits $13,566,005 $13,882,415 $14,003,728 $17,100,855

PV Total Costs $23,818,523 $27,618,325 $19,745,570 $23,248,246

Net Present Value (NPV) -$10,252,518 -$13,735,910 -$5,741,842 -$6,147,391

Benefit Cost Ratio (BCR) 0.57 0.50 0.71 0.74


Key findings:

All options presented in Table 8 have a BCR less than 1

Option B has the highest cost and almost the lowest total benefits resulting in the lowest (although very negative) NPV

Option C has the best NPV

Option D has the highest BCR of 0.74 and also generates the highest benefits due to the increased flood protection (from Q20 to a Q30 ARI flood level) from the 1.5 m increase in levee height

6.3 One Mile

6.3.1 Flood mitigation options cost The following Table 9 describes the capital and recurrent costs of each option over the assessment period of 20 years.

Table 9 One Mile mitigation options and costs in 2013 prices over a 20 year assessment period

Options A B C

Description South + North levee South + Wises Rd levee Property Buyback (20 properties)

Capital Costs $8,963,578 $10,001,074 $12,619,156

Recurrent Costs (annual) $30,000 $40,000 Nil

6.3.2 The cost of flooding The following Figure 19 presents the AAD curve for the Q2 to Q100 ADI flood levels for the flood mitigation options against the base case for the One Mile area.

It is observed in Figure 19 that a step change at Q10 and Q20 occurs for Options A and B. This step change demonstrates how the damage cost jumps above the base case due to the flood waters topping the levee. This levee is topped at Q10 for the South levee as it is at its technical limit. The benefits for each option can be observed by the area between it and the base case line up to when the levee is breached. The benefit result is penalised by the increased damage beyond a Q20 or Q30 flood, depending upon the option.


Figure 19 One Mile damage curve comparison between base case and flood mitigation options

6.3.3 CBA results The following Table 10 describes the CBA results including the Present Value Total Benefits which is the aggregate of the 20 Year Benefits assessment period. For the Option C, the buyback residual value at Year 20 has been included to price the benefit produced by the reduction in residual risk for comparison against the levee solutions.

Table 10 CBA results at 7% discount rate over a 20 year assessment period

Options A B C

Description South + North levee South + Wises Rd levee Property Buyback (20 properties)

PV Total Benefits $1,495,036 $1,564,136 $2,900,489

PV Total Costs $9,281,398 $10,424,835 $12,619,156

Net Present Value (NPV) -$7,786,362 -$8,860,699 -$9,718,667

Benefit Cost Ratio (BCR) 0.16 0.15 0.23 Key findings:

All three flood mitigation solutions have very low BCRs less than 1

In terms of the levee options, Option A has the highest BCR compared with Option B since the capital cost is the lowest

Similarly Option A has the best NPV although very negative at approximately -$7.8 million

Option C which is the buyback solution is the most economically efficient of the options with a BCR of 0.23 however it also has the highest capital cost

Option C also benefits from the reduced residual risk from levee topping since the properties no longer exist.


6.4 Monkland and Glanmire

6.4.1 Flood mitigation background Since the flood mitigation solutions for Monkland and Glanmire are focused on improving accessibility, the economic damage from flood events have monetised the economic turnover loss from the lack of accessibility rather than from property inundation. The flood modelling has indicated that businesses and industries in Monkland and Glanmire are predominately flood free up to a Q100 ARI event level. The issue is that access roads between the Gympie CBD and the industrial parks are inaccessible above a Q20 flood level. Therefore the economic loss to these businesses has been assessed for floods above a Q20 level.

Considerations for the assessment include the fact that the Bruce Highway access is cut to the south at a Q20 flood level and that in a region wide event which involves the flooding of the Mary River, the Bruce Highway will most likely be inaccessible from the north as well. This consideration has reduced the benefits from the improved accessibility options as the businesses and industries cannot access their customers and suppliers which will cause a turnover loss. Therefore most of the business turnover loss reduction benefits yield from the improved accessibility has been sourced from the benefit that employees who reside in Gympie can still access their place of employment in Monkland and Glanmire.

Another consideration in the analysis for Monkland and Glanmire is the improved access from the proposed Bruce Highway Bypass upgrade which will provide equivalent flood immunity benefits as the proposed solutions. The upgrade is planned to be constructed in at least 10 years’ time. Therefore since the accessibility is provided by Council funded road projects not related to the Bruce Highway upgrade, the assessment period has been limited to a 20 year benefit period.

6.4.2 Flood mitigation options cost The following Table 11 describes the costs of each option. For all options the Present Value (PV) Total Cost has been modelled over the 20 year period assuming that the purpose of the connection will be superseded by the construction of the Bruce Highway.

Table 11 Monkland and Glanmire mitigation options and costs in 2013 prices over a 20 year assessment period

Options A/B A/B Optimal E F

Description Upgrading Enterprise, Old Goomboorian, Lynch and East Deep Creek Roads and constructing new roads to provide a Q100 flood immune connection for light and heavy vehicles from Gympie to Monkland and Glanmire industrial estates

Construct missing section of alternate flood bypass route as a 7m wide gravel road between Penny Road and Hyland Road and improve Drummond Drive providing 100 year ARI connectivity between Monkland and Glanmire for all vehicles

Construction of Q100 flood immune bridge across Deep Creek at Union Street for light and heavy vehicles

Construction of Q100 flood immune bridge across Deep Creek at Tin Can Bay Road for light and heavy vehicles

Capital Costs $21,721,152 $4,428,320 $31,626,336 $33,316,960

Recurrent Costs $21,721 $4,428 $31,626 $33,317 Table 11 describes that Option A/B Optimal is the lowest cost option.


6.4.3 CBA results The following table provides the CBA results for the road related infrastructure options over the 20 year assessment period.


Options A/B A/B Optimal E F

Description Upgrading Enterprise, Old Goomboorian, Lynch and East Deep Creek Roads etc

Penny Road to Hyland Road

and

Drummond Drive

Construction of Q100 flood immune bridge across Deep Creek at Union Street for light and heavy vehicles

Construction of Q100 flood immune bridge across Deep Creek at Tin Can Bay Road for light and heavy vehicles

PV 20 Year Benefits $8,094,228 $8,094,227 $8,094,224 $8,094,227

PV Residual Value Nil Nil Nil Nil


PV Total Costs $22,003,699 $4,485,923 $32,037,729 $33,750,344

Net Present Value (NPV) -$13,909,474 $3,608,305 -$23,943,502 -$25,656,117

Benefit Cost Ratio (BCR) 0.37 1.80 0.25 0.24 Key findings:

Option A/B, E and F Q100 flood immune access solutions presented in Table 12 have a BCR less than 1

Option A/B Optimal achieves a BCR of 1.80 and therefore is economically efficient as it is the lowest cost solution

Option A/B Optimal easily delivers the best BCR and NPV results for the Monkland and Glanmire precincts and also across all mitigation solutions for all precincts

The Option A/B Optimal solution to provide access between Gympie and the Monkland and Glanmire industrial estates is very economically efficient over the 20 year assessment period under the modelling assumptions and these metrics support a business case to pursue this option. Option A/B Optimal involves establishing a new road at a minimum standard to provide a 20 year ARI flood immune light vehicle connection from Gympie to Monkland and Glanmire. This solution appears adequate for purpose since during 20 year ARI and above flood events, the Bruce Highway is flooded in both directions and heavy vehicle access from intrastate and interstate regions is limited. The heavy vehicle access to Gympie during these flood events will improve as the Bruce Highway is upgraded to a 100 year ARI flood immune.

6.5 Sensitivity analysis – effect of economic life The above CBA results have been modelled over a 20 year assessment period as per the Local Government Floods Response Subsidy guidelines. This assessment period can give an inaccurate result if the economic life of an asset, such as a levee, is greater than the 20 year assessment period.

Therefore a sensitivity analysis has been completed to model the CBA analysis across a 50 year assessment period (which is more accustomed to the economic life of a levee which can be up to 100 years). This sensitivity analysis has been completed for the Mary Street and One Mile options since they both contain levees.


6.5.1 Mary Street The following Table 13 describes the CBA sensitivity results including the Present Value Total Cost and Benefits over the economic life of the levee options of 50 years.

Table 13 CBA results at 7% nominal discount rate over a 50 year assessment period

Options A B C D


PV 20 Year Benefits $13,566,005 $13,882,415 $14,003,728 $17,100,856

PV 21-50 Year Residual Value

$7,043,835 $7,208,124 $7,271,113 $8,879,225


PV Total Costs* $23,982,520 $27,782,322 $19,909,567 $23,412,243

Net Present Value (NPV) -$3,372,680 -$6,691,783 $1,365,274 $2,567,838

Benefit Cost Ratio (BCR) 0.86 0.76 1.07 1.11 NOTE: * Recurrent cost includes $500,000 periodic overhaul/maintenance provision every 20 years (2013 dollars).

It can be observed that over a 50 year assessment period which is more alike to the economic life of a levee, levee Options C and D yield an economically efficient result with a result greater than 1. This result is dependent on the additional flood protection benefits from extending the assessment period to the reasonable life of the levee. For this revised lifecycle for the Option D levee alignment, a BCR of 1.11 and $2.5m NPV represent solid metrics to support a business case to pursue this option.

6.5.2 One Mile The following Table 14 describes the CBA results including the Present Value Total Costs and Benefits over the economic life of the levee options of 50 years. For the Option C, the buyback residual value at Year 20 has been included to price the benefit produced by the reduction in residual risk over the 50 year period to make a fair comparison with the levee solutions.


Options A B

Description South + North levee South + Wises Rd levee

PV 20 Year Benefits $1,495,035 $1,564,135

PV 21-50 Year Residual Value $873,720 $914,103

PV Total Benefits $2,368,755 $2,478,238

PV Total Costs* $9,551,582 $10,727,086

Net Present Value (NPV) -$7,182,827 -$8,248,848

Benefit Cost Ratio (BCR) 0.25 0.23 NOTE: * Recurrent cost includes $500,000 periodic overhaul/maintenance provision every 20 years (2013 dollars) for the levee type options.

It can be observed that over a 50 year assessment period which is more alike to the economic life of a levee, all options remain economically inefficient with results less than 1. The BCR results have increased compared to the 20 year assessment period results; however this increase is not enough to consider any option close to being economically viable. Accordingly, a business case for such works cannot be supported base on these metrics


6.6 Conclusions The CBA has assessed flood mitigation options for the 3 study precincts within Gympie. This analysis has monetised business and industry flood damage remediation and turnover loss which incorporate revenue losses for each of the base case and flood mitigation options. The economic benefit is yield from the difference between the economic damage loss of each of the flood mitigation options and the base case modelled over the 20 year assessment period. The economic cost of each option has been provided as the Present Value of the capital and operating cost over the 20 year assessment period. The analysis has utilised a 7% nominal discount rate to model the cash flows.

The CBA results suggest that Options C and D are close to being economically viable for the Mary Street CBD area with BCR results of 0.71 and 0.74 respectively. Option D is the preferred solution due to the increased flood protection to a Q30 ARI flood level. In the sensitivity analysis which extends the assessment period to 50 years, Options C and D have been demonstrated to be economically efficient with BCR results of 1.07 and 1.11 respectively when assessed over the 50 year economic life of the levee. The metrics associated with both options support a viable business case for a levee when considered over a 50 year lifecycle.

The One Mile levee options have been found to be economically unviable with BCR results significantly less than 1. The capital cost of all options, including the buyback option, significantly outweigh the benefits due to the lower value of the businesses in One Mile compared to the Monkland and Glanmire areas. The sensitivity analysis which extends the assessment period to 50 years has demonstrated that neither option is economically viable with BCR’s between 0.25 and 0.36. These metrics do not support a business case for such works.

Most of the Monkland and Glanmire Q100 flood immunity access options have been found to be economically unviable with BCRs significantly less than 1, with the notable exception of option A/B Optimal. Due to the significantly less capital cost from the low cost solution to provide minimum level of services for a light vehicle connection between Gympie and Monkland and Glanmire, option A/B Optimal delivers the best BCR and NPV results not only for the Monkland and Glanmire precinct, but most importantly across all mitigation solutions and all precincts in the study. The metrics associated with the A/B Optimal option support a business case for these works, and is the most attractive of all flood mitigation and resilience building options assessed.

Finally there are also other local and regional indirect tangible damages, disruption to transport, services infrastructure damage and broader flow-on impacts through the regional economy. These indirect damages have not been modelled in the CBA and reputable references such as RAM identifies that such damages can be in the order of 30% of the value of direct tangible damages. These additional unmonetised benefits would further improve CBA modelling results for the mitigation options, improving the attractiveness of those options already identified and NPV positive (ie A/B Optimal and Mary Street Levee options C and D), but do not improve the metrics of the options tested at One Mile such that they become economically viable.


The Queensland Government Disaster Mitigation and Resilience Funding The Queensland State Government has recently introduced a new process to deliver disaster mitigation and resilience grants to Local Government, via a joint application package for disaster mitigation and resilience funding. This provides applicants with a single point for all disaster mitigation and resilience funding administered by the state government and offers a streamlined assessment process. This new joint application incorporates the following programs:

Royalties for the Regions (R4R)

Local Government Floods Response Study (LGFRS)

Natural Disaster Resilience Program (NDRP)

Applications opened on 25 June 2013, and will close 19 July 2013. Announcement of successful projects will be from August 2013.

Application forms and guidelines are available on the Department of Local Government, Community Recovery and Resilience website. One application is required for each individual project, irrespective of the funding program being targeted. Note however that the applicant does not need to nominate a funding program in their submission.

Applications will be assessed for eligibility under the three funding programs administered by the State noted above. Those deemed eligible for more than one program will be assessed under each of the relevant programs. Funding recommendations will be made based on a single-stage competitive assessment process against all relevant programs. Successful applicants will be required to enter into funding agreements with the Queensland Government and to meet any other program prerequisites prior to receiving funds.

In 2013/2014, the state’s priorities for flood mitigation funding are to support:

Projects that are supported and informed by a completed flood management study and, where relevant, consider the potential impact of communities downstream

Projects that will achieve improved infrastructure and flood resilience outcomes/(such as levees and detention basins) that protect people and property, and essential services such as water and sewage treatment plants, hospitals and major transport facilities such as airports

Projects that reflect the needs of the catchment as a whole and provide evidence that they have been developed collaboratively across relevant councils or other organisations

Flood mitigation projects that address outstanding needs from the events of 2010-2011 and January 2013

Projects that are ready to proceed and can be delivered within the timeframe

7 Funding mechanisms


Regarding eligibility, all local governments are eligible under LGRFS and NDRP, and those local governments outside South East Queensland are eligible for R4R.

The types of flood mitigation projects that could be funded include, but are not limited to:

Flood mapping, flood management studies, reports and modelling

Infrastructure that mitigates against flood damage and inundation, such as levees, detention basins, floodgates and backflow prevention devices

The table below summarises the activity types that may be eligible for the three funding programs:

Table 15 Funding sources and their applicability

Activity Type LGFRS R4R NDRP

Mapping or studies, such as flood mapping or flood management studies

Y Not applicable Y

Mitigation infrastructure that protects residents, essential infrastructure and other assets, or improves resilience against future flooding

Y Structural works only, where consistent with R4R funding program schedule

Y

Community preparedness/resilience, such as community education programs and volunteer capability building

Not applicable Not applicable Y

Assessment criteria are five main areas:

The project informs development of appropriate mitigation strategies such as flood management studies, mapping and modelling

The project provides infrastructure that builds resilience for the community and achieves improved mitigation outcomes

The project is collaborative and based upon a regional catchment approach

The project is financially sound and is ready to be delivered

The project has demonstrated community support

Details on the specifics of the three main programs can be found in the guidelines on the website.

7.1 Recommendations The types of mitigation options that are evolving from this Study appear to be the types of projects likely to be eligible for funding under the state’s Disaster Mitigation and Resilience Funding program.

Council should apply to the Disaster Mitigation and Resilience Funding program in order to obtain funding to progress the various projects arising from this study. Given however that the funding is limited ($10 million available in 2013-2014 from R4R; $12.9 million available in 2013-2014 for flood mitigation projects from LGFRS; and an unknown amount available from NDRP), Council should prioritise the various projects and focus the applications on those top priority projects (as assessed by economic and other factors).


It is noted that property buyback type options could be eligible for funding under this program via NDRP. The Queensland Floods Commission of Inquiry – Final Report (March 2012) contained a specific recommendation (Recommendation 11.1) relating to buy-backs:

“Councils should consider implementing a property buy-back program in areas that are particularly vulnerable to regular flooding, as part of a broader floodplain management strategy, where possible obtaining funding from the Natural Disaster Resilience Program for this purpose.”


Aurecon has undertaken a comprehensive investigation of flood risks throughout the Gympie Regional Council area and identified a range of opportunities to improve flood resilience across the community. The study examines flood conditions across the Region, and evaluates a number of strategies to provide improved flood protection and/or reduce exposure to flood risks. Flood mitigation options considered include:

Levees – to annex the affected community from the floodplain

Floodways – creating preferred alternate routes for flood waters

Detention basins – capturing and reducing flood peaks

Buyback/relocation – removing the affected properties from the floodplain

Flood warning – reducing impacts by improving warning time and flood level accuracy

Waterway modification – increasing conveyance efficiency in waterways by removing bottlenecks

Improving road networks and/or bridges – where flooding brings community isolation

8.1 At the screening level Key recommendations from the screening level assessment include:

Flood levee for Goomeri – Council should undertake investigations to assess the feasibility of a levee on the southern side of town to reduce the risk of flooding due to breakout flows from Nangur Creek.

Relocation of the Kandanga Bowls Club – consideration of the relocation of the Bowls Club, buildings and greens, away from this highly at-risk floodplain area next to Kandanga Creek.

Raising of Coondoo Creek bridge on Tin Can Bay Road – Council should engage with TMR to investigate the potential for significantly reducing the isolation risks for some 4000 properties in the Cooloola coast area.

Raising River Road at Centro Way – Council should engage with TMR to further investigate statewide benefits associated with improved passage for commercial vehicles travelling south along the Bruce Highway. These improvements, via River Road and Pengellys Bridge, would allow southbound passage some six to ten hours earlier than is available via the (still flooded) Bruce Highway Inglewood Bridge.

Improved connectivity between Gympie and Southside – to address isolation risks for over 400 residential lots in Southside, via a 100 year ARI flood free crossing of the Mary River at Gympie. Council should engage with TMR to understand where this issue rests in the context of state-wide transport planning, and determine the need to develop a flood management plan to support Southside residents during times of isolation.

8 Summary and recommendations


Voluntary Buy Back – consideration should be given to the voluntary buy back of residences and businesses exposed to frequent and significant above floor flood inundation. The flood return period for which such voluntary buy back should be triggered needs to be considered within the context of resilience of the property and its land use. Nevertheless, as a minimum, it is recommended that Council consider buyback for businesses and residences exposed to above floor flooding at the 5 year ARI level.

Finally, the screening assessment and consultation process acknowledges a broader and region wide issue of flood resilience on the district’s rural roads, and that a generally poor level of flood immunity across the rural transport network has a significant impact in recovery in rural areas. These rural State Roads require improvement to offer rural areas a ready means to recover following natural disasters.

In addition to the specific recommendations above, there are a range of generic flood resilience actions that can be implemented by Council to reduce exposure to flood risks. These include:


Drainage Maintenance – an ongoing commitment to the maintenance of Council drainage infrastructure is required, to ensure existing drainage and flooding issues do not escalate as a result of declining infrastructure condition

8.2 Project recommendations The detailed cost-benefit assessment has identified two opportunities to offer a significant improvement in flood resilience for the Gympie business community. It is recommended Council pursue funding for these preferred strategies in the CBD and Monkland/Glanmire via the Queensland Government’s Disaster Mitigation and Resilience program:

Gympie CBD Flood Levee – flood levee located in the Albert Park Touch Football fields, offering protection up to 22 m on the Kidd Bridge gauge $22.7 million

Monkland Glanmire Flood Access Improvements – construction of a new road link from Penny Road to Hyland Road and improvement works for Drummond Drive $4.4 million


Key references for this assessment have included:

GHD 2012 – Final Report of Gympie Regional Flood Study. Reference 41/23403/421073 (Rev 3)

NRE 2000 – Rapid Appraisal Method (RAM) for Floodplain Management. ISBN 0 73114 515 1

SCARM Report 73 – Floodplain management in Australia: best practice principles and guidelines, CSIRO Publishing, ISBN 0 643 06034 0

NRM 2002 – Guidance on the Assessment of Tangible Flood Damages, Qld Government (reference QNRM 02081 – 15929)

9 References

Appendices

Appendix A Consultation report

Appendix B Flood maps

Appendix C Gympie CBD mines

Aurecon Australia Pty Ltd ABN 54 005 139 873 11/102 Wises Road Maroochydore QLD 4558 PO Box 5196 Maroochydore QLD 4558 Australia T +61 7 5443 4055 F +61 7 5443 7597 E [email protected] W aurecongroup.com Aurecon offices are located in: Angola, Australia, Botswana, China, Ethiopia, Ghana, Hong Kong, Indonesia, Lesotho, Libya, Malawi, Mozambique, Namibia, New Zealand, Nigeria, Philippines, Qatar, Singapore, South Africa, Swaziland, Tanzania, Thailand, Uganda, United Arab Emirates, Vietnam.

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