multifunctional stormwater projects - an innovative technical design and partnership approach

Multi-Functional Stormwater Projects – An Innovative Technical Design and Partnership Approach

Adam BerryFloodplain Management Engineer, Ipswich City Council, Ipswich, Australia

E-mail: [email protected]

David SextonPrincipal Engineer, Engeny Water Management, Brisbane, Australia

E-mail: [email protected]

1. OVERVIEW

Ipswich City Council (Council) has been pursuing sustainable development and regional stormwater solutions for a number of years through a variety of different mechanisms. Traditionally these mechanisms were limited to a water quality offset scheme.

A developer, Citimark Properties originally lodged a development application with Ipswich City Council for a residential subdivision and commercial precinct within Eastern Heights, Ipswich. As opposed to following the standard development application process, Council began discussions and negotiation with the developer in order to develop an innovative and sustainable solution to stormwater management for the site which provided benefits to multiple parties. The outcome was a regional solution on Council owned land downstream from the development which included stormwater detention, water quality improvement infrastructure, stormwater harvesting and a functional community asset. This allowed the developer to transfer the stormwater infrastructure offsite while council benefited through a more sustainable, integrated solution that complimented existing community open space and recreational infrastructure. The regional solution partnership between both parties was fostered through a non-trunk infrastructure agreement whereby site

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Multi-Functional Stormwater Projects Sexton, Berry

construction costs and land value were taken into account for final contributions.

This paper presents two key components of the project, the infrastructure agreement between council and the developer and the innovative design elements introduced during the design of the integrated stormwater system. The paper is a result of the partnership between Ipswich City Council, Citimark Properties (Citimark), Engeny Water Management (Engeny) and Griffith University that provides a new solution to incorporation of stormwater infrastructure into an urban environment. The challenge to provide feasible regional stormwater solutions are well acknowledged in the industry and this paper will present the approach used to rise to the challenge and deliver positive outcomes for Ipswich.

Figure 1. Aerial photo of works

2. OBJECTIVE

The primary objective of this project was to develop an alternative solution to the traditional means of developments providing onsite stormwater quality and quantity management measures. Engeny Water Management worked in direct partnership with Council to develop a fully integrated stormwater treatment system designed to meet a number of objectives including; improve the quality of stormwater discharging from the site, improve the overall health of Bundamba Creek and provide economic savings for the irrigation of several sports fields via a stormwater harvesting system. The intention of this partnership project was to produce an asset for the City of Ipswich, which is sustainable, economically viable and aesthetically pleasing to the community.

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Figure 2. Bulk Earthworks Complete

3. APPROACH

The approach to the entire inception, design and delivery of the Jim Donald Parklands project has ensured the underlying themes of partnerships, collaboration, integration and innovation are at the core of critical thinking. Figure 3 shows the high level master planning that occurred upfront to ensure seamless integration of the area.



Figure 3. Jim Donald Parklands Master Plan

This critical thinking was prevalent in different stages of the project and different components as listed below.

Infrastructure Agreement and Delivery MechanismDiscussions between Council and the developer were instigated in the early stages of the project, focussing around the imposition for the area required and construction costs usually incurred for site based bio-retention and detention basins. A co-operative partnering type arrangement was seen ideal for both parties due to the extensive benefits that could be derived overall. Additional consideration was given for the land value and reduction of establishment, maintenance and time burden for the developer in order to calculate a relevant and reasonable contribution value. Upon agreement between both parties, respective legal documents were finalized through a non-trunk infrastructure agreement.

The Infrastructure Agreement itself was seen as an important document not only for financial components but also setting the scene for a co-operative and partner arrangement between both parties during construction. The delivery contract included the following components:

Citimark Properties were nominated as the Principal

Council were bound through the IA as a partner and advisor to the Principal

Citimark and Council jointly assigned a Superintended, assessed tenders and awarded works



to the contractor. Both parties assigned Project Managers to the project who worked in collaboration.

Council and Citimark continually liaised throughout the whole construction process on a variety of construction and contract challenges.

The innovation on the delivery mechanism allowed the strengths of both parties to be utilised to produce a superior construction result.

Figure 4. Development Works

Project TeamThe project team for the Jim Donald Parklands project as a major driver and critical component of the multi-functional, integrated and innovative outcomes achieved. Very early on in the design process it was agreed that the procurement design model would not be traditional. Rather than Council purely sending out a Request for Quotation with scope and leaving all of the design work to the consultant, the following components were achieved:

Council completed all of the conceptual flood, water quality and water balance assessment in house along with concept designs and utilised Engeny as a peer review and value add partner on the initial design



During modelling refinement and detailed design, a hot seat was set up in the Engeny office for Council officers which was utilised frequently. The hot seat for Council’s engineers and scientists included the ability to inform, Engeny staff on design parameters were required.

This arrangement intensified collaboration, creativity and innovation between a variety of Council officers resulting in minimal delays, no reviews required and all stakeholders 100% satisfied on the end design.

DesignThe Jim Donald Memorial Park is nine hectares of parkland immediately south of the old Bremer High School redevelopment site; it is bordered by the old Bremer High School grounds to the north and Robertson Road to the south. The open space area consists of an open low flow channel incorporated with a grassed flood plain and a single sports field and club house is located to the east of the open drain. Figure 5 below shows an overview of some of the components of design.

Figure 5. Detailed Design Overview

Flooding

The residential properties downstream from the proposed site are subject to flooding and therefore the development and stormwater system will have impact downstream properties and was considered in the overall design.

The Jim Donald Parkland has a 72 ha catchment which contributes runoff to the site via a number of underground stormwater network outlet points and overland flow paths. The catchment is primarily urban residential with small areas of commercial activity across the catchment. There are no industrial areas within the catchment area. As part of the Jim Donald detailed design project, a flood assessment was undertaken to quantify the changes in peak flows as a result of the upstream development, assess the change in flood behaviour due to the proposed park works, and determine suitable mitigation measures to alleviate any associated flood impacts.



Flood modeling was originally undertaken in a sub-regional XP Rafts/Tuflow model to assess various storms, durations and reporting locations downstream of the site. Storage capacity was made available within the wetland in order to attenuate minor storms and a flood bund upstream also provided an additional 2200m3 of storage. The flood results are shown in Figure 6.

Figure 6. Flood Modelling

Water Quality

Overall the water quality treatment train consisted of the following components:

a) 717m2 Sedimentation basinb) 130m vegetated low flow swale. The low flow swale also consists of a sand substrate and sub

soil drainage, further assisting in water quality reductions. c) 3400m2 Wetlandd) 1100m3 Stormwater harvesting ponde) 300m high flow bypass swale

An extract of the MUSIC model is shown in Figure 7.



Figure 7. MUSIC Model

As the design focused on integrated and innovation, stormwater quality targets were not considered a driving primary factor in design and would have impacted on the project’s success. Regardless, the water quality targets were seen as a good benchmark for cost effectiveness and the following pollutant reductions are shown in Figure 8.

Figure 8. Water Quality Results

In order to provide a robust, conservative and long term functioning asset for Council, the wetland was designed in a way to ensure its sustainability in terms of managing the hydraulic and pollutant loadings from a large catchment relative to the size of the wetland. This was achieved via the following mechanisms:

The standard 48 hour retention/residence time was revised to 12 hours. The requirement to reduce the duration of inundation was confirmed through a wet Spell analysis using outputs



from MUSIC modelling.

Inflows into the wetland were limited to 66% of the 3 month event with the remaining 33% diverted through a heavily vegetated swale. This was achieved through the use of dual outlet pipe networks within the sediment basin. Whilst a small reduction in stormwater harvesting was encountered, this method provides even further hydraulic impact protection to the macrophyte plants by reducing hydraulic and pollutant loading on the wetland

Stormwater Harvesting

Ipswich City Council is investigating measures to reduce potable water demands through a variety of mechnisms and the Jim Donald Parklands site was seen as an attractive opportunity to achieve some of this long term goal. The parkland irrigation on average was consuming 21 megalitres of potable water per year at a cost to Council of approximately $80,000.

With the introduction of the stormwater harvesting (including the solar transfer pump), modelling results show a yield of 20 Megalitres annually resulting in a reduction of potable water usage of 95% and a cost saving of approximately $76,000 per year.

Figure 9. Stormwater Harvesting

Innovation in Design

Additional innovative design components that were incorporated into the design include;

A crescent shaped sedimentation basin shown in Figure 10 was designed which provided a more linear treatment effect while reducing the footprint of the basin required to remove sediment. Additionally, the crescent shaped design provides a more practical and cost effective method of sediment removal long term.



Figure 10. Sediment Basin Arrangement

Stormwater harvesting was included within the system to allow for irrigation of the sports fields, the irrigation system draws water from a separate pond that stores treated water that has passed through the wetland. The outlet structure of the wetland is set up in a way that allows water from the wetland to preferentially fill the harvesting pond but won’t allow water to be directly harvested from the wetland. As such, the stormwater harvesting will not impact on the dry weather resilience of the wetland. To further increase the resilience of the wetland to dry weather and improve harvesting yields, a solar pump was set up within the sediment basin to pump water from the sediment basin into the wetland and therefore into the harvesting pond.

Wetland media consisting of gravel overlaid with a 100mm layer of sand was used as the wetland planting media. This media was incorporated on the basis that plants would receive nutrients from the stormwater passing through the system and the sand/ gravel layer would encourage sub surface flows. It also avoids issues associated with anaerobic conditions developing within the wetland due to excessive nutrients associated with nutrient rich top soils.



Figure 11. Wetland Media

Introduction and trialing of a subsurface geocell foundation for the high flow bypass swale in order to assist with reducing rutting from maintenance vehicles and to assist in sub surface flow. It is also worth noting that although not modelled, this component also provides additional water quality benefits through some filtration. The installation of the geocells is shown in Figure 12.



Figure 12. Highflow Swale Geocell Installation

Multifunctional infrastructure including the sediment dry out area, the low flow weir and the flood bund doubling as a basketball court, sediment basin vehicle access and a viewing platform respectively.

The innovation and creativity present in the design is a direct result of the high level of partnering and collaboration on the project between multiple organisations.

4. RESULTS / OUTCOMES

The project outcomes are a direct result of a successful partnership arrangement between the developer, Council and Engeny’s project team which included Griffith University.

Key outcomes from the project included:

More developable land and yield is made available at the site which would have otherwise been taken up by bio retention and detention basins

More aesthetic appeal to the subdivision and increases the value of residential lots due to not having sterile detention basins etc.

May provide savings for developers with regard to a reduction in the amount of stormwater modelling, calculations and stormwater management plans

Reduces the burden of the developer not having to undertake the required 2 year maintenance and establishment period, often a very difficult component of works

Increases the yield for stormwater harvesting by collecting water from a larger catchment.

Reduces potable water consumption and Council’s open space irrigation expenditure.

Less maintenance for Council long term by reducing the amount of site based assets within the catchment

Provides a highly integrated solution with environment, engineering, open space and recreational outcomes



Enhances Council’s perception of delivering on sustainable development and leverages on strategic direction

Provides the community with a useable, attractive and educational asset that can be enjoyed by the surrounding area.

Potential to expand knowledge in this area through innovation and post construction monitoring by university students.

Increase in pollutant removal cost efficiency through treatment of a larger catchment through an undersized system.

Provides more robust and creative designs through the unique partnership approach

Will inform future designs on integrated and water sensitive urban design projects

Figure 13. Wetland Viewing Deck



5. CONCLUSIONS

This project has resulted in an innovative solution that provides management of stormwater through the construction of sustainable, useable and appealing community assets. This project has demonstrated it meets the technical design intent of providing water quality improvement, flood mitigation and stormwater harvesting. Moreover, the project provides a positive demonstration of the approach to provide an asset that serves a long term function for the community whilst at the same time emphasising a sustainable development and whole of life cycle perspective. This will set an example of the path forward in these areas for both Ipswich and the stormwater industry.

Furthermore, the unique design and partnering approach that has occurred on this project between multiple parties has allowed a creative fostering of innovation between different disciplines and organisations to produce a result that would have otherwise not been possible.


multifunctional stormwater projects - an innovative technical design and partnership approach

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