1. condition 5 detailed engineering plans...1 cal/jab - bsc file no: da 10.2016.77.1 #e2019/55108 31...

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CAL/JAB - BSC File No: DA 10.2016.77.1 #E2019/55108

31 July 2019

Mr Phil Warner Manager Assets and Major Projects Byron Shire Council Attention: Joshua Winter Dear Sir, Re: DA10.2016.77.1 Byron Bay Bypass Council has considered the various details and plans referenced below addressing conditions 5, 7, 9, 10, 13 and 22. The following comments are provided in relation to each of those conditions. 1. Condition 5 – Detailed Engineering Plans

Prior to the commencement of construction, a staging plan shall be approved by Council with detailed engineering plans for each of the stages. The plans are to be in general accordance with the approved plans under Condition No.2, as qualified by Condition 1, and incorporate the following detail to the extent relevant.

a) Each Set of drawings shall be accompanied by a Certification Report which must be signed by a suitably qualified Civil Engineer or Registered Surveyor. The Certification Report will comprise the certificate and check lists set out in Annexure DQS-A of the Northern Rivers Local Government Development & Design Manuals or the Roads and Maritime Services specifications. The information shown on the drawings shall be logically collected on discrete sheets generally in accordance with Annexure DQS-B and the Sample Drawings of the Northern Rivers Local Government Development & Designs or the Roads and Maritime Services specifications.

b) The Design Plans to demonstrate works are contained wholly within the road reserve and railway corridor crossing with appropriate clearances to property boundaries or Council shall provide appropriate road widening or open a temporary public road to accommodate the works.

c) The plans to incorporate details for any services including relocated infrastructure. Any proposed relocation water and sewer infrastructure to include details demonstrating

i) Development Design and Construction Manuals, Northern Rivers Local Government, 2009;

ii) Development Servicing Plan for Water Supply Services, Byron Shire Council, 2011;

iii) Development Servicing Plan for Sewerage Services, Byron Shire Council, 2011;

iv) Fire Flow Design Guidelines, Water Directorate, 2011;

v) WSAA Codes.

d) The plans to demonstrate compliance with the Section 138 Approval and associated concurrence from Roads and Maritime Service

e) Driveways to be reconstructed as necessary.

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f) Street lighting plan consistent with the Visual Consideration and Recommendations by Design Team Ink in the GHD Environmental Impact Statement dated January 2016.

g) The design to include the following measures to protect rail infrastructure:

i) The rail crossing to be designed, approved and constructed in full consultation with JHR and in accordance with TfNSW (2014b) Construction of New Level Crossings Policy/Change in Use of a Level Crossing or as otherwise approved by TfNSW.

ii) The design to provide ongoing access to the rail corridor for land owner staff and contractor vehicles.

iii) For applicability with the preferred drainage strategy for the town centre.

h) Water quality from the proposal meets the relevant guidelines. COMMENTS The plans listed below are approved to satisfy this condition, subject to the following: 1. The approval relates to the portion of the Byron Bay Bypass through the SEPP 14

Wetlands only. 2. The sewer and water plans have been approved separately to address condition 5c). 3. Glare shields are to be provided to all street lights. 4. Construction is to be carried out in consultation with John Holland Rail and must comply

with the Property Licenses issued by Transport for NSW. 5. Structural certification is to be provided for the major culvert structure. 6. All signage and line marking must be approved by Council’s Local Traffic Committee prior

to installation. 7. The following updates/amendments to plans be considered:

a. Rock blanket under embankment should be shown as a Type D filter material as per NRLG Spec 233 (Drawing C002).

b. The stone pitching shown is to be in accordance with RMS Spec R11 (Drawing C105).

c. The vertical curve on Road 01 Chainage 1165 is short. This area is outside the SEPP 14 wetland hence outside the scope of works for BRS (Drawing C152).

d. Pipe run F6/40 – H7/40 is shown as Class 2 pipe in plans and Class 4 in long section. Class 4 pipe should be shown in plan (Drawing C216).

e. Pit G1/45 is incorrectly noted as S1/45 (Drawing C228). f. Concrete footpath detail to be updated to match standard drawing R-07 with

100mm thick concrete and SL62 mesh (Drawing C301). g. Detail for ‘Concrete Apron’ to be updated to note compliance with RMS Spec R83

(Drawing C301).

Plans

Author Document Title Document Reference Revisio

n GHD COVER SHEET AND LOCALITY PLAN 41-29578-G001 0

GHD DRAWING INDEX 41-29578-G002 2

GHD GENERAL ARRANGEMENT 41-29578-G003 2

GHD TYPICAL SECTIONS SHEET 2 OF 5 41-29578-C002 2



GHD CONTROL LINE SETOUT DETAILS 41-29578-C010 1

GHD CONTROL LINE SETOUT TABLES 41-29578-C011 1

3

GHD CONSTRUCTION DETAIL LAYOUTS SHEET 4 OF 7

41-29578-C104 1


41-29578-C105 1


41-29578-C106 1

GHD ROAD01 LONGITUDINAL SECTION SHEET 2 OF 4

41-29578-C152 0


41-29578-C153 0


41-29578-C154 0

GHD SURFACE DRAINAGE CATCHMENT AREAS SHEET 4 OF 7

41-29578-C204 1


41-29578-C205 1


41-29578-C206 1

GHD DRAINAGE LAYOUT SHEET 4 OF 7

41-29578-C214 1

GHD DRAINAGE LAYOUT SHEET 5 OF 7

41-29578-C215 1

GHD DRAINAGE LAYOUT

SHEET 6 OF 7

41-29578-C216 1

GHD DRAINAGE LONGITUDINAL SECTIONS SHEET 8 OF 8

41-29578-C228 1

GHD DRAINAGE CROSS SECTIONS (CULVERT 5-1)

41-29578-C231 0

GHD SETOUT DETAILS SHEET 4 OF 7 41-29578-C254 1



GHD SETOUT TABLES SHEET 3 OF 4 41-29578-C260 0

GHD SETOUT TABLES SHEET 4 OF 4 41-29578-C261 0

GHD PAVEMENT AND SURFACE TYPE LAYOUTS LEGEND

41-29578-C301 2

GHD PAVEMENT AND SURFACE TYPE LAYOUTS SHEET 4 OF 7

41-29578-C305 1


41-29578-C306 1


41-29578-C307 1

GHD CROSS SECTIONS CONTROL LINE ROAD01 SHEET 7 OF 14

41-29578-C507 1


41-29578-C508 1


41-29578-C509 1


41-29578-C510 1

4


41-29578-C511 1


41-29578-C512 0

2. CONDITION 7 – Stormwater Management Plan

Prior to the commencement of construction, a detailed stormwater management plan shall be approved by Council. Stormwater is to be collected and discharged in accordance with Council’s standards, currently Northern Rivers Local Government Development Design & Construction Manuals or the Roads and Maritime Services specifications. The drawings and management plan are to include, but not be limited to, the following items:

a) Catchment plan (included in the drawing set). b) Measures to allow for the free flow of water from between the Bypass and the Rail

Corridor into the Cumbebin Swamp to the west. c) Stormwater quality must be suitable for discharge in accordance with Council's

standards and Northern Rivers Local Government Development Design & Construction Manuals or the Road and Maritime Services specifications.

d) Proposed treatment measures for any swales and grassed embankments to filter stormwater run-off from the road prior to it entering the receiving environment. The embankments and or swales to be planted using suitable non-invasive species.

e) Include details on legal points of discharge as necessary including any easements. f) Ensure drainage of water toward the rail corridor does not exceed predevelopment

levels.

The plan is to be consistent with the findings and recommendations of the flood modelling for the bypass.

COMMENTS The plans listed below are approved to satisfy this condition, subject to the following: 1. The approval relates to the portion of the Byron Bay Bypass through the SEPP 14

Wetlands only.

Plans

Author Title Reference Revision

BMT Byron Bay Bypass Stormwater Management Plan

R.B23714.002.01_Stromwater 01

BMT MUSIC Model - Developed Case existing_issued20190717.sqz -

BMT MUSIC Model – Developed Case Unmitigated

developed_rev_issued201907

23.sqz

-

BMT MUSIC Model – Existing Case developed_unmit_issued20190717.sqz

-

BMT Byron Bay Bypass Stormwater Management Plan – Letter

regarding catchment redirection

L.B23714.002.00 3 July

2019

BMT Byron Bay Bypass – Swale Performance

F.B23714.003 24 July

2019

5

Ardill

Payne

& Partners

Swale Planting Plan SK07 2

Humes Humeceptor © Sizing Detailed Report

- 4 June

2019

Humes Humeceptor © System Inspection and Maintenance Guide

- Issue 1

3. Condition 9 – Public Safety Management Plan

Prior to the commencement of construction, a Public Safety Management Plan shall be approved by Council. This public safety management plan is to include provision for (but not be limited to):

a) Pedestrian barriers, alternative footpaths and ramps as necessary; b) Lighting of the alternative footpaths between sunset and sunrise; c) Hoardings and fencing around the site; d) The loading and unloading of construction and materials; e) Parking spaces for worker vehicles; f) Removal of any such hoarding, fence or awning as soon as the particular work has

been completed. COMMENTS The plans listed below are approved to satisfy this condition, subject to the following: 1. The approval relates to the portion of the Byron Bay Bypass through the SEPP 14

Wetlands only.

Plans

Autho

r

Report Title Report Reference Revision

hazellbros Public Safety Management Plan PLN-CST-0014-Q19040 2.1

4. Condition 10 – Traffic Management Plan

Prior to the commencement of construction, a Traffic Management Plan shall be approved by Council. The plans and specifications are to include the measures to be employed to control traffic (inclusive of construction vehicles) during construction of the works. The traffic control plan is to be designed in accordance with the requirements of the Roads and Traffic Authority’s Manual, Traffic Control at Work Sites Version 2, and the current Australian Standards, Manual of Uniform Traffic Control Devices Part 3, ‘Traffic Control Devices for Works on Roads’.

The plan shall incorporate measures to ensure that motorists using road adjacent to the development, residents and pedestrians in the vicinity of the development are subjected to minimal time delays due to construction on the site or adjacent to the site.

The traffic control plan must be prepared by a suitably qualified and RTA/ RMS accredited Work Site Traffic Controller.

COMMENTS The plans listed below are approved to satisfy this condition, subject to the following:

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1. The approval relates to the portion of the Byron Bay Bypass through the SEPP 14

Wetlands only.

Plans

Author Document Title Reference Revision

hazellbros Traffic Management Plan PLN-CST-005-Q19040 2.1

hazellbros Byron Bay Bypass Project – Methodology Appendix D –

Proposed Haul Routes

FRM-CST-52 -

5. Condition 13 – Erosion and Sediment Control/ Soil and Water Quality Management

Plan

As part of the CEMP, a detailed Soil and Water Management Sub Plans shall be prepared in consultation with the EPA, NSW Fisheries, the NPWS and BSC. The Plans shall be prepared in accordance with the Department of Housing’s guideline Managing Urban Stormwater - Soils and Construction 1998, and the RTA’s Guidelines for the Control of Erosion and Sedimentation in Roadworks. The Plans shall be approved by Council prior to the commencement of construction. The Soil and Water Quality Management Sub Plans shall contain, but not be limited to:

a) management of the cumulative impacts of the development on the quality and quantity of surface, including stormwater in storage, sedimentation basins and flooding impacts;

b) details of short and long-term measures to be employed to minimise soil erosion and the discharge of sediment to land and/or waters including the exact locations and capacities of sedimentation basins;

c) detailed erosion and sedimentation controls including a strategy to manage the extent of exposed ground surface during construction;

d) identification of all potential sources of water pollution and a detailed description of the remedial action to be taken or management systems to be implemented to minimise emissions of these pollutants from all sources within the subject site;

e) detailed description of water quality monitoring to be undertaken during the preconstruction, construction and operation stages of the project including base line monitoring, identification locations where monitoring would be carried out and procedures for analysing the degree of contamination of potentially contaminated water;

f) measures to handle, test, treat, re-use and dispose of stormwater, effluent and contaminated water and soil;

g) procedures for the re-use, treatment and disposal of water from sedimentation basins and constructed wetlands;

h) measures for the use of water reclaimed or recycled on-site; i) contingency plans to be implemented in the event of fuel spills or turbid water

discharge from the site; and, j) a program for reporting on the effectiveness of the sedimentation and erosion control

system against performance goals. COMMENTS The plans listed below are approved to satisfy this condition, subject to the following: 1. The approval relates to the portion of the Byron Bay Bypass through the SEPP 14

Wetlands only.

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2. Condition 13 part 9(j) is satisfied in the hazellbros Soil and Water Management Sub-Plan Section 3.2 – Monitoring and Inspections, which forms part of the Construction Environmental Management Plan (CEMP).

3. The Soil and Water Management Plan provided by GHD notes that refuelling areas on site should be bunded. This is agreeable; however the erosion and sediment control plans do not show any bunded areas for refuelling. Designated fuelling areas on site are to be provided with appropriate bunding to ensure any spills are contained

Plans

Auth

or

Title Reference Revisio

n hazellbros Byron Bay Bypass Construction Phase Erosion and Sediment Control Plan

Stage 2 – Main Civil Works

3351-1015 Pages 1-5

2

GHD Byron Bay Bypass

Soil and Water Management Plan

22/19047 November 2017

6. CONDITION 22 - Road Signage Plan

Prior to the commencement of construction a road signage plan shall be approved by Council. The Plan to include normal road signage (speed limits, warning signs etc) plus signage restrictions to prevent car parking, prohibition on overnight camping, and littering along the southern half of the Bypass Route south of the Glen Villa Resort to Browning Street. All regulatory signage to be approved by the Local Traffic Committee.

COMMENTS The plans listed below are approved to satisfy this condition, subject to the following: 1. The approval relates to the portion of the Byron Bay Bypass through the SEPP 14

Wetlands only. 2. All regulatory signage must be approved by Council’s Local Traffic Committee prior to

installation.

Plans

Author Drawing Title Drawing

No.

Revision

Ardill Payne & Partners Sign and Linemarking Layouts Sheet 4

of 7

C354 3


of 7

C355 3


of 7

C356 3

Should you have any further queries please contact me in the first instance Yours faithfully

Chris Larkin Manager Sustainable Development

CAL/NFSx#E2019/62245

22 August 2019 Mr Phil Warner Manager Assets and Major Projects Byron Shire Council Attention: Joshua Winter Dear Sir, Re: DA10.2016.77.1 Byron Bay Bypass Further to our letter of 31 July 2019, Council confirms the approved CEMP as required under Condition 12 is the submitted plan prepared by GHD and titled “Report for Byron Shire Council – Byron Bay Bypass 22/19971 dated March 2019”. It is considered this updated plan satisfies the requirements of Condition 12 of the consent. The comments provided in our previous letter of 31 July 2019 remain relevant to the proposal. Should you have any further queries please contact me in the first instance. Yours faithfully

Chris Larkin Manager Sustainable Development

Byron Bay Bypass Stormwater Management Plan

Reference: R.B23714.002.01_Stormwater.docx Date: June 2019 Confidential

G:\Admin\B23714.g.dcc.ByronBayBypassMitigationAssessment\02_Reports\R.B23714.002.01_Stormwater.docx

Document Control Sheet

BMT Eastern Australia Pty Ltd 6/20 Byron Street Bangalow NSW 2479 Australia Tel: +61 2 6687 0466 Fax: +61 2 6687 0422 ABN 54 010 830 421 www.bmt.org

Document: R.B23714.002.01_Stormwater.docx

Title: Byron Bay Bypass Stormwater Management Plan

Project Manager: Damion Cavanagh

Author: Damion Cavanagh; Blair Filer

Client: Byron Shire Council

Client Contact: Josh Winter

Client Reference:

Synopsis: This report presents the outcomes of conceptual stormwater quality and quantity assessments completed for the current detailed civil design for the Byron Bay Bypass.

REVISION/CHECKING HISTORY

Revision Number Date Checked by Issued by

00 13 May 2019 B. Filer D. Cavanagh

01 7 June 2019 B. Filer

D. Cavanagh

DISTRIBUTION

Destination Revision

0 1 2 3 4 5 6 7 8 9 10

Byron Shire Council BMT File BMT Library

PDF PDF PDF

PDF PDF PDF

Copyright and non-disclosure notice The contents and layout of this report are subject to copyright owned by BMT Eastern Australia Pty Ltd (BMT EA) save to the extent that copyright has been legally assigned by us to another party or is used by BMT EA under licence. To the extent that we own the copyright in this report, it may not be copied or used without our prior written agreement for any purpose other than the purpose indicated in this report.

The methodology (if any) contained in this report is provided to you in confidence and must not be disclosed or copied to third parties without the prior written agreement of BMT EA. Disclosure of that information may constitute an actionable breach of confidence or may otherwise prejudice our commercial interests. Any third party who obtains access to this report by any means will, in any event, be subject to the Third Party Disclaimer set out below.

Third Party Disclaimer Any disclosure of this report to a third party is subject to this disclaimer. The report was prepared by BMT EA at the instruction of, and for use by, our client named on this Document Control Sheet. It does not in any way constitute advice to any third party who is able to access it by any means. BMT EA excludes to the fullest extent lawfully permitted all liability whatsoever for any loss or damage howsoever arising from reliance on the contents of this report.

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Byron Bay Bypass Stormwater Management Plan i Executive Summary


Executive Summary

This report has developed and assessed a conceptual stormwater quality and quantity system for the Byron Bay Bypass (BBB). The system has aimed to integrate as extensively as possible with the current proposed detailed design arrangement for the road and easement areas. Both water quality and quantity modelling have been completed as a combined exercise (albeit within different modelling packages).

Water quality modelling results indicate that the stormwater quality objectives will be achieved for the BBB. These systems integrate with those to mitigate stormwater quantity to the maximum extent possible. Further design development is required to further test aspects of civil integration for the selected stormwater management measures. Overall system performance will be confirmed again at this later stage.

Water quantity modelling results indicate that the selected stormwater quantity mitigation will enable the achievement of stormwater quantity objectives for the BBB. These devices integrate with stormwater quality devices and hence serve a dual purpose. Similar as for the stormwater quality devices, further design development is required for water quantity measures to confirm site integration and functionality.

Overall, this stormwater management conceptual design and assessment report identifies that stormwater quality and quantity measures can be accommodated in the Byron Bay Bypass project, allowing Council to achieve its stormwater management targets for water quality and quantity.

Byron Bay Bypass Stormwater Management Plan ii Contents


Contents

Executive Summary i

1 Introduction 1

1.1 Project Overview 1 1.2 Site Location 1 1.3 Existing and Proposed Development 3 1.4 Site Topography 3 1.5 Drainage 6 1.6 Flooding 6 1.7 Site Soils Assessment 6

2 Stormwater Objectives and Modelling Overview 8

2.1 Conditions of Consent (CoC) 8 2.2 Stormwater Management Objectives and Targets 9

2.2.1 Stormwater Quality Objectives 9 2.2.2 Stormwater Quantity Objectives 10 2.2.3 Extent of Application 10

2.3 Stormwater Quality Modelling Overview 10 2.3.1 Model Description 10 2.3.2 Rainfall and APET 11 2.3.3 Land Use 12 2.3.4 Rainfall-Runoff Parameters 12 2.3.5 Runoff Quality Parameters 13

2.4 Event Hydrological Modelling 14 2.4.1 Model Design 14 2.4.2 Sub-Catchments 14 2.4.3 Land Use 16 2.4.4 Parameters 16 2.4.5 Design Rainfall 19

3 Opportunities and Constraints 20

3.1 Opportunities 20 3.2 Constraints 20

4 Stormwater Assessments 21

4.1 Model Preparation 21 4.2 Selection of Stormwater Management Measures 24

Byron Bay Bypass Stormwater Management Plan iiiContents


4.3 Stormwater Quality Modelling 26 4.3.1 Changes in Load Generation and Load Reduction Targets 26 4.3.2 Stormwater Quality Measures 27

4.4 Stormwater Quantity Modelling 31 4.4.1 Calibration 31 4.4.2 Unmitigated Discharge Analysis 33 4.4.3 Mitigation Measures and Discharge Analysis 34 4.4.4 Stormwater Quantity Mitigation Limitations 35

5 Findings and Conclusions 36

6 References 38

Appendix A Stormwater Quantity Full Results A-1

Appendix B Detailed Design Drawings, General Arrangements and Cross Sections (GHD, 2016) B-1

List of Figures

Figure 1-1 Byron Bay Bypass Study Area 2 Figure 1-2 Byron Bay Bypass Elevation Model 5 Figure 2-1 Hydrologic Sub-Catchment Delineation and Key Features 15 Figure 2-2 Hydrologic Discretised Sub-catchment with Base Case Land Use 17 Figure 2-3 Hydrologic Discretised Sub-catchment with Developed Case Land Use 18 Figure 4-1 Study Area Discretised into Sub-Catchments 22 Figure 4-2 Discretised Sub-catchment with Land Use 23 Figure 4-3 Types and Locations of Selected Stormwater Management Measures 25 Figure 4-4 Byron Bay Bypass MUSIC Model Arrangement 29

List of Tables

Table 2-1 Byron Shire Council Stormwater Quality Objectives 10 Table 2-2 Adopted Average Monthly Areal PET Rates 12 Table 2-3 MUSIC Rainfall-Runoff Parameters (HLW, 2018) 13 Table 2-4 Lumped Land Use Concentration Parameters (mg/L-log10) 13 Table 2-5 Split Land Use Concentration Parameters (mg/L-log10) 13 Table 2-6 Fraction Impervious 16 Table 2-7 Model Parameters 19 Table 2-8 Model Losses 19

Byron Bay Bypass Stormwater Management Plan iv Contents


Table 4-1 Existing Land Use 26 Table 4-2 Developed Land Use 26 Table 4-3 MUSIC Modelling Results – Unmitigated Site 27 Table 4-4 MUSIC Modelling Results – Load Targets 27 Table 4-5 Specifications for swales 30 Table 4-6 MUSIC Modelling Results – Load Targets 31 Table 4-7 Rational Method Parameters 32 Table 4-8 OutSouth Hydrologic Discharge Comparison 32 Table 4-9 OutMid Hydrologic Discharge Comparison 33 Table 4-10 Base Case and Developed Case Discharge Comparison 33 Table 4-11 Mitigation Measure Details 36 Table 4-12 Base Case and Mitigated Case Discharge Comparison 36 Table A-1 OutNorth Peak Discharge Results A-1 Table A-2 OutMid Peak Discharge Results A-1 Table A-3 OutWetland Peak Discharge Results A-1

Byron Bay Bypass Stormwater Management Plan 1 Introduction


1 Introduction

1.1 Project Overview

Byron Shire Council (herein referred to as ‘Council’) has completed detailed design of the Byron Bay Bypass (BBB) project. Additionally, Council has approval for the project, which was granted with Conditions of Consent (CoC).

BMT has been engaged by Council to assess the detailed design of the BBB for integration of stormwater quality and quantity control measures as these were not included in the earlier conceptual and detailed design stages. As such this component of work forms an adjunct to the detailed design work completed to date.

This Stormwater Quality and Quantity Management Report (SQQMR) summarises the outcomes of stormwater quantity and quality assessments undertaken for the BBB. Separately a Flood Impact Assessment has been completed by BMT that investigated potential flood impacts of the detailed design (refer to Section 1.6 for more detail).

This SQQMR identifies how Council achieves the requirements of the CoC and achieves its own stormwater quality and quantity objectives (applicable to this type of project). The stormwater quality and quantity measures are to as far as practicable, integrate with the existing road design and environment such that outcomes are cost effective, maintainable and aesthetically suited.

The report is structured as follows:

• Section 2 – Stormwater Objectives and Modelling Overview.

• Section 3 – Opportunities and Constraints.

• Section 4 – Stormwater Assessments.

• Section 5 – Findings and Conclusion.

1.2 Site Location

The BBB is located immediately to the west of the Byron Bay CBD and is to the west of the North Coast Railway Line. The BBB commences in the north on the existing Butler Street alignment just south of the main roundabout into Byron Bay from Shirley Street. The BBB heads in a southerly direction for 700 m along Butler Street before passing the Glen Villa Resort and extending out over the Cumbebin Swamp before turning east and crossing the North Coast Railway Line and passing adjacent to existing buildings (such as Mitre 10) and joining Jonson Street and Browning Street at a new proposed roundabout. In total the BBB is approximately 1km in length.

The site location is shown in Figure 1-1.



1.3 Existing and Proposed Development

In reference to the study area map shown in Figure 1-1. The existing land use includes a mixture of urban areas, roads, verges and in locations undisturbed vegetation. Some of the vegetation resides within wetlands and swamp areas that ultimately form part of the Cumbebin Swamp Nature Reserve (the Reserve is around 500m to the west of the study area).

Butler Street itself is an active road which is used to access an established area of housing to the west of the town centre. The road is in generally poor condition with extensive presence of patching, potholes, variable road width and eroded edges. There exists extensive informal parking along the road edge and verge. The verge areas are a mixture of gravel and or grassed areas. Public egress facilities, such as walkways exist in locations adjacent to the road easement. The southern extent of the study area (off of Browning and Jonson Street) is used for parking to access local business primarily, as there is no through road for road traffic.

Future land use of the study area will remain the same in the northern portion with the key differences being that as part of the proposed works, the road pavement extent increases, and gravel/vegetated road easement extent will decrease. The proposed design incorporates shared pedestrian paths. Within the swamp / wetland areas the land use will change to that of road and grassed verges, while the southern area will remain largely the same as the area is already extensively hardstand (bitumen).

General Arrangement Drawings and Cross Sections are provided in Appendix B for reference.

1.4 Site Topography

The BBB crosses an extent of ground that is mostly low lying and would likely have been mostly swamp land prior to historic development.

In terms of road topography, currently the northern end of the BBB is on slightly elevated ground at 2 to 2.5m AHD from where is extends south at approximately 2m AHD across low lying ground prior to rising again over a small ridge line (3 to 4.5m AHD) centred around Burns Street. Southwards of Burns Street elevations reduce again to around 1 to 1.5m across existing swamp land prior to elevating as it crosses eastwards and over the North Coast Railway Line. Future road design sees minor changes in road levels along Butler Street with extensive fill required within the wetland areas to achieve flood immunity.

In terms of catchment topography, the BBB is crossed by two major drains (flowing east to west towards the wetland or the Butler Street drain) which originate from catchments to the east. The catchments include portions of the Byron Bay town centre which drain to the Butler Street drain (approximate chainage 220m) and areas to the south of the town centre which drain to the wetland (approximate chainage 900m). As these existing catchments drain ‘through’ the BBB they are not extensively considered in this report, rather focus is applied to local catchments which encase the BBB and will activate its local drainage and water quantity/quality control systems.

Using GIS based tools which process ground elevation datasets, the local catchments of the BBB (excluding upstream catchments) were identified for use in this study. These are further outlined in later sections of this report.



A digital elevation model of the developed site including the BBB is shown in Figure 1-2.



1.5 Drainage

As outlined in Section 1.4 this report considers local drainage relevant to the BBB. The BBB has cross drainage culverts which allow for upstream flows from catchments to the east to drain under the alignment to the west.

The roadway itself has been designed to drain water off the road (crested centre) and direct water into pits where kerb edges exist. Review of design drawings suggest that kerbs are present from chainage 0 to approximate chainage 600m. These kerb drains are collected in longitudinal running pipes which discharge periodically on either side of the road to existing ground or drainage features. In sections where drainage fall is insufficient, or no drainage features exist, road drainage is again collected in longitudinal running pipes and conveyed downslope to a suitable location for discharge.

From around chainage 600m the road is elevated over the wetland and drainage occurs over a verge to table drains where able to be accommodated.

At the southern end of the road where it ties back into the existing roads, the road is again drained via pipes.

1.6 Flooding

BMT was engaged by Council to assess the detailed design of the BBB in relation to Part 6 ‘Flooding’ of the Conditions of Consent attached to the road approval. This was undertaken concurrently with this stormwater assessment. The purpose was to hydraulically model the regional flooding around the BBB from Belongil Creek and local flooding from the town centre for a range of storm events (including climate change) and incorporate different flood mechanisms (flooding and storm tide). The overall objective was to ensure that flood levels do not increase on other properties. Overall no increases in flood levels for events modelled were observed and the Conditions of Consent were achieved.

1.7 Site Soils Assessment

The EIS for the Byron Bay Bypass (GHD, 2016) provides a summary of site soils conditions determined from a variety of desktop and site-specific assessments. Primarily the review suggests that the bypass is underlain by Coastal and estuarine plain deposits. As described in the EIS, the, “Quaternary coastal and estuarine deposits are present throughout the proposal site. These

comprise marine barrier and estuarine sediments, some with a thin veneer of Quaternary alluvium.

This unit includes estuarine basin and channel deposits, intertidal bank barrier, washover and tidal

delta deposits, beach and dune sand deposits and organic swamp deposits”.

Soil testing along the profile has confirmed the presence of primarily sandy profile soils to around 1m, although profile differences were noted at the southern and northern ends of the bypass.

Mapping of Acid Sulfate Soil (ASS) risk identifies variation along the route with higher risk in the southern portions. Soil testing along the route has identified either potential or actual acid sulfate soils at a number of locations, however, depth to ASS was in excess of 1m below the ground surface.



Groundwater was observed in boreholes at a depth between 1.1 and 2.2 metres below ground level during testing for ASS. Due to the presence of the groundwater in what is expected to be an unconfined aquifer, groundwater levels would be expected to naturally fluctuate.

Byron Bay Bypass Stormwater Management Plan 8Stormwater Objectives and Modelling Overview


2 Stormwater Objectives and Modelling Overview

2.1 Conditions of Consent (CoC)

In relation to stormwater, Council has provided the following requirements, which are believed to have been included in the CoC, although it was noted that some of these conditions were originally documented within the EIS (GHD, 2016) itself.

Byron Bay Bypass Stormwater Management Plan 9Stormwater Objectives and Modelling Overview


The key requirements for water quality and quantity objectives from the above are as below:

Achieving Council’s stormwater quality discharge load based targets as outlined in the 2014 DCP;

Achieving Council’s stormwater quantity discharge load based targets as outlined in the 2014 DCP; and

Design and operation consistent with outcomes required for flood management.

No further site-specific information was available to inform these assessments.

2.2 Stormwater Management Objectives and Targets

2.2.1 Stormwater Quality Objectives The stormwater management objectives to be achieved by this project include both quality and quantity objectives (Byron Shire Council, 2014 and 2014a).

As the proposed development involves an area of land greater than 2 500 m2, the operational phase stormwater solution for the site must provide measures to address the “key” pollutants for all stormwater flows up to 25% of the 1-year ARI peak flow from the development site. Key pollutants are outlined in Table 2-1 (BSC, 2014a).

Byron Bay Bypass Stormwater Management Plan 10 Stormwater Objectives and Modelling Overview


Table 2-1 Byron Shire Council Stormwater Quality Objectives

2.2.2 Stormwater Quantity Objectives In terms of stormwater quantity, the on-site stormwater detention requirements of Council apply (except for discharge to tidal waterways). Council’s guidelines identify requirements that post-development flow is to be controlled to be no greater than the pre-development flow for all storm events up to the critical 100-year ARI event.

2.2.3 Extent of Application The stormwater quality and quantity assessments outlined later in this report are necessarily applied only to the new or modified areas of the catchment, which for this development relates to the creation of new pavement and loss of existing ground surfaces. A significant portion of the existing site remains the same and is not subject to the stormwater quality and quantity objectives outlined above. Achieving Council’s stormwater quality and quantity objectives across the entire development (including pre-existing and proposed) would be an unnecessarily onerous requirement.

To ensure that the development is appropriately assessed, models have been extended to encompass the sub catchments it is located within.

2.3 Stormwater Quality Modelling Overview

2.3.1 Model Description Stormwater quality modelling was undertaken to estimate the hydrology and load of common stormwater pollutants (i.e. TSS, TP and TN) generated by the site using the Model for Urban Stormwater Improvement Conceptualisation (MUSIC). MUSIC includes algorithms to evaluate the hydrology and concentrations / loads in stormwater runoff from urban catchments as well as estimate the performance of selected stormwater management measures to capturing these pollutants and achieve load and/or concentration reductions in discharges of stormwater.

MUSIC was designed to continuously simulate urban stormwater systems over a range of temporal and spatial scales utilising historically representative rainfall data. MUSIC is considered within the engineering industry to be an appropriate conceptual design tool for the analysis of runoff water quality in the urban environment.



The hydrologic algorithm in MUSIC is based on the model developed by Chiew & McMahon (1997). The model simplifies the rainfall-runoff processes and requires input of the following variables to perform the hydrological assessment:

• Rainfall data (time steps varying from 6 minutes to 1 days);

• Areal potential evapotranspiration (PET) rates;

• Catchment parameters (area, % impervious and pervious areas);

• Impervious and pervious area parameters (rainfall threshold, soil and groundwater parameters); and

• Storm event and base flow stormwater pollutant concentrations.

MUSIC can be applied for comparison of alternative scenarios that adopt the same base inputs. Although the magnitude of the estimates may not be equivalent to actual site conditions (due to limitations in available data for a particular site), the relative differences between scenarios is expected to be appropriate for decision making.

The MUSIC modelling approach applied to estimate stormwater pollutant loads for the Site is described in the following sections.

It should be noted that MUSIC modelling approaches have followed those recommended within the MUSIC Modelling Guidelines developed by Healthy Land and Water (2018). While currently a consultation draft, this version is noted to be extensively similar to earlier MUSIC modelling guidelines published by this organisation.

2.3.2 Rainfall and APET The meteorological template includes the rainfall and areal potential evapotranspiration (APET) data and forms the basis for the hydrologic calculations within MUSIC.

The nearest Bureau of Meteorology (BoM) continuously recording rainfall station is located at the Federal Post Office (Station 58072) approximately 15 km west of the Site and is significantly elevated compared to the Site. This gauge was found not to be representative of the long-term rainfall at the Site due to its location.

Due to the sparsity of long-term rainfall intensity records in proximity to the site, the coastal Coffs Harbour MO (Station 59040) located approximately 200 km south of the Site (and at a similar elevation) was used. This station was found to provide more representative rainfall for the Site as its long-term rainfall average rainfall was similar to that for Byron Bay which is the key consideration in selecting a suitable rainfall record.

Review of the data indicated that the period 1995 to 2005 inclusive was relatively free of data gaps and accumulated rainfall data. The mean annual rainfall for this period is 1 555 mm which is similar to the estimated long term mean annual rainfall for the site, which from BoM records for Byron Bay Lighthouse is 1 737mm (based on data from 1950 to 2012). Rainfall data from Coffs Harbour MO (Station 59040) for the 1995 to 2005 period was adopted for MUSIC modelling.



Average monthly areal potential evapotranspiration (PET) rates adopted for the MUSIC modelling are summarised in Table 2-2. These values were obtained from BoM gridded dataset and are relevant to the Site. A 6-minute time step was adopted for the MUSIC modelling.

Table 2-2 Adopted Average Monthly Areal PET Rates

Month Mean monthly areal PET (mm)

January 194

February 164

March 157

April 109

May 73

June 53

July 56

August 69

September 103

October 156

November 178

December 205

2.3.3 Land Use Land use categories based on the existing and proposed site conditions are summarised in the respective modelling sections (refer to Section 2.3.3 and Section 2.4.3 for land use descriptions for stormwater quality and quantity modelling respectively).

For stormwater quality purposes the total land use has generally been divided into roof areas, ground level areas, roads and forests (for wetland only) in accordance with the methodology outlined in HLW (2018) and further detail on areas is included in Section 4.3.1.

2.3.4 Rainfall-Runoff Parameters Modelling of the rainfall-runoff process in MUSIC requires the definition of one impervious surface parameter and eight pervious surface parameters. The impervious surface parameter (rainfall threshold) and pervious surface parameters utilised were the default MUSIC hydrologic parameters for an urban land use, as summarised in Table 2-3.

Urban residential parameters are applied Ground Level land use, Road land use and Roof Land Use. It is important to note that for Road and Roof land use , the only parameter of relevance in Table 2-3 is the impervious area parameter, as both Road and Roof land uses are specified as 100% impervious area within the MUSIC model and as such the pervious area and groundwater parameters are irrelevant.

The Forest parameters have only been applied to areas identified as SEPP Coastal Wetlands as per the current Coastal Management Area mapping available under the Coastal Management SEPP 2018. Both Ground Level and Forest land use areas are modelled with 100% pervious area fractions



Table 2-3 MUSIC Rainfall-Runoff Parameters (HLW, 2018)

Impervious Area Parameters Urban Residential

Forest

Rainfall Threshold (mixed urban surfaces, mm)

1.0 1.0

Pervious Area Parameters

Soil Storage Capacity (mm) 500 120

Initial Storage (% of capacity) 10 10

Field Capacity (mm) 200 80

Infiltration Capacity Coefficient – a 211 200

Infiltration Capacity Exponent - b 5.0 1.0

Groundwater Properties

Initial Depth (mm) 5.0 50

Daily Recharge Rate (%) 50 25

Daily Baseflow Rate (%) 28 3

Daily Deep Seepage Rate (%) 27 0

2.3.5 Runoff Quality Parameters MUSIC requires stormwater constituent concentrations for storm flow and base flow for the various site land uses. These concentrations are converted to logarithmic values for input into MUSIC. The adopted log10 values are summarised in Table 2-4 and Table 2-5.

Table 2-4 Lumped Land Use Concentration Parameters (mg/L-log10)

Surface Type TSS TP TN

mean std. dev mean std. dev Mean std. dev

Forest Base Flow 0.51 0.28 -1.79 0.28 -0.59 0.22

Forest Storm Flow 1.90 0.20 -1.10 0.22 -0.075 0.24

Table 2-5 Split Land Use Concentration Parameters (mg/L-log10)


mean std. dev. mean std. dev. Mean std. dev.

Urban Residential – Roof Base Flow

NA NA NA NA NA NA

Urban Residential – Road Base Flow

1.00 0.34 -0.97 0.31 0.20 0.20

Urban Residential – Ground Level Base Flow

1.00 0.34 -0.97 0.31 0.20 0.20

Urban Residential – Roof Storm Flow

1.30 0.39 -0.89 0.31 0.26 0.23




mean std. dev. mean std. dev. Mean std. dev.

Urban Residential – Road Storm Flow

2.43 0.39 -0.30 0.31 0.26 0.23

Urban Residential – Ground Level Storm Flow

2.18 0.39 -0.47 0.31 0.26 0.23

2.4 Event Hydrological Modelling

2.4.1 Model Design A detailed local hydrologic model was developed for the local catchment along the BBB. The hydrologic model was developed to assess the increase in peak discharge from the additional paved surfaces associated with the BBB, and additional to size appropriate mitigation measures to return discharge back to existing conditions. For this assessment the following cases were undertaken:

• The base case (existing conditions without the BBB);

• The developed case (existing conditions with the inclusion of the BBB); and

• The mitigated case (existing conditions with the inclusion of the BBB and mitigation measures).

The hydrologic model was developed using Watershed Bounded Network Model (WBNM), which is an industry standard hydrologic software package, and has been calibrated to the Rational Method, as described in Section 4.4.1. Figure 2-1 illustrates the setup of the WBNM model and the reporting locations for both the Rational Method calibration and peak discharge analysis.

The model developed for the stormwater quantity assessments has been undertaken in accordance to the following standards unless stated otherwise:

• Byron Shire Council’s “Comprehensive Guidelines for Stormwater Management” which was adopted on 26th June 2014; and

• “Handbook of Stormwater Drainage Management – Version 3” for the Northern Rivers Local Governments which was adopted in August 2013.

2.4.2 Sub-Catchments In total, 77 sub-catchments have been delineated across the local catchment for the hydrologic model. The sub-catchment delineation is based on a 1m resolution DEM (based on LiDAR survey captured in 2010) and the BBB road alignment from GHD (4129578_12D DESIGN TIN R2.12daz).



2.4.3 Land Use The land use conditions have been informed using the Byron Shire Council zoning maps, current aerial imagery and the BBB road alignment. The land use has been used for both the model development (the fraction impervious of each sub-catchment) and the Rational Method calibration (the 10-year coefficient of discharge (C10)). The land use conditions for the base case and developed case / mitigated case change with the additional paved surfaces of the BBB. Each land use zone for the base case and the developed case is illustrated in Figure 2-2 and Figure 2-3, respectively. The parameters used for each land use are tabulated in Table 2-6.

Table 2-6 Fraction Impervious

Land Use Fraction

Impervious (%)

Coefficient of Discharge

Road 90 0.88

Business (Industrial / Commercial)

90 0.95

Open Space 0 0.71

Low Density Residential

70 0.75

Medium Density Residential

85 0.8

2.4.4 Parameters WBNM applies two hydrologic model parameters, one for channels and one for sub-catchments which are tabulated in Table 2-7. For the channels default non-linear routing has been adopted with different parameter sub-categories based on the channel type. Different sub-categories have been applied on a sub-catchment basis using aerial and DEM information.

The sub-catchment parameter is divided into two sub-categories for pervious and impervious areas (based on land use in Section 2.4.3). The pervious sub-catchment parameter has been adjusted during calibration and the default impervious lag parameters have been adopted. The sub-catchment parameters have been applied globally to all sub-catchments.

All parameters are applied in in accordance with those recommenced by Boyd et al (2012).



Table 2-7 Model Parameters

Parameters Type Sub-Category Value

Channel Routing Natural 1

Constructed / Excavated Earth

0.5

Concrete Lined / Paved

0.33

Sub-Catchment Lag

Pervious 1.9

Impervious 0.1

2.4.5 Design Rainfall The design rainfall has been simulated for each annual exceedance probability (AEP) from 1 exceedance per year (EY) to the 1 in 100 AEP. All storm durations for each exceedance probability from 15 minutes to 3 hours have been simulated for all cases.

The intensity-frequency-duration (IFD) curves to simulate the design rainfall events are based on the recommended guidelines (Section 2.4.1) for the Byron Bay & Bangalow area. The rainfall has been distributed based on the temporal patterns from Australian Rainfall and Runoff 1987 (ARR, 1987).

Initial (IL) and continuing (CL) rainfall loss parameters were adopted in the hydrologic model. The loss parameters are based on the above recommended guidelines and calibration to the Rational Method in Section 4.4.1. The loss parameters are tabulated in Table 2-8.

Table 2-8 Model Losses

AEP

(1 in X)

Pervious IL (mm)

Pervious CL (mm/hr)

Impervious IL (mm)

Impervious CL (mm/hr)

1 to 20 25 2.5 1.5 0

50 to100 15 2.5 1.5 0

Byron Bay Bypass Stormwater Management Plan 20 Opportunities and Constraints


3 Opportunities and Constraints

This section identifies site opportunities and constraints for stormwater management. The opportunities and constraints were identified through detailed review of road detailed design drawings, site inspections and discussions with Council.

3.1 Opportunities

Some of key opportunities identified for this Site include:

• Presence of open space in road reserve areas adjacent the Bypass providing opportunity for ‘at surface’ stormwater quantity and quality controls such as swales and buffer strips.

• Presence of open space outside the road reserve, but within Council control for implementation of larger stormwater quality and quantity controls such as bio-retention systems.

• Infiltration through soil profile to potentially native sandy soils which display relatively high infiltration rates (based on similar local projects and is subject to site confirmation).

• Accessibility of the piped stormwater network in locations for implementation of stormwater quality improvement devices (SQIDs) such as gross pollutant traps (primary treatment) or sediment arrestors (secondary treatment).

3.2 Constraints

Some of key constraints identified for this Site include:

• Limitations in suitable slope adjacent the road (within or external to the road easement) to promote drainage to logical stormwater management locations.

• Limitations in suitable areas adjacent the road (within or external to the road easement) to contain logical stormwater management locations.

• Potential services interference.

• Potentially high ground water levels in locations requiring confirmation. High ground water may interfere with the operation of specific types of stormwater management measures that utilise infiltration. Previous site testing has identified groundwater at depths of between 1.1 and 2.2m below the ground level.

• Acid sulfate soils may be present at depths greater than 1m below the soil profile.

In summary, the approach considered for stormwater management makes maximum benefit from the natural opportunities afforded by the road design and the verge areas in the road easement. Consideration has been applied to using areas external to the road easement where in Council control or existing stormwater infrastructure already exists.

Byron Bay Bypass Stormwater Management Plan 21 Stormwater Assessments


4 Stormwater Assessments

The following sections provide the stormwater quantity and quality assessments.

4.1 Model Preparation

To support the quality and quantity modelling the overall catchment of the proposed development has determined, and then this catchment discretised into sub-catchments for the purposes of drainage routing.

The process for preparing data for models includes:

• review of the existing site including numerous site inspections;

• review of site survey conducted by others;

• review of existing and proposed drainage details prepared by others;

• review of detailed designs produced by others;

• preparation and review of GIS data for the existing site and the post-developed site, particularly elevation data and cadastral boundary information;

• consideration of potential sites for placement of stormwater quality of quantity devices; and

• consideration of the nature of the existing land use and post-developed land use.

Without providing exhaustive information on the above model preparation steps, Figure 4-1 and Figure 4-2 have been prepared which illustrate how the developed site has been discretised into sub-catchments for the purposes of modelling. Additional imagery has been provided showing land use within these sub-catchments that has been utilised in modelling.



4.2 Selection of Stormwater Management Measures

The process of selecting potential stormwater management measures has been to primarily avoid site constraints and take advantage of identified opportunities. Some of the key considerations in selecting and siting potential management measures have included:

• Presence (or likely presence) of suitable gradient;

• Presence (or likely presence) of available space in the road verge to effect treatment;

• Presence (or likely presence) of subsurface features which may restrict treatment options including services, groundwater, etc;

• Presence of existing established features including road buffer, swales and other vegetated areas and their likely current role (and future) role in stormwater management;

• Visual appearance of the selected infrastructure in the environment and likely compatibility; and

• Ability to and ease of maintenance.

Further descriptions are provided in the following sub-sections and in Section 3.

The stormwater management measures selected for the developed site are outlined in Figure 4-3 and are described further in the following sections.



4.3 Stormwater Quality Modelling

4.3.1 Changes in Load Generation and Load Reduction Targets MUSIC models have been developed for the BBB and the catchments in which it is contained to enable accurate estimation of the effect of the BBB on pollutant discharge. The total modelled area is approximately 11.33 ha.

The existing site is comprised of a mixture of land use with areas as outlined in Table 4-1. These areas were determined using high resolution aerial imagery within a GIS platform and detailed design drawings for the BBB.

Table 4-1 Existing Land Use

Surface Type Area (ha) Imperviousness (%)

Ground Level 6.150 0%

Road 1.471 100%

Roof 1.152 100%

Forest 2.550 0%

Total 11.32 23.2%

With the proposed development, the area of pavement increases with a corresponding decrease in landscaped verge, as outlined in Table 4-2.

Table 4-2 Developed Land Use

Surface Type Area (ha) Imperviousness (%)

Ground Level 5.804 0%

Road 2.357 100%

Roof 1.153 100%

Forest 2.017 0%

Total 11.33 31.0%

The net estimated increase in road area is 0.886 ha (i.e. 60% increase on the existing extent of impervious area) with a corresponding decrease in pervious extents associated with the Ground Level and Forest land use.

Unmitigated modelling results are provided in Table 4-3.



Table 4-3 MUSIC Modelling Results – Unmitigated Site

Parameter

Annual Loads

Existing Site

(unmitigated)

Annual Loads

Developed Site

(unmitigated)

Increase

in Load % Increase

Total Suspended Solids (kg/yr) 15 300 20 300 5 000 32.7

Total Phosphorus (kg/yr) 29.5 36.8 7.3 24.7

Total Nitrogen (kg/yr) 162 183 21 13.0

Gross Pollutants (kg/yr) 846 1 130 284 33.6

Due to the nature of road design, it is not possible (or practical) to treat stormwater from just the new pavement areas. Water quality measures will capture runoff from both the existing and proposed pavement areas indiscriminately. Water quality objectives (refer Section 2.2) apply only to runoff from new pavement areas, not existing pavement areas.

As such, to estimate the load reductions required for works associated with the Byron Bay Bypass modelling has been completed to determine the change (increase) in mean annual load from the development (refer Table 4-3) and calculating the required load reductions. Table 4-4 provides details of the required load reductions and resultant load target for the developed mitigated site.

Table 4-4 MUSIC Modelling Results – Load Targets

Parameter Load Reduction

Target

Annual Required

Load Reduction A

Required Annual

Load Developed

Mitigated Site B

Total Suspended Solids (kg/yr) 80% 4 000 16 300

Total Phosphorus (kg/yr) 45% 3.28 33.52

Total Nitrogen (kg/yr) 45% 9.4 173.6

Gross Pollutants (kg/yr) 70% 199 931

A - Calculated by multiplying the ‘load reduction target’ by the modelled ‘increase in load’ B - Calculated by subtracting the ‘annual required load reduction’ from the ‘annual loads developed site (unmitigated)

4.3.2 Stormwater Quality Measures A variety of stormwater management measures have been integrated into the existing case MUSIC model as treatment nodes. Selected measures include:

• Buffer strips;

• Swales;

• Gross pollutant traps; and

• Bio-retention systems.



It has not been possible to include stormwater treatment in some catchments due to a lack of physical space to effect treatment, and as such catchment flows and loads direct discharge to the drainage system. Stormwater management measures selected are outlined in Figure 4-3. The MUSIC model structure is outlined in Figure 4-4.



Figure 4-4 Byron Bay Bypass MUSIC Model Arrangement

Buffer Strips

Buffer strips were included in locations where stormwater was able to sheet flow from the road surface and/or other paved surfaces adjacent the road (without a kerb) directly onto a vegetated strip of grass. Locations exist in the new road section where the roadway is elevated over the wetland areas (refer Buffer 7 and Buffer 10 in MUSIC model).

Buffer treatment nodes have two key inputs, namely ‘percentage of upstream area buffered’ and ‘buffer area (expressed as percentage of the upstream impervious area). The first parameter refers to the total percentage of the impervious catchment upstream of buffer strip that will be directed to it and it has been calculated using GIS tools. The second parameter relates the area of the buffer strip to the area of the upstream impervious catchment draining to it. Again, this parameter has been calculated using GIS tools.

Generally the buffer strips are sited in a way they can treat all upstream road runoff (draining to them) and they are sized in a way that they are at least 50% of the area of the upstream impervious road area.

Swales

Swales were included in several locations where cross drainage, longitude drainage and verge design allowed for swales to be considered. It is noted that on occasions swales occur outside of the road easement and are either existing (in Council control) or are represented in the detailed design drawings. Some of these locations will require further detailed design consideration and or modifications to ensure they integrate effectively with the surrounding design.

No specific criteria for flow conveyance was adopted for channel cross sectional design. The swales in their intended locations are being used to convey a portion of flow to identified outlet structures, i.e. major cross drainage lines. If the nominal capacity of the swale is exceeded it will simply spill out into adjacent low lying areas which are not utilised for habitation.



Properties for swales are provided in Table 4-5.

Table 4-5 Specifications for swales

Parameter Swale

2 * 3 7 8 9 10

Low Flow bypass (m3/s)

0 0 0 0 0 0

Length (m) 120 120 200 200 150 250

Bed slope (%)

0.5** 1 0.5 0.5 0.5 0.5

Base Width (m)

1.5 0 0 9 0 0

Top width (m)

6 3 3 3 3 3

Depth (m) 0.3 0.3 0.3 0.3 0.3 0.3

Vegetation height (m)

0.25 0.25 0.25 0.25 0.25 0.25

Exfiltration Rate

(mm/hr)

0 0 0 0

0 0

* It is assumed that existing informal drains that act like swales will be improved to represent modern constructed swales containing landscaping and planting

** Swales with very low grades may require additional drainage infrastructure to promote operation and limit ponding.

Detention Basin

A detention basin style system is proposed adjacent to Glen Villa Resort in existing road easement managed by Council. It is understood that this land will not be developed in the future for a road.

The detention basin is required primarily for water quantity management, but it will serve to capture sediments and nutrients associated with the sediments. The design of the detention basin largely replicates that described in Table 4-11. No exfiltration to groundwater has been assumed initially.

Detailed design will consider basin configurations to manage potential sediment loads and ongoing maintenance, aesthetics and safety.

Gross Pollutant Trap

A singe gross pollutant trap is proposed to capture runoff from the extensively hardstand areas at the southern end of the BBB where it reconnects to Jonson Street and Browning Street. Access in this location should be possible for suction / eductor style vehicles. The final selection of a GPT system is yet to occur however, for the purposes of MUSIC modelling the following assumptions are made.

Potential treatment systems are produced by a variety of proprietary manufacturers such as Rocla, Holcim, Ecosol and Ocean Protect. These manufacturers produce a variety of treatment system



which offer primary, secondary and tertiary treatment of stormwater. Hydraulic loading rates and treatment efficiency vary for each system with primary treatments capable of higher hydraulic loading than other systems. Conversely, these systems offer the lowest level of treatment for sediments and nutrients. The capital and operating costs of these systems all vary considerably, and costs are a necessary consideration for device selection.

For the purposes of modelling the following pollutant reductions (transfer functions) were adopted. The reductions don’t relate to any specific gross pollutant removal system, rather a combination of performances from differing systems:

• TSS – 70%

• TN – 0%

• TP – 30%

• Gross Pollutants – 100%

Catchment flows above the modelled 3 month flow were diverted around the device.

Modelled Results

The modelled results of the developed mitigated site are included in Table 4-4.

Table 4-6 MUSIC Modelling Results – Load Targets

Parameter Load Reduction

Target

Required Annual

Load Developed

Mitigated Site

Achieved Annual

Load Developed

Mitigated Site

Total Suspended Solids (kg/yr) 80% 16 300 5 170

Total Phosphorus (kg/yr) 45% 33.52 18.4

Total Nitrogen (kg/yr) 45% 173.6 163

Gross Pollutants (kg/yr) 70% 931 66.5

The outcomes of the MUSIC modelling indicate that the stormwater objectives will be achieved for the BBB. Design of the stormwater measures will ensure integration for the selected stormwater management measures with the civil design.

4.4 Stormwater Quantity Modelling

4.4.1 Calibration The hydrologic model was calibrated to the Rational Method for two catchments. These two catchments are illustrated in Figure 2-1 and details are as follows:

• OutSouth – this includes the upstream sub-catchments to north of BBB’s southern cross-drainage structures. This catchment includes parts of the Mobile Park Home.

• OutMid – this encompasses the sub-catchments to the west of Butler Street and includes Burns Street.

Byron Bay Bypass Stormwater Management Plan 32Stormwater Assessments


The rational method for each catchment was undertaken using the following:

The 10-year coefficient of discharge (C10) is based on land use stated in Section 2.4.3 for the base case and frequency factor (Fy) is based on BSC guidelines;

The time of concentration was based on the stream velocity method for channel flow and Friend’s equation for overland flow; and

The IFD curves are the same as those stated in Section 2.4.5.

Table 4-7 Rational Method Parameters

Component Parameter OutSouth OutMid

Area (ha) 5.5 1.8

Coefficient of Discharge

10 year (C10) 0.73 0.78

Time of Concentration

Channel Length (m) 500 359

Channel Velocity (m/s) 2.0 2.0

Overland Flow Length (m)

11 27

Overland Slope (%) 3.3 2.5

Horton’s Surface Roughness Factor (n)

0.060 0.015

The calibrations for two catchments are tabulated in Table 4-8, and Table 4-9. The tables illustrate that the hydrologic model compares well with the rational method and are generally within 10%. Note, that the WBNM is in general higher than the rational method. Given the good match across the two catchments, the model is fit for purpose of this assessment.

Table 4-8 OutSouth Hydrologic Discharge Comparison

AEP

(1 in X)

Rational Method

(m3/s)

WBNM

(m3/s)

Difference (%)

2 1.01 1.00 -1.0

5 1.37 1.48 8.0

10 1.61 1.73 7.5

20 1.86 2.08 11.8

50 2.32 2.41 3.9

100 2.69 2.75 2.2

Byron Bay Bypass Stormwater Management Plan 33Stormwater Assessments


Table 4-9 OutMid Hydrologic Discharge Comparison

AEP

(1 in X)

Rational Method

(m3/s)

WBNM

(m3/s)

Difference (%)

2 0.48 0.50 4.2

5 0.65 0.69 6.2

10 0.76 0.84 10.5

20 0.88 1.00 13.6

50 1.09 1.11 1.8

100 1.27 1.22 -3.9

4.4.2 Unmitigated Discharge Analysis A comparison of the peak discharge was undertaken for the base case to the developed case and mitigated case. The comparison was undertaken at three reporting locations which correspond to where discharge occurs off-site (at existing legal points of discharge). These reporting locations can be found in Figure 2-1 and are described as:

OutNorth – this encompassed all northern sub-catchments that drainage towards the northern cross-drainage structures of the BBB.

OutMid – this encompasses the sub-catchments to the west of Butler Street and includes Burns Street and is the same area as the Rational Method catchment.

OutWetland – This encompasses all sub-catchments that discharge into the wetland around the southern section of the BBB. This reporting location includes the Rational Method catchment OutSouth.

The change in peak discharge for the 1 in 20 AEP and 1 in 100 AEP are tabulated in Table 4-10 and full results are tabulated in Appendix A. The results show that at all three reporting locations increase discharge due to the increase in paved surfaces. Of note, the OutNorth location has a much higher increase in peak discharge than the other reporting locations, this is due to a larger portion of the upstream catchment changing land use. From Table 4-10, all report locations therefore required mitigation measure which are described in Section 4.4.3.

Table 4-10 Base Case and Developed Case Discharge Comparison

Location 1 in 20 AEP 1 in 100 AEP

Base Case (m3/s)

Developed Case (m3/s)

Difference (%)

Base Case (m3/s)

Developed Case (m3/s)

Difference (%)

OutNorth 0.95 1.10 15.8 1.30 1.47 13.1

OutMid 1.00 1.03 2.0 1.22 1.24 1.6

OutWetland 2.61 2.65 1.5 3.43 3.46 0.9



4.4.3 Mitigation Measures and Discharge Analysis The details of the mitigation measures are tabulated in Table 4-11 and illustrated in Figure 4-3. The mitigation measures are described as follows:

OutNorth – The swale along the western side of the BBB is converted to provide detention capacity. The constraints and assumptions in regard to the design are:

○ Slotted kerb and channel, possibly including protected drop downs;

○ The depth of the 1 in 100 AEP is limited to 0.5m. This limitation is to ensure that flooding does not impede onto the adjacent property to the east;

○ The invert of the detention basin is 1.4m AHD;

○ The detention basin is a rectangular-basin prism with 1 in 6 batters; and

○ Any discharge in the downstream drainage path (at the north cross-drainage structures) does not impede the detention basin outlet arrangement.

OutWetland – part of the discharge is diverted from the eastern stormwater drainage proposed by GHD (along Butler Street) to the OutMid catchment. Note that the pipe size proposed by GHD has been reduced where the diversion occurs. The assumptions of this measure are:

○ The stormwater pipes are the controlling constraint to the stormwater system (ie stormwater pits on the eastern side of Butler Street are oversized) and are inlet controlled. A 0.5m cover (depth) above the obvert of the stormwater pipes is assumed;

○ There is a junction chamber with zero storage capacity to discharge stormwater to the west via the diversion stormwater pipe (towards OutMid) and to the south (towards OutWetland) via GHD’s proposed stormwater pipe. No blockage on either pipe is also assumed; and

○ The inverts to the diversion stormwater pipe do create an adverse slope (i.e. the inverts of the stormwater pipes on western side of Butler Street are at the same level or lower that than the stormwater pipes on eastern side of Butler Street).

OutMid – a detention basin to the north of Mobile Home Park which detains the OutMid catchment and the diverted flow from OutWetlands (outlined in the dot point above). The constraints and assumptions in regard to the design are:

○ The depth of the 1 in 100 AEP is limited to 0.55m. This limitation is to ensure freeboard of 0.3m is not compromised on the adjacent properties to the north and south;

○ The invert to the detention basin is 1.3m AHD;

○ The detention basin is a rectangular-basin prism with 1 in 6 batters; and

○ The detention basin only captures the discharge from the OutMid catchment and diverted discharge from OutWetland. The discharge that flows north into the same easement from the Mobile Park Home and the discharge that flow south from the properties on Little Burns Street is diverted around the detention basin.



Table 4-11 Mitigation Measure Details

Parameter OutNorth OutMid OutWetland

Type Detention Swale Detention Basin Flow Diversion

Area Required (m2) 865 890 -

Volume Required (m3) 363 403 0

Culvert Arrangement (no. RCP/RCP size)1

6/0.3m 3/0.3m Diversion GHD2

1/0.225m 1/0.3m

Weir Arrangement (width @ invert)

10m @ 0.45m 5m @ 0.4m Road Alignment

Assumed Invert Level (m AHD)

1.4 1.3 Road Level minus 1.025m

Max Water Depth (m) 0.5 0.55 -

Assumed batter 1 in 6 1 A minimum cover of 0.1m was assumed between the obvert of the culverts and the invert of the weir 2 GHD proposed 1/0.525m pipe that outlets on the eastern side of the BBB. This size has been reduced to meet the mitigation required at OutWetland.

The change in peak discharge for the 1 in 20 AEP and 1 in 100 AEP are tabulated in Table 4-12 and full results are shown in Appendix A. The results illustrate that at all three reporting locations meet the requirements to reduce the peak discharge back to the base case. Both detention basins for OutNorth and OutMid are free draining with peak depths well below the safety requirements (1.2m in the 1 in 100 AEP).

Table 4-12 Base Case and Mitigated Case Discharge Comparison

Location 1 in 20 AEP 1 in 100 AEP

Base Case (m3/s)

Mitigated Case (m3/s)

Difference (%)

Base Case (m3/s)

Mitigated Case (m3/s)

Difference (%)

OutNorth 1.00 0.82 -13.7 1.22 1.13 -13.1

OutMid 0.95 0.82 -18.0 1.30 1.17 -4.1

OutWetland 2.61 2.55 -2.7 3.43 3.39 -2.0

4.4.4 Stormwater Quantity Mitigation Limitations Minimal practicality assessments have been undertaken and this is required to confirm conceptual design aspects. Additionally, further ground surface survey may be required.

All mitigation measures have been undertaken in a hydrologic model. A hydraulic model should be developed for the detailed design to assess the following:

The diversion flow split with the GHD pipe and pit arrangements.

The detailed design of the detention basin volumes with the proposed outlet (culvert and weir) arrangements.

Byron Bay Bypass Stormwater Management Plan 36 Findings and Conclusions


5 Findings and Conclusions

This report has developed and assessed a conceptual stormwater quality and quantity system for the Byron Bay Bypass. The system has aimed to integrate as extensively as possible with the current detailed design arrangement for the road and easement areas.

Stormwater objectives are relevant for both water quality and quantity, where water quality objectives include a set of pollutant load reduction targets, and quantity objectives require mitigation of peak post-development flows to existing case conditions. As the proposed development essentially includes expansion of road areas, objectives apply only to the newly developed catchment areas, not existing catchment areas (i.e. the additional road and/or verge areas).

In terms of water quality, the assessment approach has been to utilise MUSIC to test the performance of the conceptual stormwater management systems and demonstrate achievement of Council’s water quality objectives. However, it is not possible (or practical) in a road design sense to separate runoff from new and existing catchment areas and effect treatment in isolation. The water quality and quantity measures proposed capture (and or treat) runoff from both the existing and new pavement areas, and as such most of the existing and new road catchments receive some level of stormwater treatment.

MUSIC models were established to represent sub catchments in which the Bypass resides, based on topography and drainage to which rainfall, evaporation and pollutant export parameters were applied. Site inspections completed during the study furthered site understanding of opportunities, constraints, and existing site features and assisted in the identification of potential stormwater management options.

The outcomes of the MUSIC modelling indicate that the stormwater objectives will be achieved for the BBB using a variety of standard water sensitive urban design approaches and proprietary systems. Design of the stormwater measures will ensure integration for the selected stormwater management measures with the civil design..

Water quantity modelling was completed using the WBNM hydrology software package. The approach was to model individual sub-catchments in their current state (i.e. base case) and in their proposed state (i.e. a developed case), where the developed case represents the proposed development with increased pavement extent. The increase in imperviousness (resulting from increased pavement) resulted in a change in the runoff hydrograph with higher peak discharge in the developed case. The difference in the base case and developed case runoff hydrographs was then required to be mitigated back to the base case levels and meet discharge requirements. The mitigation measures to meet these objectives are:

A flow diversion to move discharge on the eastern side of Butler Street to the road reserve north of the Mobile Home Park; and

Two detention devices – one at the northern cross-drainage structures (drainage swale) and one in the easement north of the Mobile Home Park (drainage basin).

Overall, this stormwater management plan, including the design and assessment of mitigation measures identifies that stormwater quality and quantity measures can be accommodated in the

Byron Bay Bypass Stormwater Management Plan 37 Findings and Conclusions


Byron Bay Bypass project, allowing Council to achieve its stormwater management targets for water quality and quantity, subject integration of the these components into the overall civil design.

Byron Bay Bypass Stormwater Management Plan 38 References


6 References

AUS-SPEC Joint Venture (2013). Handbook of Stormwater Drainage Management – Version 3. Report prepared for the Northern Rivers Local Governments.

Boyd, M., Rigby T., and VanDie (2012), Details of the Theory used in WBNM. Report published by University of Wollongong.

BMT (2019), Byron Bay Bypass Detailed Design Flood Impact Assessment. Report published by BMT.

Byron Shire Council (2014), Comprehensive Guidelines for Stormwater Management. Report published by Byron Shire Council.

Byron Shire Council (2014a), Development Control Plan 2014, Chapter B3 Services. Report published by Byron Shire Council.

Chiew, F.H.S. & McMahon, T. A. (1997). Modelling Daily Runoff and Pollutant Load from Urban Catchments. Water (AWWA Journal) 24:16-17

GHD (2016), Byron Bay Bypass. Environmental Impact Statement. Report prepared by Byron Shire Council.

Healthy Land and Water (2018) MUSIC Modelling Guidelines, Consultation Draft. Healthy Land and Water, Brisbane.

QUDM (2017), Queensland Urban Drainage Manual 2017. Institute of Public Works Engineering Australasia, Queensland.

Water by Design (2014), Bioretention Technical Design Guidelines (Version 1.1). Healthy Waterways Ltd, Brisbane.

Byron Bay Bypass Stormwater Management Plan A-1 Stormwater Quantity Full Results


Appendix A Stormwater Quantity Full Results

Table A-1 OutNorth Peak Discharge Results

AEP ( 1 in X) Base Case (m3/s) Developed Case (m3/s) Mitigated Case (m3/s)

1 0.31 0.36 0.26

2 0.49 0.57 0.42

5 0.69 0.77 0.57

10 0.80 0.91 0.64

20 0.95 1.10 0.82

50 1.14 1.33 0.93

100 1.30 1.47 1.13

Table A-2 OutMid Peak Discharge Results


1 0.37 0.39 0.32

2 0.50 0.53 0.45

5 0.69 0.72 0.56

10 0.84 0.86 0.66

20 1.00 1.03 0.82

50 1.11 1.13 1.02

100 1.22 1.24 1.17

Table A-3 OutWetland Peak Discharge Results


1 0.72 0.75 0.71

2 1.28 1.32 1.22

5 1.87 1.92 1.81

10 2.19 2.24 2.13

20 2.62 2.65 2.55

50 3.04 3.07 2.95

100 3.46 3.50 3.39

Byron Bay Bypass Stormwater Management Plan B-1 Detailed Design Drawings, General Arrangements and Cross Sections (GHD, 2016)


Appendix B Detailed Design Drawings, General Arrangements and Cross Sections (GHD, 2016)

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