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February 2016

Geelong Central Drainage/Flood Study Report

City of Greater GeelongCentral Geelong Drainage Flood Study

DOCUMENT STATUSVersion Doc type Reviewed by Approved by Distributed to Date issued

V01 Draft Report LJC LJC Jarrod Malone 04/01/2016

V02 Final Report LJC LJC Jarrod Malone 02/02/2016

PROJECT DETAILSProject Name Central Geelong Drainage Flood Study

Client City of Greater Geelong

Client Project Manager Jarrod Malone

Water Technology Project Manager Johanna Theilemann

Report Authors Johanna Theilemann, James Newton

Job Number 3922-01

Report Number R01

Document Name document.doc

Copyright

Water Technology Pty Ltd has produced this document in accordance with instructions from City of Greater Geelong for their use only. The concepts and information contained in this document are the copyright of Water Technology Pty Ltd. Use or copying of this document in whole or in part without written permission of Water Technology Pty Ltd constitutes an infringement of copyright.

Water Technology Pty Ltd does not warrant this document is definitive nor free from error and does not accept liability for any loss caused, or arising from, reliance upon the information provided herein.

15 Business Park DriveNotting Hill VIC 3168

Telephone (03) 8526 0800Fax (03) 9558 9365

ACN No. 093 377 283ABN No. 60 093 377 283

Cover Image: Aerial View of Geelong, Geelong Central 1947, Museum of Victoria

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EXECUTIVE SUMMARY

This report details the input data, approach and outcomes for the Central Geelong Drainage/Flood Study. The study has been initiated by the City of Greater Geelong (COGG), in order to define the extent and characteristics of flooding in the Central Geelong Area so that future planning decisions may be soundly based and measures may be put in place to minimise existing and future flood risks to the community.

The study provides information on flood levels and flood risk within the Central Geelong area in order to produce accurate flood mapping for the Geelong CBD for a range of scenarios and storm events.

Detailed existing condition flood modelling for a range of flood magnitudes up to and including the 1% Average Exceedance Probability1 (AEP) was undertaken (Figure ES 0-1). Results for the 1% AEP flood event within the study area indicated a number of locations within the CBD where flood depths and extents posed a significant risk to pedestrians, vehicles and commercial and residential building assets. The risks associated with flash flooding during 1% AEP flood events indicated that at least 124 buildings are likely to experience above floor flooding with total damages estimated to reach $25,560,273, which does not include indirect cost associated with clean up, relocation and emergency response. The Average Annual Damages (AAD)2 within the study area was calculated to be $818,577.

1 The terminology of “Average Exceedance Probability” has recently been recommended in ARR, 2015 to replace the use of “Average Recurrence Interval”. What would have previously been referred to as the ‘1 in 100 year ARI storm event’ is now the ‘1% AEP storm event’. Conversion of ARI to AEP can be taken as AEP (%) = (1/ARI) x 1002 The term “Average Annual Damage” or “AAD” refers to the average annual cost of floods of a specific magnitude. It is the yearly cost of flooding likely to result from damage to the specified study area. An assessment of the AAD is used to compare the severity of flooding in terms of potential damages and the effectiveness of different management options.

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Figure ES 0-1 Existing Conditions 1% AEP Flood Depth

A community consultation session was held part-way through the study to present the preliminary flood model results to those who live, work or have business in the study area. The session was well attended and was very beneficial to the study to help verify the accuracy of the model and to hear the community’s accounts of flooding and view historical photos that were provided by some.

Following the completion of the flood modelling scenarios, a full range of structural and non-structural mitigation options were considered in order to best manage the existing flood risk within the CBD. Detailed modelling of 4 major structural mitigation options and 8 minor options was undertaken. Key to this assessment was modelling of the 4 major options which included:

Increase in storage capacity with Johnstone Park; Additional main drain from Johnstone Park to the Bay; Additional main drain from junction pit within Gheringhap Street to the Bay; and, Construction of Floodway and alteration to existing drainage along Eastern Beach Road.

All of the proposed mitigation options provided a ranging degree of decreased flood depth and extent results. Most effective in reducing the extent and depth of flooding within the study area is the inclusion of the additional main drain pipe both from Johnstone Park and/or from the junction pit within Gheringhap Street.

The proposed mitigation options were costed and an assessment of the benefit cost ratios undertaken. The benefit cost ratio assessment of the 4 major structural mitigation options indicated a very low result given the high capital investment and modest reduction in the number of expected properties to experience above floor flooding. The nature of the flood risk within the CBD is such that the expected damages resulting from flooding are uniformly spread across the study area. To this end, no single stand alone mitigation option is able to meaningfully reduce flooding within the CBD. Further to this the relatively low floor levels of the commercial premises equate to relatively low thresholds once floodwaters exceed the capacity of the urban drainage network including flow over the kerb and channel.

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Other broader benefits associated with the works to achieve drainage mitigation outcomes need to be considered. These benefits may relate to other Central Geelong initiatives and achieve a multitude of outcomes that are broader than the scope of this study.

It is important to consider however that the cost benefit assessment does not consider the non-monetary effects to the community that stormwater flooding can cause. Not only will the flooding affect building and infrastructure assets, but it can have serious implications to business function within the CBD and the mental health of those effected.

In considering future works to address the existing flood risk within the Central Geelong area, complimentary works including whole of water cycle management within the CBD and greater catchment area would be able to reduce the volume of runoff generated as well as improve the expected quality of this runoff.

Further investigation of the functional design of the proposed mitigation options is recommended. Recognising that future intensification of development within both the catchment and study area will potentially increase runoff and exacerbate existing stormwater problems, robust and thorough assessment of future development is critically important. Added to this, the risks of Climate Change of increased rainfall intensities and increased tailwater levels in the Bay will also impact the effectiveness of the drainage network. The principles of best practice environmental management for stormwater and floodplain management must be adhered to in order to ensure that the cumulative impacts of any proposed development do not unknowingly increase the flood risk to neighbouring and/or downstream locations.

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GLOSSARY OF TERMSAnnual Exceedance

Probability (AEP)

Refers to the probability or risk of a rainfall event of a given magnitude (intensity and duration) occurring or being exceeded in any given year. A 90% AEP event has a high probability of occurring or being exceeded; it would occur quite often and would be a relatively minor rainfall event. A 1% AEP event has a low probability of occurrence or being exceeded; it would be rare but it would be likely to cause extensive damage.

Australian Height Datum

(AHD)

A common national surface level datum approximately corresponding to mean sea level. Introduced in 1971 to eventually supersede all earlier datum’s.

Average Recurrence Interval

(ARI)

Refers to the average time interval between a given flood magnitude occurring or being exceeded. A 10 year ARI flood is expected to be exceeded on average once every 10 years. A 100 year ARI flood is expected to be exceeded on average once every 100 years. The AEP is the ARI expressed as a percentage.

Cadastre, cadastral base Information in map or digital form showing the extent and usage of land, including streets, lot boundaries, water courses etc.

Catchment The area draining to a site. Generally relates to a particular location and may include the catchments of tributary streams as well as the main stream.

Design flood A significant event to be considered in the design process; various works within the floodplain may have different design standards. A design flood will generally have a nominated AEP or ARI (see above).

Discharge The rate of flow of water measured in terms of volume over time. It is to be distinguished from the speed or velocity of flow, which is a measure of how fast the water is moving rather than how much is moving.

Flood Relatively high stream flow which overtops the natural or artificial banks in any part of a stream, river, estuary, lake or dam, and/or overland runoff before entering a watercourse and/or coastal inundation resulting from elevated sea levels and/or waves overtopping coastline defences.

Flood damage The tangible and intangible costs of flooding.

Flood hazard Potential risk to life and limb caused by flooding. Flood hazard combines the flood depth and velocity.

Flood mitigation A series of works to prevent or reduce the impact of flooding. This includes structural options such as levees and non-structural options such as planning schemes and flood warning systems.

Floodplain Area of land which is subject to inundation by floods up to the probable maximum flood event, i.e. flood prone land.

Flood storages Those parts of the floodplain that are important for the temporary storage, of floodwaters during the passage of a flood.

Freeboard A factor of safety above design flood levels typically used in relation to the setting of floor levels or crest heights of flood levees. It is usually expressed as a height above the level of the design flood event.

Geographical information A system of software and procedures designed to support the management, manipulation, analysis and display of spatially referenced

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systems (GIS) data.

Hydraulics The term given to the study of water flow in a river, channel or pipe, in particular, the evaluation of flow parameters such as stage and velocity.

Hydrograph A graph that shows how the discharge changes with time at any particular location.

Hydrology The term given to the study of the rainfall and runoff process as it relates to the derivation of hydrographs for given floods.

Intensity frequency duration (IFD) analysis

Statistical analysis of rainfall, describing the rainfall intensity (mm/hr), frequency (probability measured by the AEP), duration (hrs). This analysis is used to generate design rainfall estimates.

TUFLOW A hydraulic modelling tool used in this study to simulate the flow of flood water through the floodplain. The model uses numerical equations to describe the water movement.

Ortho-photography Aerial photography which has been adjusted to account for topography. Distance measures on the ortho-photography are true distances on the ground.

Peak flow The maximum discharge occurring during a flood event.

Probability A statistical measure of the expected frequency or occurrence of flooding. For a fuller explanation see Average Recurrence Interval.

Risk Chance of something happening that will have an impact. It is measured in terms of consequence and likelihood. For this study, it is the likelihood of consequences arising from the interaction of floods, communities and the environment.

RORB A hydrological modelling tool used in this study to calculate the runoff generated for design rainfall events.

Runoff The amount of rainfall that actually ends up as stream or pipe flow, also known as rainfall excess.

Stage Equivalent to 'water level'. Both are measured with reference to a specified datum.

Stage hydrograph A graph that shows how the water level changes with time. It must be referenced to a particular location and datum.

SWMP Stormwater management plan

Topography A surface which defines the ground level of a chosen area.

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TABLE OF CONTENTSExecutive Summary 2

GLOSSARY OF TERMS 4

1. Introduction 12

1.1 Objectives and Scope of Investigation 131.2 Site 132. Available Information Review and Site Visit 16

2.1 Site Visit 162.2 Available data review 162.3 Zone and Overlays 172.4 Topography 182.5 DEM Verification 192.6 Catchment Drainage 212.6.1 Pits and Pipes 212.6.2 Previous Flood Related Studies 212.6.3 Storages - Johnstone Park 223. Hydrological Modelling 23

3.1 Overview 233.2 RORB Model 233.3 External Catchment Flows 264. Hydraulic Modelling 27

4.1 Overview of Rain on Grid Modelling274.1.1 Hydraulic Model Datasets 284.2 Direct Rainfall Model Inputs 294.2.1 Rainfall 294.2.2 Fraction Impervious 304.2.3 IFD Parameters 304.2.4 Comparison of ARR – New IFD 314.2.5 Losses and Runoff Coefficient 324.2.6 Pre-wet334.3 Overview of TUFLOW Hydraulic Model 334.4 Hydraulic model construction and parameters 334.4.1 Model Version 334.4.2 2D Grid Size and Topography 334.4.3 1D Network 344.4.4 Plot Output Lines 354.4.5 Roughness 384.4.6 Pit Configuration 404.4.7 Boundary Conditions 404.5 TUFLOW Model Reconciliation 414.5.1 GIS Processing 41

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4.5.2 TUFLOW Data Processing 414.5.3 Results Processing 414.5.4 Hydraulic Model Application 414.5.5 TUFLOW model checks 435. GIS Processing 44

5.1 TUFLOW model outputs 445.2 TUFLOW Data Processing 445.3 Results Processing 445.4 Data Integrity Checks 445.5 Filtering of Results 445.6 Key Assumptions 456. Hydraulic Modelling Results 46

6.1 Existing Conditions Results 466.2 Discussion and Observations 466.3 Validation Data 476.4 Climate Change 486.4.1 Climate Change Methodology 486.4.2 Climate Change Results 497. Sensitivity Testing 52

7.1 Overview 527.2 Sensitivity Testing Results 528. Flood Hazard Assessment 53

8.1 Hazard Overview 538.2 Melbourne Water Hazard Criteria 538.3 Australian Rainfall & Runoff - Vehicle Hazard 538.4 Existing Conditions Flood Damages 568.4.1 Overview 568.4.2 Flood Damages 568.4.3 Non-Economic Flood Damages 588.5 High Risk Areas 589. Mitigation Options 59

9.1 Mitigation Overview 599.2 Structural Mitigation Options 599.2.1 Option 1 – Johnstone Park, Park Extension 619.2.2 Option 2 – Johnstone Park, Additional Pipe from Park 649.2.3 Option 3 – Johnstone Park, Additional Pipe from drainage junction 679.2.4 Option 4 – Eastern Beach Road 719.2.5 Option 5 – Brougham Street 769.2.6 Option 6 – Burrows Lane & Corner of Malop/Yarra Street 809.2.7 Option 7 – Hays Place 849.2.8 Option 8 – Kirk Place 889.2.9 Option 9 – Corner of Latrobe and Little Malop 929.2.10 Option 10 – Bayside Church/Childcare Centre 969.2.11 Option 11 – Corner Smythes/Cavendish Street 99

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9.2.12 Option 12 – Mercer Street/ Corio Street Carpark 1039.2.13 Additional Non-Standard Options 1069.3 Non Structural Mitigation Measures 1079.4 Mitigation Flood Damages 1089.5 Cost Benefit 1099.6 Flood Mitigation Process 1099.7 Discussion 11010.2.1 Overview 11210.2.2 Flood Elevation Contours 11210.2.3 Hazard Mapping 11210.3 Flood Mapping for Land Use Planning 11310.3.1 Overview 11310.3.2 Victoria Planning Provisions 11310.3.3 Flood Related Planning Overlays11310.3.4 Flood Related Planning Zone and Overlay Delineation 11411. Study Deliverables 115

11.1 Overview 11511.2 Mapping Outputs 11511.2.1 Datasets 11511.2.2 Maps 11511.3 Flood Extent Mapping (VFD Compliant) 11611.4 Land Use Planning Maps 11612. Conclusion 117

12.1 Overview 11712.2 Key Findings and Outcomes11713. References118

Appendix A Site Photographs 119

Appendix B Existing Conditions Results 125

Appendix C Critical Duration 130

Appendix D Detailed Mitigation Costing 133

Appendix E Final Deliverable Checklist 135

LIST OF FIGURESFigure ES 0-1 Existing Conditions 1% AEP Flood Depth 2Figure 1-1 Study Area (red outline) 14Figure 1-2 CoGG Drainage Catchments 15Figure 2-1 City of Greater Geelong Planning Scheme – Zone (Source: Planning Maps Online) 17Figure 2-2 City of Greater Geelong Planning Scheme - Special Building Overlay (Source: Planning

Maps Online) 18Figure 2-3 Extents of available Photogrammetry and LiDAR data 19Figure 2-4 DEM comparison – Photogrammetry minus LiDAR 20Figure 2-5 Adopted Study model DEM 20

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Figure 2-6 Historic Survey Map of Geelong (showing Western Gully) - Engineering Heritage Victoria (1950s) 22

Figure 2-7 Johnstone Park, 19 February 2014, Photograph taken by Geelong SES 22Figure 3-1 Undiverted RORB Model 24Figure 3-2 Model inflow locations 26Figure 4-1 Rain on Grid (Direct Rainfall) Methodology 28Figure 4-2 Fraction Impervious Plan 30Figure 4-3 Catchment IFD Chart 31Figure 4-4 City of Greater Geelong – Pits and pipes 36Figure 4-5 PO Line Locations 37Figure 4-6 Manning's Roughness Polygons 39Figure 4-7 Tuflow model boundaries 40Figure 6-1 Existing Conditions 1% AEP Flood Depth (meters) 46Figure 6-2 Comparison of 1% AEP Flood Extent and photographs of the February 2014 flood

event 47Figure 6-3 1% AEP Climate Change Flood Depth - 32% Increase in Rainfall Intensity 50Figure 6-4 Comparison of Climate Change (increase rainfall intensity) and Existing Condition -

1% AEP Water Surface Elevation 51Figure 8-1 Existing Conditions 1% AEP Melbourne Water Flood Hazard Category 54Figure 8-2 Existing Conditions 20% AEP Melbourne Water Flood Hazard Category 54Figure 8-3 Existing Conditions 1% AEP Pedestrian Hazard (ARR) 55Figure 8-4 Existing Conditions 1% AEP Vehicle Hazard (ARR)55Figure 8-5 Existing conditions building with above floor flooding (orange shading) 57Figure 8-6 Flooding problem areas 58Figure 9-1 Existing Conditions 1% AEP Flood Depth - Johnstone Park 61Figure 9-2 Proposed Johnstone Park Extension (aerial image, edited terrain, extent at full) 62Figure 9-3 Comparison of Existing and Mitigated 1% AEP Flood Depth 62Figure 9-4 Difference in water surface elevation - Existing Conditions and Mitigation Option 1

(1% AEP) 63Figure 9-5 Comparison of Existing and Mitigated 20% AEP Flood Depth 63Figure 9-6 Proposed additional pipe from Johnstone Park to the Bay (shown in pink) 65Figure 9-7 Comparison of Existing and Mitigated 1% AEP Flood Depth 65Figure 9-8 Difference in Water Surface Elevation - Existing Conditions and Mitigation Option 2

(1% AEP) 66Figure 9-9 Comparison of Existing and Mitigated 20% AEP Flood Depth 66Figure 9-10 Proposed additional pipe from Gheringhap Street to Corio Bay 68Figure 9-11 Comparison of Existing and Mitigated 1% AEP Flood Depth 68Figure 9-12 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 69Figure 9-13 Comparison of Existing and mitigated 20% AEP Flood Depth 69Figure 9-14 Existing Conditions 1% AEP Flood Depth - Eastern Beach Road 71Figure 9-15 Proposed changes to drainage network 72Figure 9-16 Eastern Beach Road Topography73Figure 9-17 Comparison of Existing and Mitigated 1% AEP Flood Depth 74Figure 9-18 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 74Figure 9-19 Comparison of Existing and Mitigated 20% AEP Flood Depth 75Figure 9-20 Existing Conditions 1% AEP Flood - Brougham Road 76Figure 9-21 Brougham Street Topography 77Figure 9-22 Comparison of Existing and Mitigated 1% AEP Flood Depth 78Figure 9-23 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 78Figure 9-24 Comparison of Existing and Mitigated 20% AEP Flood Depth 79Figure 9-25 Existing Conditions 1% AEP Flood Depth - Burrows Lane/Malop Street 80

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Figure 9-26 Proposed changes to drainage network 81Figure 9-27 Comparison of Existing and Mitigated 1% AEP Flood Depth 82Figure 9-28 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 82Figure 9-29 Comparison of Existing and Mitigated 20% AEP Flood Depth 83Figure 9-30 Existing Conditions 1% AEP Flood Depth - Hays Place 84Figure 9-31 Proposed changes to drainage network 85Figure 9-32 Comparison of Existing and Mitigated 1% AEP Flood Depth 86Figure 9-33 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 86Figure 9-34 Comparison of Existing and Mitigated 20% AEP Flood Depth 87Figure 9-35 Photograph Kirk Place (Google Maps) 88Figure 9-36 Existing Conditions 1% AEP Flood Depth - Kirk Place 88Figure 9-37 Proposed changes to drainage network 89Figure 9-38 Comparison of Existing and Mitigated 1% AEP Flood Depth 90Figure 9-39 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 90Figure 9-40 Comparison of Existing and Mitigated 20% AEP Flood Depth 91Figure 9-41 Existing Conditions 1% AEP Flood Depth - Latrobe Tce/Little Malop 92Figure 9-42 Proposed changes to drainage network 93Figure 9-43 Comparison of Existing and Mitigated 1% AEP Flood Depth 94Figure 9-44 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 94Figure 9-45 Comparison of Existing and Mitigated 20% AEP Flood Depth 95Figure 9-46 Existing Conditions 1% AEP Flood Depth – Bayside Church 96Figure 9-47 Comparison of Existing and Mitigated 1% AEP Flood Depth 97Figure 9-48 Difference in 1% AEP Depth - Existing and Mitigated 97Figure 9-49 Comparison of Existing and Mitigated 20% AEP Flood Depth 98Figure 9-50 Existing Conditions 1% AEP Flood Depth - Corner Smythes/Cavendish Street 99Figure 9-51 Proposed chnages to drainage network 100Figure 9-52 Comparison of Existing and Mitigated 1% AEP Flood Depth 101Figure 9-53 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 101Figure 9-54 Comparison of Existing and Mitigated 20% AEP Flood Depth 102Figure 9-55 Existing Conditions 1% AEP Flood Depth - Mercer Street/Corio Street Carpark 103Figure 9-56 Comparison of Existing and Mitigated 1% AEP Flood Depth 104Figure 9-57 Difference in 1% AEP Water Surface Elevation - Existing and Mitigated 104Figure 9-58 Comparison of Existing and Mitigated 20% AEP Flood Depth 105Figure 9-59 Sample flood signage 106Figure 9-60 Mitigation cost-benefit ratio comparison109

LIST OF TABLESTable 2-1 Data References 16Table 2-2 Key metadata for DEM datasets 18Table 3-1 RORB Parameters 23Table 3-2 Undiverted RORB Model Peak Flow Comparisons 24Table 3-3 Differences in Peak Flows Between the Undiverted and Diverted RORB Models 25Table 3-4 Peak Flows for the Design Inflow Hydrographs 26Table 4-1 IFD Parameters 30Table 4-2 Comparison of IFD depths between new and old IFD BOM tool (50%, 20% and 10%

AEP events) 32Table 4-3 Comparison of IFD depths between new and old IFD BOM tool (5%, 2% and 1% AEP

events) 32Table 4-4 Initial loss values 32

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Table 4-5 Assumed Depth of Cover 35Table 4-6 Pit Grate Dimensions (mm) 35Table 4-7 Manning's n Roughness Coefficients 38Table 4-8 Model Runs 42Table 8-1 Flood Hazard Summary Table 53Table 8-2 Existing Conditions Flood Damages Assessment 56Table 9-1 Suggested Mitigation options 60Table 9-2 Comparison of mitigation damages 108

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1. INTRODUCTION

Water Technology has been commissioned by the City of Greater Geelong (COGG) to prepare a detailed flood assessment for the Geelong Central Business District (CBD). Geelong sits alongside Corio Bay south of Melbourne and is one of Victoria’s largest municipalities. Settled in the early 1800s, the city of Geelong is central to the municipality and services the needs of the region with respect to education, health, retail and business.

Drainage within this area is serviced by conventional pit and pipe, kerb and channel, and retarding basin drainage infrastructure some of which dates back over 150 years. A significant proportion of the study area drains along the alignment of the historical Western Gully to a large outfall east of Cunningham Pier. Given the urban nature of the entire catchment, a high proportion is considered to be impervious and as a result, significant rain events inevitably cause flash flooding.

Overwhelmed subsurface drainage infrastructure and poorly defined overland flow paths have often resulted in flash flooding events within Central Geelong. Significant flooding events have been recorded in 1999, 2005 and late 2010.

With a population that is expected to reach 500,000 by 20503, the City of Greater Geelong is committed to attracting investment to Central Geelong. The Central Geelong Action Plan (2013) discusses achieving three principles within this area:

• More people living, working, learning and playing;

• Great infrastructure; and,

• Smart investment.

In achieving these principles a number of targets have been set by 2028, importantly:

• Increase the population living in Central Geelong to at least 10,000;

• Increase the number of people working in Central Geelong from 21,000 to 30,000; and,

• Increase student population from 4,500 to 10,000.

While these targets set about achieving substantial growth within Central Geelong, inevitable intensification of development and urban consolidation will place further strain on ageing infrastructure. Further to this, given the existing risks associated with flooding in Central Geelong, increasing the permanent population within the central business area will also increase the population at risk and the costs of possible damages following flooding events.

Given the location of the catchment within a busy town centre, traditional mitigation options of large flood storage basins or new conveyance infrastructure are often made increasingly difficult. This study will investigate and quantify the existing flood risk within Central Geelong and will also consider how this risk may be reduced with structural and non-structural flood mitigation.

3 Central Geelong Taskforce, Central Geelong Action Plan (2013)

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1.1 Objectives and Scope of InvestigationThis project builds on previous work undertaken in 2008 as part of the Western Gully Flood Study (WBM, 2008). The area of detailed flood mapping produced by the former study did not cover the entire Geelong CBD area. The 1% AEP flood extent produced by the study is represented in the current Special Building Overlay (SBO) which extends across the western portion of the CBD/study area.

Future redevelopment and revitalisation plans for the city are important drivers in undertaking this study. The Geelong CBD has experienced numerous flash flooding events in recent years which have resulted in property and infrastructure damage, hazardous vehicle egress along Eastern Beach Road especially, and constraints around development.

The aging drainage network and continuing increase in density of development in the catchment has also meant that the existing drainage network and identified overland flow paths are stretched to and beyond capacity.

As such, the key objective of the study is to produce detailed stormwater modelling of flooding within the CBD and to investigate means by which the risk associated with this flooding in and around Central Geelong may be reduced.

The project scope has been split into a number of stages/tasks which include:

1. Development of a digital terrain model;2. Building of “Base Case Existing Conditions” model;3. Revision of “Base Case Existing Conditions” model;4. Mitigation options modelling; and,5. Preferred options modelling and completion of reporting documentation.

Outputs from this project will be used to inform scoping of numerous other projects including capital works, redevelopment and revitalisation programs within Central Geelong (including the City in the Park Strategy (2015)) and innovative stormwater upgrades which will consider integrated water management.

1.2 SiteThe subject site is located within the Geelong Central Business District (CBD). The study area has a catchment of approximately 320 ha and drains an area defined by Latrobe Terrace to the west, Corio Bay to the north, Swanston Street to the east and McKillop Street to the south (Figure 1-2).

The study area covers the majority of the business district within Central Geelong. Whilst the City of Greater Geelong is continuing to invest in infrastructure replacement, repair and upgrades within the CBD, much of the existing drainage infrastructure is still undersized for current relatively frequent rainfall events.

Geelong has experienced a number of flash flooding events in recent years, a number of observed flooding locations in and around the Central Geelong include, Eastern Beach Road, the location of the Bayside Church, and the rail underpasses of Gordon Avenue and Brougham Street.

The study area is defined by a number of catchment areas, the majority of which forms part of the larger C150 (Western Gully Main Drain/ Cunningham Street Catchment). Smaller sections of the study area also include C176 (Yarra Street North Catchment), C177 (Bellerine Street North Catchment) and C215 (Ginn Street Catchment). Key drainage infrastructure within the study area includes Johnstone Park which is a depressed park area within the CBD of Geelong which acts as a retarding basin during periods of significant runoff. The main drainage outlet for the study area is located immediately east of Cunningham Pier. This rectangular outfall, measuring 2.4 meters wide , drains a majority of the 390 ha C150 catchment (Figure 1-3).

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Figure 1-2 Study Area (red outline)

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Figure 1-3 CoGG Drainage Catchments

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2. AVAILABLE INFORMATION REVIEW AND SITE VISIT

2.1 Site VisitA number of site visits were undertaken by Water Technology to review the existing conditions. The Water Technology project team were joined by City of Greater Geelong for a number of these visits.

Key hydraulic structures / crossings, rail underpasses, recently developed sites and areas of known flooding were visited.

This process provided invaluable input to the project. Gaining an understanding of the key areas of flooding early in the project was critical to determining the most appropriate methodology to move forward with the hydraulic modelling. Water Technology staff also gathered information on the terrain, vegetation and soil characteristics of the study area, focusing on critical inputs to the modelling stages such as Manning’s roughness coefficients, pipe and culvert locations and characteristics, as well as key topographical influencers to overland flow paths. Photos from the site visit can be found in Appendix A.

2.2 Available data review Key data used in this investigation (and its source) is shown in Table 2-1. The following Sections will cover each area of focus in the investigation and nominate key data collected and how it was used.

Table 2-1 Data References

Data Date Received Source

Drainage pits and pipes (GIS and PDF) 3/6/15 CoGG

Western Gully Flow Hydrographs 17/6/15 CoGG

Building Footprints (GIS) 24/6/15 CoGG

Land use/ land cover map/zoning 3/6/15 Vic Map

Existing Flood Extent Shape Files 10/6/15 CoGG

Aerometrex LiDAR 3/6/15 CoGG

Detailed flash flood incident list (Geelong) 6/6/15 SES

Western Gully Flood Study Report 5/6/15 CCMA

Calibration data – LiDAR & aerials 5/6/15 WT

Additional Pit Survey 20/7/15 St Quentin

Additional Pipe Survey 20/7/15 St Quentin

Finished Floor Level Survey and additional verification survey

20/7/15 St Quentin

Additional Floor Level Survey 20/12/15 St Quentin

Corio Bay Levels (tide levels) 10/6/15 BoM

Climate Change Report (Storm Tide and Storm Surge)

10/6/15 CSIRO

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2.3 Zone and OverlaysExisting land use and planning controls covering the study area include the following zones and overlays:

Zones

Activity Centre Zone (ACZ) Mixed Use Zone (MUZ) Public Use Zone (PUZ) General Residential Zone (GRZ) Public Parks & Recreation Zone

(PPRZ) Residential Growth Zone (RGZ) Road Zone (RZ)

Overlays

Design and Development Overlay Heritage Overlay Special Building Overlay

Environment Audit Overlay

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Figure 2-4 City of Greater Geelong Planning Scheme – Zone (Source: Planning Maps Online)

The Special Building Overlay which covers part of the study area represents the clipped 1% AEP flood extent as defined by the Western Gully Main Drain Flood Study completed by WBM in 2009.

The purpose of the Special Building Overlay is to ensure that development within urban areas liable to flooding is undertaken with due regard for the flood risk and to the manner in which it may affect the flood risk of others.

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Figure 2-5 City of Greater Geelong Planning Scheme - Special Building Overlay (Source: Planning Maps Online)

2.4 TopographyThe City of Greater Geelong supplied photogrammetry survey data flown by Aerometrex in December 2014 which covers the area to be flood mapped. The photogrammetry DTM (Digital Terrain Model) was supplied in the form of 3 m x 3 m XYZ points with 50 cm breakpoints located along breaklines. The XYZ points were built into a TIN (Triangular Irregular Network) and then converted to a 1 m resolution DEM by Water Technology which accurately depicts all breaklines in the Geelong CBD.

Aerial LiDAR (Light Detection and Ranging) survey is available for the Geelong area from the 2006-07 Port Phillip & Western Port LiDAR Project. The data is available as 2km x 2km tiles of 1 m-resolution DEMs (Digital Elevation Model), which have been mosaicked into a single 1 m DEM covering the Geelong CBD area.

Key metadata for the above digital elevation models (DEMs) is given in Table 2-2, and their extents are shown in Figure 2-6.

Table 2-2 Key metadata for DEM datasets

Dataset Source Date of Capture

Vertical accuracy (1 sigma)

Resolution

2014 Photogrammetry (Aerometrex)

Photogrammetry Dec 2014 0.10 m 1 m

2006-07 Port Phillip & Western Port LiDAR

LiDAR Apr-Jul 2007 0.10 m 1 m

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Figure 2-6 Extents of available Photogrammetry and LiDAR data

2.5 DEM VerificationComparison of photogrammetry DEM and LiDAR

A comparison of the 2014 photogrammetry DEM and 2007 PPB-WP LiDAR DEM where they overlap is shown in Figure 2-7. The plot shown represents the photogrammetry DEM minus the LiDAR DEM. A close inspection of the difference plot reveals large differences between the two datasets (> ± 0.3m) where there are buildings, however there is good general agreement (< ± 0.1m) in all other areas. The average difference of 0.021m shows that the two datasets agree very well, and the differences where there are buildings are attributed to different settings or techniques for masking out buildings during the processing of the photogrammetry and LiDAR by the vendors.

The building outline polygons have also been provided by Aerometrex and can be used to represent buildings during the model development. This enabled Water Technology to assign one roughness value to the building footprint and another to the remainder of the lot to represent the way in which water will runoff differently over various land uses.

Following completion of the data verification process and necessary filtering of anomalies within the dataset, a finalised Digital Elevation Model (DEM) was produced for the detailed modelling as shown in Figure 2-8.

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Figure 2-7 DEM comparison – Photogrammetry minus LiDAR

Figure 2-8 Adopted Study model DEM

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2.6 Catchment DrainageThe study area and greater catchment covers a diverse mix of land uses including high-density residential, commercial and industrial development. The majority of the catchment has a relatively steep slope grading towards Corio Bay. Given the relatively high percentage of impervious surfaces within the study area and adjoining catchment, an extensive drainage network has been constructed in order to service the stormwater runoff requirements within this catchment. Rainfall runoff generated within the catchment is drained to Corio Bay via the existing drainage network of pits and pipes and via overland flow predominantly directed along the city’s roadways. Significant drainage infrastructure within the study area includes the Western Gully main drain and outfall (Cunningham Pier) and Johnstone Park retarding basin.

Pits and Pipes As part of the data review process, the GIS datasets and PDF design plans provided by the City of Greater Geelong along with GIS pipe and pit datasets provided by WBM were compared. The review focused on the Council GIS dataset within the study area as shown in Figure 1-2. Despite the significant amount of data supplied by the City of Greater Geelong, in many cases data gaps existed within the GIS. In most cases this was in the form of missing inverts and or the absence of pits or pipes within the GIS dataset.

Of the total 2,144 pipes within the Council provided dataset, 1,657 pipes do not have any invert information. Pit depths were also only available for 337 out of a total 2,141 Council pits. Inaccuracies have also been observed with some of the invert levels in the council GIS dataset, i.e. where the pipe obvert is above the known ground surface.

Following the completion of this review, additional survey was undertaken to fill necessary data gaps. Survey was focused around data gaps along trunk drains and key flow points.

Previous Flood Related StudiesThe Western Gully Main Drain Drainage and Flood Study was completed by WBM in 2008. The Western Gully Main Drain Flood Study produced results which covered only part of the current study area.

This study produced detailed flood maps for existing conditions and three proposed structural mitigation schemes for various flood magnitudes up to and including the 1% AEP. The mapping produced by this study has been included into the City of Greater Geelong Planning Scheme and forms the Special Building Overlay which covers part of the current study area.

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Storages - Johnstone ParkJohnstone Park is located within the Central Geelong area and is an important feature both aesthetically and functionally within the study area. The current park occupies the original course of the Western Gully, a former watercourse which drained from Newtown to Corio Bay. Survey plans of the Geelong region dating back to the mid-1800s clearly identify the orientation of this waterway (Figure 2-9). In the mid-1800s, a dam was constructed at the site of the current park, within the gully. The dam was fenced and the area made into a city park in March of 1872 and named after former Geelong Major Robert De Bruce Johnstone. Originally the park extended from Gheringhap Street all the way to Latrobe Terrace, however throughout the years the size of the park has been incrementally reduced. The current park operates as an important retarding basin to stormwater flows from the Newtown area. The retarding basin is activated via a number of large surcharge pits within the main drain pipe whilst also receiving overland flow from local runoff. Surcharge flooding has been experienced within the park as recently at February 2014 (see Figure 2-10).

Figure 2-9 Historic Survey Map of Geelong (showing Western Gully) - Engineering Heritage Victoria (1950s)

Figure 2-10 Johnstone Park, 19 February 2014, Photograph taken by Geelong SES

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Western Gully


3. HYDROLOGICAL MODELLING

3.1 OverviewThe assessment of flooding within Central Geelong utilised the Direct Rain methodology. This method combines the hydrologic and hydraulic modelling. As part of this assessment of peak flows within the catchment, a RORB model was also developed and validated to ensure the parameters used in the Direct Rainfall model were adequately validated given the lack of formal flow records.

This assessment and validation of peak flows for the hydraulic modelling did not include a review of the existing WBM RORB model developed for the Western Gully Catchment. Hydrographs derived from the WBM study were used for input into the hydraulic model and not those developed by Water Technology.

3.2 RORB ModelRORB (Laurenson et al 2005) is a non-linear rainfall runoff and streamflow routing model for calculation of flow hydrographs in drainage and stream networks. The model requires catchments to be subdivided into subareas, connected by conceptual flow reaches. Design storm rainfall is input to the centroid of each pre-defined subarea. Loss parameters are applied to the model depending on the ARI event being studied and are then deducted by RORB with the excess runoff being routed through the conceptual reach network.

An undiverted RORB model was initially constructed. The sub-area breakup used in the undiverted RORB model estimates at the three main outflow points to Corio Bay. It is important to consider that the model has not been calibrated to these peak flood estimates. These have been provided to demonstrate the varying range of flows generated by the various methods. RORB derived regional parameters were adopted for this assessment and are displayed in Table 3-3.

Initial losses within the RORB model have been selected based on the density of development and land use type within the catchment. Location/Catchment 1 represents the Western Gully Catchment, the majority of which covers the urban residential areas around Newtown and West Geelong. Location/Catchment 2 represents part of the central business district from McKillop Street to the Bay and Location/Catchment 3 represents the eastern portion of the central business district.

Table 3-3 RORB Parameters

Location/Catchment Kc m IL RoC

1 4.78 0.8 10.0 0.6

2 1.35 0.8 1.0* 0.6

3 1.54 0.8 1.0* 0.6

*Catchment 2 and 3, which include the majority of the Central Geelong area were allocated a lower initial loss to account for the intensity of development within this area.

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Table 3-4 Undiverted RORB Model Peak Flow Comparisons

Location in RORB Model

Catchment Area

(km2)

100 Year ARI Rational Method

Peak Flow (Urban) (m3/s)

Regional Method4

(Urban)(m3/s)

100 Year ARI RORB Model Peak

Flow (m3/s)

1 4.7 57.9 30.9 59.6

2 0.4 10.3 5.37 10.5

3 0.5 11.6 6.3 12.1

Figure 3-11 Undiverted RORB Model

A diverted RORB model was then created, to include the Johnstone Park retarding basin and major flow diversion (out of the catchment) which occurs at a 1,200 mm diameter RCP under the railway line near Waterloo Street. The flow diversion leaving the catchment at Waterloo Street was estimated as the peak 5 Year ARI flow (approximately 4.5 m3/s) from the upstream catchment. This is a conservative approach, producing higher peak flows entering the study area in the RORB models. The changes in peak flows at the bay outfall points, between the undiverted and diverted RORB models, are presented in Table 3-5.

Table 3-5 Differences in Peak Flows Between the Undiverted and Diverted RORB Models

4 Grayson, R.B., Argent, R.M., Nathan, R.J., McMahon, T.A. and Mein, R. (1996) Hydrological Recipes: Estimation Techniques in Australian Hydrology Cooperative Research Centre for Catchment Hydrology, Australia

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Location 1

Location 2

Location 3


Location in RORB Model

100 Year ARI Undiverted Peak Flow (m3/s)

100 Year ARI Diverted Peak Flow (m3/s)

Critical Duration

1 59.6 52.3 1 hr

2 10.5 10.5 20 min

3 12.1 12.1 20 min

3.2.1 Flow ChecksFlows from the preliminary hydraulic model were compared to the RORB model outputs at the following outlets into Corio Bay:

Location 1 in the RORB Model: The main outlet adjacent to Cunningham Pier; Location 2 in the RORB Model: The outlet near the corner or Bellerine Street and Eastern

Beach Road; and, Location 3 in the RORB Model: The outlet near the corner of Yarra Street and Eastern Beach

Road.At location 1 the preliminary hydraulic model had a combined peak flow rate of approximately 21.5 m³/s. While this is considerably lower than the RORB model peak flow this is due to the fact that the BMT WBM inflows from the previous study were used to represent the external inflows. These inflows were much less than those calculated through the Water Technology RORB model.

At location 2, the preliminary hydraulic model had a combined peak flow rate of approximately 3.3 m³/s, while at location 3 the combined peak flow rate was equal to 3.8 m³/s. Whilst these flow rates are quite different to that of the other estimated peak flow methods, we anticipate that the following measures will increase the flow rates and therefore better reflect the peak flow calculations:

1. Further decreasing the initial losses applied to the rainfall hyetograph;2. Pre-wetting the catchment with a small amount (approximately 3 mm) of rainfall to fill the

depression storages; and,

3. Running the full suite of durations.

RORB assumes that every drop of rainfall (post subtraction of appropriate losses) eventuates at the catchment outlet. The RORB model uses a kc “lag” parameter which speeds up or slows down the travel time along one of the 4 pre-defined reach types, with only 2 reach types accounting for catchment slope.

The hydraulic model does not assume that all rainfall reaches the outlet. Localised depressions and storages trap runoff and generally results in lower flow rates at the catchment outlet when compared to hydrological only methods. The hydraulic model is also represented on a grid basis, with each fine scale grid applied a roughness (unlimited choice of roughness type compared to the 4 reach types in RORB) and an elevation which is relative to all surrounding grid cells and forms a much more accurate representation of catchment slope. For these reasons, it is generally expected that a hydraulic model, particularly a direct rainfall model, will produce lower flow rates than what would be seen in a hydrological model such as RORB or a Rational Method Estimate. On direct rainfall models, historical flood information and accounts from the community are one of the best methods to prove the results of the flood model, along with sensibility checks from the hydrology.

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3.3 External Catchment FlowsExternal catchment inflows have been included in the model at Latrobe Terrace, Gordon Avenue and Mercer Street as shown in Figure 3-12 and were sourced from the Western Gully Flood Study undertaken by WBM in 2008.

Figure 3-12 Model inflow locations

The inflow boundaries from the Western Gully Flood Study were applied as direct inflow hydrographs at the three specified locations. The peak design flows extracted from the hydrographs at the inflow locations are shown in Table 3-6.

Table 3-6 Peak Flows for the Design Inflow Hydrographs

Location Peak Flow (m3/s)

Overland Flow Component

(m3/s)

Pipe Flow Component

(m3/s)

Critical Duration

Gordon Avenue 17.1 6.9 10.2 2 hour

Latrobe Terrace 2.5 1.7 0.8 1 hour

Mercer Street 1.7 1.7 N/A 1 hour

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Mercer Street Inflow

Latrobe Terrace Inflow

Gordon Avenue Inflow


4. HYDRAULIC MODELLING

The following Section outlines the hydraulic modelling approach, which has been confirmed with the City of Greater Geelong.

4.1 Overview of Rain on Grid ModellingThe Direct Rain on Grid Method utilises the capability of the hydraulic modelling software to incorporate rainfall into the hydraulic model, requiring minimal hydrological input in the form of hyetographs. After subtracting initial losses, the hyetographs are applied directly on the 2D domain in the hydraulic model. Fraction Impervious (FI) and Runoff Coefficient (RoC) values are applied inside the hydraulic model.

There are a number of advantages of the Direct Rain on Grid Method compared with traditional methods and these include:

A rainfall-runoff hydrologic model such as a RORB model is not required nor is a detailed analysis of sub-catchments;

Flows are applied to the model at all points and so there is no reliance on empirical relationships; and,

Catchment storage areas are more accurately defined.

Figure 4-13 displays a simple schematic of the process of developing flood mapping based on this method. It also defines the typical input data in developing a rain on grid model.

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Figure 4-13 Rain on Grid (Direct Rainfall) Methodology

Hydraulic Model DatasetsSeveral data sets were analysed to help schematise the detailed rain-on-grid model. Key items that need to be represented in the modelling included:

Runoff Characteristics - GIS data:

- Land use mapping polygons available from Vic Map datasets;- Geo-referenced Aerial Image – Captured in December 2015, supplied by CoGG undertaken

by Aerometrex,- VicMap Base data – Land parcels, roads, designated waterway features, planning layers and

overlays etc., supplied by DEPI.

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GIS data was used to describe the physical catchment conditions; these included relative imperviousness and roughness.

Terrain data (topography);

o As indicated in Section 2.4.

Asset Data (Pits, Pipes Crossings);

o Provided by City of Greater Geelong on 3/6/2015.o Additional pit and pipe data provided by St Quentin 20/7/2015

Boundary Conditions;

o Inflow hydrographs for all ARIs provided by City of Greater Geelong on 17/6/15.o Corio Bay Water level provided by BoM

Rainfall Conditions;

o Inflow hydrographs for all ARIs provided by City of Greater Geelong on 17/6/15.o Corio Bay Water level provided by BoM

4.2 Direct Rainfall Model Inputs

RainfallThe basic hydrologic model provided design rainfall hyetographs for input to the hydraulic modelling as part of the Direct Rain on Grid method. The hyetographs were extracted from AusIFD Software using the 1987 AR&R method and processing tools developed by Water Technology.

Rainfall is input into the model via 2d_rf layers and linked to the hyetographs generated using rainfall intensities generated from the AR&R IFD data. As the catchment is in an urban environment and therefore relatively small, no areal reduction factors are used. Runoff coefficients will be calculated using Equation 1 which is the same equation used to calculate the rainfall runoff relationship in RORB. Fraction impervious values have been determined based on planning zones obtained from VicMaps.

Losses will be applied to represent infiltration losses based on the fraction impervious and runoff coefficients. Evapotranspiration losses will not be considered as the Geelong CBD catchment is quite peaky and will not allow time for evapotranspiration processes to occur.

Equation 1 Runoff Coefficient Calculation for Rainfall Layers

Where:

ROCxfinal = Final runoff coefficient for ARI of x years

FI = Fraction Impervious of rainfall polygon

ROC(x) pervious = Runoff Coefficient for ARI of x years

Hyetograph .csv files were created for both rainfall ARI events and durations using AusIFD software and in-house Excel tools. They were then applied to the TUFLOW model as appropriate.

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Fraction ImperviousFraction impervious (FI) values across all catchments were determined according to the City of Greater Geelong planning zones as per Melbourne Water’s MUSIC Guidelines (2010a) and are shown in Figure 4-14. These values were then used to calculate runoff coefficients as detailed in Section 4.2.5. Catchment-specific FI values can be found in the appendices of this report. FI values were verified by review of aerial imagery of the catchment areas and site visits. Values were judicially adjusted if they were deemed inappropriate based on the review.

Figure 4-14 Fraction Impervious Plan

IFD ParametersIFD Parameters were determined at the centroid of the catchment using the Bureau of Meteorology IFD Program with a latitude and longitude of 38.148⁰S and 144.362⁰E respectively (see Table 4-7 and Figure 4-15).

Table 4-7 IFD Parameters

Catchment

Log Normal Intensities (mm)Geographical Factors

2 year ARI 50 year ARI

1hr 12hr 72h 1hr 12hr 72h Skewness (G)

F2 F50

Geelong 18.0 3.3 0.9 34.58 6.18 1.8 0.42 4.29 14.94

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Figure 4-15 Catchment IFD Chart

AusIFD Software used IFD parameters to generate hyetographs for each required AEP event and duration. Design temporal patterns were obtained from AR&R (1987). The catchment is located within Zone 1 of the temporal pattern map as defined in AR&R (1987). The Zone 1 temporal patterns were therefore utilised and were filtered to remove embedded intensities of lower AEP events.

A uniform spatial rainfall pattern (i.e. same rainfall depths applied to the entire catchment) was adopted for the generation of design flood hyetographs for events up to and including the 1% AEP event. These were then converted to an appropriate format for the TUFLOW hydraulic model input.

Comparison of ARR – New IFDWhilst IFD parameters were extracted from the old Bureau of Meteorology IFD tools for the purposes of flood modelling for this catchment and as currently recommended by the Bureau as of early 2016, new IFD parameters are available from the Bureau of Meteorology. The new IFD utilises the revised Australian Rainfall and Runoff IFD methodology. A sensitivity analysis between the two sets of parameters was completed. Preliminary values have been extracted from the BoM online original and updated IFD extraction tool at the centroid of the study area. A comparison of the original and updated IFD depths for the study area is shown in Table 4-8 and Table 4-9. It can be seen that the revised depths are predominately lower than the original depth across the full range of events. This potentially suggests greater conservatism has been built into this study.

The current advice from the Bureau of Meteorology is to use the old IFD values until the Australian Rainfall and Runoff update is complete. This is to ensure that the other design flood estimation techniques are revised in line with the IFD update to ensure AEP neutrality across design rainfall and runoff estimation (i.e. 100 year ARI rainfall produces 100 year ARI runoff). For this reason the old IFD values were used for the hydrologic component of the study.

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Table 4-8 Comparison of IFD depths between new and old IFD BOM tool (50%, 20% and 10% AEP events)

Event50% AEP 20% AEP 10% AEP

Depth (mm) Difference Depth (mm) Difference Depth (mm) DifferenceDuration Old New mm % Old New mm % Old New mm %

5 min 5.00 4.10 -0.90 -0.22 6.83 5.90 -0.93 -0.16 8.08 7.20 -0.88 -0.1215 min 9.43 7.40 -2.03 -0.27 12.50 10.80 -1.70 -0.16 14.75 13.30 -1.45 -0.1130min 13.10 9.70 -3.40 -0.35 17.30 14.20 -3.10 -0.22 20.10 17.60 -2.50 -0.14

1Hr 17.50 12.40 -5.10 -0.41 22.70 17.90 -4.80 -0.27 26.20 21.90 -4.30 -0.202 Hr 22.00 15.80 -6.20 -0.39 28.40 22.30 -6.10 -0.27 32.80 27.00 -5.80 -0.216 Hr 31.08 24.40 -6.68 -0.27 39.84 32.90 -6.94 -0.21 45.66 39.10 -6.56 -0.1712Hr 38.64 32.70 -5.94 -0.18 49.32 43.50 -5.82 -0.13 56.40 51.30 -5.10 -0.1024 Hr 48.00 42.60 -5.40 -0.13 62.16 57.40 -4.76 -0.08 71.28 68.00 -3.28 -0.05

Table 4-9 Comparison of IFD depths between new and old IFD BOM tool (5%, 2% and 1% AEP events)

Event5% AEP 2% AEP 1% AEP

Depth (mm) Difference Depth (mm) Difference Depth (mm) DifferenceDuration Old New mm % Old New mm % Old New mm %

5 min 9.83 8.60 -1.23 -0.14 12.25 10.40 -1.85 -0.18 14.33 12.00 -2.33 -0.1915 min 17.75 16.00 -1.75 -0.11 22.00 19.70 -2.30 -0.12 25.50 22.80 -2.70 -0.1230min 23.90 21.00 -2.90 -0.14 29.50 25.90 -3.60 -0.14 34.00 29.90 -4.10 -0.14

1Hr 30.90 26.10 -4.80 -0.18 37.50 31.90 -5.60 -0.18 43.00 36.60 -6.40 -0.172 Hr 38.60 31.80 -6.80 -0.21 46.60 38.40 -8.20 -0.21 53.40 43.90 -9.50 -0.226 Hr 53.58 45.40 -8.18 -0.18 64.80 54.10 -10.70 -0.20 73.80 61.10 -12.70 -0.2112Hr 66.00 59.30 -6.70 -0.11 79.56 70.50 -9.06 -0.13 90.60 79.40 -11.20 -0.1424 Hr 84.00 78.80 -5.20 -0.07 102.00 93.80 -8.20 -0.09 116.64 105.80 -10.84 -0.10

Losses and Runoff CoefficientRainfall losses were incorporated in the modelling through two ways: Initial Loss (IL) and Runoff Coefficient (RoC). Rainfall Runoff coefficients were set in accordance with Melbourne Water guidelines (MWC, 2014).

The IL values of 10mm, 1mm and 0.6mm were tested in the initial modelling phases of the project. Following discussions with the City of Greater Geelong, and taking into consideration the recent modelling of Melbourne CBD undertaken by Water Technology as part of the Integrated Climate Adaptation Modelling with the University of Melbourne and Moroka, an initial loss value of 0.6mm was adopted. Rainfall Runoff coefficients were calculated in accordance with Melbourne Water guidelines (MWC, 2014).

Table 4-10 Initial loss values

Storm Event (AEP)

Initial Losses (mm)

RoC

20% 0.6 0.2510% 0.6 0.355% 0.6 0.452% 0.6 0.551% 0.6 0.60

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Pre-wetA pre-wet of the catchment was undertaken prior to the actual event to remove minor depressions in the topography. To do this a small amount of rain (3 mm) was applied over 10 minutes, with the model allowed to run for 10 hours following the rainfall to ensure that the model has finished “running off” prior to commencing the design rainfall event.

4.3 Overview of TUFLOW Hydraulic Model The hydraulic model routes the design flood hyetographs obtained from IFD rainfall along the proposed infrastructure and any associated overland flow path. The hydraulic model, TUFLOW, was employed in this investigation.

TUFLOW is a widely used hydraulic model that is suitable for the analysis of overland flows in urban areas. In addition to the Rainfall and Catchment loss inputs discussed in Section 5.2 TUFLOW has three main inputs:

Topography and drainage infrastructure data;

Roughness; and,

Boundary conditions.

The TUFLOW model was used to route flows within the catchment. Flow was routed along one-dimensional (1D) elements as pipes. Where the capacity of the 1D elements was exceeded, the excess flows are routed overland in a two dimensional (2D) domain. The TUFLOW model outputs flood depths/elevations and velocities.

A detailed 1D/2D TUFLOW5 model was developed to simulate existing conditions. The model allows for the accurate representation of the 20, 10, 5, 2 and 1% AEP flood extents within the Geelong CBD.

A rain-on-grid (direct rainfall) approach was employed in this model. The advantage of this type of model is that no assumption needs to be made regarding the catchment delineation; the topography, roughness and 1D networks will contribute to this delineation as the model runs.

4.4 Hydraulic model construction and parametersThe TUFLOW model was constructed in MapInfo V12.0. This Section details key elements and parameters of the TUFLOW model.

Model VersionThe single precision version of the latest TUFLOW release was used for all simulations (TUFLOW Version: 2013-12-AE-w64). This was the latest double precision build at the time of modelling (2015).

2D Grid Size and TopographyA single 2D domain was used with a grid size of 2 m. The 2d_zpt file was populated with elevations from the photogrammetry data obtained from CoGG on the 3/6/15.

Additionally, initial model results indicate that there are a number of deep holes on various locations around the study area. These have been smoothed over with z-shapes to remove any unrealistic ponding. The roughness has been adjusted accordingly over these locations. It is noted that there

5 TUFLOW is a standard hydrodynamic modelling package used extensively by Melbourne Water to undertake urban flood investigations.

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were gaps in the photogrammetry where data was missing. Z-shapes were used to smooth over these areas, so as to reduce instability or inaccuracy in the model.

Underground car parks were not considered in the model. The main car park beneath Westfield (eastern section, Big W) is however included, as the building carpark area is at or above ground level. The level was taken from available topographic information. In light of this, inclusion of this area within the model is considered reasonable. It is important to note that none of the underground car parks within the CBD appear to have flood gates.

ARR Project 15 recommends a number of options for the representation of buildings in 2D modelling:

a) Set the building elevations to pad or floor level;b) Raise the grid to represent the building;c) Model the building floor levels, and walls with openings; or,d) Model the buildings as porous elements.

It is advised not to null cells (make them inactive) in direct rainfall models. This is because doing so would remove the rainfall volume which falls on these cells from the model, as such this has not been done.

Note that residential, commercial and industrial properties and buildings will be modelled the same way. There is not really a difference between these building types within the CBD.

A combination of the methods described above has been used as part of this assessment. The majority of the buildings footprints have been roughened and raised with known floor levels. In a number of cases buildings have been raised to block flow paths so as to represent more accurately an important flow obstruction or constriction.

1D NetworkPit and pipe data was obtained from the City of Greater Geelong Asset Management GIS. Additional data was also provided from the GIS drainage layers produced as part of the Bridge Street Drainage Flood Study (WBM). As mentioned previously, of the total 2,144 pipes within the Council provided dataset, 1,657 pipes do not have any invert information. Pit depths were also only available for 337 out of a total 2,141 Council pits. Inaccuracies have also been observed with some of the invert levels in the council GIS dataset, i.e. where the pipe obvert is above the surface.

Where noted data gaps or inaccuracies were present additional data including pit configuration and invert levels were obtained from detailed ground survey undertaken by St Quentin. In some cases pipe configuration was confirmed and assumed based on construction plans provided by the City of Greater Geelong.

Alignment differences between the Council dataset and the WBM dataset, meant that manual ‘snapping’ was required to align the WBM models pipe and pit dataset to match the Council GIS data.

Where survey and or ground truthing was not possible, Water Technology inferred missing data such as the pipe and pit invert information using the cover rules shown in Table 4-5 .

The data has been checked to ensure there are no clashes and key drainage and overland flow paths have had invert levels reviewed to ensure the flow will be directed downstream, without downstream invert levels higher than upstream invert levels. A plan of the adopted pit and pipe network has been prepared for the purposes of the model, and can be seen in Figure 4-16.

Rail underpasses along Brougham Street and Gordon Avenue have been modelled within the 2D network and not within the 1D networks with the exception of the existing drainage network

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through these locations. Losses were applied as per the standard Melbourne Water method found in the Land Development Manual.

Gross Pollutant Traps (GPTs) have not been included in this model as there is insufficient information available about them for hydraulic modelling purposes. Open sewer pits have also been ignored for the purposes of this project.

Pits have had depth-varying flow relationships applied to them depending on whether they are grated side entry pits, side entry pits, double grated side entry pits or overflow kerb pits. The overflow kerb pits within the study area have all been assumed to be of rectangular shape, on-grade with the street, and a longitudinal grade of 1% for the purpose of calculating the relationship.

Table 4-11 Assumed Depth of Cover

Pipe Diameter (mm) Assumed Depth of Cover (mm)Less than or equal to 900 mm 600

Greater than 900 mm 750

Connectivity to the 2D was achieved using SX and CN lines for outflow points, and SX connections for pits. Loss parameters around building structures are managed in 2D by TUFLOW with no additional losses. 1D losses are applied as per the standard methods recommended by Melbourne Water in the Land Development Manual.

Other important parameters to note include the following:1. Pits that are open are modelled as if they are not blocked;2. All base flow is ignored;3. 1D HT (Head Timeseries) conditions were imposed at 1D outlets based on the obvert of the

downstream end of the pipe outlets (only for pipe which drained south towards the Barwon River); The remainder of the pipes drained to the bay where a bay level was applied;

4. Pit classes provided by CoGG have had assumed grate dimensions applied in the model as per Table 4-12; and,

5. It is assumed that Side Entry Pits (SEP) are 0.9 m long by 0.11 m high at the entrance in the kerb.

Table 4-12 Pit Grate Dimensions (mm)

Class Grate width Grate lengthDouble grated Varies VariesDouble GSEP (provided and assumed) Varies VariesGrated pit (assumed) 900 600GSEP – where no info, 900 * 430 900 600LT – Litter Trap 900 600Side entry pit 900 110Johnstone Park – Special GP1 1800 1800Johnstone Park – Special GP2 1500 1500Johnstone Park – Special GP3 1200 1800Yarra Street – Special GP 2250 2250

Plot Output LinesPlot Output or PO lines have been set up to record flow and water surface elevations at strategic locations throughout the model as per Figure 4-17.

3922-01 / R01 V02 - 04/01/2016 36

Figure 4-16 City of Greater Geelong – Pits and pipes

Figure 4-17 PO Line Locations

RoughnessLand use zonings available from the VicMap dataset were used to determine the Manning’s roughness coefficients as per Table 4-13 and Table 3.2 of Melbourne Water’s Guidelines and Specifications for Flood Modelling. Refer to Figure 4-18 for the layout of the Manning’s roughness polygons.

It is noted in ARR Project 15 that for direct rainfall modelling ‘n’ values may be input into the model in a few ways:

a) Standard ‘n’ process;b) Vary ‘n’ values for buildings depending on whether they are in the floodplain or not. This

allows a very low value of n to be used for buildings to represent fast runoff from the building’s roof outside the floodplain, and a high value to represent the much higher resistance to flow within the floodplain. With this option, building elevations should be set to building pad or floor level; or,

c) Vary ‘n’ with depth of flow.

As little data exists for Options b and c, Option a has been utilised in this model.

Table 4-13 Manning's n Roughness Coefficients

Land UseManning’s n Roughness Coefficient

Roads 0.020

Parking 0.020

Open Space of Waterway (minimal vegetation) 0.040

Open Space of Waterway (moderate vegetation) 0.060

Open Space of Waterway (heavy vegetation) 0.090

Activity Centre Zone (Paved Open Areas) 0.020

Activity Centre Zone Building Footprints 0.450

Industrial / Commercial 0.300

Railway Line 0.125

Residential Property (With Building Footprint) 0.100

Residential - Urban 0.350

Residential Building Footprint 0.400

Concrete Lined Channels 0.016

Figure 4-18 Manning's Roughness Polygons

Pit ConfigurationPits along the 1D pipe section were connected to the 2D using the “SX” option for the 1d_nwk pit Conn_2D attribute.

Boundary ConditionsAs mentioned, the model setup utilises a combination of direct inflow and direct rainfall (generated from AR&R 1987 IFD data). TUFLOW Height versus Time (HT) boundaries have been included at the various outlets to Corio Bay based on the estimated 10% storm tide level information provided by the 2008 CSIRO Report “The effect of climate change on extreme sea levels in Port Phillip Bay”. The adopted 10% AEP storm tide level used for the modelling was 0.91 meters AHD.

Initial water level DEMs for each AEP event were produced as per the method described above. Note that Corio Bay has not been modelled. That is; dynamic tidal influence, swell and wave actions have not been incorporated into the model.

1D initial water levels were sampled from the compiled 2D IWL DEMs. These were applied, along with HT boundaries, maintaining this water level for the duration of the model run, at each 1D outlet point outside of the model boundaries.

HQ boundaries have been applied along the model boundaries with a grade of 1 in 100 to allow water to flow out of the model. This HQ boundary has been offset from the assumed catchment boundary by 10m to ensure they do not interact with the area of interest. This boundary does not remove any of the water from the catchment, but allows water to flow out of the model unimpeded from areas outside the catchment, as defined by the topography. This was refined based on preliminary model results. The TUFLOW boundary layers used in the existing conditions model are shown below in Figure 4-19.

Figure 4-19 Tuflow model boundaries

4.5 TUFLOW Model Reconciliation

GIS ProcessingThe TUFLOW model was set up using a projection in the Map Grid of Australia, Zone 55. All GIS layers were projected within this coordinate range and the projection file read into the TUFLOW Control File (TCF).

TUFLOW Data ProcessingThe TCF was set up to process the following:

TUFLOW Event File (TEF) including the ARI and duration of each event as described below;

TUFLOW Geometry File (TGF) including the cell size, grid size, LiDAR, material roughness GIS layers, initial water levels and Z-shapes used to correct for initial water levels;

Estry Control File (ECF) including the 1D timestep network, boundary conditions, initial water levels and the location of 1D results;

TUFLOW Materials File (TMF);

Plot Output lines;

2D boundary conditions including the boundary conditions database and GIS layer; and,

2D Simulation parameters, including the timestep, recording times and displays, start times and cell wet/dry depth.

Results ProcessingThe results to be generated were set up in the TCF including water elevation, velocities, flows, depths, hazard mapping and mass balance checks as described below. The map cut-off depth command in TUFLOW was set to 0.0 m, with all future thinning of results post processed from the full raw dataset. The cell wet-dry depth was set at 0.0002 m as required for the very shallow depths seen in direct rainfall modelling.

Hydraulic Model ApplicationThe TUFLOW model was run for a suite of storm durations for each of the required ARIs in the existing conditions and various scenarios as per Table 4-14. In initial runs, the full suite of storm durations from 10 minutes to 72 hours was run, but iterations after the initial runs focused on durations between 15 minutes and 9 hours, as this is where the peak events occur.

All TUFLOW model runs were controlled through a TUFLOW Event File (.tef) and a series of batch files constructed for use in this project. The use of the .tef file and batch files ensures that the base .tcf (TUFLOW Control File) does not change between runs, with all event specific parameters specified in the .tef file. This reduces the potential for error and also assists in reducing model run and processing times. A full explanation of the use of the .tef files will be provided along with a batch file for future running of the models by City of Greater Geelong.

Table 4-14 Model Runs

Scenario ARI’s Duration Model Nomenclature

Existing Conditions 20%, 10%, 5 % 2% and 1%

15min, 20min, 25min, 30min, 45min, 1hr, 1.5hr, 2hr, 3hr, 4.5hr, 6hr and 9hr

Existing

Climate Change (Rainfall) 1% 15min, 20min, 25min, 30min, 45min, 1hr, 1.5hr, 2hr, 3hr, 4.5hr, 6hr and 9hr

CC

Blockage 1% 2 hr SensBlock

High Tailwater (1% Storm tide) 1% 2 hr Sens_TW

Mitigation Option 1 20%, 10%, 5 % 2% and 1%


Mit1_JP2b



Mit1_JP1



Mit1_JP1b



Mit3_EBR



Mit_Add3



Mit_Add1



Mit_Add2



Mit_Add4



Mit_Add3



Mit_Add1



Mit_Add2



Mit_Add4

TUFLOW model checksAs outlined in the Melbourne Water Guidelines and Technical Specifications (MWC, 2014) Section 3.4.4, the following checks have been undertaken on the TUFLOW model parameters and outputs:

2D grid size is 2 m, within the range of 2-3m for urban catchments; 2D time step is 1 s, within the range of ¼ - ½ of the grid size; 1D time step is 0.5 s, within the range of no less than 1/10 – 1/5 of the 2D time step, and

generally no less than 0.5 seconds; Model mass errors within the range of +/- 1.2% for all ARIs and durations; Errors messages do not occur; Pipes properly connected; 2D model extent does not constrain the flood extent; Warning messages are eliminated or explained; and Pipes flowing full –

Checks and fixes were undertaken prior to the final model runs being launched on the locations of any 1D or 2D negative depths, significant depths of water and any warnings and checks contained within the log files. All instabilities have been found and rectified.

Velocity checks were undertaken and pits and pipes examined where velocities were over 5 m/s. The TS file was imported into MapInfo and the relevant pits and pipes reviewed to determine whether the actual maximums were a result of a small instability. If so, these were either accepted, if the TS plot showed actual peak velocities less than 5 m/s, or the issue corrected where possible. Generally, where velocities are higher than 5 m/s, it is a result of steep pipes.

Changes in elevation caused by the Z-shapes were checked and deemed acceptable.

Pipe capacity was reviewed using the CCa results file in MapInfo. Locations where pipes were flowing less full downstream than upstream were reviewed and deemed acceptable, generally owing to increases in cross-sectional area in downstream pipes, or increases in slopes in the pipes.

Duration %FullAv

100y15m 75.956

100y20m 77.627

100y25m 78.260

100y30m 77.473

100y45m 76.117

100y1h 77.770

100y1_5h 77.752

100y2h 77.038

100y3h 71.224

100y4_5h 67.891

100y6h 63.281

100y9h 59.327

5. GIS PROCESSING

5.1 TUFLOW model outputsTUFLOW provides times-series of depths (m), water surface elevations (m AHD), flow velocities (m/s) and flood hazard (m/s,m) at each link location within the 1D element, and at the grid points within the 2D domain.

The raw model output data was processed in order for it to be easily viewed in GIS. Processing occurred in two stages – firstly processing the raw data using TUFLOW utilities and then processing the resulting data within a GIS environment. These processes are detailed below.

5.2 TUFLOW Data ProcessingTUFLOW contains a number of utilities for processing output data. The following utilities were used:

Res_to_Res.exe: This utility has a number of functions and in this instance was used to extract the maximum value for depth, velocity and water elevation at each grid point across the durations run for each event. The maximum values are then placed into a new data file.

TUFLOW_to_GIS.exe: This utility converts TUFLOW data into GIS formats and in this instance was used to convert TUFLOW data into the MapInfo mid/mif interchange format.

5.3 Results ProcessingMapInfo was used to import and then compile the data into an appropriate format. Initially the depth, velocity, water surface elevation and duration layers were amalgamated into a single layer for each event. Final maps were produced from ASCII plots in Arc-GIS v10.3.

5.4 Data Integrity ChecksThe results were checked to ensure that larger events corresponded with increased depths, flood level and velocity in each cell.

Results must conform to the following: 100yr > 50yr >20yr > 10yr >5yr.

5.5 Filtering of ResultsThe results obtained from the modelling were filtered in two stages.

Level 1 filtering involved removing areas where depth and velocity * depth criteria were below a threshold, as per the Melbourne Water Guidelines and Technical Specifications for Flood Mapping Projects:

1. Minimum Depth Threshold – any flooded cells with depths less than 0.02 m were removed; and2. Velocity * Depth Criteria – The results were filtered to remove any cells where both the depth is less than 0.05 m and the V*D is less than 0.008.

All cells considered as flooded after the application of the above filters (1 & 2) are then combined into a flood extent that connects neighbouring cells.

In addition to the filtering mentioned, a second level of filtering was also applied, however this was only used in the development of a proposed Special Building Overlay (SBO). Level 2 filtering involved

removing puddles of less than 100m2 from the final flood extents.

5.6 Key AssumptionsA number of key assumptions underpin the model, including the following:

Interaction between the surface and ground water has not been modelled; Modelled scenarios did not account for potential pipe blockages, although the 1% AEP

critical storm event was modelled as part of the sensitivity testing; No underground car parks have been considered in the model; The hydrology and hydraulics of the Corio Bay were not analysed as part of this project; 1987 IFD rainfall parameters were used in the modelling; Additionally, other assumptions are detailed in sections below throughout the report as

relevant.

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