BANYULE CITY COUNCIL · BANYULE CITY COUNCIL STORMWATER MANAGEMENT – CATCHMENT ANALYSIS V2000_048 Stormwater Management – Catchment Analysis DOC PATH FILE: REV DESCRIPTION AUTHOR

BANYULE CITY COUNCIL

Stormwater Management – Catchment Analysis

September 2013

V2000_048


STORMWATER MANAGEMENT – CATCHMENT ANALYSIS

V2000_048 Stormwater Management – Catchment Analysis DOC PATH FILE: REV DESCRIPTION AUTHOR REVIEWER APPROVED BY DATE

Rev 0 Client Issue Scott Dunn Paul Clemson Scott Dunn 13/09/13

Signatures

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DISCLAIMER

This report has been prepared on behalf of and for the exclusive use of Banyule City Council and

is subject to and issued in accordance with Banyule City Council instruction to Engeny Water

Management (Engeny). The content of this report was based on previous information and studies

supplied by Banyule City Council.

Engeny accepts no liability or responsibility whatsoever for it in respect of any use of or reliance

upon this report by any third party. Copying this report without the permission of Banyule City

Council or Engeny is not permitted.



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CONTENTS

1. INTRODUCTION AND STUDY OBJECTIVES ...........................................................1

1.1 Expansion of Original Study .......................................................................................2

1.2 Banyule Municipality Description................................................................................2

2. SCOPE OF WORKS ..................................................................................................4

2.1 Original Scope of Works ............................................................................................4

2.2 Expanded Scope of Works ........................................................................................4

3. DATA PROCESSING ................................................................................................6

3.1 Sources of Data .........................................................................................................6

3.2 Data Review ..............................................................................................................6

3.3 Site Visit.....................................................................................................................7

4. HYDROLOGY .......................................................................................................... 15

4.1 Sub-Catchment Data ............................................................................................... 15

4.2 Intensity-Frequency-Duration Data .......................................................................... 17

4.3 Adopted RORB Model Parameters .......................................................................... 17

4.4 RORB Outputs ......................................................................................................... 18

4.5 External Melbourne Water RORB Models ................................................................ 18

5. HYDRAULIC MODELLING ...................................................................................... 19

5.1 Digital Terrain Model (DTM) ..................................................................................... 19

5.2 Catchment Topography ........................................................................................... 19

5.3 1-D Network Data .................................................................................................... 21

5.4 Surface Roughness ................................................................................................. 23

5.5 Boundary Conditions................................................................................................ 24

5.6 TUFLOW Parameters .............................................................................................. 26

5.7 Model Validation ...................................................................................................... 27

6. FLOOD MAPPING ................................................................................................... 28

6.1 Filtering of Flood Modelling Results ......................................................................... 28

6.2 Results Analysis: Flooding ‘Hot Spots’ ..................................................................... 28



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6.3 Comparison to Melbourne Water Flood Extents ....................................................... 32

6.4 Flood Mitigation ....................................................................................................... 34

7. FUTURE DEVELOPMENT & PLANNING OPPORTUNITIES .................................. 35

7.1 Opportunities for Stormwater Detention ................................................................... 35

7.2 Opportunities for Stormwater Treatment .................................................................. 35

8. SUMMARY AND CONCLUSIONS ........................................................................... 39

8.1 Future Works ........................................................................................................... 39

9. QUALIFICATIONS ................................................................................................... 41

Appendices

APPENDIX A – CATCHMENT DELINEATION FOR ENTIRE MUNICIPALITY

APPENDIX B – 5 YEAR ARI FLOOD DEPTH CONTOUR PLANS

APPENDIX B – 100 YEAR ARI FLOOD DEPTH CONTOUR PLANS

List of Tables

Table 1-1 Catchment Summary .............................................................................................3

Table 4-1 IFD parameters for the Banyule Municipality ....................................................... 17

Table 5–1 Surface roughness values .................................................................................. 24

List of Figures

Figure 3.1 Henry Street low point ..........................................................................................9

Figure 3.2 Henry Street drain outlet .......................................................................................9

Figure 3.3 Kardinia Street low point and overland flow path (Beatrix Street Drain) .............. 10

Figure 3.4 Elmo Road Drain through rear of residential properties ...................................... 10

Figure 3.5 Hyacinth Street low point (Elmo Road Drain) ...................................................... 11

Figure 3.6 Para Road Retarding Basin (Lower Plenty Drain) ............................................... 11

Figure 3.7 Para Road Retarding Basin outlet (Lower Plenty Drain) ..................................... 12

Figure 3.8 Goulburn Grove Sediment Pond outlet (Watsonia Drain) .................................... 12



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Figure 3.9 Upstream of Melbourne Water Watsonia Drain Retarding Basin ......................... 13

Figure 3.10 Melbourne Water Watsonia Drain Retarding Basin Outlet ................................ 13

Figure 3.11 Watsonia Drain downstream of Lower Plenty Road .......................................... 14

Figure 4.1 Example of sub-catchment delineation ............................................................... 16

Figure 5.1 DEM Extent for 4 Catchments ............................................................................ 20

Figure 6.1 St Helena Drains 100 year ARI flood extent comparison .................................... 33

Figure 6.2 Andersons Creek 100 year ARI flood extent comparison at the North Ringwood RB 34

Figure 7.1 Example of Water Sensitive Urban Design street scaping .................................. 36

Figure 7.2 Example of bioretention swale within car park .................................................... 37



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1. INTRODUCTION AND STUDY OBJECTIVES

Banyule City Council has committed to a long term strategic catchment management strategy. The first component of this strategy is to assess the hydraulic drainage performance of the Council drainage network in both minor and major storm events. Banyule City Council engaged Engeny to begin this work by undertaking a catchment analysis of the following 4 catchments (Contract No. 0771-2013) within the Banyule municipality:

Lower Plenty Drain Catchment.

Elmo Road Drain Catchment;

Watsonia Drain Catchment; and

Beatrix Street Catchment.

The outcomes of this study and analysis will assist Council to:

prioritise and allocate funds for drainage improvements;

guide policy decisions concerning catchments; and

guide the assessment of future developments.

The modelling that Engeny has performed as part of this study has been undertaken to assist Council in determining flood extents, flood depths and appropriate mitigation options for these catchments. Modelling the catchment in TUFLOW hydraulic modelling software has enabled identification of inadequacies in the existing drainage infrastructure and locations where drainage improvement works could be implemented to reduce the severity of flooding.

This report documents the modelling process employed to deliver this study including the key modelling parameters and assumptions. The assistance that Melbourne Water has provided, as documented throughout this report, has been critical to the successful delivery of this study. The structure of the report is as follows:

Section 2 – details the scope of works

Section 3 – details the sources and review of data

Section 4 – details the hydrological modelling process

Section 5 – outlines the hydraulic modelling process

Section 6 – summarises the flood modelling results



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Section 7 – provides details of the WSUD assessment for the 4 catchments study

Section 8 – summarises the work undertaken across the Banyule municipality

The following information has been provided to Council to support this report:

A CD containing the relevant GIS output from this study;

A0 flood maps for each catchment (depicting peak 5 year an 100 year ARI flood extent); and

A detailed report documenting the mitigation assessment completed as part of the scope of works for the original four catchment study which also includes mitigation assessment for the Irvine Road Drain catchment.

1.1 Expansion of Original Study

In the early stages of the study Banyule City Council saw a need and opportunity to expand the study and apply a similar scope of works across the entire municipality. Banyule City Council engaged Engeny to undertake this work (the scope of works for the expanded study is described in more detail in Section 2.2).

1.2 Banyule Municipality Description

The Banyule municipality covers an area of approximately 63 square kilometres north-east of Central Melbourne and is made up of 21 suburbs. The municipality is bounded by the Yarra River in the south and in the west by Darebin Creek. Banyule has a diverse community of approximately 119,000 residents, and although the number of people living in Banyule is expected to remain stable between 2001 and 2015 the total number of households is expected to increase. An increase in the number of households will put added pressure on the existing drainage network through an increase in the imperviousness of the catchment resulting in an increase in stormwater runoff. The results from this study will assist Council in guiding the assessment of future developments to ensure that existing flooding ‘hot spots’ are not adversely impacted by future development within the municipality.

1.2.1 Major Waterways

Within the Banyule municipality there a number of major waterways, these include:

Yarra River to the south;

Plenty River running north-south through the middle of the municipality;

Darebin Creek to the west; and

Salt Creek in the western portion of the municipality.



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1.2.2 Banyule Drainage Catchments

To undertake detailed flood modelling across the entire municipality, Engeny divided the municipality into a number of discrete catchments as shown in Appendix A. Engeny used Melbourne Water catchment boundaries to assist in this process. Table 1.1 below summarises each of the catchments modelled by Engeny and the Melbourne Water catchments that make up these catchments (the total modelled area is 63.3 km2). Table 1-1 Catchment Summary

Catchment Name Melbourne Water Catchments within Modelled

Catchment

Approx. Catchment

Area (km2)

Watsonia Drain Watsonia Drain 2.94

Beatrix Street Drain Beatrix Street Drain 0.97

Lower Plenty Drain Lower Plenty Drain 0.70

Elmo Road Drain Elmo Road Drain 0.81

South West Catchments Lillimur Avenue Drain, Southern Road Main Drain,

Heidelberg West Main Drain and Darebin Creek (Lower)

9.51

Irvine Road Drain Irvine Road Drain and Locksley Road Main Drain 4.78

Bundoora Drain Bundoora Drain and Janefield Drain 2.85

Salt Creek Salt Creek, Banyule East Drain, Banyule Drain, Macleod

High School Drain, and Banksia Street Drain

15.62

Upper Plenty River Upper Plenty River, Carolyn Street Drain, Yando Street

Drain, Kempston Street Drain, and Diamond Creek Road

Drain

7.48

Lower Plenty River* Lower Plenty River, Castleton Road Drain, and Cleveland

Avenue Drain

9.66

Eltham Drains Eltham West Drain, Eltham Park Drain, and Bolton Street

Drain

3.31

St Helena Drains St Helena West Drain and St Helena East Drain 4.64

* The larger Lower Plenty Catchment modelling did not include the smaller Melbourne Water Lower Plenty Drain Catchment as this catchment was modelled as part of the original four catchments study.



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2. SCOPE OF WORKS

2.1 Original Scope of Works

The following scope of works was adopted for the project (Contract No. 0771-2013):

Data collection (assist Council in the collection of pipe diameter information);

Site visit;

Identify locations within the study catchments which are subject to flooding;

Hydrologic and hydraulic analysis of the stormwater network across the four catchments;

Geospatial depiction of capacity issues within the catchments with a rating for priority;

Geospatial depiction of the stormwater network, including overland flow paths, with a visual rating for hydraulic adequacy for 5, 10, and 100 year ARI storm events;

A visual safety rating to be allocated to overland flows associated within flooded roads and reserves (High, moderate and low risk ratings);

Supply of all geospatial results compatible with Council’s GIS;

Produce a capital works program based on an assessment and prioritisation of identified locations with cost estimates;

Future planning and development recommendations including the following (4 Catchments only):

Potential GPT locations Flood mitigation measures including retarding basins All cleansing issues

A detailed report.

2.2 Expanded Scope of Works

Through consultation with Council the following scope of work was adopted to assess the remainder of the municipality:

Data collection (assist Council in the collection of pipe diameter information);

Hydrologic and hydraulic modelling for the remaining catchments (for 5 year and 100 year ARI events);



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Risk ratings for overland flow paths in each catchment.

The catchments making up the remainder of the municipality included:

Irvine Road Drain;

Bundoora Drain;

Salt Creek;

Upper Plenty River;

Lower Plenty River;

Eltham Drains; and

St Helena Drains.

Outputs for each of the above catchments was agreed to be:

Provision of GIS pipe layer with diameters included (completed as part of the data collection phase of work);

Flood extents for 5 year and 100 year ARI events; and

GIS based risk outputs.



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3. DATA PROCESSING

3.1 Sources of Data

3.1.1 Council Data

Banyule City Council provided the following GIS data for use on this project:

Pit and pipe data;

Cadastre boundaries;

Aerial photography; and

Road alignments with names.

The LiDAR data used to create Digital Terrain Models (DTMS) for the catchments was used under license from the Department of Sustainability and Environment and by agreement with Banyule City Council. The DTMs are discussed further in Section 5.1.

3.1.2 Melbourne Water Data

The following GIS data was obtained from Melbourne Water for use on this project:

Catchment boundaries;

Pit and pipe data;

Hydrological models; and

Available flood extent and contour information.

Engeny obtained and used asset data in Melbourne Water’s GIS to model all Melbourne Water’s drainage assets within the Banyule municipality.

3.2 Data Review

Engeny undertook a broad scale review of the existing information to identify risks and opportunities within the Banyule municipality. This is summarised in the following sections.

3.2.1 Previous Studies and Reports

Engeny is not aware of any similar detailed flood mapping studies previously conducted by Council within the municipality. Previous studies conducted by or on behalf of Melbourne Water produced 20, 50 and 100 year ARI flood extents for their underground



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drainage assets within each of the modelled catchments. The majority of these studies were conducted in the late 1990’s with the majority of the modelling performed within a 1D hydraulic model. Flood extents from Engeny’s current study for Banyule City Council have been compared to the existing Melbourne Water flood extents as a form of model validation (see Section 6.3).

Engeny is aware that Melbourne Water is currently completing a detailed 2D flood study for the Eltham West Drain catchment which includes the St Helena East and West Catchments within the Banyule municipality. Melbourne Water provided Engeny with draft flood extents for this study for comparative purposes (see Section 6.3).

3.2.2 Topography

Engeny reviewed LiDAR data for the municipality using aerial photography and MapInfo cross-sections. Whilst the LiDAR was flown in 2008-2009, Engeny believes that the LiDAR provides sufficient definition of the key topographical features in the catchment such as open channels, waterways and roadways. Development within the municipality over the last 4-5 years has had some impact on the topography, such as the WaterMarc development on Grimshaw Street. Minor modifications were required to reflect the altered topography in these instances, as discussed in the hydraulic modelling section of this report.

3.2.3 Missing or Inconsistent Data

Engeny reviewed the pipe and culvert diameters and connectivity provided in Banyule City Council’s GIS. This assessment identified that the majority of the drainage within Council’s GIS system was missing diameter information which is a critical input to the hydraulic modelling.

Missing network data was resolved by Engeny providing Council with assistance to review design drawings and input pipe diameters into Council’s GIS system. Upon the completion of this process it was also necessary to use engineering judgement to fill some data gaps where the pipe was not critical and could be reasonably estimated. In some instances where this process was not able to resolve the remaining data gaps, Banyule City Council provided additional information or conducted field inspections. No invert details were used from the design drawings due to time constraints and the likelihood that due to the age of the design plans, the majority are based on an old datum. As agreed with Council, Engeny generated invert levels based on an assumption of 600 millimetres cover. Further details of this process are provided in Section Error! Reference source ot found..

3.3 Site Visit

Engeny completed a site visit of the four original contract catchments on the 29th of April 2013. With preliminary flood modelling results at this stage, a number of key flooding



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hotspots were visited. Figures 2.1 to 2.11 show some of the areas of flooding and catchment features.



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Figure 3.2 Henry Street drain outlet Figure 3.1 Henry Street low point



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Figure 3.4 Elmo Road Drain through rear of residential properties Figure 3.3 Kardinia Street low point and overland flow path (Beatrix Street Drain)



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Figure 3.6 Para Road Retarding Basin (Lower Plenty Drain) Figure 3.5 Hyacinth Street low point (Elmo Road Drain)



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Figure 3.8 Goulburn Grove Sediment Pond outlet (Watsonia Drain) Figure 3.7 Para Road Retarding Basin outlet (Lower Plenty Drain)



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Figure 3.10 Melbourne Water Watsonia Drain Retarding Basin Outlet Figure 3.9 Upstream of Melbourne Water Watsonia Drain Retarding Basin



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Figure 3.11 Watsonia Drain downstream of Lower Plenty Road



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4. HYDROLOGY

Hydrology forms an integral component of this project. Catchment hydrology provides a key input into the hydraulic (TUFLOW) model in the form of hydrographs which TUFLOW models at each inlet pit within the catchment. RORB hydrological modelling software was used to model each of the catchments within the Banyule municipality and is the preferred hydrological modelling software used by Melbourne Water. MiRORB (Mapinfo RORB) was utilised to develop the RORB models. MiRORB was developed by Melbourne Water to:

Draw RORB catchments using Mapinfo, allowing automatic generation of physical catchment data such as reach lengths and impervious fractions; then

Collate this data and create a catchment file, which can be used directly by RORB.

4.1 Sub-Catchment Data

Sub-catchment data is not directly utilised by TUFLOW. However, it is utilised indirectly to generate inflow hydrographs for each of the inlet pits along the pipe network. These hydrographs were generated from each of the RORB models and manipulated into a format able to be read by TUFLOW.

The procedures used for determining sub-catchment boundaries are discussed below.

4.1.1 Sub-catchment Boundary Delineation

Sub-catchment boundaries have been defined based on contours, pipe alignments, and property boundaries, not specifically for the 100 year overland flow path. Given that all council assets (except for minor rear of allotment/private drainage) within the municipality are modelled in the hydraulic models it is important that the catchment hydrology resolution is detailed enough to ensure accurate inflows at the pit level.

Figure 4.1 below shows an example of how the sub-catchments were drawn in MiRORB. The result is an accurate representation of the sub-catchment areas draining to a particular branch of the pipe network. Engeny believes that the sub-catchment delineation chosen is the best way to represent how flow gets from each property and road to the drainage pits. The flow from the pits, via pipes and/or overland flow can then best be modelled in the hydraulic model.



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Figure 4.1 Example of sub-catchment delineation

4.1.2 Fraction Impervious

Engeny assigned a fraction impervious value to RORB sub-catchments based on typical fraction impervious values for the planning zones within each sub-catchment. The typical values are based on fraction impervious provided in Melbourne Water’s MUSIC guidelines. Fraction impervious values applied for typical land uses within the catchment include:

Residential (mid-high density) - 70% impervious

Residential (low density) - 45% impervious

Open parklands & reserves - 10% impervious

Local roads & car parks - 60% impervious

Major roads - 70% impervious

Commercial & industrial - 90% impervious

Railway - 50% impervious

The fraction impervious value for a sub-catchment was obtained by calculating a weighted average of the fraction imperviousness of land types within the sub-catchment. These results were then checked using aerial photography and some adjustments made where necessary.



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4.2 Intensity-Frequency-Duration Data

Intensity-Frequency-Duration (IFD) data for the Banyule municipality was sourced from the Bureau of Meteorology using the online IFD request tool. The same IFD data was used for the hydrological modelling of each catchment. This methodology was adopted so that consistent design rainfall is applied to each catchment. There is only a marginal, and insignificant, variation in design rainfall if the centroid of each flood mapping catchment was used to determine design rainfall data.

This Bureau of Meteorology tool provided the IFD variables shown in Table 4-1. Table 4-1 IFD parameters for the Banyule Municipality

Parameter Value

Intensity - 1 hour duration, ARI = 2 years (2I1) 19.41






Skew (G) 0.35

F2 4.29

F50 14.97

4.3 Adopted RORB Model Parameters

The primary purpose of hydrological modelling is to determine sub-catchment inflow hydrographs for the hydraulic model. These hydrographs are obtained directly from the RORB sub-catchments, and are not influenced at all by routing along the reaches in the RORB models. All routing of flows has been accurately accounted for in the hydraulic models using the digital terrain model and surface roughness. Therefore, the RORB routing parameter kc has no impact on the flood mapping results. Engeny adopted a RORB kc value based on the MMBW/DVA plot of kc versus catchment area, based on previous calibrations, which is a Melbourne Water recognised method for determining kc. This method defines kc using the following relationship:

kc = 1.53 × A0.55.

Other key RORB parameters adopted in the models are (as per Melbourne Water Guidelines and Technical Specifications):

m = 0.8

Initial loss = 10 mm

Runoff coefficients:

100 year ARI runoff coefficient = 0.60



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5 year ARI runoff coefficient = 0.25

4.4 RORB Outputs

The key focus of the hydrological modelling was to ensure that the RORB models are delineated in such a way that will allow for good representation of inflows into the hydraulic models. The key outputs from the RORB models are the sub-catchment hydrographs, not the total flow from the downstream outlet of the RORB models.

4.5 External Melbourne Water RORB Models

There are a number of instances where flow originates outside of Banyule and flows through the municipality. Some of these catchments are only marginally beyond the municipal boundary and have been accounted for in the modelling undertaken by Engeny. However, there were also some instances where the external catchment area was significant and hydrological models were able to be obtained from Melbourne Water. Engeny obtained the hydrological models for the following catchments from Melbourne Water for use in the study:

Eltham West Catchment; and

Lillimur Avenue Drain Catchment.

These models were used to apply flow from the external catchments to the Banyule City Council catchment being modelled by Engeny.

For some of the major waterways that run along the edge of through the municipality, RORB models were unable to be obtained. These waterways include:

Yarra River;

Plenty River; and

Darebin Creek.

Section 5.5 provides further information on how the hydraulic model’s boundary conditions accounted for these waterways.



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5. HYDRAULIC MODELLING

5.1 Digital Terrain Model (DTM)

LiDAR (Light Detection And Ranging) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. LiDAR data for the study catchment consists of a regularly spaced grid (one metre in this case) of levels.

The digital data set was triangulated and processed to produce a surface grid for carrying out the hydraulic modelling in TUFLOW. The purpose of this grid was to enable spot levels to be allocated to points within the 2-D grid layer which is utilised directly by TUFLOW. Figure 5.1 shows the DTM generated for the extent of the 4 catchments (Appendix A provides a plan showing the DTM for the entire municipality). The orange areas indicate the greatest elevation whilst the blue areas designate the lower elevations.

A three metre grid size was chosen to accurately model surface flows in TUFLOW. Such a cell size proved to be sufficiently small to enable the effects of raised roadside verges and medians to be modelled while at the same time providing reasonable model run times.

5.2 Catchment Topography

As discussed in Section 3.2.2 Engeny believes that the LiDAR provides sufficient definition of the key topographical features in the catchment such as open channels, waterways and roadways. Development within the municipality over the last 4-5 years has had some impact on the topography used in the modelling such as the WaterMarc development on Grimshaw Street. In this instance the topography was modified based on Council design drawings to ensure that a kerb in front of the development was adequately defined in the hydraulic model given that it was not well represented in the LiDAR data. Other instances where minor modifications were required include:

Heidelberg Road where it crosses Darebin Creek. The LiDAR had picked up some road levels and as a result without adjustment flow within Darebin Creek was effectively being blocked at this point.

Bonds Road (south of Cleveland Avenue) where the LiDAR did not adequately pick up the invert of the creek. It was necessary to adjust the topography to ensure that the culvert crossing could discharge to the creek and have sufficient cover beneath Bonds Road.



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Figure 5.1 DEM Extent for 4 Catchments



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5.3 1-D Network Data

GIS data of the existing stormwater network was obtained from Banyule City Council. Verification and manipulation of this data was necessary to ensure consistency throughout the drainage network.

5.3.1 Pipe Sizes

As discussed in Section 3.2.4 missing pipe diameter data was resolved by Engeny providing Council with assistance to review design drawings and input pipe diameters into Council’s GIS system. Upon the completion of this process it was also necessary to use engineering judgement to fill some data gaps where the pipe was not critical and could be reasonably estimated. In some instances where this process was not able to resolve the remaining data gaps, Banyule City Council provided additional information or conducted field inspections.

A number of local detention systems were identified throughout the catchment. Generally, these detention systems consist of one or more large diameter pipes with a smaller pipe outlet and can store flow during large rainfall events. Plans for these systems were provided by Council and Engeny has included them in the hydraulic model.

5.3.2 Pipe Direction

It is necessary in a TUFLOW model that all pipes point in the direction of upstream to downstream. It was necessary to change the direction of some pipes to eliminate errors in the TUFLOW model. A MapBasic program assisted in this task. By highlighting an individual pipe the direction was able to be changed.

5.3.3 Snapping Pipes Together and Pits to Pipes

It is crucial to the accuracy of TUFLOW modelling that all pipes are linked to one another as well as being fully connected to the inlets. If gaps are present in the model data, stormwater flowing throughout the model network is able to exit the underground system and result in flooding in an area which otherwise may not be flood prone. A MapBasic program was used to eliminate these gaps and reduce possible errors in the modelling output.

5.3.4 Invert Level Generation

Information regarding the upstream and downstream invert levels of each pipe was not present throughout the entire pipe network. Therefore, it was necessary to artificially generate invert levels. The following formula was used to Engeny estimated inverts by adopting the following formula:

Invert level = Ground level RL – 600mm (pipe cover) – pipe diameter



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A MapBasic program was used to grade the drainage network such that there are no pipes within the network with negative grade.

Invert levels provided in Melbourne Water’s GIS for Melbourne Water assets within the Banyule municipality have been generally been adopted in the model, with some adjustments made to ensure connectivity with the LiDAR data (e.g. to allow water to discharge from a Melbourne Water main drain into an open channel, creek, or waterway).

5.3.5 Open Channels/Waterways

Within the Banyule municipality there a number of major waterways, these include:

Yarra River to the south;

Plenty River running north-south through the middle of the entire municipality;

Darebin Creek to the west; and

Salt Creek in the western portion of the municipality.

There are also a number of smaller open channels and minor waterways within the Banyule municipality including the Beatrix Street Drain, Watsonia Drain, and Banyule Drain.

Review of the LiDAR found that it provides a satisfactory definition of the waterways and as such it was determined that the waterways can be effectively modelled in the 2-D domain. Several culverts have been included in the model along the waterways at road crossings, with the culverts modelled as pipes in the 1-D domain.

5.3.6 Retarding Basins

Melbourne Water managed retarding basins within the Banyule municipality include the following:

Watsonia Drain Retarding Basin;

Parade College Sportsground Retarding Basin;

Yando Street Retarding Basin;

Briar Hill Retarding Basin; and

Salt Creek Retarding Basin.

There are some smaller Council managed retarding basins, dams and lakes throughout the municipality, including:



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Retarding Basin at AK Lines Reserve (corner of Grimshaw Street and Greensborough Bypass);

Para Road retarding basin;

Pecks Dam; and

Kalparrin Gardens.

LiDAR data, Melbourne Water and Council drainage information were reviewed to ensure that the retarding basins, dams and lakes were included and modelled accurately.

5.3.7 Drainage Inlets

Engeny has modelled pits at the junction of every pipe in the models. The models allow stormwater to overflow from these pits once the level in the pit exceeds the ground level.

While it is known that some pits are junction pits, and would not have an opening to the surface, it is a conservative approach to allow water to overflow from these pits as pressure build up underneath the pit lid is likely to dislodge the pit cover so that water can escape the drainage system.

All pits were assumed to be 900 mm by 1200 mm in size, with a 100 millimetre by 900 millimetre opening.

5.3.8 Pipe and Pits Losses

The 2010 release of TUFLOW includes a new way of computing pit losses. A manhole layer can be either automatically or manually created and used to apply the losses and nodes created in the one dimensional network layers in a variety of different ways. Engeny has used the automatically generated manhole layer, applying losses using the Englehund Method. This method recalculates losses at each time step using the angle of the entry and exit pipes, water levels and flow distributions. Engeny checked the losses calculated by this automatic approach to ensure they are reasonable and flow patterns have been checked to ensure that the pit losses have not resulted in any unexpected surcharges.

5.4 Surface Roughness

Within TUFLOW a land use (materials) layer was utilised to import surface roughness information into the model. A surface roughness has been assigned to each property polygon based on land use and aerial photography. In some instances, Engeny added additional shapes or split property polygons (such as parks with buildings and open space) in order to accurately model surface roughness.



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Table 5–1 provides the Manning’s ‘n’ roughness values applied across the Banyule municipality (based on Melbourne Water Guidelines and Technical Specifications, November 2012).

Table 5–1 Surface roughness values

Land Use Manning’s n

Mid-high density residential where the buildings and remainder of parcel are modelled together

0.35

Low density residential where the buildings and remainder of parcel are modelled together

0.2

Residential building footprints (where building footprint and remainder of parcel are modelled separately)

0.5

Residential remainder of parcel (where building footprint and remainder of parcel are modelled separately)

0.10

Parks or large properties with a small coverage of building/s 0.10

Commercial or industrial 0.5

Open paddock / parkland – minimal vegetation 0.035

Open paddock / parkland – moderate vegetation 0.06

Open paddock / parkland – high density vegetation 0.09

Railway line 0.12

Car parks and roads 0.03

Open waterways with moderate/reedy vegetation 0.06

5.5 Boundary Conditions

5.5.1 1-D Boundary Conditions

1-D boundary conditions are required at pipe outlets where drains discharge out of the catchment, generally into a neighbouring Council catchment. A head versus time (HT) boundary was applied to the downstream ends of the pipe network to represent the assumed tailwater level at this point of the model. The assumed tailwater levels adopted in the models are:

Pipes exiting catchments

5 year ARI events: tailwater level at obvert of downstream pipe; and 100 year ARI events: tailwater level at ground surface level.

1-D boundary conditions were required at pipe outlets where drains discharge into one of the major waterways for which Melbourne Water was unable to provide a hydrology model. As Melbourne Water could not provide a hydrology model for the major waterways, the flow within these waterways have not been included in the hydraulic



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model. Appropriate boundary conditions have been adopted in the hydraulic model to ensure that the tailwater effect of the waterways is applied to Council drainage.

Outlets to waterways

100 year ARI events – Top of bank level 5 year ARI events – 0.3m reduction in 100 year ARI flood level

The 1-D boundary condition layer was also used to read the RORB inflow hydrographs for each sub-catchment. The polygons created for the sub-catchments in the hydrological models have been included in the 1-D boundary layer, which allows for the inflow hydrograph to be split equally across each of the pits within a particular sub-catchment.

In some instances where there were no or very few inlet pits within a RORB sub-catchment, a 2-D source area was used to apply flows in the TUFLOW model. Section 5.5.3 provides further information on 2D source areas.

5.5.2 2-D Boundary Conditions

The TUFLOW models include a series of 2-D boundary conditions to control points where flow enters or leaves the 1-D pipe network. SX lines are drawn at locations where flow interacts between the 1-D network and the 2-D floodplain, with CN lines drawn to connect the SX lines to the 1-D network.

As part of the 1-D network, 2-D SX boundaries have been assigned to each pit to allow discharge of water from the pipe network to the 2-D surface and also allow runoff to enter the 1-D network if the pipe capacity allows it.

HQ (head versus flow) were drawn at the catchment boundaries to allow overland flow to leave the models at the catchment outlets.

5.5.3 2-D Source Areas

Some RORB sub-catchments used to define the hydrological model do not have any or have very few inlet pits. These include areas that are undeveloped open land such as parks. In these instances 2-D source areas were used to apply flow to the hydraulic models.

A 2-D source area is a polygon drawn within the sub-catchment where an inflow hydrograph is applied to the 2-D domain. Flow from the source area travels overland until it reaches the 1-D network, or may flow overland to the catchment outlet.

5.5.4 Initial Water Levels

Initial water levels were used near boundary conditions to specify a fixed water level, such as where a pipe exists a catchment In these cases the initial water level was set to the same level as the boundary condition (i.e. at the obvert of the pipe for 5 year ARI events



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and at surface level for 100 year ARI events). Applying initial water levels helps to prevent a “backflow wave” from the boundary condition which would fill up low lying sections of the drainage network near the boundary conditions, thereby reducing mass balance errors.

5.6 TUFLOW Parameters

5.6.1 Grid Size

A three metre grid size was chosen to accurately model surface flows in each catchment and to ensure consistency. Such a cell size proved to be sufficiently small to enable the effects of raised roadside verges and medians to be modelled while at the same time providing reasonable model run times. A 3m grid size is within Melbourne Water guidelines for the modelling of urban areas.

5.6.2 Time Steps

Melbourne Water guidelines recommend that the 2-D time step should generally be one quarter to half of the TUFLOW grid size. The majority of catchments fall within this range except for Salt Creek, Bundoora Drain, and South West catchments where steep terrain caused some fast flowing shallow flow and some 2-D error. Reducing the time step to one fifth of the grid size helped reduce the error to acceptable levels.

5.6.3 Modelled Storm Durations

Engeny modelled up to at least the 18 hour duration storm event for both the 5 year and 100 year ARI events for the majority of the catchments. The Salt Creek and Lower Plenty River catchments were modelled up to the 24 hour duration storm event given the larger catchment areas whilst the smaller catchments modelled as part of the four catchments study were modelled up to the 12 hour storm event. Appropriate checks were made to ensure that peak flood levels were captured for all catchments.

5.6.4 Model Log File

The TUFLOW log file provides a summary of key information while the model is running in order to assess the ‘health’ of a model. Two items that are reported in the log file are percentage error and change in volume of water in the model (dV).

Engeny have been able to keep the 2-D and cumulative errors within acceptable limits for all runs. The model log files show relatively smooth changes in volume over the simulation duration, which is a sign of a stable model as water is entering and leaving the model with few ‘wobbles’ or fluctuations.



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5.7 Model Validation

Engeny has validated the model by checking that flows and water depths produced by the TUFLOW model are reasonable. Any unexpectedly large or small flow results were investigated to understand whether or not they were reasonable.

Model result files such as the 1-D capacity check (ccA), time series (TS) and time series loss (TSL) have been investigated for a sample of runs from each ARI event. These files were used to check that pipes are flowing full in the 5 year event and if not flowing full then to confirm that the level of overland flow was minor. The pipe flows in the 100 year event were also checked to ensure that the network had been modelled correctly and that there were no ‘brick walls’ where pipes had not been correctly connected to the next pipe downstream.

Results were also checked to ensure that TUFLOW was not producing high velocities or depths where they are not expected.

The TUFLOW model was reviewed internally at different stages of its development using QA processes developed by Engeny to ensure that consistent best practice modelling has been applied and that the model is as accurate as reasonably possible.

Engeny has also discussed the results of the modelling with Banyule City Council at various stages throughout the study.



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6. FLOOD MAPPING

The 5 year and 100 year ARI events were selected for modelling. The 5 year ARI event was modelled to highlight the performance of the underground drainage network, which has typically been designed to cater for the peak 5 year ARI flow. The 100 year ARI event was modelled to assess the pattern of overland flooding.

6.1 Filtering of Flood Modelling Results

Filters have been applied to the raw TUFLOW results in order to produce the flood extents and maps for each of the catchments. The aim of the filters is to remove areas of non-critical inundation from the final flood extents. The applied filters are in accordance with Melbourne Water filtering guidelines and have been adopted by other local councils in Melbourne.

For each modelled scenario a flood extent has been produced, with data included only if it meets the following conditions:

Depth ≥ 0.05m or Velocity × Depth ≥ 0.008 m2/s

Areas of Flooding > 100m2 included in the flood extent

Surrounded dry islands < 100m2 included in the flood extent

Appendix B and Appendix C provide the flood inundation plans obtained by the hydraulic modelling conducted as part of this study. A0 plans of the flood inundation maps have also being provided to Council.

6.2 Results Analysis: Flooding ‘Hot Spots’

6.2.1 Identification of Hot Spots

Engeny and Council have worked together to identify hot spots throughout the municipality which correspond to under capacity Council drainage assets. As a general guide, an area is considered a flooding hot spot if there is in excess of 100 millimetres of overland flow for a 5 year ARI event, which is part of a connected and defined flow path impacting several properties.

6.2.2 Four Catchments Study (Including Irvine Road Drain Catchment)

The major flooding ‘hot spots’ within the 4 catchments (including Irvine Road Drain catchment) are identified and discussed in the Stormwater Management – Catchment Analysis: Mitigation Assessment report, a summary of these hot spots is provided below.

Lower Plenty Drain Catchment



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Byron Avenue; and Main Road.

Elmo Road Drain Catchment

Sherbourne Road; Baldwin Avenue (two mitigation options presented); and Calrossie Avenue.

Watsonia Drain Catchment

Warralong Avenue Harborne Street; Elder Avenue (two mitigation options presented); and Cooinda Crescent

Beatrix Street Drain Catchment

De Blonay Crescent; Nell Street; Medbury Avenue; and Lyell Parade to Henry Street.

Irvine Road Drain Catchment

Thoresby Grove; Maltravers Road; and Wilfred Road

6.2.3 South West Catchments

The following areas have been identified as being at risk of flooding within the South West Catchments for a 5 year ARI event:

Swanston Street

- Engeny have assisted Council in assessing mitigation options in this area, an overland flow path with a depth in excess of 250 millimetres is predicted within a residential property.

Ponding in excess of 700 millimetres predicted within Liberty Parade directly south of Southern road with inundation of residential properties downstream of ponded area.

Miller Street and Alfred Street, an overland flow path up to 300 millimetres deep is predicted within several residential properties between Bell Street and St Hellier Street.



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Waterdale Road near the intersection Malahang Road, ponding in excess of 300mm within the road reserve is predicted causing adjacent properties to be inundated.

6.2.4 Salt Creek Catchment

The following areas have been identified as being at risk of flooding within the Salt Creek Catchment for a 5 year ARI event:

Somers Avenue and Stewart Terrace, an overland flow path up to 300 millimetres deep is predicted within residential properties west of Hinkler Avenue.

Carwarp Street and Ferguson Street, an overland flow path up to 300 millimetres deep is predicted within residential properties north of Chapman Street.

Rosemar Circuit and Ironbark Street an overland flow path with a depth in excess of 500 millimetres is predicted within residential properties directly south of Northwood Drive.

Rutherford Road and Diane Crescent, an overland flow path with a depth in excess of 200 millimetres is predicted within residential properties south of Warren Road. Residential properties downstream of this location are also at risk of flooding due to accumulation of runoff along this flow path.

Kathleen Street and Rosanna Road, an overland flow path with a depth in excess of 200 millimetres is predicted, resulting in a ponded depth of over 500 millimetres within residential properties north of St James Road.

Durham Street and Devon Street, an overland flow path with a depth in excess of 500 millimetres is predicted within residential properties west of Lower Heidelberg Road.

Cleve Grove and Bronte Street, on overland flow path with a depth in excess of 300 millimetres is predicted within residential properties.

Whilst there are other areas within this large catchment at risk of flooding within a 5 year ARI event the list of areas above highlight the higher priority Council drainage areas.

6.2.5 Bundoora Drains Catchment

The following areas have been identified as being at risk of flooding with the Bundoora Drains Catchment for a 5 year ARI event:

Ponding in excess of 350 mm is predicted within Cameron Parade east of the intersection with Sandhurst Crescent.

Grimshaw Street and Noorong Avenue, an overland flow path between these roads with a depth in excess of to 400 millimetres is predicted within residential properties



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east of Balaka Place. There is also a smaller overland flow path to the west of Balaka Place.

Ponding in excess of 550 millimetres predicted within Flannery Avenue directly east of Noorong Avenue.

Decathlon Street and Morwell Avenue, an overland flow path between these roads with a depth in excess of 200 millimetres is predicted within residential properties.

6.2.6 St Helena Drains Catchment

The following areas have been identified as being at risk of flooding with the St Helena Drains Catchment for a 5 year ARI event:

Intersection of St Helena Road and Mountain View Road, an overland flow path up to 400 millimetres is predicted within residential properties adjacent to Mountain View road.

St Helena Road and Marden Drive, flooding in excess of 500 millimetres is predicted within residential properties north of Marden Drive.

Nulgarrah Crescent, ponding in excess of 500mm within the road reserve with residential properties downstream through to St Helena Road impacted.

Child care centre on the corner of St Helena Road and Parkland Avenue is affected by an overland flow path up to 400mm deep.

6.2.7 Eltham Drains Catchment

The following areas have been identified as being at risk of flooding with the Eltham Drains Catchment for a 5 year ARI event:

West of Intersection between Bolton Street and Sackville Street, an overland flow path with a depth of over 300 millimetres is predicted. This flow path runs parallel to Bolton Street within residential properties.

Reichelt Avenue and Allens Road, an overland flow path between these two roads with a depth in excess of 400 millimetres within residential properties.

6.2.8 Upper Plenty River Catchment

The following areas have been identified as being at risk of flooding with the Upper Plenty River Catchment for a 5 year ARI event:

Weatherlake Street and Rushworth Street, flooding in excess of 350 millimetres is predicted within residential properties east of Watsonia Road.



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Reeves Street and Meagher Street, flooding in excess of 200 millimetres within residential properties both north of Nell Street.

6.2.9 Lower Plenty River Catchment

The following areas have been identified as being at risk of flooding in the Lower Plenty River Catchment for a 5 year ARI event:

Alexander Street and Para Road, an overland flow path with a depth in excess of 400 millimetres is predicted resulting in a ponded depth of 800 millimetres directly south of Rattray Road.

Mayona Road and Starling Street, an overland flow path with a depth in excess of 500 millimetres is predicted, resulting in a ponded depth of 1.45 metres directly south of Sherbourne Road.

Intersection of Castleton Road and Sherlowe Crescent, an overland flow path with a depth in excess of 300 millimetres is predicted within a residential property south of Castleton Road.

6.3 Comparison to Melbourne Water Flood Extents

Engeny compared the results of the current study to flood extends provided by Melbourne Water from previous studies conducted on their assets within the municipality.

Figure 6.1 show a comparison of the 100 year ARI flood extents within the St Helena Drains catchment and Figure 6.2 shows a comparison within the Watsonia Drain Catchment at the Watsonia Drain Retarding Basin. In each figure, the Melbourne Water flood extent is shown as a black polygon, with the flood extent from the current study contoured by depth as shown in the legend. It is important to note that the Melbourne Water studies included Melbourne Water assets only with no Council drainage assets modelled.



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Figure 6.1 St Helena Drains 100 year ARI flood extent comparison



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Figure 6.2 Andersons Creek 100 year ARI flood extent comparison at the North Ringwood RB

The flood extents from the two sources show a good level of correlation and give confidence in the accuracy of the current study’s outputs.

6.4 Flood Mitigation

Refer to Stormwater Management – Catchment Analysis: Mitigation Assessment report provided to Council.



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7. FUTURE DEVELOPMENT & PLANNING OPPORTUNITIES

7.1 Opportunities for Stormwater Detention

Stormwater detention (or retarding) is used to temporarily store floodwaters during a storm event and to release the flow at a rate that protects downstream areas from high peak flows.

Available opportunities for stormwater detention within the study area for the original scope (four catchments) have been investigated. Given the relatively high level of development across the area and relatively small catchment areas, opportunities are limited, especially areas that will be provide a measurable difference. The areas identified for stormwater detention are:

Greensborough College, Nepean Street (Beatrix Street Drain catchment);

Porter Street Reserve (Elmo Road Drain catchment);

Elder Street Reserve (Watsonia Drain catchment);

Expansion of Watsonia Drain retarding basin (Melbourne Water asset, Watsonia Drain catchment);

Cheverton Road Reserve (Lower Plenty Drain catchment); and

3 & 4 Kett Street (existing residential properties identified by Council within the Lower Plenty Drain catchment).

7.2 Opportunities for Stormwater Treatment

Available opportunities for stormwater treatment within the four catchments have also been assessed. Stormwater treatment within the catchments could include the following:

Wetlands. Wetland treatment areas should be considered during further investigation of the areas available for stormwater detention. It is efficient to combine detention and wetland within the same area to achieve mitigation and water quality objectives.

Raingardens and/or bioretention swales within road reserves. When a flood mitigation project involves the disturbance of the road reserve there is a great opportunity to consider the use of raingardens within the reinstated road reserve. Figure 7.2 shows an area where Melbourne Water have implemented water sensitive urban design street scaping for the dual purpose of providing detention during low flow events and stormwater treatment.



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Figure 7.1 Example of Water Sensitive Urban Design street scaping

Bioretention swales within car parks. An example of a bioretention swale within a car park is shown in Figure 7.1 below. The Diamond Village Shopping Centre or the commercial precinct on the corner of Watsonia Road and Morwell Avenue, both within the Watsonia Drain Catchment are areas where this could be implemented.



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Figure 7.2 Example of bioretention swale within car park

Gross Pollutant traps are designed to capture and retain gross pollutants, litter, grit, sediments and associated oils. Potential locations identified for GPTs and/or litter traps are:

Henry Street drainage outfall to Beatrix Street Drain (Beatrix Street Drain catchment).

Upstream of Para Road retarding basin (Lower Plenty Drain catchment). Commercial precinct west of Greensborough Bypass (Watsonia Drain catchment),

within commercial areas the gross pollutant and sediment load is highest posing risk to blockage of downstream drainage assets if they are collected.

Sherbourne Road outfall to Eltham West Drain (Elmo Road catchment).

7.2.1 Pipe Cleansing Issues

Engeny investigated areas within the Council drainage network where cleansing of the network is an issue and subsequently where the risk of pipe blockage is highest. A pipe with a velocity of less than 0.6m/s is considered to be at risk of blockage. In all instances where pipes in the drainage network area at risk of blockage the following is evident:

There is only a small upstream catchment area. This results in small flow rates arriving at the upstream sections of drainage presenting a risk of blockage during frequent and low intensity rainfall events. Areas of high tree coverage exacerbate this problem with leaves collecting in drainage inlets and pipes.



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Small pipe diameter. This presents a risk of blockage particularly for diameters 300mm or less where the pipe flow and subsequently velocity is low, particularly during frequent and low intensity rainfall events.

Blockage risk is highest at ‘sag’ locations within a drainage catchment. In urban areas this is a significant risk at these points in the drainage network given that there is no opportunity for runoff to bypass. If the drainage network is blocked or under capacity overland flow will pond in these locations and potentially overtop into adjacent properties.

Results from the hydraulic modelling can be used to help Council develop a maintenance program for those areas where cleansing issues and blockage risks are highest across the municipality.



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8. SUMMARY AND CONCLUSIONS

Engeny has completed the following work for Banyule City Council across the Banyule municipality:

Data collection (Council’s GIS system now contains pipe diameter information for every pipe except for minor rear of allotment/private drains);

Detailed hydrology analysis to estimate flows in the catchment for 5 year and 100 year ARI rainfall events for existing catchment conditions;

2-D flood modelling for 5 year and 100 year ARI rainfall events for existing catchment conditions;

Flood mapping of the 2D modelling results, including application of data filtering (to Melbourne Water specifications);

Flood risk analysis over the entire municipality; and

Compilation and supply of results in GIS format.

Based on the full range of flood mapping results produced in this study, Engeny identified areas within the Banyule municipality that are flood prone (refer to Section 6.2).

Overall, Engeny considers the Banyule flood models provide a good representation of the hydraulic behaviour across the municipality.

8.1 Future Works

The information contained in this report and the modelling results will assist Council to develop flood mitigation priorities across the catchment. Engeny has developed a mitigation priority for the following catchments:

Lower Plenty Drain;

Elmo Road Drain;

Watsonia Drain;

Beatrix Street Drain; and

Irvine Road Drain.

Similar work should be undertaken across the remainder of the municipality to encompass all catchments and prioritise all mitigation works across the municipality.



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In many instances where properties are affected by flood water, capital works may not be justified. The main reason for this is that excessive capital upgrades to alleviate flooding for a small number of properties would result in an unfavourable cost-benefit ratio and could be better managed by non-structural means. In these instances, Engeny recommends that development be controlled through the use of planning overlays.

Special Building Overlays (SBOs) are appropriate for identifying overland flow paths for 100 year ARI storms along drainage lines in urban areas. Use of SBOs across the entire the municipality is recommended to control future development and to reduce the flood risk for new buildings. The use of SBOs are considered the highest priority as they do not have any capital cost and will overall result in the most effective measure across the municipality.



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9. QUALIFICATIONS

a. In preparing this document, including all relevant calculation and modelling, Engeny Management Pty Ltd (Engeny) has exercised the degree of skill, care and diligence normally exercised by members of the engineering profession and has acted in accordance with accepted practices of engineering principles.

b. Engeny has used reasonable endeavours to inform itself of the parameters and

requirements of the project and has taken reasonable steps to ensure that the works and document is as accurate and comprehensive as possible given the information upon which it has been based including information that may have been provided or obtained by any third party or external sources which has not been independently verified.

c. Engeny reserves the right to review and amend any aspect of the works performed

including any opinions and recommendations from the works included or referred to in the works if:

(i) additional sources of information not presently available (for whatever reason)

are provided or become known to Engeny; or

(ii) Engeny considers it prudent to revise any aspect of the works in light of any information which becomes known to it after the date of submission.

d. Engeny does not give any warranty nor accept any liability in relation to the completeness or accuracy of the works, which may be inherently reliant upon the completeness and accuracy of the input data and the agreed scope of works. All limitations of liability shall apply for the benefit of the employees, agents and representatives of Engeny to the same extent that they apply for the benefit of Engeny.

e. This document is for the use of the party to whom it is addressed and for no other

persons. No responsibility is accepted to any third party for the whole or part of the contents of this report.

f. If any claim or demand is made by any person against Engeny on the basis of

detriment sustained or alleged to have been sustained as a result of reliance upon the report or information therein, Engeny will rely upon this provision as a defence to any such claim or demand.

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