Report - · Model build 2016-03-AC-iDP-w64 Survey LiDAR (2007) Survey Data (’11 Acre’ Site) Upstream boundary Rain-on-Grid Rainfall data (‘2d_rf’ layer with 15 mm initial

Final Report

Logans Beach Strategic Framework Plan Flood Modelling

Warrnambool City Council

June 2017

Warrnambool City Council | June 2017 Logans Beach Strategic Framework Plan Flood Modelling

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Document Status

Version Doc type Reviewed by Approved by Date issued

01 Draft ADV ADV 14/03/2017

02 Final ADV ADV 23/06/2017

Project Details

Project Name Logans Beach Strategic Framework Plan Flood Modelling

Client Warrnambool City Council

Client Project Manager Julie Glass

Water Technology Project Manager Bertrand Salmi

Water Technology Project Director Luke Cunningham

Authors Bertrand F. Salmi

Document Number 4710-01_R02V01

COPYRIGHT

Water Technology Pty Ltd has produced this document in accordance with instructions from Warrnambool City Council 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 Drive

Notting Hill VIC 3168

Telephone (03) 8526 0800

Fax (03) 9558 9365

ACN 093 377 283

ABN 60 093 377 283


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CONTENTS

1 INTRODUCTION 5

1.1 Objectives 5

2 CATCHMENT DESCRIPTION 6

3 METHODOLOGY 9

3.1 Hopkins River and Coastal Interfaces 10

3.2 Climate Change Scenario 11

3.2.1 Hopkins River and Coastal Interfaces 11

3.3 Development Scenario 11

4 RESULTS 14

4.1 Existing Conditions 14

4.1.1 Minor Flood Event 14

4.1.2 Major Flood Event 16

4.2 Climate Change Scenario 18

4.3 Development Scenarios 21

4.3.1 Limitations of Model 25

5 DISCUSSIONS 26

5.1 Infrastructure Requirements 26

5.1.1 Rural Living Areas 26

5.1.2 Coastal Zones 27

5.1.3 Funding 27

5.2 Non-structure Controls 28

5.3 Acid Sulfate Soils 28

6 SUMMARY 31

7 REFERENCES 32


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APPENDICES Appendix A - Development Scenario (500 Lots)

Appendix B - Maps – Flood Velocities

LIST OF FIGURES Figure 2-1 Subject Site (source: Warrnambool City Council) 6

Figure 2-2 Site Topography 7

Figure 2-3 Existing Drainage Infrastructure 8

Figure 3-1 Model Extent and Hydraulic Roughness 10

Figure 3-2 Indicative Development Precincts Yield Plan (Source: Insight Planning). 12

Figure 4-1 Existing Flood Depths for the 5 year ARI Critical Event 14

Figure 4-2 Existing Flood Hazard for the 5 year ARI Critical Event 16

Figure 4-3 Existing Flood Depths for the 100 year ARI Critical Event 17

Figure 4-4 Existing Flood Hazard for the 100 year ARI Critical Event 18

Figure 4-5 Flood Depths for the 100 year ARI Critical Event with Climate Change 19

Figure 4-6 Difference in 100 year ARI Flood Depths between Existing and Climate Change Scenarios 20

Figure 4-7 Flood Hazard for the 100 year ARI Critical Event with Climate Change 21

Figure 4-8 Flood Depths for the 100 year ARI Development Scenario (500 lots). 22

Figure 4-9 Difference in 100 year ARI Flood Levels between Existing and ‘Maximum’ Yield Scenarios (500 lots) based on Critical Flood Maps 23

Figure 4-10 Flood Hazard for the 100 year ARI Development Scenario (500 lots) 24

Figure 4-11 Maximum Yield Plan (source: Niche Planning Studio) 25

Figure 5-1 Hot Spots 26

Figure 5-2 Potential Coastal Acid Sulfate Soils (source: Ground Science, 2017) 29

LIST OF TABLES Table 3-1 –Baseline Scenarios Key TUFLOW Modelling Information 9

Table 3-2 – Development Scenarios - Key Modelling Information 13

Table 4-1 – Flood Safety Hazard Risk 15


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1 INTRODUCTION Warrnambool City Council is developing a Strategic Framework Plan for the Logans Beach coastal area in

Warrnambool, Victoria. Warrnambool City Council Commissioned Insight Planning to lead a multi-disciplinary

team to assist decision-makers in considering potential impacts of planning decision. The team included traffic

engineers, environmental scientists and planning consultants. Water Technology was commissioned to

undertake a hydraulic study of the catchment and inform the Strategic Framework Plan in respect to flood

risks.

The following report presents the result of the hydraulic modelling undertaken to inform the Strategic

Framework Plan, including flood depths, water surface elevations and flood hazards. The study considers flood

risk for existing, climate change and developed flood conditions.

1.1 Objectives

The objective of the hydraulic study was to:

Identify opportunities for stormwater management in the Logans Beach Study Area;

Assess existing and future flood risks; and

Inform adaptation strategy and Strategic Framework Plan.


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2 CATCHMENT DESCRIPTION The Logans Beach coastal area is located to the east and south of the Hopkins River in Warrnambool, as

shown in Figure 2-1. The area is predominantly zoned Rural Living (RLZ) to the south of Hopkins Point Road

and includes land zoned Public Park and Recreation Zone (PPRZ) and Public Conservation and Recreation

Zone (PCRZ). Morang Estate (an area zoned Rural Living) and Riverview Terrace (zoned General Residential

Zone) are located north of Hopkins Point Road and abut Hopkins River.

The area is also a popular tourist spot because the Logans Beach whale watching platform is located in the

catchment.

Figure 2-1 Subject Site (source: Warrnambool City Council)


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The Coastal / Hopkins River Environment growth area is located east of the Logans Beach coastal area and

was rezoned in 2014 to allow residential development at conventional densities. These areas have recently

been subdivided and new road infrastructure has recently been constructed.

Hopkins Point Road is located through the catchment. The subject area is about 300 ha in size and Figure 2-2

below shows the site topography. The catchment is general steep, with surface levels ranging from about

65 m AHD down to sea levels. The Logans Beach Study Area generally drains to Hopkins River to the west,

via the low-lying area immediately south of Hopkins Point Road, or the sea to the south.

Figure 2-2 Site Topography

The current drainage regime for the catchment will be via underground pipes, infiltration (including via

soakaways), and overland flow paths for excess stormwater runoff. There is limited existing Council drainage

infrastructure located in the catchment, except along Riverview Terrace, Hopkins Point Road and Logans

Beach Road, as shown in Figure 2-3.

Low lying area (includes “11 Acres” site)


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Figure 2-3 Existing Drainage Infrastructure


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3 METHODOLOGY A hydraulic model (TUFLOW) of the site was constructed to model overland flooding under existing conditions.

TUFLOW is widely used software that is suitable for the analysis of overland flows in urban areas. The

TUFLOW model routes flows overland across the topographic surface (2D Domain) to create flood extents,

depths and velocities.

Table 3-1 below shows key modelling information used in the development of this hydraulic model, including:

• Topography data;

• Manning’s roughness;

• Hydrological input (upstream boundary conditions); and

• Drainage Infrastructure.

Table 3-1 –Baseline Scenarios Key TUFLOW Modelling Information

Model type TUFLOW 2D

Model build 2016-03-AC-iDP-w64

Survey LiDAR (2007)

Survey Data (’11 Acre’ Site)

Upstream boundary Rain-on-Grid Rainfall data (‘2d_rf’ layer with 15 mm initial loss) for event duration between 1 hr and 18 hr. All routing, including constant loss, is completed in the hydraulic model.

Downstream boundary ‘HQ’ boundaries for the east boundary

Constant height boundary ‘HT’ at interfaces with Hopkins River and Warrnambool Bay.

‘QT’ for modelled soak wells (‘1d_bc’ layer)

Roughness parameters Roughness (manning ‘n’ values) was based on planning zone types and was validated using recent aerial imagery within the Logans Beach Study Area

• Roads/car parks - 0.02;

• Rural Living Areas residential – Between 0.05 and 0.35 dependent on dwelling density;

• Drainage Easement – 0.05;

• Open Space – between 0.04 and 0.09 dependent on vegetation.

Model timestep (2d) 0.5 seconds


Critical Duration

(predominantly)

12 hour for Existing Scenarios (5yr and 100yr ARI)

18 hour for Climate Change Scenario (100yr ARI)

Model grid size 3 m x 3 m

Final cumulative mass error <0.32% Existing Scenario (100yr)

<0.10% Climate Change Scenario (100yr)

<1.04% Existing Scenario (5yr)


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The model extent for the catchment is shown in Figure 3-1. It should be noted that Hopkins Point Road Bridge

is not captured in the LiDAR used for this model. As a result, it is shown to be inundated in the results, however,

levels of the bridge mean that it is flood-free during the modelled design events.

Figure 3-1 Model Extent and Hydraulic Roughness

3.1 Hopkins River and Coastal Interfaces

The western, northern and southern downstream ends of the model are located near the interface between

the catchment and Hopkins River, and the coastal environment (i.e. Warrnambool Bay). A constant HT

boundary was applied based on the 100 year ARI (1% AEP) design tidal levels estimated for Warrnambool;

CSIRO has estimated a 100 year ARI design tidal levels of 1.06 m AHD (CSIRO, 2009). A similar boundary

condition was adopted for all scenarios (including the 5 year ARI scenarios), except for the climate change

scenario.


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3.2 Climate Change Scenario

A 12% increase to rainfall intensity was adopted for 100 year ARI event with climate change. This is the same

increase used by the Engeny-led hydraulic study of other parts of the municipality1 and may facilitate

comparison of risks between results (although modelling methodology may differ). Most of the Representative

Concentration Pathways (RCP) scenarios concur that the total warming will be in the “Hotter” range of 1.5-3

degrees Celsius with a median value of 2.25 degrees Celsius for the Southern Slopes Mainland Cluster. We

understand that, consequently, a 5% increase was applied by Engeny to the 1987 AR&R rainfall patterns for

each predicted degree of warming.

3.2.1 Hopkins River and Coastal Interfaces

A constant HT boundary of 1.88 m AHD was applied for the 2010 climate change, based on the IPCC 2007

A1FI scenario which predicts a sea level rise of 0.82 m (CSIRO, 2009)

3.3 Development Scenario

Following validation of the baseline (existing conditions) model, the TUFLOW model was adjusted to reflect

potential development scenarios for the 100 year ARI event (maximum of two storm durations) with smaller

‘low density’ lot sizes. Following discussion with Council, Insight Planning and Council agreed on a ‘maximum’

yield potential for the precinct that would result in a total yield of 700 lots in the subject area. An indicative

development plan is shown in Figure 3-2.

1 Email correspondence from Glenn Ottrey (Engeny) dated 8 December 2016.


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Figure 3-2 Indicative Development Precincts Yield Plan (Source: Insight Planning).

Two development scenarios were modelled:

A ‘maximum’ yield potential of the precinct resulting in 700 lots; and

A yield potential resulting in 500 lots.

Table 3-2 below shows key modelling information used in the development of this hydraulic model.


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Table 3-2 – Development Scenarios - Key Modelling Information

Model type TUFLOW 2D

Model build 2016-03-AC-iDP-w64

Survey LiDAR (2007)

Survey Data (’11 Acre’ Site)

Upstream boundary Rain-on-Grid Rainfall data (‘2d_rf’ layer)

Fraction impervious increased to reflect proposed yield

Initial Loss reduced to 10 mm (from 15 mm) for event duration between 12 hr and 18 hr.

Downstream boundary ‘HQ’ boundaries for the east boundary

Constant height boundary ‘HT’ at interfaces with Hopkins River and Warrnambool Bay.

‘QT’ for modelled soak wells (‘1d_bc’ layer)

Roughness parameters Roughness (manning ‘n’) values were updated to reflect the increase in density;

• 0.35 for lots smaller than or equal to 500m2; and

• 0.15 for lots greater than 500m2.



Critical Duration

(predominantly)

18 hours for Development Scenarios (100yr ARI)

Model grid size 3 m x 3 m

Final cumulative mass error <0.29% 500 lots Development Scenario

<0.29% 700 lots Development Scenario


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4 RESULTS Detailed TUFLOW modelling was completed for the site, for existing and future conditions, including climate

change scenario and development scenarios. The flood modelling results (filtered to exclude shallow flood

depth and isolated ponding areas less than 100 m2) are discussed in this section.

4.1 Existing Conditions

4.1.1 Minor Flood Event

The catchment is shown to suffer from localised flooding during the 5 year ARI event, as shown in Figure 4-1.

In the steeper section of the catchment, flooding is confided within well-defined overland flow paths. Ponding

occurs north of the dunes located along the south, and the low-lying areas located between Logans Beach

Road and Hopkins Point Road.

As an example, most of the “11 Acres” property is flood-prone, with overbank flood depths exceeding 500 mm.

The property at 1 Logans Beach Road, provides significant flood storage and has flood depth of about 350 mm.

The adjacent property at 5 Hopkins Point Road (i.e. “11 Acres” is also flood-prone, however, most of the

floodwater on this property is conveyed and contained within the existing channel.

Figure 4-1 Existing Flood Depths for the 5 year ARI Critical Event


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We noted during our site inspection that the recent development at 1 Logans Beach and associated infill may

have disrupted existing conveyance channel and reduce storage capacity at the site. This may have been

avoided with stronger planning controls in place, as discussed in Section 5.2.

Levels in Hopkins River influences flooding in the catchment. Thus, the critical flood extent in the low-lying

areas of the catchment was estimated to be 12 hours.

Flood hazard is generally assessed in terms of flood depth and flood velocity. The product of flood depth and flood velocity is often referred to as the flood hazard, with flood depth also being considered to drive the hazard if above the velocity and depth product. Melbourne Water Flood Mapping Technical Specifications (2012) classifies flood hazard as shown in Table 4-1.

Table 4-1 – Flood Safety Hazard Risk

Hazard Category Velocity x Depth Criteria (m2/s) Depth Criteria (m)

High Risk > 0.84 > 0.84

Moderate to High Risk 0.60 – 0.84 0.60 – 0.84

Moderate Risk 0.40 – 0.60 0.40 – 0.60

Low to Moderate Risk 0.20 – 0.40 0.20 – 0.40

Low Risk < 0.20 < 0.20

As flooding is limited during the 5 year ARI event, the flood hazards are generally between Low to Moderate,

as shown in Figure 4-2. Hazard are higher in the low-lying areas and localised depressions, with some areas

rated as High. Roads within the catchment are all considered to have a Low or Low to Moderate Hazard rating

and egress requirements for the area are met during minor flood events.


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Figure 4-2 Existing Flood Hazard for the 5 year ARI Critical Event

4.1.2 Major Flood Event

Like the 5 year ARI event, the catchment is shown to suffer from localised flooding during the 100 year ARI

event, as shown in Figure 4-3. As expected, the larger flows and volumes (relative to the 5 year ARI event),

result in a larger flood extent, with flood depths exceeding 1 m in spots.

In the steeper section of the catchment, flooding is confided within well-defined overland flow paths. Ponding

occurs north of the dunes (located along the south) and in the low-lying areas located between Logans Beach

Road and Hopkins Point Road.

As an example, most the “11 Acres” property is flood-prone, with overbank flood depths exceeding 500 mm.

The adjacent site (1 Logans Beach Road) also provides significant flood storage and has flood depths

exceeding 1.0 m.


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Figure 4-3 Existing Flood Depths for the 100 year ARI Critical Event

In the 100 year ARI event, levels in Hopkins River influence flood depths in the catchment and therefore the

critical flood extent in the low-lying areas of the catchment was estimated to be 12 hours.

Similar to the 5 year ARI event, hazard is generally considered between Low to Moderate, as shown in Figure

4-4. Hazard are higher in the low-lying areas and localised depressions, with some areas rated as High. Roads

within the catchment are generally considered to have a Low or Low to Moderate Hazard rating. Safety

requirements for the main roads are met during minor flood events. Flood depths towards the south-western

section of Henderson Way and sections of Logans Beach Road exceed 500 mm, and this may result in access

difficulty to local properties.

Henderson Way

Logans Beach Road


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Figure 4-4 Existing Flood Hazard for the 100 year ARI Critical Event

4.2 Climate Change Scenario

The flood mechanism in the 100 year ARI event, considering climate change (Figure 4-5), is comparable to

the existing 100 year ARI results. The higher water levels in Hopkins River and Warrnambool Bay result in

greater flood depths in the low-lying areas and localised depressions. The flood extents increase slightly,

however overbank flood depths across these properties increase markedly. Flood depths exceed 1 m at the

low-lying area south of Hopkins Point Road, as shown in Figure 4-5. Critical flood depths increase by up to

250 mm in areas, as shown in Figure 4-6.


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Figure 4-5 Flood Depths for the 100 year ARI Critical Event with Climate Change


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Figure 4-6 Difference in 100 year ARI Flood Depths between Existing and Climate Change Scenarios

The 18 hour duration result in the greatest flood risk in the catchment, increasing the-risk period and volume

of floodwaters during major flood events.

The flood risks (Figure 4-7) in the climate change 100 year ARI event, are similar to existing conditions

100 year ARI event, with High Hazards at the following locations:

Low lying properties south of Hopkins Point Road;

Eastern section of Logans Beach Road; and

South-western end of Henderson Way.


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Figure 4-7 Flood Hazard for the 100 year ARI Critical Event with Climate Change

4.3 Development Scenarios

Development within the Logans Beach coastal area is currently being guided by the Logans Beach Local Plan

1998 (Local Plan). The Local Plan includes a minimum lot size of 6,000 m2 and an average of 10,000 m2 for

multiple lot subdivisions (implemented within the schedule to the Rural Living Zone). The coastal areas

currently zoned Public Park and Recreation Zone (PPRZ) and Public Conservation and Recreation Zone

(PCRZ) would remain protected and unchanged.

A ‘maximum’ yield scenario (500 lots) modelled as part of this study would result in:

Approximately 50 new lots in the Motang Estate, immediately north of Hopkins Point Road;

500 m2 lots at the western end of Logans Beach Road;

2,000 m2 lots at the southern end of Blue Hole Road; and

1,000 m2 lots elsewhere in the catchment.

Results from the second development scenario (with a maximum of 700 lots) are shown in Appendix A.

The 100 year ARI flood depths for the maximum yield development scenario are shown in Figure 4-8. The

increase in impervious areas result in a greater volume of stormwater runoff within the catchment. The impact

is generally marginal in the steeper sections of the catchment, where flooding is confided within well-defined


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overland flow paths. As with the climate change scenario, the modelled developed scenario results in greater

flood depths in the low-lying areas and localised depressions, as shown in Figure 4-9. Flood depths increase

by up to 200 mm in areas.

Figure 4-8 Flood Depths for the 100 year ARI Development Scenario (500 lots).


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Figure 4-9 Difference in 100 year ARI Flood Levels between Existing and ‘Maximum’ Yield Scenarios

(500 lots) based on Critical Flood Maps

The 18 hour duration result in the greatest flood risk in the catchment, increasing the-risk period and volume

of floodwaters during major flood events.

Flood risks in the modelled 500 lot development scenario (Figure 4-10) are similar to that in the existing

100 year ARI event, with High Hazards at:


Eastern section of Logans Beach Road; and



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Figure 4-10 Flood Hazard for the 100 year ARI Development Scenario (500 lots)

This development scenario was run as a sensitivity. It should be noted that the capacity of the existing Hopkins

River Bridge may limit yield in the study area to 450 dwellings (Traffix Group, 2017), when considering the

planned growth in the Coastal Hopkins Environmental Growth Area. Other constraints further reduce potential

development in the area. As a result, the maximum yield for the catchment, shown in Figure 4-11, only allows

for between 172 and 197 additional lots within Logans Beach coastal area. A lower yield may reduce the

overall increase in flooding, however, low-lying areas are likely to be further at risk.


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Figure 4-11 Maximum Yield Plan (source: Niche Planning Studio)

4.3.1 Limitations of Model

The flood extent in the steeper parts of the catchment is shown to be reduced compared to the baseline

scenarios. This is likely to be the result of only the 12 hour and 18 hour having been run rather than a smaller

flood extent. Shorter duration events are likely to be the critical event in steeper sections of the catchments.

Our development scenario models allowed for changes in roughness coefficient and stormwater generation

associated with increase in imperviousness. It must be noted however, that there is no allowance for local road

or additional drainage infrastructure in these models. Roads are likely to be designed to convey the major flow

in new subdivision, whilst underground pipes or open drains would convey some flows even during extreme

rainfall events.

It is considered that overland flow paths may change due to development, especially as roads would act as

preferential flow routes. The stormwater volume however, is likely to be a critical factor influencing flood depth

and risk in the low-lying areas of the catchment. Although a high-level assessment, the modelling is considered

sufficient to support decision-making at a planning stage. A more detailed model may be required once the

road and drainage infrastructure have been designed.


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5 DISCUSSIONS This section discusses possible implications re: drainage to:

Mitigate existing drainage hot spots;

Alleviate future pressure on existing systems resulting from development; and

Minimise impact on coastal acid sulfate soils, once susceptible areas have been identified.

5.1 Infrastructure Requirements

5.1.1 Rural Living Areas

The steep nature of the majority of the catchment results in well-defined overland flow paths. Additionally, the

existing pipe system appear to provide sufficient capacity to convey minor flows (1 in 5 year ARI flows). The

model results indicate that there are a number of hot spots within the subject area (shown in Figure 5-1),

generally associated with low lying areas and depressions. One of the flood constraints is that levels in Hopkins

River, and therefore the bay, are likely to influence drainage.

Figure 5-1 Hot Spots

Logans Beach Road

Logans Beach Road (east)

Logans Beach Road

Low lying areas or depressions


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Logans Beach Road is badly flooded in a couple of locations, which may render access difficult during flood

events. It is therefore considered that the flood-prone area located between Hopkins Point Road and Logans

Beach Road is the most significant drainage hot-spots of the subject area. Drainage upgrade may not

significantly alleviate existing flood risk, except for the installation of a pump system. A pump system would

have significant maintenance, operation (i.e., energy) and renewal implications.

The hot spots on Henderson Way may be alleviated by pipe upgrade, however, consideration of impact on

downstream flood risk should be considered. Drainage upgrade elsewhere in the catchment, such as the

construction of pipes, may alleviate localised flooding but this type of infrastructure will also convey floodwater

to the aforementioned hot spots.

Impacts from new development should be managed via appropriate on-site stormwater management strategy.

These should allow for:

Detention/storage to retard peak stormwater runoff to below pre-development conditions;

Loss of flood storage (e.g., via infill) to be off-set;

Provide water quality treatment, ideally in accordance with the requirements of the Urban Stormwater Best

Practice Environmental Management Guidelines (Victorian Stormwater Committee, 1999);

Ensure proposed dwellings are protected from flooding (elevated finished floor levels); and

Minimise (if possible) total volume of stormwater runoff discharged from the site.

Whilst the first four points are generally included as conditions in town planning permits, the last

recommendations may have significant impact on the size of the systems. It may be achievable with systems

promoting water re-use, whether via rainwater tanks or stormwater harvesting systems to irrigate open space

area. There will be opportunity for these systems, including for development off-set open spaces.

The implementation of flood controls within the Planning Scheme such as Special Building Overlay (SBO) or

a Land Subject to Inundation Overlay (LSIO) are recommended to manage future development and are

discussed in Section 5.2.

5.1.2 Coastal Zones

Flood-prone areas are also located behind coastal dunes, as local topography may prevent water flowing freely

to the beach. It is however, likely that these areas would drain via infiltration in the (likely) sandy soils of the

areas. Additionally, these low points are within areas zoned Public Park and Recreation Zone (PPRZ) and

Public Conservation and Recreation Zone (PCRZ), and may not be easily accessible. These High Hazard

areas are therefore not considered an issue as they are not associated with residential parcels.

5.1.3 Funding

Council may generate funds for capital works via Development Contribution Plans (DCP). These plans will be

based on a catchment-specific drainage plan that outlines the functional designs of the infrastructure required

to service urban growth, and a pricing arrangement that details how Council plans to recoup the infrastructure

costs through financial contributions paid by developers. The adopted rates per Net Developable hectare within

the catchment would need to reflect both the ability to generate “revenue” and infrastructure requirements.

Infrastructure requirements may be more substantial based on a higher yield and this should be reflected in a

DCP for the area.

Council may also look to fund projects through the special charge provisions in the Local Government Act

1989. It will be crucial for Council to identify who benefits from the new infrastructure. This may require some

level of community engagement to be undertaken as part of the design of the system and may require

cooperation between Council and the community. If the proposed drainage infrastructure only provides flood


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benefits, it will be reasonably easy to identify properties directly benefiting from the works. It may be more

difficult to identify a sufficient number of beneficiaries to make Special Charge Scheme viable, as the overland

flow paths in the upstream reaches of the catchment are well defined, and thus only a limited number of

properties (though more may contribute to the problem) are considered at-risk.

The wider community benefits from drainage assets that improve urban aesthetics, may be easier to support

a special planning scheme. This may be achieved via distributed systems, such as streetscapes raingardens.

Recent research from the Water Sensitive Cities has shown that the improved amenity associated with WSUD

assets and the rehabilitation of creeks may benefit house sale prices of adjacent properties (UWA and CRC

WSC, 2015). We note that similar Water Sensitive Urban Design are being constructed in the new subdivisions

east of the subject area.

5.2 Non-structure Controls

The current planning scheme includes a number of overlays protecting future land use in the area, including

an Environmental Significant Overlay and a Significant Landscape Overlay however, it does not include a

flood-related overlay. Sub-divisions and other development may impact on flood risk by building in flood-prone

areas, increasing stormwater runoff and/or disrupting existing overland flow paths. The absence of a relevant

overlay may result in development being progressed without consideration of existing flood risk.

It is therefore recommended that the Warrnambool City Council planning scheme maps be amended to show

flood-prone areas and existing overland flow paths during the 1% AEP flood extent (100 year ARI event). This

will facilitate the assessment of future planning application and should ensure that:

Overland flow paths are maintained to prevent detrimental impact on future dwellings;

Finished Floor Levels for new dwellings are set above applicable flood levels with sufficient freeboard;

Stormwater Management Strategy are developed for subdivision sites (as discussed in Section 5.1); and

Infrastructure requirements are designed with due consideration of existing flood risk.

The planning scheme may also be used to ensure that future development considers potential coastal acid

sulfate soils, which have been identified within the catchment. Drainage implication of coastal acid sulfate soils

are discussed in the following section.

5.3 Acid Sulfate Soils

Acid Soils are defined in the Industrial Waste Management Policy (Waste Acid Sulfate Soils) 1999 as “… any

soil, sediment, unconsolidated geological material or disturbed consolidated rock mass containing metal

sulfides which exceeds criteria for acid sulfate soils specified in Publication 655 entitled ‘Acid Sulfate Soil and

Rock’ published by the Authority in 1999 as amended from time to time or republished by the Authority”.

Disturbance of acid sulphate soils can cause detrimental impact to land, water and ecosystems, including

corrosion to infrastructure, environmental degradation and human health impacts (EPA, 2009).

Three areas have been identified within the catchment with potential coastal acid sulfate soils (Ground

Science, 2017), as shown in Figure 5-2. These include:

The estuarine area south of the Hopkins Point Road bridge;

The costal dune formations along Warrnambool Bay; and

The low-lying land generally following the alignment of Logans Beach Road.

Whilst the estuarine area and the coastal dune formation are protected by current planning overlays (i.e.,

Environmental Significance Overlay) and unlikely to be developed in the future, the parcels along Logans


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Beach Road may be developed. We understand that several developments are currently being considered

(e.g. “11 Acre” property) with the study area.

Figure 5-2 Potential Coastal Acid Sulfate Soils (source: Ground Science, 2017)

The hierarchy of management for Potential Acid Sulfate Soils (PASS) is:

1. Avoid disturbance

2. Minimise disturbance

3. Prevent oxidation

4. Treat to reduce or neutralise acidity

5. Offsite reuse or disposal

These will need to be considered for development in the area along Logans Beach Road. Preferably

development in areas with PASS should avoid disturbance and, if not possible, minimise disturbance. Potential

options available to minimise disturbance are listed below, though the list is not exhaustive:

Minimise excavation of land and/or filling land over in situ potential acid sulfate soil (PASS). Consideration

of bio-retention systems (e.g. raingarden) over constructed wetlands may result in smaller asset footprints.

Alternative technologies, such as floating wetlands, may also be considered as they may reduce the

overall footprint of a wetland system.


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Use strip filter drains instead of conventional circular ag pipes. Its narrow profile results in significantly

narrower trenches compared to traditional circular ag pipes. Consequently, there is less material to

excavate and dispose of, volume of trench fill material required is reduced. These filters drains should not

result in compromise on conveyance capacity (refer to IPWEA article and Austroads’ Guide to Pavement

Technology Part 10: Subsurface Drainage) and may have the added benefits of reducing capital costs

associated with installation (Dassanayake, 2016).

Promote stormwater management strategies that rely on infiltration systems, such as soak wells. To

prevent lowering of the groundwater table Proprietary products, such as HydroCon permeable concrete

pipes Sandy soils of the areas are likely to facility stormwater management system via infiltration. The use

of infiltration system must, however, be supported by on-site testings to ensure soils are adequate for

infiltration.

Should disturbance be unavoidable, a site-specific acid sulfate soil management plan should be prepared and

implemented. Additional information is available in EPA Publication 655.1.


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6 SUMMARY The following report presents the result of the hydraulic modelling undertaken to inform the Strategic

Framework Plan which Warrnambool City Council is currently progressing. The Logans Beach coastal area is

located to the east and south of the Hopkins River in Warrnambool and its low lying areas are shown to be

flood-prone. In the steeper section of the catchment, flooding is confided within well-defined overland flow

paths. Five hot spots were identified as part of this study, including:


Sections of Logans Beach Road; and


One of the key flood constraints is that levels in Hopkins River, and therefore the bay, are likely to influence

drainage. As a result, larger volume of stormwater runoff, which will occur under both the development and

climate change scenarios tested as part of this study are likely to exacerbate existing flood risk in low lying

areas. Additionally, forecast higher levels in the Hopkins River and Warrnambool Bay (CSIRO, 2009), will

increase flood risk in these areas. This may restrict development potential for the affected properties.

As a result, planning controls should be strengthened in the area to ensure future development does not

increase flood risk in the catchment. It is therefore recommended that the Warrnambool City Council planning

scheme maps be updated to show flood-prone areas and existing overland flow paths during the 1% AEP flood

extent (100 year ARI event).

Flood Hazard in the catchment, especially within Logans Beach Road and Henderson Way may also restrict

accessibility during flood events. It may be necessary for Council to consider emergency access as

development of the catchment may increase residents at risk.


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7 REFERENCES ,

CSIRO (1999), Best Practice Environmental Management Guidelines. Victorian Stormwater Committee.

CSIRO (2009), Effect of Climate Change on Extreme Sea Levels along Victoria's Coast.

Dassanayake D. (2016), Accounting for Water in Sustainable Road Designs. Proceedings of 2016

Sustainability in Public Works Conference (extended abstract).

Ground Science (2017), Coastal Acid Sulfate Soils, Preliminary Hazard Assessment, Logans Beach

Coastal Area. Prepared for Warrnambool City Council. EPA (2009), Publication 655.1 Acid Sulfate Soil

and Rock.

Traffix Group (2017), Traffic Impact Assessment – Logans Beach Strategic Framework Plan. Prepared

for Warrnambool City Council.


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- DEVELOPMENT SCENARIO (700 LOTS)


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Flood Depths for the 100 year ARI Development Scenario (700 lots).


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Flood Hazard for the 100 year ARI Development Scenario (700 lots).


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Difference in Flood Depths between Existing and ‘Maximum’ Yield Scenarios (700 lots)


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- MAPS – FLOOD VELOCITIES


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Flood Velocities for the Existing Conditions 5 year ARI Critical Event.

Flood Velocities for the Existing Conditions 100 year ARI Critical Event.


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Flood Velocities for the 100 year ARI Critical Event with Climate Change

Flood Velocities for the 100 year ARI Development Scenario (500 lots).


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Flood Velocities for the 100 year ARI Development Scenario (700 lots).


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Melbourne 15 Business Park Drive Notting Hill VIC 3168 Telephone (03) 8526 0800 Fax (03) 9558 9365

Brisbane Level 3, 43 Peel Street South Brisbane QLD 4101 Telephone (07) 3105 1460 Fax (07) 3846 5144

Wangaratta First Floor, 40 Rowan Street Wangaratta VIC 3677 Telephone (03) 5721 2650

Perth PO Box 362 Subiaco WA 6904 Telephone 0438 347 968

Geelong PO Box 436 Geelong VIC 3220 Telephone 0458 015 664

Gippsland 154 Macleod Street Bairnsdale VIC 3875 Telephone (03) 5152 5833

Wimmera PO Box 584 Stawell VIC 3380 Telephone 0438 510 240

www.watertech.com.au

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Documents

Report - · Model build 2016-03-AC-iDP-w64 Survey LiDAR (2007) Survey Data (’11 Acre’ Site) Upstream boundary Rain-on-Grid Rainfall data (‘2d_rf’ layer with 15 mm initial