separate cover attachment · 2019. 6. 3. · 170 greenhill road parkside sa 5063 p 08 8172 1088 f...

City of Onkaparinga Agenda for the Strategic Directions Committee meeting to be held on 19 August 2014 _________________________________________________________________________________________

ATTACHMENT UNDER SEPARATE COVER (39 pages)

Strategic Directions Committee 19 August 2014

Item 7.4 - Attachment 5

Seaford District Centre Strategic Management Plan 2014-35

This attachment is separate to the main agenda and is numbered autonomously. If inserted into the main agenda, note that the numbering will then not be sequential.

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170 Greenhill Road Parkside SA 5063

P 08 8172 1088 F 08 8271 2055 www.southfront.com.au

Summary Report Seaford District Centre Strategic Management Plan Stormwater Investigations

DRAFT

Item 7.4 - Attachment 5 Seaford District Centre Strategic Management Plan 2014-35 of 39

© Southfront 2014

Summary Report

Seaford District Centre Strategic Management Plan Stormwater Investigations

City of Onkaparinga

Our Ref.: 14001-2B

Revision Date Approved Details

A 11 August DK Incomplete draft for initial comment

B 12 August 2014 DK Revised Incomplete Draft


Summary Report for City of Onkaparinga i

Contents

Executive Summary i

1 Introduction 1

2 Catchment Modelling 2

2.1 Site Description 2 2.2 Wetland Catchment 3 2.3 Baseline Data 4 2.4 Digital Terrain Model 4 2.5 DRAINS Modelling 4 2.6 Existing Modelling Results 9 2.7 Future Development 10 2.8 Climate Change Impacts 10 2.9 Ultimate Scenario Modelling Results 11 2.10 MUSIC Modelling 12 2.11 Floodplain Mapping of Existing Wetland 15 2.12 TUFLOW Modelling Results 17 2.13 Summary and Recommendations 18

3 Scenario Modelling 20

3.1 Wetland Reserve Concept 20 3.2 Floodplain and Hazard Mapping 21 3.3 Indicative Cost Estimate 31

4 References 32

Tables

Table 2.1 Existing Wetland Elevation vs Storage Volume 6 Table 2.2 Typical Impervious Fractions for Different Types of Development 7 Table 2.3 Rainfall IFD data for Seaford 9 Table 2.4 Existing DRAINS Modelling Results 9 Table 2.5 Ultimate Scenario Residential Catchment Sub-area Fractions 10 Table 2.6 Ultimate Scenario DRAINS Modelling Results 12 Table 2.7 MUSIC Modelling Results 15 Table 2.8 Water Quality Improvement Performance 15 Table 2.9 Bed Resistance Parameters 16 Table 3.1 Peak Floodwater Level at Outlet 21

Figures

Figure 2.1 Wetland Site 2 Figure 2.2 Catchment Boundary 3 Figure 2.3 Digital Terrain Model 5 Figure 2.4 Catchment Samples Areas Impervious Fractions 7 Figure 2.5 Existing DRAINS Model Layout 8 Figure 2.6 District Centre Master Plan and Directly Connected Impervious Fractions 11


Summary Report for City of Onkaparinga ii

Figure 2.7 Existing Wetland Layout 13 Figure 2.8 MUSIC Model Layout 14 Figure 2.9 Floodplain Map Inundation Depth Colour Ranges 17 Figure 2.10 100 Year ARI Flood Inundation of Existing Wetland (Ultimate Scenario) 18 Figure 3.1 Proposed Wetland Concept 20 Figure 3.2 1 Year ARI Floodplain Map 22 Figure 3.3 5 Year ARI Floodplain Map 23 Figure 3.4 10 Year ARI Floodplain Map 24 Figure 3.5 100 Year ARI Floodplain Map 25 Figure 3.6 Flood Hazard Legend 26 Figure 3.7 1 Year ARI Flood Hazard 27 Figure 3.8 5 Year ARI Flood Hazard 28 Figure 3.9 10 Year ARI Flood Hazard 29 Figure 3.10 100 Year ARI Flood Hazard 30

Appendices

Appendix A Initial Option Cost Estimate Schedules


Summary Report for City of Onkaparinga i

Executive Summary

Introduction

A three park reserve concept is currently being developed within the City of Onkaparinga’s Seaford District Centre Strategic Management Plan. A 20 year shared vision for this District has been recently developed and endorsed by Council’s Strategic Direction Committee. The three park concept proposes redevelopment of the wetland detention area on the corner of Seaford and Commercial Roads, at Seaford. Southfront was commissioned by the City of Onkaparinga to provide stormwater investigations for this existing wetland site, including hydrological, hydraulic and water quality modelling techniques. The aims of the project include the following:

To assist in determining the potential for development of the wetland site area; To maximise amenity outcomes; To minimise Council’s investment costs, and; To provide detail for development of the final Strategic Management Plan report and

spatial plan.

Development of Catchment Models

Catchment models for the wetland site were developed to confirm the ultimate development scenario parameters. 3 types of models were developed including a hydrological model (DRAINS), a hydraulic floodplain model (TUFLOW) and a water quality model (MUSIC). Baseline data were obtained from Council including survey of the site, as well as GIS databases of all drainage infrastructure. A Digital Terrain Model of the site was also procured to assist with the modelling. The DRAINS model was developed to determine the magnitude of flows entering the wetland site as well as assess the detention storage capacity of the existing wetland. The modelling showed that the existing wetland has a detention volume of approximately 60,000 m3, and a high capacity outlet that discharges to the ocean via Rye Street and Seaford Road. The results of the DRAINS modelling found that even in the ultimate development scenario, assuming the high capacity outlet is maintained, the required storage volume is 37,000m3 which is significantly less than existing. The water quality modelling was undertaken to ensure that any proposed development is designed to satisfy specific water quality improvement criteria at the outlet. Various water quality improvement measures such as Gross Pollutant Traps (GPT’s) have been recommended at all stormwater outlets, and existing measures such as vegetated swales the sedimentation basins and a heavily vegetated macrophyte zone were recommended to be maintained within the new concept. The floodplain modelling confirmed that the existing wetland had sufficient capacity to detain the 100 year ARI ultimate development flood flows without overtopping. Based on the results of our modelling we recommended that the following minimum parameters be adopted:


Summary Report for City of Onkaparinga ii

The existing 1500mm diameter outlet drain to Rye Street is to remain. This outlet is to have an invert of no higher than 21 mAHD. Existing invert is shown to be 20.46 on Council survey

A peak allowable flood water elevation of 24 mAHD during the 100 year ARI event A detention storage volume of 37,000 m3 A treatment train with a GPT at each pipe outlet and vegetated swales leading in to the

wetland. An inlet / sedimentation pond with a 1 metre deep permanent water level and a

minimum surface area of 4,000 m2. The ratio of length to width within this sedimentation pond ideally would be around 3:1 to allow for sufficient settling time. This pond is proposed to spill into the macrophyte zone.

A minimum 12,000 m2 macrophyte zone which is heavily vegetated with an extended detention depth of 600mm.

Vegetated swales with a base width of no less than 1.5 metres from the macrophyte zone to the outlet.

Any additional permanent water storage needs to be located outside of the pond and macrophyte zone.

Scenario Modelling

Council engaged Wax Design to develop a concept for the wetland site based on the parameters determined from the development of the catchment models. The reshaped wetland is proposed to function in a similar way to the existing wetland with the exception being that there are two macrophyte zones proposed to be separated by an embankment. All wetland inlets and the outlet are proposed to remain in a similar location to the current layout to minimise redevelopment costs. The concept has been developed to contain the 100 year ARI flood flows within the embankments. In the event that the flood waters are not contained, the wetland will spill onto Commercial Road in the vicinity of Rye Street which forms the current overland flow route. The design contours provided were used to develop a new 2D domain within the TUFLOW models. The revised wetland surface was modelled by applying the ultimate scenario inflows for the 1, 5, 10 and 100 year ARI events. Both floodplain and hazard maps have been created for these events as shown in Section 3.

Peak floodwater elevations at the outlet are shown below.

Peak Floodwater Level at Outlet

Storm ARI Peak Floodwater Level at Outlet

1 Year 22.42 mAHD

5 Years 22.80 mAHD

10 Years 22.95 mAHD

100 Years 23.73 mAHD

The modelling shows that the proposed wetland will not overtop and spill onto Commercial Road during the 100 year ARI event. Any ground level development proposed to occur adjacent to the wetland will need to consider these flood levels when determining the elevation of finished floor levels.


Summary Report for City of Onkaparinga 1

1 Introduction

A three park reserve concept is currently being developed within the City of Onkaparinga’s Seaford District Centre Strategic Management Plan. A 20 year shared vision for this District has been recently developed and endorsed by Council’s Strategic Direction Committee. The three park concept will incorporate redevelopment of the wetland detention area on the corner of Seaford and Commercial Roads, at Seaford. Community consultation undertaken as part of the Seaford District Centre Strategic Management Planning Project identified that whilst there is a need to provide a suitable detention volume and water quality improvement measures within the site, there is an opportunity to further consider options for improved utilisation of the site, including increasing surrounding residential development and providing higher value recreational use. Southfront was commissioned by the City of Onkaparinga to provide stormwater investigations for this existing wetland site, including hydrological, hydraulic and water quality modelling techniques. The aims of the project include the following:

To assist in determining the potential for development of the wetland site area; To maximise amenity outcomes; To minimise Council’s investment costs, and; To provide detail for development of the final Strategic Management Plan Report and

spatial plan. This report summarises the stormwater investigations that were undertaken and provides a technical basis for the further development of the wetland site.



2 Catchment Modelling

2.1 Site Description

The subject site is located on the south-eastern corner of Seaford and Commercial Roads in Seaford. The site consists of a large Council reserve that contains an existing wetland as well as large, grassed open area with some perimeter paths. The existing wetland has a storage volume of approximately 60,000 m3 and is fed by large diameter drains (up to 1350 mm) from the north, south and east. The outlet from the wetland consists of a 1500 mm diameter drain that discharges to the Ocean via Rye Street and Seaford Road. The existing site and wetland are shown in Figure 2.1.

Figure 2.1 Wetland Site



2.2 Wetland Catchment

The catchment draining to the wetland covers an area of approximately 1.4 km2 and is bounded by Commercial Road to the west, South Pacific Drive to the North, Griffiths Drive to the south and the Seaford Railway Line to the east. The catchment to the north of the wetland consists of residential development within Seaford Meadows. There is also some residential development to the east of the wetland, as well as a retirement village on the south-eastern corner of Grand Boulevard and Seaford Road. The southern part of the catchment consists of a predominantly commercial area with the Seaford Central Shopping Centre, the Seaford Railway Station as well as some schools and recreational facilities in the south eastern portion of the catchment. The catchment boundary is depicted in Figure 2.2.

Figure 2.2 Catchment Boundary



2.3 Baseline Data

Council have supplied GIS databases that provide details of existing pits and pipes within the catchment. In addition to the GIS data, Council provided DRAINS models of certain catchments within Seaford Meadows as well as a DRAINS model of Catchment 36 that contains the wetland site. Original design drawings of the wetland as well as drawings of upgrades to the wetland designed in 2007 were also provided. The layout, pipe diameters and inverts of any drainage infrastructure contained within the GIS layers at the wetland site were compared against ground survey to ensure that the most up to date information was used in the modelling. A review of the baseline data found a major discrepancy between the wetland outlet drain diameter shown in Council’s GIS data and the outlet diameter picked up in the recent ground survey. The outlet diameter in Council’s GIS database, the wetland design drawings and the existing Catchment 36 DRAINS model was shown to be 900 mm. The diameter of the outlet measured on the ground was 1500 mm. Further investigation of the outlet drainage network by Council confirmed that the pipe diameter is 1500 mm at the outlet from the wetland, 1650 mm at the upstream end of Rye Street, and 1800 mm at the outlet to the ocean.

2.4 Digital Terrain Model

Southfront engaged Aerometrex to supply photogrammetric survey data of the wetland site that was obtained in December 2012. The photogrammetric survey data was then converted into a Digital Terrain Model (DTM) of the site containing 0.5 metre contours. This data has been used as base data for the flood plain modelling. This information highlights that the wetland is approximately 3 metres deep, with a base level of approximately 21 mAHD and top of bank levels of approximately 24 mAHD. The DTM shows that any overflows from the wetland site would discharge onto Commercial Road adjacent to Rye Street, which follows the alignment of the underground system discharging from the wetland. The digital terrain model is shown in Figure 2.3.

2.5 DRAINS Modelling

2.5.1 Modelling Approach

The performance of the existing wetland under existing and ultimate scenarios was assessed using the DRAINS modelling platform. As described in the model documentation (Watercom, 2011), DRAINS is a multi-purpose Windows program for designing and analysing urban stormwater drainage systems and catchments. Working through a number of time steps that occur during the course of a storm event, DRAINS simulates the conversion of rainfall patterns to stormwater runoff hydrographs and routes these through networks of pipes, channels and creeks. In this process, it integrates:

Design and analysis tasks; Hydrology (four alternative models) and hydraulics (two alternative procedures); Closed conduit and open channel systems; Headwalls, culverts and other structures; Stormwater detention systems; and Large-scale urban and rural catchments



Figure 2.3 Digital Terrain Model 2.5.2 Hydrological Model

The ILSAX model has been adopted as the default hydrological model within DRAINS, with depression storages of:

paved = 1mm supplementary paved = 1mm grassed = 40mm

A custom soil type was selected, with values entered to achieve a continuing loss of 3mm/hour.



2.5.3 DRAINS Model Establishment

Council provided 14 separate DRAINS models that were produced for various stages of the Seaford Meadows development as well as a DRAINS model of Catchment 36 that included the existing wetland detention storage. An assessment of these models found that they were not able to be combined into a large “master” model of the catchment due to spatial location issues as well as inconsistencies between pit databases and several other modelling parameters. As a result it was decided to develop a coarse DRAINS model of the wetland catchment that utilised the drainage layout in Council’s GIS database as well as the survey data obtained for the wetland site. Sub-catchment boundaries for the model were determined from an analysis of the topography of the catchment as well as site inspection. The purpose of this model was to provide hydrological simulations that could be interrogated to determine the following:

The magnitudes of incoming flows to the wetland site for various storm durations and rainfall events;

The critical storm duration for the wetland for each event; The performance standard of the existing wetland Whether the existing amount of detention storage is appropriate for the ultimate

development scenario. Figure 2.5 illustrates the model layout. Flood flows were directed in accordance with surface gradients, with those overland flows accumulating in the wetland site directed to a detention basin node that was established to reflect the surface storage available. The elevation versus surface area relationship of this detention basin was developed from the design drawings of the wetland, and matches the relationship used within the existing Catchment 36 DRAINS model. The total detention storage available was determined to be approximately 60,000 m3. The elevation versus surface area relationship is summarised in Table 2.1 below. Table 2.1 Existing Wetland Elevation vs Storage Volume

Elevation (mAHD)

Surface Area

(m2)

Cumulative Volume

(m3)

21 200 -

22 1600 900

23 34500 16,000

24 57000 60,000

2.5.4 Catchment Characteristics

The catchment area was delineated into sub-catchments based on the existing stormwater drainage network and gutter flow directions. The time of concentration (time for runoff from the entire catchment to be contributing) for each sub-catchment was calculated based on the length and longitudinal grade of the longest flow path within each catchment. In order to establish the impervious and pervious area fractions for each sub-catchment, a sample residential area was selected for examination. The impervious area fractions were measured using current aerial photography. Figure 2.4 shows the pervious and impervious areas associated with this sample area.



Figure 2.4 Catchment Samples Areas Impervious Fractions Based on the results of the analysis a similar procedure was applied for other sub-catchments dominated by other types of development. Typical impervious fractions applied across the catchment are shown in Table 2.2. Table 2.2 Typical Impervious Fractions for Different Types of Development

Catchment Type Paved Area % Supplementary Paved Area %

Grassed Area %

Residential 61 9 30

Commercial 90 0 10

Schools / Recreation 45 10 45

Wetland Site 10 0 90



Figure 2.5 Existing DRAINS Model Layout



2.5.5 Design Rainfall

Rainfall intensity data for a range of storm durations and Annual Recurrence Intervals (ARI) were incorporated into the model using Intensity Frequency Duration (IFD) data from the Bureau of Meteorology (http://www.bom.gov.au/water/designRainfalls/ifd/index.shtml). The IFD data for the Seaford area is summarised in Table 2.3. Table 2.3 Rainfall IFD data for Seaford

Duration

Rainfall Intensity (mm/hr) for various ARIs

1 2 5 10 20 50 100

5 mins 44 59.3 82 98.7 122 156 187

6 mins 41 55.2 76.1 91.6 113 145 173

10 mins 33 44.3 60.8 73.1 89.6 115 137

20 mins 23.5 31.5 42.8 51 62.4 79.4 94.2

30 mins 18.8 25.1 33.9 40.3 49.1 62.2 73.6

1 hour 12.4 16.4 22 26 31.6 39.8 46.9

2 hours 7.82 10.4 13.9 16.5 20 25.2 29.6

3 hours 5.93 7.89 10.6 12.6 15.2 19.3 22.7

6 hours 3.67 4.9 6.62 7.87 9.59 12.2 14.4

12 hours 2.3 3.06 4.11 4.87 5.92 7.49 8.84

24 hours 1.45 1.9 2.49 2.92 3.49 4.35 5.07

48 hours 0.893 1.15 1.45 1.65 1.93 2.35 2.69

72 hours 0.65 0.83 1.03 1.15 1.34 1.6 1.83

2.5.6 Model Simulation

Model simulation was completed for rainfall events with ARIs of 1, 5, 10 and 100 years. A range of storm durations from 5 minutes to 72 hours were assessed for each ARI to determine the critical duration for flood storage.

2.6 Existing Modelling Results

The DRAINS model has been executed for the full range of events, and it was found that the existing wetland catered for all events up to the 100 year ARI. Peak storage levels and critical storm durations for the existing wetland are shown in Table 2.4. Table 2.4 Existing DRAINS Modelling Results

ARI (years) Peak Wetland Level (mAHD)

Peak Flood Volume (m3)

Critical Storm Duration

1 22.42 4,000 1.5 hours

5 22.76 12,000 1.5 hours

10 22.84 14,000 1.5 hours

100 23.41 30,000 3 hours



2.7 Future Development

The future development potential of the contributing catchment was assessed in two separate stages. The first stage consisted of the residential catchment to the north of Seaford Road which was determined through discussions with Council to have a moderate potential for infill development. The second stage of the assessment consisted of the catchment to the south of Seaford Road, within which future development will be dictated by the Seaford District Centre Strategic Management Plan and recent changes to policy in this area within the City of Onkaparinga Development Plan through the gazettal on 31 July 2014 of the Seaford District Centre DPA. A diagram of the proposed Master Plan is shown in Figure 2.6 (left). Council’s stormwater engineers provided input into the selection of the ultimate scenario modelling parameters. For the residential catchment to the north of Seaford Road, the ultimate scenario impervious area fractions are shown in Table 2.5. These fractions are conservative to support all forms of development potential envisioned for the catchment. Table 2.5 Ultimate Scenario Residential Catchment Sub-area Fractions

Catchment Type Paved Area % Supplementary Paved Area %

Grassed Area %

Residential 70 10 20

For the sub-catchments to the South of Seaford Road, the directly connected impervious area fractions used in the ultimate scenario modelling are shown in Figure 2.6 (right).

2.8 Climate Change Impacts

In order to protect the long-term sustainability and performance standard of the upgrade solution, the impacts of climate change on rainfall intensity have been considered. Rainfall Intensity The Guidelines for Developing a Climate Change Adaptation Plan and Undertaking an Integrated Climate Change Vulnerability Assessment (LGA SA, 2012) has been reviewed in relation to advice on changes in future design rainfall intensity. This document states:

In addition to the changes in the amount of rainfall received, it is also important to consider the changes in how the rain falls – intensity, frequency and seasonality. The more intense the rainfall events, the more likely there will be floods, while if the frequency of the rain is reduced there may be the risk of drought. For many areas in the tropics and sub-tropics it is expected that the intensity of rainfall will increase. However, for South Australia, projections for rainfall intensity suggest only minor increases of perhaps only 2% above current levels (Darren Ray, BOM, Head Climatologist, South Australia, personal communication).

No changes to rainfall intensity are proposed to be considered in modelling the ultimate development scenario.



Figure 2.6 District Centre Master Plan and Directly Connected Impervious Fractions

2.9 Ultimate Scenario Modelling Results

The DRAINS model has been executed for the full range of events with the revised impervious fractions. As for the existing scenario, it was found that the wetland in its current configuration catered for all events up to the 100 year ARI. Peak storage levels and critical storm durations for the existing wetland are shown in Table 2.6.



Table 2.6 Ultimate Scenario DRAINS Modelling Results

ARI (years) Peak Wetland Level (mAHD)

Peak Flood Volume (m3)

Critical Storm Duration

1 22.57 5,000 1.5 hours

5 22.86 14,000 1 hour

10 22.96 15,000 1.5 hours

100 23.56 37,000 3 hours

The results of the DRAINS modelling have found that the existing wetland is oversized when assessed against the requirement to provide a 100 year ARI standard of protection against flooding. With a peak 100 year ARI flood volume in the wetland of 37,000 m3, the modelling showed that there is approximately 23,000 m3 of available storage that is surplus to requirements.

2.10 MUSIC Modelling

A MUSIC water quality model was prepared for the catchment and the wetland.

As can be seen in Figure 2.7, the existing wetland has multiple inflows, with two inflows from the north entering the sedimentation pond, one inlet from the east entering the macrophyte zone, and other inlets from the east and the south bypassing the sedimentation pond and macrophyte zone and flowing through vegetated swales directly to the outlet. The MUSIC model for the proposed wetland has been developed with the assumption that the current drainage layout and wetland configuration will remain largely unchanged during any proposed modifications to the wetland site. The water quality improvement at the outlet from the wetland was assessed against the requirements outlined in Section 2.10.1 below.

2.10.1 Council Water Quality Guidelines

The City of Onkaparinga Water Resources Service Levels and Service Standards Guideline Document states the following in relation to Water Quality: Our service levels for water quality management acknowledge the influence of the Adelaide Coastal Waters Study and regional natural resources management plans on our Stormwater management obligations under the agreement with the State Government. Water quality in outflows from new development shall have load reduction (when compared to untreated Stormwater outflows) improvement equivalent to:

80% reduction in suspended solids 45% reduction in total nitrogen 60% reduction in phosphorous 90% reduction in litter. Water quality modelling is undertaken based on works proposed either as new development or infrastructure extension, renewal or upgrades.



Figure 2.7 Existing Wetland Layout

2.10.2 Execution of MUSIC Model

The layout of the MUSIC model is shown in Figure 2.8. The model assumes that in the ultimate development scenario, all pipe outlets contain Gross Pollutant Traps and that any existing swale areas along Commercial Road and within the reserve will be maintained as a form of pre-treatment. Our model assumes an inlet / sedimentation pond with a 1 metre deep permanent water level and a minimum surface area of 4,000 m2. The heavily vegetated macrophyte zone in our model has an area of 12,000 m2 with an extended detention depth of 600mm. The



dimensions of the sedimentation basin and macrophyte zone are similar to those of the existing wetland.

Figure 2.8 MUSIC Model Layout 2.10.3 MUSIC Modelling Results

The water quality parameters at the receiving node within the model were assessed with the results shown in Table 2.7.



Table 2.7 MUSIC Modelling Results

WQ Parameter Source Load Residual Load % Reduction

Total Suspended Solids (kg/yr) 59,800 7,800 87.0

Total Phosphorus (kg/yr) 120 36.0 70.1

Total Nitrogen (kg/yr) 870 378 56.6

Gross Pollutants (kg/yr) 15,200 0.00 100.0

Table 2.8 shows the performance of the modelled wetland against the target water quality improvement parameters. It can be seen that the water quality targets have been met. Table 2.8 Water Quality Improvement Performance

WQ Parameter Target Reduction Modelled Reduction Criterion Met

Total Suspended Solids 80% 87% Yes

Total Phosphorus 60% 70.1% Yes

Total Nitrogen 45% 56.6 % Yes

Gross Pollutants 90% 100.0% Yes

2.11 Floodplain Mapping of Existing Wetland

2.11.1 Modelling Software

Hydraulic floodplain modelling was carried out using the TUFLOW (and ESTRY) computer program jointly funded and developed by BMT WBM and The University of Queensland in 1990. TUFLOW (Two-dimensional Unsteady FLOW) is specifically orientated towards establishing flow and inundation patterns in coastal waters, estuaries, rivers, floodplains and urban areas where the flow behaviour is essentially 2D in nature and cannot or would be awkward to represent using a 1D model (BMT WBM, 2010). A powerful feature of TUFLOW is its ability to dynamically link to 1D networks using the hydrodynamic solutions of ESTRY. The user sets up a model as a combination of 1D network domains linked to 2D domains. The models were developed so that any underground stormwater drains (in this case the outlet from the wetland) were modelled in 1D using ESTRY, while the floodplain on the surface was modelled in 2D using TUFLOW. The pipe network was hydro-dynamically linked to the floodplain, allowing flows in both domains to interact. Inflows to the wetland were generated on the surface using hydrographs obtained from the DRAINS modelling. The model area was divided into fixed rectangular cells that can be either wet or dry during a simulation. The model can simulate the variation in water level and flow inside each cell once information regarding the ground resistance, topography and boundary conditions is entered.

2.11.2 Modelling Scope

The scope of this investigation involved floodplain mapping the existing wetland for the 100 year ARI event. Various storm durations were modelled in order to determine the critical durations for the 100 year ARI event. The storm durations modelled ranged from 1 hour to 6 hours.



2.11.3 2D Cell Size

Determining an appropriate 2D cell size to be used by TUFLOW requires a compromise between the accuracy of modelling necessary to sufficiently reproduce the hydraulic behaviour of the floodplain as well as limitations in computing power and processing time. Due to the relatively small size of the model domain, and the desire to accurately represent the shape of the existing wetland, a 1 metre cell size was chosen. The 2D domain consisted of approximately 140,000 cells.

2.11.4 Time Step

The time step selection in the 2D domain is an important aspect of TUFLOW modelling as it is directly proportional to the running time of a model. A small time step will create more accurate results and is less likely to cause instabilities, however the simulation time can often stretch to days for long duration storm events. On the other hand, a large time step will shorten simulation times but may lead to meaningless results. A general rule for TUFLOW models (although this is not a necessity) is to use a time step (in seconds) equal to approximately half the cell size (in metres). For our model, the time step used was 0.5 second. It should be noted that 99% of the computational effort is in solving the 2D surface flow equations and hence the impact of the time step on simulation times is negligible in the 1D domain. Thus the 1D ESTRY time step was also set to 0.5 second.

2.11.5 2D Topography

A DTM of the wetland was provided by Aerometrex as described in Section 2.4. The DTM was used to assign elevations to individual cells within the 2D domain. These elevations are assigned at the cell centres, corners and mid-sides to enable interaction with surrounding cells.

2.11.6 1D Domain Pipe Network

The 1D domain was constructed from Council’s GIS database as well as the information output from the DRAINS model.

2.11.7 Resistance Parameters

The bed resistance is an essential element used to define the flow and hence the water depth at any location within the 2D model domain. In TUFLOW, bed resistance values for 2D domains are most commonly created by using GIS layers containing polygons with varying Materials values. The Materials values specified in GIS correspond to a user defined Manning’s n value described in the Materials File. This approach allows for a relatively quick and simple adjustment of Manning’s n values, especially during model calibration. The bed resistance values used in the modelling are specified in Table 2.9. Table 2.9 Bed Resistance Parameters

Type of Land Use Manning's Roughness Coefficient

Roads 0.030

Sparsely Vegetated Open Space 0.050

Densely Vegetated Open Space 0.080



The Manning's n value used for modelling of underground drains was 0.012.

2.11.8 Boundary Conditions

As part of the modelling, consideration was given to the boundary conditions within the 1D and 2D domains. The 1D boundary condition is the HGL elevation at the downstream end of the outlet pipe which was assumed to be the top of the 1650 mm diameter drain in Rye Street. Within the 2D domain, the boundary condition is the edge of the model around the perimeter of the reserve. The boundary condition adopted in the 2D domain was a "HQ" (stage-discharge) type boundary with a water surface slope of 1%. The purpose of this approach was to allow water to “disappear” once flood flows reached the model boundaries and ensure that results in the floodplain were not affected at model edges.

2.11.9 Inflows

The inflow hydrographs at each inlet to the wetland were derived from the DRAINS modelling. The hydrographs for each storm event were applied as surface inflows directly into the 2D domain Due to the hydro-dynamic links between the 1D and 2D domains, this arrangement allowed for flows from the 2D wetland surface to enter the 1D drainage outlet and conversely any backing up of the 1D network due to downstream boundary conditions was able to enter the 2D domain and be represented on the mapping.

2.12 TUFLOW Modelling Results

The flood extents and depths of the existing wetland have been mapped for the 100 year ARI ultimate scenario. Flood depth ranges are represented in colour bands as shown in Figure 2.9.

Figure 2.9 Floodplain Map Inundation Depth Colour Ranges The mapping shows that the existing wetland does not overtop in the 100 year ARI event, with floodwaters being contained within the banks. The maximum depth of flood flows is between 1.5 and 2.5 metres in the vicinity of the outlet which corresponds to peak flood level of less than 23.5 mAHD. This result correlates well with the results of the DRAINS modelling. The floodplain extents are shown in Figure 2.10.



Figure 2.10 100 Year ARI Flood Inundation of Existing Wetland (Ultimate Scenario) 2.13 Summary and Recommendations

Upon completion of the catchment models, the results were presented to Council to provide a starting point for the development of a revised concept for the wetland site. Based on the results of our modelling we recommended that the following minimum parameters be adopted:

The existing 1500mm diameter outlet drain to Rye Street is to remain. This outlet is to have an invert of no higher than 21 mAHD. Existing invert is shown to be 20.46 on Council survey



A peak allowable flood water elevation of 24 mAHD during the 100 year ARI event A detention storage volume of 37,000 m3 A treatment train with a GPT at each pipe outlet and vegetated swales leading in to the

wetland. An inlet / sedimentation pond with a 1 metre deep permanent water level and a

minimum surface area of 4,000 m2. The ratio of length to width within this sedimentation pond ideally would be around 3:1 to allow for sufficient settling time. This pond is proposed to spill into the macrophyte zone.

A minimum 12,000 m2 macrophyte zone which is heavily vegetated with an extended detention depth of 600mm.

Vegetated swales with a base width of no less than 1.5 metres from the macrophyte zone to the outlet.

Any additional permanent water storage needs to be located outside of the pond and macrophyte zone.



3 Scenario Modelling

3.1 Wetland Reserve Concept

Council engaged Wax Design to develop a concept for the wetland site based on the parameters determined from the development of the catchment models. The proposed concept is shown in Figure 3.1.

Figure 3.1 Proposed Wetland Concept



The reshaped wetland is proposed to function in a similar way to the existing wetland with the exception being that there are two macrophyte zones separated by an embankment. All wetland inlets as well as the outlet are proposed to remain in a similar location to the current layout to minimise redevelopment costs. The bottom of the embankment within the sedimentation pond and the macrophyte zones has an elevation of 22 mAHD. The top of embankment level around the perimeter of the wetland and near the proposed turf open space area have an elevation of 24 mAHD. A 25 mAHD elevation contour has been proposed on the eastern and western sides of the reserve to provide flood protection to any new development in the vicinity of the wetland. In the event that the flood waters exceed an elevation of 24 mAHD, the wetland will spill onto Commercial Road in the vicinity of Rye Street which forms the current overland flow route.

3.2 Floodplain and Hazard Mapping

The design contours provided were used to develop a new 2D domain within the TUFLOW models. The surface topography was modified using TUFLOW’s built in “Create Tin Zpts” procedure which triangulates the revised area based on the supplied contour information. In order to maintain the functionality of the existing outlet, we provided an additional contour at elevation of 21 mAHD to allow flows to exit the wetland using the full flow capacity of the outlet. This contour covers a small area and is considered to have a negligible impact on the overall detention volume available. The revised wetland surface was modelled by applying the ultimate scenario inflows for the 1, 5, 10 and 100 year ARI events. Both floodplain and hazard maps have been created for these events.

3.2.1 Flood Inundation Extents

Floodplain maps are shown in Figures 3.2 – 3.5. As expected, the modelling results indicate that flooding depths and extents become more pronounced with an increase in the ARI. In all events, the sedimentation basin is shown to overtop into the northern macrophyte zone. In the 1 and 5 year AR events, flood waters are shown to be contained with each separate macrophyte zone, but in the larger 10 and 100 year ARI events it can be seen that the entire wetland becomes inundated. Peak floodwater elevations at the outlet are shown in Table 3.1.

Table 3.1 Peak Floodwater Level at Outlet

ARI of Storm Event Peak Floodwater Level at Outlet

1 Year 22.42 mAHD

5 Years 22.80 mAHD

10 Years 22.95 mAHD

100 Years 23.73 mAHD

The modelling shows that the reshaped wetland will not overtop and spill onto Commercial Road during the 100 year ARI event. There is 270 mm of freeboard between the peak 100 year ARI flood level of 22.73 mAHD and the spill level of 24 mAHD. Any ground level development proposed to occur adjacent to the wetland will need to consider these flood levels when determining the elevation of finished floor levels.



Figure 3.2 1 Year ARI Floodplain Map



3.2.2 Flood Hazard Mapping Results

The criteria adopted for determining the flood hazard rating at a particular location is shown in Figure 3.6. The flood hazard is dependent on both flow velocity and flood depth. These hazard classes can be found in the SCARM Report 73 (CSIRO, 2000).

Figure 3.6 Flood Hazard Legend Flood hazard maps are shown in Figures 3.7 – 3.10. As expected, the modelling results indicate that flooding hazards become more pronounced with an increase in the ARI. The low and moderate hazards are typically in the shallower areas at the edges of the wetland and in the macrophyte zones, with high and extreme hazards shown in the deeper sections of flooding such as within the sedimentation basin and near the outlet. Given the wetland consists of a large body of water, it is not expected that large flow velocities will occur on the site with the exception of the swales immediately adjacent to the inlets.



Figure 3.7 1 Year ARI Flood Hazard



3.3 Indicative Cost Estimate

Indicative construction costs for the wetland are proposed to be developed following further discussion with Council and Wax Design regarding the scope of any proposed civil works.



4 References

Aecom (2010) Hydrological Services and Investigations - Coastal Sub-Catchments Study Report Bureau of Meteorology IFD Program http://www.bom.gov.au/water/designRainfalls/ifd/index.shtml) City of Onkaparinga Water Resources Service Levels and Service Standards Guideline Document City of Onkaparinga (2013) Seaford District Centre Development Plan Amendment - Explanatory Statement and Analysis CSIRO (2007), The Adelaide Coastal Waters Study – Final Report, Volume 1, Summary of Study Findings for the South Australian Environment Protection Authority CSIRO (2000) Floodplain Management in Australia – Best Practice Principles and Guidelines SCARM Report 73, CSIRO Publishing. LGA SA (2012) Guidelines for Developing a Climate Change Adaptation Plan and Undertaking an Integrated Climate Change Vulnerability Assessment Watercom Pty Ltd (2011) DRAINS user Manual BMT WBM (2010) TUFLOW (and ESTRY) User Manual


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