casey growth area ballarto road psp 57 stormwater ...€¦ · casey growth area . ballarto road psp...

Neil M Craigie Pty Ltd ACN 074 582 282 ABN 29 074 582 282

Waterway Management Consultants

Director Neil McKinnon Craigie BE(Civil), MEngSci, MIEAust, CPEng Email: [email protected]

40 Jamieson Court, Vic. 3939, Australia Mobile: 0427 510 053

CASEY GROWTH AREA

BALLARTO ROAD PSP 57

STORMWATER MANAGEMENT STRATEGY (SWMS)

(DRAFT-Version 1)

5 March 2013

Neil M Craigie

PSP57 SWMS (Draft V1)

Neil M Craigie Pty Ltd

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TABLE OF CONTENTS

1. INTRODUCTION 1

2. REQUIREMENTS OF THE BRIEF 1

3. BACKGROUND INFORMATION 3

4. DESIGN CRITERIA AND ASSUMPTIONS 4

4.1 Flood Storage Systems 4

4.2 Water Quality and Quantity Treatment Protocol 6

4.3 Water Reuse 7

4.4 Cross PSP Boundary Issues in the Western Port Catchment 7

5. CATCHMENTS AND DRAINAGE OUTFALLS 8

6. THE OUTFALL 4 WLRB 9

6.1 Levels 9

6.2 Wetland Sizing 9

6.3 Flood Mitigation Storage Volume 10

6.4 Allowable Peak Flows 10

6.5 WLRB Reserve Area 10

6.6 Is there an alternative? 12

7. STAGING/IMPLEMENTATION CONSIDERATIONS 14

7.1 General Principles 14

7.2 Development Application Requirements and Compliance 14 Figures 1-4



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1. INTRODUCTION In partnership with Casey City Council (CCC), the Growth Areas Authority (GAA) is managing the preparation of Precinct Structure Plans (PSPs) for the Casey Growth Area. These precincts have been created as a result of the extension of the Urban Growth Boundary through Amendment V68 passed by the Victorian Government in July 2010 and the boundaries are shown on Figure 1. Management of surface water (drainage, flooding, water quality, waterway values) and protection of local environmental values and Western Port Bay are key issues across all PSP’s.

This study has been commissioned by Newland Developers (owners of 365 Clyde-Five Ways Road, Clyde) acting on behalf of landowners in PSP 57 to:

• assess the current drainage and hydrological makeup of the land in PSP 57;

• develop a surface water management strategy (SWMS) that responds to and effectively mitigates the impacts of urban development on receiving waterways and environments;

• provide concept designs for stormwater management that detail alignment of drainage lines, location and size of retarding basins, water quality treatment wetlands, waterway setbacks, and key habitat protection and enhancement measures;

• confirm the allocation of land take in the draft urban structure plan for stormwater management infrastructure and associated mitigation works and to confirm potential future stormwater assets for Melbourne Water (MW) which will be included in future MWC Development Services Schemes (DSS).

The northeast sector of PSP 57 is part of the Clyde Creek catchment in PSP 54. The Clyde Creek catchment has been considered in detail as part of the PSP 53 and PSP 54 investigations and the outcomes and recommendations have been adopted for this report. The balance of PSP 57 drains to the south and into PSP 58 on the south side of Ballarto Road. All of the lands form part of the greater Clyde Creek/Western Contour Drain catchment which outfalls to Western Port Bay. Figure 2 shows the lands included in PSP 57, with the likely expansion of Casey Fields.



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Figure 1 Casey Growth Area and Precincts

PSP 57

PSP 53

PSP 54

PSP 55

PSP 58

PSP 56



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Figure 2 PSP 57 boundaries (red dash)

with Option 3 Layout for Casey Fields Expansion, and showing

existing drainage outfall alignments (blue dash)

Catchment boundaries



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2. REQUIREMENTS OF THE BRIEF Although not commissioned by the GAA, this study for PSP 57 has been carried out in sympathy with the general requirements applied to PSP 53 and 54 drainage investigations. With suitable modifications to suit PSP 57 circumstances these are as set out below.

The drainage strategy shall:

1. Provide a drainage design solution that generally complies with the Precinct Structure Planning Guidelines and:

• Incorporates design methods and innovation where possible to minimise land take.

• Maximises the ability to utilise land set aside for drainage for open space, recreation and other urban purposes.

• Contributes to a green urban environment with extensive tree and other planting.

• Integrates with other proposed elements of the PSP such as town centres, education and community facilities, as well as, the quarry site.

• Allows for appropriate maintenance access and integrates this with recreation uses where practical (e.g. access tracks/pedestrian and cycle paths).

2. Provide an overall drainage design solution that also incorporates a time allowance for iterative local drainage system design refinement in consultation with the GAA (to achieve the objectives outlined in point one above).

3. Retard water flow to pre-development levels for flow events up to the 1 in 10 year Average Recurrence Interval (ARI) level and Best Practice treatment performance, prior to entering Clyde Creek and the Western Outfall Drain.

4. Retard stormwater runoff flows to the capacity of the downstream system or pre-developed flows up to 1 in 100 year events.

5. Ensure adequate buffers are provided around wetland habitats and waterways depending on the quality of terrestrial habitat or dispersal corridors (remnant native vegetation to be retained within the buffers).

6. Enable existing channels within the CGA to be re-modelled to cater for increased flows as they have low geomorphological value (GGF and DG do not rely on these in-channel habitats).



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7. Include the construction of waterways designed to protect movement opportunity of fish and other aquatic fauna between suitable habitats.

8. Determine locations and sizing of in-catchment treatment of stormwater (to Best Practice - 80% Total Suspended Solids, 45% Phosphorus and 45% Nitrogen reduction) prior to water entering Clyde Creek and Western Outfall Drain or any existing or constructed wetland for habitat purposes.

9. Undertake an options analysis for the potential location of retarding basins and stormwater quality treatment assets within the CGA.

The drainage strategy should:-

10. Quantify volumes of water that can be harvested at key locations in the system to protect waterway values, it is suggested that developments should harvest stormwater to prevent excess run-off from events up to the 1 in 1 year ARI. MW will continue to work with the GAA, South East Water and Council to ensure that any stormwater harvesting opportunity is not lost as part of the PSP planning for the area.

11. Ensure designs of RB within the CGA maximise connectivity for aquatic species.

12. Comply with SEPP F8 water quality targets for Western Port Bay. It is intended that the proposed RB/wetland downstream of the CGA will provide additional water quality treatment to achieve SEPP F8 targets.

Based on other modelling in Western Port, the SEPP target are likely to be as high as:

- 93% reduction in suspended solids loads. - 66% reduction in total phosphorus loads. - 63% reduction in total nitrogen loads. However, Melbourne Water in consultation with stakeholders will adopt a practical water quality treatment target for the stormwater discharging from the proposed large RB/wetland. It is intended that the entire stormwater system will provide stormwater treatment above the current Best Practice guidelines.



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3. BACKGROUND INFORMATION In late 2011 MW engaged SKM to assess the Casey Growth Area (CGA). The assessment included a study of environmental issues of the area including aquatic ecology, geomorphology, water quality, hydrological, cultural heritage and groundwater issues affecting Clyde Creek, Muddy Gates Drain and the Western Outfall Drain. All these drain into Western Port Bay. The SKM 2012 report ‘Casey Growth Area Planning: Assessment of the risk to water dependant environmental values from development of the Casey Growth Area (Part A – Clyde Creek/Western Outfall Drain and Muddy Gates Drain)’ has been used to inform the SWMS. A further study of water use and reuse across the CGA is being carried out in parallel with the SWMS. South East Water (SEW), MW and Southern Rural Water (SRW) have developed a draft Integrated Water Management (IWM) strategy for the South East Region of Melbourne which coincides with the extension of the urban growth boundary in the municipalities of Casey and Cardinia. The aim is to develop the optimal mix of water related solutions for the community in this region taking account the many values of water, the associated environmental footprint, optimisation of resources and infrastructure, supporting customer choice and providing community value. SEW is preparing a Servicing Master Plan for the Casey Cardinia Growth Expansion. The aim of this master plan is to develop a servicing plan with consideration given to meeting demand through non-conventional means incorporating elements of IWM. The non-conventional means include local sewerage treatment and reuse, stormwater harvesting (with potential treatment and re-use), and rainwater tanks. SKM acting as consultants for SEW are currently investigating 9 options for service delivery. Outputs from the SWMS investigations (volumes and quality of surface water, location and sizing of various storage and treatment systems, will be used to directly inform SKM’s options analysis. MW have supplied detailed LiDAR survey overlaid on aerial photography for use in the investigations. In addition to the draft reports and modelling completed by Neil M Craigie P/L for PSP 53 and 54, other reports and studies that have influenced development of the SWMS for PSP 57 include:

• “Drainage, An analysis of opportunities and constraints in Investigation Areas”. Beveridge Williams and Neil M Craigie P/L February 2009. This study was part of the Melbourne@5Million investigations and it defined an initial drainage layout for the CGA areas.



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• Southeast Urban Growth Zone, The Proposed Western Contour Drain

Wetland/Retarding Basin Strategic Investigation Discussion Paper, for Melbourne Water, Neil M Craigie P/L, 13 March 2011.

The RORB1 hydrologic model and MUSIC2 water quality model have been used for all investigations in the wider catchment studies for Clyde Creek/Muddy Gates Creek. This work is reported in the PSP 53 and 54 SWMS documents and is not repeated herein.

4. DESIGN CRITERIA AND ASSUMPTIONS

4.1 Flood Storage Systems The process of urbanisation dramatically increases impervious areas (increasing the proportion of rainfall which becomes surface runoff) and modifies the drainage system characteristics through piping of minor catchments and construction of open waterways (typically reducing the response time for surface runoff). The net hydrologic impact in the absence of appropriate countermeasures is significantly increased peak discharges, volumes and frequency of surface runoff events. To offset the impacts of increased peak discharges, flood retarding storages are provided and constructed open waterways are designed to slow velocities. However retarding storages have little effect on volumes or frequency of runoff events and it can be these impacts which are of most concern (a) for ecological conditions in receiving natural waterways, and (b) for agricultural operations on lands downstream. The SKM report has found that the receiving waterways in the CGA in the greater Clyde Creek/Muddy Gates Creek catchments have been so modified that there are negligible remnant environmental values, apart from the Category 1 Growling Grass Frog (GGF) corridor in the lower end of Clyde Creek. However the report also emphasises that there are opportunities to restore and rehabilitate naturalised open waterways with improved values as part of overall drainage planning.

1 RORB is the name given to an industry-standard Runoff Routing Model originally due to Laurenson EM and Mein RG. It is an interactive runoff and streamflow routing program that calculates catchment losses and streamflow hydrographs resulting from rainfall events and/or other forms of inflow to channel networks. It is used for flood estimation, spillway and retarding basin design and flood routing. 2 MUSIC is the acronym used for the Model for Urban Stormwater Improvement Conceptualisation software developed by the Cooperative Research Centre for Catchment Hydrology to model urban stormwater quality management schemes.



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Hence for the purposes of developing this strategy for PSP 57 it has been assumed that:

• there are no ecological obstacles to construction of pipelines or new naturalised waterways to replace the existing (largely artificial) minor drainage lines;

• design of new waterways should aim at maximising potential habitat,

recreational and landscape values, in addition to providing flood conveyance at minimum velocities;

• water quality treatment will be required on all tributary catchments to protect the ultimate downstream receiving environments;

• online wetland and pondage systems may also be implemented as part of the

water quality treatment trains where topography creates the opportunities, provided that such assets can be designed to comply with MWC guidelines and do not create significant ecological barriers;

• online wetland and pondage systems can and should be integrated with flood

storage wherever feasible. It is recognised that land values in town centres and other activity centres and industrial zones will usually favour the use of pipelines rather than open waterways. The same applies for drainage lines passing through schools and major active open space land. Integrated flood retarding storage and water quality treatment pondages take two main forms:

• Sediment basin/retarding basins (SBRB’s); • Wetland/retarding basins (WLRB’s).

Wetlands and sediment basins that are not intended to provide flood retarding storage (ie., those for which airspace volume is only sufficient to provide for the necessary extended detention storage for water quality treatment) are labelled as WL’s and SB’s respectively. Water surface areas are sized to comply with specified water quality treatment standards. In conjunction with the relevant level constraints, this in turn then



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determines the potential flood storage capacity in the airspace above the water surfaces. It follows then that selection of the appropriate design forms is dependent to a large degree on the adopted water quality treatment protocol to be followed across the CGA, as well as on topographic opportunity, environmental values and planning proposals.

4.2 Water Quality and Quantity Treatment Protocol Water quality treatment standards are quoted as percentage removal of the typical urban loads of Gross Pollutants/Total Suspended Solids/Total Phosphorus/Total Nitrogen (GP/TSS/TP/TN). Current best practice standards are 70/80/45/45 percent removals of the typical urban loads. Gross pollutants are removed in either purpose built traps or sediment basins and wetlands. Whatever technique is used GP removal does not impact on water surface area or flood storage systems. Hence the focus of this strategy is on TSS/TP/TN removals. From the guiding objectives set out in the brief and after consideration of the results of the SKM study for the southerly draining catchments to Western Port, the appropriate water quality treatment protocol for lands in PSP 57 have been derived. Lands draining north into the Cranbourne East PSP will form part of the extension of Casey Fields and no further work is required in this SWMS. Those parts of PSP 57 draining south across Ballarto Road, pass firstly into PSP 58 (future industrial land), and thence through PSP 56. The downstream drainage infrastructure is over-taxed under existing conditions. Rural grazing dominates across PSP 58 lands. Intensive agriculture enterprises dominate along the drainage lines through PSP 56. Development of PSP 57 is likely to precede the ultimate development of PSP 58 and the residential development in PSP 56 further downstream. Therefore, to protect downstream rural industries it will be necessary to provide sufficient retardation storage in PSP 57 to maintain existing peak discharges for all events up to and including the 100 year ARI event. Having regard to the fact that PSP 57 land is remote from eventual outfall to Western Port, it is considered that conventional best practice water quality treatment standards (80/45/45) will suffice in all outfalls at Ballarto Road.



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For the balance lands in the north east sector which drain into the Clyde Creek catchment at Clyde-Five Ways Road, all permanent water quality treatment infrastructure is to be provided downstream within PSP 54. Hence lands within PSP 57 will be required to pay contributions to the MWC DSS on an equivalent basis to those lands in PSP 54, according to zoning.

4.3 Water Reuse The beneficial impacts of water reuse in support of or in lieu of treatment and disposal to the drainage system have not been factored into the above protocol. It is considered that until such time as an agreed strategy is in place locking in such reuse, the drainage strategy must assume no reuse for the purposes of sizing treatment facilities across the drainage system.

4.4 Cross PSP Boundary Issues in the Western Port Catchment Topographic opportunities for surface water management assets do not always match the PSP boundaries. In some drainage lines overall drainage scheme planning could favour locating wetland systems downstream of the PSP boundaries. Cost offsets for all such asset transfers can be resolved via the MWC DSS. It has been agreed by MWC/GAA that the strategy across the CGA should be founded on best matching topographic opportunity and constraints (and planning constraints) to location of main drainage assets. This means that some site-specific variations to the water quality treatment protocol set out in Section 4.2 could occur. Of most benefit for PSP 57 is the location of all major drainage management assets for land draining east to Clyde-Five Ways Road downstream in PSP 54.



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5. CATCHMENTS AND DRAINAGE OUTFALLS The tributary drainage subcatchments for the Clyde Creek/Muddy Gates Creek RORB model (as used in the original SE WLRB study) are shown on Figure 3. It can be seen that PSP 57 is split three ways. The north-westerly part drains into Casey Fields and the Cranbourne East PSP (Clyde Creek) catchment. All land draining north in PSP 57 will form part of the extension of Casey Fields and is not considered further in this report. The bulk of PSP 57 is basically represented by one subarea in the northeast sector (PSP 54 outfall) and one other for the balance land to Ballarto Road. For the recent PSP 53 and 54 studies the northeast sector was split into two subcatchments, reflecting two outfall crossings of Clyde-Five Ways Road. No further modelling is required for this part of PSP 57 because all major drainage management assets are to be located east of Clyde-Five Ways Road within PSP 54. Permanent drainage works comprising conventional 5 year ARI capacity pipelines and overland flowpaths are all that is required in the easterly draining parts of PSP 57. Figure 4 shows a more detailed subcatchment layout and likely alignments for the main pipelines draining to the Clyde-Five Ways Road outfalls labelled 1 and 2. The total developable land area draining to Clyde-Five Ways Road is estimated at 46.0 ha. For the balance southerly draining land there are two outfall points shown on Figure 4; one in the far southwest corner of PSP 57 (outfall 3-5.5 ha developable catchment), and one adjacent to the existing major dam (outfall 4-56 ha developable land plus 28.7 ha Casey Fields extension). A major wetland/retarding basin (WLRB) is required at outfall 4. In theory another minor WLRB will be required at outfall 3. However this site will have high land values given its immediate proximity to the main Casey Fields Boulevard crossing of Ballarto Road. The LiDAR contour data indicates that a 5 year ARI capacity pipeline could be used to divert flows eastwards to the outfall 4 WLRB with overland flows passing across Ballarto Road via future culverts. This will increase the size of the outfall 4 WLRB but will prove more economic to construct than a separate WLRB.



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For the purposes of this report it has been assumed that this option will be adopted and pipe flows from outfall 3 will be directed eastwards along Ballarto Road. Hence total developable land area directed to outfall 4 is 61.5 ha.

6. THE OUTFALL 4 WLRB

6.1 Levels This will be the only major drainage management asset required in PSP 57. LiDAR data shows the existing lowpoint level of Ballarto Road to be about 37 m. Downstream of Ballarto Road a large on-stream dam is present with normal water level of 35.5+ m and top of bank 36.0+ m. To avoid the need for modification of the dam and outlet works it would be prudent to adopt a Normal Top Water Level (NTWL) not lower than 35.7 m for the proposed WLRB. With less than 1.3 m airspace storage depth above NTWL to overtopping level of Ballarto Road it may be necessary to construct a new embankment parallel to and just north of Ballarto Road to provide the necessary flood mitigation storage capacity in the WLRB.

6.2 Wetland Sizing The MUSIC Version 3 model was used to size the wetland water surface area, using Koo Wee Rup 1996 6 minute rainfall data and extended detention depth for the wetland of 0.3 m. The results in Table 1 comply with the water quality treatment protocol and were obtained with total inlet pond volume of 3,000 m3 and wetland macrophyte area of 1.8 ha. Mean annual flow volumes can be used as inputs into the water reuse study being carried out by SEW.

TABLE 1 MUSIC Model Results (PSP 57 South KWR 2004 6 min) Location/Asset/Parameter Catchment

Source Loads Residual

Loads % Load removal in

system to asset outlet Outfall 4 WLRB

Flow (ML/yr) 395 372 6 Suspended Solids (Kg/yr) 71,600 14,100 80 Total Phosphorus (Kg/yr) 151 49 68

Total Nitrogen (Kg/yr) 1,090 590 46 Gross Pollutants (kg/yr) 12,300 0 100



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6.3 Flood Mitigation Storage Volume Given the lack of a proposed urban design layout for PSP 57, no new RORB modelling has as yet been carried out for this small catchment. Instead reliance has been placed on accepted estimating relations for the southeast areas draining to Western Port Bay, but which are not affected by tidal conditions. To maintain peak discharges as for existing conditions for all events up to and including the 100 year ARI flood a retarding storage capacity of not less than 500 m3/ha of developed land will be required. Hence for the total developed catchment area of 61.5 ha the airspace flood storage volume must be at least 61.5*500=30,750 m3. Note that this estimate assumes Casey Fields will manage surface runoff to maintain existing conditions flows within its own confines.

6.4 Allowable Peak Flows Using the Rational Method with Adams Equation and runoff coefficients from Book 2 of Australian Rainfall and Runoff, the peak flows generated under existing conditions from the 61.5 ha catchment to Ballarto Road are 0.4 m3/s, 0.9 m3/s, and 1.8 m3/s for 1, 10 and 100 year ARI events respectively. These peak flows are the design objectives to be applied for discharge from the outfall 4 WLRB.

6.5 WLRB Reserve Area Sediment Drying Area The “WSUD Engineering Guidelines-Stormwater” (WSUDEG-SW) procedures (Chapter 4) are used to determine the required basin sizing as per MWC conditions. The relevant parameter values and design sizings have been assessed as follows, with target particle retention being 90% of 125 um size:

Site Urban Catchment Area (ha)

% impervious

Tc (mins) 1 year ARI peak flow

(m3/s)

Sediment loading rate (m3/ha/yr)

Storage period (yrs)

Sediment Accumulation

Volume required (m3)

Outfall 4 WLRB

61.5 60 18 2.0 1.6 5 442



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MWC are now applying very conservative requirements for the provision of drying area to temporarily store and dry wet extracted sediments, pending eventual removal and disposal. The area required is determined as the 5 year cleanout interval/50% accumulation volume divided by average wet storage depth of 0.5 m. For this asset the sediment drying area is thus 442/0.5=884 m2. The sediment accumulation volume of 442 m3 has to be less than or equal to 50% of the total inlet pool storage volume at NTWL. This is easily satisfied by the 3,000 m3 adopted. Site Area requirements Site requirements will also depend on batter treatments and access. For the purposes of this exercise it is assumed that a 6:1 average slope will be applied to embankments and excavated batters plus 4 m wide access/maintenance trails around the wetland edge and the reserve perimeter. An access ramp will be provided at 10:1 down to the drying bench and then into the base of the sediment pond. With total water surface area of 2.1 ha for the wetland and inlet pond the likely reserve area can be estimated using the following considerations:

• Not less than 20 m setback between water’s edge at NTWL and any road or lot boundary;

• Sediment drying area of about 1,000 m2; • 3:1 edges to NTWL +0.3 m, 6:1 minimum above; • Landscaping and recreational space of about 0.5 ha.

The concept layout shown on Figure 4 provides for a reserve area of up to 4.8 ha. This area estimate is sensitive to the assumptions used in its derivation and hence is subject to variation with more detail functional design after preparation of an urban design for PSP 57. A key assumption to be tested in the future is the final land area required for the Casey Fields extension. If this is less than the area shown on Figure 4 then developable land area increases and so does wetland area and flood storage volume.



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Total flood storage volume available at 37.0 m on Figure 4 exceeds 33,000 m3 so it may be possible to delete the embankment along Ballarto Road. This would reduce site area by at least 0.6 ha.

6.6 Is there an alternative? The layout shown on Figure 4 complies with known site constraints posed by Casey Fields extension, downstream drainage inverts, flat east-west topography, and limited flood storage depth to the level of Ballarto Road. It also meets the water quality and quantity treatment protocols. However Figure 4 should still be considered as a starting point for further modification as the PSP layout evolves following confirmation of cultural/heritage and environmental values and Casey Fields extension area, and other planning and transport objectives progress, and especially after landowner inputs are received. Receipt of field survey of existing drainage culverts will also be of benefit for design. If a deeper outfall invert is able to be negotiated at the time development occurs then the wetland NTWL can be lowered. This allows extended detention depth to be increased which will reduce wetland area. The increased airspace flood storage depth may also permit a lower embankment or no embankment at all along Ballarto Road. It may be feasible to relocate part or all of the WLRB into PSP 58 downstream. However given the higher density land uses proposed south of Ballarto Road it is not considered likely that this option could succeed.



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Figure 4 Proposed Main Drainage Layout for PSP 57 (based on Casey Fields Extension Option 3). Drainage outfalls listed as 1-4.

Figure 3 RORB

subcatchment layout

(Clyde Creek and Muddy Gates

Catchments

PSP 57

Outfall 4 WLRB



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7. STAGING/IMPLEMENTATION CONSIDERATIONS

7.1 General Principles The location of the stormwater management assets shown on Figure 4 has been arranged to minimise overall capital and ongoing costs and to utilise as much land which is already encumbered by flooding as is practicable (rather than otherwise developable land), having regard to other constraints such as flora/fauna values, main roads, railway, and the Casey Fields extension. It is possible to split WLRB’s into segments to better suit staged or “out-of-sequence” development, or to resolve property ownership demarcations. However there is an “efficiency” penalty in doing this. As storage depths are basically fixed by flood levels and creek levels, and batter lengths are increased, splitting storages directly increases land area requirements. Other studies indicate storage capacity requirements rise by about 20% on average when a WLRB is split into two segments. Similarly there is a penalty for ongoing operation and maintenance costs with increased numbers and total areas of assets. Subject to suitable arrangements being put in place to cover any capital cost or ongoing cost penalties and the same performance standards being met, there is no technical reason why a storage cannot be split to better suit development layouts or land ownership differences. It is standard practice in urban development contributory drainage schemes across the greater Melbourne area, for any temporary management facilities that may be required to service “out-of-sequence” development (as may be required to protect downstream undeveloped land and/or the environment) to be funded by the proponents of that development without reimbursement from the scheme. Timing of construction of WLRB’s (and connecting pipelines or waterways) is entirely governed by the progress, rate and staging of development. The need for, and extent/size of any temporary management facilities that may be required to service “out-of-sequence” development is similarly affected. Subject to MWC and Council agreement (as the ongoing responsible bodies for operation and maintenance), flexibility should always be retained to allow different landowners to negotiate changes to drainage layout and design of assets-with any extra capital costs outside the DSS also being negotiated between them.

7.2 Development Application Requirements and Compliance Applications for development approval for lands within the PSP areas may include construction of permanent works included in the DSS, or temporary works to adequately service “out-of-sequence” developments or to defer major works expenditure.



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The following principles will be applied by MWC and Council in responding to all applications:

• Temporary works do not form part of the DSS and hence are to be fully funded by the development proponent, unless they are part of ultimate drainage design works (eg., partial excavation of a larger SBRB or waterway or wetland that are to be funded as part of the DSS).

• Development proponents are required to show in any application how the

development proposal affects, or is affected by the requirements of this SWMS.

• Development proponents must provide Stormwater Environmental Management Plans (SEMP) which identify potential waterway stability/environmental/drainage/flooding problems and constraints arising from their development proposals (including upstream or downstream impacts on existing receiving environments, waterways, land uses and assets/works), and quantify and recommend what is required to ensure compliance with best practice water management objectives.

• Unless otherwise pre-approved by MWC and Council, temporary works are not to be designed in a manner which prevents free invert drainage and/or which causes under-design surcharging for any permanent pipeline system (eg., a 5 year ARI capacity pipeline being surcharged in lesser storm events because of outfall capacity restrictions). Every SEMP must include computations verifying compliance with this requirement.

• Every SEMP must deal explicitly with control over stormwater sediment loads

and monitoring of same during estate construction works, and demonstrate how the works comply with best practice whilst addressing high construction-era sediment loads, potential acid sulphate soils and dispersive soils management issues.

• Potential acid sulphate soils and dispersive soils management issues are to be identified and appraised by suitably qualified geotechnical personnel.

• Where the proposed development drainage management measures do not form

part of the DSS schedule, the development proponents are required to investigate, design, construct and fund all costs of establishment of the temporary works, including monitoring and reporting of water quality testing as may be required by MWC, Council, or DSE, and ongoing maintenance requirements and costs.

• Statements of compliance will be conditional, in part, on cleanout and

resetting of sediment management assets before handover to MWC/Council for ongoing responsibility, and on satisfactory financial arrangements being reached with MWC/Council for ongoing maintenance and eventual reclamation of temporary works.



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Methodology and performance standards Best practice water quality performance is to be demonstrated using the MUSIC model. For hydrologic assessments (flow and storage computations), the RORB model or equivalent is to be used. For hydraulic modelling of one-dimensional flow systems the HEC-RAS model will suffice for water level, velocity and channel shear stress computations. For more complex hydraulic situations (generally wider floodplains), two-dimensional hydrodynamic models are to be used such as TuFlow or Mike 21 or their approved equivalents. Where no gauged streamflow data exists to allow peak flows for existing rural conditions to be derived, the Rational Method may be applied for this purpose using Adams equation for estimation of time of concentration with matched runoff coefficients, all in accord with the recommendations set out in Australian Rainfall and Runoff (ARR). The 10 year ARI runoff coefficients provided in Volume 2 of ARR should be used and not those listed in other references such as VicRoads Design Manuals. Unless specifically directed otherwise by MWC and/or Council:

• temporary drainage management works are to be sized to maintain existing conditions peak flows for all events up to and including 100 years ARI, on lands downstream of the subject property.

• temporary drainage management works are to be designed to maintain existing conditions flood levels for all events up to and including 100 years ARI, on lands upstream and downstream of the subject property.

• best practice water quality objectives must be achieved prior to water exiting

from the boundaries of the relevant development. This allows for options such as overland flow dispersal across vegetated areas within a larger development to be implemented.

Bioretention or infiltration systems will not be accepted as temporary sediment management works for any development application. Neil M Craigie

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