J. WYNDHAM PRINCE CONSULTING CIVIL INFRASTRUCTURE … · J. Wyndham Prince Consulting Civil...

CONSULTING CIVIL INFRASTRUCTURE ENGINEERS & PROJECT MANAGERS

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J. WYNDHAM PRINCE

Our Ref: 110350_2_Elara ISWMS.docx

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31 August 2017 Stockland Level 23, 133 Castlereagh Street SYDNEY NSW 2000

Attn: Mike Milligan

Subject: Elara Precinct 5 Marsden Park (JRPP-15-2324);

Amended Stormwater Management Strategy

Dear Mike,

In order to support the ongoing construction Precinct 5 at Elara there is a need to amend the stormwater management strategy for the precinct, in particular the relocation of the detention basin that exists to support the current Elara development to a location that can service a greater portion of the development in the interim. The strategy presented below supports the. approval for the entire Elara Precinct 5 development together with the existing approved stages of the development. It also considers the management of external catchments that will need to utilise the proposed stormwater management device as the overall development in the Marsden Park Precinct progresses.

1. BACKGROUND

An existing temporary detention basin known as temporary Basin 3 was designed and documented by Cardno in August 20141 to support the MPP1 Stockland development. An amendment to the temporary Basin 3 arrangement was prepared by J. Wyndham Prince in December 2016 to support the Elara Stages 39 & 40 subdivision2.

Ultimately the stormwater quality and quantity management of the catchment discharging to the existing temporary Basin 3 will be provided in regional detention Basin ML 1.03 (previously known as Basin 3). It is therefore proposed to relocate this temporary detention basin to the footprint of the ultimate device.

1 Engineering Services and Civil Infrastructure Report – Proposed Subdivision – MPP1, Marsden Park, Cardno 25 August 2014.

2 Elara Stages 39 + 40 Development; Stormwater Management Letter, J. Wyndham Prince 21 December 2016.

3 Section 94 Contributions Plan No.21 – Marsden Park, Blacktown City Council, 14 December 2016.

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J. Wyndham Prince Consulting Civil Infrastructure Engineers & Project Managers

Document: 110350_2_Elara ISWMS.docx 2 of 10

Blacktown City Council confirmed (email correspondence dated 24 July 2017) that temporary basins can be located on future SP2 drainage land while it is still in the developer’s private ownership. They note that the temporary works are to ensure that the embankment is not within the footprint of the ultimate permanent embankment, and are to be fully removed as part of any subsequent Section 94 works. A copy of email correspondence from Blacktown City Council is provided in Appendix A.

It is noted that Winten Property Group’s adjacent Newpark Development utilises OSD storage over an approved BEW sediment Basin 15 (LDC Ref. #14446). Both sediment basin 14 and 15 approved as part of Winten’s Bulk Earthworks Construction Certificate (LDC Ref.# 14108) will be superseded by this new device. The proposed OSD storage over the Elara Sediment Basin/relocated temporary Basin 3 will therefore need to consider stormwater management of Winten’s land as well as the existing developed catchment currently managed by Temporary Basin 3. Plate 1-1 provides an overview of the site.

Plate 1-1 Detention Basin Locality and Site Overview

Section 2 and section 3 below detail the interim stormwater management strategy to support the approval of the Elara Precinct 5 development.



2. STORMWATER QUANTITY MANAGEMENT

An on-site stormwater detention system (OSD) has been sized to limit the post-developed flows to the pre-developed flows for the 0.5 EY (2 year ARI) and 1% AEP (100 year ARI) storm events.

The existing conditions XP-RAFTS hydrologic model that supported the Marsden Park Water Cycle Management Strategy4 that informed the rezoning processError! Bookmark not defined. was prepared using a 15% increase in rainfall intensity to reflect climate change conditions. This model has been updated to reflect current climate rainfall intensity.

To maintain consistency with the hydrologic modelling methodology for the Marsden Park Precinct, current climate design rainfall intensity-frequency-duration (I.F.D.) data for the site were obtained using methods set out in Australian Rainfall and Runoff (A.R.R.) (1987), and are presented in Table 3-1 below. All other parameters are as per the modelling approach documented in the rezoning strategy4.

Table 2-1 Marsden Park Current Climate IFD Data

Plate 3-1 provides an overview of the existing conditions XP-RAFTS model layout. Details of the detention basin configuration is presented on J. Wyndham Prince amended Bulk Earthworks (BEW) Construction Certificate plan 11035002/CC092 (Rev. E). The full list of amended BEW drawings is provided on drawing 11035002/CC001 (Rev. C).

4 Marsden Park Residential Precinct – Post Exhibition Water Cycle & Flood Management Strategy Report, J. Wyndham Prince July 2013.



Plate 2-1 Existing Conditions XP-RAFTS model [110350-02_RA_02(Ex).xp]

The existing conditions (current climate) XP-RAFTS hydrologic model was then updated to reflect the inclusion of the development shown in Plate 2-1 to determine the on-site stormwater detention required to ensure that post development flows are no greater than existing conditions flows downstream of the development (Winten Property Group/Clydesdale property boundary).

It is proposed that a portion of the future permanent pond within future Basin ML 1.0 will be utilised to provide the sediment management for the catchment during the construction phase. Detention storage will be provided above the permanent pool level of 13.4 m AHD.

Plate 3-3 provides an overview of the developed conditions XP-RAFTS model.



Plate 2-2 Developed Conditions XP-RAFTS model [110350-02_RA_05_ISWMS (Dev).xp]

The XP-RAFTS models were run for all durations from 10 minutes to 40 hours for the 0.5 EY (2 year ARI) and the 1% AEP (100 year ARI) storm events.

Table 3-2 provides a summary of peak flows at key comparison locations downstream of the development. Please see Plate 3-4 for reporting locations.



Table 2-2 Flow Summary Table

Plate 2-3 XP-RAFTS Reporting Locations

The proposed outlet arrangement to ensure post development flows are no greater than existing conditions flows is detailed in Table 3-3.



Table 2-3 Basin Outlet Arrangement

Details of this outlet are provided on J. Wyndham Prince engineering drawing 11035002/CC092 (Rev. E) which supports the Bulk Earthworks amendment.

Table 3-4 provides details of the interim detention basin performance.

Table 2-4 Basin Performance

Results of the XP-RAFTS hydrologic modelling indicate that the proposed 49,991 m³ interim detention storage and proposed outlet arrangement will ensure that developed conditions flows are no greater than existing conditions flows in the 0.5 EY and 1% AEP storm events.



3. STORMWATER QUALITY MANAGEMENT

Blacktown City Council’s Development Control Plan (DCP) Part J – Water Sensitive Urban Design requires all developments to achieve a minimum percentage reduction for a series of stormwater pollutants. Details of these minimum reductions are provided below:

• Gross Pollutants – 90% reduction

• Total Suspended Solids (TSS) – 85% reduction

• Total Phosphorous (TP) – 65% reduction

• Total Nitrogen (TN) – 45% reduction

• Total Hydrocarbons – 90% reduction

The DCP requires that the construction of stormwater treatment measures only occur once 90% of the catchment is developed, and refers to Blacktown City Council’s WSUD handbook for guidance on treatment measures to achieve the treatment performance objectives.

Due to the imminent design and construction approval of the regional Section 94 infrastructure (Basin ML 1.0 and RG ML 1.1) that will service the greater Marsden Park catchment, and given the likelihood that the regional devices will be in place before 90% of the catchment is developed, a detailed water quality assessment has not been completed. MUSIC modelling Figure 20 of Blacktown City Council’s WSUD Developer Handbook has been utilised to inform the size of the rain garden required for the development.

In accordance with Figure 20, based on the following assumptions;

• a 0.5 m filter depth;

• saturated hydraulic conductivity of 100 mm/hr (modelled, not actual);

• a target particle size of 0.5 mm;

• an extended detention depth of 0.3 m; and

• Total Nitrogen being the limiting pollutant.

A raingarden sized at 1.1% of catchment area is anticipated to provide the required percentage pollution reduction. The device will cater for:

• the existing Elara Development currently managed in Temporary Basin 3;

• The southern portion of Winten’s Newpark Precinct 2 development;

• The eastern portion of Winten’s Precinct 4 and Precinct 6 development; and

• All of Elara Precinct 5.

Delivery of the 4EY (6 month ARI) stormwater runoff to Gross Pollutant Traps (GPTs) within the street drainage network of the development will ensure the bulk of the coarse sediment, gross pollutants and hydrocarbons are removed prior to secondary treatment in the raingarden. The GPTs form part of the subdivision delivery, and will ultimately discharge into regional detention Basin ML 1.0 and raingarden ML 1.1 for secondary treatment.

Table 2-1 provides details of the raingarden sizing for the development. Plate 2-1 provides an overview of the development managed by the proposed raingarden.



Table 3-1 Raingarden Size

Plate 3-1 Development Catered for in Raingarden

In the unlikely event that 90% of catchment is developed before the future regional device is constructed, there is ample room within the 20,841 m² sediment pond area to de-commission the sediment pond and construct the temporary 13,809 m² raingarden.

Details of the raingarden configuration are provided on J. Wyndham Prince engineering drawings 11035001/CC584 and CC585 (Rev. A).



4. SUMMARY/CONCLUSION

This stormwater management strategy has been prepared to support the ongoing development within the Marsden Park Precinct being undertaken by both Stockland and Winten Property Group. This strategy ensures that the bulk of the development that will be catered for in the future regional device are appropriately managed in the interim. The specific elements of this strategy are listed below.

Temporary On-site Stormwater Detention (OSD) - A 49,991 m³ interim detention storage and proposed low pipe/high weir outlet arrangement will ensure that developed conditions flows are no greater than existing conditions flows in the 0.5 EY and 1% AEP storm events.

Temporary stormwater quality treatment system - A temporary 13,809 m² rain garden has been sized based on 1.1% of catchment area. There is ample room in the footprint of the 20,841 m² sediment basin to construct this rain garden if required, however it is likely that the Section 94 Contributions Plan rain garden ML 1.1 will be under construction before 90% of the catchment area is developed.

We trust that this information is sufficient to support the future approval of the Elara Precinct 5 and Newpark development.

Should there be any queries regarding this matter please do not hesitate to contact David Crompton on 4720 3340 or [email protected]

Yours faithfully

J. WYNDHAM PRINCE

DAVID CROMPTON

Manager – Stormwater and Environment Group

mailto:[email protected]

APPENDIX A – Email Correspondence

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Francis LaneFrom: Chris MudieSent: Thursday, 27 July 2017 11:55 AMTo: David Crompton; Phillip Stoddart; Francis LaneSubject: [110350-2] Fwd: Elara Marsden Park JRPP-15-02324 Condition 5.12.1 and 5.13.1

Sent from my iPhone Begin forwarded message:

From: Georg Eberl <[email protected]> Date: 24 July 2017 at 9:25:35 am AEST To: Michael Johnson <[email protected]> Cc: Tana Sainsbury <[email protected]>, Chris Mudie <[email protected]>, Graham Mallison <[email protected]>, Robert Peterson <[email protected]>, Peter Shahatit <[email protected]> Subject: RE: [110350-2] Elara Marsden Park JRPP-15-02324 Condition 5.12.1 and 5.13.1

Hi Michael, Temporary basins can be located on future SP2 drainage land while it is still in the developers ownership. There will be conditions imposed as part of the land transfer to deal with remediation of the temporary works etc. The temporary works should be designed to ensure that the embankment is not located within the footprint of the permanent embankment as the temporary works would not be part of the WIKA and would have to be fully removed as part of any subsequent S94 works. The temporary works would generally need to remain operational while the permanent works are constructed. Regards GEORG EBERL I Senior Engineer – Forward Planning BLACKTOWN CITY COUNCIL PO Box 63 I Blacktown NSW 2148 T 02 9839 6382 F 02 9831 1961 E [email protected] I www.blacktown.nsw.gov.au From: Michael Johnson [mailto:[email protected]] Sent: Monday, 24 July 2017 8:15 AM To: Georg Eberl Cc: Tana Sainsbury; Chris Mudie; Graham Mallison Subject: RE: [110350-2] Elara Marsden Park JRPP-15-02324 Condition 5.12.1 and 5.13.1 Hi Georg, Attached is a sketch showing the proposed location of temporary OSD and sediment Basin. The proposed areas of the basin is currently being sized to accommodate the relevant catchments to service the adjoining land and developments. Could you please confirm that council has no objections to allowing the implementation of temporary OSD and Water Quality Devices within SP2 Land?

2

We will be seeking an amendment to the existing bulk earthworks Construction Certificate to obtain formal approval for these temporary works. Regards Michael Johnson – Project Manager ------------------------------------------------------------------------------------------------------------------------------------------

J. WYNDHAM PRINCE CONSULTING CIVIL INFRASTRUCTURE ENGINEERS & PROJECT MANAGERS P 02 4720 3317 M 0400 058 145 W www.jwprince.com.au 580 High Street, Penrith NSW PO Box 4366 PENRITH WESTFIELD 2750 From: Georg Eberl [mailto:[email protected]] Sent: Wednesday, 12 July 2017 11:40 AM To: Michael Johnson <[email protected]> Cc: Graham Mallison <[email protected]>; Tana Sainsbury <[email protected]>; Chris Mudie <[email protected]> Subject: RE: [110350-2] Elara Marsden Park JRPP-15-02324 Condition 5.12.1 and 5.13.1 Hi Michael, The condition requires plans showing the location of the temporary facilities. Can you send these through please so I have the complete picture of what is being proposed thanks. Regards GEORG EBERL I Senior Engineer – Forward Planning BLACKTOWN CITY COUNCIL PO Box 63 I Blacktown NSW 2148 T 02 9839 6382 F 02 9831 1961 E [email protected] I www.blacktown.nsw.gov.au From: Michael Johnson [mailto:[email protected]] Sent: Wednesday, 12 July 2017 11:21 AM To: Georg Eberl Cc: Graham Mallison; Tana Sainsbury; Chris Mudie Subject: [110350-2] Elara Marsden Park JRPP-15-02324 Condition 5.12.1 and 5.13.1 Good Morning Georg, As discussed with Graham Mallison earlier today, we are seeking clarification on Condition 5.12.1 and 5.13.1 (JRPP-15-02324 and MOD-17-00029) regarding locating temporary stormwater quality control devices on land zoned for future public use. Our understanding of this condition is that Council wants to ensure that temporary deceives are not present at the time of handover to Council. We are proposing to locate temporary stormwater quality control devices within the proposed Basin ML 1.0 footprint. These temporary devices will be completely decommissioned prior to handover of land to Council as the ultimate design will be constructed. Could you please confirm that these conditions are not required and it is permissible to locate temporary stormwater quality control devices within the SP2 Land at Elara Marsden Park? Regards

3

Michael Johnson – Project Manager ------------------------------------------------------------------------------------------------------------------------------------------

J. WYNDHAM PRINCE CONSULTING CIVIL INFRASTRUCTURE ENGINEERS & PROJECT MANAGERS P 02 4720 3317 M 0400 058 145 W www.jwprince.com.au 580 High Street, Penrith NSW PO Box 4366 PENRITH WESTFIELD 2750 ********************************************************************** This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This footnote also confirms that this email message has been swept for the presence of computer viruses. ********************************************************************** ********************************************************************** This email and any files transmitted with it are confidential and intended solely for the use of the individual or entity to whom they are addressed. If you have received this email in error please notify the system manager. This footnote also confirms that this email message has been swept for the presence of computer viruses. **********************************************************************

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