j. wyndham prince consulting civil infrastructure engineers

CONSULTING CIVIL INFRASTRUCTURE ENGINEERS & PROJECT MANAGERS

ABN 67 002 318 621

PO Box 4366

PENRITH WESTFIELD, NSW 2750 ISO 9001:2008 – Quality P 02 4720 3300 AS/NZS 4801:2001 - Safety

W www.jwprince.com.au ISO 14001:2004 - Environment

E [email protected]

J. WYNDHAM PRINCE

Our Ref: 109985-12-Precinct 7 SW Compliance Letter.docx

DC:fl

14 December 2017 Blacktown City Council PO Box 633 Blacktown NSW 2148

Attn: The Development Engineer

Subject: Newpark Precinct 7 Subdivision Development Application;

Stormwater Management Strategy

Dear Sir/Madam,

This letter is to confirm that the Newpark Precinct 7 Development Application (DA), together with the water quality treatment devices proposed as part of this development are generally in accordance with the “Marsden Park Residential Precinct – Post Exhibition Water Cycle & Flood Management Strategy Report” (JWP, July 20131) developed to support the overall Marsden Park Precinct development.

Newpark Precinct 7 is approximately 92 hectares in size and is located on the Western Peninsula of the Marsden Park Residential Precinct.

The proposed development will consist of the subdivision of the site into 1864 residential lots together with a road and street drainage network to service the development.

Stormwater management will be provided by the following devices set out in Marsden Park Section 94 Contributions Plan No. 21 (CP21):

• Detention Basins MS 2.0 and MS 3.0;

• Bioretention Raingardens MS 2.1, MS 3.1, ML 3.0, ML 6.0, and ML 7.0;

• Gross Pollutant Traps MS 3.2, MS 3.3, ML 3.1, ML 6.1, and ML 7.1.

Plate 1 provides a context of the site locality. Full details of the proposed Precinct 7 subdivision development are provided on engineering design drawings 998512/DA00 to DA185.

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

http://www.jwprince.com.au/

J. Wyndham Prince Consulting Civil Infrastructure Engineers & Project Managers

Document: 109985-12-Precinct 7 SW Compliance Letter.docx 2 of 8

Plate 1 Newpark Precinct 7 Overview and Locality Plan

1. BACKGROUND

The Marsden Park Residential Precinct Post Exhibition Water Cycle & Flood Management Strategy Report1 modelled the eastern portion of Precinct 7 (which interfaces with Little Creek) as a undetained catchment, with compensatory detention management provided in regional Basin ML 1.0 (formerly known as Basin 3) and Basin ML 5.0 (formerly known as Basin 2). The western portion of the site was to be managed in a series of detention basins prior to discharge toward South Creek. A series of bio-retention raingardens located along the edge of the development were proposed to ensure the statutory stormwater pollution reduction targets were achieved.

The rezoning strategy indicates that Precinct 7 was to be serviced by two (2) regional detention basins and five (5) bio-retention raingardens for stormwater quantity and quality management.

The Marsden Park Section 94 Contributions Plan No.21 (CP21) made some amendments to the detention volume and bio-retention raingarden filter media areas, merging some devices and generally reducing the size of the devices.

It is our understanding that Blacktown City Council recently completed a further assessment of the need for detention management more broadly in the Northwest Growth Centres but particularly for the Marsden Park Precinct. This report is currently being considered by Council’s Executive Committee. A possible outcome is the removal of the detention management for the entire Precinct 7 catchment. However, confirmation of this amendment to the rezoning strategy is not expected to be released by Council until early 2018. For the purposes of this assessment, stormwater detention arrangements as proposed at rezoning have conservatively been maintained.



Detailed hydrologic, hydraulic and water quality modelling will be prepared as part of the separate future Section 94 concept design process for all devices within Precinct 7.

2. STORMWATER MANAGEMENT

To confirm consistency of the concept design with the rezoning strategy, we have undertaken a comparison between the current catchment areas and the assumptions made for the rezoning catchment areas discharging to each stormwater management device in Table 2.1.

Table 2.1 Flood Level Assessment – 1% AEP + 1% AEP Regional

Detailed modelling of the stormwater management devices and their contributing catchments will be provided to Blacktown City Council’s engineers as part of the Concept Design Report for the Section 94 Contributions Plan items as part of a separate approval process.

It is proposed that the 2 EY (6 month ARI) stormwater runoff from the Precinct 7 development will be collected via the street drainage network and be directed to a series of Gross Pollutant Traps and Bio-retention raingardens for stormwater quality treatment.

For the eastern portion of the precinct, flows in excess of the 2 EY event will discharge directly to Little Creek. Appropriate scour protection will form part of the drainage design to ensure velocities are reduced to acceptable levels prior to discharge to the watercourse.

For the western portion of the site, two (2) regional detention basins will provide peak flow management for the 0.5 EY (2 year ARI) and 1% AEP (100 year ARI) storm events. The location of the devices is provided on engineering drawing 10998512/DA04 and DA05.

3. FLOOD LEVEL ASSESSMENT

Whilst there have been some internal grading changes to provide a continuous rising grade out of the Marsden Park Residential Precinct for flood evacuation purposes, the proposed Precinct 7 development extent within the floodplain is relatively consistent with the development extent considered as part of the rezoning assessment. Hence the flood model that supported the “Marsden Park Residential Precinct - Post Exhibition Water Cycle & Flood Management Strategy Report’ (JWP 2013) is still considered valid to confirm flood levels for Newpark Precinct 7. Further detailed modelling will be provided as part of the detailed design of the bridge (CP21 item ML 4.0) between Precinct 4 and Precinct 7.

Figure 6.15 from the 2013 Water Cycle and Flood Management report provides the developed conditions flood level for the combined 1% AEP + 1% AEP regional tailwater condition (see Appendix A for a copy of this figure). It is noted that all flood modelling in the 2013 assessment considered a 15% increase in rainfall intensity to cater for climate change conditions. An interrogation of the flood results adjacent to Precinct 7 indicate that the proposed development levels provide the minimum 0.5 m freeboard to the proposed lots. Table 3.1 refers to locations shown in Plate 3.1 where flood levels have been compared with engineering design levels.



Table 3.1 Flood Level Assessment – 1% AEP + 1% AEP Regional

Plate 3.1 Flood Level Reporting Locations

Bridge ML 4.0 was previously part of an adjacent development application for the Newpark Precint 4 development adjoining the eastern end of the bridge. The subsequent development consent (DA-16-05360) included a condition (5.3.4) that the soffit of the bridge deck is to be located above the 100 year ARI regional flood level or the local 2000 Year Flood with 1 m freeboard.



Engineering drawing 998512/DA185 shows that the centreline design level of the bridge deck (ML 4.0) is approximately 19.80 m AHD at the lowest point, and it is estimated that the soffit of the bridge at the lowest point will be 18.30 m AHD, which is 1 m above the regional 100 year ARI flood level (17.3 m AHD). Furthermore, the Local PMF level is 18.25 m AHD which is also below the soffit. Further information is provided on drawing 998511/SK13-1 in Appendix A.

4. FLOOD EVACUATION

At rezoning stage, a flood evacuation specialist (Molino Stewart) was engaged to develop an appropriate flood evacuation strategy for Precinct 7 (formerly known as Precinct 1 in Molino Stewart’s original assessment). At that time, it was anticipated that access to land above the regional Probable Maximum Flood (PMF) level (i.e. 26.4 m AHD) within the Air Services Land to the south would be possible under an emergency evacuation situation.

Recent liaison with Air Services Australia has confirmed that a suitable access arrangement to the Air Services land cannot be achieved at this time. Therefore, with the exception of 173 lots between a collector road (Road A100) and the Air Services Land, a continuous rising grade is provided along Road A101 to Richmond Road to provide appropriate flood evacuation.

Given the level of area southwest of The Collector which ranges between 18 and 26.5 m AHD there is still sufficient time for all occupants of Precinct 7 to safely evacuate the precinct via vehicle. Flood water would not enter this area south of the collector run until close to the peak of the PMF event.

Notwithstanding the considerable difference between the Regional Flood Evacuation Level of 17.3m in which an evacuation would be triggered and the lowest point in this portion of Precinct (ie. 18.0 m AHD), a “safe haven” has been provided within the southern portion of Precinct 7 which is above 26.4 m AHD and a rising grade is provided to this refuge point for all 173 dwellings.

A 1,520 m² pad is provided above the PMF level of 26.4 m AHD, which is ample room for the anticipated 500 residents to take refuge. Plate 4.1 provides an overview of the flood evacuation route for pedestrians south of Road A100.

Plate 4.1 Flood Evacuation



This alternate flood evacuation strategy was discussed with Molino Stewart and a letter of support for this strategy is provided in Appendix B. A full flood evacuation plan is provided on engineering drawing 998512/DA03.

5. COMPLIANCE WITH CLAUSE 19 OF SEPP (SYDNEY REGION GROWTH CENTRES) 2006

Clause 19 of the State Environmental Planning Policy (Sydney Region Growth Centres) 2006 requires the following heads of consideration (in italics) to be addressed. The subdivision development application has been prepared giving due consideration to the approved Marsden Park Residential Precinct Post Exhibition Water Cycle & Flood Management Strategy Report (J. Wyndham Prince 2013). We provide the following additional comments as to how these specific sub-clauses of clause 19 of the SEPP have been addressed:

(a) whether or not the development will adversely affect flood behaviour resulting in detrimental increases in the potential flood affectation of other development or properties,

Response: The precinct wide strategy1 mapped the changes that are likely to occur both within and external to the precinct in July 2013. This assessment confirmed that no detrimental impacts will occur to other developments or properties external to the site. The development extent of Precinct 7 is generally consistent with the extent proposed as part of this strategy1.

(b) whether or not the development will alter flow distributions and velocities to the detriment of other properties or the environment of the flood plain,

Response: Appropriate scour protection and erosion mitigation works are proposed to ensure that flood velocities are reduced to acceptable levels where the proposed works interface with the receiving waters. Therefore we do not anticipate that the proposed development will alter flow distributions and velocities to the detriment of other properties or the environment of the flood plain.

(c) Whether the development will enable safe occupation of the flood prone and major creeks land,

Response: All housing development will be constructed to be above the Flood Planning Level (FPL). The flood Planning Level is defined as the combined local 1% AEP event (with 15% increase in rainfall intensity) + regional 1% AEP tailwater flood level (17.3 m AHD) with 0.5 m freeboard. A continuous raising grade is provided for all residents to a level above the regional Probable Maximum Flood (PMF) level of 26.4 m AHD. This arrangement ensures that the Flood Prone Land can be safely occupied and appropriate flood evacuation of the Flood Prone Land is catered for.

(d) whether or not the development will detrimentally affect the flood plain environment or cause avoidable erosion, siltation, salinity, destruction of riparian vegetation or a reduction in the stability of the riverbank/watercourse,

Response: The Soil Salinity and Aggressivity Assessment undertaken by DLA Environmental (May 2015) indicates that there is a low potential for salinity impacts in the vicinity of drainage channels, creeks and water dams at depths of 2 – 3 m below ground level, and that a soil salinity management plan will need to be implemented as part of the proposed works. However there are no long term salinity or siltation issues expected. Appropriate scour and erosion protection works will need to be provided at the site discharge location/s to ensure avoidable erosion is mitigated.

(e) whether or not the development will be likely to result in unsustainable social and economic costs to the flood affected community or general community, as a consequence of flooding,



Response: All proposed development is provided above the Flood Planning Level (see item c above for definition of FPL). The proposed works will enhance the existing environment which is a benefit to the greater community. Therefore we do not anticipate that the proposed development will be likely to result in unsustainable social and economic costs to the flood affected community or general community, as a consequence of flooding.

(f) whether or not the development is compatible with the flow conveyance function of the floodway,

Response: A 142 m bridge crossing is proposed to maintain the flow conveyance function of Little Creek and an unnamed adjacent tributary of Little Creek and to provide access to Precinct 7. No residential development is proposed within a floodway. Therefore the development is compatible with the flow conveyance function of a floodway.

(g) whether or not the development is compatible with the flood hazard,

Response: the proposed development is consistent with the approved Marsden Park Residential Precinct Post Exhibition Water Cycle & Flood Management Strategy Report1 which shows all development is clear of the high hazard mapping in the 1% AEP event.

(h) in the case of development consisting of the excavation or filling of land, whether or not the development:

i). will detrimentally affect the existing drainage patterns and soil stability in the locality, and

Response: flows from the proposed development are safely managed via overland flow paths and a street drainage network and we do not anticipate that there will be any detrimental affect on the existing drainage patterns and soil stability in the locality.

ii). will significantly impact on the likely future use or redevelopment of the land, and

Response: the proposed development supports the proposed future use, and re-development of the land.

iii). will adversely impact on the existing and likely amenity of adjoining properties, and

Response: The proposed development is consistent with the planning proposal and will not have any adverse impact on the existing and likely amenity of adjoining properties.

iv). will minimise the disturbance of relics, and

Response: AHIP No. C0001857 has been issued. The salvage works were strictly carried out in accordance with the AHIP.

v). will adversely impact on any watercourse, drinking water catchment or environmentally sensitive area.

Response: The site is not located in a drinking water catchment. A series of gross pollutant traps and a water quality devices will ensure that the expected stormwater pollutants can be treated and managed to the appropriate standards prior to discharge to any environmentally sensitive areas.



6. CONCLUSION

This report details the investigation completed in order to support the proposed subdivision of Precinct 7 Newpark. Further details of the proposed development is provided in J. Wyndham Prince’s engineering design drawings 998512/DA00 to DA185.

The following Marsden Park Section Contributions Plan No. 21 will provide stormwater quantity and quality management for this development:

• Detention Basins MS 2.0 and MS 3.0;

• Bioretention Raingardens MS 2.1, MS 3.1, ML 3.0, ML 6.0, and ML 7.0;

• Gross Pollutant Traps MS 3.2, MS 3.3, ML 3.1, ML 6.1, and ML 7.1.

As the proposed development is generally consistent with the development extent considered as part of the rezoning process, the previous flood level assessment (JWP 2013) is considered an appropriate basis on which to assess flood planning levels and suitable to inform a development approval. Table 3.1 in Section 3 of this report confirms that the required 0.5 m freeboard to the proposed development is achieved in the 1% AEP + 1% AEP tailwater flood planning level event. Furthermore, the proposed bridge crossing ML 4.0 has sufficient conveyance capacity to ensure that it is flood free in the 0.2 % AEP flood event.

This proposed stormwater management strategy is consistent with the Marsden Park Residential Precinct Post Exhibition Water Cycle & Flood Management Strategy Report (J. Wyndham Prince 2013), and considers the objectives of Council’s policies and provides best practice stormwater management for Newpark Precinct 7.

Detailed flood modelling of the bridge and other Section 94 Contributions Plan items will be provided to Blacktown City Council’s engineers as part of the detailed concept design process for these works.

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 – FIGURES

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PLAN No:

DATEAMENDMENT

J. WYNDHAM PRINCE CONSULTING CIVIL INFRASTRUCTURE ENGINEERS

PO Box 4366 PENRITH WESTFIELD NSW 2750

P 02 4720 3300 F 02 4720 3399 W www.jwprince.com.au E [email protected]

& PROJECT MANAGERS

DES DRN CKD APR

FILE No:

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CLIENT:

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ORIGIN:THIS DRAWING MUST NOT BE USED FOR

CONSTRUCTION UNLESS SIGNED AS PART OF AN

APPROVED CONSTRUCTION CERTIFICATE.

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NOTES:

- BRIDGE TYPE & CONFIGURATION IS INDICATIVE ONLY

AND SUBJECT TO DETAIL BRIDGE DESIGN.

- LOCAL PMF LEVEL ADOPTED FROM MARSDEN PARK

RESIDENTIAL PRECINCT WATER CYCLE AND FLOOD

MANAGEMENT STRATEGY (JWP, 2013)

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APPROX. LOCAL PMF LEVEL RL.18.25

APPENDIX B – REVISED FLOOD EVACUATION STRATEGY

MOLINO STEWART PTY LTD ABN 95 571 253 092 ACN 067 774 332

PO BOX 614, PARRAMATTA CBD BC, PARRAMATTA NSW 2124 TEL: (02) 9354 0300 FAX: (02) 9893 9806

www.molinostewart.com.au

13th

October, 2017

David Crompton

Senior Water Resources Engineer

J.Wyndham Prince

580 High Street

Penrith NSW 2750

Dear David

Re: Marsden Park Precinct Revised Flood Evacuation Strategy

I refer to your recent request to provide an opinion on the evacuation implications of some proposed

modifications to the Marsden Park development. This letter sets out my views based on my review of

the documentation which you sent me.

Background

In a letter of 6th

June 2012, I advised that it was my opinion that:

“An evacuation plan can be developed for the Marsden Park Precinct which is consistent with the SES

strategy for the Valley as set out in its Hawkesbury Nepean Flood Emergency Sub Plan (2005) if the

current regional development and road networks are maintained and Richmond Road is upgraded to a

four lane road between Marsden Park and the M7 Motorway.”

That remains my opinion if the assumptions set out in that letter with regards to dwelling yield, the

evacuation Strategy and evacuation triggers are still valid.

The report went on to say:

“There are a number of reasons that the evacuation of the site might not proceed according to the

evacuation plan. This could include residents deciding not to evacuate or delaying evacuation beyond

what has been modelled, obstructions to the evacuation path or a lack of resources to effectively carry

out the evacuation. “

We analysed the potential for pedestrian evacuation from dwellings if that were delayed until

floodwaters reached the doorstep of those dwellings and concluded that too would be possible

providing the following recommendations were integrated into the final form of the development:

Fill all residential land to at least the 1 in 100 level

Ensure all new roads are above the 1 in 100 flood level and that there is a rising pedestrian

route (and preferably vehicular route) from every residential property to land above the

regional PMF level.

Fill the low points which cause Locations 1 and 2 to become isolated under the existing

topography so that they are no longer low flood islands

Raise the roads from Precincts 1 and 2 which cross the internal creeks to as high a level as

practicable so that these locations have less chance of being isolated

Provide a suitable pedestrian access from the top of Precincts 1 and 2 to flood free land within

the Airservices Australia site to the south. It is also recommended that negotiations on this

commence as soon as practical so as to avoid issues with security fencing obstructing

evacuation paths when required.

Ensure all residential buildings within the environmental living zones have minimum floor

levels, including garage floor levels above the 1 in 100 ARI flood level and that vehicular

P AGE 2 O F 5

access from the buildings to the local road is above this level. They should be constructed

within 50m of the road.

Figures 2 and 3 from the original report are attached to show Locations 1 and 2 and Precincts 1 and 2

referred to above.

The reason for the recommendation of providing access to Air Services land was because Precincts 1

and 2 could become completely inundated and the nearest flood free land is to the south within the

Airservices Australia site (Figure 3). Therefore, to be able to evacuate on foot, residents within

Precincts 1 and 2 would require pedestrian access into the Airservices Australia site.

This was reiterated in later advice regarding pedestrian evacuation of the site in a letter dated 23rd

October, 2014.

Design Modifications

Since the report was prepared, development of Marsden Park has proceeded in stages. You have

advised that it is proposed to modify the design and construction of compared to what was assessed by

me in 2012 and 2014. In, particular, changes are proposed to the layout and topography of what my

previous report referred to as Precincts 1 and 2.

Former Precinct 2

This is now referred to as Newpark Precinct 5. This has been reviewed and regraded to provide a

continuous rising grade out of the precinct via a link into the rest of the development to the east as per

J. Wyndham Prince plan 998511SK06-1 (attached).

As this now provides a continuously rising gradient within the development beyond the extent of the

PMF it is my opinion that this part of the development no longer requires access to the Air Services

land to the south.

Former Precinct 1

This is now referred to as Newpark Precinct 7. You advised that your design review has confirmed that

it is too remote to be able to provide a continuous rising grade out of the precinct to flood free land

within the development.

You have also advised that it has not been possible to obtain formal approval to enter Air Services

Australia land for failsafe pedestrians flood evacuation.

Your alternative proposal is therefore to provide a pedestrian flood evacuation pad in the sourthern

most portion of the precinct set above the regional PMF level of 26.4m AHD. This is shown in J.

Wyndham Prince plan 998512SK20-2 (attached).

This proposed option would provide a pedestrian flood evacuation pad for all lots south of the collector

road in Precinct 7 which if the residents choose to remain in their homes and not evacuate via vehicle

and wait for floodwater to be at their doorstep, the residents can still rise up to an area that is above the

PMF level.

You have stated that the affected lots in this area, based on the current lot layout, is approximately 173

low density residential dwellings (zoned R2) which could accommodate between about 450 and 500

residents. The proposed pad is approximately 1,520 m2 which would provide more than 3m

2 per

person. Which would be sufficient space for people to stand, sit or lie.

I also understand that while this option does not rely on the consultation and agreement to access Air

Services land to the south, emergency services are likely to be able to access this refuge through the Air

Services land if required.

I also note that the areas within this precinct which are below about 22.5m AHD would have a

continuously rising pedestrian route through the development to flood free land. This means that it

would only be those people who failed to evacuate in time in an event exceeding about a 1 in 5,000

AEP flood who may need to take refuge on the elevated pad. Furthermore, if the dwellings in this area

are of two storey construction, many would have their upper floor above the reach of the PMF.

P AGE 3 O F 5

Conclusion

In light of the above discussion, I consider the design changes proposed to provide a suitable

alternative to providing a written agreement for pedestrian access through the Air Services Land.

If you require any further assistance in assessing and managing flood risks at the site please do not

hesitate to ask.

Yours faithfully

For Molino Stewart Pty Ltd

Steven Molino

Principal

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82.362

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AREA ABOVE PMF -

1,520m²

A

IR

S

E

R

V

IC

E

S

L

A

N

D

R

O

A

D

A

100

ROAD A100

R

O

A

D

A

1

1

3

R

O

A

D

A

108

R

O

A

D

A

112

R

O

A

D

A

124

RO

AD

A

126

PARK

PLAN No:

DATEAMENDMENT

J. WYNDHAM PRINCE CONSULTING CIVIL INFRASTRUCTURE ENGINEERS

PO Box 4366 PENRITH WESTFIELD NSW 2750

P 02 4720 3300 F 02 4720 3399 W www.jwprince.com.au E [email protected]

& PROJECT MANAGERS

DES DRN CKD APR

FILE No:

AZIMUTH:

CLIENT:

DATUM:

ORIGIN:

THIS DRAWING MUST NOT BE USED FOR

CONSTRUCTION UNLESS SIGNED AS PART OF AN

APPROVED CONSTRUCTION CERTIFICATE.

SHEET SIZE: A1 ORIGINAL

Plotted: 20 S

eptem

ber , 2017 11:16:08 A

M F

ile N

am

e: J:\9985D

\S

K - C

oncept S

ketch D

esigns\9985S

K - P

K12\998512S

K20.dw

g

ADVANCE COPY ONLY

NOT FOR CONSTRUCTION

DRAFT ISSUE ONLY

PRELIMINARY DESIGNS SUBJECT TO CHANGE

NEWPARK

PRECINCT 7

PMF OPTION GRADING PLAN

998512/SK20 2

998512SK20

AM

AM

AM

AM 20/09/17

14/09/17ISSUE FOR INFORMATION

GENERAL AMENDMENTS

1

2

METRES

1:1000 (AT A1)

20 10 0 10 20 30 40 50 60 70 80 90 100

1:2000 (AT A3)

ROUTE TO PEDESTRIAN REFUGE

NOTE:

THERE AREA 173 LOTS THAT

WOULD EVACUATE TO THE AREA

ABOVE PMF.

j. wyndham prince consulting civil infrastructure engineers

Documents