Proposed New Car Showroom and Car Storage Facility

101-109 Parramatta Road, Homebush

Stormwater Management Report

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Disclaimer

This report has been prepared on behalf of and for the exclusive use of Suttons Motor Group, and is subject to and issued in accordance with the agreement between Suttons Motor Group and Northrop Consulting Engineers. Northrop Consulting Engineers accepts no liability or responsibility whatsoever for it in respect of any use of or reliance upon this report by any third party. Copying this report without the permission of Suttons Motor Group or Northrop Consulting Engineers is not permitted.

166286: 101-109 Parramatta Road, Homebush – Stormwater Management Report

Rev Description Prepared by

Reviewed by

Issue Date

Client App

Approval Date

A Draft for Client Review BL SF 13.04.17

B Issued for Council Approval BL SF 27.06.17

C Issued following Client Comments

BL SF 28.06.17

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Table of Contents

1. Introduction ............................................................................................................ 4

2. Site Description ..................................................................................................... 4

3. Concept Stormwater Drainage Plan ....................................................................... 5

4. Stormwater Quality Management ........................................................................... 8

5. Conclusion ........................................................................................................... 11

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1. Introduction

Northrop Consulting Engineers (Northrop) have been engaged by Suttons Motor Group to prepare Civil Documentation in accordance with Development Application conditions outlined by Strathfield Council Council) for the proposed development at 101-109 Parramatta Road, Homebush. The proposed Homebush showroom and car storage facility development will involve:

A new commercial building housing a showroom, car storage area and reception

The construction of external car display areas, as well as new entrances, ramps and internal car parking areas

Provision of internal stormwater drainage including the addition of water quality treatment tanks, as well as an on-site detention tank.

This report covers Northrop’s scope to support the development application:

Preparation of a site specific WSUD Strategy as per the requirements of the Strathfield Council WSUD Reference Guideline (March 2011)

Undertake a site specific stormwater design for the proposed development & the provision of a hydrologic/hydraulic model incorporating OSD as per the requirements of the Strathfield Council Stormwater Management Code (October 1994).

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2. Site Description

2.1 Existing Site Description

The existing site comprises an ancillary vehicle service center, small office, outdoor paved car display area, as well an unsealed gavel car parking area to the rear (north) of the site. Vehicular access is gained via Powell Street (directly south of the M4 Motorway) which forms the property boundary to the north.

A Sydney Water owned stormwater channel passes through the site (contained within an 3.66m wide easement) that is partly covered prior to passing under Powell Street and the M4 further downstream.

The site location is shown in Figure 1 as shaded in orange.

Figure 1 - Site Locality Plan

2.2 Proposed Development

The proposed development will incorporate the construction of a new car showroom, service centre as well as various outdoor car display areas. These display areas retain some form and function of the original site, however, will require regrading in order to better service the new car showroom.

It is proposed that the existing Sydney Water stormwater channel be retained in its current form (cross-section/capacity) and alignment. The proposed building footprint is not to extend over the existing stormwater channel easement held by Sydney Water over the site. Proposed car parking area (outdoor – flexible pavement) is proposed over the channel where it is currently covered. An accompanying flood study (also prepared by Northrop Consulting Engineers) assesses flood levels both pre-and post-development – and should be referenced in conjunction with this report. The interaction of the on-site stormwater

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design and associated flood levels forms the basis of the stormwater management design approach taken for the site, and for which the performance of the existing stormwater channel is critical.

3. Concept Stormwater Drainage Plan

3.1. Overland Flows

A Sydney Water owned asset (overland flow path) exists through the site – the impact of the proposed development and the associated flood levels within and surrounding the site have been investigated (using 2D analysis software) to ensure no adverse effect on flood water levels. This study has been included as an accompanying report as previously referenced.

3.2. Internal Site Drainage

The proposed internal site drainage has been designed to have capacity do convey all runoff from the 1% AEP (100 year) event via an above ground pit and pipe network. All proposed roof areas are to drain via internal pipework to a water quality treatment chamber, prior to discharging to an OSD system before discharging to Council’s stormwater network. Hydraulic analysis includes consideration of tail water levels adopted from the accompanying flood study. This ‘worst-case’ scenario has been adopted to ensure the OSD is not outlet controlled as per Strathfield Council’s Stormwater Management Code.

The outdoor car display or ‘podium’ area also drains to a series of stormwater pits and pipes, however, due to the flat nature of the site – these flows were unable to be directed back to the water quality treatment/OSD tank as described above.

The proposed outdoor car parking area (located at the north-east of the site), is to include above ground OSD (via three separate storage areas) that does not pond over 150mm in height. The most downstream pit is to include an orifice (55mm diameter) prior to discharging to Council’s stormwater network. Any emergency overflow (above the major event), will flow above the proposed ‘speed hump’, towards Powell Street.

Designs were undertaken using 12d 1D Dynamic Drainage (ILSAX) software adopting the following parameters in accordance with the Strathfield Council Stormwater Management Code (and accompanying OSD specification) on Technical Specification:

Paved (impervious) area depression storage = 1mm

Supplementary area depression storage = 0mm

Grassed (pervious) area depression storage = 3 mm

Soil type = 3

AMC = 3

Rainfall intensity duration and frequency data was adopted from the 1987 version of Australian Rainfall and Runoff & Council’s Stormwater Management Code (Supplement 1).

3.3. Permissible Site Discharge

The permissible site drainage has been adopted as per Strathfield Council’s On-Site Detention Storage Requirements for the ARI 2, 10 and 100 year events. Table 1 provides a summary of the adopted PSD for the total development site for a Commercial/Industrial – Small Development (Strathfield Council Stormwater Management Policy – S2.1). Discussions with Council Engineers have confirmed that the below rates & category of PSD ‘rate’ is suitable for the proposed development.

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Table 1 - PSD & Minimum Storage Requirements

ARI (YR)

PSD Rate (L/s per 1000m2)

Maximum PSD Flow (L/s)

Storage Rate (m3/1000m2)

Required Storage (m3)

2 20 93.00 7 32.55

10 25 116.25 11 51.15

100 33 153.44 18 83.70

The total site area is 4,692 m2. The time of concentration for the site was modelled using the kinematic wave equation within the 12d Software (ILSAX).

3.4. On-Site Stormwater Detention

A series of enclosed OSD tanks connected hydraulically (totaling 83m2 footprint which may be further optimised – including water quality treatment area, varying in depth, outlet controlled with a flood gate; 150mm low level outlet and emergency overflow weir -100mm high, 2.0m wide discharging to the adjacent car park) will be required to service the development. The invert of the tank is to sit on approx. existing surface level, access gained via chambers placed in the carpark/ramp above (upper ground floor).

Approximately 620 m2 of car display (podium) and landscaping area will bypass the OSD tank due to grading constraints on site. This accounts for approximately 13% of the site area. All roof areas will first drain to a water quality treatment chamber prior to discharging to the OSD tank. The above car podium will drain to a series of stormwater pits and pipes within the site, prior to a piped connection to council’s stormwater network

In accordance with Council requirements, on site stormwater detention (OSD) has been designed for the site using Full Computation Method.

The drainage models used in this assessment have been provided with this report.

Modelling results are summarised in Table 2 below which shows that the proposed OSD configuration will comply with Council’s flow targets. Table 2 - Stormwater Quantity Management Performance

ARI (YR) Site Discharge (m3/s) Comment

PSD (see Table 1) Post Development

2 0.093 0.053 Complies

10 0.116 0.097 Complies

100 0.153 0.132 Complies

Northrop file reference: "T:\2016 jobs\166286 - Suttons Homebush - 101-109 Parramatta Road\D-Drawings\D-Northrop\C-CIVIL\6-12d\D-Project\Suttons Homebush”

The OSD tank has been designed in accordance with (Strathfield Council Stormwater Management Policy – S2.2). The OSD tank is to include an outlet with a flood gate, sit adjacent the lowest point on the site and fulfill habitable floor level freeboard requirements (situated beneath an internal carpark) – any overflow would discharge to the directly adjacent stormwater channel.

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4. Stormwater Quality Management

In accordance with Council’s Development Control Plan, the site is required to achieve stormwater pollutant load reductions.

The site incorporates and existing stormwater quality chamber which will be retained. Additional stormwater quality devices are required to provide stormwater quality management for the expanded site and car parking areas.

A stormwater ‘treatment train’ has been developed using MUSIC software, with the appropriate catchment & treatment parameters as per the Strathfield Council WSUD Reference Guideline, Table 3 provides a summary of the land use characteristics of the site. Table 3 - Land Use Characteristics

Surface Type Area (Ha) Proportion of Total Site

Roof Area 0.332 71%

Paved Area (Car Display) 0.036 7%

Landscaped bypass area 0.026 6%

Paved Area (Car Park) 0.074 16%

TOTAL SITE 0.4680 100%

To appropriately manage the volume of pollutants discharged from the site, a treatment train has been developed to capture and remove a percentage of pollutants from the site prior to discharging to receiving infrastructure.

The water quality objectives for the site, as defined by Council’s WSUD Reference Guideline, can be achieved by the combined benefits of the following:

Showroom roof runoff – 50% into the treatment chamber directly and 50% into a rainwater tank before discharging into the treatment chamber and into an OSD tank

Podium area to second treatment chamber via trash screens 5 x Enviropods)

Car park area to third treatment chamber via trash screens 3 x enviropods).

Landscaped area to bypass water quality treatment

The following input parameters have been adopted in this model:

Pluvio Rainfall data from the NSW Bureau of Meteorology Sydney Airport 6-minute

rainfall data (1979-1988)

Pollutant Event Mean Concentrations (EMC) from the Draft NSW MUSIC Modelling

Guidelines

Rainfall-Runoff Parameters from Strathfield Council WSUD Reference Guideline

Figure 2 (see over) provides a depiction of the proposed treatment train design as adopted in MUSIC stormwater quality modelling software.

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Figure 2 - MUSIC Treatment Train

Table 4 below shows the results from MUSIC modelling indicating that that the prescribed water quality objectives are achieved.

Table 4 - Stormwater Quality Management Performance Results

Surface Source Loads

Residual Loads

Reduction (%)

Target Reduction (%)

Comment

Total Suspended Solids (kg/yr) 362 47.1 87 85 Complies

Total Phosphorus (kg/yr) 0.928 0.213 77 60 Complies

Total Nitrogen (kg/yr) 8.76 4.15 52.6 45 Complies

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4.1. Proposed Stormwater Treatment Train

Stormwater360 Psorb (MMC) Stormfilter Cartridges The showroom runoff will be treated by the equivalent of 10 Stormfilter Cartridges with PSorb media (690mm high units with a total treatable flow rate of 9L/s). The cartridges are located within the treatment chamber with a total internal footprint of 14 m2.

The car podium area runoff will be treated by an additional ‘small’ water quality chamber with 3 Stormfilter cartridges (Psorb media 690mm high units) with a treatable flow rate of 2.7L/s.

The carpark area (rear) runoff will be treated by an additional ‘small’ water quality chamber with 3 Stormfilter cartridges (Psorb media 310mm high units) with a treatable flow rate of 1.2L/s.

Trash Screens/ Stormwater360 Enviropod 200 inserts A total of 5 trash screens or Stormwater360 Enviropod 200 pit inserts in grated pits will be used as pre-treatment for stormwater runoff to capture litter and coarse sediment from the podium area of the site. 10kL Rainwater Tank A 10kL rainwater tank will be implemented to capture 50% of stormwater runoff generated off the roof of the showroom building. The collected rainwater will be used for car washing services and bathroom usage. The rainwater tank structure will be installed prior to the OSD tank. Overflows from the rainwater tank would discharge into the OSD and then into Council’s stormwater infrastructure. The rainwater tank water re-use is distributed for car washing services and bathroom purposes. The sizing of the Rainwater tank is based on the following usage calculations:

Car washing services: The site contains 4 detailing bays, each servicing approximately 2 vehicles per day and requiring 20L/day. This results in 160L/day of water required for car washing services. Bathroom Usage: Each of the 14 staff members are approximated to each require 3 flushes per day multiplied by an average of 3.5L of water usage per flush, resulting in 147 L/day of water required for bathroom use. A total of approximately 80.1 kL/year has been determined as the total amount of water to be reused by the showroom from the rainwater tank. This figure has been used within the rainwater tank node in the MUSIC model accordingly.

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5. Conclusion

Stormwater models have been produced in 12d 1D Dynamic Drainage Software (ILSAX) to determine the hydraulic performance of the post development network under a range of flood events (2, 10 & 100 yr ARI). The modelling shows that the proposed OSD tanks will preserve the permissible site discharge rates for the range of ARI’s as required by Strathfield Council’s Stormwater Management Code.

A treatment train has been developed using MUSIC software to demonstrate that the stormwater pollutant load reduction targets are achieved by the proposed treatment train to the requirements as set out by the Strathfield Council WSUD Guideline.

Benjamin Lawrence

Civil Engineer (BE Civil & Environmental, Hons1, GradIEAust)

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