16697E

Page | 1

STORMWATER MANAGEMENT PLAN

Development of Two Lot Subdivision 23 Hopkins Street, Birregurra

East Parcel

19 August 2020

Job No: 16697E

Report Version: 001

16697E

Page | 2

CONTENTS

SECTION PAGE

1.0 INTRODUCTION ................................................................................................................................................. 3

2.0 LEGAL POINT OF DISCHARGE ............................................................................................................................. 3

3.0 PROPOSED DRAINAGE STRATEGY ...................................................................................................................... 3

4.0 STORMWATER DETENTION ................................................................................................................................ 4

5.0 STORMWATER QUALITY .................................................................................................................................... 4

6.0 SUMMARY ......................................................................................................................................................... 6

Appendices EXISTING SITE SURVEY AND PROPOSED DEVELOPMENT LOT LAYOUT Appendix 1 TYPCIAL RAINTANK PROVIDING DETENTION STORAGE Appendix 2 STORMWATER DETENTION CALCULATION Appendix 3

16697E

Page | 3

1.0 INTRODUCTION

St. Quentin Consulting was commissioned by the client for a stormwater management report for the proposed two lot subdivision at 23 Hopkins Street, Birregurra. The site has street frontage (Hopkins Street) on north and street frontage (Prime Street) on south. The existing site consists of one shed and grassed backyard. Geometry of the site is flat and falls gently towards the Hopkins street frontage (Refer Appendix 1). The development will not retain the existing shed and incorporate no new buildings as part of a two-lot subdivision. The proposed two lots will be accessed from Hopkins Street and Prime Street respectively (Refer Appendix 1). The Colac Otway Shire is responsible for stormwater infrastructure and managing storm water quality and quantity requirements. This report demonstrates how the proposed development will manage quantity and quality of the stormwater runoff, thus satisfying the requirements for the planning permit application.

2.0 LEGAL POINT OF DISCHARGE

According to the draft engineering conditions of the planning permit, the legal point of discharge is to the existing stormwater drainage network dissecting the site between Hopkins Street and Prime Street. (Refer Appendix 1).

3.0 PROPOSED DRAINAGE STRATEGY

The drainage strategy focuses on the stormwater quantity and quality and ensures the stormwater management to be:

• Designed to ensure that flows downstream of the site are restricted to predevelopment levels (1 in 5 years event).

• Designed to meet the current best practice performance objectives for stormwater quality as contained in the Urban Stormwater - Best Practice Environmental Management Guidelines (Victorian Stormwater Committee, 1999). Payment of WSUD contribution to council is leu of onsite treatment is acceptable within the Colac Otway Shire.

The client proposes to install rainfall tank for each building to collect and reuse rainwater. Considering the exiting condition of the site and the client’s proposal, the following stormwater detention strategy is proposed:

• The runoff from the roof of each building is to be captured with a rainwater tank. This will provide stormwater detention storage/permanent storage in the rainwater tank (refer Appendix 2). The tanks will be connected to the internal plumbing for on-site reuse around the new dwellings. Installation of the tanks will be governed by a Section 173 agreement.

• It is proposed to connect orifice flow and overflow pipes from the rainfall tank to the existing underground drainage that dissects the site.

• It is proposed the on-site reuse of rainwater will partially meet the current stormwater quality best practices, with the balance being meet through a Water Quality Contribution.

The existing drain invert is approx. 0.9 meters deep, a preliminary grading design of the proposed property connection(s) suggests the site is able to drain to the existing pipe via gravity.

16697E

Page | 4

4.0 STORMWATER DETENTION

Stormwater detention for a 1 in 5-year rainfall event is provided by the rainfall tank for each building to restrict the overall site discharge to the pre-development discharge rate. Runoff generated from paved area and garden area is considered as unrestricted flow and is calculated in the overall site discharge. The existing roof/paved/garden areas have been calculated from Nearmap Images, while the proposed development scenario has been based on the IDM fraction impervious values for the relevant lot sizes. All roof areas will be required to drain via a sealed system to a rainwater tank attached to each building. Overflow from each tank will discharge to the existing underground drainage that dissects the site. The Swinburne method has been used to calculate detention storage for each building, refer Appendix 3 for detail. The required detention storage for each building is listed in Table 4-1. Table 4-1: Detention Required for Each Building

Building Detention Storage Required

Suggested Rainwater Tank Size

Existing Lot 1 2200 litre 8000 litre

Lot 2 2200 litre 8000 litre

5.0 STORMWATER QUALITY

It is proposed the on-site reuse of water through the instillation of rain tanks will partially meet the current stormwater quality best practices, with the balance being meet through a Water Quality Contribution. In order to determine the effectiveness of the proposed treatment measures in meeting the water quality objectives, stormwater quality determination was performed using the Melbourne waters STROM calculator. The percentage treatment meet against best practices for each lot is listed in Table 5-1.

Lot Achieved Water Quality objectives

Portion of Water Quality objectives require additional offset

Lot 1 67% 33%

Lot 2 67% 33%

The STORM treatment results are shown in Figure 5-1.

16697E

Page | 5

Figure 5-1: STORM output

Figure 5-1 shows the overall effectiveness of the treatment. The result indicates that the percentage reduction for TSS, TP, TN and GP meet 67% of the criteria specified at the Best Practice Environmental Management Guidelines – 33% offset required

16697E

Page | 6

6.0 SUMMARY

The Stormwater Management Plan report completed by St Quentin for the proposed development at 23 Hopkins St Birregurra, demonstrates how the stormwater management requirements of the Colac Otway Shire Council can be addressed within the proposed development. The report summarises the calculation and modelling undertaken to determine the required detention volume and stormwater quality treatment measures required to achieve both stormwater flow rate and quality performance objectives. Prepared by:

Cameron Walsh Civil Engineer St. Quentin Consulting Pty Ltd

Page |7

Appendix 1: EXISTING SITE

EXISTING PIT.

LEGAL POINT OF

DISCHARGE

PROPOSED LOT 2 HOUSE

CONNECTION INTO

EXISTING PIPE NETWORK

Page |8

Appendix 2: TYPCIAL RAINTANK PROVIDING DETENTION STORAGE

Stormwater detention storage

Stormwater permanent storage

Orifice Flow Outlet

Page |9

Appendix 3: STORMWATER DETENTION CALCULATION

REF: 16697E VERSION: 01 DATE: 20/08/2020

Designed by: CW Checked by: KM

ADDRESS:

PLANNING PERMIT NO:

NUMBER OF LOTS:

LEGAL POINT OF DISCHARGE:

TOTAL SITE AREA:

DETENTION METHOD:

1) COMPUTATIONS

a) Pre development Discharge

- Time of Concentration;

- Rainfall Intensity; 73.2 mm/hr , 5yr ARI (BOM IFD charts for Birregurra grid cell)

- Existing Site conditions based on survey and NearMap.

Site

Roof area (c= 0.9) 54.00 (m2), 0.0054 (Ha)

Impervious surface (Paved Area c = 0.85) 0.00 (m2), 0.0000 (Ha)

Pervious surface (c = 0.35) 1,970.00 (m2), 0.1970 (Ha)

Total Area 2,024.00 (m2), 0.2024 (Ha)

Weighted Coefficient of runoff; 0.36

- Pre-developed runoff;

Q Total = 0.01501 (m3/s) 15.01 (l/s)

ON SITE DETENTION COMPUTATIONS

23 Hopkins St, Birregurra (East)

2

CONDITION NO:

Based on Infrastructure design manual version 5.3

Existing pit in Hopkins St

2024m2

6mins (section 16.6 of the infrastructure design manual)

Install raintank for each lor. Restrict runoff to pre

development rate

Page1of6

b) Post development Discharge- Time of Concentration;

- Rainfall Intensity; 73.2 mm/hr , 5yr ARI (IFD charts for Greater Geelong)

- Proposed Site conditions based on the IDM coefficient of runoff

-

Lot 1

Lot area (c= 0.55) 1012.00 (m2), 0.1012 (Ha)

Total Area 1,012.00 (m2), 0.1012 (Ha)

Weighted Coefficient of runoff; 0.55

- Post-developed runoff;

Q Total = 0.01132 (m3/s) 11.32 (l/s)

Lot 2

Lot area (c= 0.55) 1012.00 (m2), 0.1012 (Ha)

Total Area 1,012.00 (m2), 0.1012 (Ha)

Weighted Coefficient of runoff; 0.55

- Post-developed runoff;

Q Total = 0.01132 (m3/s) 11.32 (l/s)

6mins (section 16.6 of the infrastructure design manual)

Common property area is been averaged and weighted across all nine properties

Page2of6

METHOD OF DETENTION

Rainwater Tank X

Below ground Tank

- Post developed runoff;

c) Un-restricted post development runoff

Permissible runoff from Lot 1 = 7.50 (l/s) (PSD Lot 1)

Impervious surface (c = 0.85) 95.00 (m2), 0.0095 (Ha)

Pervious surface (c = 0.35) 635.36 (m2), 0.0635 (Ha)

Total Area 730.36 (m2), 0.0730 (Ha)

Weighted Coefficient of runoff; 0.422

Q Lot 1 (unrestricted) = 0.00626 (m3/s) 6.26 (l/s)

Permissible runoff from unit 2 = 7.50 (l/s) (PSD Lot 2)

Impervious surface (c = 0.9) 95.00 (m2), 0.0095 (Ha)

Pervious surface (c = 0.35) 635.36 (m2), 0.0635 (Ha)

Total Area 730.36 (m2), 0.0730 (Ha)

Weighted Coefficient of runoff; 0.422

Q Unit 2 (unrestricted) = 0.00626 (m3/s) 6.26 (l/s)

Comparing pre and post development discharge, it was determined that detention is required for this

development. It is determined that detention Is best provided via use of

Above Ground Depression

Oversized pipe

Lot 1 and 2 areas are based on the IDM coefficient of runoff.

(95 sq.m of paved areas, 281 sq.m of roofed area, 635 sq.m of impervious area)

Page3of6

d) Allowable runoff from new dwelling roofAllowable Runoff = Permissible runoff - Unrestricted runoff

Allowable Runoff Lot 1 = 0.00124 (m3/s) 1.24 (l/s)

Allowable Runoff = Permissible runoff - Unrestricted runoff

Allowable Runoff Lot 2 = 0.00124 (m3/s) 1.24 (l/s)

Page4of6

Required detention volume

e) (Stormwater detention calculator output)

1&2

3) SUMMARY

Lot 1

Lot 2

It is recommend that detention be provided by way of an above ground rainwater detention tank for lots 1 & 2

A rainwater tank with minimum 2200 L detention space with a 25 mm orifice and 0.858m head over the orifice

will meet OSD requirements for this development. (Variation to orifice size and head must maintain PSD of 1.24

l/s.)

A rainwater tank with minimum 2200 L detention space with a 25 mm orifice and 0.858m head over the orifice

will meet OSD requirements for this development. (Variation to orifice size and head must maintain PSD of 1.24

l/s.)

Page5of6

Page6of6