14 October 2019

MB180072

Scott Patroni Salta Properties Level 26, 35 Collins Street Melbourne VIC 3000

Dear Scott,

Level 3, 520 Collins St, Melbourne VIC 3000 03 9600 2645

melbourne@northrop.com.au www.northrop.com.au ABN 81 094 433 100

Re: 12 Nexus Court, Mu!grave — Concept Stormwater Management Strategy

This letter does not attempt to provide detailed design solutions to all issues; rather it will investigate the feasibility of solutions based on information that we have gathered to date from a number of sources and provide outcomes which will be developed further at Construction Certificate and Construction phases of the project.

1. Proposed Stormwater Management Strategy

The proposed stornnwater management strategy for the development, can be summarised as follows;

• The Stormwater quantity targets will be achieved by providing a minimum of 45 kL of On Site Detention. Refer to section 2. Stormwater quantity assessment for further concept details;

• A rainwater reuse tank is proposed on the upper basement level, located toward the western portion of the development. Runoff from the roof areas will be collected and directed to the reuse tank, with minimum volume of 30 kL. The entire roof runoff is deemed to be satisfactory for harvesting purposes, with plant areas separated and directed to the sewer under a trade waste arrangement or via the surface water detention system.

• The harvested runoff is to be reused onsite for toilet flushing only throughout the building facilities and potential irigation. Overflow from the tank will discharge to the detention tank under the internal road.

• The concept strategy proposes two points of discharge in the north-west and southern portion of the site to best accommodate for the natural surface levels. Refer to Figure 1 below.

• A 5 m2 raingarden is proposed for treatment of the northern portion of the internal road with a low flow control where the larger flows will bypass treatment and directed to the detention tank.

• Runoff from the southern portion of the internal road is to be treated through stormwater pit inserts prior to connection to the southern point of discharge.

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LPC)D 1

INDICATIVE LOCATION OF DETENTION TANK (APPROX Em WIDE x 9m LONG x 1.0m DEEP)

5 . 7 1 1 0 VIVRE. M I A I I M P 1 0 I S MIMED

NDRTH GA,I;HMH,'

III H III II

LPOD 2

A L 1 0

.46661f6W " . ail iN

Mil

!WSW& M•11.11111.01IGI 00.11211 .11111111. 0 0 A S S O.N1.121.11G

Figure 1 — Catchment/points of discharge plan.

2. Stormwater Quantity Assessment

In order to minimise the impact of the development on the wider catchment runoff regime, hydrological and hydraulic modelling has been undertaken using the software package OSD4W. In particular, the modelling was undertaken to achieve the objectives outlined in the City of Monash's Planning Scheme. The output screen of the OSD4W model is shown below in Figure 2.

Version Home directory = Q:13 Engineering \ Civil \ osd4

Site parameters

Total Area • sq.M

OSD System Details Flow Calculations Storage Calculations

Td • min 1-4184 Sr Calculate PSD

Td • min 11r— l'

C existing 11:1 r Nominate PSD _

I • mmihr M. I • mm/hr PC. C proposed

Storage Qa • Lis

Type

10.7 Oa • U s 192.-t 11.

1.1 ITank '•

l• Du -L/s I— PI:' Tc - mm. Vtc • cub.M

Flow Control Device IMC2 Multi•Cell F-- Op -1./s 311 F14.3 Too - min. Stmage Vol - cub.M

Tcs • min J PSD Calc. - Lb i Time to FM • min. 121.E

RainfaR Zone 'MELBOURNE PSD Nom. • Lis I— Time to Empty - min. 1J

— ARI Flow Reference Storage Period - min. F87

M I Storage I2C Job Name IJobName Runoff Volumes cub.M

aptot - L/s 3r1 Job Reference 10SO4W-2008•001 Existing Irl

Zone-ARI Flow 'MELBOURNE 5 ..:i JobFele /JobNotes IJob180072 Filename

Proposed - Flow Fir—

Zone-ARI Store 'MELBOURNE 20 Pic:posed - Stole Date 114/10/2019

.Z-I

Site Details I Save Time I—

Calc. Time 10H-1-6 Job Details I

Units I Load all zones File Menu I SaveJob File I Load Job File I

Preferences I Show all zones I Report menu I Save Job Notes I LoadJob Notes I

Ness the ENTER KEY after editing fields

Notes Box - Scrol down to view all text in the Notes Box - Click "Save Job Notes" to save contents r default folder or click File Menu for more options.

Clear Notes Bort I

ICalculate I

Back

CIVIL

Figure 2— OSD4W Calculator output screen.

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A few key considerations in preparing the OSD4W model include;

The existing site land area is 4,184 m2, which is made up 1,100 m2 of asphalt carpark or 26% impervious percentage. The pervious area was assumed to have a runoff coefficient of 0.4, typical to mowed grass and 0.9 for the impervious carpark area. The proposed development was assumed to be 65% impervious with a runoff coefficient of 1.0 for all roof and paved areas and 0.4 for all landscaped areas.

The detention system has been designed to over detain flows by replicating the existing flows in a 5- year ARI with proposed flows in a 20-year ARI. This approach ensures that the proposed development has a beneficial effect on the wider catchment.

The target objectives can be achieved for the development by implementing a minimum 45 kL of On Site Detention. The detention is proposed via an underground detention tank with flows being restricted via the outlet chamber configuration. Note, this is subject to Counicil review and approval, final volume may be larger than indicated in this report.

OSD4W data files can be provided upon request.

3. Stormwater Quality Assessment

In order to minimise any adverse impacts upon the ecology of the downstream watercourses; stormwater treatment devices have been incorporated into the design of the development. The stormwater quality treatment strategy has been assessed against The State Environmental Protection Policies (SEPPs), Clause 44 of SEPP Victoria, of which the treatment targets for pollutant removal and replicated below in Table 1.

Table '1 — SEPP Victoria water quality reduction targets

Pollutant % post development average annual load reduction Total Suspended Solids (TSS) 80 %

Total Phosphorous (TP) 45 %

Total Nitrogen (TN) 45 %

Gross Pollutants (Litter) 70 %

The performance of the proposed stormwater management strategy was assessed against these targets using the conceptual software MUSIC (Version 6). The MUSIC model was developed in accordance with the Melbourne Water MUSIC Guidelines.

A schematic of the MUSIC model can be seen below in Figure 3.

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LPOD 1

NORTH LANDSCAPE [1000m2] [PV.

OOF TO TANK [2050m2] [Root

Raingarden

NORTH HANDSTAND [474m2] [Mixed] ri0

SOUTH HANDSTAND [200m21 [Mixed] SOUTH LANDSCAPE [46Orn2] [Mixed]

SPEL Stormsack (1)

Figure 3— MUSIC Schematic

The results from the MUSIC model are shown below in Table 2.

Table 2— MUSIC Results

Source Load (kg/yr)

Residual Loads (kg/yr)

Percentage Reduction

Target Objectives

Total Suspended Solids (TSS) 136 26.4 80.7 % 80 %

Total Phosphorous (TP) 0.331 0.132 60.1 % 45 %

Total Nitrogen (TN) 2.93 1.26 57.0 % 45 `)/0

Gross Pollutants (Litter) 477 0.0 100 % 70 %

Table 2 shows that the proposed stormwater management strategy is predicted to achieve the SEPP load reduction targets, as estimated by MUSIC. The model adopts a daily average reuse demand of 1500L/day to service the development with flushing for toilets or for external irrigation.

MUSIC data files can be provided upon request.

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4. Conclusions

Given the results of the above investigations, it is concluded that the development meets the best practice stormwater management requirements through providing;

• The attenuation of stormwater runoff to improve on the pre developed scenario has been achieved via a minimum 45 kL of On Site Detention.

• A minimum of 5 m2 of raingarden with 100mm of extended detention depth for treatment of the northern portion of the internal roadway.

• Stormwater pit inserts (SPEL stormsack or approved equivalent) for treatment of the southern portion of the internal road.

• A minimum of 30 kL of harvested roof runoff for toilet flushing, equivalent to an average daily draw down of 1500Uday.

I trust the above meets your requirements; however, if you would like to discuss the development further, then please do not hesitate to contact me.

Yours faithfully,

Justin Lanz Civil Engineer Northrop Consulting Engineers

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