Cape Paterson North Proposed Drainage Strategy

Cape Paterson North

Proposed Drainage Strategy

REVISION F

1 February 2016

Report by: Valerie Mag BE Civil (Hons), M Eng Sci

Stormy Water Solutions

Ph 9511 5911, M 0412 436 021

Contents

1. INTRODUCTION AND BACKGROUND .................................................................................. 1

2. ASSUMED DRAINAGE SYSTEM REQUIREMENTS ............................................................ 4

2.1 HYDROLOGICAL REQUIREMENTS ............................................................................................. 4

2.2 WATER SENSITIVE URBAN DESIGN REQUIREMENTS ................................................................ 4

2.3 ECOLOGICAL CONSIDERATIONS ............................................................................................... 5

3. DRAINAGE STRATEGY DESCRIPTION ................................................................................ 6

4. HYDROLOGIC FLOOD FLOW MODELLING ....................................................................... 9

4.1 EXISTING CONDITIONS............................................................................................................. 9

4.2 POST DEVELOPMENT CONDITIONS ......................................................................................... 13

5. STORMWATER POLLUTANT MODELLING ...................................................................... 19

6. POTENTIAL ESTUARY IMPACTS......................................................................................... 23

6.1. FLOW FREQUENCY ANALYSIS ................................................................................................ 24

6.1.1 Analysis Methodology ....................................................................................................... 24

6.1.2 Scenarios Investigated ...................................................................................................... 24

6.1.3 Rainfall Data .................................................................................................................... 25

6.1.4 Catchment Delineation and Specification ........................................................................ 25

6.2 ANALYSIS RESULTS ............................................................................................................... 27

7. CONCLUSIONS AND FURTHER WORK REQUIRED ........................................................ 28

8. ABBREVIATIONS AND DEFINITIONS ................................................................................. 30

APPENDIX A – RETARDING BASIN CONCEPT DESIGNS ....................................................... 31

A.1 RETARDING BASIN RB1 ......................................................................................................... 32




APPENDIX B – RETARDING BASIN MODELS ............................................................................ 40

APPENDIX C FLOOD FREQUENCY ANALYSIS RESULTS ................................................. 44

1

1. Introduction and Background

This report pertains to the drainage system which discharges to Seaward Drive, north of Cape

Paterson. In particular it considerers the implication of development of the subject site for

residential purposes as detailed in Figure 1 below.

Figure 1 Subject Site

Figure 1 shows:

• The subject site,

• The piped trunk drainage within the existing township (denoted by the bold red line),

• The indicative location of informal farm drainage systems as denoted by the bold blue

lines (in some cases these systems are made up of many very small paddock drains),

and

• The major outfall points from the subject site at Seaward Drive.

The drainage system configuration was formulated given,

• Site observations,

2

• Google map information, especially in relation to defining existing farm drain locations

and outfall points,

• One metre lidar contour information,

• 0.5 metre site survey information,

• 10 metre Vic Map contour information,

• Bass Coast Shire Council Drainage plans within Cape Paterson Township,

• Details of the existing drainage system within the proposed Eco Village located west

of the township (Bass coast Planning Scheme Amendments C53, C93 and C98 Panel

Report, April 2010, Figure 5.4)

• Detailed survey of Wilsons Road (including all culverts) and the Cape Paterson Road

culvert systems.

Actual detailed drainage configurations may differ slightly from those shown. However, it is

considered that enough analysis has been undertaken to give a reasonable indication of the

drainage system configuration and operation north of Cape Paterson.

Outfall Point 5 denotes the location of the major external catchment inflow north of the subject

site.

Point 6 is a formal site outfall point. Flow at this point is conveyed via a culvert under Cape

Paterson Road to a road drain at Point 7, where is joins a flow path conveying rural flow from

the catchment east of the road. As shown, this drainage system forms the major outfall to the

sea via a naturalistic drainage path east of the township. South of Cape Paterson Road, the

drain flows through Parks Victoria land (downstream of Point 8) and is relatively large and,

although overgrown, exhibits ecological and landscape attributes.

The only other “real” outfall to the sea is the outfall from the township drainage system as

shown in red. Council have indicated that the township drainage system only accounts for

drainage of the town, and makes no provision for external upstream catchments. However, it

appears that some minor input may occur at Points 2, 3 and 4 (possibly though small road

culverts into the township drainage system). Whatever the case, in relatively large storm

events, when flow does occur over Seaward Drive from the catchment to the north, nuisance

flooding in the township would be expected.

Outfall Point 1 denotes the location of the outfall from the subject site across Seaward Drive

and into the proposed Eco Village (located west of the existing Cape Paterson township).

Council have confirmed that discharge can occur from Point 1 into the future Eco Village

provided flow retardation to predevelopment flow rates occurs.

3

This report details a proposed a site drainage strategy developed by Stormy Water Solutions

and Beveridge Williams designed to mitigate stormwater impacts downstream of Point 7 and

to improve the flood impact within the Cape Peterson township.

The November 2015 (Revision E) of this report specifically addresses minor changes in

wetland/retarding basin location and extents given landowner consultation process which has

occurred throughout 2015.

4

2. Assumed Drainage System Requirements

Stormy Water Solutions and Beveridge Williams met with Bass Coast Shire Council (Council)

and The West Gippsland Catchment Management Authority (WGCMA) on 3 September 2013.

This meeting confirmed that:

• The had been no formal drainage strategy submitted to council for the proposed eco

village located south of Point1,

• Any future drainage strategy for the eco village should account for existing external

flows, including those entering the eco village at Point 1,

• There is no provision within the existing township piped drainage system to accept

ANY additional flows from north of Seaward Drive (i.e. flows from existing outfall

points 2, 3 and 4),

• The most obvious site outfall point was to Point 6, with site flows then being directed

to the drain downstream of Point 7 and into the Parks Victoria land, and

• The WGCMA confirmed an appropriate drainage strategy would have to show no

impact on the creek through the Parks Victoria Land.

If any flow were to be directed into the existing township, piped drainage system

augmentation would be required to ensure no increased flood effect, and if possible an

increased level of flood protection should be provided. Subsequent investigations indicated

that the cost of drainage system augmentations through the township would be prohibitive in

regard to the subject site drainage strategy implementation.

Given the above the following drainage strategy requirements are assumed. It should be

noted that these requirements are in line with existing best practice in Victoria for simular

developments.

2.1 Hydrological Requirements

It is assumed that:

• The 100 Year ARI flow directed into the eco village site should be retarded to below

existing flow rates at Point 1,

• No site drainage discharges into the township should occur, and

• Any flow directed to Point 6 must be retarded to ensure the 100 Year ARI post

development flow is not greater than the existing 100 Year flow at this point.

2.2 Water Sensitive Urban Design Requirements

It is assumed that all subject site runoff must be treated to at least best practice before

discharge from the subject site. As such the following requirements must be met

5

• 80 % retention of subject site Total Suspended Solids (TSS) on site,

• 45 % retention of subject site Total Phosphorus (TP) on site , and

• 45 % retention of subject site Total Nitrogen (TN) on site.

2.3 Ecological Considerations

The existing waterway downstream of Point 8, though the Parks Victoria land is relatively

degraded and exhibits many weed species etc. It is understood that this waterway was

originally constructed to provide a concentrated drainage outflow point for the land located to

the north. Prior to this there was no large outflow connection directly to the sea.

The developer has recently extended the site Flora and Fauna Report (Ecology and Heritage

Partners) to include the proposed drainage outfall for the development (to its estuary) and

provide an assessment as to whether or not there are any adverse effects from directing the

flows from another catchment through this outfall. It is understood by Stormy Water Solutions

that the main issue that was highlighted was that there are a mating pair of hooded plovers

that nest in the path of the existing outfall on the beach.

Advice from Ecology and Heritage Partners, on 27/03/2015 regarding this issue suggest that:

• The outfall is usually blocked but there is, on average, two storm events during the

hooded plover’s breeding period (September – March) that are large enough to blow

out the sand and wash away eggs or young chicks, and

• It takes the hooded plover two and a half months to raise a chick from when the eggs

are laid. If the sand is blown out and the outfall flows during this time, the eggs (or

chicks) are washed away. If there is not two and a half months between these events

the hooded plover does not reproduce for the year.

It is required to be shown that the frequency of the times in which a flow event occurs capable

of opening the estuary does not increase due to the drainage strategy proposals.

6

3. Drainage Strategy Description

Stormy Water Solutions has used the constraints and requirements detailed above to develop

a drainage strategy for the subject site. The strategy is presented as shown in Figure 2. The

strategy incorporates the following:

• Retarding Basin 1 ( RB1),




• A bypass pipe along Seaward Drive directing site outflow from retarding basins RB1,

RB2 and RB3 in an easterly direction,

• A grassed swale in parkland west of RB3, directing a flow from a localised low point

into the RB3 inlet pipeline as shown,

• Discharge of RB4 outflows south through the Cape Paterson Eco Village (this

assumes the Eco Village accounts for an inflow at the Seaward Drive low point at

predevelopment flow rates),

• A vegetated swale along the eastern boundary of the subject site designed to convey

external flows around the site from Point 5 to Point 6,

• 2000 litre tanks for toilet flushing on all new lots within the subdivision, and

• Increasing the size of the existing road culvert at Point 6 to ensure 100 year flow

conveyance across the road at this point and to ensure free outflow from RB1 in all

events up to and including the 100 Year ARI event.

All retarding basins incorporate a wetland within its base to ensure maximum flood storage

given the site area available and to provide stormwater treatment. It is assumed that each

retarding basin and wetland system treats and stores water from its local catchment only (as

defined in Figure 2), before discharge of flows.

All retarding basin and wetland concept design parameters are detailed in Appendix A. It

should be noted that the outflow from each system has been minimised (given site and

topography constraints), to minimise site discharge downstream. All concept designs have

been formulated given actual site outfall invert level constraints and topographical constraints

(e.g. 1 in 5 batters to cut lines etc).

The requirements of the proposed lot stormwater harvesting tanks for toilet flushing are

detailed in Section 5 of this report.

The strategy:

• Ensures NO discharge south into the existing township in all events up to the 100

Year event, thus reducing the current flood impact in the township considerably,

7

• Ensures no increase in the 100 Year flood flows downstream of Point 6, thus

addressing flood impact considerations in the waterway through the Parks Victoria

Land (see Section 4),

• Meets current best practice in regard to stormwater treatment (See Section 5), and

• Shows that the frequency of the times in which a flow event occurs capable of

opening the estuary does not increase, thus addressing the ecological requirement

detailed in Section 2.3.

The main issues in regard to implementation of the drainage strategy is the construction of

the bypass pipe along Seaward Drive which is required to be relatively deep to meet

upstream wetland discharge requirements and downstream invert level constraints,

All designs are to the concept design stage only at this stage. All proposals are subject to

change during the design process. However, enough analysis has been conducted to clearly

show that development of the subject site is achievable without adverse stormwater impact on

downstream properties or ecological habitats. In fact, by formally constructing an outfall for

the catchment north of Seaward Drive, flooding impacts though the existing township should

be reduced when the drainage strategy is implemented.

It should be noted that the strategy developed is a “worst case” scenario in terms of

downstream stormwater impact. There is some scope in the design to increase flood

storage/wetland provisions (if required due to site or subdivision design constraints) during

the design process.

The remainder of this report details the hydrological calculations relating to the formulation of

the above drainage strategy.

8

Figure 2 Proposed Drainage Strategy Note: 2000 litre tanks for Toilet Flushing are to be incorporated on each lot

9

4. Hydrologic Flood Flow Modelling

The analysis detailed below explains the RORB models and storage parameters used for this project.

4.1 Existing Conditions

The RORB model parameters are based on the regional parameter developed by Melbourne Water for the

South East region of Melbourne.

• Kc = 1.53A 0.55 = 2.57

• m = 0.8

• Initial loss = 10 mm

• Pervious area runoff coefficient (Cperv) 100 year ARI Cperv = 0.6

Figure 3 details the RORB model for the existing situation. Tables 1 and 2 detail the tabulation of the RORB

model setup (i.e. catchment areas, fraction imperviousness and reach lengths etc). Cape Paterson rainfall

intensities were utilised.

Reaches have been defined as either “natural” or “excavated unlined” to represent the farm and road

drainage network.

10

Table 1 Existing Situation RORB Model Setup - Subareas

Sub Area Area (ha) Area (km2) Fraction

Imperviousness

(Existing)

A 2.8 0.028 0.10

B 3.1 0.031 0.10

C 3.5 0.035 0.10

D 2.8 0.028 0.10

E 4 0.04 0.10

F 5.5 0.055 0.10

G 4.9 0.049 0.10

H 3.1 0.031 0.10

I 3.3 0.033 0.10

J 2.6 0.026 0.10

K 4.5 0.045 0.10

L 7.4 0.074 0.10

M 3.8 0.038 0.10

N 6 0.06 0.10

O 6.6 0.066 0.10

P 8.5 0.085 0.10

Q 3.7 0.037 0.10

R 4.8 0.048 0.10

S 3.3 0.033 0.10

T 10.5 0.105 0.10

U 12.2 0.122 0.10

V 9 0.09 0.10

W 5.3 0.053 0.10

X 8.7 0.087 0.10

Y 5.9 0.059 0.10

Z 5.5 0.055 0.10

AA 6.3 0.063 0.10

AB 6.6 0.066 0.10

AC 4.5 0.045 0.10

AD 5.4 0.054 0.10

AE 31.8 0.318 0.10

AF 30.8 0.308 0.10

AG 17.6 0.176 0.10

AH 12.7 0.127 0.10

257.0 2.57

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Table 2 Existing Situation RORB Model Setup - Reaches

Reach Length (m) Length (km) Reach Type - Existing

100 Year

1 220 0.22 NATURAL

2 120 0.12 NATURAL

3 270 0.27 NATURAL

4 100 0.1 NATURAL

5 320 0.32 EXCAVATED UNLINED

6 200 0.2 NATURAL

7 170 0.17 NATURAL

8 20 0.02 NATURAL

9 190 0.19 NATURAL

10 130 0.13 NATURAL

11 100 0.1 NATURAL

12 250 0.25 NATURAL


14 190 0.19 NATURAL



17 20 0.02 NATURAL

18 300 0.3 NATURAL


20 140 0.14 NATURAL

21 270 0.27 NATURAL

22 80 0.08 NATURAL




26 20 0.02 NATURAL

27 110 0.11 NATURAL

28 20 0.02 NATURAL

29 280 0.28 NATURAL

30 270 0.27 NATURAL

31 200 0.2 NATURAL

32 170 0.17 NATURAL











43 70 0.07 NATURAL


45 320 0.32 NATURAL


47 240 0.24 NATURAL



12

Figure 3 RORB Model - Existing Conditions

Note: Reaches and catchments may vary slightly from those detailed above. However the model is considered to give a reasonable representation of the existing conditions given the consideration detailed in Section 1 above.

The following 100 Year ARI flows were obtained for the existing conditions:

• Point 1 – 0.9 m3/s (critical duration = 2 hours)




13



• External Catchment AE – AH – 2.9 m3/s (critical duration = 9 hours)


• Dummy Combined outflow from all outfall points = 8.3 m3/s (critical duration = 9 hours)

The total dummy combined flow was compared with flows estimated using the rational method and the DSE

regional flow estimate graphs. This check was done to ensure the parameter set adopted was valid for its

application in this area. The rational method obtained a 100 Year flow estimate of 11.1 m3/s (C100 = 0.4 and

a tc = 30 mins). The DSE regional flow estimate graph (Q100 = 4.67×A 0.763) resulted in a flow estimate of

9.6m3/s. Given the sandy nature of the soils in this catchment, the dummy RORB estimated flow of 8.3 m3/s

is deemed comparable with flows obtained using the above two methods.

4.2 Post Development Conditions

As above, the RORB model parameters for the post development scenario were based on the regional

parameter developed by Melbourne Water for the South East region of Melbourne.

• Kc = 1.53A 0.55 = 2.6 (slightly increased catchment area)

• m = 0.8

• Initial loss = 10 mm

• Pervious area runoff coefficient (Cperv) 100 year ARI Cperv = 0.6

Figure 4 details the RORB model for the post development situation. Tables 3 and 4 detail the tabulation of

the RORB model setup. Cape Paterson rainfall intensities were utilised. Reaches have been defined as

either “piped” within the developed area and as per the “existing conditions” model otherwise.

Flood storage models for RB1 - RB4 were developed given:

• Site survey and topography constraints at all sites,

• 1 in 5 batters from cut lines to embankment crests to the wetland normal water levels,

• The approximate space allocated to each area at the present time by Beveridge Williams, and

• Estimated downstream culvert and pipe invert levels.

Appendix B details the Stage/Storage/Discharge relationship determination for each site.

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Table 3 Post Development Situation RORB Model Setup - Subareas

Sub Area Area (ha) Area (km2) Fraction

Imperviousness

(Future)

A 2.8 0.028 0.10

B 5.2 0.052 0.10

C 2.3 0.023 0.60

D 3.5 0.035 0.10

E 2.8 0.028 0.50

F 6.8 0.068 0.60

G 8 0.08 0.10

H 3.1 0.031 0.50

I 2.5 0.025 0.60

J 4.9 0.049 0.60

K 4.4 0.044 0.50

L 5 0.05 0.60

M 6.9 0.069 0.50

N 6.3 0.063 0.60

O 5.4 0.054 0.30

P 7.8 0.078 0.60

Q 2.9 0.029 0.60

R 4.8 0.048 0.30

S 3.4 0.034 0.10

T 10.5 0.105 0.10

U 12.2 0.122 0.10

V 9 0.09 0.10

W 5.3 0.053 0.10

X 8.7 0.087 0.10

Y 5.9 0.059 0.10

Z 6.4 0.064 0.60

AA 7 0.07 0.60

AB 4.8 0.048 0.60

AC 4.5 0.045 0.60

AD 5.4 0.054 0.60

AE 31.8 0.318 0.10

AF 30.8 0.308 0.10

AG 17.6 0.176 0.10

AH 12.7 0.127 0.10

261.4 2.61

15

Table 4 Post Development Situation RORB Model Setup - Reaches

Reach Length (m) Length (km)

Reach Type -

developed

100 /5 Year

1 320 0.32 NATURAL

2 290 0.29 PIPED

3 140 0.14 PIPED

4 250 0.25 PIPED

5 420 0.42 PIPED

6 170 0.17 PIPED

7 270 0.27 PIPED

8 190 0.19 NATURAL

9 100 0.1 PIPED

10 220 0.22 PIPED

11 150 0.15 PIPED

12 100 0.1 PIPED

13 220 0.22 PIPED

14 170 0.17 PIPED

15 270 0.27 PIPED

16 250 0.25 PIPED

17 220 0.22 PIPED

18 90 0.09 PIPED

19 440 0.44 NATURAL IN PARK

20 100 0.1 PIPED

21 210 0.21 PIPED

22 270 0.27 PIPED

23 110 0.11 PIPED

24 110 0.11 PIPED

25 130 0.13 PIPED

26 90 0.09 PIPED

27 110 0.11 PIPED

28 250 0.25 PIPED

29 280 0.28 NATURAL

30 270 0.27 NATURAL

31 200 0.2 NATURAL

32 170 0.17 NATURAL


34 180 0.18 NATURAL


36 120 0.12 PIPED

37 140 0.14 PIPED

38 180 0.18 PIPED

39 160 0.16 PIPED

40 300 0.3 PIPED

41 210 0.21 PIPED

42 160 0.16 PIPED

43 70 0.07 PIPED


45 320 0.32 NATURAL


47 240 0.24 NATURAL



16

Figure 4 RORB Model - Post Development Conditions

Note: Reaches and catchments may vary slightly from those detailed above. However the model is considered to give a reasonable representation of the post development conditions given the consideration detailed in Section 1 above.

17

The following 100 Year ARI flows were obtained for the post development conditions:

Retarding Basin 4

• 100 Year Inflow = 3.3 m3/s (critical duration = 25 mins)

• 100 Year Outflow = 0.85 m3/s (critical duration = 9 hours)

• Maximum 100 Year flood storage = 3,720 m3

• Maximum 100 Year flood level = 28.45 m AHD

Reach 19 (assuming conveyed by swale in parkland) = 0.2 m3/s (100 yr.)

Retarding Basin 3





Bypass pipe along Seaward Drive between RB3 outlet and RB2 outlet = 0.36 m3/s (100 yr.)

Retarding Basin 2





Bypass pipe along Seaward Drive between RB2 outlet and RB1 outlet = 0.52 m3/s (100 yr.)

External Flow at Point 5 = 2.1 m3/s

(Note differs slightly from existing flow estimate due to different catchment setup)

Retarding Basin 1





Site Outflow at Point 6 = 2.5 m3/s (9 hr.)

Note: Existing Q100 = 2.7 m3/s at Point 6.

18

External Catchment AE – AH – 3.2 m3/s (critical duration = 2 hours)

(Note differs slightly from existing flow estimate due to different catchment setup)

Flow downstream of site at Point 7 = 5.6 m3/s (9 hr.)

Note: Existing Q100 = 5.6 m3/s at Point 7.

As detailed above, the proposed drainage strategy results in the post development 100 Year flow estimates

at Points 6 and 7 being at or below the existing flow estimates.

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5. Stormwater Pollutant Modelling

Retarding Basins RB1, RB2, RB3 and RB4 will incorporate wetlands within the base of each system. These

wetlands maximise flood storage given the area available. However, they also serve the dual function of

providing stormwater treatment for pollutant loads generated in the developed area. Wetland parameters

are:

RB1

• Wetland area (average area between Normal Water Level (NWL = 3,970 m2) and Top of Extended

Detention (TED = 4,350 m2 )) = 4,160 m2

• Extended Detention = 0.5 m

• Detention Time = 36 hours

• Sediment pond (to be located at upstream end of the system) = 1000 m3

RB2

• Wetland area (average area between NWL = 7,080 m2 and TED = 9,560 m2) = 8,300 m2




RB3




• Sediment pond (to be located at upstream end of the system) = 1,500 m3

RB4





It should be noted, that at 36 hours, the detention times in the wetland ED range (RB1 to RB3) are less than

usually specified. However, it is considered that this is enough time to facilitate adequate stormwater

pollutant update, while producing a downstream flow regime similar to the existing situation in low flow

events. This is required to meet the site ecological requirements (See Section 6).

20

On site stormwater harvesting for toilet flushing was modelled. This harvesting initiative was seen as easy to

implement if required via (say) a 173 agreement on the development.

Catchment, demand and storage size assumptions are as detailed below. Of course, these may be subject

to change given the final development form. However they are considered enough to give a reasonable

indication of the benefits of this initiative.

Cape Paterson North Development Catchment Breakdown

Area

(ha)

Approx

Dwellings*

Roof to

Toilet* Roof Fimp

Non Roof

Catchment

Non roof

Fimp

Catchment (ha) (ha) (ha) (ha)

C, E, F 11.9 178.5 1.8 1.0 10.1 0.5

H to O 38.5 577.5 5.8 1.0 32.7 0.5

P, Q, R 15.5 232.5 2.3 1.0 13.2 0.5

Z to AC 22.7 340.5 3.4 1.0 19.3 0.5

AD 5.4 81.0 0.8 1.0 4.6 0.5

.*Assumes 15 lots/ha = 600 m2 lots (say)

.** Assumes 100 m2 of roof to 2000 litre tank for toilet flushing

.*** Assumes overall catchment Fimp = 0.6

Toilet Demand and Tank size

Number of people per house = 2

Duel flush toilet demand = 22 l/person /day

(Coomes Consulting Group-Epping Project 2002)

storage - 2000 litre tanks for toilet flushing

Area

(ha)

Approx

Dwellings*

Approx

Population

Approx toilet

Demand

Approx

toilet

Demand

Tank

volume (m3)

Catchment litres/day ML/yr

C, E, F 11.9 179 357 7854 2867 357

H to O 38.5 578 1155 25410 9275 1155

P, Q, R 15.5 233 465 10230 3734 465

Z to AC 22.7 341 681 14982 5468 681

AD 5.4 81 162 3564 1301 162

The performance of the WSUD system detailed in Appendix A and Figure 2 was analysed using the Model

for Urban Stormwater Improvement Conceptualisation model (MUSIC – Version 6). Figure 5 details the

MUSIC model developed for the catchment.

21

Catchments are defined as per the catchment layout detailed in Section 4 above. The model utilised the

Melbourne Water reference year for Narre Warren North 1998 at 6 minute intervals. This rainfall and

evaporation data is considered valid for areas south east of Melbourne exhibiting annual rainfall between

850 – 1100 mm. As Cape Paterson receives about 950 mm/yr rainfall, it is considered that this gauge

information is valid for this application. The default base flow generation parameters have been used as they

represent reasonably sandy soils.

Figure 6 shows the MUSIC model developed. Table 5 details the performance of the strategy.

Figure 5 MUSIC Model Note external catchment T – Y not treated and conveyed in a vegetated swale along the eastern edge of the development. Wetlands treat subject site and external catchments A, B, D and G

22

Table 5 Music Results

Point 1 - RB4 outflow to Eco Village

Point 6 – Combined Eastern Site outfall

Table 5 shows that the load reduction objectives for Total Suspended Solids (80% retention of loads

produced on site), Total Phosphorus (45% retention) and Total Nitrogen (45% retention) will be met by the

proposed wetland and sediment ponds located within the RB1, RB2, RB3 and RB4.

It should be noted that the load pollutant reduction will still be met even without tanks for toilet flushing.

However, this initiative has been specified to meet the ecological requirements detained in Section 6 below.

In addition, although the stormwater pollutant reduction targets are exceeded, no reduction in wetland size is

proposed. The wetlands are required to also maximise the flood storage provisions detailed in Section 4

above.

23

6. Potential Estuary Impacts

The existing waterway downstream of Point 8, though the Parks Victoria land is relatively degraded and

exhibits many weed species etc. It is understood that this waterway was originally constructed to provide a

concentrated drainage outflow point for the land located to the north. Prior to this there was no large outflow

connection directly to the sea.

In 2015 the developer extended the site Flora and Fauna Report (Ecology and Heritage Partners) to include

the proposed drainage outfall for the development (to its estuary) and provide an assessment as to whether

or not there are any adverse effects from directing the flows from another catchment through this outfall. It is

understood by Stormy Water Solutions that the main issue that was highlighted was that there are a mating

pair of hooded plovers that nest in the path of the existing outfall on the beach.

Advice from Ecology and Heritage Partners, on 27/03/2015 regarding this issue suggest that:

• The outfall is usually blocked but there is, on average, two storm events during the hooded plover’s

breeding period (September – March) that are large enough to blow out the sand and wash away

eggs or young chicks, and

• It takes the hooded plover two and a half months to raise a chick from when the eggs are laid. If the

sand is blown out and the outfall flows during this time, the eggs (or chicks) are washed away. If

there is not two and a half months between these events the hooded plover does not reproduce for

the year.

It is required to be shown that the frequency of the times in which a flow event occurs capable of opening the

estuary does not increase due to the proposals.

Development of a catchment usually results in an increase in frequency of low and medium range flow

events at the estuary mouth. This may lead more storm events during the hooded plover’s breeding period

(September – March) that are large enough to blow out the sand and wash away eggs or young chicks.

The drainage strategy proposes to use the retarding basin/wetland systems to mitigate the effect of

increased flood frequency downstream at the estuary.

The wetlands within the base of each retarding basin provide a stormwater treatment function. However,

through storing water in their extended detention ranges they also act to retard and mitigate frequent flow

events.

The extent of these retardation/stormwater harvesting benefits during frequent rainfall events is investigated

below.

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6.1. Flow Frequency Analysis

6.1.1 Analysis Methodology

The following methodology has been used to assess estuary impacts due to development.

1. Determine the flow magnitude (in m3/s) which is exceed, on average, twice per breeding season for

the existing catchment,

2. Assess the increase in flows exceeding the rate calculated in 1. due to the Cape Paterson North

development and diversion to the estuary without wetlands and tanks for toilet flushing in the

development , and

3. Assess the reduction in impact of 2. Given the drainage strategy wetland proposals (3 wetlands in

RB1, RB2 and RB3 (500 mm extended detention and 36 hours extended detention period) and tanks

for toilet flushing utilised in the development.

It should be noted that an extended detention of 36 hours was deliberately chosen to balance the stormwater

pollutant retention function of the wetlands with the low flow range retardation function. At a detention time of

36 hours, the low flow wetland outlet flow rate is measurable (i.e. typically 15 – 45 l/s) and the wetland TED

storage range can be modelled as flood storage. This allows control of minor flow events within the extended

detention range of the wetland, rather than bypass of these minor events due to the wetland being full. If the

wetland bypasses too much (which could happen with wetlands of long detention times), the mitigation of the

required number of low flow events may not occur.

The performance of the drainage strategy in relation to mitigation of frequent flow events was analysed using

the MUSIC model. The water balance component of the model was utilised to conduct the analysis as it is an

appropriate tool to examine the effect of low flow events.

6.1.2 Scenarios Investigated

The scenarios analysis were:

1. Existing situation. No development or diversion of Cape Paterson North Flows.

2. Development of Cape Paterson North and diversion of flows east. No wetlands, rainwater tanks or

retardation initiatives.

3. Development of Cape Paterson North and diversion of flows east. 2000 Litre Tanks for toilet flushing

on all lots. Wetlands and retarding basins as detailed including an extended detention range of 500

mm and an extended detention period of 36 hours.

25

6.1.3 Rainfall Data

The closest Bureau of Metrology (BoM) rain gauge to Cape Paterson is Wonthaggi. The BoM web site

information for this gauge suggests an average long term rainfall for this gauge of 938 mm/yr. Eight years of

Croydon rainfall and evaporation data (BoM site 86234) at 1 hour intervals was used to undertake a water

balance analysis of the catchment. This gauge, duration and time interval were used as:

• This is considered a long enough duration to give an indication of long term average effect,

• The 1980’s is a good period to look at long term averages in and around Melbourne as they

contained some dry years, some wet years and an average rainfall per year of around the expected

long term average,

• 1 hour is less than the time of concentration of the total catchment to the estuary,

• Gauge 86234 (Croydon) recorded 874 mm rainfall/yr. during this period which is within 7 % of the

expected rainfall in Wonthaggi,

• An extension of the information contained within the current Melbourne Water rainfall distribution

map for Melbourne and surrounds suggests that Croydon rainfall can be a reasonable indication of

the rainfall distribution in the Cape Paterson region, and

• The MUSIC rainfall data was assessed as adequate for use (i.e. no significant gaps in the rainfall

record) between 1980 and 1988.

6.1.4 Catchment Delineation and Specification

The MUSIC Model delineation is as detailed in Figures 6 and 7 below.

A fraction imperviousness of 0.1 was used for existing rural areas, and as per the RORB model for the Cape

Paterson North Development.

Wetland areas are as detailed in the 2014 Appendix A.

Catchment areas for the development are as per the RORB model in Section 4. Catchment areas for the

extensive existing rural catchment are as detailed in Figures 6 and 7.

Soil infiltration parameters are as per the default MUSIC parameter set as they are known to be

representative of relatively sandy soil profiles. Minor infiltration at the wetland sites of 10 mm/hr is assumed.

This is on the lower end of infiltration expected for sandy clay profiles, and a conservative assumption in

regard to the sandy profiles expected at each site.

26

Figure 6 Catchment Deliniation – Scenario 1 (588 ha)

Figure 7 Catchment Deliniation – Scenario 3 (677 ha)

27

6.2 Analysis Results

The flow output (in m3/s) can be extracted at 1 hour interval at the estuary for the above scenarios. This was

done, and the output cropped to just consider the month of September to March inclusive (i.e. the breeding

months). For Scenario 1, it was found that a flow rate of 1.2 m3/s occurred on average 2.0 times per breeding

season over the eight years. As such, this seems a reasonable flow rate to assess as the flow rate capable

resulting in sand being washed out of the estuary and estuary flows occurring directly to the sea. This flow

also matches the rule of thumb check for a 3 month ARI flow for a catchment of this size. Again this seems

reasonable as the flow rate to assess as the event which may be exceeded twice over a breeding season.

Appendix C details the detailed results of each scenario. Table 6 below summarises the results.

Table 6 Analysis Results

Scenario

Average number times during a

breeding season that the flow at

the estuary is greater than 1.2 m3/s

=

Number of breeding seasons

incorporating a flow of 1.2 m3/s

more than two times in that

season

1

Existing situation. No development

or diversion of Cape Paterson North

Flows

2.0 3 out of 8

2

Development of Cape Paterson

North. No wetlands or retardation

initiatives

6.9 8 out of 8

3

Development of Cape Paterson

North. 2000 Litre Tanks for toilet

flushing on all lots. Wetlands as

detailed - Extended detention range

= 500 mm, detention time = 36 hours

2.0 3 out of 8

As detailed above incorporating tanks for toilet flushing on all lots and the wetland provisions as per this

strategy should ensure replication of existing estuary flow frequencies in regard to opening of the estuary to

the sea after a storm event.

It should be noted that, if either tanks for toilet flushing cannot be applied, and/or the required wetland

extended detention provisions cannot be applied (possibly due to unforseen site or design constraints) there

is scope in the drainage strategy to still complement the low flow frequency reduction benefits of the scheme

via provision of an additional wetland/retardation basin at Point 5 (Figure 1).

As such it is concluded that mitigation of the impact of the development on the existing estuary dynamics can

occur going forward.

28

7. Conclusions and Further Work Required

The proposed rainwater harvesting tanks, retarding basin / wetland systems detailed in this report

represents:

• a best practice application of water sensitive urban design principles to the mitigation of stormwater

pollutant impacts given future development of the subject site,

• a strategy which will mitigate storm flows such that the 100 year ARI flow at the catchment outlet is

reduced below the predevelopment flow rate, and

• a strategy which can mitigate the impact of the development on the existing estuary dynamics in

relation to the ecological requirements.

The retarding basin/wetland concept designs have been considered to enough detail to set the following

given topography and outlet invert level constraints:

• Normal water level

• Top of extended detention level

• Normal water level areal extents

• Top of extended detention level areal extents, and

• Cut line extents given 1 in 5 batters (min) to NWL

Work required to develop these to a functional design includes (but is not limited to):

• Confirming the size (and depth to natural surface level) of the bypass pipe system along Seaward

Drive to convey the 100 Year ARI outflow from RB2 and RB3,

• Determining if any drain cleanout, revegetation or enlargement works are required downstream of

Point 7,

• Setting wetland bathymetry (i.e. shape and levels) below normal water level,

• Confirming the retarding basin outlet sizes and configurations,

• Designing the sediment pond areas in all retention basins giving due consideration for cleanout

access provision and storage of sediment for drying during cleanout maintenance activities,

• Sizing all retarding basin inlet pipes (outfall invert into retarding basins should be above the set

concept design normal water level),

• Finalising the subdivision layout around the retarding basins giving due consideration of:

o The assumed contributing catchments to each basin as described in this report,

o The assumed location of the inlet pipe system to each system,

o The social and landscape benefits of locating boulevard roads adjacent to the retarding

basin systems

o The road levels, if any roads are to form part of a retarding basin embankment,

• Obtaining detailed survey of each site to ensure all design levels detailed in this report are

reasonable,

29

• Undertaking full testing of the site to determine soil characteristics, and

• Completing the site landscape design including access track and (potentially) boardwalk locations.

It should be noted that the retarding basin concept designs have optimized each site in regard to the

retardation and wetland requirements. In particular, incorporating a wetland in the base of the retarding

basins has reduced the retarding basin site requirements, as the flood storage is provided within the vertical

airspace, rather than the areal expanse during flooding.

All designs are to the concept design stage only at this stage. However, enough analysis has been

conducted to clearly show that development of the subject site is achievable without adverse stormwater

impact on downstream properties. In fact, by formally constructing an outfall for the catchment north of

Seaward Drive, flooding impacts though the existing Cape Paterson township should be reduced if the

drainage strategy is implemented.

The strategy developed is a “worst case” scenario in terms of downstream stormwater impact. There is

scope in the drainage strategy to complement the scheme (if required) via provision of an additional

wetland/retardation basin at Point 5 (Figure 1).

30

8. Abbreviations and Definitions

The following table lists some common abbreviations and drainage system descriptions and their definitions

which are referred to in this report.

Abbreviation

Descriptions

Definition

AHD - Australian Height Datum

Common base for all survey levels in Australia. Height in metres above mean sea level.

ARI - Average Recurrence Interval.

The average length of time in years between two floods of a given size or larger

BoM Bureau of Meteorology

DoT Department of Transport

Evapotranspiration

The loss of water to the atmosphere by means of evaporation from free water surfaces (e.g. wetlands) or by transpiration by plants

Groundwater All water stored or flowing below the ground surface level

Groundwater Level The level of groundwater below the surface level at a particular point of interest (usually given in AHD or relative to surface level)

Inlet Pond See Sediment Pond

Hectare (ha) 10,000 square metres

Hec Ras A one dimensional, steady state hydraulic model which uses the Standard Step Method to calculate flood levels and flood extents

Kilometre (km) 1000 metres

m3/s -cubic metre/second

Unit of discharge usually referring to a design flood flow along a stormwater conveyance system

Megalitre (ML) (1000 cubic metres)

1,000,000 litres = 1000 cubic metres Often a unit of water body (e.g. pond) size

MUSIC Hydrologic computer program used to calculate stormwater pollutant generation in a catchment and the amount of treatment which can be attributed to the WSUD elements placed in that catchment. Can also be used to calculate water body turnover period and wetland draw downs etc

NWL Normal Water Level – invert level of lowest outflow control from a wetland or pond.

PET Potential Evapotranspiration – potential loss of water to the atmosphere by means of evaporation or transpiration from wetland or pond systems.

Retarding Basin Drainage element used to retard flood flows to limit flood impacts downstream of a development. Can include complementary WSUD and ecological site benefits if wetland incorporated within the site.

Sedimentation basin (Sediment pond)

A pond that is used to remove coarse sediments from inflowing water mainly by settlement processes.

Surface water All water stored or flowing above the ground surface level

TED Top of Extended Detention – Level to which stormwater is temporarily stored for treatment in a wetland or pond (above NWL).

TSS Total Suspended Solids – a term for a particular stormwater pollutant parameter

TP Total Phosphorus – a term for a particular stormwater pollutant parameter

TN Total Nitrogen – a term for a particular stormwater pollutant parameter

Wetland

WSUD elements which is used to collect TSS, TP and TN. Either permanently or periodically inundated with shallow water and either permanently or periodically supports the growth of aquatic macrophytes

WSUD Water Sensitive Urban Design.

31

Appendix A – Retarding Basin Concept Designs

All plans detailed should not be scaled. A set scale in this report has not been assigned.

The designs detailed below are concept designs only. They have been considered to enough detail to set the following

given topography and outlet invert level constraints:

• Normal water level

• Top of extended detention level

• Normal water level areal extents

• Top of extended detention level areal extents, and

• Cut line extents given 1 in 5 batters (min) to NWL

Work required to develop these to a functional design includes (but is not limited to) those aspects discussed in

Section 7 above.

It should be noted that 2000 litre tanks for toilet flushing on all lots is also proposed as part of this strategy.

32

A.1 Retarding Basin RB1

34


36


38


40

Appendix B – Retarding Basin Models

RB1 Stage Storage Discharge Relationship

Normal Water Level 18.75 m AHD

Normal Water Level Area = 3971 m2

Extended Detention level = 19.25 m AHD

Top of Extended Detention Area = 4349 m2

Outflow relationshipsRetarding Basin Outflow below TED m AHD

Average wetland treatment storage area = 4160 m2

Extended Detention height = 0.50 m

Extended detention volume = 2080 m3

Detention time = 36 hours - maximise outflow in ED range while

still allowing adequate treatment time

Outflow at TED = 0.016 m3/s

Therefore orifice control at invert level = 18.75 m AHD to limit outflow to

0.016 m3/s in ED range

ORIFICE CALCULATION Q = 0.6× A×(2gH) 0.5

orifice dia = 0.13 m

Area = 0.01389471 m2

Elevation H (m) Q(m3/s)

19 0 0.0

19.25 0.2 0.016 130 mm orifice hole required at IL = 18.75 m AHD

Culvert acting under inlet control


pipe dia = 0.375 m

Area = 0.110460938 m2

Elevation H (m) Q(m3/s) Q(m3/s)

ONE 450 mm dia THREE 450 mm dia

18.75 0 0.000 0.000 wetland outflow at TED

19.25 0.3 0.02

20 1.1 0.303 0.91

20.5 1.6 0.367 1.10

nb IN THIS CASE INLET AND OUTLET CONTROL

Storage Relationship PRODUCE ESSENTIALLY THE SAME US WL

Level (m AHD) Area (m2) Average Area (m2) Height Diff (m) Volume (m3) Cumulative Volume (m3)

18.75 3971 0

19.25 4349 4160 0.5 2080 2080

20 5800 5075 0.75 3806 5886

20.5 6813 6307 0.5 3153 9039

Stage/Storage/Storage Relationship

Level (m AHD) Q(m3/s) Storage (m3)

18.75 0.000 0 Normal Water Level

19.25 0.016 2080 Top of Extended Detention

20 0.908 5886

20.5 1.101 9039

41




Extended Detention level = 25 m AHD





Extended detention volume = 4160 m3








Area = 0.01767375 m2


24.5 0 0.0

25 0.4 0.031 150 mm orifice hole required at IL = 23.5 m AHD



pipe dia = 0.225 m

Area = 0.039765938 m2


25 0 0.031 wetland outflow at TED

25.05 0.4 0.070

25.5 0.9 0.100

26.5 1.9 0.145

Storage Relationship


24.5 7080 0

25 9560 8320 0.5 4160 4160

25.5 10800 10180 0.5 5090 9250

26.5 14000 12400 1 12400 21650



24.5 0 0 Normal Water Level

25 0.031 4160 Top of Extended Detention

25.05 0.070 4500

25.5 0.100 9250

26.5 0.145 21650 Cut line at lowest natural surface level

42







Average wetland treatment storage area = 11232.5 m2


Extended detention volume = 5616.25 m3








Area = 0.0254502 m2


26.5 0 0.0




pipe dia = 0.375 m

Area = 0.110460938 m2


27 0 0.043 wetland outflow at TED

27.05 0.4 0.177

27.5 0.8 0.265

28.5 1.8 0.395



26.5 10125 0

27 12340 11233 0.5 5616 5616

27.5 13593 12967 0.5 6483 12100

28.5 16100 14847 1 14847 26946





27.05 0.177 6000

27.5 0.265 12100


43









Extended detention volume = 1100.75 m3








Area = 0.007089138 m2


27.75 0 0.0




pipe dia = 0.45 m

Area = 0.15906375 m2


27.75 0 0.000 wetland outflow at TED

28 0.0 0.008

28.5 0.5 0.919 TRIPLE outlet pipes under road



27.75 4140 0

28 4666 4403 0.25 1101 1101

28.5 6100 5383 0.5 2692 3792






44

Appendix C Flood Frequency Analysis Results

Scenario 1

• Existing situation

• No development or diversion of Cape Paterson North Flows

No times > 1.2 m3/s = 16 .= 2.0 per breeding season Looks OK - 1.2 m3/s is approx 3 month flow

No Breeding seasons incorporating a flow of 1.2 m3/s more than two times = 3 out of 8

0

200

400

600

800

1000

1200

1

10

81

21

61

32

41

43

21

54

01

64

81

75

61

86

41

97

21

10

80

1

11

88

1

12

96

1

14

04

1

15

12

1

16

20

1

17

28

1

18

36

1

19

44

1

20

52

1

21

60

1

22

68

1

23

76

1

24

84

1

25

92

1

27

00

1

28

08

1

29

16

1

30

24

1

31

32

1

32

40

1

33

48

1

34

56

1

35

64

1

36

72

1

37

80

1

38

88

1

39

96

1

41

04

1

42

12

1

43

20

1

44

28

1

45

36

1

46

44

1

47

52

1

48

60

1

49

68

1

50

76

1

51

84

1

52

92

1

54

00

1

55

08

1

56

16

1

57

24

1

58

32

1

59

40

1

60

48

1

61

56

1

62

64

1

63

72

1

64

80

1

65

88

1

66

96

1

68

04

1

69

12

1

45

Scenario 2

• Development of Cape Paterson North

• No wetlands or retardation initatives

No times > 1.2 m3/s = 55 .= 6.9 per breeding season

No Breeding seasons incorporating a flow of 1.2 m3/s more than two times = 8 out of 8

0

200

400

600

800

1000

1200

1

10

81

21

61

32

41

43

21

54

01

64

81

75

61

86

41

97

21

10

80

1

11

88

1

12

96

1

14

04

1

15

12

1

16

20

1

17

28

1

18

36

1

19

44

1

20

52

1

21

60

1

22

68

1

23

76

1

24

84

1

25

92

1

27

00

1

28

08

1

29

16

1

30

24

1

31

32

1

32

40

1

33

48

1

34

56

1

35

64

1

36

72

1

37

80

1

38

88

1

39

96

1

41

04

1

42

12

1

43

20

1

44

28

1

45

36

1

46

44

1

47

52

1

48

60

1

49

68

1

50

76

1

51

84

1

52

92

1

54

00

1

55

08

1

56

16

1

57

24

1

58

32

1

59

40

1

60

48

1

61

56

1

62

64

1

63

72

1

64

80

1

65

88

1

66

96

1

68

04

1

69

12

1

46

Scenario 3

• Development of Cape Paterson North

• 2000 Lite Tanks for toilet flushing on all lots

• Wetland extended detention range = 500 mm, extendeded detention period = 36 hours

Cape Paterson North Proposed Drainage Strategy

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