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STORM WATER MANAGEMENT STRATEGY:

Marong Township

City of Greater Bendigo

December 2017

Marong Township: SWMS 2

Document history

Revision: Revision no. 02 Author/s Harry Virahsawmy Dan O’Halloran Checked Dan O’Halloran Approved Jonathon Mclean

Distribution: Revision no. 02 Issue date 19 December 2017 Issued to Brendan Aikman

Strategic Planner - Design City of Greater Bendigo

Description: Final report

Revision no. 01 Issue date 26 October 2017 Issued to Brendan Aikman

Strategic Planner - Design City of Greater Bendigo

Description: Draft for comment

Ref: R:\Projects\2017\085_Marong_stormwater_management_plan\1_Deliverables\Final data package\P117085_R02V02_MarongTownship_SWMS.docx


Contents

1 Introduction 5

1.1 Background 5

1.2 Background information 6

2 Existing conditions 7

2.1 Existing land use 7 Planned development areas (Stage 1) 8 Proposed area for development (Stage 1) 8 Proposed future area for development (Stage 2) 8 Potential area for development 8

2.2 Existing drainage characteristics and infrastructure 10 Sub-division A 11 Sub-division C 13 Sub-division D 14 Sub-division E 16 Sub-division F 19

2.3 Flooding - existing condition 20

2.4 Summary 20

3 Principles of the strategy 21

3.1 Stormwater conveyance 21

3.2 Flow retardation 22

3.3 Water quality treatment 22

3.4 Opportunities 23

3.5 Strategy criteria 24

4 Modelling and analysis 25

4.1 Hydrologic modelling 25

4.2 Stormwater treatment modelling 26

5 Proposed strategy – Option description 27

Option 1 27

Option 2 27

6 Proposed Strategy 28

6.1 Minor and major stormwater conveyance 28 Option 1 28 Option 1 Costing 30 Option 2 33 Option 2 costing 33

6.2 Culverts 35 Option 1 35 Option 2 35

6.3 Stormwater treatment 36 Option 1 36 Option 2 37

6.4 Flow retardation 37 Option 1 37


Option 2 40

7 Summary of assets proposed 41

Option 1 41

Option 2 43

8 Cost 45

9 Recommendations and conclusion 46

Appendix A 47

Flooding photos 47

Appendix B 52

Retarding Basin Stage Storage Relationships 52


1 Introduction

The City of Greater Bendigo (CoGB) is planning for the growth and development of the Marong Township (Marong). Marong lies approximately 15km to the west of Bendigo and is currently home to approximately 1,100 people (profile.id.com.au). The long-term vision for Marong is for it to become a satellite township of Bendigo with a population of approximately 8,000 people.

The management of surface water is a critical element in planning new townships and developments, principally to ensure that development does not exacerbate flooding and that stormwater generated from paved surfaces is treated to a suitable standard so as not to detrimentally impact receiving environments, principally Bullock Creek.

This Stormwater Management Strategy for the Marong Township has been prepared to provide the CoGB with a holistic drainage strategy that sets out flood mitigation and stormwater treatment requirements across the township. Whilst delivered at a strategic level, the plan sets out drainage asset requirements, locations and dimensions to provide certainty in terms of asset footprints. This can contribute to Council’s thinking of broader liveability aspects, including the interaction of the community with waterways and wetlands, to ensure that Marong develops in accordance with the vision and objectives set out in the Marong Township Structure Plan.

1.1 Background The Marong Township Structure Plan (Figure 1) guides the future development of the township defining areas for residential development, retail and commercial activities, public spaces and transport infrastructure including proposed traffic bypasses. It can be observed that the town is built around an existing town centre and public space network featuring Bullock Creek and Malone Park.

Bullock Creek flows south to north, bisecting the township and forming a prominent feature in the precinct plan. Bullock Creek also contributes to a valuable network of public and recreational spaces.

Figure 1. Draft Marong Township Structure Plan (modified by Alluvium to include additional proposed area for development outlined by red box)


Land development within Marong is broken down into two stages (see the dark to light blue/ grey shaded areas in Figure 1).

Stage 1 will consist of:

o Planned development areas to zoned “General Residential Zone”.

o A proposed area for development which will consist of residential zone (GRZ), Special Use Zone (SUZ) and Mixed-Use Zone (MUZ).

Stage 2 will consist of proposed future area for development which will be an extension of the General Residential Zone (GRZ).

In developing the stormwater management strategy, a potential future area for development (outlined in red in Figure 1) will also be considered. This area does not currently form part of the draft Marong Township Structure Plan. Consultation with the landowners is currently underway regarding the potential future development of this area.

1.2 Background information The following sources of information have been referenced in the development of the Stormwater Management Strategy:

The Draft Marong Township Structure Plan (March 2017)

The Marong Plan (City of Greater Bendigo June 2015)

The North Central Catchment Management Authority (NCCMA) Flood Maps of Bullock Creek

Submission to the City of Greater Bendigo in response to the Draft Marong Structure Plan from Whatley, Pollock and Harrison families

• Site visit and inspection by Alluvium and Council on 28th

June 2017

• Drainage plans of recently approved sub-divisions (e.g. Discover Marong, Marong Links)

• Flow paths for the Marong Proposed Industrial Site

• Waterway Corridors, Guidelines for greenfield development zones within the Port Phillip and Westernport Region – Melbourne Water

• Australian Rainfall & Runoff (1997) – Engineers Australia

• Australian Rainfall & Runoff (2016) – Engineers Australia


2 Existing conditions

Defining existing conditions is the first step in defining the stormwater management conditions for the township with a view to anticipating the impact of land use change and future development on surface and stormwater.

2.1 Existing land use For the purposes of this stormwater management plan, the development areas have been labelled as sub-divisions A to F (see Figure 2). At the time of writing drainage strategies for sub-divisions B and F had been completed. This strategy therefore excludes analysis of those sub-divisions. The strategy does however account for the implications of upstream land development on the planned stormwater infrastructure within sub-divisions B and F.

The drainage strategy for sub-division E was under-development at the time of writing this strategy. The strategy for sub-division E (and F) includes a main drain from Marong Links RB to Bullock Creek. Whether the main drain from Marong Links RB to Bullock Creek is adopted or not, presents the remainder of the strategy area with two possible future options.

Option 1: includes analysis and recommendations for sub-division E (including design of water quality and retarding basin assets incorporating the alignment of the proposed main drain).

Option 2: excludes analysis and recommendations for sub-divisions E assuming the drainage strategies for sub-divisions E (and F) are progressed.


Planned development areas (Stage 1) Sub-division B: 23.4 ha off Calder Highway currently zoned “Township Zone”. To be rezoned “General Residential Zone”. This area has been approved for residential sub-division and is currently under development as ‘Discover Marong’ (see Figure 4).

Sub-division F: 8.1 ha off Calder Highway currently zoned “Township Zone”. To be rezoned “General Residential Zone”. This area has been approved for residential sub-division and is currently vacant. It is adjacent to the recent sub-division of Marong Run (Figure 6).

Figure 2. Marong Township sub-divisions

Proposed area for development (Stage 1) Sub-division E: 31 ha bisected by Calder Highway, dividing this sub-division into sections south of the highway (20 ha) and north of the highway (9.3 ha). This area is currently zoned into three groups – Township zone, Special Use Zone and Industrial 3 Zone. The “Marong Caravan and Cabin Village” and “Big 4 Bendigo Marong Holiday Park” are currently located in sub-division E1. The area will eventually consist of residential zone (GRZ), Special Use Zone (SUZ) and Mixed Use Zone (MUZ).

Proposed future area for development (Stage 2) Sub-division A: 96 ha adjacent to Calder Highway. This sub-division is roughly bounded by Hills Road (west), Calder Highway (north), Birchalls Road (east), and Salvarezza Road and Lockwood Marong channel (south). It is currently zoned as “Farming Zone” and is currently proposed to be rezoned to “General Residential Zone”.

Sub-division C: 136 ha south of the town centre intersected by Calder Alternative Highway and Bullock Creek. Sub-division C1 (96 ha) is east of the Highway and sub-division C2 (35 ha) is to the west. This area is currently zoned as “Farming Zone” and is currently proposed to be rezoned to “General Residential Zone”.

Potential area for development Sub-division G: 115 ha area for development bounded by Fletchers Creek Rd, Calder Highway, Frankels Lane and an unnamed road. This area is proposed for future low-density housing subject to further planning.


Sub-division D: 62 ha west of the town centre is a potential area for development but not currently included in the draft Marong Structure Plan. The area is bounded by McCreddons Road (west), the railway (north), Landry Lane (east) and Wimmera Highway (south) that is currently “Farming Zone” (Figure 5). It has been suggested that the 1 km radius buffer from the intensive animal industry may be reduced to 750 m, however this is not confirmed. This sub-division is to the west of the recent sub-division, Marong Links (Figure 6).

The land north of the railway line is not planned for residential development to maintain a buffer between potential residential development and the proposed Marong Business Park to the north west. The land south of Wimmera Highway and north of the proposed Western Bypass alignment is also not planned for conventional residential development, but other opportunities may exist such as low density residential options or a small local enterprise precinct.

Figure 3. Land development and stormwater management infrastructure at Discover Marong.

Figure 4. Drainage works and drainage paths downstream of ‘Discover Marong’


Figure 5. Existing site condition for land parcels west of Landry Lane and south of the railway line including culvert

Figure 6. Recent residential sub-divisions – Marong Links with retarding basin (left) and Thomas Drive in the Marong Run Estate (right)

2.2 Existing drainage characteristics and infrastructure The drainage characteristics of each proposed sub-division was investigated. This process was informed by aerial photography, the site visit and catchment mapping. The catchment within and upstream of each sub-division was mapped. There are two catchment types considered: the “External Catchment” and “Internal Catchment.” The term “External Catchment” refers to any land outside the relevant proposed sub-division boundary. Table 1 summarises the catchment area for each sub-division and existing (pre-development) flows at the nominated outfall locations.


Table 1. Catchment area and existing flow conditions at nominated outfalls

Sub-division Stage Outfall

location

Internal

Area

(ha)

External

Area

(ha)

Total Area

at outfall

(ha)

Existing flow

1 in 100-year ARI

(m3/s)

Sub-division B Planned B1 23.4 49.81 73.21 3.55

Sub-division F Planned F1 8.45 9.98 18.43 1.18

Sub-division E Proposed

(stage 1)

E1 14.06 47.32*

61.38 3.08

E2 9.97 61.38*

71.35 3.47

E3 9.98 - 9.98 0.73

Total 34.01 108.70 142.71

Sub-division A Proposed (stage 2)

A1 8.34 - 8.34 0.64

A2 14.82 - 14.82 0.99

A3 44.60 58.48 103.08 4.61

A4 25.30 116.37 141.67 5.79

Total 94.50 174.85 269.35

Sub-division C Proposed (stage 2)

C1 20.23 25.39 45.63 2.41

C2 35.47 11.66 47.13 2.47

C3 6.59 7.59 14.18 0.95

C4 10.79 - 10.79 0.77

C5 25.61 5.61 31.22 1.76

C6 11.98 - 11.98 0.83

Total 110.67 50.25 160.93

Sub-division D Potential

D1 32.55 3.90 36.50 1.99

D2 18.10 - 18.10 1.16

D3 12.95 214.06 227.01 7.99

D5 0 214.06 214.06 7.60

Total 63.60 269.13 332.73

* Includes Marong Links and golf course

Sub-division A Sub-division A drains in a general northwest direction towards the Calder Highway and the tributary of Bullock Creek (see Figure 7). The existing Lockwood Marong Channel currently forms the southern boundary of sub-division A.

Runoff from the catchments contributing to sub-division A outfalls at five locations (A1 through to A5). There are three existing culverts along the Calder Highway at location A3, A4 and A5. The largest culverts are located at location A3 (tributary of Bullock Creek) with 3 culverts of approximately 1200 mm width and 900 mm height (see Figure 8).

The Lockwood Rural system form the southern boundary of sub-division A with approximately 1,200 ML/year flowing through this system. The Lockwood Rural system serves about 160 customers including major chicken producers. This system is likely to continue into the foreseeable future however Coliban Water regularly evaluates the need for rural system reconfiguration. There is also a proposal to supply Laanecoorie with either raw or treated water from the Bendigo rural system and a decision on this is likely to influence the timing of reconfiguration of this system.

External catchments (largely to the south of the Lockwood Rural system) are planned to remain undeveloped. The assumption is that stormwater infrastructure will enable surface water flows from the external catchment


to bypass the rural channel and enter the sub-division A. Given that the external catchments are planned to remain undeveloped, external flows will therefore not be attenuated before entering sub-division A.

Figure 7. Sub-division A drainage and catchment characteristics

Figure 8. Existing culverts (1200×900 mm) under Calder Highway at the tributary of Bullock Creek (location A3) (City of Greater Bendigo)


Figure 9. Existing culverts under Calder Highway at location A4 (City of Greater Bendigo)

Sub-division C The Calder Alternate Highway and Bullock Creek divides sub-division C (Figure 10). The land on the eastern side of the highway generally drains towards Bullock Creek, except for the easternmost section that drains north towards sub-division B (outfall location C1). This land section was assumed to remain as rural land when sub-division B (‘Discover Marong’) was planned.

Post-development flows at location C1 will therefore need to be retarded back to pre-development flow rates. It is assumed that the proposed street network in sub-division B will handle the overland flows from sub-division C associated with a 1 in 100-year average recurrence interval (ARI) rainfall event. It is also assumed that the railway drain at Malone Park (Figure 4) has sufficient capacity to convey the overland flows to Bullock Creek.

It is also likely that post-development flows from sub-division C at locations C2, C3 and C4 will need to be retarded to pre-development levels before reaching Bullock Creek to minimise impact on downstream flooding. Flow retardation will also need to be sized to accommodate flows from sub-catchments external to the development areas (Table 1).

The land on the western side of the highway drains in a general north to northwest direction towards the golf course and sub-division D at outlet location C5 (Figure 10). It is likely that the developed flows will need to be directed north-west towards sub-division D instead of North towards the “Marong Links” sub-division which has a known drainage limitation (being currently serviced by a pumped retarding basin). The drainage strategy for this land parcel will also need to account for external catchment flows.


Figure 10. Sub-division C drainage and catchment characteristics

Sub-division D Sub-division D drains in a north to northwest direction towards the unused railway line (Figure 11). There are three outfalls: D1 at the intersection with the railway, D2 at the north-west corner and D3 on the western boundary at the corner of Barnes Rd and McCreddons Rd. There are two existing culverts under the unused railway line at location D1 and D4 (Figure 11).

A large external catchment of 214 ha – the area bounded by the Wimmera Highway, Calder Alternate Highway and the higher ground to the south west – contributes flows to the western portion of sub-division D which drains to the western boundary at outfall D3. An existing channel conveys the external flows through sub-division D (Figure 14) to outfall D3.

The eastern portion of sub-division D drains to the north towards the railway at outlet location D1. The drainage strategy for this eastern portion needs to account for external flows from sub-division C (outfall C5) as discussed above). It is likely that flows at outfall location D1 will need to be retarded back to pre-development levels and drainage works (e.g. channel) required through the private property to the north of railway line to enable discharge (at pre-development levels) through that property.

Given that the large external catchment contributing flows to the western half of subdivision D is planned to remain undeveloped and will not receive flow attenuation (or treatment), it is proposed that only the eastern portion of sub-division D is included in the development of the drainage strategy.

It should be noted that the land west of Landry Lane (which includes sub-division D) has been identified within the Marong Structure Plan as being poorly drained and this area does not currently form part of the draft Marong Township Structure Plan. However, it has been included in this stormwater management strategy given that it is a ‘logical inclusion’ from a catchment perspective – particularly given that developed flows from sub-division C at outfall C5 will need to be directed through sub-division D, and not through “Marong Links” which has a known drainage limitation (being currently serviced by a pumped retarding basin).


Figure 11. Drainage paths (Sub-division D)

Figure 12. Existing two 300 mm diameter pipes under railway (location D4)


Figure 13. Existing culvert (600 × 400 mm) under unused railway line (location D1))

Figure 14. Existing channel and flooding at Wimmera Highway close to McCreddons Rd- Southern boundary of sub-division D (Location D5)

Sub-division E Sub-division E is divided by the Calder Highway. Part of the sub-division drains in north-east direction towards Goldie Street and Thomas Drive towards Bullock Creek (Figure 15). The northern part of the sub-division E drains to the north at outfall E3 towards Bullock Creek through sub-division F (Figure 15).

External flows drain through sub-division E to outfalls E1 and E2. The external catchment includes the Marong Links sub-division, the upstream golf course plus a small area south of the Wimmera Highway (Table 1). It is understood that development of Marong Links assumed that the upstream catchment (golf course) would continue to discharge at the existing undeveloped rate. These flows would transfer through to the retarding basin via the road network.

There are six existing culverts located under the railway line north of Marong Links’ retarding basin (RB) (Figure 18). There are existing culverts under the Calder Highway along the boundary of sub-division E at outfall locations E1 and E3 (Figure 16 and Figure 17). As indicated in “Drainage Study for Western Marong Township” (Spiire, 2015), the 6 x 600mm diameter culverts are designed to convey flow from a surcharged pit connected to the RB’s below ground storage chamber. The gravity fed flow from the pumped RB is to be conveyed through the railway culverts and join the existing drainage line.


It is understood that should the basin/pump system fail, there is a flow path from Ormond Drive to McKimmie Road which will follow the low point through to the Calder Highway and on to Bullock Creek.

There is a proposal in the Spiire report for a new pipe system to carry the flows from the Marong Links RB and developed flows from sub-division E and F to Bullock Creek. The pipe transitions in diameter from 825 mm to 1350 mm and passes through sub-division E in the direction of Goldie Street and Thomas Drive and eventually to Bullock Creek through sub-division F. The stormwater management strategy will need to account for this proposed main drain.

It should also be noted that at the time of this strategy development, the individual drainage strategy for sub-division E was also progressing within Council. In particular, it includes the proposed main drain from Marong Links RB to Bullock Creek.

Figure 15. Drainage paths (Sub-division E)


Figure 16. Existing 300 mm diameter pipe under Calder Highway North of Goldie Street looking south-west

Figure 17. Existing 300 mm diameter pipe under Calder Highway North of Torrens Street looking west


Figure 18. Railway culverts north of Marong Links retarding basin (Source: Spiire, 2015)

Sub-division F Sub-division F drains to the north towards Bullock Creek at outfall location F1 (Figure 19). Part of the sub-division drains to an existing, meandering channel prior to joining Bullock Creek at outfall F2. External flows enter sub-division F at outfall E2 and E3. The external catchment includes the northern portion of sub-division E (outfall E3) and the southern portion of subdivision E, “Marong Links” sub-division and the golf course at outfall E2.

It should be noted that the drainage plans for sub-division F – a planned extension to the Marong Run sub-division – have been completed but not submitted for approval at the time of writing. The plan does not appear to allow for conveyance of external flows entering subdivision F at outfall E3. However, it does include the proposed main drain pipe from Marong Links RB to Bullock Creek as discussed in the section above.

Whilst, this strategy will exclude analysis and recommendations for sub-division F, it will consider the implications of upstream land development (i.e. sub-division E) on the stormwater infrastructure required to service sub-division F.

Figure 19. Drainage paths (sub-division F)


2.3 Flooding - existing condition The North Central Catchment Management Authority (NCCMA) provided the 1% Average Exceedance Probability (AEP) flood extent maps for the Marong North and Marong South areas. These overlays show the areas subject to flooding (based on the January 2011 flood event). These maps show substantial areas through the township along Bullock Creek and Fletchers Creek that are subject to flooding. The main areas subject to flooding that are relevant for this stormwater management strategy are in sub-division C (an estimated 300m flood width in the northern section), sub-division G, and the land directly west of the Allies Road and Calder Highway intersection. A number of additional sites are known to be impacted by floods (Appendix A).

The NCCMA has also recently undertaken flood modelling and the 1% AEP modelled flood extents correspond with the flood extents observed during the January 2011 flood event. The observed flood maps together with the modelled flood extents were used to inform the development of the stormwater management strategy. It should be noted that only the flood extents associated with the 1% AEP event were modelled by the NCCMA and made available for this investigation.

2.4 Summary Based on the existing condition, the following key issues have been identified.

Table 2. Existing condition summary

Category Issue Proposed response

Drainage Sub-division D (land west of Landry Land) is identified within the Marong Structure Plan as being poorly drained.

Determine a suitable stormwater conveyance solution (noting that sub-division D does not form part of the draft Marong Township Structure Plan)

Large external catchments contributing flows to sub-division D

Limit the investigation to the eastern half of sub-division D

Stormwater runoff from sub-division D and sub-division C (outfall C5) draining into private property north of railway line.

Retard flow from sub-division D and sub-division C (outfall C5) to pre-development rates at outfall D1 and allow drainage works (e.g. drainage channel) in the private property to the north of the railway line to enable discharge (at pre-development levels) through that property.

The approved drainage plans for sub-division F does not allow for conveyance of external flows entering the subdivision at outfall E3.

Convey developed flows at outfall E3 (retarded to pre-development levels) along Goldie Street to the proposed pipeline from Marong Links RB and Bullock Creek.

Flooding Existing flood risk and the likelihood of exacerbated flooding conditions post development

Flow attenuation e.g. by retarding basins required to meet pre-development flow rates

Stormwater quality and waterway values

Stormwater quality Treatment assets require to meet BPEMG (Best Practice Environmental Management Guidelines) pollutant reduction targets


3 Principles of the strategy

3.1 Stormwater conveyance The minor-major stormwater flow conveyance principle was adopted for this strategy. The piped (or minor drainage) system is assumed to be designed to carry flows associated with a 1 in 5-year average recurrence interval (ARI) event. Flows in excess of the minor drainage capacity surcharge resulting in excess stormwater having to travel overland within road reserves. Road reserves are therefore assumed to convey the ‘gap flows’ for the 1 in 100-year ARI event minus the 1 in 5 year ARI event (that is conveyed within sub-surface pipes).

Based on typical road width and slope, and the maximum allowable nature strip cross-fall of 10%, the capacity that can be contained within the main road reserves is shown in Table 3. A conservative upper limiting road reserve capacity of 4 m

3/s has been allowed for this strategy. Where flows exceed the pipe capacity plus

4 m3/s road capacity in the 1 in 100-year ARI rainfall event, it is assumed that it cannot be contained within a

combined pipe and designated road reserve, and as such a constructed waterway is required.

Table 3. Road capacity flows*

Road width Slope Road capacity (m3/s)

16 m 1.5 % 4.0

16 m 1.0 % 4.5

16 m 0.5 % 4.5

20 m 0.5 % 5.5

* This capacity has been determined using HEC-RAS based on the Melbourne Water floodway safety criteria for residential streets used as floodways and Council’s requirement that flows associated with the 1 in 100-year ARI event must be contained within the road reserve and must not enter any part of private allotments. Criteria include Manning’s ‘n’ = 0.020, average velocity time average depth of less than 0.35 and average depth of less than 0.30 m.

The strategy assumes that each undeveloped property (that is > 0.4 ha) within the sub-division is required to convey and treat stormwater via a pipe network from its lowest point or points to a waterway or stormwater treatment asset (wetland or retarding basin). Where drainage lines traverse properties the strategy provides for stormwater carriage through the property. There is no allowance for internal connections. Landowners wishing to connect to the proposed pipes at points other than the designated low point(s), or to relocate the pipes to suit their sub-division, are assumed to bear any associated additional cost.

Pipes were sized to accommodate flows associated with a 1 in 5-year ARI event in residential areas, using:

The Rational Method (AR&R 1987) to estimate the runoff from individual lots and land parcels

Runoff coefficients of 0.6 for residential land and 0.1 for undeveloped land. Note that a runoff coefficient of 0.1 has also been used for the pervious areas of the existing golf course, in the north of sub-division C.

Design rainfall parameters (Intensity-Frequency-Duration, or IFD) for Marong were obtained from Meteorology’s Intensity Frequency Duration (IFD) Tool (Table 4)

Pipe sizes determined assuming pipes are flowing at slightly less than full capacity

Pipe slope was estimated using LiDAR provided

Pipe hydraulic roughness (Manning’ n) of 0.013 was adopted for all pipe design calculations.


Table 4. AR&R Design Rainfall parameters

Parameter Value

Rainfall intensity

1hr 2yr 18.9 mm/h

12hr 2yr 3.36 mm/h

72hr 2yr 0.879 mm/h

1hr 50yr 41 mm/h

12hr 50yr 7.12 mm/h

72hr 50yr 1.83 mm/h

Skew 0.17

F2 4.34

F50 14.96

Zone 1

3.2 Flow retardation Given the known flooding issues within Marong, it is possible that additional flows from the proposed sub-divisions (particularly sub-divisions A and C) could exacerbate downstream flooding issues. Alternatively, flood storage available in Malone Park may be sufficient to avoid retarding flows from sub-division C (outlets C2, C3 and C4). Two options are therefore provided for this strategy (discussed further in section 5).

Option 1 – Flow retardation required at outlets C2, C3 and C4, discharging into Bullock Creek

Option 2 – Flow retardation not required at outlets C2, C3 and C4. Should Council adopt this option, it is recommended that additional flood modelling is undertaken to ensure downstream flooding issues along Bullock Creek is not exacerbated.

There are also several locations where concentrated flows from the proposed sub-divisions will cross roads or the EagleHawk-Inglewood railway line (A3, C1, C5, and D1). Retarding basins are proposed at these locations to retard post-development flow rates to pre-development levels and ensure drainage issues at these locations are not exacerbated.

3.3 Water quality treatment The Best Practice Environmental Management Guidelines (BPEMG) (CSIRO, 1999) establishes stormwater quality objectives to help determine the level of stormwater management necessary to meet the State Environment Protection Policy (Waters of Victoria) objectives. This SEPP is a statutory policy under section 16 of the Environment Protection Act 1970 that identifies the beneficial uses of Victoria's waterways.

As discussed further in section 3.4, centralised constructed stormwater treatment wetlands are suitable for the proposed sub-divisions around Marong Township to manage stormwater quality. A constructed wetland typically consists of a sediment basin and a vegetated macrophyte zone.

For this stormwater management strategy, the wetlands have been designed in accordance with Water Sensitive Urban Design (WSUD) Engineering Procedures (Melbourne Water, 2005) to achieve annual pollutant loads reduction targets set out in BPEMG:

45% reduction in total nitrogen (TN) from typical urban loads

45% reduction in total Phosphorus (TP) from typical urban loads

80 % reduction in total Suspended Solids (TSS) from typical urban loads

70% reduction in litter from typical urban loads


Design parameters adopted for the wetland and sediment basin sizing have been based on the Melbourne Water Constructed Wetland Design Manual including:

Wetland Extended Detention Depth of 0.35 m

Wetland width to length ratio of approximately 1 to 4

Wetland average depth of 0.4 m

Wetland detention time of 72 hrs

Sediment basin Extended Detention Depth of 0.35 m

Sediment width to length ratio of approximately 1 to 4

Sediment basin average depth of 1.0 m

Sediment basin sediment capture efficiency of at least 95% (coarse particles ≥ 125 µm diameter)

Sized to allow for treatment of flows associated with a 3-month ARI rainfall event (with the assumption that flows are directed to the sediment basin and macrophyte zone, and gap flows (flows associated with a 1 in 5 year ARI minus flows associated with a 3 month ARI event) bypassing the wetland system.

It should be noted that the drainage system provided in this strategy is preliminary and does not represent a final stormwater network layout, nor does it replace the need for further design of individual assets.

3.4 Opportunities Based on consultation with Council and the community at a workshop held on 31

st August 2017, the following

opportunities are considered appropriate in the development of the stormwater management strategy:

Centralised stormwater management assets: combined retarding basin and stormwater quality assets are preferred to provide flow retardation and stormwater treatment while being less complex for Council to manage.

A combined retarding basin / wetland arrangement involves the wetland being positioned at the base of the retarding basin (i.e. the wetland is cut into the base). The base of the retarding basin design is therefore dictated by the wetland footprint. The required retarding basin storage is based on the cut volume from the existing surface to the wetland normal water level. Whilst a larger land take is generally required for flow retardation function compared to stormwater treatment, combining the two functions into one asset results in an optimised land take outcome when compared to building separate assets.

Potential for wetland / retarding basins to improve open spaces and serve to connect the community to Bullock Creek, as well as contributing to the development of habitat corridors along Bullock Creek.


3.5 Strategy criteria The criteria to be used in the development of the stormwater management strategy:

1. The drainage network is designed according to the minor/major drainage system philosophy

o the sub-surface (or minor drainage network) will be designed to convey the estimated 1 in 5-year ARI flowrate at full development.

o the surface or major drainage network, including roadways, will be designed to convey up to the 1 in 100-year average recurrence interval (ARI) flowrate at full development.

2. New connections: Where there are new connections to existing drainage infrastructure, flowrates will be limited to pre-development conditions to ensure existing drainage delivers expected levels of service.

3. Stormwater quantity: The traditional approach is to retard post-development flowrates to pre-development flowrates associated with the 1 in 100-year ARI event. Retarding basins are proposed to control the peak flowrates generated by future development to provide flood protection downstream, and where there are infrastructure capacity issues (e.g. at railway, highway or freeway crossing) and where topography suits the construction of such an asset.

4. Flood risk: Where there is a risk of flooding downstream of a future development, there is a requirement to retard flow associated with a 1 in 100-year ARI rainfall event back to existing conditions. We have relied on the flood extent provided by NCCMA to inform the development of the stormwater management strategy, with the objective that the developable areas should not exacerbate localised and downstream flooding.

5. Stormwater quality: New developments will be required to meet pollution reduction requirements as specified within BPEMG:

o 80 % removal of Total Suspended Solids

o 45 % removal of Total Nitrogen

o 45 % removal of Total Phosphorus

o 70 % removal of gross pollutants.

6. Development density: it will be assumed that the growth sub-division will have a similar residential density as recently completed residential sub-divisions including Marong Links and Discover Marong (i.e. 10-12 lots per hectare or 650-800 m

2 per lot).

7. Rural irrigation channel: it is assumed that the Lockwood rural channel (operated by Coliban Water) will remain, however surface water from upstream catchments continue over or under the rural channel via stormwater infrastructure.


4 Modelling and analysis

4.1 Hydrologic modelling The catchment drainage and hydrologic analysis has been informed from site inspections and hydrologic modelling. The kinematic wave equation and rational method was used to estimate the peak design flows at relevant locations (outfalls) under existing (i.e. pre-development) conditions.

Recent research on the estimation of peak flood flows for rural catchments for Engineers Australia has been published in Australian Rainfall and Runoff (ARR) Project 5, Stage 2 Report, dated June 2012. This report recommends that ARR move to a regional regression analysis approach for calculating pre-development peak flood flows. The report also considered the accuracy of the current ARR method (the Adams Rural Rational Method) and found that this method was appropriate, but suggested adjustment of the results for very small catchments as per the relation shown on Figure 5.3.6 of the ARR 2012 report (see Figure 20 below). The latest (2016) edition of ARR provides guidance on how to use the Regional Flood Frequency Estimate (RFFE) for design flood estimation in ungauged catchments. However, the RFFE model has several constraints which limit its applicability to Marong.

Figure 20. Relationship between scale correction factor (SCF) and catchment area.

Peak flows for existing rural conditions are therefore to be derived using the current Australian Rainfall and Runoff (ARR) method with the Adams equation for estimation of time of concentration with matched runoff coefficients, all in accordance with the recommendations set out in ARR. The Figure 20 correction factors are then to be applied to calculated discharges. This will be compared for consistency to the estimate from the RFFE model, noting there will be high uncertainty in the latter result.

RORB software (v6.31) was used to model the effect on flowrates of development in each sub-division to enable sizing of the retarding basin storages. RORB is a rainfall-runoff routing model that simulates catchment influences on runoff through translation and attenuation of rainfall inputs. RORB was used to estimate the changes in peak flowrates associated with the 1 in 100 year ARI event and required retarding basin design to retard flowrates to pre-development levels.

The RORB model was initially developed using sub-catchment characteristics of the pre-development (existing) sub-division. Existing conditions were informed by aerial photography, previous site visits and professional judgement. The catchment upstream of each sub-division outfall was broken into at least four sub-catchments.


The RORB model rainfall inputs were based on parameters in Table 4. An initial loss of 18 mm and continuing loss model of 2.4 mm/hr were assumed. The RORB model for each sub-division was “calibrated” to existing conditions at the sub-division drainage outfall using an estimate of peak flow from the Adams Rural Rational Method. Table 5 shows the results for the Rational Method, and the peak flowrates from the calibrated RORB model and the calibrated catchment parameters.

Table 5. Comparison of flow estimates associated with the 1 in 100-year ARI event

Outfall

Peak flow associated

with 1 in 100-year ARI rainfall event Comments

RORB Calibrated Kc parameter

Rational Method

(m3/s)

RORB*

(m3/s) Kc

A3 9.23 9.23 Based on the internal and external flows for sub-division A

2.33

C1 2.41 2.40 1.45

C2 2.47 2.49 1.65

C3 0.95 0.95 1.32

C4 0.77 0.76 0.75

C5 2.30

2.30 Based on the combined flows from outfall C5 and C6

1.63

D1 3.79

3.80 Based on the combined flows from outfall C5, C6 and D1

0.73

E1 0.95

0.96 Based on internal flows at outfall E1

0.81

E2 0.73

0.72 Based on internal flows at outfall E2

0.75

E3 0.73 0.72 0.84

*RORB model calibrated to match Rational Method estimate at each sub-division drainage outfall

The RORB models were then updated for developed conditions. Residential development was simulated primarily by varying the fraction impervious parameter, changing reach types to match road and pipe design, and varying the loss model. Table 6 below shows the fraction impervious adopted for different land use. An initial loss of 15 mm and runoff coefficient (proportional loss) model of 0.6 were assumed for developed conditions. The hydrologic modelling considered a range of design storms, from 6 minutes duration through to 72 hours, to determine the critical duration event with respect to reach and other major storage.

Table 6. Fraction impervious for different land use

Land use type Fraction impervious adopted

Residential – standard densities 0.75

Undeveloped 0.05

4.2 Stormwater treatment modelling A MUSIC (Model for Urban Stormwater Improvement Conceptualisation) model was developed to estimate the pollutant loads generated from developed catchments to enable sizing of the wetland assets required to meet the pollutant reduction targets (section 3.3). Climate data (6-minute rainfall) was obtained from the Bendigo Airport station for the years 1992-2010. The MUSIC model includes both internal and external catchment flows. The fraction impervious value for each land use type has been adopted based on the Melbourne Water MUSIC Guidelines (Table 6).


5 Proposed strategy – Option description

As discussed above, the progress that has been made in planning for sub-division E and F presents two options for this overall stormwater management strategy.

Option 1

Incorporates development of the eastern half of sub-division D with external flows from sub-division C (land parcels to the west of the Highway) conveyed through the developed portion of sub-division D.

Flow retardation is required at all identified outlets with the objective not to exacerbate downstream flooding, particularly along Bullock Creek

Option 2

Excludes sub-division E on the basis that the drainage strategy for sub-division E (and F) are well progressed within Council at the time of writing.

Sub-division D remains undeveloped, with developed retarded flows from sub-division C (land parcels to the west of the Highway) piped along Wimmera Highway to location D5 (close to the corner of Wilsons Hill Road and Wimmera Highway) and through sub-division D along an existing channel.

Whist this requires flow to be transferred between catchments, flows from sub-division C at outlet C5 will be retarded to ensure peak developed flow rates entering sub-division D at D5 are less than pre-developed flow rates. To enable this, a 12.5 ha portion of the land parcel associated with sub-division C (west of the Calder Alternate Highway) has to be drained towards Bullock Creek (under the Calder Alternate Highway) instead to outlet C5.

Flow retardation is not required for land development associated with outlets C2, C3 and C4. The assumption is that flood storage available in Malone Park is sufficient to avoid this. Should Council adopt this option, additional flood modelling should be undertaken to confirm this.

Stormwater treatment assets (i.e. wetlands) at outlets C2, C3 and C4 are located outside of developable land i.e. below the 1% AEP flood level in Bullock Creek. Should Council adopt this option, it is recommended that the wetlands are located above the 10-year flood level. Additional flood modelling should be undertaken to determine the 10-year flood level.


6 Proposed Strategy

6.1 Minor and major stormwater conveyance The approach to designing the stormwater pipe network was based on the principles outlined in section 3. The road reserves throughout the sub-divisions will convey the overland flows paths or ‘gap flows’ associated with the 1 in 100-year ARI event (see Figure 21).

Option 1 An indicative minor drainage system is proposed in Table 7 for Option 1. Based on Table 7, all overland flows should be safely contained within the proposed road reserves (with the assumption that multiple roads will be constructed where gap flows exceed the upper limiting road reserve capacity of exceed 4 m

3/s).

It is proposed that the street network in sub-division D handles the developed flows from sub-division C at outfall C5 (i.e. flow associated with a 1 in 100-year ARI event retarded to pre-development levels). It is also proposed that the minor developed flows from sub-division C at outfall C5 (i.e. flows associated with a 1 in 5-year ARI) is conveyed via an underground pipe (P21-P25) to outfall D1 along Landry Lane (bypassing sub-division D and wetland retarding basin WLRB_D1). This is shown in Figure 22.

A drainage channel is also proposed in the private property to the north of the railway line to enable discharge (at pre-development levels) from outfall C5 and D1 through that property. This channel is approximately 540 m long, with an upstream base width of 4 m and depth of 1.1 m transitioning to a narrower channel with a shallower depth downstream. At the downstream end, the channel invert ties in with the existing surface.

Table 7. Indicative minor drainage, major flows and gap flows (Option 1)

Flow path reference Indicative minor drainage

(maximum pipe diameter) *

Flow associated with the

1 in 100-year ARI event (m

3/s)

Gap flow (m3/s)

FP1 2 × 1050 mm 10.15 5.73**

FP2 4 × 825 mm 20.24 14.90**

FP3 1 × 600 mm 2.19 1.21

FP4 2 × 675 mm 4.40 2.33

FP5 3 × 600 mm 4.44 2.04

FP6 2 × 750 mm 4.81 2.34

FP7 2 × 600 mm 2.19 1.06

FP8 2 × 750 mm 2.60 1.30

FP9 2 × 825 mm 3.04 1.28

FP10 1 × 1050 mm 2.28 0.75

FP11 3 × 900 mm 8.97 6.2**

FP12 2 × 825 mm 3.25 1.87

FP13 2 × 750 mm 2.85 1.56

FP14 2 × 750 mm 2.57 1.31

FP 15 – Swale 1.5 m base width

0.8 m depth

4.08 -

FP16 – Channel 540 m long

Upstream section (4 m base width, 1.1 m depth)

1.86 -

* Where more than 1 pipe is indicated, multiple pipe alignments are assumed to drain to outfall

** Where gaps flows exceed 4 m3/3, multiple roads are assumed to drain to outfall


Figure 21. Option 1 - Overland flow paths


Option 1 Costing For the Developer Contribution Plan (DCP), costing has been limited to pipes connecting the lowest point or points of each undeveloped property (that is > 0.4 ha) to a waterway or stormwater treatment asset (wetland or retarding basin) and pipes connecting external catchments to a waterway or treatment asset.

This indicative pipe network is shown in Figure 22 and Figure 23. The maximum pipe size required for each minor drainage flow point is also provided below (Table 8). The pipeline from the existing RB in Marong Links to Bullock Creek has been included. This pipeline transitions from 850 mm (P42-P43) to 1350 mm (P44-P45).

Table 8. Option 1 - Minor drainage for the Developer Contribution Plan (DCP)

Pipe Reference

Contributing-catchment area (ha)

Design ARI

(years)

Design flow (m3/s) Maximum pipe size

required (mm)

P1-P2 21.60 5 0.46 525

P2-P4 6.73 5 1.29 825

P3-P4 94.77 5 1.49 825

P5-P7 4.09 5 0.48 600

P6-P7 58.48 5 0.50 750

P7-P8 24.50 5 2.98 1050

P9-P10 6.09 5 0.68 675

P10-P11 9.91 5 1.04 900

P12-P14 25.39 5 0.52 525

P13-P14 3.00 5 0.38 525

P15-P17 70.14 5 0.28 450

P16-P17 5.80 5 0.66 600

P18-P19 7.59 5 0.20 525

P20-P21 5.61 5 0.16 450

P21-P25* 43.21 5 0.50 750

P28-P29 0.42 5 0.05 300

P29-P30 8.31 5 0.81 900

P31-P32 0.80 5 0.10 375

P32-P34 1.72 5 0.19 450

P33-P34 0.39 5 0.05 300

P35-P36 0.63 5 0.08 375

P36-P37 1.50 5 0.17 450

P39-P40 8.13 5 0.69 825

P42-P43 Marong Links 5 0.50 825

P43-P44 14.06 + Marong Links 5 1.44 1200

P44-P45 24.03 + Marong Links 5 2.17 1350

P45-P46 24.03 + Marong Links 5 2.17 1350

*Bypass pipe from outfall C5 to outfall D1


Figure 22. Option 1 - Minor pipe network, and locations of retarding basin/wetland assets and new culverts. Sub-division A draft open space and shared use path shown.


Figure 23. Option 1 - Minor pipe network (sub-division E)


Option 2 It is proposed that the major developed flows from sub-division C at outfall C5 (i.e. flow associated with the 1 in 100 year ARI event retarded to pre-development levels) and the minor developed flows from sub-division C at outfall C5 (i.e. flows associated with a 1 in 5 year ARI) is conveyed via an underground pipe (P47-P48) to location D5 and through sub-division D along an existing earthen channel (Figure 24). It is likely that this existing channel would need to be deepened by approximately 0.75m to tie in with the pipe (P47-P48) downstream invert level.

Option 2 costing Similar to option 1, costing has been limited to pipes connecting the lowest point or points of each undeveloped property (that is > 0.4 ha) to a waterway or stormwater treatment asset (wetland or retarding basin) and pipes connecting external catchments to a waterway or treatment asset.

Table 9. Option 2 – Minor drainage for the Developer Contribution Plan (DCP)

Pipe Reference

Contributing-catchment area (ha)

Design ARI

(years)

Design flow (m3/s) Maximum pipe size

required (mm)

P1-P2 21.6 5 0.46 525

P2-P4 6.73 5 1.29 825

P3-P4 94.77 5 1.49 825

P5-P7 4.09 5 0.48 600

P6-P7 58.48 5 0.50 750

P7-P8 24.50 5 2.98 1050

P9-P10 6.09 5 0.68 675

P10-P11 9.91 5 1.04 900

P12-P14 25.4 5 0.52 525

P13-P14 3.00 5 0.38 525

P15-P17 70.1 5 0.28 450

P16-P17 5.80 5 0.66 600

P18-P19 7.59 5 0.20 525

P47-48 25.1 100 0.29*

750

*Based on output from RORB model. This pipe can cater for both the 1 in 5 and 1 in 100-year retarded flows. It was found that if the pipe

was designed for the 1 in 5 flow events only, the pipe diameter had to remain at 750 mm to enable suitable flow velocities for pipe self-

cleansing purposes given the pipe grade of 1 in 650.


Figure 24. Option 2 - Minor pipe network, and locations of retarding basin/wetland assets and new culvert. Sub-division A draft open space and shared use path shown.


6.2 Culverts Culverts are proposed under major roads to convey flows associated with the 1 in 100-year ARI event ARI.

Option 1 Under Option 1 new culverts are required at the nine locations listed in Table 10 and shown in Figure 22.

Table 10. Option 1 culvert details

Outfall location

Culvert Reference

Description

Flow

1 in 100-year ARI

(m3/s)

Length

(m)

Grade

(1 in ...) Size

A3 CV1 Under Calder Highway 8.17 30 300 Two additional

1200 x 900 mm

box culverts at same invert level as

existing culverts

C1 CV2 Under Salvarezza Rd 2.06 34 136 1 x 900mm

C1 CV3 Under Sterry Rd 2.19 20 200 1 x 900mm

C5 CV4 Under Wimmera Highway

0.50 25 300 1 x 750mm

D1 CV5 Under unused railway line

1.86 20 240 1 x 1200mm

E1 CV6 Under Calder Highway 1.44 32 320 1 x 1200mm

E2 CV7 Under Goldie Street 2.17 30 300 1 x 1350mm

E3 CV8 Under Calder Highway 0.69 32 320 1 x 825mm

CV9 Under Wimmera Highway

1.26 25 500 1 x 1200mm

Option 2 Under Option 2 new culverts are required at four locations listed in Table 11 and shown in Figure 22.

Table 11. Option 2 culvert details

Outfall location

Culvert Reference

Description Flow

1 in 100 year ARI

(m3/s)

Length

(m)

Grade

(1 in ...)

Size

A3 CV1 Under Calder Highway 8.17 30 300 Two additional

1200 x 900 mm

box culverts at same invert level as

existing culverts

C1 CV2 Under Salvarezza Rd 2.06 34 136 1 x 900mm

C1 CV3 Under Sterry Rd 2.19 20 200 1 x 900mm

D5 CV10 Under Wimmera Highway

0.29*

25 500 1 x 750 mm

* Based on RORB model for retarded developed flows associated with the 1 in 100 year event from WLRB_C5.


6.3 Stormwater treatment Stormwater quality assets have been specified to receive stormwater from post-development sub-divisions (and external undeveloped catchments where applicable) to achieve best practice stormwater treatment targets (Table 12) and flow retardation performance (Table 14) as described in section 3.

The location and Normal Water Level (NWL) of each wetland considered site topography and known constraints. The NWL is primarily controlled by the depth and cover levels of the incoming drainage pipes and invert level of existing infrastructure at the outfall (e.g. existing culvert where applicable). The designs include provision for a 4-metre access path surrounding the wetland and sediment dewatering areas.

Option 1 Ten combined retarding basin/wetland assets are proposed as part of the drainage strategy (see Figure 22).

Table 12. Option 1 - Constructed wetland design parameters and performance

System arrangement System treatment performance (removal)

WSUD Treatment Measure

Catchment Contributing

(ha)

Treatment Area (m

2)

Total Suspended Solids (%)

Total Phosphorous

(%)

Total Nitrogen (%)

Gross Pollutant (%)

WLRB_A3%

126.24 ha SB:3,300

WL:16,000 85.6 70.7 44.5 100

WLRB_A4 141.67 ha SB:1,700

WL:11,500 79.5 67.8 45.6 100

WLRB_C1

45.63 SB:800

WL:5,500 79.9 68.2 46.3 100

WLRB_C2 47.13 SB:1,200

WL:9,000 79.9 69.2 49.5 100

WLRB_C3 14.18 SB:270

WL:1,800 79.7 68.0 47.3 100

WLRB_C4 10.79 SB:300

WL:2,800 80.1 69.6 51.3 100

WLRB_C5* 43.2 SB:850

WL:10,000 79.9 69.6 50.8 100

WLRB_D1** 36.60 SB:800

WL:8,500 79.7 70.3 51.1 100

WLRB_E1***

14.06 SB:450

WL:3,700 80.4 70.3 52.1 100

WLRB_E2**** 9.97 SB:350

WL:2,500 80.1 70.1 50.9 100

WLRB_E3 9.98 SB:350

WL:2,500 80.0 70.1 51.1 100

% Wetland WLRB_A3 NWL has been set to enable discharge to the existing culvert under the Calder Freeway. As a result,

the wetland permanent pool can only be drained (for maintenance purposes once every 5 years typically) by pumps. The retarding basin can continue to drain by gravity. *Consists of contributing catchment for outfall C5 and C6

**Minor flows from sub-division C (outfall C5) bypasses WLRB_D1 via pipe along Landry lane (P21-P25)

*** Minor and major flows from external catchments are conveyed in pipe P42-P43 and bypass WLRB_E1

****Minor and major flows from external catchments are conveyed in pipe P43-P44 and bypass WLRB_E2.


Option 2 Seven wetland assets are proposed under Option 2. The footprints are similar to those described under Option 1 except for WL_C4 and WLRB_C5.

Table 13. Option 2- Constructed wetland design parameters and performance

System arrangement System treatment performance (removal)

WSUD Treatment Measure

Catchment Contributing

(ha)

Treatment Area (m

2)

Total Suspended Solids (%)

Total Phosphorous

(%)

Total Nitrogen (%)

Gross Pollutant (%)

WLRB_A3%

126.24 ha SB:3,300

WL:16,000 85.6 70.7 44.5 100

WLRB_A4 141.67 ha SB:1,700

WL:11,500 79.5 67.8 45.6 100

WLRB_C1

45.63 SB:800

WL:5,500 79.9 68.2 46.3 100

WL_C2 47.13 SB:1,200

WL:9,000 79.9 69.2 49.5 100

WL_C3 14.18 SB:270

WL:1,800 79.7 68.0 47.3 100

WL_C4* 28.89 SB:700

WL:6,000 79.5 69.0 50.0 100

WLRB_C5** 25.10 SB:700

WL:6,500 80.0 70.0 51.6 100

% Wetland WLRB_A3 NWL has been set to enable discharge to the existing culvert under the Calder Freeway. As a result,

the wetland permanent pool can only be drained (for maintenance purposes once every 5 years typically) by pumps. The retarding basin can continue to drain by gravity. *Includes flows from contributing catchment from part of subdivision C west of Calder Alternate Highway (12.5 ha)

*Includes flows from contributing catchment at outlet C6

6.4 Flow retardation As described in section 4.1, a RORB model was used to size retarding storages. The hydrologic modelling considered a range of design storms, from 10 minutes through to 72 hours, to determine the pre-development and post-development (with and without flow retardation) peak flow rates. The designs include provision for a 4-metre access path surrounding the wetland and sediment dewatering areas.

Option 1 The RORB modelling results and retarding basin design parameters are summarised in Table 14 and Table 15. The spillway parameters and downstream pipe connections were initially assumed in RORB based on existing site contours and preliminary grading of drainage assets. An iterative method was applied to size pipe outlet connections until the peak flow rates was similar to the estimated pre-development conditions.

The overall footprint of the assets (Table 15 and

Table 17) have been developed without undertaking earthwork modelling. As such the implications of site topography on batter extent and therefore overall footprint, have not been accounted for. The footprint shown within the tables below should be interpreted as the minimum footprint required. The wetland retarding basin assets WLRB_A3 and WLRB_A4, which have been shaped as a linear system to accommodate the layout of a planned school/oval and shared used path, is expected to have a maximum width that varies between 75 m to 100 m based on the site topography. Detailed earthwork modelling at the functional design stage of these assets will confirm the final maximum width required.

Appendix B shows the stage-storage relationship used for each retarding basin in the RORB model, informed by wetland asset design.


Table 14. Option 1 - Retarding basin performance

Retarding Basin

Peak flow rate 1 in 100-year ARI

Pre-development conditions (m

3/s)


Post-development conditions without retardation

(m3/s)


Post-development with retardation

(m3/s)

WLRB_A3+WLRB_A4 8.48 40.4 8.37

WLRB_C1

2.41 7.32 2.06

WLRB_C2 2.47 8.42 2.26

WLRB_C3 0.95 2.21 0.83

WLRB_C4 0.77 1.48 0.72

WLRB_C5 1.76 6.65 1.76

WLRB_D1 2.00 8.97 1.86

WLRB_E1

0.95 2.91 0.94

WLRB_E2 0.73 2.11 0.73

WLRB_E3 0.73 2.05 0.69


Table 15. Option 1 - Retarding basin design parameters

Basin

Minimum RB area

(m2)

Road deck level

(m AHD)

Peak flood level

(m AHD)

Spillway crest level

(m AHD)

Peak storage

(m3)

Outlet details

WLRB_A3 31,000 189.5 189.24

189.5

(260 mm freeboard)

35,400

Side winding penstock at 187.85 m AHD

5x900 mm pipes at 188.20 m AHD

WLRB_A4 20,200 193.5 193.33

193.5

(170 mm freeboard)

21,000



WLRB_C1 11,600 194.0 193.87

194.00

(130 mm freeboard)

10,300



WLRB_C2 16,200 190.0 189.97

190.0

(No freeboard required)

14,900



WLRB_C3 4,200 190.0 189.93

190.0


2,720



WLRB_C4 5,900 190.0 189.85

190.0


3,680



WLRB_C5 19,000 190.0 189.81

190.0

(190 mm freeboard)

15,600



WLRB_D1 18,750 187.0 186.67

186.85

(330 mm freeboard)

13,500


3x1.05 mm pipes at 185.75 m AHD

WLRB_E1 9,400 186.0 185.71

185.91

(290 mm freeboard)

5,700



WLRB_E2 7,000 186.0 185.82

186.00

(180 mm freeboard)

3,850



WLRB_E3 7,000 185.0 184.81

185.00

(190 mm freeboard)

3,830




Option 2 In option 2, retarding basin performance are similar to option 1 with the exception of WLRB_C5. With flows from sub-division C conveyed to location D5 (transfer of flow across catchment), the peak developed flow rates are below the existing peak flow rates (Table 16). For option 2, there is also no flow retardation requirement assumed at outlet C2, C3 and C4.

Table 16. Option 2 - Retarding basin performance

Retarding Basin

Peak flow rate 1 in 100-year ARI Pre-development

conditions (m

3/s)


Post-development conditions without retardation

(m3/s)


Post-development with retardation

(m3/s)

WLRB_A3+WLRB_A4 8.48 40.4 8.37

WLRB_C1

2.41 7.32 2.06

WLRB_C5 0.90 4.27 0.29

Location D5 7.60 7.56

Table 17. Option 2 -Retarding basin design parameters

Basin

Minimum overall area

(m2)

Road deck level

(m AHD)

Peak flood level

(m AHD)

Spillway crest level

(m AHD)

Peak storage

(m3)

Outlet details

WLRB_A3 31,000 189.5 189.24

189.5

(260 mm freeboard)

35,400



WLRB_A4 20,200 193.5 193.33

193.5

(170 mm freeboard)

21,000



WLRB_C1 11,600 194.0 193.87

194.00

(130 mm freeboard)

10,300



WLRB_C5 15,500 190.0 189.23

190.0

(80 mm freeboard)

14,500




7 Summary of assets proposed

The above strategy outlines the assets required to manage surface water quality and quantity associated with future residential sub-divisions proposed for the Marong Township. It should be noted that asset footprints (Table 18 and Table 19) have been developed without undertaking detailed earthwork modelling. The overall footprint in the tables should therefore be interpreted as the minimum footprint required.

Option 1 The key asset are summarised below and in Table 18:

Ten combined retarding basin/wetland assets are proposed to meet best practice water quality pollution reduction targets and attenuate flows to pre-development levels.

The minor flows managed by an underground pipe system while the major flows will be conveyed via road reserves throughout the developable areas and directed to Bullock Creek.

A swale is proposed to convey major (retarded) flows from WLRB_A4 to outfall A3 where the existing culvert under the Calder Highway will be graded (Culvert CV1). The swale base width required is 1.5 m with a depth of 0.8 m.

A drainage channel is also proposed in the private property to the north of the railway line to enable discharge (at pre-development levels) from outfall C5 and D1 through that property. This channel is estimated to be 540 m long, with an upstream base width of 4 m and depth of 1.1 m transitioning to a narrower channel with a shallower depth downstream.

A pipeline is also proposed to convey peak flows from the existing RB within Marong Links and major (retarded) flows from two retarding basins (WLRB_E1 and WLRB_E2). The pipeline transitions from 825 mm to 1350 mm in diameter.

New culverts are required at nine locations to convey major flows under roads and the Eaglehawk-Inglewood railway line.


Table 18. Option 1 - Summary of assets proposed

Asset type Description Asset size Contributing catchment

Pipe network Minor flows pipe network for internal

catchments

See Table 8 See Table 8

Swale Between WLRB_A4 and outfall location A3

1.5 m with a depth of 0.8 m

Drainage channel

540 m long, with an upstream base width of 4 m and depth of 1.1 m transitioning to a narrower channel with a

shallower depth downstream

Pipeline from Marong Links to Bullock Creek

Convey peak developed flows from

Marong links to Bullock Creek

Transitions from 850 to 1350 mm in diameter

Stormwater treatment and retardation

Base footprint (m2)

Minimum

overall footprint* (m

2)

WLRB_A3 SB:3,300

WL:16,000 31,000 126.2 ha

WLRB_A4 SB:1,700

WL:11,500 20,200 141.7 ha

WLRB_C1 SB:800

WL:5,500 11,600 45.6 ha

WLRB_C2 SB:1,200

WL:9,000 16,200 47.1 ha

WLRB_C3 SB:270

WL:1,800 4,200 14.2 ha

WLRB_C4 SB:300

WL:2,800 5,900 10.8 ha

WLRB_C5 SB:850

WL:10,000 19,000 43.2 ha

WLRB_D1 SB:800

WL:8,500 18,750 36.6 ha

WLRB_E1 SB:450

WL:3,700 9,400 14.1 ha

WLRB_E2 SB:350

WL:2,500 7,000 10.0 ha

WLRB_E3 SB:350

WL:2,500 7,000 10.0 ha

* Overall footprint of the assets have been developed without undertaking detailed earthwork modelling. As such the implications of topography on batter extent and in

turn over overall footprint have not been accounted for. The overall footprint in the tables should therefore be interpreted as the “minimum” footprint required.


Option 2 The key assets are summarised below and in Table 18:

Four combined retarding basin/wetland assets to meet best practice water quality pollution reduction targets and attenuate flows from the proposed sub-divisions to pre-development levels.

Three wetland assets to meet best practice water quality pollution reduction targets (no flow retardation function required)

The minor flows managed by an underground pipe system while the major flows will be conveyed via road reserves throughout the developable areas and directed to Bullock Creek.

A swale is proposed to convey major (retarded) flows from WLRB_A4 to outfall location A3 where the existing culvert under the Calder Highway will be graded (Culvert CV1). The swale base width required is 1.5 m with a depth of 0.8 m.

The existing drainage channel in sub-division D is proposed to be deepened by 0.75 m at Wimmera Highway, with the intent that the channel invert ties back in with the existing invert level at McCreddons Road.

New culverts are required at four locations to convey major flows under roads.


Table 19. Option 2 – Summary of assets proposed

Asset type Description Asset size Contributing catchment

Pipe network Minor flows pipe

network for internal catchments

See Table 8 See Table 8

Swale Between WLRB_A4

and outfall location A3 1.5 m with a depth of 0.8 m

Deepening of existing drainage channel Deepening of channel by 0.5 m over a length of 530 m

long

Stormwater treatment and

retardation

Wetland/RB

Base footprint (m2)

Wetland/RB

overall footprint* (m

2)

WLRB_A3 SB:3,300

WL:16,000 31,000 126.2 ha

WLRB_A4 SB:1,700

WL:11,500 20,200 141.7 ha

WLRB_C1 SB:800

WL:5,500 11,600 45.6 ha

WL_C2 SB:1,200

WL:9,000 14,600 47.1 ha

WL_C3 SB:270

WL:1,800 4,200 14.2 ha

WL_C4 SB:700

WL:6,000 10,900 28.9 ha

WLRB_C5 SB:700

WL:6,500 15,750 25.1 ha

* Overall footprint of the assets have been developed without undertaking detailed earthwork modelling. As such the implications of topography on batter extent and in

turn over overall footprint have not been accounted for. The overall footprint in the tables should therefore be interpreted as the “minimum” footprint required.


8 Cost

Costs have been estimated at a concept level for the purpose of informing the Developer Contribution Plan (DCP). The (DCP). The key items are summarised in Table 20 and

Table 21. Costs are indicative only and do not include land acquisition. The wetland/retarding basin costs Include wetland planting costs, excavation and pipe/control structures. The costing for the culvert has not allowed for pipe jacking under VicRoads assets and traffic management costs.

What is evident is that Option 2 has a significantly lower cost given that it does not include servicing infrastructure for sub-division E (and F) as well as fewer culverts under main roads.

Table 20. Option 1 cost estimate

Asset type Total cost Asset included

Pipes $ 3,745,000 See Table 8

Culverts $ 715,000 See Table 10

Swale $72, 000 Between WLRB_A3 and WLRB_A4

Drainage channel $ 58,000 Downstream of outfall D1

Wetland/Retarding basins

$ 12,957,000 See Table 12 and

Table 15

Total $17,475,000

Table 21. Option 2 cost estimate

Asset type Total cost Asset included

Pipes $ 1,989,000 See Table 9

Culverts $ 410,000 See Table 11

Swale $72,0 00 Between WLRB_A3 and WLRB_A4

Drainage channel deepening

$ 83,000 Downstream of outfall D5

Wetland/Retarding basins

$ 9,217,000 See Table 13 and

Table 17

Total $11,700,000


9 Recommendations and conclusion

The Marong Stormwater Management Strategy sets out management strategies and asset requirements to manage future surface water volumes, flowrates. Two options have been provided depending upon whether current and progressed plans for some subdivisions are adopted by Council or not.

Further recommendations are:

Sub-division E: adopt Option 1. We understand that the individual drainage strategy being developed within Council for sub-division E is already well in progress but is yet to be approved. As a result, Option 1 should be adopted to ensure water quality and flow retardation objectives as set out in section 3 are achieved.

Assets located along Bullock Creek (WLRB_C2, WLRB_C3 and WLRB_C4): should Council adopt Option 2 and locate these assets below the 1% flood level with no flow retardation function, it is recommended that additional flood modelling be undertaken to confirm that there is no risk of exacerbated flooding downstream.

It is also recommended that wetlands are located above the 10-year flood level to ensure wetland plants are not at risk of inundation for long periods and scouring.

Serviceability: That is, the perceived ease with which these catchments can be serviced), all are expected to be able to be serviced without significant upgrades to existing infrastructure. The exceptions to this are sub-divisions C and D, that drain to the west of Landry Lane. Whilst sub-division C is within the bounds of the Marong Township Structure Plan, sub-division D is not. In order to service sub-division C without transferring flows across catchments (Option 1), the outflow has to be directed through sub-division D with culverts required under Wimmera Highway and the railway line to the north of sub-division D. Under this option, the eastern portion of sub-division D could be described as a logical inclusion within the Structure Plan and we will leave this for Council’s consideration.

If Option 2 is preferred (involving the transfer of flows across catchments), it is possible for outflow from sub-division C to be piped to location D5 and through sub-division D along an existing drainage channel. However, this channel will require deepening to tie in with the pipe invert level at location D5. This will require approval from the private property owners.

The next stage of work for Council to consider in greater detail how these assets are designed and constructed such that they are aligned with the vision and objectives set out in the Marong Township Structure Plan. This may include preparing design standards and principles that provide clear direction to developers as to what is expected in terms of assets under this plan.

The opportunity is to ensure that all assets are designed to maximise their liveability benefits to the current and future community. This includes requiring retarding basin and wetland reserves be designed to be aesthetically pleasing, to incorporate passive and active recreation space sand be sensitive to their surrounds including Bullock Creek and proposed future schools.

What is counterproductive is when sub-divisions gain approvals external to or independent of the requirements set out in this strategy. This undermines the principles of the strategy such as ensuring that best practice stormwater quality outcomes are achieved, and that the cost of doing so is equitably shared. It is therefore recommended that wherever possible sub-divisions are subject to the recommendations set out within this strategy.


Appendix A

Flooding photos


Figure 25. Flooding (Alana Court – Marong Run)

Figure 26. Flooding (High Street Bridge)


Figure 27. Flooding (Intersection of Calder Highway and Bullock Creek)

Figure 28. Flooding (Malone Park)


Figure 29. Flooding (Wimmera Highway close to McCreddons Rd- Southern boundary of sub-division D)

Figure 30. Flooding (Landry Lane and Railway intersection)


Figure 31. Flooding (McCreddons Rd and Barnes Rd looking South)


Appendix B

Retarding Basin Stage Storage Relationships


OPTION 1

Table 22. WLRB_A3 H-S relationship

Height (m AHD) Storage (m3)

187.85 0.00

187.9 1086.82

187.95 2187.31

188 3301.53

188.1 5571.38

188.2 7896.78

188.3 10278.17

188.4 12715.99

188.5 15210.66

188.6 17762.63

188.7 20372.31

188.8 23040.14

188.9 25766.56

189 28551.99

189.1 31396.87

189.2 34301.64

189.3 37266.71

189.4 40292.53

189.5 43379.52



192.10 0.00

192.15 733.68

192.20 1477.25

192.25 2230.77

192.35 3767.88

192.45 5345.42

192.55 6963.83

192.65 8623.55

192.75 10325.00

192.85 12068.63

192.95 13854.85

193.05 15684.10

193.15 17556.82

193.25 19473.43

193.35 21434.37

193.45 23440.08

193.55 25490.97


Table 24. WLRB_C1 H-S relationship


192.80 0.00

192.85 388.83

192.90 785.37

192.95 1189.67

193.05 2021.75

193.15 2885.51

193.25 3781.39

193.35 4709.80

193.45 5671.20

193.55 6666.00

193.65 7694.64

193.75 8757.56

193.85 9855.17

193.95 10987.93

194.05 12156.25



188.90 0.0000

188.95 598.72

189.00 1206.9

189.05 1824.6

189.15 3088.9

189.25 4391.9

189.35 5734.1

189.45 7115.9

189.55 8537.8

189.65 10000.

189.75 11503.

189.85 13048.

189.95 14634.

190.05 16263.



189.10 0.0000

189.15 127.00

189.20 259.00

189.25 395.00

189.35 681.00


189.45 985.00

189.55 1308.0

189.65 1650.0

189.75 2012.0

189.85 2395.0

189.95 2798.0

190.05 3222.0



189.05 0.0000

189.10 190.13

189.15 385.57

189.20 586.37

189.30 1004.2

189.40 1444.2

189.50 1906.7

189.60 2392.1

189.70 2900.9

189.80 3433.5

189.90 3990.3

190.00 4571.9



188.80 0.0000

188.85 654.00

188.90 1319.0

188.95 1997.0

189.05 3391.0

189.15 4833.0

189.25 6326.0

189.35 7870.0

189.45 9464.0

189.55 11110.

189.65 12807.

189.75 14557.

189.85 16360.

189.95 18216.

190.05 20125.


Table 29. WLRB_D1 H-S relationship


185.40 0.0000

185.45 567.00

185.50 1144.0

185.55 1734.0

185.65 2946.0

185.75 4204.0

185.85 5509.0

185.95 6860.0

186.05 8258.0

186.15 9704.0

186.25 11198.

186.35 12741.

186.45 14332.

186.55 15972.

186.65 17662.

186.75 19403.

186.85 21194.

186.95 23036.

187.05 24929.

Table 30. WLRB_E1 H-S relationship


184.80 0.0000

184.85 253.38

184.90 513.57

184.95 780.62

185.05 1335.5

185.15 1918.5

185.25 2529.9

185.35 3170.4

185.45 3840.1

185.55 4539.8

185.65 5269.6

185.75 6030.1

185.85 6821.7

185.95 7644.9

186.05 8500.0

186.15 9387.5

186.25 10308.

186.35 11261.




184.95 0.0000

185.00 172.86

185.05 351.47

185.10 535.89

185.20 922.38

185.30 1332.7

185.40 1767.4

185.50 2226.8

185.60 2711.4

185.70 3221.6

185.80 3757.9

185.90 4320.6

186.00 4910.2

186.10 5527.1

186.20 6171.9

186.30 6844.8

186.40 7546.4

186.50 8277.0



183.95 0.0000

184.00 172.86

184.05 351.47

184.10 535.89

184.20 922.38

184.30 1332.7

184.40 1767.4

184.50 2226.8

184.60 2711.4

184.70 3221.6

184.80 3757.9

184.90 4320.6

185.00 4910.2

185.10 5527.1

185.20 6171.9

185.30 6844.8

185.40 7546.4

185.50 8277.0


OPTION 2



187.85 0.00

187.9 1086.82

187.95 2187.31

188 3301.53

188.1 5571.38

188.2 7896.78

188.3 10278.17

188.4 12715.99

188.5 15210.66

188.6 17762.63

188.7 20372.31

188.8 23040.14

188.9 25766.56

189 28551.99

189.1 31396.87

189.2 34301.64

189.3 37266.71

189.4 40292.53

189.5 43379.52



192.10 0.00

192.15 733.68

192.20 1477.25

192.25 2230.77

192.35 3767.88

192.45 5345.42

192.55 6963.83

192.65 8623.55

192.75 10325.00

192.85 12068.63

192.95 13854.85

193.05 15684.10

193.15 17556.82

193.25 19473.43

193.35 21434.37

193.45 23440.08

193.55 25490.97




192.80 0.00

192.85 388.83

192.90 785.37

192.95 1189.67

193.05 2021.75

193.15 2885.51

193.25 3781.39

193.35 4709.80

193.45 5671.20

193.55 6666.00

193.65 7694.64

193.75 8757.56

193.85 9855.17

193.95 10987.93

194.05 12156.25



188.80 0.00

188.85 530.03

188.90 1070.17

188.95 1620.47

189.05 2751.75

189.15 3924.31

189.25 5138.59

189.35 6395.00

189.45 7694.00

189.55 9036.00

189.65 10421.44

189.75 11850.76

189.85 13324.37

189.95 14842.73

190.05 16406.25

STORM WATER MANAGEMENT STRATEGY - City of Greater … · The City of Greater Bendigo (CoGB) is...

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