Living Hobsons Bay:
an Integrated Water
Management Plan
Technical Support Manual
2014-2019
Acknowledgements
November 2014
This strategy was compiled by Hobsons Bay’s Strategy and Advocacy Department. For further information
contact Hobsons Bay City Council on 9932 1000 www.hobsonsbay.vic.gov.au
Council acknowledges the people of Yalukit Wilum of the Boon Wurrung Country that makes up the Greater
Kulin Nation as the traditional owners of these municipal lands.
Council acknowledges the legal responsibility to comply with the Charter of Human Rights and Responsibilities
Act 2006 and the Equal Opportunity Act 2010. The Charter of Human Rights and Responsibilities is designed
to protect the fundamental rights and freedoms of citizens. The Charter gives legal protection to
20 fundamental human rights under four key values that include freedom, respect, equality and dignity.
Contents
1. Overview
1
2. Investment scenarios
2
3. Prioritisation system
4
4. Strategy delivery methodology
7
5. Index of river conditions
8
6. Water balance and future demand
9
7. Strategic redevelopment precincts
10
8. Projected pollutant load reductions
15
Integrated Water Management Plan Technical Support Manual
i
1
Overview
The Technical Support Manual contains the
background data and assumptions made during the
development of the 2014-2019 Living Hobsons Bay:
An Integrated Water Management Plan (IWMP). Key
sections within the support manual include:
Investment scenarios
The investment scenarios are the cost benefit
analysis used to determine the targets for meeting
best practice stormwater management and
determining the financial investment needed to meet
the objectives of the IWMP.
Prioritisation system
The prioritisation system uses an excel tool to
assist Council staff to incorporate the objectives
of the IWMP into Council’s works. It enables staff
to incorporate decisions around waterway health,
flooding risk and current and planned large scale
projects into their day to day decision making.
Strategic redevelopment precincts
Council’s Industrial Land Management Strategy 2008
was developed in light of both requests to rezone
industrial land, as well as resident concerns about
the operations and impacts of some industries.
Land development projections have been provided
in this table. This data is used in appendix six to
project stormwater volume and quality implications of
redevelopment.
Projected pollutant load reductions
Changes in stormwater quality and volumes are
modeled and predicted for:
• industrial land rezoned and redeveloped
as residential
• infill of existing residential areas due
to population growth
• industrial land redeveloped for industrial purposes
Strategy delivery methodology
Council is committed to the continuous improvement
of project delivery and ongoing management of
assets. The methodology in this section outlines the
process the Council will follow to achieve this.
Index of river conditions
The index of river conditions is the tool used by
Melbourne Water to assess the health of local
waterways. This section contains the raw data used
in section 6.5 of the IWMP and a link to Melbourne
Water’s full assessment of waterways health within
the region.
Water balance and future demand
The data that supports the water balance is
detailed here. The table of data includes:
• water type and volumes for Council,
residential and non-residential uses
• the year and source of the data
• any assumptions made about the data
Integrated Water Management Plan Technical Support Manual
1
2
Investment scenarios
The investment scenarios are the cost benefit
analysis used to determine the targets for meeting
best practice stormwater management and
determining the financial investment needed to meet
the objectives of the IWMP. A total of six options
were investigated. These options begin from the
baseline expected to be achieved by the end of 2013
with the completion of the Council’s Laverton Park,
Paisley Park and Cyril Curtain Reserve Stormwater
Harvesting systems.
The Urban Stormwater Best Practice Environmental
Management Guidelines 1999 defines best practice
stormwater management as:
•
an 80% reduction in suspended solids
•
a 70% reduction in litter
•
a 45% reduction in Total Phosphorus (TP)
•
a 45% reduction in Total Nitrogen (TN)
•
reducing stormwater runoff for a rainfall event that
happens on average every 1.5 years to what it
would have been before the land was developed
Of these targets, the nitrogen target is considered to
be the hardest to achieve. As a result, it is commonly
accepted science that if the nitrogen has been
removed then all of the other pollutants have been
removed as well. Therefore, nitrogen has been used
as the indicator for meeting best practice stormwater
management.
These trajectories are shown in Figure 1, and
describe the following scenarios:
Case 1: $100,000 investment per annum in water sensitive urban design infrastructure by Council.
Case 2: $300,000 investment per annum in water sensitive urban design infrastructure by Council.
Case 3: $300,000 investment per annum in water sensitive urban design infrastructure by the Council; plus the
achievement of best practice stormwater management in all new residential developments.
Case 4: $300,000 investment per annum in water sensitive urban design infrastructure by the Council; plus the
achievement of best practice stormwater management in all new residential developments; plus the
achievement of best practice stormwater management in all new industrial developments.
Case 5: $300,000 investment per annum in water sensitive urban design infrastructure by the Council; plus
the achievement of best practice stormwater management in all new residential developments; plus
the achievement of best practice stormwater management in all new industrial developments; plus
rainwater tanks in 50% of new dwellings and an extra 15% of existing dwellings.
Case 6: $600,000 investment per annum in water sensitive urban design infrastructure by the Council; plus
the achievement of best practice stormwater management in all new residential developments; plus
the achievement of best practice stormwater management in all new industrial developments; plus
rainwater tanks in 50% of new dwellings and an extra 15% of existing dwellings.
Figure 1: Trajectory of investment required to meet 45% Total Nitrogen load reduction
Integrated Water Management Plan Technical Support Manual
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2 Investment
scenarios
Case 1 indicates that if the Council invests $100,000
per annum in water sensitive urban design, it will
take 290 years to meet best practice stormwater
management.
years to meet best practice stormwater management.
This demonstrates the importance of working
closely with developers and land owners to improve
stormwater outcomes.
Case 2 indicates that if the Council invests $300,000
per annum, it will take 100 years to meet best
practice stormwater management.
Case 6 demonstrates what could be achieved if
the Council invested $600,000 per annum in water
sensitive urban design and worked closely with
developers and land owners to improve stormwater
outcomes. To achieve this outcome the Council
would need co-investment from partner organisations
and/or grant funding. The Council will continue
to work closely with partner organisations and to
investigate funding opportunities as and when they
become available.
Cases 3, 4 and 5 highlight the combined effect of
Council investment and effective planning controls on
waterway health. The projection for Case 3 is that it
will take 80 years to meet best practice stormwater
management; Case 4 indicates that it will take 70
years and Case 5 indicates that it will take around 60
Graph 1: Trajectory of investment required to meet 45% Total Nitrogen load reduction
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3
Prioritisation system
To enable the objectives of the IWMP to be
incorporated into the Council’s works the Checklist
for Water Planning has been developed. This tool
is a prioritisation system that enables Council staff
to quickly assess which elements of Water
Sensitive Urban Design (WSUD) are a priority
for a given project.
priority is given based on the river health objectives
discussed in section 6.5 of the IWMP. This can be
seen in Table 1 and Table 2 below.
Having a small scale upstream treatment like a
water tank on a building can directly compete
with a downstream treatment such as stormwater
harvesting. The existence, or the planned existence,
of a downstream system must therefore be taken
into consideration when prioritising projects.
This is shown in Table 1.
The Checklist begins by assessing what the
sustainable water management priorities are for the
location in which the project is taking place. An initial
SKELETON
CREEK
LAVERTON
CREEK
CHERRY
CREEK
KOROROIT
CREEK
STONY
CREEK
YARRA
RIVER
Stormwater harvesting system
already in operation
very low
very low
very low
very low
very low
very low
Stormwater harvesting system
identified in for development in
current capital works program
very low
very low
very low
very low
very low
very low
very low
very low
Stormwater harvesting system
identified to be built in next 5 years
low
Stormwater harvesting system could
be build at some time but there are
no definite plans
medium
medium
medium
No stormwater harvesting system
identified in this drainage catchment
very high
very high
high
low
low
low
medium
high
low
medium
low
medium
Table 1: Priority for improved water management based on Melbourne Waters river condition index
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3
Prioritisation system
The Checklist provides prompts for each waterway catchment. These prompts remind staff of the key actions
needed to improve or protect the health of each of the waterways. These prompts are listed in Table 2.
SKELETON CREEK
LAVERTON CREEK
CHERRY CREEK
Please pay particular
consideration to:
• mimic natural
flows by reducing
stormwater runoff
• protect water
quality by removing
pollutant loads
• protect aquatic
life by preventing
sediment and
litter from entering
waterways
Please pay particular Please pay particular
consideration to:
consideration to:
• mimic natural
• protect water
flows by reducing
quality by removing
stormwater runoff
pollutant loads
• protect water
quality by removing
pollutant loads
KOROROIT CREEK
STONY CREEK
YARRA RIVER
Please pay particular
consideration to:
• protect water
quality by removing
pollutant loads
• protect aquatic
life by preventing
sediment and
litter from entering
waterways
Please pay
particular
consideration to:
• protect from
litter
Please pay
particular
consideration to:
• protect from
litter
Table 2: Prompts for key actions needed to protect the health of each waterway.
The priority for undertaking works to improve water
management in the project area is then refined
depending on whether the project is in a location
where the stormwater drains are at or near capacity. If
the stormwater drainage system is at or near capacity
then there is an increased risk of flooding either at
the project location or downstream of it. It is therefore
a priority for the project to ensure that stormwater
runoff from the site is reduced. The exception to
this is if there is a large scale downstream project
planned, such as a stormwater harvesting system.
This is because stormwater harvesting systems can
be designed in such a way as to reduce nuisance
flooding. An example of this is the Laverton Park
stormwater harvesting system.
Not all flooding is nuisance flooding. Nuisance
flooding is the flooding that occurs from low rainfall
events such as those that happen, on average, once
every one to two years. It is noted that large scale
flooding, such as flooding that happens on average
once every 20 to 100 years, is the responsibility of
Melbourne Water Corporation.
The changed prioritisation for any project in any
waterway catchment where the drainage system is at
or near capacity is shown in Table 3.
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3 Prioritisation system
STORMWATER
DRAIN IS AT
OR NEAR
CAPACITY
Stormwater harvesting system already in operation
very low
Stormwater harvesting system identified for development in current capital works program
very low
Stormwater harvesting system identified to be built in next 5 years
low
Stormwater harvesting system could be built at some time but there are no definite plans
high
No stormwater harvesting system identified in this drainage catchment
very high
Table 3: Priority for reducing stormwater runoff from a project site
The priority for using water in the landscape to maintain vegetation and create cool environments is determined
by a combination of the intensity of the use of the site and its heat vulnerability index. This can be seen in
Table 4. The heat vulnerability index is a measure of an area’s susceptibility to heat related illness as derived by
Monash University. The heat vulnerability index for a postcode can be found on the following website
www.mappingvulnerabilityindex.com/home
HEAT
VULNERABILITY
INDEX RATING
10-9
HIGH PROFILE
OR HIGH USE SITE
MEDIUM PROFILE OF
MEDIUM USE SITE
LOW PROFILE
OR LOW USE SITE
very high
very high
medium
8-7
very high
high
low
6-5
high
medium
low
4-3
medium
low
very low
2-1
low
very low
very low
Table 4: Priority for using water in the landscape for evapotranspirative cooling
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4
Strategy delivery methodology
Council is committed to the continuous improvement of project delivery and the ongoing management of its
assets. To this end, the framework methodology below outlines the broad process that the Council will follow.
This framework will be detailed further as Council specifies all project delivery processes and continuously
improves and documents its processes for ongoing asset management.
Integrated business cases
Lifecycle costing
• Identify operating context
• Undertake a net present value
analysis incorporating both the
capital costs and operational
costs over the life of the asset
• Identify potential benefits
• Investigate options
• Assess options based on
the number and priority of
benefits achieved
• Estimate ongoing maintenance
costs and requirements
Project delivery
Asset management
• Verify scope of works
• Develop asset management plans
in accordance with Australian
Standards
• Develop and implement project
management plan
• Verify quality of works during
specified hold points
• Update the asset register
• Commission and hand over
Maintenance
Review project success
• Review maintenance requirements
• Compare project outcomes
against project objectives
• Submit budget submissions for
the ongoing maintenance of new
assets as required
• Record learnings for the
development and delivery of the
project
• Incorporate learnings into
new projects
Diagram 1: Project delivery framework
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5
Index of river conditions
The index of river conditions is the tool used by Melbourne Water to determine the health of a waterway. It is
made up of five separate categories. According to this tool, waterways across Hobsons Bay rate from moderate
to very poor, as seen in Table 5 below. This is normal for the lower reaches of waterways that pass through
highly urban environments.
PARAMETER
DEFINITION
SCORE (/10)
Yarra
Stony
Kororoit
Cherry
Laverton
Skeleton
Hydrology
Flow volume and
seasonality of flow
2
2
2
5
8
7
Physical form
River bank and bed
condition, presence of
and access to physical
habitat, artificial barriers
2
3
9
8
8
9
Streamside
zone
Quality and quantity of
streamside vegetation
2
3
2
3
2
2
Water quality
Key water quality
indicators compared
against Victorian
Government
environment protection
policy water quality
objectives
4
4
4
5
6
6
Aquatic life
Diversity of macro
invertebrates
1
4
16 (very
poor)
5
Overall health score =
11 (very
poor)
21 (poor)
4
25
(poor)
4
29
(mod)
7
31
(mod)
A full analysis of the health of local waterways within Hobsons Bay can be found on the Melbourne Water
website www.melbournewater.com.au/waterdata/riverhealthdata/Pages/river-health.aspx
Table 5: River condition index of the six main creeks traversing Hobsons Bay (Melbourne Water 2004)1
1
Melbourne Water 2004, Index of River Condition, available online at http://www.melbournewater.com.au/content/rivers_and_creeks/river_
health/measuring_environmental_condition_of_rivers/measuring_environmental_condition_of_rivers.asp?bhcp=1
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6
Water balance and
future demand
The data that supports the water balance is detailed in table 6 below. The table includes:
• water type and volumes for Council, residential and non-residential uses
• the year and source of the data and
• any assumptions made about the data
WATER TYPE
Potable water
YEAR
2013
2031
Ground water
(licensed
extractions only.
No data exists
on unlicensed
extractions)
2013
2031
USER
CATEGORY
Council
WATER
CONSUMPTION
(ML/YR)
428
ASSUMPTIONS
Estimated post drought water needs:
Buildings approximately 45 ML/yr as Parks and Open Space
(everything except sports grounds) approximately 50 ML/yr Sports
Grounds (assuming 66 ha at 5ML/yr) is approximately 333 ML/yr
Residential
5,400
Based on residential water consumption for 2012/2013.
Data provided by City West Water
Non-residential
6,700
Based on non-residential water consumption for 2012/2013.
Data provided by City West Water
Assumes 2013 water demand; plus
And extra 14 ha of sports grounds at 5 ML/yr = 70 ML; minus
stormwater harvesting at the Cyril Curtain Reserve (10 ML), Laverton
Park (68 ML) and Paisley Park (75 ML but half will be used by
the golf course) and projected additional 140 ML of stormwater
harvesting.
Council
242
Residential
6,370
Projected population in 2031 times water consumption assuming
170 L per person per day (this would only be 5,804 ML if the 160 L
target was used). Water restrictions have been lifted and residents
have increased their water use, hence the higher figure was used.
Non-residential
7,600
Based on a 4% increase in potable water demand in a five year
period, forecast by City West Water in their Water Plan 3. Note:
industry is anticipated to expand by 10% over five years but
alternative water supply will account for some of this demand.
Council
Residential
19.3
-
Based on 2010/2011 billing information
Not licensed therefore volumes unknown
Non-residential
1024
Data obtained from Southern Rural Water, data provided 21/2/2013
Council
19.3
Assumes no increase in ground water consumption
Residential
Non-residential
1024
Not licensed
Assumes no increase due to poor quality of ground water
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6 Water balance and future demand
WATER TYPE
YEAR
Recycled water
(class A)
2013
2031
USER
CATEGORY
WATER
CONSUMPTION
(ML/YR)
ASSUMPTIONS
Council
5
Current supply agreement between Hobsons Bay and City West
Water. Unfortunately City West Water has had difficulty meeting
their HACCIP requirements and so supply is no longer available. It
is unclear how long this will continue for. The Parks department has
indicated a willingness to use the recycled water when it is available.
Residential
0
Data supplied by City West Water
Non-residential
990
Data supplied by City West Water on what was delivered in
2011/2012. During the exhibition period City West Water’s Water’s
Innovations division indicated that 2,200 ML could be supplied to
uses within Hobsons Bay. Due to the current HACCIP issues that are
preventing the supply of recycled water the historical data will be
used and the new data will be referred to as future potential supply.
Council
14
City West Water’s Water Plan 2013-2017 indicates that it will supply
the Council with 14.3 ML per year.
http://waterplan.citywestwater.com.au
Residential
0
City West Water does not currently have plans to supply Hobsons
Bay residents with recycled water.
Non-residential
2,200
Refer to 2013 comments.
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6 Water balance and future demand
WATER TYPE
YEAR
Rainwater and
stormwater use
2013
2031
USER
CATEGORY
Council
WATER
CONSUMPTION
(ML/YR)
ASSUMPTIONS
8
Estimated water capture from Altona Green stormwater harvesting,
Bayfit rainwater harvesting and water tanks. No measured data
available. Data estimated from design plans and stormwater models.
Residential
201
Sustainability Victoria’s Green Light report (2011) states that 37% of
households had rainwater tanks in 2010. In 2010 Hobsons Bay had
35,408 private dwellings.
Yarra Valley Water’s Future Water report (2012) states that demand
is 19L/p/d and that supply is 90% reliable
Non-residential
Council
267
Unknown
12 community buildings are likely to be updated over next decade and
2 pavilions per annum will be upgraded up until 2030.
It is assumed that the tank installed for each community building
and pavilion is 90% reliable in meeting toilet flushing demands. The
toilet flushing demand is based on 200 flushes per day for each.
This is estimated to provide 3 ML of rainwater bringing the total from
2013 up to 11 ML in 2031.
Stormwater harvesting from Cyril Curtain Reserve, Laverton Park
and Paisley Park is predicted to yield 153 ML/yr – half of the water
harvested at Paisley Park has been included as the other half will be
used by the golf course which leaves 116 ML
Assuming that Council follows the investment scenario in case 5 and
commits to the development of another 140 ML/y of stormwater by
2031 this would increase rainwater and stormwater use to 267 ML/yr
Residential
348
Assume 50% of all new residential developments have a rainwater
tank. Forecast id predict an additional 7,541 new private dwellings in
Hobsons Bay.
Assume an additional 15% of residents based on 2010 dwelling
figures have adopted rainwater tanks. This assumption is based on
data given in Sustainability Victoria’s Green Light report (2011).
As per the current water balance, assume that each household has
2.6pax, a toilet flushing demand of 19L/p/d, and the rainwater tank
is 90% reliable)
Non-residential
248
Assume 20% of rainwater generated off industrial roof area is used for
industrial purposes. The total industrial area was calculated in the Big
Roof project.
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6 Water balance and future demand
WATER TYPE
Wastewater
YEAR
2013
2031
Stormwater
runoff
USER
CATEGORY
WATER
CONSUMPTION
(ML/YR)
Council
41
ASSUMPTIONS
A 0.9 sewerage discharge factor was applied to mains water
consumption in buildings.
Residential
4,860
A 0.9 sewerage discharge factor was applied to mains water
consumption. The 0.9 factor is applied by City West Water to
residential mains water consumption to calculate the sewerage
charges.
Non-residential
5,500
Data provided by City West Water is that 10.4 ML of sewerage was
produced in Hobsons Bay in 2012/2013. The residential production
has been subtracted from this amount.
Council
41
Assume unchanged
(needs to account to 12 new community buildings)
Residential
5,730
A 0.9 sewerage discharge factor was applied
to mains water consumption.
Non-residential
6,390
Assume a 4% increase in wastewater aligned with increase in
potable water. CWW 2013, City West Water, Water Plan 2013-2017,
http://waterplan.citywestwater.com.au
2013
Total
11,096
Calculated through MUSIC modeling done by E2Design Lab in 2013.
2031
Total
10,726
Increased imperviousness minus increase in
rainwater and stormwater harvesting
Table 6: Current and projected water consumption data
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7
Strategic redevelopment
precincts
Council’s Industrial Land Management Strategy 2008 intends to consolidate the Council’s position on industrial
land. It was developed in light of the release of the State plan, Melbourne 2030, and in light of requests to
rezone industrial land and resident concerns about the operations and impacts of some industries.
The Table below is based on the Industrial Land Management Strategy and provides a summary of the precincts
currently identified for redevelopment and their known status to date. This information, combined with ID data,
was used to determine the expected number of new developments within Hobsons Bay and the expected
changes in impervious areas due to that development. This informed the water balance and future demand figures.
STRATEGIC REDEVELOPMENT PRECINCT
AREA
(HA)
PREFERRED FUTURE LAND USE
Residential
Part Precinct 9 - land fronting Millers Road, Altona North
Part Precinct 13 - land at corner of Maddox Road and
Kororoit Creek Road, designated strategic redevelopment
area, Williamstown North.
Part Precinct 13 - land at Akuna Drive industrial
developments and 92-100 Champion Road, Williamstown
North.
Part Precinct 14 - land bounded by Geelong Road, Millers
Road, Francis Street and Cemetery Road in Brooklyn
Part Precinct 15 - Part of land bounded by Blackshaws
Road, Kyle Road
New Street and the Westgate Freeway in Altona North
Precinct 16 - Industrial land bounded by Blackshaws
Road, Stephenson Street and the railway line in South
Kingsville
Part Precinct 17 - land at southwest corner of Birmingham
Street and Melbourne Road, Spotswood.
Part Precinct 18 - land in Hudson Road/Hall Street,
Spotswood
Part Precinct 18 - land at the southeast corner of
Melbourne Road and McLister Street, Spotswood
Part Precinct 20 - land at Ann Street, Nelson Place,
Kanowna Street, Aitken Street, south of Nelson Place and
Battery Road, Williamstown
Part Precinct 21 - Former Newport Mill, Blackshaws Road
and McRobert Street, Newport

744
4.7

12.6

66.5
9.9
4.7
2.3
1
6600
(currently 63%
impervious)


Undecided
(but possible)
Undecided
(but unlikely)
Permit
approved for
mixed use
Permit
approved for
mixed use
2200
Estimated 1200

23.4
1.8
Possible
number of
residents

7.9
27.7
Light
industrial /
commercial

528
1760

Table 7: Strategic redevelopment precincts. Last updated April 2013
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Proposed Residential
Areas in Hobsons Bay
129 Millers Road,
Altona North,
11 dwellings (VCAT)
(PA1123894)
185 Millers Road
Altona North
10 dwellings
(PA0817257)
16/04/13
Approx 3000 dwellings
(Dons Site) Precinct 15.
Proposed Structure Plan and
Planning Scheme Amendment
being facilitated by DPCD
637 Melbourne Road
Spotswood, approx 74
dwellings
285 dwellings, Precinct 17
(Proposed concept plan)
14
14
14
7
15
7
31-69 McLister Street, Spots
240 dwellings (NOD issues fo
PA1021678)
19
Golf Course
McLean Res
Crofts Res
15
Memorial Park
16
Scienceworks
17
Approx 422 to 600 dwellings
Part Precinct 16
Caltex Site
18
Riverside Park
18
21
Newport Lakes
Fowles Auction
6
4
6
Power Station
Paisley Park
9
Native Grass Reserve
Approx 339 dwellings for the
remainder of Precinct 16
Newport Park
Golf Course
Bladin Street
verton, 48 dwellings
CAT appeal)
1224379)
10
Air Liquide
5
10
2
10
3
12
10
3
1
450 Melbourne Road New
31 dwellings approved
13
10
5
Cemetery
11
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2
184-190 Ferguson Street
Williamstown. 40 dwellings
(PA1225021) at VCAT
Town Hall
Cherry Lake
Altona Costal Park
2
Melbourne Ballpark
2
Civic Centre
Jawbone Conservation
2
20
Botanic Gardens
Point Gellibrand Coastal Heritage Park
Truganina Swamp
Approx 800 dwellings.
Precinct 20 (Former P
Phillip Mills Site)
Cricket Ground
Bruce Comben Res
Golf Club
112-114 Pier Street
Altona
40 dwellings
under Construction
(PA0919539)
Truganina Explosives Res
H.D. Graham Res
53 Sargood Street
Altona 14 dwellings
(PA0919649)
Former Altona Tip
Cheetham Wetlands
14
Queen/Merton Street
Altona Meadows
80 dwellings
(PA1124010)
On-hold VCAT
105-113 Pier Street
Altona 95 dwellings
under Construction
(PA1020478).
Approx 66 lots
Arundel Stud and
Hobsons Bay
Caravan Park
Precinct 13
Amendment C54
Approx 121 lots
Winky Pop
Precinct 13
Amendment C74
42 Kororoit Creek Road
Williamstown
19 dwellings
(PA1225020)
LEGEND
Core Industrial Area
Secondary Industrial Area
Strategic Redevelopment
Non-Residential Use
Strategic Redevelopment
Potential Residential Land
Proposed Residential
Development Sites
8
Projected pollutant load
The data in this section of the report shows the
impact of projected increases in impervious areas
due to residential development on stormwater runoff
volumes and water quality outcomes. Equally, it
shows the benefits of using best practice stormwater
management to reduce stormwater runoff volumes
and improve or protect water quality.
PAISLEY PARK STORMWATER HARVESTING
It can be seen in the analysis below that meeting best
practice stormwater management in industrial and
residential developments plays an important role in
preventing the increase of pollutant loads. It is noted
however that best practice stormwater management,
without the incorporation of rainwater or stormwater
harvesting, is not enough to completely mitigate
increased stormwater runoff.
Pollutant load reduction associated with
Council stormwater harvesting works
Source
load
Residual
load
%
Reduction
Flow (ML/yr)
Total Suspended
Solids (kg/yr)
213
119
44.1
4110
281
93.2
Total Phosphorus
(kg/yr)
Total Nitrogen (kg/yr)
84.8
15.7
81.5
620
282
54.6
Table 8: Summary of pollutant loads from Paisley Park
stormwater harvesting
If these results are extrapolated across all three
stormwater harvesting systems then it can be
assumed that pollutant loads will be reduced as
shown in Table 9 below:
The Council commissioned three stormwater
harvesting projects systems in 2013. They are:
• The Paisley Park stormwater harvesting system,
expected to harvest 75 ML of water per year. This
water will be shared between the Altona Lakes
Public Golf Course and the Council
• Laverton Park stormwater harvesting system,
expected to harvest 68 ML of water per year
• The Cyril Curtain Reserve stormwater harvesting
system, expected to harvest 10 ML of water per
year.
A MUSIC model is currently only available for the
Paisley Park stormwater harvesting system. It
has been assumed that each of the stormwater
harvesting systems reduces stormwater flows and
pollutant volumes at a similar rate to the Paisley Park
stormwater harvesting system.
Source
Residual
load
%
Reduction
Flow (ML/yr)
434.52
242.76
44.1
Peak Flow (m3/s)
Total Suspended
Solids (kg/yr)
Total Phosphorus
(kg/yr)
8.772
0.47124
94.6
83844
5732.4
93.2
172.992
32.028
81.5
Total Nitrogen (kg/yr)
1264.8
575.28
54.5
Table 9: Projected pollutant load reduction from the Councils
2013 stormwater harvesting projects (153 ML of water
harvested per annum, this includes water used by the
Altona Lakes Public Golf Course)
The results of the Paisley Park model are shown right:
Integrated Water Management Plan Technical Support Manual
15
8 Projected pollutant load reductions
If the Council commits to an additional 140 ML of
stormwater harvesting by 2031 then the additional
pollutant load reduction that will be achieved is as
shown below in Graph 1 and Table 10:
Load reduction (t/yr)
total suspended
solids (t/yr)
total phosphorus
(kg/yr)
total nitrogen
(kg/yr)
Graph 1: Projected pollutant load reduction from
140 ML of stormwater harvesting
Source
Flow (ML/yr)
397.6
Residual
load
222
Precinct size developments
The Industrial Land Management Strategy identifies
111 ha of land that is likely to be developed as
residential by 2023. Through the analysis of aerial
photographs, 52 ha of this land have been identified
as being impervious. Clause 55 of the Victorian
Planning Provisions requires residential developments
to be at least 20% permeable. It has been assumed
in these calculations that developers will maximize
that floor area of their developments and leave only
the minimum permeable area permissible.
650
600
550
500
450
400
350
300
250
200
150
100
50
0
flow
(ML/yr)
Pollutant load reduction if residential
subdivisions meet best practice
A MUSIC model has been developed to show the
stormwater runoff and pollutant loads of this land
under current conditions, after being developed
without treatment and after being developed with
treatment.
The assumptions that have been made during the
analysis are:
%
Reduction
44.1
• that the site will increase to 80 per cent
impervious (site planning provisions)
Total Suspended
Solids (kg/yr)
76,700
5245
93.2
• that each dwelling will have a 4 kL water tank
connected to the toilet
Total Phosphorus
(kg/yr)
158.2933
29.3
81.5
• that demand is 19L/p/d as per Yarra Valley
Water’s Future Water report (2012)
Total Nitrogen (kg/yr)
1157.333
526.4
54.5
• that the net roof area of the developments is 40 ha
Table 10: Projected pollutant load reduction from an additional
140 ML of stormwater harvesting
To achieve a 140 ML increase in stormwater
harvesting by 2031 the Council would need to
commit to an additional 42 ML of stormwater
harvesting over the life of this Strategy.
• that the remaining impervious surfaces have
been treated with buffer strips as this is a
low cost option.
It is noted that different treatment options will yield
slightly varied results. However, these results serve as
a guide in lieu of formal proposals from developers.
Integrated Water Management Plan Technical Support Manual
16
8 Projected pollutant load reductions
Phosphorus
kg/yr
current load
load without action
load under best
practice management
It can be seen from the Graph 2 above that treating
stormwater to the current best practice standards reduces
stormwater runoff from the site from 350 ML per year to
244 ML/yr. This is a significant reduction. However, the net
development area only had 210 ML/yr of stormwater runoff
when the average imperviousness was only 47 per cent.
This is a net increase of 34 ML.
It is noted that meeting best practice stormwater
management mitigates some of the risk of increased
stormwater loads from developments that increase
flooding. However, this does not guarantee that all of the
risk will be mitigated.
kg/yr
load under best
practice management
Nitrogen
1200
1000
800
600
400
200
0
current load
load without action
load under best
practice management
Graph 5. projected nitrogen from precinct size residential
developments
Suspended solids
80000
70000
60000
50000
40000
30000
20000
10000
0
load without action
load without action
Graph 4: Projected phosphorus from precinct size
residential developments
Graph 2: Projected stormwater runoff from
precinct size residential developments
current load
160
140
120
100
80
60
40
20
0
current load
kg/yr
1000,000 of litres per year
Stormwater runoff
400
350
300
250
200
150
100
50
0
load under best
practice management
Graph 3: Projected suspended solids from precinct
size residential developments
It can be seen from Graph 3 that meeting best
practice stormwater management significantly
reduces the volume of suspended solids that end up
in the Bay and waterways. Suspended solid loads
reduce from 71 tonnes if developments don’t meet
best practice to 14 tonnes if they do.
It can be seen from Graph 4 and Graph 5 that
meeting best practice stormwater management
effectively reduces the volume of phosphorus and
nitrogen that end up in the Bay and waterway.
Phosphorus loads reduce from 146 kg/yr if
developments don’t meet best practice to 54 kg/yr
if they do. Similarly, nitrogen loads reduce from 1000
kg/yr if developments don’t meet best practice to 511
kg/yr if they do.
Reducing these three pollutants, suspended
solid, phosphorus and nitrogen, will be particularly
important for developments identified in the Kororoit
Creek catchment. This is due to the moderate
condition of aquatic life in this waterway and its high
recreational value.
No strategic development sites have been identified
for catchments in the municipality’s highest quality
waterways, Skeleton Creek and Laverton Creek.
Integrated Water Management Plan Technical Support Manual
17
8 Projected pollutant load reductions
Table 11 provides a summary of the data shown in the
graphs discussed above.
Current load
Load
without
action
Load under
best practice
management
Flow (ML/yr)
210
350
244
Suspended
solids (kg/yr)
42600
71900
14000
Phosphorus
(kg/yr)
86.6
146
53.7
Nitrogen
(kg/yr)
604
Implementing best practice stormwater management
for these developments will have the impact as shown
in Graph 6 below on pollutant loads:
Stormwater runoff
1000
511
Table 11: Summary of pollutant loads from precinct
size residential developments
Small scale developments
ID forecasts the development of 7,541 new private
dwellings within Hobsons Bay by 2030. It is estimated that
6,078 of these dwellings will come from the large scale
precinct developments identified in Section 7. Therefore,
the remaining 1,463 developments will be supplied by
infilling existing areas. Assuming that these dwellings
average between 300 and 350 square meters per dwelling,
this gives an additional 49 ha of infill development.
Assuming that:
• the average subdivided site increases from
30 per cent impervious to 70 per cent impervious
• the average subdivided site is between
350 and 300 m2
• the average roof area is 120 m2
• each building has a 5 kL tank
connected to the toilet
• the average occupancy rate is
2.5 people per house
• impervious areas that are not connected to
a water tank flow over a buffer strip before
connecting to stormwater
1,000,000 of litres per year
12
10
8
6
4
2
0
current load
load without action
load under best
practice management
Graph 6: Projected stormwater runoff from small scale residential
developments per ha.
The increase in impervious surfaces increases
stormwater runoff from 5.125 ML/yr to 11.275 ML/yr.
With best practice, this is reduced down to
8.569 ML/yr. Most of the reduced flow comes from
the installment of water tanks. Each tank reduces
the flow from each roof by approximately one third.
As seen previously, although best practice stormwater
management does reduce stormwater runoff, it
does not completely mitigate the risk of increased
flooding from increases in imperviousness. In areas
where drainage infrastructure is at or near capacity,
additional requirements may be needed. Similarly,
in the Skeleton and Laverton Creek catchments
additional requirements may be needed to protect
the hydrology of these waterways. This is important
as they are the only waterways in the municipality
that currently have good hydrology. Good waterway
hydrology is difficult to restore once damaged.
As can be seen in Graph 7, meeting best practice
stormwater management has a significant impact on
reducing suspended solids. Meeting best practice
stormwater management reduces the suspended
solid load below the current load. This is despite
significant increases in the total impervious area.
Integrated Water Management Plan Technical Support Manual
18
8 Projected pollutant load reductions
Suspended solids
Nitrogen
2500
35
2000
30
25
kg/yr
kg/yr
1500
1000
20
15
10
500
5
0
0
current load
load without action
load under best
practice management
current load
load without action
load under best
practice management
Graph 7: Projected suspended solids from small scale
residential developments per ha.
Graph 9: Projected nitrogen from small scale
residential developments per ha.
As shown in Graph 8, meeting best practice
stormwater management reduces phosphorus loads
to just below the current level, while Graph 9 shows
that nitrogen loads increase only slightly.
Reducing these three pollutants, suspended
solid, phosphorus and nitrogen, will be particularly
important for developments in the Skeleton Creek,
Laverton Creek, Cherry Creek and Kororoit Creek
catchments. This is due to the high condition of
aquatic life in Skeleton Creek and Kororoit Creek.
Removing these pollutants from Laverton Creek
and Cherry Creek catchments is an important
part of protecting the moderate water quality of
these catchments.
kg/yr
Phosphorus
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
Current load
current load
load without action
load under best
practice management
Graph 8: Projected phosphorus from small scale
residential developments per ha.
Load
without
action
Load under
best practice
management
Flow (ML/yr)
5.125
11.275
8.569
Suspended
solids (kg/yr)
1005
2308
476
Phosphorus
(kg/yr)
2.0705
4.674
1.845
15
32
17.835
Nitrogen
(kg/yr)
Table 12: Summary of projected yearly change in pollutant loads
from small scale residential developments (1 ha of
development)
It is important to note that building extensions,
new carports and sheds, verandas and concreted
yards will also have a similar impact on pollutant
loads and particularly stormwater runoff as seen
in Table 12 above. Currently these works are
not required to meet best practice stormwater
management. The magnitude of this impact and
the actions that can be undertaken to mitigate
them should be further investigated.
Integrated Water Management Plan Technical Support Manual
19
8 Projected pollutant load reductions
Pollutant load reduction if industrial
developments meet best practice
Stormwater runoff reduces from 170 ML/yr to
129 ML/yr. The majority of this comes from harvesting
water from the roof.
The vast majority of industrial developments that
have been identified for strategic redevelopment
are already highly impervious. It is estimated
that these sites are already 87% impervious on
average. The greatest amount of pervious land can
be found in Precinct 9. Therefore the redevelopment
of Precinct 9 will have the greatest impact on
water quality and flow volumes.
As shown in Graph 11, Graph 12 and Graph 13
below, suspended solids, phosphorus and nitrogen
loads all drop dramatically from these developments.
This is because these sites are already highly
developed and untreated. There is significant gain in
water quality and flow volumes to be had in treating
existing pervious areas.
Suspended solids
The results in Graph 10 below are an analysis
of what the pollutant loads might look like from
these developments if best practice stormwater
management was met. It has been assumed that:
kg/yr
It has been assumed that all sites once
redeveloped will be 98% impervious.
• most sights would choose to use a water tank
as this would provide them with an additional
resource
• demand for rainwater is half that of the annual
runoff from the roof. Half that annual runoff from
the roof would provide a high level of reliability
of supply. It is considered unlikely that industry
groups would wish to invest in a low reliability
of supply
current load
Phosphorus
kg/yr
current load
load without action
load under best
practice management
Graph 12: Projected phosphorus from precinct size
industrial developments
Nitrogen
600
500
400
current load
load without action
Graph 10: Projected stormwater runoff from
precinct size industrial developments
load under best
practice management
kg/yr
flow in 1000,000 of litres per year
load under best
practice management
90
80
70
60
50
40
30
20
10
0
Stormwater runoff
200
180
160
140
120
100
80
60
40
20
0
load without action
Graph 11: Projected suspended solids from precinct size
industrial developments
• roof areas take up half the land area
• the remaining 2% of pervious land is used as a
buffer strip to treat stormwater runoff from the
remaining hard surfaces. A buffer strip has been
chosen due to its low cost
45000
40000
35000
30000
25000
20000
15000
10000
5000
0
300
200
100
0
current load
load without action
load under best
practice management
Graph 13: Projected nitrogen from precinct size industrial developments
Integrated Water Management Plan Technical Support Manual
20
Reducing these pollutant loads is particularly
important in the Kororoit Creek catchment.
This is due to the moderate condition of aquatic
life in this waterway and its high recreational value.
No strategic developments have been identified for
the catchments in the municipality’s highest quality
waterways, Skeleton Creek and Laverton Creek.
Table 13 provides a summary of the data shown in
the graphs discussed above.
Current
load
Flow (ML/yr)
Suspended solids
(kg/yr)
170
Load
without
action
187
Load under
best practice
management
129
35500
38600
9470
Phosphorus
(kg/yr)
70.3
77.7
30.4
Nitrogen (kg/yr)
489
542
266
Table 13: Summary of pollutant loads from precinct size
industrial developments
Integrated Water Management Plan Technical Support Manual
21
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