ProgramsStormwater Quality Offsets
Offsetting the Impact of Urban Development
The Victorian Government has introduced new objectives for stormwater quality management to protect the environment from the impacts of urban development. These objectives are contained in "Urban Storm Water: Best Practice Environmental Management Guidelines" (Victorian Stormwater Committee 1999). These Guidelines are now a referenced document in the State Planning Policy Framework and form part of the attainment program for State Environment Protection Policies including Waters of Victoria.
To apply these stormwater quality objectives to all forms of urban development whilst recognising and rewarding the contribution of onsite stormwater quality treatment, Melbourne Water is implementing a Stormwater Quality Offset Strategy. This strategy will ensure best practice stormwater treatment is provided to protect Port Phillip Bay and Western Port from pollutant loads associated with urban growth. The following diagram provides a basic overview of the concept.
- Concept for Melbourne Water Offset Strategy Diagram (PDF, 251kb)
To isolate the pollutants of concern in any catchment, the designer needs to closely examine receiving water degradation in light of the area's land-use and current management practices. The performance objectives (see table below) set out in Chapter 2 of the Urban Stormwater Best Practice Environmental Management Guidelines (CSIRO 1999) are a guide to the typical pollutant load reductions required to contribute to State Environment Protection Policy (SEPP) compliance. The draft Australian Runoff Quality guidelines (http://www.arq.org.au) provides assistance for decision making in selecting, integrating and locating stormwater treatment measures.
Stormwater quality performance objectives
|Pollutant||Receiving water objective:||Current best practiceperformance objective:|
|Post construction phase:|
|Suspended solids (SS)||Comply with SEPP (eg. not exceed the 90thpercentile of 80 mg/L) (1)||80% retention of the typical urban annual load|
|Total phosphorus (TP)||Comply with SEPP (eg. base flow concentration not to exceed 0.08 mg/L) (2)||45% retention of the typical urban annual load|
|Total nitrogen (TN)||Comply with SEPP (eg. base flow concentration not to exceed 0.9 mg/L) (2)||45% retention of the typical urban annual load|
|Litter||Comply with SEPP (eg. no litter in waterways) (1)||70% reduction of typical urban annual load (3)|
|Flows||Maintain flows at pre-urbanisation levels||Maintain discharges for the 1.5 ARI* at pre-development levels|
|Suspended solids||Comply with SEPP Effective treatment of daily run-off events (eg. >4 months ARI).||Effective treatment equates to a 50%ile SS concentration of 50 mg/L.|
|Litter||Comply with SEPP (eg. no litter in waterways) (1)||Prevent litter from entering the stormwater system|
|Other pollutants||Comply with SEPP||Limit the application, generation and migration of toxic substances to the maximum extent practical|
|1 An example using SEPP(Waters of Victoria1988), general surface waters segment.
2 SEPP schedule F7-Yarra Catchment-urban waterways for the Yarra River main stream.
3 Litter is defined as anthropogenic material larger than five millimetres.
Questions and Answers
Q. Why is stormwater an issue?
A. Stormwater is a major source of pollution in rivers and creeks. The volume of stormwater runoff delivered to receiving streams is much greater and is delivered much quicker in urban catchments where hard surfaces do not allow rainfall to filter through the soil. The rapid delivery of water to the stream can scour in-stream habitat and cause streambank erosion.
The main pollutants in urban stormwater are large quantities of already naturally occurring nutrients and particles such as nitrogen and phosphorus, heavy metals and fine sediments. These pollutants originate from a variety of sources, such as atmospheric deposition, synthetic and organic litter, construction sites and cars and are washed into waterways during storm events. Pollutants from stormwater are known to have a detrimental effect on water quality and the overall health of waterways.
Q. What are the best practice stormwater quality objectives?
A. Best practice stormwater quality management objectives are to retain:
- 80% of the suspended solid annual load
- 45% of total phosphorus annual load
- 45% of total nitrogen annual loads
The document, "Urban Storm Water: Best Practice Environmental Management Guidelines", prepared by the Victorian Storm Water Committee and the CRC for Catchment Hydrology (1999) describes these objectives that should be achieved by all development. The guidelines can be obtained from CSIRO publishing www.publish.csiro.au/nid/197/issue/3822.htm.
Q. What is a stormwater quality offset?
A. An offset is an action performed elsewhere in the catchment that counter-balances a polluting or environmentally degrading activity on-site. Offsets facilitate economic development and environmental protection at the same time. An offset mechanism gives the polluter the option of undertaking action themselves or paying others to do so on their behalf.
A stormwater quality offset has been established for urban development in the Port Phillip Bay and Western Port catchments. Nitrogen has been established as the offset currency and a price has been determined based on the cost to Melbourne Water to provide regional water quality treatment. Nitrogen is typically the limiting pollutant and hence if objectives for nitrogen are met then objectives for phosphorus and suspended solids will usually be met also.
Under Melbourne Water's Stormwater Quality Offset Strategy, a developer has the option of meeting best practice objectives for nitrogen within the development or paying an offset contribution to Melbourne Water, who will construct treatment systems elsewhere in the catchment to achieve the equivalent load reduction not met onsite.
Q. What type of developments does the offset strategy apply to?
A. Stormwater Quality Offsets will apply to all urban developments referred to Melbourne Water for subdivision approval.
On 9 October 2006, the new Sustainable Neighbourhoods Provisions (Clause 56) came into effect. These provisions apply to applications to subdivide land in the Residential 1 Zone, Residential 2 Zone, Residential 3 Zone, Mixed Use Zone, Township Zone and any Comprehensive Development Zone or Priority Development Zone that provides for residential development.
The new provisions require that the urban stormwater management system ‘must be designed to meet the current best practice performance objectives for stormwater quality, as contained in the Urban Stormwater Best Practice Environmental Management Guidelines (Clause 56.07-4).
It may be possible to satisfy the water quality requirements of Clause 56 through a financial contribution towards regional works (stormwater quality offsets). Within the Port Phillip and Westernport catchments, some existing drainage schemes already provide for water quality treatment at a regional scale and offsets may be available for subdivisions, which are located inside these schemes. Outside of schemes or in schemes that don’t include water quality treatment, offsets may be available for smaller developments (less than 1 ha). Councils, as the Responsible Authority for implementing Clause 56 will determine whether a development may satisfy the stormwater quality objectives through offsets, whether treatment will need to be provided within the development or whether a combination of on site works and offsets are appropriate.
Industrial or commercial developments not subject to requirements under Clause 56 are eligible for offsets for sites less than 5ha or where a greenfield scheme provides for water quality treatment.
Q What is the basis of this new strategy?
A. The Victorian Government has recently introduced new requirements for stormwater quality management to protect the environment from the impacts of urban development. These requirements are contained in "Urban Stormwater: Best Practice Environmental Management Guidelines" prepared by the Victorian Stormwater Committee (1999). The Guidelines are now a referenced document in the State Planning Policy Framework and form part of the attainment program for recent State Environment Protection Policies including Waters of Victoria.
Nitrogen has been established as the charge currency and a price has been determined based on the cost to Melbourne Water to provide regional water quality treatment.
Nitrogen was chosen because it is the pollutant most difficult to treat. If objectives for nitrogen are met then objectives for phosphorus and suspended solids will usually be met also.
Q. Where does the money go?
A. Melbourne Water will use offset contributions to construct stormwater quality treatment measures elsewhere in the Port Phillip and Westernport Bay catchments.
Off-site stormwater treatment measures will typically comprise wetland or bioretention / infiltration type systems. These systems will be managed by Melbourne Water or will have maintenance agreements between Melbourne Water and the landowner (eg councils).
The works will balance the short fall of nitrogen reduction not achieved on-site by developers.
Q. Why is this strategy being put into practice/introduced now?
A. The introduction of the strategy is the result of several factors.
- Best practice objectives for stormwater now exist and are referenced in the State Planning Policy Framework and form part of the attainment program for recent State Environment Protection Policies including Waters of Victoria.
- Recent research shows that distributed Water Sensitive Urban Design (WSUD) can achieve better environmental outcomes at a lower total cost to the community than large regional stormwater quality treatment facilities.
- WSUD is becoming standard practice for many large urban development projects in Melbourne.
- Up until now Melbourne Water has serviced urban growth stormwater treatment (typically large constructed wetland systems) through developer contributions. There is now a need for a mechanism to recognise and reward efforts made by developers who meet water quality objectives within the development.
Q. What proof is required to demonstrate that treatment measures either have been built or will be built?
A. Melbourne Water will accept a range of documentation that can demonstrate treatment measures have been used or are proposed for the site. These include, but are not limited to:
- "As constructed" plans
- A Section 173 agreement
- A Restriction on title
- Condition in contract of sale
- Evidence of a Municipal ‘development plan’ or ‘structure plan’ that requires best practice treatment for new development
Q. How do I assess the water quality performance of different on-site treatment measures?
A. For smaller developments with single treatment measures simplified treatment curves can be used to assess treatment performance. The curves give a conservative estimate of performance against water quality objectives for a given treatment surface area. The curves were derived from simulations untaken using the Model for Urban Stormwater Improvement Conceptualisation (MUSIC) V3 a computer model developed by the Cooperative Research Centre for Catchment Hydrology for simulating complex stormwater management treatment trains.
The simplified curves are available via a web based application known as STORM.
For larger developments there are a range of recognised computer programs (such as MUSIC) that can be used to model the performance of stormwater treatment measures.
Q How has the new water quality offset contribution been calculated?
A. Outside of greenfield drainage schemes and inside redevelopment schemes the water quality offset rate is based on the average cost of treating nitrogen (currently $800/kg for 05/06 and 06/07).
To be consistent with existing processes the $800/kg rate has been translated into a standard residential rate ($/ha). A standard rate is representative of an average lot size which broadly corresponds to an impervious fraction of 0.5. The standard rate is factored up or down depending on development density. The following table shows the different density factors that are applied to different types of development.
|Development type||Density Ratio|
(450 m² <lot< 1000 m²)
|High density residential sub division
(Lot < 450 m² including dual occupancy)
|Low density residential subdivision
(1000 m² < Lot <10,000 m²)
|School site on plans of subdivision||1.00|
|Retirement village development||1.25|
|Industrial / commercial development||1.50|
Q. Why do water quality rates vary across municipality areas?
A. Outside greenfield drainage schemes rates have been derived for each municipality. Rates for each municipality will vary largely because pollutant loads are different across the region based on rainfall averages. More rainfall produces more stormwater and hence more pollutant loads. Municipalities with higher rainfall will have higher rates because there are more pollutants to treat.
Municipalities with higher rainfall will have a higher rate, because there will be more pollutants in stormwater entering the drainage system. For example MUSIC modelling shows that in Hume one square metre of impervious surface generates 0.00125 kg/y of total nitrogen compared with double this amount ie 0.0029 kg/y in the Shire of Yarra Ranges. At $800/kg to treat a kilogram of nitrogen it would cost twice as much to achieve a 45% reduction in nitrogen for the SYR compared with Hume as there is twice as much nitrogen to treat. Hence to treat a one hectare to best practice in the Shire of Yarra Ranges will cost $4,489 compared with only $2,450 in Hume.
Q. Do I need to pay contributions for the subdivision of existing buildings?
A. No. Currently contributions for existing development, or proposed development with an approved planning permit will not be required. This is because incorporation of stormwater treatment should be considered at an early stage in the development design process. If planning has already occurred prior to application for subdivision then an offset contribution will not be required.
Q. Will rural zoned properties be subjected to the offset contribution?
A. The offset strategy applies only if there is a change in the zoning ie from rural to residential.
Q. Where can I get more information from?
A. Further information about WSUD, including tools such as STORM and Fact sheets can be found on Melbourne Water's Water Sensitive Urban Design web site.
Information regarding Melbourne Water's referral process and offset contribution rates can be found in Melbourne Water's Land Development Manual.