Rain that falls on hard surfaces like roofs, driveways and roads cannot soak into the ground. Instead, it is carried via stormwater pipes to waterways such as rivers creeks and wetlands.
This fast-moving runoff can:
These impacts occur because urban stormwater systems alter the way water moves through a catchment. In natural landscapes, most rainfall soaks into the soil and makes its way slowly to waterways once the soil is saturated. In contrast, urban stormwater systems concentrate runoff and carry it rapidly into receiving waterways. These sudden, high flows – often called ‘flashy’ flows – can scour the riverbanks and erode the creek beds that are connected to urban stormwater pipes and send more water downstream at once than the waterways can handle.
In many parts of the Corangamite Region, such as the Geelong and Bannockburn growth areas, the risk from stormwater to waterway health is especially high given the rate of urban growth. Managing stormwater in these areas is critical to protecting rivers, wetlands and other significant and sensitive environments including internationally significant Ramsar wetlands.
Without proper management, increasing urban development can lead to more stormwater entering waterways, affecting their health and the plants and animals that depend on them.
It is very important to appropriately manage urban stormwater, especially in greenfield development areas where appropriate infrastructure can be incorporated into development plans early in the planning process.
In new developments, stormwater can be a valuable resource. It can be:
It is also possible that collected stormwater could be used to send controlled flows downstream to benefit things like fish spawning and habitat connectivity. This opportunity is particularly relevant in a drying climate, and in systems like the Moorabool and Barwon Rivers, which we know experience flow stress because of upstream uses.
The challenge is that traditional stormwater systems don't provide this steady, controlled flow of water. Instead, they quickly channel poor-quality stormwater into waterways at harmful rates that can cause damage. For stormwater to be beneficial to a waterway, it needs to be treated to remove pollutants, picked up from roads, roofs and other hard surfaces. It also needs to be captured and released in a way that supports the specific species living in that system.
A helpful analogy is the difference between watering your garden with a fire hose full of polluted water for 30 seconds versus a gentle sprinkler over 30 minutes – the same volume of water would have very different effects on the soil and plants in the garden bed.
To support appropriate urban stormwater management, Corangamite CMA has published Stormwater Management Priority Areas for the Geelong and Bannockburn growth areas.
These are areas where additional stormwater is most likely to cause significant harm to downstream waterways and wetlands.
Developments within these priority areas need to achieve higher stormwater harvesting and infiltration outcomes to help protect these important environments.
The Victorian EPAs Urban stormwater management guidance (Publication 1739.1, 2021), sets targets for how much stormwater should be captured (harvested) or allowed to soak into the ground on site (infiltrated). The targets vary according to average rainfall, with a higher percentage of overall runoff volume required to be harvested and infiltrated in areas that receive less annual rainfall.
Following are the applicable targets for the Geelong and Bannockburn areas, which receive an average of 500-600mm annual rainfall.
In ‘priority areas’, developments should aim to:
In ‘other areas’, developments should aim to:
These targets help reduce the impact of urban runoff on downstream waterways.
Meeting these targets is required under the General Environmental Duty – part of the Environment Protection Act 2017 – for new developments.
The approach to meeting these stormwater harvesting and infiltration targets in each development will need to consider the development type, opportunities for precinct-scale infrastructure, and other site conditions, in a similar way that options to meet the existing stormwater quality targets are considered. These decisions are made through the planning process in consultation with planning authorities. That decision-making should also include considering what is ‘reasonably practicable’ in each case.
To ensure development is compliant with the General Environmental Duty and identify the most cost-effective solutions, stormwater harvesting and infiltration targets should be considered as early as possible in the planning process.
Priority areas were defined following the science-based process developed by Melbourne Water in their Healthy Waterways Strategy. The three-step process provides:
This process helps ensure stormwater management efforts are focused where they will deliver the greatest environmental benefit.
Available on request:
Please email communications@ccma.vic.gov.au
Work to determine Stormwater Management Priority Areas for other urban and urbanising areas of our catchment, including Ballarat and Winchelsea, is currently underway through DEECA-supported projects, and will be published here when available.
64 Dennis Street, Colac VIC 3250
PO Box 159, Colac, VIC 3250
Open to the public: 8.30am - 4pm, Mon to Wed only
Staff are available Monday to Friday during business hours
T: 1800 002 262
E: info@ccma.vic.gov.au
BARWON WATER OFFICES
Hours: 8:30am – 5pm, Monday to Friday
All mail must be sent via our Colac office
PO Box 159, Colac, VIC 3250
T: 1800 002 262
E: info@ccma.vic.gov.au
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