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  • 'Flashy' flows a problem for urban creeks

    11 April, 2016

    While the rain last week in Melbourne was very welcome and provided some much needed input to many streams, it also highlighted one of the key problems with urban waterways.

    Under natural conditions, rainfall would usually seep into the ground and slowly drain into the local waterways, with some being diverted to groundwater and some retained by the soil. As a result water flow in creeks increases gradually and then subsides over an extended period. Soil and vegetation also act as natural filters, removing many pollutants from the water as it percolates through.

    In urban areas, natural surfaces have been largely replaced with hard, impervious surfaces (roofs, roads, concrete). Flow patterns in urban areas are highly altered as rainfall quickly runs off the impervious surfaces and directly into creeks via drains. This results in a very rapid increase and decrease in flows, higher peak flow, and extended periods of low flows between rainfall events as shown below. This increased ‘flashiness’ in the flow regime is one of the key factors degrading urban waterways leading to a loss of aquatic biodiversity and ecological function.

    Typical flow pattern in an urban waterway following rain (Diamond Creek, Eltham). Source: melbournewater.com.au/waterdata/rainfallandriverleveldata


    Alteration of natural flow regimes can have a variety of impacts including poor water quality, increased bank erosion, physical displacement of aquatic fauna (macroinvertebrates, fish, platypus - platypusspot.org/blog/updates-from-the-field/stormwater-impacts-on-urban-platypuses), altered behaviour (foraging, migration, breeding), and channel ‘scouring’ which removes organic matter and habitat from the bottom of creeks and rivers. In addition, increased run-off collects a lot of pollutants and rubbish (which are also higher in urban areas) from the surrounding catchment and deposits them directly into the nearest creek.

    Actions that reduce or slow the run-off following rainfall can reduce the impacts of altered flow regimes. Solutions include stormwater harvesting, rain gardens (melbournewater.com.au/raingardens), domestic water tanks, or diverting stormwater through artificial wetlands. 


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