The last few years in Melbourne have been dry. In each of 2006, 2007 and 2008, only about 450 mm of rain fell in the city centre, down from the long-term average of about 650 mm.
While the dry has resulted in worrying declines in the volumes flowing into our water-supply reservoirs, it has had much less of an effect on the volume of damaging stormwater flowing into Melbourne's urban waterways. Here is a table detailing the very large volumes of stormwater that are making our urban waterways sick.
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Annual rainfall |
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Average
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Dry |
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Runoff from all roofs and roads of the Melbourne region |
608 GL |
400 GL |
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made up of... |
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Volume that the roofs and roads stop from soaking into the ground to provide dry-weather flows to the city's waterways (water that ideally should be allowed to flow to the waterways through the soils or through constructed filters like rain-gardens) |
145 GL |
50 GL |
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Volume that would have been taken up by plants to be released back to the air if they hadn't been replaced by roofs or roads (water that ideally should be kept in the catchment to water our gardens and parks, or to substitute for mains water) |
463 GL |
350 GL |
A GL is a gigalitre - 1,000,000,000 litres - and the people of Melbourne currently use about 420-450 of them each year, imported from reservoirs in distant catchments. The Victorian Government is about to augment these supplies with 150 GL each year from a new desalination plant at Wonthaggi.
Almost all of the runoff from Melbourne's roofs and roads runs down our stormwater drains, damaging the health of our rivers and creeks, and threatening the health of our bays. As Melbourne expands and its existing suburbs become more densely developed, the volume of damaging stormwater that Melbourne produces will only increase, even in a drier future.
The health of our rivers and creeks depends on us retaining as much urban stormwater as we can in our catchments. If we were to turn to this abundant, secure water source to water our gardens and our parks, we could help keep our city green and cool. If we were to use a moderate proportion of it as a substitute for our main water supply, we could avoid expensive water supply augmentation projects that only serve to make our long-term response to climate change more difficult.
*These annual rainfall values are for central Melbourne. The runoff volumes were calculated allowing for variation in annual rainfall across the Melbourne region. For more information on how these numbers were calculated see the next tab at the top of this page (How runoff vols are calculated), and to work out how much damaging stormwater runoff is produced by a particular building or road of interest, see the third tab (Runoff from an area you know).
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The volumes of stormwater runoff shown on the first tab were estimated from three sets of data that are illustrated below.
Melbourne is spread across a strong rainfall gradient with long-term average annual rainfall ranging from ~600 mm in the west to over 1200 mm in the upper Yarra Valley. To calculate runoff from the 964 square kilometres of Melbourne's roofs and roads, each surface was first attributed an average annual rainfall. Not all rainfall runs off impervious surfaces: it usually takes about a millimetre of rain before the surface is wet enough to allow water to run off it. The relationship between annual rainfall and annual runoff from impervious surfaces is fairly consistent across the Melbourne region, with as little as ~75% of rain running off in dry years and dry places, up to as much as ~90% in wet years and wet places (see the figure above right). So runoff volume (in litres) from each surface was calculated by converting its annual rainfall to impervious runoff (in mm) and multiplying this by its area (in square metres). If stormwater runoff from a building or road is directed directly to a stormwater drain, then it is contributing to the damage being done to the waterway at the end of the pipe. To remove the impact that the building or road is having on its waterway, a small proportion of its runoff should be delivered slowly through a filtration system to restore lost dry-weather flows, and as much of the remainder of its runoff as possible should be used to water plants or to replace the potable water supply. |
