The Problem......

Urban Stormwater

    

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Contents
 

Why is stormwater a problem?

Why is Stormwater a problem?

  1. Pollution cocktail

   2. Wrong time

   3. Too much

The solution to Stormwater
 

Stormwater is a problem and a challenge to manage for three reasons:
 

How much stormwater does all of Melbourne produce?

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Every time it rains, stormwater runs off the roofs and paving and roads of our suburbs, eroding and polluting our creeks and rivers.
 
1. Stormwater is a pollution cocktail
       

When stormwater runs off roofs, and particularly paving and roads, it picks up everything in its path, including:

 

# dirt;

# leaves and other organic matter that might # contain pathogens;

# fertilizers and pesticides;

# oil and grease;

# potentially toxic metals (like copper, cadmium, lead and zinc).

 

By the time the stormwater gets to the creek it is a toxic cocktail of pollutants.

The stormwater drainage system is designed to send every drop of rain to the end of the pipe. So, even in dry weather, if there is an oil spill on a road, or a sewerage pipe inadvertently gets connected to a stormwater pipe, the pollution will flow directly to the creek, even if the accident is high up in the catchment.

The solution is to have drainage systems that retain the dry weather spills, and small, frequent rainstorms, but let the big storms drain away to avoid flood risk.  Rain-gardens do this really well.  And because rain-gardens and rainwater tanks hold back small storms, they help reduce the size of bigger floods
                   
       
2. Stormwater Flows at the wrong time
       

In natural, forested landscapes, creeks and rivers receive virtually all of their flow from water that percolates through the soils, to provide a steady base flow throughout the year.  Once or a few times a year, it might rain so much that parts of the forest floor get wet enough to start overland flow.  This sort of storm is enough to produce a flood that carries sediments and assorted contaminants down the creek.  The animals and plants living in the creek are adapted to these infrequent disturbances and have a variety of strategies to survive occasional floods.

But stormwater pipes send a flush of water to the stream every time it rains more than a millimetre or so: 121 days in an average year.  So stormwater increases the frequency of disturbance to the stream from once or a few times a year, to once or a few times a week

The good news is that most rain events are small, and rainwater tanks and rain-gardens can be very efficient at retaining water from those small, frequent events.  This retention of frequent events is the most important part of our environmental benefit index
       
3. There is too much stormwater
       

When Mt Evelyn was a forest of stringybark trees, the forest drank 80% of the water that fell on the catchment as rain.

In an average year, at Polat Court, where the creek has a catchment of 185 hectares: 1,388 million litres of water were drawn up by the trees of the catchment and released back to the air.

Just 369 million litres flowed down the creek.

Now, with 21 hectares of roofs and roads replacing lots of trees, much less water is drawn up by tree roots to be returned to the air: only 1,257 million litres.

And because the roofs and roads do not let water filter into the soils, less water gets to the creek through natural subsurface flows: only 327 million litres

The remaining 174 million litres, that is no longer taken up by the trees, and no longer filters into the ground is rushed straight to the creek by the underground stormwater drainage system.

Even if we were to augment the subsurface flows with some of that water going down the pipes to get the sub-surface-fed stream flow back to a more natural 369 million litres, we would still have an extra 131 million litres causing us and the creek a problem.

We can most easily harvest rainwater from roofs (it is more difficult from paving and roads). Roofs make up about a half of the hard surfaces of the catchment, so we could conceivably harvest about 87 million litres in Little Stringybark Creek catchment in an average year (half of the 174 million litres currently flowing down the stormwater pipes)

That is, from roofs alone, we could harvest three-quarters of the water used by the people of the catchment. Even in a very dry year, we could harvest more than half of our water demand from our roofs, so we have the potential to make serious, secure, long-term water savings with rainwater tanks.

If we could keep that 87 million litres out of the stormwater pipes, it would greatly help our chances of adequately treating the remaining stormwater runoff from paving and roads. But even if we are that successful, and completely retain all roof runoff, we still have another 87 million litres of water flowing down our stormwater pipes from the paving and roads of the catchment. If we can manage to retain and treat this runoff from the roads to adequately protect the stream, we are still delivering 45 million litres more to the stream than it received before Mt Evelyn was urbanized.

We cannot possibly take too much water from the Little Stringybark Creek by harvesting roof runoff. We are therefore encouraging people to harvest as much water as they possibly can from their properties.
       

The Stormwater Solution
       

The solution to protecting creeks from the ravages of stormwater runoff is to retain as much of it as possible in the catchment to allow it to filter through the soils, to get to the creek slowly, to provide a more natural, clean base flow. Rain-gardens are the perfect tools for achieving this.

But the large excess volumes of stormwater mean that, if our rain-gardens are to treat stormwater adequately to protect the creek, we also need to harvest and use as much stormwater in the catchment as we possibly can. In the Little Stringybark Creek catchment, it will be impossible for us to harvest enough stormwater to get the creek back to its pre-urban flows.

If we are to restore Little Stringybark Creek, we need to harvest as much stormwater as we possibly can.

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