A little rain, a lot of disturbance

In past posts about my rain-garden system, I’ve tended to focus on the water saving, food production and flood-mitigation benefits of systems like ours.  But my main motivation and interest has always been how such systems can make our local rivers and streams healthier. Today, with a (very) little rain I witnessed, on my ride to work, an example of the problem with not having stormwater retention systems in our catchments.

I updated my rain-garden diary the other day, noting how dry the last 6 weeks have been (tank empty for the last week, both rain-gardens still going strong without any need for mains water [although the front rain-garden needed a couple of top-ups from the charged downpipe]).  After such a long dry, warm spell, we have enough capacity to capture more than 30 mm of rain should it arrive.  This morning we received a disappointing 1.8 mm, just enough to refill the downpipes and add 100 L or so to the tank.

What’s important here is that our property produced zero runoff to the street drainage (and therefore to the river) today.  The parks and gardens of the city gratefully soaked up the 1.8 mm of rain and didn’t produce any runoff either.  Any tank or raingarden in the neighbourhood would have produced zero runoff.  But the roofs and roads without retention systems downstream produced a lot, and carried a cocktail of nasty pollutants to the river.  Here is a picture of the Hawthorn main drain this morning, producing a plume of soapy, polluted water to the Yarra River.

Stormwater retention systems can (and should) easily be designed to retain small storms like this.  Small rain events of up to a few mm are the most common types of events, and we can greatly reduce the frequency of disturbances like this to our valued waterways like the Yarra, reducing the risks of disease to people who want to swim or fish in the river and the bay downstream.

We really do not need a drainage system that sends this sort of nastiness to our rivers and bays after less than 2 mm of rain.  It is time we started thinking of the stormwater drainage system as a problem—a threat to the health of waterways and bays—and started doing drainage differently.

Leave a Reply