I think part of the problem is the propensity to have a "flash" flood.
If you reduce the ratio of water permeable ground (grass, trees, etc) to
non-permeable ground (concrete, roads, roofing, etc) the runoff from a
sudden downpour tends to rush to the waterways, causing a sudden and
violent torrent. Tanks would help to hold this up, by taking out the
first rush of water (which may or may not be the whole amount). A
prolonged rain event would tend to be less violent and would still
refresh the waterways, clearing out areas of stagnant or more toxic
water.
This doesn't solve the whole problem, which would involve diverting as
much runoff from all sources to more extensive land as possible.
Difficult in an increasingly dense urban environment.
Bill
-----Original Message-----
From:
On Behalf Of Peter Shute
Sent: Monday, 25 August 2008 11:48 AM
To:
Subject: Water tanks and wildlife (Victoria)
There is, apparently, a proposal to "drop" water tanks as part of
Victoria's water strategy. This is front page news in today's Age:
http://www.theage.com.au/national/desal-and-water-tank-wars-20080824-41e
t.html
and there is a full page about it in the Opinion and Analysis section of
the paper (not online yet).
They suggest that previous studies underestimated the amount of water
household tanks could collect by 2/3, so their installation should be
encouraged, not halted.
The main article says:
"Tanks can be important in reducing stormwater run off, which damages
and pollutes waterways."
Does this sound right? I would have thought that if you remove the
fairly clean roof component of the flows in urban waterways, but still
allow runoff from the roads, then the water that does reach the local
creeks will be far more polluted than before.
I've been wondering what effect it would have on the volumes reaching
local waterways if all roof runoff was removed, as the tank proponents
seem to be advising. I'm guessing at least a 10% reduction, far more in
inner urban areas. So are we really "saving" water by doing this, or
just stealing it from the local crakes and rails (etc)?
Does anyone have any thoughts about this, and whether I need to worry
about it at all? Should this issue be part of the water tank debate, or
is the possible effect too small to matter?
Peter Shute
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