Queensland Flood Commission says to lower Wivenhoe Dam levels to 75% on basis September forecasts
August 3rd, 2011 by Warwick HughesThis of course assumes that a reliable forecast exists – on which to base this proposed very serious action. The only forecasts I am aware of are the BoM Rainfall Outlooks which are published usually after the middle of the month and cover the next 3 calendar months.
I have just checked the BoM September Rainfall Outlooks for the 11 years of their archive, for the Brisbane catchment area. I make the score 5 wins, 5 failures and 1 too close to call.
If valuable dam water is to be released from say October onwards – on the basis of useless BoM 3 month forecasts – then it is inevitable there will be a considerable risk that this will be in vain.
But it would help to keep upward pressure on the price of water.
From mid 2010 I have several articles critical of BoM Outlooks:
BoM incompetence again – for the 5th month in a row
Ongoing BoM utter incompetence
#2 Ongoing BoM utter incompetence
#3 Ongoing BoM utter incompetence – month after month
#4 Temperature too – not just rain – ongoing BoM utter incompetence – month after month after month
#5 BoM Spring Outlooks – rain and temperature – more hopelessly wrong models
Posted in BoM Australia, Water | 2 Comments »
August 7th, 2011 at 9:17 pm
5 wins, 5 losses and 1 too close means their forecasting is basically random, which means that the science is wrong, or at best incomplete.
August 10th, 2011 at 3:31 pm
There are a number of problems with Wivenhoe Dam. One is the contradiction in its dual function of flood mitigation and water storage. For it to be used for flood mitigation it has has to be held at reduced capacity eg 75% but its water storage function requires that it be held at full capacity.
Another problem is that Wivenhoe relies on extreme weather events to be filled and these are random events occurring on average 3.7 years but can be as long as 11 years. But these are also the events one has to mitigate because they cause the flooding. The same weather events which are necessary for filling the dam can also be the source of the flooding!
As I have pointed out in an earlier comment, Wivenhoe was built in reaction to the 1974 floods. BUT the flooding in 1974 occurred in sub catchments below the site of Wivenhoe Dam. Most of the 1974 flooding was caused by record flooding in the Bremer, Lockyer and lower Brisbane sub catchments which Wivenhoe is powerless to mitigate!
So if we get another flood in the sub catchments below Wivenhoe keeping it at 75% or 50% capacity will be useless as far as flood mitigation is concerned and it may compromise its water storage function!
Half the “big floods” have been caused by the sub catchments below Wivenhoe – the second 1893 and 1974 floods. The first 1893 and 2011 floods were all Wivenhoe sub catchment floods.