Monday, January 17, 2011

Lessons from flooding

I fear that lessons from the Brisbane catchment flooding may not be heeded for the Murray Darling basin at the moment.

The main thing I have seen is that, for the Brisbane River at least, it is enough to refine procedures (or the rule book, as I have heard it called in the media) to have made the catastrophic major flood of the lower Brisbane river a moderate flood (albeit a longer lived moderate flood). The rule book seems to have been written at the time the Wivenhoe was built, and it essentially awaits for upstream data of inflows to enable releases in advance to allow room for incoming floods. This was fine when weather predictions days in advance were highly unreliable (as they were in the eighties) or for inflows of a volume that is not several times the capacity of the dam, nor when "La Nina" conditions point to aggressive early releases being more "conservative", than a wait and see what runoffs eventuate policy.

This lesson, which I am sure will be heeded for the Brisbane River catchment because catastrophic floods make it obvious, seems to be being ignored for the Murray-Darling. At the moment, the calculations are quite similar - with the Menindee lakes at over 100% and three major floods on their way in the next few months.

In 1956, another La Nina year, catastrophic floods hit the lower Murray when floods of the Darling and Upper Murray converged at the same time - Similarly a reaction was to build flood mitigation storages (in that case the Menindee lakes). They really should be emptied at a much faster rate right now, not just to absorb the peaks coming down already, but to have enough capacity to hold back the flood such that it doesn't coincide with possible autumn upper murray floods. The analogy with the Brisbane catchment is that the Wivenhoe may have been able to actually slow down the outflow as the Bremer and Lockyer floods peaked through, mitigating them rather than exacerbating them (If enough pre-emptive releases were made).

Of course, the bigger the flood, the less dams can feasibly hold them back, regardless of strategy.

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