From Mike Muller, Department of Water Affairs and Forestry
“It is frighteningly clear” that Fred Pearce has not understood the extreme
nature of the recent southern African floods
(25 March, p 16).
It has not just been a wet year down here, with 75 per cent more rain than
normal. Years with 75 per cent more (or less) runoff than average are the norm
in our highly variable rivers. What was abnormal was to have twice the average
annual flow going down the Limpopo (to quote one example) in just eight days.
Even in our terms, this is an extreme event.
It was the exceptional rainfall that resulted in the unprecedented floods.
The volumes involved would have filled all the dams in the river basin concerned
five times over even if they had been completely empty to start, with minimal
effects on flooding.
Dams built to store water to keep farmers going during inevitable dry periods
will damp down “normal” flood flows even if they are full, as any first-year
hydrology student will confirm. But to suggest that they could somehow be
managed to mitigate floods of the severity of those recently experienced is,
politely put, misleading.
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We hope that an independent UNESCO-led review of the flood events will be
carried out at the request of Mozambique’s government. What we believe this will
show is that the flooding was an extreme event, that no actions to manage dams
or catchment land use would have had much impact, and that the appropriate
response is to strengthen existing monitoring and early warning systems, to
control settlement on the flood plains (so called for good reason) and promote
activities to limit human and economic casualties.
We also hope for guidance on the likelihood that global climate change will
mean this kind of extreme event becomes more common in the future.
Pretoria, South Africa
