From ROBERT J. NICHOLLS, KAREN C. DENNIS, STEPHEN P. LEATHERMAN
Your recent article on ‘The rivers that won’t be tamed’ (13 April) provides
an interesting commentary on efforts to ‘tame’ nature in dynamic natural
environments and their likely negative consequences. However, it fails to
note one of the most serious problems associated with conventional flood
protection schemes in deltaic environments: namely subsidence. Deltaic environments
naturally subside and the land surface can only be maintained at a constant
elevation if sedimentation occurs. If the sediment is prevented from accumulating
by levees or dikes, the ground sinks, producing a relative rise of sea level
which in the relatively flat deltaic environment causes large coastal land
losses. This is happening in the Mississippi Delta, Louisiana, where locally
sea level is rising and there is the largest coastal erosion problem in
the US.
Bangladesh is always cited as one of the major losers to accelerated
sea-level rise and, if successful, these flood protection schemes can only
exacerbate these problems. (Loss of the sediment supply due to upstream
dam construction would be equally serious). We support Fred Pearce’s suggestion
that soft methods of flood control should be investigated as an alternative
to hard stabilisation.
In this regard, it is interesting to note that large-scale water (and
hence sediment) diversions are being constructed on the Mississippi to flood
some of the areas protected by the levees in a controlled manner. It is
hoped that this will reduce wetland loss.
Let us hope that Bangladesh can learn the lessons of the Mississippi
Delta and manage the problems of flooding in the delta in a manner which
does not increase the future vulnerability of the country to sea-level rise.
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Robert J. Nicholls, Karen C. Dennis, Stephen P. Leatherman University
of Maryland at College Park, US
