Wetlands associated with rivers reduce
storm flows by absorbing excess surface
water and releasing it slowly. In this way, flooding is localised in an environment
adapted to high water tables, and the potentially damaging effects of floods further
downstream are reduced. Removal of wetlands into which excess water can drain,
therefore, increases the frequency and magnitude of floods. All around
Wetlands may also be important in recharging
aquifers. In southern Florida, for
example, the Everglades wetland system overlies a large expanse of porous
limestone, the Biscayne aquifer, into which much of the water seeps. The area is
very low-lying and channelisation and drainage during the 20th century have
disrupted this recharge, allowing intrusion of saline water from the sea, a process
compounded by heavy extraction from the aquifer for domestic supply.
Saltmarshes play a valuable role in coastal
protection, dissipating the energy from
storm surges and thereby reducing the requirement for expensive artificial flood
defence structures. Saltmarshes are accreting environments, the vegetation
slowing water flow over the marsh so thai the sediment burden of the incoming tidal
water; is deposited. This often leads to a marked change in level at the edge of the
vegetated marsh compared to the adjacent tidal flats, this 'micro-cliff and the
extensive areas of gently sloping marsr acting to dissipate wave action and so
preveni erosion of the coast at the high water mark Concern has been expressed
that increasing se£ levels will cause submergence of the marsh and sc reduce its
ability to provide coastal defence while, in other areas, establishment of new
Saltmarshes is being activel) encouraged, by flooding coastal meadows, ir order to
provide a coastal defence function.
