Coastal flooding

Coastal flooding: transient and permanent

Coastal flooding: transient and permanent

Fifty percent of the US population lives near the coast and fastgrowing Third World populations show strong migratory tendencies towards coastal plain cities. Thus a knowledge of coastal processes and the mitigation of the hazards inherent in living and working in coastal zone should be a key element of future environmental planning strategies, particularly given the accelerated rise in highly dynamic areas and is subject to rapid fluctuation of elevation due to sstorm surges and tsunami waves generated by earthquake shocks and volcanic explosions. In the longer term, sea- level is a response to climatic change, regional tectonic, basin sedimentation and the continued adjustment of the Earth’s crust to the redistribution of ice and water loads following the end of the last glaciation.

In many parts of the world holding the shoreline at its present position requires large coastal defence structures. This approach is well illustrated, by the Dutch Delta Plan, initiated in 11958 after 1,835 people in the Netherlands lost their lives in the 1953 North Sea floods and not completed until 1986. A series of barriers, designed to withstand a 1 in 5,000 year flood, now close the northern tributaries of tthe Rhine-Meuse-Scheldt delta. Such schemes are expensive- the East scheldt barrier alone cost/ 2.3 billion (1986prices)- and require a commitment to ongoing repair and maintenance. They may also generate new problems: part of high cost the Scheldt barrier was due to the need to provide 65 movable gates to preserve tidel ecosystems that would have been lost to a freshwater lake had a cheaper ficed dam been constructed. Furthermore, such structures may increase the risk of inundation by interfering with natural processes of sedimentsupply and transport thereby leading to the loss of important beach and marshland buffer zones in front of artificial barriers. Infrastructural development behind supposedly secure defences means that when such structures are breached replacement costs are high: HHurricane Hugo caused US 7 billion worth of damage in South Carolina (USA) alone in 1989 and the cost of the 1992 landfall of Hurricane Andrew in southern Florida and Louisiana is expected to exceed thisfigure. A less costly and more ecologically sound strategy is that of `managed retreat’. This approach is currently being evaluated by English Nature at Northey island on Essex’s Blackwater estuary: here, by artificially breaching the most seaward wall, the shoreline has been ` set back’ bby 75m, allowing tidal inundation and creation of new saltmarsh in the abandoned zone between the old new defence structures.

Fro some of the world’s poorest countries such flexibility does not presently exist. In Bangladesh, rapidly growing rural population is increasingly squeezed into a highly dynamic physical environment. The large river discharges of the Ganges-Brahmaputra system result in distributary channels moving laterally at up to 200myr-1 and massive sediment supply to a delta front subject high wave attack (mean wave height = 1.4m) and high tidal range (4-9m) Against such a social ...