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 storm 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 1958 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 the 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: Hurricane 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’ by 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 and physical backdrop, the impact of tropical cyclones, induced storm surges and earthquake- caused tsunamis is carastrophic (Table 1 b). well- intentioned control ofmonsoonal flooding, to prevent the loss of exiting crops and to promote multiple cropping, prevents natural fluvial sediment inputs reaching inter-distributary areas. These inputs normaly counterbalance the high rates of sea-level rise which result from sediment compaction and downwarping under sediment loading in the delta. Dominance of subsidence threatens agriculture further by leading to the loss of land by marine erosion and increased saltwater intrusion of the freshwater lens. Only imaginative schemes to stabilise the delta through mangrove plantings offer the possibility, following vertical swamp growth, of creating new land for agriculture.
All these problems need to be viewed in the context of present trends in sea level. Sea-level rise is predicted to accompany the `greenhouse effect’ as a result of the combination of seawater expansion due to ocean warming and enhanced ice-melt of mid-latitude glaciers and ice sheets in a warmer world. Current estimates of sea-level rise represent considerable downward revisions of the catastrophic flooding predictions of the mid-1980s, but uncertainties remain, not least over the behavior of the Greenland and Antarctic ice sheets.
Immediate effects from sea-level rise will include (I) shoreline recession; (ii) saltwater intrusion of the freshwater lens- with implications for agriculture, water supply and, in extreme cases, human habitation; (iii) distribution of ricer systems adjusted to former baselevels, and increase in the freaquency and extent of coastal flooding.
Future physical changes in coastal zone will be taking place in environments severely altered and pre-stressed by 200 years of postindustrial human activity. The understanding and prevention of coastal flooding in future will therefore require not only use of analogues from the reconstruction os past environmental change but also the detailed monitoring of present processes.