Coastal Erosion

Coastal erosion occurs when sediments from upstream fail to replenish shore sediments. The latter are constantly eroded by wave action and, thus, coast lines undergo dramatic change. Reduction of sediments is primarily due to the construction of structures for hydrological modifications and diversions, in particular dams. The effect of dams on the relative ecology of an area is a subject which has been studied in detail. This is due to the dramatic consequences seen in all rivers once large-scale dams have been constructed. Given below is a detailed account of the effects of dams on aquatic habitats.

Dams are constructed for various reasons and very few are built for a single purpose. Agriculture may be viewed as a primary factor, however, as it is responsible for the majority of water abstracted from reservoirs created by dam construction. The effect of damming a waterway is to effectively halt the transport of sediment from upstream of the dam as it settles within the reservoir. This sediment accumulation has a negative effect on the reservoir as over time it dramatically reduces the volume of water that can be stored, thereby reducing the dam's effectiveness. The 'flushing' of sediments from the dam sometimes controls this but can result in a sudden flush of anoxic water and sediments with detrimental results as they smother remaining fertile soils downstream.

Downstream of the dam, lands which were once replenished by fertile silts from upstream, lose fertility and organic matter in topsoil can be reduced. This can increase erosion and sedimentation into the river. However, due to the reduced volume and rate of flow of the river these sediments are not carried as far into the coastal zone as before. Instead they settle in slack areas of waterways and deltas, increasing the need for dredging. It has been estimated that following dam construction there is 2 - 16 times more erosion than was originally estimated and that immediately downstream sediment yields increase by 50% per decade. This effect is due to the decreasing stability of soils immediately downstream of dams through the lack of replenishment of fertile soils.

Silts from upstream not only physically replenish delta areas, but also the fertility of the marine environment, providing nutrition for the basis of many food webs, particularly in relatively nutrient poor seas such as the Mediterranean. These nutrients provide fertilizer for marine plant growth, which, together with organic matter and detritus from land, provide food for fish and shrimps. Dam construction has reduced this source of nutrition and has had deleterious effects on coastal fisheries as their stocks have undergone dramatic reduction.

Lack of sediment replenishment increases the vulnerability of the foreshore areas of delta to wave erosion. Sediments eroded by wave action are now being deposited further upstream as the slower moving rivers no longer have the power to carry them out to the delta front. Such erosion can occur at an alarming rate - 250 metres per annum has been reported from areas of the Nile Delta. As the front erodes it threatens areas of human settlement and other construction. The encroachment of seawater into coastal agricultural land, previously buffered by fresh water, increases soil salinity and decreases fertility. Losses to coastal agriculture of around 10% per annum have been quoted.

Increased sedimentation poses a serious health hazard to human inhabitants of estuaries and coasts as the sediments accumulate toxins, such as heavy metals. This problem is increased as areas downstream of the dam, which were once unsuitable for habitation due to relatively frequent flooding, are now occupied by people attracted by the opportunity of being able to exploit the remaining fertile soils and relatively flat land. The road building, drainage, construction and sanitation works that follow human settlement contribute significantly to the erosion of the area.