Page:Niger Delta Ecosystems- the ERA Handbook, 1998.djvu/99

Sand-Barrier Islands

containing swamps, small rivers or, on the estuarine edges of the islands, tidal creeks. (Plan and Section diagrams of a sand-barrier island are given in Figures 3A.)

8.2 THE BALANCE BETWEEN DEPOSITION AND EROSION

Long-shore drift involves erosion and deposition of sediment. Formation of new islands depends on the amount of sand deposited being greater than the amount of erosion.

Erosion by rivers: the strength and speed of flowing water is able to wear away the material of the riverbank and bottom, and carry it as sediment, in smaller or larger particles. The action of the moving sediment itself acts as an abrasive which may accelerate further erosion downstream. When the river slows down sufficiently, the sediment is dropped; heavier particles are deposited first, while the smaller lighter particles are carried further. Sediment may be repeatedly eroded and deposited over shorter and longer periods of time. Erosion by the sea: the physical action of storms, currents and waves may loosen and break up the material of cliffs and beaches. This debris may then further add to the abrasion. Other forms of coastal erosion include the compression and release of air trapped in joints and crevasses by repeated wave action, the chemical solution of rocks in sea water, and salt crystallisation that may then loosen rock fragments and make them vulnerable to wave erosion.

This balance between deposition and erosion is affected by long-term geological factors. Over shorter time periods, variations in rainfall and in human activities will differentially affect different parts of individual islands and different sub-ecozones. The two human activities that have the most profound impact on this balance are forest clearance and the construction of dams.

#Forest Clearance

Clearing an area of forest vastly reduces the amount of vegetation protecting the soil from rainfall damage. It also largely removes the 'sponge' that moderates the release of rainwater into river systems. The result is increased erosion and increased sediment loads carried by rivers, and therefore an increase in the amount of sediment available for deposition. There may also be an increased risk of flooding, for two main reasons.

• Firstly, because the forest 'sponge' is no longer acting, rainfall runs straight off the land into streams and rivers. These may now disappear during the dry season, but break their banks or overflow in the rainy or wet season.
• Secondly, especially where river flow is sluggish, some of the increased sediment load may settle out on the riverbed. As the riverbed rises, the volume available within the banks for the actual flow of water is reduced and again the risk of overflow is increased.

In many areas of the world, including parts of China, India and Europe, forests have been cleared and entire landscapes devoted to agriculture for hundreds or even thousands of years. In some places, earth walls or dykes have been built to contain floods, and sometimes riverbeds are now actually higher than the surrounding land. When such rivers break their banks, the floods can be catastrophic and may not only destroy fields and buildings but cause the deaths of thousands of people.

However, just as forest clearance can accelerate erosion and increase the sediment load of a river system, the construction of dams often reduces this load by

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