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Sand Dunes as a Coastal DefenceIn Britain sand dunes are a feature confined to the coast, but in other parts of the world they can form inland as well.There are several basic requirements for the formation of sand dunes, these are:
Dunes normally form at the top of a beach, when the onshore winds blow sand from the lower shore to the top of the shore. The sand builds up at the top of the shore, especially around small objects such as strandline material. These small objects slow the speed of the wind so that it is no longer able to blow the sand. These mounds of sand are colonised by salt tolerant plants, which further reduce the wind speed and increase sand deposition. With increased dropping of sand the height of these 'embryo dunes' increases. These embryo dunes are less frequently covered by the tide and waves and subsequently more plant species are able to grow here. The embryo dunes continue growing, unless they are destroyed by waves or storms. When the dunes are no longer covered by the highest tides, they are generally known as 'mobile dunes'. These mobile dunes are less salty than the embryo dunes as rain washes out some of the salt. Sand continues to be blown from the beach into and over these dunes and the mobile dunes appear to move backwards. Mobile dunes are characterised by areas of sand and a few species of dune building plants. As they get further from the sea the amount of salt in the dunes becomes less and a larger variety of plants can grow. When the dunes are almost totally covered with vegetation they are classed as 'fixed dunes'. Erosion Erosion of the dunes can and does occur naturally often following disease or damage to vegetation. There are two main types of erosion: There are clear changes in beach profile as they become flatter as a result of destructive wave action. The upper beach level falls allowing the high tide mark to reach the toe of the dunes. Waves impact directly onto the dune toe, causing the front face of the dune to collapse onto the beach, and sand is then carried down the beach. These eroding dunes are characterised by steep high frontal dunes. However, erosion can also occur and be accelerated as a result of human activities including excessive trampling, driving vehicles over dunes, removal of sand, mechanical beach cleansing, changes to training banks/coastal defence works and dredging. Sand dunes as a coastal defence A dune based sea defence is much preferable on the undeveloped coast to any man-made system as artificial defences have a limited lifespan and are costly to construct. The dunes, if carefully and sensitively managed, are often self-sustaining. If their condition is monitored, erosion and weaknesses can be better predicted. The dunes act as a buffer against erosion, they form a sand reservoir, replenished when the beach levels are high and released to nourish the foreshore during storm erosion.
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