For some areas a more effective form of protection may be to build groins. These are less expensive and have been used to great effect in some areas. Groins slow down the process of long shore drift, with the up drift side of the structure trapping sand preventing sand from being transported down drift. Groins are built at right angles to the shoreline. It is argued that groins only benefit areas in the up drift regions, and don’t have a positive affects on the down drift areas. If this argument were correct erosion would seem be accelerated in down drift regions, therefore the greater number of groins that are constructed on a stretch of bay the greater the erosion down drift, for this reason groins in some locations are not encouraged. Although Groins do trap sediment, they can redirect beach material off shore. This occurs as currents around the structure force sand outwards to the end of a groin, this changes the topography of the surrounding area making it more complex and can lead to mounds of sand a the sea end of groins. Minikin (1952) suggested that groin arrangement and characters must be suited to the beach concerned. This suggests that groins can be affective if situated properly. Although this method of ceasing beach erosion can be very effective, its drawbacks are obvious. Wherever sand on one beach is increased, other beaches down the coast loss a lot of sand. This process often leads to a domino effect. Once one groin is put into place, another is needed at the beach next door.
With the limited affect of the more technical forms of protection becoming more and more evident, trends of coastal protection have now turned to beach nourishment. This basically is replacing lost sand with sand taken from suitable sized sand banks and from deep seawaters. This is an expensive alternative, and may only works on a short-term basis. The sand used to replenish beaches is often withdrawn at the expense of beaches further up coast. This may not be a problem if the beaches up coast are stable in nature. Sand may be dredged from one point, and may inadvertently be increasing erosion and removing sand from another bay. This problem may arise when not enough observations and sampling are carried out. More often than not the erosion, which first cased the need for nourishment, will act upon the newly placed sand, and the situation will not improve greatly.
Another solution to coastal erosion, which has been discuses widely in recent years, is the construction of off shore breakwaters. Breakwaters are constructed by strategically placing, offshore, a line of large boulders parallel to the shoreline. The effect these have is to limit the energy of the waves before in reaches the coastline and create an area of calm water behind them called a wave shadow. A breakwater off Santa Monica, California resulted in interference with the long shore drift; this produced a large deposition of sand behind the breakwater, and increased erosion down current. Up coast from breakwaters the shoreline may advance but in contrast to this the down drift side of the shoreline can erode quiet severely As well as this not enough research has been carried out as to where the best place to situate them is in respect to the beach, or cliff line.
Conclusion
There are many types of coastal defence schemes each of which have positive and negative affects on protecting land and property from the sea. Whatever man made features are made there is a good chance that they will interfere with the natural movements of beach sediments. This may lead to increased erosion in certain areas eventhough the structure was originally desighned to prevent this problem.
REFERENCES
Paul d. Komar (1976) “beach Processes and sedimentaion”
Bird, E.C.F (1987) “Coasts”
Simon K. Haslett (2000) “Coastal systems”
Karl F. Nordstrom (2000) “Beaches and Dunes of Developed Coasts”
Bird E.C.F (1985) “Coastline Changes”
