Human impacts at the coast intended to stabilise a coastline can also lead inadvertently to increased coastal erosion. Discuss.

The construction of sea walls is one of the most popular methods of coastal defence employed.  Sea walls tend to work when it comes to protecting the cliffs or settlement situated behind it but it can also have a damaging effect on the natural defence that is the beach.  If the high water mark does not reach the sea wall then generally it will work very well as a defence.  If however the sea water comes right up to the sea wall then the waves begin to be reflected back which will cause a process of undercutting the beach known as scour.  The effects of scour tend to happen fairly quickly and the beach will gradually become lower and lower till, in an extreme circumstance, there is no more beach or only very little visible at the low water mark (see diagrams (c) (d), (e) below).  An example of a place where a sea wall has caused increased coastal erosion is near Brighton, England.  Cliff top houses were being threatened by the collapse of the cliff due to coastal erosion so a large sea wall was built and a series of large groynes.  The houses on the top of the wall are now protected but the beach below has suffered.  The beach has not lowered as much as it would have if there were no groins but it has shrunk in its size.  The addition of groynes with the sea wall however has starved downdrift beaches of sediment though resulting in increased beach loss and cliff collapse.

Deforestation and fires, grazing of sheep and cattle and the use of off-road recreational vehicles along the cliff tops, beach edges and dunes also play a major part in increasing coastal erosion.  With all processes a lot of vegetation is killed off, much more so with burning.  The effect of none or little vegetation will result in the removal of sand by wind which can cause grey dunes or even younger fore dunes to disappear leaving no natural defence behind the beach causing the land behind it to have the full force of the sea upon it thus increasing the erosional processes on the land.

Dredging a beach for heavy minerals or to provide aggregates for construction can have a very dramatic effect on the coastline as a beach is the best form of coastal defence and if it is removed or changed the effects could be catastrophic.  If there is little or no beach the rate of cliff collapse can be extremely fast, especially if the cliffs are made of a weak porous stone.  An excellent example where dredging a beach has caused dramatic erosional effects is in Hallsands in Devon, England.  In 1887 the beach was dredged to provide material for the construction of the docklands in Plymouth.  So much shingle was taken away that the beach showed no signs of natural replenishment, this resulted in further loss of sediment and in consequence the shore level was reduced by approximately 4 metres.  As the beach was no so little the waves had a direct effect on the cliffs behind and in the period from 1907-1957 the cliff had retreated by 6 metres.  The village is now in ruins as houses and infrastructure on the cliffs has collapsed into the sea (Goudie 2000).

It is not just human processes directly on the coast that result in increased erosional processes.  Retaining walls on river mouths can also lead to increased erosion.  Two things can happen at a retaining wall. Firstly the retaining wall could act in a similar fashion to a sea wall creating scour and a loss of sediment as the base of the wall.  Secondly the wall could interrupt longshore drift in a similar way to a groyne resulting in a loss of sediment downdrift therefore there will be increased coastal erosion downdrift.

There are many ways in which human activity impacts the coastline but it is however humans seldom attempt to accelerate coastal erosion deliberately.  Defences like some of those described above are placed in good faith that it is the right thing to do in order to protect that particular stretch of coastline.  However, although most of the time the defence in place does protect the coastline it is intended to protect there is often a huge increase in erosional effects downdrift from the protected site as one way or another the area may be starved of sediment for example.  It would seem from evidence above and many other case studies that the best option when it comes to protecting the coastline is to leave it alone as the best method of protection is a broad beach.  If it comes down to it though and a defence must be built only wait until it is very necessary and the defence that is built must be built with many factors included in its equation.  The main factors that need to be taken into account are direction of longshore drift, the layout of the coastline downdrift and the sizes of settlements, if any, downdrift.  The last point is a very important one as, although no one likes to see coastal erosion, it would not matter as much if it took place downdrift where there were no settlements.

Bibliography

Nordstrom, K F.  (1994) Beaches and Dunes of Human Altered Coasts.  Progress in

Physical Geography

Goudie, A.  (2000) The Human Impact on the Natural Environment