Beach erosion on Jamaica's north-east coast. - CONTRIBUTED PHOTOS
SEASONAL EROSION results mainly from bad weather conditions along the coast, such as winter northers and summer tropical depressions and hurricanes. These raise storm waves that erode the beach and either deposit the eroded sand offshore or move it inland, out of the beach area. On an eroding beach, the position of the actual shoreline may recede, or the erosion may only involve the removal of the top layer of sand from the beach area. Usually both kinds of erosion take place and the lateral extent of sand removal may be indicated by the formation of an erosion scarp at the back of the beach.
Normally, in calm weather, any sand deposited offshore is gradually brought back to the beach by gentle wave action. In a stormy year, the recovery of the beaches may be delayed or inhibited. A severe hurricane can cause excessive beach erosion that may not be repaired for several years, and the offshore reserves of beach sand may be swept over the edge of the island shelf into deep water - gone forever. Beach recovery can also be inhibited by poor beach management, such as removing protective seagrass beds, mining beach sand and constructing the wrong kind of hardened sea defences, such as seawalls and groynes.
Apart from the annual cycles imposed by the weather, there are longer-term cyclical changes governing whether beach erosion or deposition will be dominant. These are driven by long-term patterns in the circulation of the ocean's warm and cold currents, and the climate. Some of these may extend over a few years. Others may take centuries to complete one cycle. The pattern of annual hurricane frequency and intensity is one such cycle that appears to extend over several decades.
NOT ALL BEACHES ARE THE SAME
Whether or not a beach will survive a period of unusually intense erosion depends on the nature of the beach and the sediments of which it is composed. The white sand beaches of the north and west coasts are made up of fragments of the skeletons of marine organisms, mainly algae. As the waves wash to and fro over the beach, these particles become abraded and will eventually become so small that they are washed from the beach out to sea. Thus, it is essential for the beach's well-being that the marine organisms supplying this sediment are not killed off by poor beach management. Disposal of chemical pollutants, such as untreated sewage into the sea, and removal of the seagrass that supports the growth of these organisms are both factors that, in the end, will lead to a white sand beach's disappearance.
On the other hand, the beaches common along the south and northeast coasts are composed of sand and pebbles eroded and brought down by rivers from the rocks of the island's interior. If this material does not reach the coastline to replace beach sand lost into deep water through severe storms, or transported along the coast to other places, then these beaches will also shrink and disappear. The intensification of sand and gravel mining in the river beds is one way in which the quantity of sediment reaching the beaches might be reduced. Building dams that trap the sediment on its way to the coast is another.
The amount of erosion that is observed to occur for any given beach will help to determine the 'safe' distance from the shoreline, behind which one can build a dwelling without the fear that the sea might destroy it. This so-called setback distance is defined as the distance from the shoreline behind which it is permitted to build permanent structures, such as houses or hotels. In front of the setback limit it is advisable, even a legal requirement, to build only temporary or easily replaceable structures. Most countries, including Jamaica, impose setback limits by law. Here the recommended limit has recently been revised to a minimum of 50 metres. Legal setback distances are usually set in reference to predictions about the amount of erosion and other coastal damage expected to result from a storm of a particular intensity. The 'hundred year storm' is a popular basis for evaluating this distance. It takes into account an estimate of the amount of erosion that might be caused by the most severe storm likely to occur in any hundred year period. A typical setback is shown in diagram A.
Although it is normal to impose a setback distance that is generally applied to the whole coastline of a region or country, it must be realised that all coastlines are not equal. In addition to imposing a general limit of 50 m, it may be necessary to increase this minimum due to the special conditions of a particular bit of coastline. For example, where a beach or cliff is being rapidly eroded, say because of the weak nature of the rocks in that area, it may be that long-term erosion would eat away the legal setback distance to the point where new houses would be affected within, say, 20 years' time, unless the legal limit were to be increased. In reality each part of the coast needs its own evaluation, but this costs time and money, so a generalised setback is applied in Jamaica. However, in Cuba, the setback distance is defined as the distance from the waterline to a point behind the coastal sand dunes, so the actual setback distance may vary a lot from place to place. This policy ensures that the dunes, which provide a shield against storm erosion, are not degraded by human interference. In Jamaica, coastal dunes are usually subdued or absent. But it is worth noting that, if such a rule were to be applied to Negril, many permanent structures could be sited no closer to the beach than the coastal highway. Our own surveys of historical erosion have shown several places along the Jamaican coast where erosion rates are such that 100 metres or more of beach have been lost over the past 60 years and others where no significant erosion has taken place.
The kind of damage experienced by housing also depends on the nature of the coastline. On low-lying coasts with beaches, damage from storms is likely to result from breaking waves causing beach erosion that undermines the foundations of houses or other buildings, as has happened at places like St. Margaret's Bay and Orange Bay in Portland, and Caribbean Terrace in Harbour View. If a house is built on a cliff of hard limestone, there will probably be little or no erosion of the coastline, but the force of the storm waves surging against the cliff can generate breaking waves that overtop the cliff and damage structures some distance from the cliff top, as happened recently at the West End of Negril from Hurricane Wilma, and has been documented for past storms in such communities as Galina and Manchioneal.
