Environment and development in coastal regions and in small islands

Hurricane impact on beaches in the eastern Caribbean Islands 1989 - 1995

3.  BACKGROUND  

3.1 Beach Changes

Beaches are one of the most dynamic and fast changing environments existing in nature, they can change significantly in a matter of hours, such changes may be reversible or they may be one-way changes.  In the islands of the eastern Caribbean, the coastal zone, of which the beach is a sub-system, is a very important area where much of the islands’ infrastructure and industry is concentrated.  In most islands the beaches form an important part of the tourism product and an area for local recreation.

Most beaches consist of sand or stones or a mixture of both.  Sand beaches may be composed of coral sand or terrestrial sand, the latter may be volcanic or silica based.  Stone beaches may consist of coral fragments or volcanic stones/boulders.  Sometimes the beach material may change after a particular storm e.g. at Rockaway Beach on the west coast of Dominica, during the high winter swells the sand used to be stripped from the beach and replaced with stones and boulders, while in summer the constructive waves would bring the sand back.  This seasonal variation ceased after Hurricane David in 1979, since that event stones and boulders predominate all year (James, 1996).

Figure 1 shows a typical beach profile or cross section.  The beach system extends from the offshore zone to the land behind the beach.  The offshore zone extends from the low water mark out to a water depth of 14 m (46 ft).  The sand or rock material in this shallow area near the beach is part of the natural reserve of the beach.  Near the point where the waves break there is a feature known as the offshore step which may not always be present.  The beach itself is divided into the foreshore and the back beach.  The foreshore is the area between high and low tide marks.  Above the foreshore there is sometimes a small crest, called a berm crest.  Beyond this there is the rest of the back beach which extends to the permanent vegetation line.  Behind this there are the dunes, cliff face, rock outcrop or sometimes a low platform.

Figure 1. Typical Beach Cross Section

Many of the islands of the eastern Caribbean have their long axes in a north-south direction.  There are characteristic difference between the beaches on the east (windward) and west (leeward) coasts.  The east coast beaches are exposed to the waves generated by the Northeast Trade Winds and are usually wider.  The west coast beaches, in contrast, are generally narrow and steeper. The material of which the beach is composed also influences its shape, in general the coarser the material, the steeper the slope.

Beaches change as a result of waves, tides and currents.  Waves are a function of wind speed, wind direction and fetch (area of open water over which the wind blows).  Since the Caribbean Islands lie within the regime of the Northeast Trade Winds, waves generally approach from directions between northeast and east, although there is some variation throughout the year.  These winds are strongest from December through March and from June to July, thus these are the periods of highest wave energy on the east coast beaches.  The west coasts of the islands are sheltered from the Trade Wind waves, however, they do receive some wave energy as the wave fronts are "bent" around the tips of the islands. 

During the winter months, October to March, the Eastern Caribbean Islands often experience high waves (swells) as a result of extra-regional storms.  Very intense low pressure systems in the North Atlantic Ocean generate storm waves which travel south as swells to affect the islands.  Such swell waves can travel thousands of kilometres.  These waves approach from a northerly direction and are therefore felt on north, west and east coasts.  Their effects are most severe on the west coasts since these coasts experience only low wave energy for most of the year.  About ten swell events can be expected on average each winter season with each event lasting between two and seven days. 

Hurricanes are the most severe storms to affect the islands, these occur during the period June 1st to November 30th, with August and September being the most active months.  Hurricane damage results from the high winds, high waves, storm surge and heavy rainfall.  A fully developed hurricane may result in a raised water level (storm surge) of more than 1 m e.g. computer simulations indicate that the height of the surge in Antigua during Hurricane Luis in 1995 was between 2.4 and 3.7 m (8 - 12 ft).  Hurricane waves are the most damaging waves to affect beaches.  A fully developed hurricane could develop deepwater significant wave heights of 8 m (26 ft), this represents the highest third of the waves, with a maximum wave height of 16 m (52 ft).  Such waves cause catastrophic beach erosion, often sand may be washed offshore into water so deep that normal constructive waves can never return it to the beach system.  Hurricane waves may also damage seagrass beds and coral reefs which stabilise and provide a source of beach sediments.

Besides the onshore/offshore movement of material, sand is also moved along the beach especially when the waves approach the beach at an angle, this is called longshore drift.  Oceanic currents and tidal currents also affect beach processes, but to a lesser degree than wave processes.

As a result of all these processes, beaches change from day to day, month to month, year to year.  Beaches may get smaller (erosion) or larger (accretion).  It is useful to view erosion and accretion as changes in direction.  Reference to Figure 2 shows the position of the beach prior to a storm (a).  The second profile (b) shows the beach immediately after the storm, note that the beach has been eroded and the sand has moved offshore to form a small bar.  The third profile (c) shows the beach a few weeks later when constructive waves have moved the sand back onto the beach.  The beach in profile (c), looks to the casual observer, much the same as it did before the storm.  However, the actual position of the beach has changed, it is further inland.  Thus erosion may be defined as a movement of the beach form inland, similarly accretion may be defined as a seaward movement of the beach form.

Figure 2. Diagram of Beach Changes Before and After a Storm
1. Before the storm
2. During the storm The waves reach higher up  the beach and start eroding  the beach and dunes and  depositing the sand offshore.
3. After the storm The beach has re-established  its pre-storm profile, it looks the same as before the storm,  but its position is further inland.

Beaches change seasonally as a result of variations in wave energy.  Usually Caribbean beaches show erosion in the winter months and accretion in the summer.  However, summer is also the time when tropical storms and hurricanes occur, these rarer events usually cause severe beach erosion, which therefore disrupts the normal seasonal cycle.  Beaches may also change on a long term basis.  Beach changes may be naturally induced or a result of man’s interference such as through the construction of jetties and groynes or the mining of beaches and dunes for construction material.

3.2  Coast and Beach Stability in the Lesser Antilles (COSALC) Project

The COSALC project was started in 1985 in the small islands of the Lesser Antilles by UNESCO’s COMAR (Coastal Marine) project in response to a request from these islands for help with serious beach erosion problems which impacted their vital tourist industries.  In 1994, the University of Puerto Rico Sea Grant College Program (UPR/SGCP) became a co-sponsor and coordinating centre for the project.   In 1996 UNESCO’s Coasts and Small Islands Programme (CSI) replaced COMAR.  The goal of COSALC is to develop the in-country capabilities within the island states to measure, assess and manage their beach resources within an overall framework of integrated coastal management. 

One of the specific activities has been to help the islands measure and assess their beach resources and to monitor on a regular basis how the beaches are changing over time.  Beach monitoring programmes have been established in each island, local government personnel measure their beaches on a regular basis using standard surveying techniques.  The data are analysed and interpreted to determine local beach changes, this information is then incorporated into the planning process to assist decision making on issues such as sand mining, the positioning of new coastal development, sea defence design and environmental impact assessments.

Beach monitoring programmes are now running in eleven island states : Anguilla, Antigua-Barbuda, British Virgin Islands, Dominica, Grenada, Montserrat, Nevis, St. Kitts, St. Lucia, St. Vincent & the Grenadines, Turks and Caicos Islands.  Table 1 lists the countries, the dates the monitoring was initiated, the number of sites monitored, the length of the data record and the agencies involved in each island.

In some instances monitoring programmes were suspended for a year or longer, the figures in brackets in column 2 refer to the year when monitoring was resumed. 

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