in coastal regions and in small islands
Hurricane impact on beaches in the eastern Caribbean Islands 1989 - 1995
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 islandsí
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,
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
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
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
|3. After the storm
The beach has re-established
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 manís interference such as
through the construction of jetties and groynes or the mining of beaches and
dunes for construction material.
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
UNESCOís 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 UNESCOís
Coasts and Small Islands Programme (CSI) replaced
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
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
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.
|Country||Start Year||No. of Sites||Length of Data Record||Agencies Involved in Monitoring|
|Anguilla||1992||37||4 years||Department of Fisheries & Marine Resources Lands & Surveys Department|
Development Control Authority
|B.V.I.||1989||47||1989-1994 6 years data||Conservation & Fisheries Department|
|22||1987-1992 6 years data
1994-1996 3 years data
|Forestry & Wildlife Division
|21||1985-1991 7 years data
1994-1996 3 years data
|National Science & Technology Council
Lands & Surveys Department
Land & Water Resource Unit
Hillsborough Secondary School, Carriacou
|Montserrat||1990||11||7 years||Ministry of Agriculture, Trade & Environment|
Historical & Conservation Society
|One data set only
1991-1996 5 years data
|Southeast Peninsula Board
|1990-1991 1 years data
1994-1996 3 years data
St. Lucia National Trust
Soufriere Marine Management Area
|St. Vincent & Grenadines||1995||17||1995-1996||Soufriere Monitoring Unit|
|Turks & Caicos Islands||1995||16||1995-1996||Coastal Resources Department|
|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.|