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Environment
and development in coastal regions and in small islands |
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Environment
and development in coastal regions and in small islands |
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Hurricane impact on beaches in the eastern Caribbean Islands 1989 - 1995
5.
METHODOLOGY
This report covers three countries affected by Hurricane
Hugo in 1989 : Dominica, Nevis and the British Virgin Islands, as well as six
countries which were most severely affected by the 1995 hurricanes : Anguilla,
Antigua-Barbuda, Dominica, Montserrat, Nevis and St. Kitts.
In each country beach monitoring programmes had been
established prior to the hurricanes. Sites
had been selected for monitoring based on the following factors :
Beach dynamics differ between windward and leeward coasts;
Sites where beach and/or dune sand mining has taken place and may
still be occurring;
Sites at beaches which are important for recreation and tourism;
Beaches which may be changing as a result of tourism development
and/or coastal construction;
Control sites where there is no human development.
When the monitoring programmes were initially established,
some islands elected to try and cover all the major beaches, these islands were
British Virgin Islands, Dominica, Montserrat, Nevis and St. Kitts.
In Anguilla and Antigua-Barbuda it was decided to concentrate only on
certain beaches.
At each beach, permanent reference points were established
behind the beach, these act as the starting points for the beach profiles.
Profiles are surveyed from these points along fixed orientations and
finish in the water at the offshore step with usually co-incides with the wave
breakpoint. The equipment used
consists of hand held Abney levels, tape measures and ranging poles.
Staff from the island government and non-government agencies (see Table
1) have been trained in the monitoring techniques.
The profiles are surveyed every three months in most islands.
After a major storm or hurricane the beaches are surveyed as soon as
possible. The data are analysed
using specially designed software based on a series of macros on Lotus 123.
The data are stored at the agencies within the individual islands and at
the COSALC regional
centre at the UPR/SGCP.
The software used in the analysis calculates the cross
sectional area under the beach profile adjusted to a standard vertical drop
below the reference point, as well as the width of the profile, see Figure 4. The programme also draws the profile graphically and these
plots were used to determine the retreat of the dune base or the land edge.
| Figure 4. Beach Profile Parameters |
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Changes in profile area are calculated as percentages.
The first full year of measurement at a particular site is referred to as
the baseline year. The mean profile
area for the baseline year is calculated. Then
the mean for the subsequent years is calculated and averaged.
The two means are then compared and the percentage change determined.
The change in beach profile area is expressed as a positive or negative
percentage, a positive figure indicates an increase in profile area or
accretion, and a negative figure indicates a decrease in profile area or
erosion.
The profile width measurements are treated in a similar
manner. Profile width is measured in metres. The first year of measurement is the baseline year and the
mean profile width is calculated. The
mean width for subsequent years is then calculated.
The difference between the two mean figures is determined and divided by
the number of years to give the change in profile width expressed as metres per
year. A negative change indicates
the profile has narrowed or eroded, and a positive change indicates accretion.
It is important to realise that while a reduction in
profile width is usually accompanied by a reduction in profile area, this is not
always the case. A reduction in
profile width can occur without material loss in profile area simply by a
reconfiguration of the shape of the profile.
This has been demonstrated on the west coast of Barbados (Atherley
et al., 1993). Thus
both parameters should be included in the data interpretation, profile width and
area, as well as the shape of the profile.
In this report beach change trends prior to the hurricane
are discussed. Then the data set
prior to the hurricane are compared quantitatively with the post hurricane data
set to show the direct effects of the hurricane.
The following parameters were determined:
change in profile area (%)
change in profile width (m)
change in position of the dune base/land edge (m).
These three parameters were used to assess the impact of
the hurricanes on the beaches in each island.
Beach recovery is difficult to assess, particularly the
period over which the recovery should be measured.
In this report recovery has been assessed by comparing the pre-hurricane
data set with the data set one year later, provided it was available.
This provides only a limited picture since recovery may continue for
several years and a true picture can only be obtained by analysing the trends in
the years after the hurricane. In
addition it is sometimes difficult to distinguish recovery from normal seasonal
and annual variations.
