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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
8.
DISCUSSION
Hurricanes are major events in the islands of the eastern
Caribbean, not only as regards beach changes but also as a major disruption of
day to day activities. They affect
a country’s economic, social and environmental well being.
Even the number of days spent preparing and waiting for hurricanes that
never come is a normal part of life in these islands.
Population increase and infrastructural development are
associated with growth and development. Twenty
years ago, the number of people living near the coast and the amount of coastal
development was considerably lower than it is today.
With so many of the islands depending on tourism as a major income
generator and with the region facing the likelihood of increased hurricane
activity in the future (Gray et al.,
1996), the need to plan for
hurricanes has never been greater.
Various regional agencies and projects help the islands in
the field of disaster preparedness and mitigation.
Both the government and private sector are affected by hurricanes and it
necessary for all to learn from the lessons of 1995 and the predictions of
increased hurricane activity for the next twenty years.
The data presented in this report has shown that a major
category 4 hurricane passing within 180 km (113 miles) of a particular island
will cause major erosion to the beaches of that island.
This was seen after Hurricane Hugo in 1989 and again after the hurricanes
of 1995. When the data for a
particular island are averaged, then in every case it can be seen that serious
erosion of the beaches has taken place.
However, this is not to say that every single beach in a
particular island experiences erosion. In
almost every island it was found that there were some beaches where accretion
took place. At some beaches, one
end would erode and the other end accrete as huge volumes of sand were moved
around. Notwithstanding these local
variations, the overall trend in each island was for erosion.
While proximity to the hurricane centre is an important
variable influencing the degree of erosion, it is not the only factor.
This was seen in 1989 after Hurricane Hugo. This hurricane passed a similar distance away from Dominica
and the B.V.I.
Yet the beach damage
in Dominica was much more severe than in the B.V.I. Other factors influencing the degree of damage include
coastline shape, the width of the offshore shelf as well as the characteristics
of a particular hurricane such as the position of the strongest quadrant.
For instance the strongest sector of Hurricane Hugo was the southwest
quadrant. Since the hurricane
passed north of Dominica, the southwest quadrant impacted that island, but the B.V.I.
was affected by the northern quadrants, hence the damage was less in this
island group.
For the two islands, Dominica and Nevis, where beach
change data exist from Hurricane Hugo in 1989 and the 1995 hurricanes, it can be
seen that the average amount of erosion experienced in these two islands was of
a similar order of magnitude in 1989 and 1995.
In both years, major category 4 hurricanes were experienced.
Furthermore, in Dominica and Nevis it was found that the
pattern of beach changes was similar during both 1989 and 1995.
For instance in Dominica, although almost the entire west coast was
eroded in 1989 and 1995, one beach, Batalie, received little damage. In 1989 it accreted, while in 1995 it showed only slight
erosion. Similarly in Nevis, the
most severely impacted beaches in 1989 and 1995 was Pinneys Beach on the west
coast. This begins to provide a
useful predictive tool for planning beach resources in a particular island.
Examination of the data from the 1995 storms has shown
that two parameters are particularly useful in assessing hurricane impacts on
beaches : profile area change and the change in the position of the land/dune
edge. Profile width was not always
a useful parameter. This was
evident particularly in Barbuda, one of the most severely impacted islands.
Here the extensive sand dunes behind the beach were eroded and much of
that reservoir of sand was transferred to the lower part of the beach and the
surf zone. In other cases profile
width did not change significantly although there were major changes in profile
area and the size of the sand dunes.
The change in profile area provided a good indicator of
the amount of beach erosion and could be related to the proximity of the
hurricane centre. Similarly there
was a clear relationship between the retreat of the land/dune edge and the
proximity of the hurricane centre. The
data indicated a threshold at a distance of 40 km from the storm centre.
Between 0 and 40 km from the centre of the hurricane, the dune/land edge
retreated inland between 18 m and 5 m. Between
40 and 180 km from the hurricane centre, the dune/land edge retreated between 5
and 2 m.
This retreat of the land or dune edge is seen as a
permanent change in the coastal zone. Moderately
high dunes such as those in Anegada, Anguilla, Antigua and Barbuda take decades
to form as the wind moves the sand across wide dry sand beaches to accumulate
behind the beach. Against a
predicted background of rising sea level and increased hurricane frequency, it
is unlikely that these dunes will be rebuilt to their former extent. Where the beach is backed by a low sand platform, as in
Nevis, or a low soil platform as at some beaches in Dominica, data from
Hurricane Hugo has shown that once the low sand/soil platform behind the beach
is eroded back in a hurricane, this becomes the new coastline.
This has very important implications for coastal planners
and for coastal land owners. Attempts
to re-establish and maintain the pre-hurricane coastline will most likely end in
failure and loss of the beach in the medium term (5 - 10 years). The pre-hurricane coastline is now part of the active beach
zone.
The loss of coastal land also has implications for new
developments in the islands and the design of coastal development setbacks.
In the light of hurricane predictions for the next 20 years, it appears
likely that each of the east Caribbean Islands will be impacted by a major
hurricane (category 3 or higher) over this period. This does not mean that a hurricane centre has to pass
directly over every island, rather as has been seen in this report, significant
erosion of the land/dune edge may occur when a major hurricane passes within 180
km of an island. Thus the likely
erosion to be expected from a major hurricane should be built into coastal
development setback guidelines. This
report can provide the necessary database to calculate this component of a
setback.
After Hurricane Luis, a setback methodology was designed
for Anguilla which included hurricane impact as well as six other factors.
The actual retreat of the dune/land edge during Hurricane Luis was built
into the setback calculation. Other
factors included : historical coastline changes, recent beach profile
measurements, sea level rise, offshore features, coastal geomorphology and
planning considerations (Cambers,
1996b). These
seven factors were used to design setbacks for each sand beach in Anguilla using
the "permanent" vegetation line as the baseline.
If these setbacks are implemented, then damage to the beaches and new
coastal infrastructure should be less severe in the eventuality of another
category 4 hurricane hitting the island.
The 1995/96 database showed that the beaches had recovered
to 90% of their pre hurricane level in four islands, eight months after the
hurricane. This is particularly
important as regards the post-hurricane responses of coastal land owners. After the 1995 storms it was found that several owners of
residential properties and hotels on the coast desired to take action
immediately to restore the beach and/or protect their property. Such action included the construction of walls, revetments,
groynes and in some cases beach renourishment projects.
Some of these measures were implemented, with and without the permission
of local planning authorities and with varying degrees of success.
The database presented here clearly indicates that natural
processes will restore the beach to a large extent in the eight months following
the hurricane(s). Thus the best
advice to concerned land owners should be to wait and let nature restore the
beaches. However, sometimes it is
necessary to take additional action especially since the peak tourism season
starts in December which means that there has only been a two month period for
beach recovery - most hurricanes affect the islands in September.
This was the case at Pinneys Beach, Nevis, where sand was dredged from
the offshore zone to replenish the beach before the start of the tourist season
in December 1995. A similar project
was undertaken at Maundays Bay in Anguilla.
| Figure 17. Average Beach Recovery in Dominica Following Hurricane Hugo Northwest Coast Beaches - Coconut Beach to Toucarie 1987 - 1991 | Figure 18. Beach Profile Changes at Nevis, Pinneys Beach Cotton Ground, 1989 - 1996 |
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In this report the calculation of beach recovery has been based on single measurements taken in 1996. In view of natural beach fluctuations this can only provide a partial picture. A more comprehensive assessment of recovery may be obtained by comparing beach change trends in the years following a hurricane, as can be done in Nevis and Dominica following Hurricane Hugo in 1989. In these two islands, the calculation of beach change trends indicates that the beaches never returned to their pre-hurricane levels. Figure 17 shows an average trend for four beaches on the northwest coast of Dominica. After Hurricane Hugo the beaches recovered, but not to their pre-hurricane levels, a new lower threshold value was established. Similarly Figure 18 shows the beach change trends at Pinneys Beach Cotton Ground in Nevis. Here again, a new, lower threshold was established after Hurricane Hugo and it appears likely that a similar change is now taking place after the 1995 hurricanes.
