The University of the West Indies
Organization of
American States
PROFESSIONAL DEVELOPMENT PROGRAMME:
COASTAL INFRASTRUCTURE DESIGN, CONSTRUCTION AND
MAINTENANCE
A COURSE IN
COASTAL DEFENSE SYSTEMS II
CHAPTER 4
DESIGN OF COASTAL DEFENSE WORKS
DAVID SMITH, PhD
Smith Warner International Limited,
Consulting Engineers,
Kingston Jamaica.
Organized by Department of Civil Engineering, The University of the West Indies, in conjunction with Old
Dominion University, Norfolk, VA, USA and Coastal Engineering Research Centre, US Army, Corps of Engineers,
Vicksburg, MS, USA.
Dominica, West Indies, July 30-August3, 2001
Design of Coastal Defense Works:
Caribbean Marine and Coastal Processes
David A.Y. Smith, Ph.D., P.Eng.1
Part I
Overview of the Processes
The islands of the Eastern Caribbean stretch from the Virgin Islands in the north, to Trinidad in the
south. Geologically, these islands differ, however the majority have volcanic origins. Exceptions to
this majority include Barbados and Antigua, which have large coral caps. These islands all have
dual weather exposure, with their eastern shorelines exposed to the Atlantic Ocean and their
western shorelines open to the Caribbean Sea.
From an overview perspective, these islands are exposed to the following forces and elements:
•
The Trade Winds;
•
Waves which are generated by: the Trade Winds; by passing hurricanes; and by North
Atlantic storms;
•
Oceanic and tidally driven currents; and
•
Sea level change.
These four parameters are the primary driving forces that contribute to ongoing marine and coastal
processes in the islands of the Eastern Caribbean. They therefore need to be understood and/or
quantified in order to properly design coastal defense works.
1.1
The Trade Winds
The Trade Winds blow with great constancy primarily from the north-east to the south-east. Some
seasonal changes occur within this pattern as a result of the relative position of the sun and the
earth’s surface. On March 21st, the sun is overhead at the equator. It moves overhead the Tropic of
Cancer (22 ½oN) on June 21st, and returns overhead the equator again on September 21st. Between
September 21st and March 21st the sun is overhead south of the equator. These celestial movements
result in a natural division of the annual wind climate into four seasons:
a.
December to February: Winds are primarily from the NE to ENE.
b.
March to May: Winds are mainly from the East.
c.
June to August: Winds are primarily from the E to ESE.
d.
September to November: Winds are mainly from the E to SE.
Wind speeds are also influenced by the location of the Inter-tropical Convergence Zone, or ITC.
The ITC is formed as a result of the convergence of north-east and south-east Trade Winds in a belt
around the equator. This belt migrates north or south of the equator along with the sun’s motion.
Since the ITC is characterized by wind uplift (as a result of convergence), surface wind speeds tend
to be low in the vicinity of this feature. The ITC is closest to the Eastern Caribbean Islands
1
Smith Warner International Ltd.
Unit 2, Seymour Park, 2 Seymour Avenue
Kingston 10, Jamaica
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between June and November. These months, therefore, have the lowest average wind speeds as
compared with the rest of the year.
These seasonal variations in wind directions result in a corresponding variation in wave directions.
Disturbances to this normal circulation occur throughout the year as a result of the passage of
easterly waves, hurricanes, tropical storms, and localized meteorological phenomena such as
thunderstorms.
1.2
Wave Climate of the Eastern Caribbean
The wave climate of the Eastern Caribbean Islands has three primary components:
•
Day-to-day (or operational) waves;
•
Swell waves; and
•
Hurricanes.
1.2.1 Operational and Swell Waves
The day-to-day wave climate occurs as a result of the action of the Trade Winds on the waters of
the Atlantic Ocean, and is observed throughout the year, primarily from directions NE, through E,
to SE. Because of the constancy of the winds, the windward shores of these islands are exposed to
high-energy wave conditions on a near-constant basis. Interestingly, recent work on available wave
energy has shown that the frequency of occurrence of a given wave height has increased over the
past three decades.
By contrast, and again as a result of the directional characteristics of the Trade Winds, the west
coasts of these islands are relatively sheltered (compared to their east coasts), and the day-to-day
wave climate along such coastlines are largely as a result of diffracted waves traveling around their
north and south tips. Because of the predominance of the north-easterly component of the Trades,
the south-going diffracted wave climate typically prevails, although there are times of the year
when the predominant diffracted wave direction is to the north.
Between November and March, the islands, and in particular their west coasts, are subjected to
swell waves which are generated by extra-tropical storms occurring in the North Atlantic.
Specifically, during these winter months, a large number of cyclones originate over the Gulf of
Mexico and track in a north-easterly direction along the east coast of the USA, as far north as
Newfoundland. These extra-tropical depressions are slow-moving and so the winds under their
influence have ample time to generate an active sea state. These generated waves move
southwards, away from the cyclones, and travel over 1,000 km to Eastern Caribbean shorelines.
During this passage, the waves become regular in shape (i.e. sinusoidal) and have long wave
lengths (i.e. long period waves). Because of their direction of travel (i.e. from the north to NNE),
they have the most impact on leeward shorelines, which are otherwise sheltered, and can cause a
great degree of damage to these shorelines. These swell events last on average between 1 to 3 days,
and there are usually 5 to 9 of them in any one swell season. Because of the long wave period
characteristics of these waves, they experience a great degree of shoaling and refraction in the
nearshore waters of these leeward coasts, and can contribute substantially to the movement of sand
in the surf zone, in a southerly direction.
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Recently measured wave data collected on the lee side of the Barbadian coast has shown that
during the summer months, there is some evidence of swell from the south-west. These events
appear to be caused by tropical waves and storms passing through the southern Caribbean. The
water waves generated by these tropical waves travel in a north-easterly direction and impact on
the leeward shores of these islands, moving sand in the surf zone in a northerly direction. They are
similar in wave height characteristics to the north swell, with the exception that their wave periods
are somewhat shorter. This is to be expected, since they travel over a smaller area of water than the
swell originating in the North Atlantic. These events are more rare than northern swell and occur,
on average, once every two to three years or so. These occurrences are also known to create severe
erosion on the lee side beaches
1.2.2 Hurricane Waves
The third component of the wave climate that affects Eastern Caribbean shorelines is due to the
passage of tropical storms. These meteorological features traverse the Caribbean between June and
November (the hurricane season). They have an organized circulation structure and are
characterized by winds rotating around a central core, or “eye”. In the northern hemisphere, the
winds rotate in an anti-clockwise direction, whereas in the southern hemisphere they rotate in a
clockwise direction and are called typhoons. The majority of cyclones that affect the Caribbean
have their genesis on the African continent (Sahara region) and travel across the Atlantic. Usually,
as they make this trans-Atlantic crossing, they gain energy from the waters over which they travel,
and develop a more organized structure. For these storms, their first landfall are the islands of the
Eastern Caribbean. Less frequently, tropical cyclones originate in the south-west of the Caribbean
Sea. These usually affect the north-western Caribbean but, as in the case of Hurricane Lenny, can
travel eastward across the Caribbean sea. These storms can also be quite damaging.
The term “tropical cyclone” refers to any non-frontal, low pressure, large-scale weather system that
develops over tropical or sub-tropical waters, and possesses a definite organized circulation. They
have historically been classified according to their maximum sustained wind speeds. Cyclones with
wind speeds below 34 knots (63 km/hr) are known as Tropical Depressions. Those with wind
speeds between 34-64 knots (63-118 km/hr) are termed Tropical Storms, while the term Hurricane
is used for tropical cyclones with sustained wind speeds over 64 knots (118 km/hr). In the USA, a
further classification of hurricanes is in common usage. This classification system, known as the
Saffir/Simpson Hurricane Scale, describes five scales of hurricane strengths according to wind
speed. These range from Category No. 1, starting at 74 mph (119 km/hr) up to a Category No. 5,
with speeds in excess of 155 mph (250 km/hr). Historically, however, Category No. 5 hurricanes
have not been observed in the Eastern Caribbean.
It is interesting to note that an earlier categorization was developed for the Eastern Caribbean
(Depradine and Rudder, 1973), and is given in the following Table 1.1.
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Table 1.1
Regional Caribbean Classification of Tropical Cyclones
Storm Category
Wind Speed (knots)
Central Pressure (mb)
I
20-44
1006+
II
45-70
1000-1006
III
65-109
970-1000
IV
110+
<970
In addition, this classification was aided by a division of categories III and IV into three latitude
groups. Group A encompassed a range of from 10-15oN, Group B from 15-20oN and Group C was
north of 20oN. Observation of historical hurricane data revealed that for Category I and II storms,
the influence of latitude on the parameters given above was minimal. For Category III and IV
storms, however, there were notable changes in the storm parameters with latitude. Generally,
central pressure and forward speed of these storms decreased with increasing latitude (i.e. the
cyclones became more intense with increasing latitude), whereas radius to maximum winds
increased with increasing latitude (potentially larger storms).
1.3
Currents and Tides
1.3.1 Oceanic Currents
The primary ocean currents that influence the islands of the Eastern Caribbean are the North
Equatorial and Guyana Currents. The former current crosses the Atlantic, running in an east to
west direction just north of the equator, at a mean speed of 0.5 knot. The Guyana Current runs
parallel to the coast of South America, generally flowing from the south-east to the north-west at
speeds of up to 1 knot. The Guyana Current appears to dominate flow around the southern
Caribbean between January and April, when it brings low salinity waters north from the Amazon
and Orinoco Rivers. This influx of fresh water has been found have a profound effect on nearshore
water levels.
1.3.2 Tidal Action
In general, tides are caused by the gravitational effects of the sun and moon on the oceans of the
world. The periodicity of the rise and fall of the tide, known as the flood and ebb, is determined by
the periodicity of these gravitational effects. Additionally, the height of the tide is a function of the
sum of water displacements produced by these gravitational forces.
Closer in to shore, the effect of the tides becomes noticeable, as their amplitudes and velocities
(but not frequencies) are modified by coastal bathymetry. Typically, tidal fluctuations occur once
daily (diurnal tide) or twice daily (semidiurnal tide), with unequal amplitudes. Observations of
tides over a lunar month reveal periodic variations in the tidal range (difference between
successive high and low tides). From these records, times of maximum and minimum tidal range
are observed. The maxima are called spring tides and the minima, neap tides. These occur at
approximately two-week intervals.
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1.4
Sea Level Change
Two primary forms of quantitative information are available for the assessment of sea-level change
in the Caribbean. These are changes deduced from the sedimentary record, and from in situ tide
gauge measurements.
Sea-Level Movements Deduced From the Sedimentary Record of Coastal and Shelf Areas: Event
timing has been generated through radiometric dating of sedimentary samples that were deposited
at, or near to, sea level. The nature of these records is such that resolution greater than 100 years is
difficult to achieve, and inherent measurement errors are typically on a scale of 100 years. Thus,
while these records are useful as indicators of longer-term sea-surface changes for pre-historical
periods, they cannot be used accurately for detection of sea-surface changes on a decadal, annual
or intra-annual frequency.
Records from Tide Gauges: Within the wider Caribbean region there are sixty-four tide-gauge
stations with data of sufficiently high quality to have their data placed in the revised local reference
file at the Permanent Service for Mean Sea Level (Hanson and Maul, 1993).
There is a marked inconsistency in both the direction and rate of sea-level change from these recent
historical records. This is typical for the Caribbean in general and is caused by local variation in
factors that include the rate of tectonic displacement (land movement) at each location.
For example, within the wider Caribbean, Port-au-Prince records one of the highest historical rates
of sea-level rise. This appears to be due primarily to locally rapid subsidence of the crustal block
on which Port-au-Prince is located.
The global warming factor also needs to be taken into account. At present, while there are
indications of atmospheric warming, there is as yet no definitive signal in the tide gauge data for
the Caribbean to indicate accelerations in the rate of sea-level change.
For the purpose of estimates, however, it is considered advisable to err on the side of caution when
predicting future change, by taking into account some element of potential sea level rise from this
phenomenon.
Assumptions for future global sea-level rise under a "greenhouse" scenario vary substantially
between 0.3 cm/yr. to 1.0 cm/yr. The UNEP/IOC Task Team adopted a figure of about 0.5 cm/yr.
for modeling purposes. The actual rate of change will obviously vary enormously depending on the
local nature of tectonic movement, subsidence, fluid withdrawal and other contributors to relative
sea level rise, and needs to be considered on an island-by-island basis.
1.5
Coastal Process/Shoreline Interactions in the Eastern Caribbean
The elements and driving forces described in the previous sections have interacted with Eastern
Caribbean shorelines in a number of different ways, and have influenced, in a profound manner,
their present-day morphology. First, the sand dunes found on the windward shores of these islands
form as a direct result of aeolian transport of sand. Essentially, sand that is carried to the back of
the beach, by wave action, is dried and transported further inland by the prevailing wind. Any trees
or shrubbery in the back beach area will trap this sand, resulting in the start of a sand dune. Once
the dune forms, it will continue to grow (under the action of wind), and will only be eroded either
by sand mining or during periods of high water levels and wave action.
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The waves that are generated in deep water undergo a number of changes as they approach these
shorelines. These include:
•
Shoaling and refraction, which occur when the waves “feel” the seabed;
•
“Whitecapping” which takes place when the waves become over-steepened;
•
Interaction with, and reflection from, reefs where these occur adjacent to a shoreline;
•
Diffraction around headlands and obstructions (in fact, at a larger scale, this occurs
around the tips of the islands); and
•
Run-up on shorelines.
These changes affect, in a major way, the amount of wave energy remaining at the shoreline that is
able to influence the shoreline morphology and to inflict damage to coastal infrastructure. For
example, the emergence of headlands and bays comes about as an interaction between the
shorelines’ geological formation and nearshore wave climate, and typically occurs over geologic
time scales. These interactions further result in shoreline erosion and accretion and therefore are an
integral part of the development of a sediment budget, with impacts on the medium and long-term
equilibrium of the shoreline. Additionally, it is important to be able to quantify the nearshore and
extreme wave climate, in order to facilitate the design of any coastal/marine works. The extreme
wave climate, which is used in the design of any coastal structure, is usually derived from an
analysis of hurricane parameters.
Finally, an understanding of water levels is important. These can be divided into two main
categories, day-to-day and event related. Water levels change on a daily basis as a result of tidal
action. Typically, two high tides and two lows occur on a daily basis in the Eastern Caribbean.
This daily movement of the mean sea level affects the shaping of shorelines, and the extent of the
normal beach processes. During extreme events, such as a hurricane, water levels can increase as a
result of storm surge. The components of storm surge are:
•
Wind-induced surge;
•
Inverse barometric pressure rise; and
•
Wave set-up.
Storm surge can be devastating for a shoreline because it causes:
•
More wave energy to be transmitted over bank reefs into the nearshore zone, and,
•
Inundation of shoreline areas that would not normally be flooded. This increases the
potential for backshore erosion.
Another type of extreme event is tsunami, a phenomenon of long period ocean waves. In deep
water, these waves may be hundreds of kilometres in length and only a metre or more in height. As
they enter shoaling coastal waters, their wave lengths diminish and wave heights increase. This
phenomenon can be devastating for low-lying coastal areas. Tsunamis may be generated by
submarine earthquakes, volcanic eruptions, landslides, slumps and explosions. In the eastern
Caribbean, the most likely source of tsunamis is the nearly continuous belt of shallow-focus
seismicity, which can be traced from Central America through the Greater and Lesser Antilles, to
north-east Venezuela.
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In addition, a third category of water level change must be considered in the context of long term
development planning. This is global sea level rise, which is a long-term phenomenon.
As outlined above, these phenomena all combine to interact with, and shape, the Eastern Caribbean
shorelines. These shorelines, however, differ, and may consist of a number of different types: sand
beaches, rocky cliffs or mangrove marshes.
The beaches of the Eastern Caribbean are typically formed from one, or (a combination), of three
sources:
•
Volcanic sand which produces black sand beaches;
•
Riverine (i.e. terrestrial), sand which forms brown sand beaches, and
• Coral fragments, which form white sand beaches.
In many locations, sand from these sources mix at the shoreline. The ongoing transport of sand in
an alongshore direction (as a result of wave action) contributes to the sediment budget, whereby
sand is brought both into, and out of, a section of shoreline. Depending on the relative differences
between these two rates, the shoreline may either erode or accrete. In the development of a Coastal
Zone Management Plan, or in the preparation of coastal infrastructure designs, it is essential to
have some knowledge of the sediment budget and the impact(s) of any proposed structures on it.
As is to be expected, rocky shorelines are much more resistant to erosion than their sandy
counterparts. These rock cliffs may be coral, or may be composed of a harder substrate such as
granite. Coral cliffs have been documented to erode at the rate of approximately 5 mm/year,
however, harder substrate types are much more resistant to erosion.
Mangrove/marsh type shorelines typically occur in low wave energy environments. Because of the
complex root structure of these plants, they tend to anchor the shoreline substrate. They also
facilitate the settlement of silt and mud, which may be washed out of backshore areas during times
of heavy rainfall.
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Part 2
2.1
Quantification of Coastal Process Elements
Reasons for Quantification
In Part I, a description was given of the various coastal process components that contribute to the
shaping of Eastern Caribbean shorelines. In order to be proactive in the evaluation of development
proposals that are coastal or marine related, or to produce designs of coastal/marine projects which
work in harmony with their environment, it is essential that a proper understanding of these
processes be gained. This requires both an appreciation for the way in which these processes work
and for their relative magnitude.
In the Caribbean region, there is generally a lack of sound data on which to base a design project,
to properly evaluate such a project, or to develop a coastal zone management plan. This means that
short-term measured data must often be combined with empirical or regional observations to
complete the background picture. It is important, therefore, to be aware of what data does exist,
how this may be accessed and where the data gaps in the database lie. Further, it is important to be
aware of the techniques of field measurement, what should be measured, where the limitations lie
and what can reasonably be expected from such measurement programs.
Because there have been few comprehensive data collection programs mounted in the Eastern
Caribbean, additional techniques are often required to interpret measurements collected at spot
locations, within the context of a spatial basis. Very often, numerical modeling (computer)
techniques are used to fill in the spatial “picture” of coastal processes. It is important, however, to
realize that these models should never be used in a “black box” manner. In other words, they
should either be calibrated or their limitations should be fully understood.
In the following sections, the available database is explored and methods are given for the
quantification of the various parameters.
2.2
Wind Data
At a regional level, wind data archives are held by the Caribbean Meteorological Institute (CMI),
although not necessarily in a form which is directly suitable for computer analysis. On an islandby-island basis, wind data may be obtained from local airports. These data are typically in the form
of wind speed and direction observations that have been made at specific intervals which range
from once per hour, per day or per month. If a good digital database is to be collected and analysed,
then it is often necessary to convert the airport wind data from an analog to a digital form.
This information may be used to obtain an overview of the prevailing wind direction(s), and
consequently of the impact of wind on coastal features or facilities. Further, it may be used in a
wave hindcasting procedure to evaluate the characteristics of locally generated waves. In using this
collected wind data, it is important to properly interpret the data in the context of the location of
data collection vis-à-vis the site under consideration.
2.3
Wave Data
The wave climate at a given location is typically a collection of wave statistics, which represents
the long-term average frequency of occurrence of gravity wave conditions at that location. The
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wave climate usually includes ordinary wave conditions, and/or special events such as hurricanes.
The data within the wave climate are usually defined in terms of:
•
The event or season;
•
The wave direction;
•
A measurement of wave height; and
• A measurement of wave period.
Ideally, the wave climate at a location is best obtained through a long-term period of measurement,
spanning years. For the evaluation of hurricane waves, it is customary to review records of greater
than 100 years.
At this point it is instructive to present some definitions of and statistics for, water waves. Waves
generated in the oceans are known as gravity, or wind-generated, waves, with periods ranging from
1 to 30 seconds typically. These waves form when winds blow over a flat area of sea, and change
from ripples to full waves, through an extremely complex development process. Other factors that
play a part in this process are depth of water, storm duration and distance over which the wind
influences the waves (known as the “fetch”). For conditions where water depth, wind duration and
fetch are unlimited, a Fully Arisen Sea develops.
Waves that are still within the area of generation, are termed sea, whereas swell wave are those
which have traveled some distance away from the area of generation into the observer area. During
this period of travel, the smaller, short-period waves are “eaten up” by the longer period
components of the sea. In addition, the energy of the individual waves is dissipated to some extent.
The net result is that when the swell waves are observed after traveling some distance away from
their generation area, they have longer wave periods and smaller, more regular wave heights.
Because a sea state is comprised of waves with differing heights and periods, a tremendous amount
of research has gone into the statistical and/or probabilistic definition of these waves. One of the
most common, and more important, definitions of wave height is the significant wave height, Hs.
This definition seems to some closest to the visual estimate that would be made by an experienced
observer.
Hs - significant wave height, or average height of the highest 1/3 of the waves
In the design of a coastal or marine facility, there is interest not only in what takes place during a
particular storm, but also over the expected life of the structure. Research has shown that a
binomial distribution will give the probability that an event will not occur during the life of a
structure. The return period for this is calculated as:
T= 1
1
1 − (1 − R) N
Where T = design return period;
R = permissible risk of failure; and
N = expected project life.
For example, if a structure with an expected 50-year life was designed for a 20-year return period
storm, there is a 33% chance this event will occur during the life of the structure.
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Measured wave data may be subjected to a wave spectrum analysis (done by computer analysis
only). In this method, the wave record is digitized and a wave spectrum computed. This is a
measure of the distribution of wave energy with frequency. Integration of the wave spectrum gives
another estimate of the variance of the water surface from the mean water level (σ), which can be
used to estimate characteristic wave heights (note that Hs = 4.0σ). The wave spectrum can also be
used to give an energy density spectrum, which gives the distribution of energy with frequency.
This allows the study of resonant systems. It is normal to characterize the spectrum by its peak
frequency, fp. From this, the peak period is given as,
Tp = 1/fp.
2.3.1 Deep Water Waves
In the absence of detailed wave measurements at a particular location, it is possible to rely on long
term observations of sea state conditions that have been collected by volunteer observer ships
(VOS). Essentially, this data has been derived from a quality enhancing analysis of a massive
number of visual observations of both waves and winds, which are reported from ships in normal
service all over the world, using a computer program called NMIMET. The statistics are presented
for waves only, but the wind data has been used to improve the reliability of wave statistics. The
parameters measured correlate to what are known as “significant wave heights” and “zerocrossing” wave periods (described in the previous section).
The areas of wave parameter recordings that are applicable to the Eastern Caribbean are Areas 47
and 48.
For the islands of the Eastern Caribbean, this data source is considered to be quite reliable for the
exposed, or windward, shorelines. For the sheltered shorelines, however, it is not so reliable as
information on secondary wave trains is typically masked in the visual observations. In reality,
these secondary wave trains combine with diffracted waves, around the tips of these islands, to
produce a leeward coast wave climate. These secondary wave trains tend to be of the same order of
magnitude as the diffracted waves, on these leeward shorelines.
A NOAA wave buoy moored in the south-central Caribbean Sea provides one potential data source
for the lee coast wave climate. This buoy has been measuring wave heights and periods, and wind
directions, since 1994. This data is available on the internet, from the NOAA web site. The one
drawback to this data is that it contains no directional wave data. The assumption most therefore be
made that the wave and wind directions are coincident.
At a few locations within the Caribbean, such as Barbados, extensive wave measurements have
been made on both leeward and windward shoreline. These data have shed quite some insight into
the characteristics of swell that is experienced on leeward coastlines.
For locations where no measured data exists, a hindcast model data may be obtained. Two such
avenues are:
•
It is possible to specially order VOS wave statistics for small areas adjacent to any of
the Eastern Caribbean islands, from British Maritime Technology (BMT);
•
It is also possible to order wave data generated from a Global Spectral Ocean Wave
Model (GSOWM). These data are available in 12-hour intervals since June 1986, and
consist of a time series of directional frequency spectra with a directional resolution of
15o and a frequency range giving wave periods ranging from 3 to 25 seconds.
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In order to assess a hurricane wave climate, the recommended procedure would be to search the
database of storms held at the National Climatic Data Centre (in cooperation with the National
Hurricane Centre). These records date back to 1871 and include nearly 1000 storms. The records of
these storms are updated on an annual basis, and include data on position, wind speed and central
pressure.
A regional analysis of hurricanes was done as part of a Feasibility Study on Coastal Conservation
for the Government of Barbados. This work gave the following values of regional hurricane wave
parameters:
Table 2.2
Extremal Analysis of Maximum Regional Hurricane Wave Heights
Return Period
(Years)
Wave Height, Hs (m)
95% Confidence
Interval (m)
Peak Period, Tp (sec)
5
9.5
8.2 - 10.9
12.0
10
11.8
10.3 - 13.7
14.0
25
14.8
12.6 - 17.0
15.3
50
16.9
14.5 - 19.4
16.0
100
19.0
16.2 - 21.8
16.6
2.3.2 Nearshore Wave Climate
In order to develop a wave climate that is representative of nearshore conditions in the vicinity of a
project or beach, it is necessary to undertake a wave transformation procedure on the deep water
waves.
Waves begin to “feel” the seabed when they travel into water depths characterized by:
Lo
5 <d<
Lo
2
As these waves propagate in to shore, they are subject to: shoaling; refraction; diffraction around
seabed and shoreline features; and energy losses due to bed friction and “whitecapping”. These
changes primarily affect the wave height, and to a lesser extent the wave period.
There are a number of computer programs that are currently in use to carry out this type of
transformation procedure. These typically require bathymetric input as well as deep-water wave
characteristics, and fall generally into one of two categories. For the first, the tracks of wave rays
are computed in reverse, starting at the inshore location and ending in deep water. A large number
of wave rays may be subjected to this procedure, to include all possible wave directions. In
addition, a representative range of wave spectrum peak periods is typically used. For this
procedure, it is possible to determine the range of offshore directions that are able to arrive at a site
in question. Further, it is possible to compute the inshore wave spectrum corresponding to a
particular offshore spectrum. For a particular offshore wave condition (wave height, period and
direction), this procedure can be used to give the corresponding nearshore conditions of wave
height and direction.
The second method (known as the forward tracking procedure) is more traditional, and traces the
travel of wave rays (which are perpendicular to the wave crests) from deep water in to shore. For
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this method, a different wave direction and period are used for each simulation, with simple
monochromatic waves as input. The strong point of this method is that it gives a good graphical
representation of the patterns of wave propagation over the nearshore shallow water regions. This
is useful in identifying areas along the shore where wave energy concentrates or disperses.
2.4
Water Levels
The development of coastal zone management plans, emergency response plans and the design of
coastal structures rely heavily on an assessment of extreme water levels. For design purposes, the
50 or 100 year return period events are usually considered.
The design water level is made up of four major components:
•
Tidal fluctuations;
•
Inverse barometric effect;
•
Wind surge; and
•
Wave setup.
For long-term planning scenarios, global sea level rise should also be included.
These components are discussed in detail following.
2.4.1 Tidal Fluctuations
As previously described, tides in the Eastern Caribbean are primarily semi-diurnal in nature (i.e.
two highs and two lows per day). The parameters that are usually used to characterize the tide are:
•
Mean high high water;
•
Mean low high water;
•
Mean sea level;
•
Mean high low water; and
•
Mean low low water.
In addition, the tidal range is usually of importance, for both spring and neap conditions. In the
Eastern Caribbean, the tidal range is typically less than 1.0 metre.
2.4.2 Inverse Barometric and Wind Surge Effects
As a zone of low pressure (from a tropical cyclone or hurricane) moves over a body of water, the
reduction in atmospheric pressure (below ambient levels) results in the raising of the mean water
level. The empirical formula used to predict this effect is given by:
Sh =
Where: Sh
Po
Pn
R
(
01
. ( Pn − Po ) 1 − e
R
r
)
= The increase in water level (metres)
= The central pressure (kPa)
= The ambient pressure (kPa)
= Radius to maximum winds (km)
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r
= Radius to a particular location (km)
The wind set-up component can be evaluated using a one-dimensional surge model, given by:
Sw
Where, Sw
W
F
d
K
= KW2 F d
= The wind surge
= The wind speed
= The fetch length
= The water depth
= A calibration coefficient.
These expressions are given in the Shore Protection Manual (1977). The water level increase
resulting from low pressure is a maximum at the centre, or “eye”, of the cyclone. By contrast, the
increase resulting from wind set-up is a maximum at the radius of maximum winds. The two
effects are, therefore, not directly additive. Investigation into the relative additive strengths of these
two components has shown that the inverse barometric pressure effect predominates when the two
effects are combined.
2.4.3 Wave Set-up
Wave set-up is defined as the increase in average water level due to waves breaking in the surf
zone. This phenomenon results from the conversion of dissipated kinetic wave energy into
potential energy, which takes the form of an increase in the average water level.
The calculation of wave set-up is therefore triggered at the point at which wave breaking starts. In
reality, the larger waves break further offshore and the smaller waves travel closer in to shore. All
breaking waves, however, contribute to the wave set-up.
2.4.4 Tsunami
Three submarine volcanoes have been reported as being active in recent times in the Eastern
Caribbean. These are:
•
Kick-em-Jenny, north of Grenada
•
Holder’s Volcano, west of St. Lucia and
•
An un-named volcano north of Marie Galante.
A historical summary of tsunamis affecting the Eastern Caribbean has been prepared (Deane et al,
1973). These date back to 1530. In recent years, tide gauge anomalies have been compared with
data on earthquakes prepared by the US Department of Commerce and the Seismic Research Unit.
These have resulted in an updating of the historical records.
An evaluation of tsunami return periods was developed by Deane et al (1973). These were
translated to design water levels (from tsunami) based on the assumption that tsunami will undergo
little or no shoaling within the Eastern Caribbean islands. The tsunami wave can then be treated as
a solitary wave with most of the height being above the mean sea level (Delcan, 1994). This
approach resulted in the following design values.
Table 2.3
Design Tsunami Water Levels
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Return Period (years)
2.5
Water Levels Relative to LLW (m)
4
1.3
20
1.7
75
2.5
200
4.1 - 5.7
Currents
There is no long-term database of current data for the Eastern Caribbean that can be accessed in the
same way that is available for wave data. Reliance must, therefore, be placed on the collection of
measured data. A number of different options exist for the measurement of currents in the field.
These are:
•
Using drogues
•
Deploying moored current meters.
Drogue tracking techniques give an estimate of the spatial picture of nearshore current
characteristics. The drogues are deployed with sails set at varying depths, and are then tracked to
record speeds and directions. These measurements, therefore, give rise to an estimate of water
particle movements in a spatial sense.
Currents may also be measured using moored current meters. These may be moored close to the
seabed, or within the water column. These meters measure currents at the location of measurement.
Where flow is 2-dimensional, a single (or double) meter deployment will give a good
representation of the current characteristics within the water column.
An acoustic doppler current meter may also be used to measure currents throughout the water
column. These are usually mounted on the seabed and scan upwards to the surface. This type of
instrument gives a good indication of any 3-dimensional characteristics within the flow.
2.6
Sediment Transport Characteristics
An evaluation of the sediment transport regime is essential in developing an understanding of the
sediment budget along a shoreline, or in estimating the impact of proposed coastal/marine
developments on adjacent shorelines. Given that there are very few locations within the Caribbean
where sediment transport rates have been actively measured, it is necessary, in most cases, to
estimate this parameter through empirical methods or computer modeling. Sediment transport rates
may be measured when a groyne, for example, is constructed in the surf zone and the resulting
sand fillet surveyed over known periods of time. In the computation of transport rates, and also to
aid the understanding of beach erosion and/or accretion trends, the following input data is required:
•
Beach profile data;
•
Sediment size characteristics; and
•
Nearshore wave data or deep-water data plus offshore bathymetry.
Since 1988, a series of quarterly beach profile measurements has been carried out at 21 locations
throughout the Eastern Caribbean. This has been done as part of a long-term regional beach
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monitoring program (COLSAC). Where no such data is available at a project location, it is
necessary to carry out a beach profile survey.
Such a survey should extend from the back of beach area (landward of expected wave uprush) out
to approximately a 1 metre water depth.
Sediment samples should be collected from the active beach face and from the nearshore area in
approximately 0 to 1.0 metre water depth. These samples should then be air dried (if sand) and
subjected to a standard sieve analysis. Where there is an appreciable quantity of silt in the beach
sample, it may be necessary to do a hydrometer analysis.
The computation of potential sediment transport may be carried out using either a “bulk” predictor
or a detailed predictor model. The bulk model uses one characteristic wave height, the deep-water
significant wave height, to compute wave energy and subsequently potential alongshore sediment
transport. As may be expected, this method typically overestimates the actual rate of sediment
transport, and works best in situations where there is a virtually unlimited supply of sand.
The detailed predictors, by contrast, provide for the division of the cross-shore profile into
segments. The incoming waves are tracked through each segment and the amount of wave breaking
and remaining wave energy accounted for. This approach, therefore, provides a profile of
alongshore sediment transport throughout the surf zone. It is possible, with these predictions, to
include the effects of tidal or oceanic currents on sand transport.
It is also possible to model, in a spatial sense, the response of a shoreline or beach to the addition
of a groyne, breakwater, beach nourishment or dredge programme. These morphological models
provide good insight into the changes that can take place to a shoreline in those development
scenarios. They are, therefore, useful planning tools.
Often, in the development of a coastal project, both types of sediment transport models will be
used. The 1-line model, to get a sense of shoreline adjustment potential, in a cross-shore sense, and
the planform model, to estimate the impacts on adjacent properties.
2.7
Computer Modeling in the Marine Zone
A number of computer modeling techniques have been described in the foregoing sections. These
have dealt primarily with wave and sediment transport processes. It is also possible to model, by
computer, current hydrodynamics. Essentially, 2- or 3-dimensional hydrodynamic models may be
used to model current phenomena in a spatial manner. These models use either finite element or
finite difference techniques. The finite element models have become more popular in recent years
for the following reasons:
•
It is possible to provide better representation of the model land boundaries;
•
it is possible to obtain better detail of specific areas within the model grid, then with
finite difference models which have uniform grid spacing.
As may also be expected, the 2-dimensional models are much more efficient to run than the 3dimensional models. These models, as with all computer models, should be calibrated. The driving
parameters are usually tide and/or wind action, although river inflows may also be simulated.
Results obtained give current patterns over large-scale areas that should include points of
measurement.
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References
Battjes, J.A. and Stive, M.J.F., (1985). Calibration and Verification of a Dissipation Model for
Random Breaking Waves. J. Geophys. Res. Vol. 90, No. C5, pp. 569-588.
Deane, C.A.W., (1969). Hindcast Wave Statistics for Atlantic Coasts of Trinidad and Tobago.
Journal assocn. Of Prof. Engrs. (Trinidad and Tobago) Vol. 8, No. 2, pp. 27-50.
Deane, C.A.W., Thom, M., Edmunds, H., (1973). Eastern Caribbean Coastal Investigations (197073). Volume II - Natural Forces. Regional beach Erosion Control Programme, Faculty of
Engineering, U.W.I.
Delcan (1994). Water Levels for Barbados. Feasibility Studies on Coastal Conservation.
Government of Barbados/In ter-American Development Bank.
Depradine, C.A., Rudder, G.M., Lamming, S.D., (1973). Some Characteristics of Hurricanes in the
Eastern Caribbean. Caribbean Meteorological Institute.
Harrison, K., and Maul, G.A., (1993). Analysis of temperature, precipitation and sea-level
variability with concentration on Key West, Florida, for evidence of trace-gas-induced climate
change. Pp. 193-211 in Maul, G.A., editor, Climatic Change in the Intra-America’s Sea, Edward
Arnold, London, p. 389.
U.S. Army Coastal Engineering Research Centre (CERC). 1977. Shore Protection Manual.