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Seas at the Millenium 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 Coastal Defense Works – Caribbean Marine and Coastal Processes 2 Coastal Defense Systems II CDCM Professional Development Programme, 2001 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. Coastal Defense Works – Caribbean Marine and Coastal Processes 3 Coastal Defense Systems II CDCM Professional Development Programme, 2001 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. Coastal Defense Works – Caribbean Marine and Coastal Processes 4 Coastal Defense Systems II CDCM Professional Development Programme, 2001 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.
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