Environment and development in coastal regions and in small islands

Gillian Cambers

Table of contents

INTRODUCTION

WHEN A HURRICANE OCCURS

COUNTERING THE EFFECTS OF HIGH SEAS IN WINTER

STRUCTURES FOR BEACH PROTECTION

ADDING MORE SAND TO THE BEACH

	NEW WAYS TO REDUCE BEACH EROSION
	NOTES AND REFERENCES

Coastline changes due to natural processes and human intervention represent a major concern of coastal planners the world over, and indeed of ordinary citizens who have real or potential interests in beaches and seaside property. While beaches may be the substance of vacation dreams for some, their disappearance through erosion can lead to nightmares for those who live, work, relax and build close to the shore. In the Caribbean region, one collaborative research activity has focused since the mid-1980s on issues related to coast and beach stability. One recent product has been guidelines to help coastal planners and stakeholders to establish their own specific measures for dealing with coastline instability.

Shorelines are areas of continuous change where the natural forces of wind and water interact with the land. Though the shifts between water and land have been taking place for centuries1 - and are the result of both natural forces and human activities such as sand mining and beach construction - these changes have taken on paramount importance in the Caribbean islands since tourism became the major industry in the 1970s. Despite their economic value in a region where tourism is dependent for the most part on sun, sea and sand, beaches have unfortunately not been perceived as areas needing management, protection and funding, but rather as permanent features of the landscape.

Environmental awareness has been growing slowly. International events, such as the United Nations Conferences on Environment and Development (Rio de Janeiro, 1992) and on Sustainable Development of Small Island Developing States (Barbados, 1994), have helped to focus public attention on the need for environmental management. Yet is was the dramatic events of 1989 and 1995 that gave the region the most direct demonstration of the need for beach management; Hurricane Hugo in 1989 and the numerous hurricanes in 1995 brought home to everyone the impermanence and vulnerability of the region's beaches.

Within such a context, UNESCO has been involved in the search for answers to coastal problems for well over two decades. Under the auspices of its Coastal Marine programme, a project was developed and eventually entitled Coast and Beach Stability in the Caribbean (COSALC)2. Since 1993, the project has benefited from the co-sponsorship of the Sea Grant College Program of the University of Puerto Rico. More recently, UNESCO's support for the project's activities has been provided through the Organization’s platform for cross-sectoral action: Environment and Development in Coastal Regions and in Small Islands (CSI). One tangible output of a decade and a half of co-operation and field studies is a practical guide on shoreline protection and other management measures. The 119 page illustrated guide, Coping with Beach Erosion 3, presents environmental background and practical guidelines on what can be done in response to disappearing and degrading beaches. The information is organized in terms of eleven generic cases:

• Countering the effects of high seas in winter
• When a hurricane occurs
• When stones have replaced a sand beach
• Assessing the impact of coastal structures
• Adding more sand to the beach
• When sand has been mined from the beach
• When sand dunes have been destroyed
• When vegetation has been removed from behind the beach
• Stabilizing the river mouth or tidal inlet n Conserving reefs
• New ways to reduce beach erosion.

This article provides glimpses into some of these situations, and to the sorts of approaches that have commended to those concerned with erosion-prone beach areas. While the focus is on the Caribbean, many of the conclusions and measures may also be relevant to beach systems elsewhere in the world and especially in small islands.

WHEN A HURRICANE OCCURS

Extreme events such as hurricanes are the major cause of shoreline changes in the Caribbean.Many hurricanes originate as tropical waves off the west coast of Africa and travel across the Atlantic Ocean gaining strength from the warm ocean waters. As tropical waves streng then, they pass through several stages, including tropical depression and tropical storm before reaching hurricane strength. Once a system reaches tropical storm strength, it is named. Hurricanes are further classified into five categories based on wind speed. The wind speeds in categories one to five refer to sustained wind speeds. Actual gusts may be much higher. Most hurricanes are experienced in September in the Caribbean .

Hurricane categories and wind speed

Category Wind speed (km hr-1) Wind speed (mph) 1 118 - 152 74 - 95 2 153 - 176 96 - 110 3 177 - 208 111 - 130 4 209 - 248 131 - 155 5 249+ 156+ Most of hurricanes affecting the Caribbean islands move in a westerly to northwesterly direction. As the hurricane approaches, winds usually blow from the east and north. Once the centre, or eye, of the hurricane has passed, the wind blows from the west and south. Figure adapted from Bacon (1978).4

Based on study of weather patterns and climate records over the past hundred years, it appears that hurricane generation in the North Atlantic Ocean occurs in 20-25 year alternating cycles of activity and relative in activity. The evidence indicates that we have now entered an active hurricane cycle, which is expected to continue for the next two decades. The years 1995 and 1996 were especially active, 1995 registering as the second-most active year on record with nineteen named storms. However, even within an active cycle, there are years with below - average hurricane activity, such as 1997. Tropical storms and hurricanes are such intense, well - organized systems that they generate waves which move out of the immediate vicinity of the hurricane to a ffect other islands as swell waves. Considerable damage may occur to beach systems as a result of high seas, raised water level (known as a storm surge), high winds and heavy rain fall. Hurricane waves erode the beaches and penetrate farther into the land behind the beach causing flooding, erosion of sand dunes and destruction of coastal highways and buildings.

Typical beach cross-section before and after a hurricane. The hurricane waves overtopped the seaward dune and the building on the dune collapsed. Sand was moved offshore and inland. Figure adapted from Bush et al. (1995).5

Beach erosion after a hurricane may vary from slight to severe. In Barnes Bay, Anguilla, pre - hurricane beach conditions in 1994 featured a wide sandy beach backed by palm trees . Post hurricane, in 1995, the sand had been stripped from the beach during Hurricane Luis, leaving rocky outcrops exposed and just a few pockets of sand. The palm trees were lost.

COUNTERING THE EFFECTS OF HIGH SEAS IN WINTER

Waves, tides and ocean currents are among the several natural forces that cause beach changes. One cause of considerable beach erosion are swell waves - waves that travel out of the area in which they were generated. In the Caribbean, such swell waves are usually caused by intense midlatitude storms in the North Atlantic Ocean, which travel thousands of kilometres south affecting the west, north and east coasts of the islands in the winter months from October to April.

During the summer months, when the seas are calmer and the waves smaller, beaches tend to build up. This process is known as accretion. If the amount of winter erosion exceeds summer accretion, there is overall erosion with the land behind the beach being eroded as the beach retreats inland. The rate of retreat is called the erosion rate. Overall erosion may be due to one or more factors: for example, a particularly severe winter swell, a recent hurricane, the death of an adjacent coral reef or interference in the supply of sand. Conversely, if accretion exceeds erosion, the beach gets wider over time and accretionary features, such as cupsate forelands, tombolos, spits and bars, may develop. Figure adapted from Komar (1976)6

Winter beach conditions at Folkestone (Barbados). Following a seve re winter swell, the waves have eroded the beach and cut into the edge of the land, leaving tree roots exposed. In the background, one large tree has fallen as a result of erosion.

Summer beac h conditions Folkestone, Barbados, 1980. The dead trees have been removed and the beach has shown significant build-up of sand. However, the exposed roots of the tree in the fore ground indicate that the beach has not fully recove red from winter erosion .

STRUCTURES FOR BEACH PROTECTION

There are three main groups of solid structures which protect land and/or beaches : structures built parallel to the shore (seawalls , bulkheads, revetments); structures built at right angles to the shore (groynes and jetties); and offshore structures like offshore breakwaters .

Structures built parallel to the shore are generally made of steel, concrete, rock or wood, and are designed to protect land and buildings from erosion by the sea. They are the most common means of protection found on island beaches. They come in all shapes and sizes. It is important to remember that these structures only protect the land. They do not promote beach accretion. In this sense, they can only be regarded as a means of buying time, since erosion will continue in front of the seawall and the beach will narrow or even eventually disappear.

Groynes are structures built at right angles to the shore, designed to trap the moving sand transported in longshore currents which move parallel to the shore. Their main function is to promote beach build-up by trapping sand or slowing down its movement along the beach . Groynes are usually constructed of rock or concrete and usually result in sand accretion on one side and erosion on the other. Groynes function most effectively along coastlines where the direction of longshore transport is constant. In the Caribbean islands with their prevailing North easterly Trade Wind regimes, the predominant longshore transport direction is from east to west. Experience has shown that groynes work best along north or south facing coast lines. For example, groynes have worked reasonably effectively on the north coast of Nevis. Similarly, much of the south coast of Barbados has been stablized with groynes. Figure adapted from US Army Corps of Engineers (1981) 7

ADDING MORE SAND TO THE BEACH

Beach nourishment consists of adding large volumes of sand to the beach . The sand may be obtained from an inland or offshore source. Since land sources of sand are limited in the Caribbean, the sand is usually obtained from the offshore zone. The sand and water mixture is then pumped via a floating pipeline onto the shore. Beach nourishment has been little used in the Caribbean islands, unlike in North America and other parts of the world. Generally speaking, it is an expensive technique and not an option individuals choose, since it includes an entire beach. In the islands, the cost of dredged sand ranges from US  $  5-16m - 3. In addition, mobilization costs for the dredge may range from US$100,000 to US$300,000 depending on the location of a suitable dredge .

Pre - beach nourishment conditions , Pinneys Beach, Nevis, 1995. One week after Hurricane Luis, the beach had been severely eroded.

Post - beach nourishment conditions, Pinneys Beach, Nevis, March 1996. Three months after a sand replenishment project, there was a wide beach. However, it should be noted that the observed sand accretion will also have been partly due to natural recovery following the hurricane.

NEW WAYS TO REDUCE BEACH EROSION

Several newer techniques are now being used to protect beaches, including the use of beach face dewatering to promote drainage of incoming waves. Beach face dewatering basically consists of continuously pumping water away from the beach face. The system is based on the idea that, when the water table under the beach is lower than under the ocean, sand accretion is enhanced. As each wave rushes up the beach, water from the wave easily drains through the dry beach, leaving part of its suspended sand load on the beach. Less water drains back into the ocean taking less sand with it, and the beach accretes. The technique involves the installation of a specially designed drainage system under the beach, with pumps removing ground water from under the beach.

Artificial seaweed is another technique that has been tried in the Caribbean, as in other parts of the world. In one pilot project in Barbados10, several hundred units of artificial seaweed were placed in the water. These units consisted of 1.3 m long fronds atta ched to an anchor tube filled with sand. The seaweed units reduce the speed of the current, there by allowing sand to be deposited around and on top of the seaweed units. The hoped-for end result is that the seaweed units are buried by an offshore sand bar, and that this offshore sand deposit will then protect the beach from wave action. However, experiments in Barbados with one type of artificial seaweed showed that the particular material was not suitable for Caribbean waters, since it showed signifi cant deterioration within eight months of installation .

The example serves as a conclusion to this short article. A fair amount of progress has indeed been made in understanding the processes of beach instability and in developing techniques and technologies for combatting erosion and for rehabilitating degraded beaches. But much remains to be done to translate scientific understanding into practicable management .

NOTES AND REFERENCES

By Gillian Cambers, pages 43-49 in Nature & Resources Vol. 35 No.4, October-December '99, UNESCO-ELSEVIER

***

Gillian Cambers is a geographer and geomorphologist by training, with special interests in coastal processes and in regional activities related to coastal management and research in the Carribean.

Her address is:

Dr. Gillian Cambers, COSALC Coordinator,
Sea Grant College Program (SGCP-UPR), University of Puerto Rico,
P.O.Box 9011, College Station, Mayaguez, Puerto Rico 00681.
Fax: (1) 7872652880;
E-mail: g_cambers@rumac.uprm.edu or g_cambers@hotmail.com

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