management sourcebooks 1
When stones have replaced a sand beach
N V I R O N M E N T A L
B A C K G R O U N D
Beaches can vary in size, shape, colour and composition. There are two main sources of beach material: land-based and offshore sources.
Rivers and streams drain the land surface and carry with them particles of the underlying rock. The size of particles reaching the sea depends on the structure of the parent rocks and the extent to which the particles were worn or broken down during transport to the sea. The composition of the parent rocks determines the colour of these sediments. For example, volcanic rocks are typically dark grey to black. On the high volcanic islands of the Lesser Antilles, beach materials are commonly formed from these glistening dark minerals. Particles of rock may also be eroded from cliffs by wave action.
The size and colour of material on a beach depend on the types of materials available in the coastal area.
Along tropical coasts, where the land extends offshore to form a shallow shelf and there are few land-based sediments to cloud the water, coral reefs and seagrasses grow in abundance. These environments play host to many species of plants and to many animal species with a skeleton formed from calcium. (See also Case 10.) When the organism dies, this material becomes sand and may be transported to the beach area by wave and current action. This material is predominantly white in colour.
In many Caribbean islands, both types of sand sources are found; beaches may be light, dark or a mixture of both mineral types. Once on a beach, particles are subjected to wave and current action which causes further breakdown in particle size. Longshore currents may move the sand far from its original sources. (See also Case 4.)
Photograph 10. Black sand
beach, Byera, Saint
A beach may be covered with large rock particles. These may be chunks of coral derived from the breakdown of a coral reef or boulders and stones from a river or cliff. Other sources for coarse sediment may be man-made. They can include material derived from: ships' ballast, coastal quarrying activities, offshore dredging or boulders and stones from broken gabion baskets (See also Case 1.) or other sea defence structures.
|A beach covered by coarse sediments year-round may be one which is only supplied with coarse sediment to start with or it may be going through an extended erosive cycle.|
Sediments are deposited in layers on the beach. Size may vary between layers, reflecting the energy of the conditions at the time of deposition, as well as natural variation in the size of particles available in the particular beach system. These layers can be easily observed by scooping or digging a small trench through the beach.
A beach may be covered with stones at one time of the year and with sand at another time. Such changes in grain size will most likely reflect one of two causes: seasonal erosion or seasonal deposition.
During winter swells or hurricanes, finer grained sediments are often moved offshore or to the back beach area. The larger, heavier sediments are left as a residual layer on the beach.
Coarse sediments may also be thrown up onto the beach from offshore during a winter swell or hurricane. Coarse sediments may be covered up when calmer wave conditions return sand to the beach.
Sand and stone beach,
P R A C T I C A L
R E S P O N S E S
Obtain information on the beach before purchasing.
Observe the nature of the beach material on initial inspection of a beachfront property. Whether the beach consists of sand, stones or a mixture of sediment sizes, establish if this is a seasonal phenomenon or the normal condition for this location. Planning authorities, research organizations, local area residents and coastal landowners may all have useful information.
Avoid hasty reactive responses.
Before considering what action to take on a beach that has changed from sand to stones, determine the possible cause of these changes. Establish whether there was a seasonal swell. If so, there is a very strong possibility that the sand will begin to come back to the beach within a few days. Natural sand replenishment is also likely to follow a tropical storm or hurricane, although this may take several months. (See also Case 2.)
Be aware that, on many Caribbean beaches, material size varies seasonally. For example, a stony beach in the winter months may be replaced by a sandy beach in the summer months. This is a response to seasonal changes in the wave conditions. Little can be done to alter these natural variations. Unfortunately, the stony periods often correspond to the winter months, the peak tourist season.
Some Caribbean beaches may consist predominantly of stones in the winter months and sand in the summer months.
Consider whether any response is necessary.
After assessing the short- or long-term nature of the beach changes, determine whether to proceed with corrective measures, or whether to leave the beach to restore itself naturally. Any intervention must be carefully designed and involve consultation with the planning authorities.
The removal of stones from a particular beach is rarely successful because more stones will be moved in from adjacent areas when the high energy waves return.
Consider the engineering options.
Firstly, it is important to determine that the absence of sand is not a short-lived seasonal phenomenon. Only once this has been determined should consideration be given to engineering options. The most likely option will be to supply the beach with sand from an offshore or inland source. This option is called beach nourishment and is discussed in Case 5.
The artificial supply of beach sand must be carefully designed by experienced professionals.
Sometimes a combination of engineering solutions is required at a particular site. Examples include nourishing the beach with sand and constructing groynes to prevent the newly placed sand from being moved to other coastal stretches.
Unplanned dumping of sand is unlikely to provide a lasting solution. The absence or loss of sand in the first place indicates that any sand added to the beach is also likely to be washed away.
Consult with the planning authorities.
In the Caribbean, the seabed belongs to the government. Any action involving alteration of the seabed, such as dredging it to provide sand for beach nourishment, will require planning permission. In approaching the authorities, be aware that they will want the following information:
analysis as to the cause of the observed beach condition;
design specifications for the proposed intervention;
environmental impacts of the proposed intervention on
neighbouring shorelines both during construction and during operation.