management sourcebooks 1
Adding more sand to the beach
N V I R O N M E N T A L
B A C K G R O U N D
Beach nourishment consists of adding large volumes of sand to the beach. (See Figure 21.) 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 is pumped up often using a suction dredge. The sand and water mixture is then pumped via a floating pipeline onto the shore. (See Figure 22.)
Suction dredge (adapted from Clark, 1995).
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$5–16 per cubic yard/metre. 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.
Beach nourishment should not be viewed as a ‘once only’ operation, since periodic renourishment will be required at intervals of between two and eight years depending on the dynamics at a particular beach.
|Periodic renourishment is required after the initial effort to put sand back.|
Several factors should be considered in determining the feasibility and design of a beach nourishment operation. These include the following:
Offshore sand is the main source in the Caribbean islands. In order to be cost-effective, the dredge site should be as near the beach to be replenished as possible. Sand in shallow waters close to the beach is part of the natural reserve and may eventually replace eroded sand through natural beach processes. It should therefore not be disturbed. Sand found at depths of 15–40 m (49–131 ft) is generally the best source, since it lies beyond the natural beach sand replacement system and is relatively easy to dredge.
22. Beach nourishment with dredged sand. A
16. Settling pond, Peter Island, British Virgin
Disturbance to coral reefs and seagrass beds
Dredge sites should not be located on or near critical habitats such as coral reefs and seagrass beds. Dredging causes a great deal of siltation and turbidity, which can damage important marine ecosystems and local fisheries.
Quantity of sand to be used as fill
During the sand replacement process, it is necessary to place about 50 per cent more sand than is actually needed, since much of the sand will be lost over time as the waves form a natural slope over the beach and offshore zone.
Quality of the sand
The size of the sand grains used for replenishment should be the same as the original beach sand or slightly coarser.
Supply of sand to the beach
Once the sand has been pumped onto the beach, several settling ponds should be constructed to receive the slurry. Silt curtains can also be used to reduce the amount of sediment returning to the sea.
|The impacts of offshore dredging are often difficult to assess because they go unseen except by divers.|
Beach nourishment has the advantage of providing a new beach quickly. However, there is the expense to consider and it requires detailed preliminary studies. In addition, beach nourishment will interfere with normal beach activities while the operation is actually in progress. Depending on the size of the project, beach nourishment may take from one to six months.
|An environmental impact assessment should always be conducted before permits are issued for an offshore dredging operation. The assessment should include a detailed dredge plan and programme for mitigation.|
A C T I C A L
R E S P O N S E S
Determine if dredging is a suitable option.
At some sites, dredging may not be a suitable option, perhaps because there are no suitable offshore deposits of sand available. Offshore dredging and beach nourishment are not necessarily options for every beach erosion problem.
|Consult professionals to determine if beach nourishment is a suitable solution for the particular beach erosion problem.|
Consult with other property owners.
Beach nourishment is a major undertaking and should always be done in consultation with other property owners.
Obtain permits from the necessary authorities.
It may be necessary to obtain permits from several different government agencies. Your planning authority will be able to advise you. In most islands a full environmental impact assessment must be prepared and evaluated before permission is given to dredge. The assessment may prove expensive and time-consuming; usually, it will have to be paid for by the developer or those wishing to dredge. It will usually be necessary to allow for at least twelve months to obtain the necessary permits and have the environmental impact assessment completed.
|Allow sufficient time for the completion and review of the environmental impact assessment.|
Obtain information about previous nourishment projects on your island.
If nourishment projects have previously been undertaken on your island or those adjacent, it is well worthwhile finding out details of these projects, such as costs, location of suitable dredges and time interval before a further nourishment was necessary. This information may well help you to determine the feasibility of your own project and provide a cost-estimate prior to the undertaking of the necessary studies and design, which are themselves an additional expense.
Be aware of the limitations of dredges.
Dredges usually need fairly calm conditions in which to operate, so sand sources on an exposed or windward coast may not be suitable. On leeward coasts, the best time of year for dredging will usually be in the summer months, provided there is no hurricane activity (the hurricane season runs from 1 June to 30 November).
Budget for additional replenishments.
Nourishment is almost never a once-only operation. Experience in Anguilla and other parts of the world has shown that the nourished beach may disappear in the next storm, which may very well occur the following year.
Make provisions for beach users during dredging.
The dredging operation will cause considerable disruption to the beach. For example, the settling ponds will take up most of the beach area. During dredging, the water may look ‘dirty’ because of the heavy silt load. It is necessary to advise beach users in advance and, if necessary, restrict access to the beach for safety reasons.
Consult local biologists about turtle nesting.
If the beach you plan to nourish is an important turtle nesting site, schedule the nourishment project outside the nesting season. Local biologists and the fisheries department will have information about turtles.