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Environment and development in coastal regions and in small islands |
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Environment and development in coastal regions and in small islands |
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Coastal
management sourcebooks 1
Case 4
Assessing the impact of coastal structures
E
N V I R O N M E N T A L
B A C K G R O U N D
Beaches are constantly changing formations. They change over hourly, daily and yearly time-periods. Any sea defences built on or near the beach will influence the natural processes. Sea defences may result in beach accretion or erosion, or both of these on different parts of the beach. Sometimes, the changes may be seen just months after the structure is built. At other times, it may take years.
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.
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Any solid sea defence structure built on or near the beach will interfere with natural changes in the beach. |
Structures built parallel to the shore
Generally, such structures are 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. The main types have already been described in Case 1. (See also Figure 6.) 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. New, stronger structures may then be required to withstand the increased wave impact, making maintenance costs higher.
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Seawalls and related structures only protect the land and property behind the beach. They do not promote beachbuild-up. |
In the Caribbean, a retaining wall is often built adjacent to the beach to define property boundaries and to provide privacy for hotels and residential property. When beach erosion takes place, these retaining walls come under wave attack and they may collapse. They are designed to retain land and soil, but not to withstand wave impact. As one wall fails, it often takes an adjacent wall with it. A properly designed bulkhead, however, will have additional features, such as filter cloth, toe protection, tie backs and drainage holes. (See Figure 13.)
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Figure 13. Features of retaining walls and bulkheads. |
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A seawall, bulkhead or revetment protects only the land and buildings immediately behind it. Erosion will continue in front of the unprotected land on either side of the structure and the waves will eventually cut in behind it. (See Figure 14.) This is why it is usually necessary to build return walls, which provide flank protection.
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Figure 14. Bulkheads and flank protection. Without flank |
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Photograph 14. Seawall and
groynes, Cockburn Town, |
As erosion continues, however, the protected section of coast will become like a small headland projecting seaward of the main coastline. Wave energy will become concentrated on the adjacent unprotected shoreline. A community approach is often favoured. (See Figure 15.) This can reduce the need for individual return walls, as well as being more economical and efficient.
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A co-operative approach to shoreline |
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Figure 15. Community
approach to shore protection. |
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Photograph 15. Boulder
groynes, Nisbett Plantation, |
Nature does not recognize property boundaries; erosion or accretion can occur anywhere. It will usually affect more than one property along a particular beach.
In most of the Caribbean islands formerly under British rule, property owners have the right to protect their land. This is the case under British Common Law. However, in some of the islands, more recent coastal legislation may have established other rights.Throughout the Caribbean, it is necessary to obtain planning permission before building any structure on or near a beach.
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Planning permission is required before any structure can be built on or near the beach. |
Structures built at right angles to the shore
When waves approach the shoreline at an angle, they generate a longshore current which transports sand suspended in the water along the shore. (See Figure 16.) In addition, the wave-generated turbulence carries sand up the beachface in the general direction of the wave approach. As the water returns seaward, it moves directly down the beachface in response to gravity. Thus, sand moves along the beachface in a zig-zag motion. (See Figure 16.)
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Figure 16. Longshore sediment transport. When the |
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Figure 17. Effect of a single groyne. The beach has |
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Structures designed to trap this moving sand are called groynes. They are low walls constructed perpendicular to the shoreline which extend out into the water. 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. Sometimes, gabions are used, although, as mentioned in Case 1, they are not recommended as materials for groyne construction because of the need for continual maintenance. Wood or steel may also be used for groyne construction. Groynes may be built singly, or in groups known as groyne fields. (See Figures 17 and 18.)
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Figure 18. Effect of a groyne field. Sand moves |
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While groynes trap sediment on one side, they cause a sand deficit and therefore erosion on the other side. (See Figure 17.) In effect, one property owner may gain at the expense of a neighbour.
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Groynes usually result in sand accretion on one side and erosion on the other side. |
A properly designed groyne field should ensure that, as the sand builds up to the end of the groyne, material passes over or around the end of the groyne to the downdrift shore, but at a slower rate than before its construction. (See Figure 18) It is sometimes necessary to combine groyne fields with beach nourishment. In such cases, the groynes help to reduce the rate of longshore transport along the beach and thereby retain the sand where it was placed during the nourishment project.
Groynes function most effectively along coastlines where the direction of longshore transport is constant. In the Caribbean islands with their prevailing Northeasterly 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 coastlines and are least effective on east- or west-facing coastlines. For example, groynes have worked reasonably effectively on the north coast of Nevis. Similarly, much of the south coast of Barbados has been stabilized with groynes.
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Groynes only function well under certain specific conditions, they are not a ‘cure-all’ for every beach erosion problem. |
Sometimes, solid structures built as jetties or piers, essentially used for boat docking, have inadvertently functioned as groynes. Piled structures are recommended for jetties used for boat docking.
Groynes are often seen as a ‘cure-all’ for any erosion situation. However, this is not the case. Groynes are just one of several ways to control beach erosion. Thus, as with all coastal structures, careful design by professionals experienced in coastal processes is required.
 Source: Crowns of Thorns Newsletter, 1990a. |
Offshore
breakwaters are placed out in the water to intercept the energy of the
approaching waves, thereby sheltering the shoreline on their landward side.
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Figure 19. Plan view of an offshore breakwater. The |
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Figure 20. Floating tyre breakwater. These
are only |
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Breakwaters are built out in the sea, usually parallel to the coast or sometimes angled to the shoreline. They are usually made of interlocking concrete shapes or large boulders. Generally, they are more expensive than groynes or seawalls because they are of massive construction designed to withstand breaking waves. Also, they must be constructed in the sea, rather than placed on land. Construction material must be moved by barge or by building a temporary ramp from the land out to the construction location.
Breakwaters may emerge above the sea surface or they may be submerged. Floating breakwaters, which are constructed of buoyant materials like used tyres, may be used in sheltered wave environments like marinas to reduce the wakes caused by passing boats. (See Figure 20.) They are not suitable for open water conditions.
While it is impossible to provide specific costs for the structures described in this section since conditions and materials vary from island to island and from site to site, Table 3 provides some general cost ranges to give an order of magnitude.
TABLE
3
General cost ranges for sea defence structures in the Caribbean islands
(1998 estimates).
| TYPE OF STRUCTURE | COST PER METRE LENGTH OF STRUCTURE (US$) |
| Rock revetments | 650 – 975 |
| Groynes (rock) | 650 – 975 |
| Groynes (pre-cast concrete) | 680 – 1,170 |
| Offshore breakwaters (rock) | 2,925 – 3,900 |
| Offshore breakwaters (pre-cast concrete) | 3,250 – 4,225 |
P R
A C T I C A L
R E S P O N S E S
A response is always possible, although it will differ according to whether the structure exists, is under construction or still in the planning phase.
If a structure exists or is under construction:
Act at the first sign of construction.
The best time to act is as soon as there is any sign of construction, such as a surveyor measuring the site. Once the actual coastal structure is in place, it is very difficult to get it modified or removed.
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Take necessary action as soon as there is any sign of construction on the beach. |
Check whether the structure has the necessary permits.
While property owners in most Caribbean islands have the right to protect their land, planning permission is required for a structure on or behind the beach or out in the sea. Find out if permits exist and whether there are any conditions attached to the permits. Sometimes, the conditions relate to possible changes to neighbouring properties. You are in a much better position to negotiate if you are in possession of the facts.
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If a newly built coastal structure is affecting your property or a beach you visit, first check whether planning permission was granted. |
Take photographs during and after construction.
Take photographs as the structure is being built and afterwards. Always date the photographs and try to take them from the same position, so that they always show the view from the same angle.
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Photographs showing conditions before and after the structure was built often provide backing for a possible complaint. |
Consult with the owner of the coastal structure that is affecting you.
This can be done verbally or in writing. It often pays to have evidence in writing and thus to build a ‘paper trail’ of the complaint.
If the owner does have plans to modify the structure to reduce the impact on your property, determine the nature of the planned modifications and when they will be implemented.
| Determine whether the owner of the coastal structure has any plans for modification, expansion or compensation for you. |
Consult with planning and other authorities.
While you may have already checked with planning authorities concerning the permits for the structure, it will be necessary to lodge an official complaint with them if the owner of the structure is not willing to co-operate with you by modifying the structure or providing compensation. It may also be useful to make your complaint known to other authorities, such as your parliamentary representative. At this stage, you may wish to involve the local media and have them publicize the situation.
| Any complaint should be made in writing with a full description of the relevant facts, such as conditions and dates. |
Consult with other homeowners who are affected by the structure or who might become affected in the future.
The effects on shorelines of most coastal structures are cumulative. While only one property may be affected initially, the erosion may extend to several other property owners along the shoreline in the following months or years. Although it may be difficult to achieve a consensus approach among adjacent property owners, it is ultimately the most effective way of dealing with all shoreline problems.
Carefully consider your options.
The erosion that caused your neighbour to build a seawall, revetment or similar structure in the first place may well progress in front of your property, so you may need to consider protecting your property. If this is the case, have your structure properly designed, obtain the necessary permits and try to co-operate with other property owners, since this will be in everyone's interests.
Similarly, if a groyne has been constructed, there may be opportunities for sand bypassing, which may involve moving sand from the updrift side of the groyne to the downdrift, eroded side. Alternative solutions include trucking the sand or pumping a sand and water mixture around the structure.
A similar opportunity may exist with accretion behind an offshore breakwater. These approaches will require cooperation and cost-sharing among all the property owners involved.
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Co-operate with other property owners to ensure coastal structures are effective and economical. |
If you are planning to build a structure:
Act before the situation becomes an emergency.
Once the situation becomes an emergency, your options are going to be limited and the actual construction will be more difficult and costly.
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It is always more effective and less expensive to take the necessary action before the waves start eroding your property. |
Obtain good technical advice about design before any construction is started.
Discuss all available options at your site. In particular, ask about: maintenance costs, the life expectancy of the structure, how the structure will perform under hurricane conditions, likely impacts on adjacent properties and environmental impacts.
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Obtain advice from a professional with experience in coastal structures and their environmental impact on your island. Your planning agency will be able to recommend qualified persons. |
Consult with the planning authorities for advice and the necessary permits for the proposed structure.
Always inform your neighbours about your project, since they may be affected and there may be the possibility of a less costly co-operative approach.
