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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 7
When sand dunes have been destroyed
E N V I R O N M E N T A L
B A C K G R O U N D
Sand dunes are mounds of sand that often lie behind the active part of a beach. In the Caribbean, they range from very low formations 0.3–0.6 m (1–2 ft) high to large hills of sand up to 6 m (20 ft) high. There may be several parallel rows of dunes which are then usually named primary, secondary and so forth, starting from the most seaward line. Dunes form when sand is carried by the wind from the beach towards the land. When the wind encounters an obstacle, like a clump of vegetation, the wind slows down and the sand is deposited.
| Dunes may best be regarded as reservoirs of sand. They have even been likened to sand savings accounts (South Carolina Coastal Council, 1987). |
The rate of sand movement depends on the wind speed, sand grain size and the amount of moisture in the sand. Fundamental research on desert dunes (Bagnold, 1954) shows that significant sand movement will take place when the wind speed measured at a height of 1 m above ground level exceeds 12 knots (6 m per second). In the Caribbean, average wind speeds equal or exceed this value, especially in the months from June to July and December to March.
If, during a storm or hurricane, the beach is eroded and the waves reach the dunes, then the dunes will also suffer erosion. The sand is then carried into the water and possibly to a deposit offshore. (See Figure 23.) This sand deposit then absorbs some of the destructive wave energy that would otherwise focus on the beach and dunes. Following the storm, the sand is moved back onto the beach. As the beach accretes, the process of sand dune formation starts over again.
| Figure 23. Storm wave attack on a beach and dune. During the storm, the beach is eroded and the waves begin to attack the dune, resulting in a vertical dune face. The dune sand is deposited offshore and will eventually return to the beach (adapted from Coastal Engineering Research Center, 1984). |
| Before
the storm
During the storm
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This process was seen at many sites in the Caribbean during Hurricane Luis in 1995. Dunes and beaches were severely eroded and the sand was deposited in offshore sand bars. These sand bars migrated onto the beach in the weeks following the hurricane.
| Sand dunes and beaches must be managed as one system. Dunes depend on beach sand for their formation and beaches need the reservoir of dune sand during storms. |
Sand dune formation is a slow process when compared to beach changes. Four months after Hurricane Luis, which occured in September 1995, measurements in Anguilla showed that beaches had recovered to 75 per cent of their pre-hurricane levels (Cambers, 1996a). However, the dunes, which showed an average retreat of 9 m (30 ft) after the passage of Hurricane Luis, will take decades to recover to pre-hurricane volumes.
| Sand dune formation is a slow process. It may take decades. |
Dune vegetation promotes the large-scale trapping of sand. Dune plants have to adapt to harsh conditions that include high temperatures, dryness, occasional inundation by saltwater and the accumulation of sand. Generally, native beach grasses, such as seashore dropseed (Sporobolus virginicus) and trailing vines like beach morning-glory (Ipomoea pes-caprae) are the most hardy species, often being found on the seaward face of the dunes.
| Dune vegetation has to survive harsh natural conditions. It cannot withstand heavy trampling. |
| Sand dunes are temporary features. They are reservoirs of sand that feed the beach during storms. |
Unfortunately, sand dunes are often seen as prime sites for sand mining activities. (See also Case 6.) Like wetlands, sand dunes have often been perceived as ‘useless’ areas. Their many functions, which include acting as a sand reservoir for the beach and protecting land areas from harmful salt-laden winds, have been little understood. Many dunes in the Caribbean have disappeared altogether as a result of mining activities like those formerly present at Josiah's Bay in Tortola, British Virgin Islands, and at Diamond Bay in Saint Vincent. At the latter site, dunes more than 6 m (20 ft) high consisting entirely of black sand have been completely mined out, leaving the sea to cut into the lowland that once lay behind the dunes. (See also Case 6.)
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| Photograph 21. Breach in
the dune line after high seas, allowing flooding of the land behind the dune, Isabela, Puerto Rico, 1996. Extensive mining has left only a narrow strip of dune. |
At Isabela on the north coast of Puerto Rico, the mining of an extensive system of dunes has left a narrow strip of dunes between the land and the sea. Erosion during winter swells results in breaching of the dune line, leaving the land behind the dune vulnerable to flooding.
Dunes are the result of decades of slow accretion. The artificial pushing up of mounds of sand to form ‘instant’ dunes does not usually work well. Under normal conditions, as dunes grow naturally, each new layer of sand compacts the layers below, so that a firm structure forms. However, there are ways to speed up the process, such as building sand fences. While they are used in other parts of the world, sand fences have been little used in the Caribbean. Studies in other parts of the world have shown that a sand fence with a 50 per cent porosity (ratio of open space to total area) can result in a 1.2 m (4 ft) high dune forming in twelve to twenty-four months (Clark, 1995).
| The formation of sand dunes can be speeded up with sand fences and revegetation. |
Obviously, dunes will not form at every beach. One of the most important criterion is that there be a large area of dry-sand beach over which the wind can blow and pick up the sand grains. Unless a wide dry-sand beach is present, dune formation is unlikely to take place.
| There must be a large area of dry-sand beach over which the wind can blow in order for dunes to form. |
P R A C T I C A L
R E S P O N S E S
Conserve existing sand dunes.
While sand dunes are temporary features, every effort should be made to conserve existing dunes. Proposals to mine the primary dune or to lower it to provide for improved views should not be entertained.
However, where several rows of dunes exist, it may be possible to alter or use the secondary or tertiary dunes to build or supply construction sand. Nevertheless, special care must always be taken in developing such areas, so it is wise to consult your planning agency. In the case of mining activities, mitigation plans and site restoration activities should play a prominent role in any applications.
Do not build on the seaward or primary dune.
This is the first line of defence in a storm and should be left undeveloped so that it can fulfill that function. Remember that dunes are temporary features of the landscape. Always build behind the primary dune (See Figure 24.) and, if at all possible, well behind it. Building on piles is also recommended. This allows for some sand movement without the risk of building collapse.
| Figure 24. Recommended
construction on a dune. Here, the primary dune has been
left intact. The building has been built on piles to allow for the uninterrupted flow of floodwater and positioned behind the primary dune (adapted from the US Department of Housing and Urban Development, 1981). |
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On some islands, it may not always be feasible to avoid building on the primary dune. For instance, there may be only one dune between the beach and a salt pond or wetland. Where this dune land is privately owned, it willprobably be zoned for development. In such cases, the most important guideline is to try to build as far back from the sea as possible.
| Do not build on the primary dune. |
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| Source: Crown of Thorns Newsletter, 1990b. |
Encourage dune planting.
If low dunes are beginning to form on a beach, these can often be enhanced by planting grasses and vines to hold the sand and encourage further sand accumulation. Consult local horticulturists to determine which plants will best survive.
After a hurricane, regrade the slope and replant the dunes.
If a recent hurricane has left vertical cliffs of sand with no vegetation, it may be appropriate to regrade the slope and replant. However, it will be necessary to consult your planning agency to obtain permission first.
Consider whether sand fences could be used to speed up the process of dune recovery.
After a hurricane, the best solution for beaches is often to let them recover naturally. (See also Case 2.) However, sand fences can be erected to help dune recovery after a hurricane or a mining operation. Some guidelines for sand fences (adapted from Craig, 1984 and Clark, 1995) are as follows:
Remember to find out if the beach is a sea turtle nesting site. If it is, you may need to schedule the sand fence construction outside the nesting season, since the physical construction of the fences may disturb the turtle nests.
Avoid walking on sand dunes. Construct wooden walkways.
Dune vegetation cannot withstand the continual pressure of foot traffic. This can kill the vegetation and result in low spots in the dune line. These depressions then become the lines of weakness that waves can break through during a storm. Wooden walkways should be constructed over the dunes along well-used paths that follow the existing dune contours. Notices may also be posted to advise people not to trample the dune vegetation.
