Environment and development in coastal regions and in small islands

Coastal management sourcebooks 1
Case 2

When a hurricane occurs

E N V I R O N M E N T A L
B A C K G R O U N D

Most hurricanes are experienced in September 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 strengthen, 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. (See Table 2.)

TABLE 2
Hurricane categories

The wind speeds in categories one to five refer to sustained wind speeds. Actual gusts may be much higher. While the higher category storms cause the most damage, even a tropical storm or category one hurricane may do considerable damage depending on its particular characteristics. For example, Tropical Storm Debbie resulted in significant flooding of Saint Lucia in September 1994.

Indeed, lesser storms also have the potential to cause serious beach erosion, especially when the waves approach from an unusual direction, such as from the west.

Based on a study of weather patterns and climate records over the past century, it appears that hurricane generation in the North Atlantic Ocean occurs in 20–25-year alternating cycles of activity and relative inactivity. The evidence indicates that we are now entering an active cycle. The years 1995 and 1996 were especially active, 1995 registering as the second-most active year on record with 19 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 affect other islands as swell waves. (See also Case 1.) For example, Hurricane Erika in September 1997 did not directly affect any of the Caribbean islands as it followed a track north of the Leeward Islands. (See Figure 8.) However, swells from this hurricane affected islands as far south as Grenada and as far west as Hispaniola. Thus, waves from a particular hurricane can affect islands several hundred kilometres distant from its immediate path.

Figure 8. Track of Hurricane Erika, 4–10 September 1997. Islands as far south as Grenada and as far west as Hispaniola experienced high swell waves from this hurricane (adapted from Bacon, 1978).

Tropical storms and hurricanes may cause catastrophic damage to beach systems as a result of: high seas, raised water level (known as a storm surge), high winds and heavy rainfall.

Strong winds generate high seas. 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. (See Figure 9.)

Figure 9. 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 (adapted from Bush et al., 1995).

Within a hurricane, wind circulates counter-clockwise around a central eye. (See Figure 10.) Most hurricanes affecting the Caribbean islands move in a westerly to northwesterly direction. (See Figure 11.) As the hurricane approaches, winds usually blow from the east and north. Once the centre, or eye, of the hurricane has passed, the wind direction reverses and the wind blows from the west and south.

Figure 10. Satellite view of Hurricane Iris, 1995. The winds blow counter-clockwise around the central eye (shown in black). In the eye the winds are calm (from National Geophysical Data Center, 1996).

Figure 11. Main hurricane tracks in the Caribbean islands. Most hurricanes follow a westerly to northwesterly direction in the Caribbean region (adapted from Bacon, 1978).

The high winds associated with hurricanes cause considerable damage to infrastructure and vegetation in both coastal and inland areas. The heavy rainfall often associated with these systems causes flooding of low-lying areas.

High rainfall often results in rivers cutting new channels through the beach. After a hurricane, beaches are littered with debris, which may include mounds of dead seagrass, rubble torn from the coral reefs and huge tree trunks which sometimes originate in other islands. After Hurricane Luis in 1995, a new species of iguana arrived in Anguilla from neighbouring islands, presumably via rafts of logs (Dudley, 1996).

Beaches have been measured on a regular basis in many of the Caribbean islands over the past ten years as part of the Coast and Beach Stability in the Caribbean (COSALC) project. Monitoring has provided considerable information about the effects of hurricanes on beaches. Information collected has shown several major types of beach changes resulting from hurricanes. One or more of these changes may affect a particular beach.

Following a hurricane, beaches are usually lower and narrower. In the aftermath of Hurricane Luis in 1995, the average beach size shrank by 28 per cent on seven islands (Cambers, 1996c). Thus, waves may be able to reach roads or buildings that were tens of metres away from the active wave impact zone before the hurricane. Some beaches may be totally stripped of their sand leaving rock outcrops exposed. Rocky ledges often consisting of beachrock may become exposed where they were not evident before. (See also Case 10.) Considerable quantities of beach sand will have been moved inland and deposited on coastal highways, in swimming pools and on beachfront properties. Large volumes of sand will have moved offshore. The water depth in the offshore zone may be much shallower than before the hurricane. Sand dunes which were previously gently sloping and covered with vegetation will now stand at a near-vertical angle bereft of vegetation. (See Figure 12.) In small sandy bays, sometimes all the sand will have been transported to one end of the beach. The material composing the beach may have changed from sand to stones and boulders. (See also Case 3.)

Figure 12. Dune retreat after a hurricane. The dune face has been eroded leaving a near vertical slope. In the months following the hurricane, the sand will slump to form a more stable slope (adapted from Cambers, 1995).

Photograph 8. Pre-hurricane beach conditions,  Barnes Bay, Anguilla, 1994. There is a wide  sand beach backed by palm trees.	Photograph 9. Post-hurricane beach conditions, Barnes Bay, Anguilla, 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.

The preceding list of effects of hurricanes on beaches is by no means exhaustive. Indeed, on rare occasions, there may be no change or even accretion at a particular beach. For instance, two islands impacted by Hurricane Luis in 1995, Antigua and Anguilla, each showed accretion at only one beach.

The land behind the beach is also impacted by the hurricane waves and storm surge. The maximum dune retreat recorded during Hurricane Luis in 1995 was 30 m (98 ft).

Dune or land retreat is important because features like sand dunes take decades to form. Thus, such changes may be regarded as semi-permanent.

Beaches, on the other hand, usually recover to a large extent after the hurricane. For instance, within eight months of Hurricane Luis in 1995, an 80 per cent average recovery was recorded (Cambers, 1996c). However, not all beaches show complete recovery after a hurricane. This was evident in Prince Rupert Bay in Dominica after Hurricane Hugo in 1989. Several beaches did not recover to pre-hurricane levels over a three-year period (Cambers and James, 1994). In addition, accretionary features such as spits and tombolos, when impacted by successive hurricanes, show major changes. In the two decades since Hurricane David struck in 1979, the Scotts Head tombolo in Dominica has shown continuous lowering to the point where it is now easily breached by a small storm. Similarly, in Saint Kitts, since Hurricanes David and Frederick in 1979 and subsequent hurricanes, Dieppe Bay spit has virtually disappeared.

Hurricanes also affect offshore systems, such as seagrass beds and coral reefs. In these environments, damage often goes unseen and its effects may not become apparent for several years. Coral reefs provide protection for beaches by acting as natural breakwaters. They also produce sand for many beaches. Thus, hurricane damage to a coral reef may result in beach erosion several years after the actual hurricane. (See also Case 10.)

P R A C T I C A L
R E S P O N S E S

Before the hurricane:

Hurricanes are inevitable and Caribbean islanders must plan for their impact.

A hurricane may not pass directly over a particular island but may pass close enough to cause a severe impact. People living in the Caribbean islands should know about these predictions and plan for their consequences.

Maintain a wide and stable beach.

A wide beach is the best protection against the high waves and storm surge generated by a hurricane. A beach is a flexible barrier which will be eroded during the storm but rebuilt quickly afterwards. Any measures which help to protect a beach or dune area, such as setting new buildings well back from the active wave impact zone, conserving natural beach and dune vegetation, dune stabilization practices and preventing beach sand mining, will help to conserve the beach as a natural storm barrier.

Establish maximum setbacks for new buildings, roads and other infrastructure.

If you are planning a beachfront property, obtain information from local planning and environmental authorities about beach changes during recent hurricanes and advice concerning safe setback distances for your property. (See also Chapter 2.) A coastal setback is a prescribed distance between a coastal feature, like the vegetation line, and a building. The depth of the land parcel permitting, the greater the setback, the safer the property.

Consult local insurance agencies regarding coverage for coastal property.

It is always best to obtain this information before investing in coastal land and starting to build, since insurance rates are increasing at a significant rate. On some islands, insurance – particularly for beachfront properties – has been difficult to obtain because of recent hurricanes.

After the hurricane:

Beach damage after a hurricane is impressive, but should not be cause for panic.

As soon as the hurricane has passed out of the region, waves will usually begin to bring the sand back onto the beach. Most of the beach recovery will have occurred within six months of the hurricane. Most beaches do recover about 80 per cent of their pre-hurricane size. There are always exceptions. If a beach has not shown significant recovery within twelve months of the hurricane, it may be necessary to take some direct action along the lines of that described in Cases 1, 4, 5 and 7.

Consider the soft engineering option of beach nourishment.

In some instances, it may not be feasible to wait for natural beach recovery. Since hurricanes usually occur in September in the Caribbean, the start of the important winter tourist season is often less than three months away. A particular hotel owner may consider it essential to try to get a beach back in time for the start of the winter tourist season. In such instances, it is certainly advisable to consider replenishing the beach with sand dredged from the offshore zone. During the hurricane, much of the beach sand may have been deposited just offshore. However, such measures are expensive and require detailed preliminary studies. (See also Case 5.)

Before starting any beach nourishment, it is advisable to wait until the end of the most active hurricane period. For instance, if the hurricane that caused the damage occurred in late August, wait until the end of October before implementing any remedial measures. In particularly active hurricane years, a second hurricane may well bring some of the eroded sand back to the beach system naturally. This happened in Saint Lucia in 1995.

Consider other soft engineering options like dune rehabilitation.

While the best recommendation for beaches is to allow them to recover naturally, dunes are another matter, since they often take decades to form. If the dunes have been eroded leaving vertical cliffs of sand, then it may be appropriate to regrade the slope to a stable angle (around 20 degrees) and to replant with natural grasses and vines. Alternatively, sand accretion may be promoted by the use of sand fences. (See also Case 7.)

Reposition hurricane-damaged structures.

If a particular building adjacent to the beach has been structurally damaged by the hurricane, consider all options before reconstruction. Evidence has shown that hurricanes result in the permanent loss of beachfront land and/or dunes. The land edge may be several metres farther inland after the hurricane.

If you have lost a building or fixtures, obtain the advice of coastal professionals and planning authorities concerning if, where and how to rebuild. For instance, sometimes the building can be rebuilt farther back from the beach, behind or above existing buildings.

Have your property resurveyed after the hurricane.

It may be advisable to have the property resurveyed six months or so after the hurricane, so as to record permanently any changes in the size of the land parcel.

Return sand to the beach after the hurricane.

Sand may have been washed onto beachfront property, into the swimming pool or into the building during the hurricane. Do not sell this sand for construction or other purposes. Returning this sand to the beach will help the beach recovery process and is the best way to conserve the beach.

Be aware that the full hurricane impacts may not be experienced until several years after the event.

When nearshore coral reefs have been damaged during hurricanes, the resultant impacts on beaches may not be experienced immediately. It may be several years before the beach starts to erode as a result of a sand deficit or a reduced reef breakwater effect, both of which may be caused by a past hurricane. It therefore pays to be vigilant and to use simple measurement methods and photography to record beach changes, as described in Case 1, Table 1.

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