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close this bookManaging Natural Disasters and the Environment (World Bank, 1991, 232 p.)
close this folderRisk management
View the documentDisaster response: generic or agent-specific?
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Coastal zone management

John R. Clark

Bangladesh, St. Lucia, the Philippines, and dozens of other countries are vulnerable to serious storms and flooding. The damage from these and other hazards could be reduced through programs that control the type, density, and location of coastal settlements. It is particularly important that such programs preserve natural landforms that take the brunt of storms and thus protect lives and community structures.

Many coastlines are at high risk of damage from natural disasters - particularly death and property loss from the winds and waters of hurricanes or cyclones. These violent storms born at sea strike the coast with winds up to 200 miles per hour (mph). Tsunamis and certain types of soil liquefaction, land sinkage, and landslides are also peculiar to coastal zones. Environmental characteristics such as daily tides, mangrove forests, coral reefs, tidal flats, and barrier islands are found only at the coast. In coastal zones, critical habitats have been carelessly destroyed, ecosystem processes disrupted, and waters heavily polluted - often as a result of donor-supported coastal development.

Strategies to reduce coastal hazards should take advantage of environmental planning initiatives. Many critical ecosystems and habitats - such as coral reefs, mangroves, and sandy beaches - are also key defenses against storm damage. The United States and other countries, in efforts to build sustainable coastal societies, have begun to experiment with combining natural disaster prevention and environmental management for coastal zones in a single comprehensive, multisectoral program called “coastal area management and planning” (CAMP) or “coastal zone management” (CZM). (For details, see case studies on Sri Lanka and Mexico.)

In many densely populated nations, population growth and development projects are increasing the risk of natural disasters to inhabitants of the coastal lowlands. Coastal people become more susceptible to natural hazards such as floods, typhoons, or tsunamis when land reclamation projects encourage settlement in dangerously low-lying areas, or when land clearing and construction remove protective vegetation, reefs, or sand dunes. A particularly disastrous example is Bangladesh, where more than 300,000 people were lost in major sea storms and floods in the recent past (Wijkman and Timberlake 1984).

Reducing losses from hazards begins with preservation of coastal landforms that provide natural resistance to wave attack, flooding, and erosion from hurricanes and storms. These landforms differ significantly around the world. Human activities that remove or degrade protective landforms - for instance, by removing beach sand, weakening coral reefs, bulldozing dunes, or destroying mangrove swamps - diminish the coast’s natural protection (Clark and others 1980). Removing dunes to mine sand or to improve ocean views, for example, increases the risk to coastal development behind the former dunes. Similarly, mangroves serve to dissipate wave energy and to protect the land behind them from the erosive forces of storms. The value of these natural resources in hazard prevention reinforces the need to identify them as critical areas and give them strong protection. They also serve a unique role in coastal ecosystems.

Measures to conserve ecological resources are often the same as measures to preserve the natural landforms that serve as barriers to storms and flooding. Consequently, many communities have found that combining hazards and resource management simplifies coastal management and leads to more predictable decisions about what constitutes sustainable development.

The same setback requirement that protects beachfront settlements from erosion and storm waves, for example, could also preserve turtle nesting sites. Similarly, a zoning restriction on development of mangrove swamps would both conserve an economically valuable resource and help maintain a defense against storm waves. In a final example, a seashore or coral reef park can protect these natural landforms as both natural resources and hazard protection (Salm and Clark 1984). Well-developed CAMP programs are authorized in most U.S. coastal states by the U.S. Coastal Zone Management Act of 1972 (P.L. 92-583). There has also been progress toward truly integrated programs in many other countries (Sorensen and others 1984).

Because large-scale development can increase coastal hazards beyond natural levels (Hausner and Sorensen 1984), it is the responsibility of governments and the development community to see that these additional risks are controlled and cost-effectively minimized, whether from cyclonic storm (hurricane) attacks, tsunamis, shore erosion, coastal river flooding, land and mudslides, or soil liquefaction. The main risk in the coastal zone is tropical cyclones, which can equal earthquakes in potential for property damage and deaths. With rapid population growth, more people inhabit coasts, increasing the risk of damage, disturbance, and death.

Hazards and natural defenses

The short-lived but intensive winds of hurricanes and cyclones exert enormous pressure on natural and constructed systems. They drive before them rising water, known as storm surges, which can sometimes elevate water levels to 20 feet or more. A moderately intense storm (such as Hurricane Alicia, which struck Galveston, Texas, in 1983 with maximum winds of about 110 mph) can raise water levels six to 12 feet above normal.

The winds build waves on top of the storm surge. As they strike the coastline, waves can increase flood elevations as much as 55 percent over the surge level. In an open ocean, waves 40 to 60 feet high and higher have been observed by seamen and operators of offshore oil rigs. But these wave heights depend on great depths of water beneath them. (A three-foot wave needs at least four feet of water beneath it to be sustained; a six-foot wave needs eight or nine feet, and so on.) As the wave enters shallow water the sea bottom slows the submerged portion of the wave and reduces the sustainable wave height, so the wave finally breaks.

The wave’s energy is quickly dissipated as the wave strikes the coastline, beaches, dunes, vegetation, and structures built in the wave zone that absorb this energy. At the boundary of land and sea, beaches and low-lying dunes may be scoured by waves and the scoured sand washed overland hundreds of feet or deposited seaward, where shallower water acts to trip the waves. By yielding to waves, beaches are efficient dissipators of wave energy. If the sand is deposited seaward, the water depth will be decreased, causing storm waves to break farther from shore. Thus, dunes are important suppliers of sand.

When dunes are removed by sand mining or to improve ocean views, the risk to coastal developments behind the former dunes is greatly increased. Similarly, mangroves dissipate wave energy and protect the land behind them from the erosive forces of storms. Reefs, beaches, dunes, and mangroves are important natural defenses against the ravages of wave action.

Reefs also act to trip waves. The reach between the reef and the shore is often too short and shallow to permit waves to build to the heights they reached before striking the reef. So important are reefs that many countries have special reef conservation programs. Sri Lanka, for example, organized a nationwide management program to protect reefs and save its southwest shoreline (see case study).

Development management

Two guidelines that are particularly important in coastal management and planning are:

· Conserve protective features - protect as much as possible all the natural elements that protect the coast from storm surge and waves in hazardous areas. For example, prohibit sand removal, avoid mangrove clearing, and protect coral reefs. (Conservation of these protective features is administered in the same way as conservation of natural habitats.)

· Establish a coastal construction setback line. Delineate a “high hazard zone” for the coast and keep all coastal construction inland of it.

The “high-hazard zone” is that part of the coast that is periodically subject to flooding (rising still water) from storm surges and to the effects of fierce storm waves (including erosion and property damage). The periodicity of storm and flood events is calculated as the chance that a hazard event will strike in any one year. The result, often called the “recurrence rate” or “return probability,” is given as the percentage chance that an event will occur in any one-year period. Thus, a recurrence rate of 0.10 at a particular site means there is a 10 percent chance that a damaging storm will occur there in any one year.

With that information, the CZM authority can place the coastal setback at a particular risk point far enough back from the high water line that all structures behind it have only a 0.04 (4 percent) probability of being hit by a flood or storm waves. The 4 percent probability level is sometimes called a 25-year event because four chances in 100 equals one chance in 25. In placing the setback line, the CZM authority could pick the 20-, 50-, or 100-year event (0.05, 0.02, or 0.01 probability) as the controlling risk factor, and establish a corresponding distance inward from the high water line. The degree of precision needed to delineate and map the line depends on the program. The setback line is only for storm hazards. If there are also boundaries and buffer areas for essential habitat types, the two should be combined in a single setback line.

The most troublesome erosion of beaches occurs in developed areas where buildings and roadways have been placed too close to the water’s edge and are being undermined or threatened by storm-induced erosion. In such cases, the beach is often “armored” - that is, seawalls or groins are built to protect threatened properties or jetties are built to keep inlets open. But these structures are expensive and may even worsen the general erosion.

Planning approaches should reduce the damage from future disasters. Virtually any development project in a coastal area will be affected by, and will have an effect on, the risk of hazards. Roads are expensive to build but easily washed away in a flood. Roads in coastal areas should be designed not just to be safe from flood damage, but to be adequate to evacuate local populations when a severe storm is anticipated. Similarly, housing built in hazard-prone areas should be built on sites and to standards that assure personal safety.

Coastal development projects attract more people to and around a project site, increasing the number of lives and the amount of personal property in jeopardy. For many reasons, the principal city in most tropical nations is a port, and people migrate to such cities for economic potential that cannot be found upland. Development in coastal areas around the Bay of Bengal, for example, continues to stimulate enormous population growth despite recurrent cataclysmic cyclones and floods in the area.

Poorly controlled development often has destructive effects on coastal natural resources. Demands for waterfront land have been intense in many countries. Developers have encouraged and satisfied these demands and, in so doing, have frequently imposed high capital and service costs on coastal communities. Moreover, poorly planned development can be destroyed quickly and at great cost in floods, severe storms, and hurricanes (Clark and others 1980).


Fully comprehensive CZM programs aim both to prevent or mitigate natural hazards and to conserve coastal resources (Sorensen and others 1984). These two purposes are both compatible and mutually supportive. They both do the following:

· Require integrated approaches to influence where development occurs and what types of structure are built, at what density.

· Should involve all levels of government, national to local, and international cooperation when appropriate.

· Stress preservation of the natural elements - such as mangrove forests, dunefields, and coral reefs - that protect coastal populations from cyclonic winds and storm surges.

Case study: Sri Lanka

John R. Clark

The use of coastal zone management (CZM) for hazard prevention in Sri Lanka was motivated by persistent coastal erosion and storm damage caused by the mining of coral reefs along the south-west coast. Sri Lanka’s Coast Conservation Department recently completed four years of intensive work on a plan to prevent erosion and the loss and degradation of coastal natural habitats and to protect scenic areas and cultural and religious sites. The plan, developed in coordination with other Sri Lankan agencies responsible for coastal resources, represents the best reaction to the nation’s coastal problems. It provides a policy framework and a practical strategy for dealing with the problems (Olsen 1987).

For each important issue, the plan presents management strategies, which include regulation, research programs, better intergovernmental coordination, and public education. The erosion management strategy, for example, establishes a setback line to ensure that structures are not placed so close to the shoreline that they contribute to or are affected by erosion. Regulatory measures prohibit the construction of shoreline protection works in some locations and establish review procedures for building such structures along the rest of the coast. Coral and sand mining are also regulated because they accelerate coastal erosion. Other elements of the erosion management strategy are a public education campaign to make coral and sand miners aware of the impact of their activities, a program to identify alternative employment for displaced coral miners, and research to identify alternative sources of lime for the building industry. Complementing these management efforts is a public investment program to build shoreline protection works where appropriate.

Agencies responsible for the prevention and mitigation of natural hazards and agencies responsible for resource conservation and environmental protection should both be interested in advancing CZM programs.

The dual goals of CZM - conserving coastal resources and maintaining nature’s hazard protection systems - can save money, lives, and property. As growth along the coast accelerates through development, the publicly assumed liability for storm damage increases in many countries. Reversing this trend means preventing increased exposure to hazards and reducing the public assumption of liability. Prevention of natural hazards should be part of CZM planning. If no other government agency is dealing with the maintenance of natural storm defenses, CZM should be. Officials who are responsible for coastal hazards may concern themselves mostly with emergency response and postdisaster relief, ignoring the condition of coastal protective resources. That is why it is often essential for CZM to play a primary role in prevention.

Because of the link between development and disasters, an important aim of CZM is to integrate knowledge of coastal hazards and risks into planning for development. Guidelines for estimating how a project or program affects risk of coastal hazards should be applied to every development proposal. Many actions can be taken to assure that any project does not increase risk and, further, that the project can be implemented in a way that even reduces existing hazards and that is cost-effective. An example worth studying is the massive runoff from rainstorms associated with a major “El Ni148; event in 1983 that caused heavy property damage on Ecuador’s coast and disabled much of the aquaculture industry. Hazard assessment can be accomplished through CZM mechanisms for project review and environmental impact assessment (Hausner and Sorensen 1984).

Case study: Hurricane Gilbert in Yucatan, Mexico

John R. Clark

Hurricane Gilbert of 1988 was the record cyclonic storm of the Western Atlantic, with the lowest internal barometric pressure (885 mb) ever measured in the Western hemisphere. Wind speeds of more than 200 mph were recorded by National Oceanic and Atmospheric Administration (NOAA) aircraft flying at 10,000 feet east of the Yucatan peninsula, when the eye was eight to 10 miles in diameter. Because of a ridge of high pressure to the north, Hurricane Gilbert held to an unusual, almost straight-line, west-northwest track, rather than curving north through the straits between Yucatan and Cuba.

Gilbert reached hurricane status on September 10 and struck the Yucatan coast on the morning of September 14, with its center near Cozumel. The cyclone had broadened and wind speed had diminished so that, on arrival, the wind speed at ground level was 115-150 mph. Hurricane Gilbert then moved west-northwest across the peninsula, exiting in the vicinity of Progreso on the northwest Yucatan coast. Hurricane-force winds covered a swath of about 100 miles, rotating around an eye about 25 miles in diameter. Ocean surge heights averaged an estimated eight to 10 feet and wave heights 10 to 15 feet. Damage was extensive in the resort areas of Quintana Roo (for example, Cancun) and in rural and coastal Yucatan.

There were north winds on the front edge of Hurricane Gilbert, a prolonged calm in the eye, and southeast winds on the back edge. Damage was caused by both wind components but the north winds caused more wind, wave, and surge damage. Return flow damage across barrier islands from north coast lagoons was greater on the southeast component. Flooding occurred up to 12 miles inland in Yucatan.

Only 27 persons died (20 more were listed as missing) but 35,000 people suffered property damage - 13,000 homes in 63 towns. And losses in natural resources, habitats, and species were high. For example, as many as 3,000 to 8,000 adult flamingos were dead or missing out of a pre-Gilbert population of 20,000 to 25,000 - and 150 fledglings died. (A week after Gilbert struck we counted 16,000 survivors in an aerial survey.) An estimated 15,000 hatchling sea turtles (greens, hawksbills) were lost and three turtle hatching pens were destroyed. But the most serious effects on the north Yucatan coastal ecosystem were from 21 cuts through the barrier island strand into the estuaries - including cuts in Rio Lagartos. The Rio Lagartos cuts have serious implications because that hypersaline river provides the only nesting environment and a favored feeding area for the Yucatan population of flamingos.

After Hurricane Gilbert passed, leaving Rio Lagartos open to the sea’s entry, salinity dropped dramatically. The Industria Salinera salt works - Los Colorados’ main industry on Rio Lagartos - was put out of commission for nearly two years because of the drop in salinity, nearly total destruction of the charcos (salt pans), the destruction of infrastructure and roads, and the demolition of mills, warehouses, and pumps.

High winds destroyed the important lumber industry in Colonia Yucatan, leaving many jobless. Losses to the fishing industry varied. Because of a well-organized predisaster program - all persons were evacuated from the coast and most small boats were hauled ashore or put in safe harbor - small-scale fisheries sustained little damage and within seven days 90 percent of the boats were ready to operate. But the port of Progreso/Yucalpeten, with larger craft, sustained serious losses: 200 boats were damaged altogether, and 85 boats driven aground in the harbor required major salvage operations (about $10,000 each).

Agriculture also suffered. Half of the area’s 3 million chickens were killed, and 100,000 beehives were destroyed, for a loss of 3,500 tons of honey valued at about US$3 million. The corn crop loss was estimated at about 95 percent of the total planting of 150,000 hectares. There were also extensive losses in fruit, sorghum, rice, and beans.

Perhaps the worst economic effect was a midterm loss in tourism revenues in areas such as Cancun and Cozumel. Almost all tourist reservations at Cancun were canceled - most with refunds. It took two to 20 months to rebuild the hotels and resorts. The power and telephone systems were heavily damaged. The beaches at Cancun lost an average two meters deep of sand, with no practical way to restore them other than to wait for natural processes to act. Sixty percent of corals were detached from reef strongholds. Ninety percent of mangroves were destroyed. It may take several more years to restore operations and regain tourist confidence.