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Coastal erosion's influencing factors include development, dams, wells, and climate change.

The demographic flight to the coast, begun in early civilization, continues unabated worldwide according to latest studies. The percentage of population living on the coast is expected to remain relatively constant over the next few decades, but the total numbers will increase as the population increases. Recent coastal battering by hurricanes and extratropical storms poses questions about coastal habitability and the real economics of coastal development. Hurricanes Hugo (1989) and Andrew (1992) in the southern US and Bob (1991) in the northeast, as well as various coastal storms, recently caused widespread damage to coastal property. Repair costs are borne by private individuals as well as the public in various direct and indirect ways. As these costs escalate, it is fitting to ask what the future portends for storm and coastal-flood damage.

We know that development pressures will continue to increase along the coast, but what will happen concurrently to natural-hazard threats to this infrastructure? Though much emphasis has been placed on sea-level rise, the broader issue is climate change in general. Here, we consider climate change in both its natural and anthropogenic perspectives. Without becoming mired in the debate about the greenhouse effect and human influence on climatic shifts, we can examine some of the broad classes of natural hazards that might accompany climate change. There are several categories of possible global-change effects on coastal erosion.

Relative Sea-Level Rise

In the early 1980s, an Environmental Protection Agency (EPA) report postulated increases in global sea level up to 4 meters during the next 100 years. Though balanced somewhat by other, lower estimates of sea-level rise, this higher extreme grabbed public attention. During the next decade, scientists attempted to concur on a more reasonable estimate of global sea-level rise due to climate change. Recent credible estimates suggest that approximately 10 to 20 percent of EPA's earlier maximum estimate is most reasonable. This estimate is for global sea-level rise in the ocean basins. Geologists have argued for more than a decade that relative sea-level rise, the combination of land movement and ocean rise, is a more critical coastal management consideration than sea-level rise alone. In some places, land subsidence due to tectonism (reshaping of Earth's surface through rock movements and displacements) and other factors increase relative sea-level rise; in others, the coast is actually rising compared to sea level. Climate change will primarily affect the global sea-level rise signature, and hence is of concern for the future. However, in many places local effects will continue to dominate global sea-level rise for decades to come, and effective coastal management requires recognition of this factor.

For example, a global map of relative sea-level rise, estimated from tide gauge records, shows anything but a uniform rate of rise. Rather, these numbers suggest that coastal managers must consider such local effects as tectonism, and not rely solely on present and anticipated global average values. In the future, any climate-induced increase in rate (if it occurs) will have to be factored into local considerations.

Human Influence on Local Subsidence

Human activities such as groundwater withdrawal and hydrocarbon extraction can cause land to subside. These very local phenomena can have severe effects. For example, oil extraction at Terminal Island in Wilmington, California, resulted in nearly 8.8 meters of subsidence in three decades! As a consequence, roads and bridges had to be rebuilt, levees were constructed, and drilling was modified. A global subsidence map clearly shows many cases of local coastal subsidence due to groundwater withdrawal. Perhaps the most celebrated case is Venice, Italy. During the 1950s, surface water gave way to higher-quality groundwater as the primary source for household and industrial water supplies. With today's greater awareness of the effects of concentrated groundwater withdrawal, wells can be engineered to minimize such impacts. In Bangkok, Thailand, for example, where subsidence rates reached 10 centimeters per year in some areas in the early 1980s, well-field re-engineering prevented canals that had been filled to form roads from reverting to canals during river floods.

Coastal Storms

Presently coastal storms cause major damage to coastal facilities and infrastructure. Tropical cyclones are the most feared: One 1970 typhoon in Bangladesh caused more than 250,000 deaths, and a similar storm there in 1991 left more than 100,000 dead. Though most storms have less-adverse impacts, there is concern about storm-activity trends if the global climate changes. There are three storm aspects to consider: changes in frequency, intensity, or path. Any one of these can cause an increase or decrease in coastal erosion. Unfortunately, we are unable to predict the storm trends well. Work done in the mid-1980s by Kerry Emmanual (Massachusetts Institute of Technology) shows that tropical-storm intensity can be expected to increase under certain assumptions of climate change. However, storm frequencies and pathways are not as predictable. How should shorelines be managed in the face of such uncertainty? As recent storms have shown, the storm climate is strongly variable from year to year, and coastal managers must plan for such variability. Ten years of benign weather don't argue for another ten years of such weather. This longer-term variability and susceptibility must be incorporated adequately into policy considerations. Human nature being what it is, we often tend to discount distant history in favor of our more recent experience.

Altered Sediment Delivery to the Cost

Coastal development viability depends not only on the energy of the destructive natural forces, but also on the ability of coasts to heal themselves after storms by providing an adequate sediment supply. Increased river diversion and damming have reduced the flow of sediment to many coasts. Reduced sediment delivery leaves a deficit of sand on beaches, lessening their self-healing capabilities. There are many examples of human activities reducing sediment supply and consequent increase in coastal erosion. Some sediment-supply change may be linked to climatic variations: a shift toward a more-or-less arid climate can increase or decrease sediment supply, depending on the region. Long-term variations in precipitation and river flow, as well as human activities affecting these parameters, will therefore continue to concern coastal managers.

Destruction of Coral Reefs and Other Biogenic Sources

Biogenic sediment is an important component of the general sediment supply. In many tropical areas, the primary beach material consists of degraded shells, reefs, or other biogenic materials. As climate changes, the ability of corals to keep up with sea-level rise may also change, altering the amount of sediment they produce. Furthermore, naturally nutrient-poor tropical island waters are being altered by continuing development, which tends to increase nutrient levels and change the structure of coastal tropical ecosystems, leading to loss of corals in some areas. This chain of events eventually will result in increased erosion, both through alteration of sediment supply and loss of direct wave sheltering by the massive offshore reefs. A geographically related issue is storm protection provided by marshes and mangroves. Human activities--such as marsh destruction and using mangroves as a local fuel source--are eliminating these resources in many areas, thereby reducing the biogenic shield against erosion. The importance of these resources for shoreline protection as well as for ecosystem balance is not well understood the world over (see Zedler and Powell, page 22).

Though climate change may have some adverse effects on corals and other biogenic sediment sources, it can also have a positive impact. Warmer water temperatures may expand the corals' geographic range beyond the tropics. ever, coral bleaching (a recent disease killing many corals) and related problems also have been linked to increased temperatures, so the balance of biogenic input is unclear in a warming-ocean scenario. Finally, scientists speculate that beach rock, the hardening of beach sands into rock that lines the coasts of some tropical regions, may become more abundant as water temperature rises, thereby increasing protection and reducing erosion. These areas of research must be addressed before we can accurately estimate the effects of climatic changes on tropical beaches and their sediment supplies.

Environmental pressures on the world's coasts will continue to increase, with or without global change. Increased habitation, greater development, more pollution-these are the byproducts of denser coastal population. Changes in natural disasters affecting the coasts will accompany rising population: Storm climate, sediment supply, and natural coastal protection all may change along with climate. How do we plan for this? Is there something we can do now to prevent or anticipate such change?

Fortunately, simple options can help alleviate some of the longer-term surprises before they become too damaging. Developing uniform guidelines and practices for shoreline management is a useful step in this direction. What types of guidelines? Simple exercises, such as identifying dominant sediment sources, clarifying exposure to natural hazards such as storms, and quantifying the shoreline's susceptibility to various human pressures (such as nutrient supply and coastal armoring), can all help us understand the vulnerabilities. Once the vulnerabilities are understood, measures can be taken to monitor the system for future changes, implementing mitigation appropriate for that concern. For instance, reduced thermal discharge or nutrient input near coral reefs, improved river management practices and sediment bypassing techniques, or introduction of alternate sediment sources, may all play a role in mitigation if there is early warning. By recognizing the specific susceptibilities of different coastal areas, we can develop land- and water-use practices as well as mitigating factors that can retard coastal erosion and loss of coastal resources. Such guidelines could be developed under international aegis, such as the United Nations, and distributed widely for implementation into national plans and programs. These guidelines must recognize the value of the shoreline, its sediment sources, and the biogenic components to the continued health and viability of our shorelines.

Dave Aubrey, a Senior Scientist at Woods Hole Oceanographic Institution, has enjoyed the opportunity to visit many world beaches and coasts for his research. Some unique experiences include being tear-gassed in Korea, chased by water buffalo in China, nearly being arrested in South Africa, and dining on interesting fare in many places, all for the sake of science.
COPYRIGHT 1993 Woods Hole Oceanographic Institution
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Author:Aubrey, David G.
Date:Jun 22, 1993
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