Coastal Erosion's Influencing Factors Include Development, Dams, Wells, and Climate Change

Article excerpt

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. …