CHALLENGES TO QUANTIFYING COASTAL CHANGE HAZARDS

What processes are causing the long-term net erosion of U.S. shorelines? Many coastal scientists argue that the ~20 cm of net sea-level rise over the past century has forced the observed erosion. But how about other potential processes? For example, consider the south Atlantic coast where large estuaries trap sand supplied by rivers. Virtually no new sand is reaching the beaches from the rivers through the estuaries. At the same time, sand is being permanently lost from the littoral system into shoals offshore of the Atlantic coast's prominent Capes. On the west coast, particularly in central and southern California, there is a similar sand deficit along the beaches, although caused by different processes. Rather than large estuaries, fluvial dams trap sand, diminishing the natural supply to the beaches; and sand already on the beaches is lost down submarine canyons that extend close to the zone of nearshore transport. Hence, both sea-level rise and deficit of sand supply likely contribute to erosion of the Nation’s beaches, but what is the contribution of each? Along a sand-starved barrier system in Louisiana, a sand budget study has shown virtually all of the coastal erosion can be accounted for by a deficit in sand supply set up by gradients in sand transport. This was a surprising result, because, due to subsidence, the relative rate of sea level rise in Louisiana is about 1 cm/year, among the fastest rates found anywhere in the country!

Does it matter whether we know the relative contributions of the different processes? Will estimates of past changes, based on historical records, suffice to extrapolate erosion into the future with no understanding or quantification of the actual causes? The sensible development of our coasts demands reasonable estimates of coastal change. But if acceleration of sea-level rise occurs, as predicted by models of global climate change, we cannot sensibly forecast erosion based on past changes without first quantifying the contibuting processes. Major research challenges must be overcome to develop the means to forecast long-term coastal change hazards.