Future Shoreline Retreat Cannot Be Predicted with Accuracy

We know that sea level rise drives shoreline retreat. But, it will be the increased inland reach of coastal storms and the salinization of groundwater that will be the direct cause of any global relocation of people and infrastructure from the coast. Understandably, predicting the future rate of shoreline retreat is of huge interest, especially for management of sandy shorelines in developed countries. The Bruun Rule (and its variants) is the most widely used mathematical model for predicting sea-level-driven coastal change. Unfortunately, the Bruun Rule is seriously flawed. The rule's basic assumption is that the slope of the shoreface is the sole determiner of erosion rates. This assumption is baseless. The actual causes of erosion are many, complex and not completely understood. The Bruun Rule generally under-predicts erosion rates and its use has resulted in a lack of public appreciation for the magnitude of the problem.

In a time of rapid sea level rise, the mechanisms of shoreline retreat will differ from those in a time of gradual rise. For example, the shorefaces of today's sandy coasts formed over the last 3000 to 4000 years during a relative pause in sea level rise. During this time, the inner shelf steepened as sand swept ashore, forming the modern shoreface and widening most barrier islands. During past periods of rapid, postglacial sea level rise, it is unlikely that the shoreface maintained its current 10 to 20 m relief. Thus, with an anticipated future sea level rise rate of one meter per century or more perhaps, the upper shorefaces should begin to flatten again. This may, at least temporarily, slow shoreline retreat rates due to a combination of increased sand supply to the beach and increased wave/sea floor friction.