THE ROLE OF SHOREFACE DYNAMICS IN BEACH NOURISHMENT DECISIONS: INSIGHTS FROM A SIMPLE MODEL

Along the US Atlantic and Gulf coasts, rates of sea-level rise are expected to increase significantly over the coming century. Higher sea levels heighten the risks of flooding during storm events, resulting in shoreline erosion and the loss of coastal development, including properties, businesses, and public infrastructure. A basic management question is whether to protect existing coastal development or to retreat as the sea level rises and the shoreline erodes. Protection comprises the periodic placement of sand fill on the beach (beach nourishment) or the construction of dikes, seawalls, or revetments. Generally, humans have decided to protect highly valuable coastal developments instead of retreating, but these decisions typically lack scientific justification.

The objective of the work presented here is to incorporate beach nourishment into a recently developed morphodynamic model of barrier beach response to sea-level rise. A key feature of the model is its dynamic shoreface profile, which can be perturbed out of equilibrium due to the extraction of sediment from the upper shoreface during storms, gradients in alongshore sediment transport, or beach nourishment practices. Consequently, the model allows a two-way dynamic feedback between economically based nourishment decisions and the evolution of the barrier system. We use this model to explore both economic and geologic parameters, resulting in differing optimal management scenarios over a 100-year planning horizon. Scenarios range from the periodic widening of the beach through nourishment to letting erosion proceed unabated (passive management). Results demonstrate that the efficiency of beach nourishment is affected mainly by the dynamic state of the shoreface at each nourishment episode. This result reinforces a need to refine numerical coastal management tools to incorporate two-way feedbacks between natural processes and human activities.