Investigating Coastal Processes Responsible for Large-Scale Shoreline Responses to Human Shoreline Stabilization

Abstract:

Human shoreline stabilization practices, such as beach nourishment (i.e. placing sand on an eroding beach), have become more prevalent as erosion threatens coastal communities. On sandy shorelines, recent experiments with a numerical model of shoreline change (Slott, et al., in press) indicate that moderate shifts in storminess patterns, one possible outcome of global warming, may accelerate the rate at which shorelines erode or accrete, by altering the angular distribution of approaching waves (the `wave climate'). Accelerated erosion would undoubtedly place greater demands on stabilization. Scientists and coastal engineers have typically only considered the site-specific consequences of shoreline stabilization; here we explore the coastal processes responsible for large-scale (10's kms) and long-term (decades) effects using a numerical model developed by Ashton, et al. (2001). In this numerical model, waves breaking at oblique angles drive a flux of sediment along the shoreline, where gradients in this flux can shape the coastline into surprisingly complex forms (e.g. cuspate-capes found on the Carolina coast). Wave "shadowing" plays a major role in shoreline evolution, whereby coastline features may block incoming waves from reaching distant parts. In this work, we include beach nourishment in the Ashton, et al. (2001) model. Using a cuspate-cape shoreline as our initial model condition, we conducted pairs of experiments and varied the wave-climate forcing across each pair, each representing different storminess scenarios. Here we report on one scenario featuring increased extra-tropical storm influence. For each experiment-pair we ran a control experiment with no shoreline stabilization and a second where a beach nourishment project stabilized a cape tip. By comparing the results of these two parallel runs, we isolate the tendency of the shoreline to migrate landward or seaward along the domain due solely to beach nourishment. Significant effects from beach nourishment reached several tens of kilometers away from the nourishment project. The magnitude of these effects rivaled the erosion we expect from sea-level rise alone over the coming century. Furthermore, the nature of the effects were unexpected: where we expect beach nourishment sand to spread laterally in the direction of net alongshore sediment transport (e.g. to the right looking off-shore), coastline segments to the right of the cape should tend to migrate seaward, while segments to the left of the cape might naively be expected to feel little effects. We observed, however, that shoreline segments to the left (right) of the stabilized cape tip tended to migrate seaward (landward). Two statistics we collected for each model run--the extent of wave shadowing and the net flux of sediment at each alongshore position--helped explain the surprising behavior. By pinning the location of the cape tip, beach nourishment altered the way in which the cape shadowed adjacent coastlines. The stabilized cape-tip shadowed segments to the left more often, increasing the influence from left-approaching waves. These shoreline segments shifted seaward, relative to the non-nourishment case, through a convergence in alongshore sediment transport from increased transport from the left, rather than from laterally-spreading beach nourishment sand. The stabilized cape-tip shadowed segments to the right less often, increasing the influence of left-approaching waves. These segments shifted landward through a divergence in alongshore sediment transport from increased transport to the right.