Extreme hurricanes and extratropical storms have illustrated the importance of coastal dunes for providing protection against flooding and erosion hazards. Yet, with changing storm patterns, sea level rise, and coastal development, the need for coastal protection continues to grow. To better utilize dunes as green infrastructure for coastal protection, it is important to quantify foredune morphological variability and identify the underlying geomorphological, ecological, and anthropogenic processes that contribute to foredune variability. The US Pacific Northwest coastal dune system presents an excellent case study for examining this eco-geomorphological interaction because it contains two dune-building, invasive, congeneric beachgrasses (Ammophilaspp.), a diverse range of geomorphological conditions, and variable beach and foredune morphology. Here, we use a Bayesian network to model complex interactions between numerous ecological and geomorphological processes on US Pacific Northwest coastal dunes. We examined the relationship between foredune morphology (foredune toe elevation, foredune crest elevation, foredune volume) and ecological (beachgrass species composition and abundance), geomorphological (shoreline change rates, nearshore slope, beach slope, sediment grain size distributions), and anthropogenic (invasive beachgrass removal) drivers. We further examined possible coastal management scenarios to explore how beachgrass invasions, foredune restoration, and climate change might impact foredune morphology and resultant coastal hazard exposure.
Results/Conclusions
We found that ecological, geomorphological, and anthropogenic variables each contribute to foredune morphological variation. Among geomorphological variables, foredune height was directly influenced by a combination of shoreline change rates, nearshore slope, and rates of dune volume change. Foredune height was maximized under neutral and slightly negative shoreline change rates and decreased non-linearly as shoreline change rate increased. Among ecological variables, American beachgrass (Ammophila breviligulata) was associated with shorter foredunes than European beachgrass (A. arenaria). Moreover, foredune height increased with beachgrass density, but the effect of beachgrass density was conditionally dependent upon the underlying shoreline change rate. Consequently, beachgrass density may exert a greater effect on foredune height under high shoreline change rate than low shoreline change rate conditions. Regarding future coastal management scenarios, further invasion by A. breviligulata may further shorten dunes relative to their current A. arenaria dominated status. Overall, this Bayesian Network model allowed for integration of multiple datasets for analyses of both ecological and geomorphological processes in a coastal sand dune system. Moreover, it provides a useful method for examining how future coastal management and climate change scenarios may alter foredune morphology and coastal hazard exposure along Pacific Northwest dunes.