Assessing the impact of extreme storms on barrier beaches along the Atlantic coastline: Application to the southern Rhode Island coast

In this work, we use the 2D model XBeach to dynamically simulate coastal erosion due to a synthetic 100-year storm impacting a typical North Atlantic barrier beach located in southern RI. This storm was extracted from the North Atlantic Coast Comprehensive Study (NACCS) database, based on results of an extreme value analysis of more than 1000 NACCS storms. XBeach parameters are first calibrated/validated by simulating Hurricane Irene (August 2011), for which both nearshore wave data and pre- and post-storm beach profiles were available in the study area. Comparing results to observations allowed calibration of the wave asymmetry and skewness parameter (gamma(ua) = 0.3) in the model, resulting in a 6% mean relative error between the simulated and measured subaerial eroded volumes along 4 transects. In the 100-year storm XBeach simulations that include overwash, effects of land cover on beach erosion, in particular vegetation, are assessed by specifying a spatially varying bed friction function of high-resolution land cover. Results show that healthy back-dune vegetation is essential to prevent the dune crest from being fully eroded down to its toe level. The predicted median 100-year eroded volume is 46 m(3)/m for the entire barrier beach, in good agreement with FEMA's empirical value of 50 m3/m at the two "official 1D transects" within the study area; mean post-storm reductions in dune crest elevations are in similar agreement. The model, however, predicts very large along-shore variations of these parameters, with eroded volumes over 1000 m3/m where breaching and the opening of surge channels occurs. Overall, dune segmentation simulated in the model for the 100-year storm appears to be realistic and consistent with dune topography and land cover. XBeach thus provides an improved 2D methodology for assessing the impact of extreme storms on Atlantic barrier beaches, predicting changes in dune morphology, and quantifying the protective role of vegetation, and effects of land cover in general.