Effects of nourished beach morphology on storm response

Abstract

Field observations on storm induced beach changes are important to improve our effort on beach management. This study compared storm induced beach changes caused by hurricane Hermine in 2016 (4 years after a beach nourishment) and Tropical Storm Eta in 2020 (2 years after a beach renourishment) along the barrier-island coast of west-central Florida. Pre-Eta beach were 1 to 2 times wider than that of pre-Hermine. Since Hurricane Hermine and TS Eta generated a similar hydrodynamic condition for the study site, comparing beach changes induced by these two storms provides a unique opportunity to investigate the response of different antecedent beach conditions to energetic events. The shore protection effect of beach nourishment is apparently evidenced by the fact that post-Eta shoreline was located seaward of those post-Hermine at half of the beach-profile locations in the study area. The shore protection effect in the subaerial portion of the beach, however, is not obvious for the other half of beach profiles where shoreline positions were retreated to similar locations after these two storms. Instead, their shore protection effect occurred in the sub-aqueous portion of the beach and was indicated by higher sandbar crests located closer to the shoreline, which can dissipate and reduce incoming wave energy. The shoreline elevation needs to be properly defined (Mean High Water vs Mean Low Water line) as it is used as a proxy to represent beach volume loss. For Hermine induced beach change, no significant correlation exists between MHW line change and beach volume loss. While a significant correlation exists between MHW line change and beach volume loss induced by TS Eta. This correlation pattern switched if the shoreline here is defined as mean low water line. For efficient beach/shoreline management, multiple proxies (e.g., sandbar height and location of its crest and trough) in addition to shoreline change should be used to assess the performance of beach nourishment project.