Empirical Assessment of Remotely Sensed Beach Slopes

The slope of the beach face is one of the main parameters influencing how the beach responds to wave energy. This makes it an important quantity for coastal scientists, coastal engineers, and flood modelers to consider. The Climate Resiliance Initiative at the University of Hawaiʻi at Mānoa is working to implement CoSMoS-COAST, a shoreline change model, in the state of Hawaiʻi. Shoreline position and beach slope are primary inputs to the model. However, there is a lack of beach slope data. CoastSat is a toolkit that automatically retrieves shoreline positions and derives beach-face slopes for CoSMoS-COAST from satellite imagery. This study investigated the accuracy of CoastSat slope values by comparing them to field survey data along four shorelines with differing wave regimes on Oahu; however, we found poor correlation (R²= 0.22). To further investigate the best slope input, we compared slope calculations from existing LiDAR and beach profile data, as well as additional surveys using unoccupied aerial systems (UAS) and survey-grade GPS. We found that slope values varied between the different datasets and across locations. These variations may be due to lack of temporal data, differences in beach characteristics, and wave regimes. We will continue this work to determine the best input into CoSMoS-COAST.