USING HIGH-RESOLUTION BEACH SURVEYS TO ASSESS GEOMORPHOLOGIC CHANGE WITHIN THE SALINAS SUB-CELL (MONTEREY BAY, CA) BEFORE AND AFTER THE 2015/2016 EL NIÑO

Littoral cells characterize California’s coast along with many other tectonically active coastal regions around the world. Long-term (120 years) studies of the California coast show that Monterey Bay has experienced the highest erosion rates in the state (-0.6 m/y). This study focuses on strategically chosen beaches of the Salinas sub-cell (part of the Southern Monterey Bay Littoral Cell) between the mouth of the Salinas River (sediment point source) and the head of the Monterey Submarine Canyon (sediment sink). Elevation changes are assessed at two beaches within the sub-cell during and after the 2015/2016 El Niño, combining high-resolution (<10cm) surveying techniques such as terrestrial laser scanning (TLS) and unmanned aerial vehicle (UAV) areophotogrammetry. Molera, located approximately in the middle of the sub-cell, and Moss Landing, situated at the northern extent of the sub-cell just next to the head of the canyon, were surveyed approximately each month, and before and after major storm events. The 3D topographic data of the subaerial beach obtained from each survey are sliced into ten alongshore compartments (about 5 m wide); mean elevation was determined for each compartment to account for discrepancies in area covered by each survey. Differential elevation between surveys was calculated by averaging elevations across all ten alongshore compartments and subtracting the later survey from the previous. Mean absolute differential elevation across all sections of beach between surveys was 0.448 m at Molera and 0.296 m at Moss Landing, with peak differentials of 0.860 m and 0.448 m respectively (both peaks represent erosion, rather than deposition). Extrapolating these differentials across the entire sub-cell suggests that short-term beach variability (weeks to a month) may be comparable to previous estimates of yearly beach erosion and littoral transport at adjacent beaches.