Using Vessel-Based LIDAR to Quantify Coastal Erosion during El Nino and Inter-El Nino Periods in Monterey Bay, California

Vessel-based light detection and ranging (LIDAR) was employed to collect coastal topography data and to quantify the rates of erosion and spatial distribution of coastal retreat around Monterey Bay, California during the 2008-09 (non-El Nino) and 2009-10 El Nino. These data were compared with pre/post-El Nino LIDAR data from 1997-98 to assess shoreline change and to test the following hypotheses: (1) that broad-scale (km) spatial distribution of erosion rates is positively correlated with wave energy, and (2) that fine-scale erosion hot spots (segments of the coastline exhibiting considerably higher rates of erosion than adjacent areas) shift at predictable alongshore wavelengths between consecutive El Nino and inter-El Nino periods. Broad-scale erosion was found to be significantly higher during the 2009-10 El Nino vs. the 2008-09 non-El Nino period in both the south (1.8 m vs. 0.1 m average) and north bays (0.5 in vs. 0.0 m average). The broad-scale distribution of erosion rates during the 2009-10 El Nino was positively correlated with wave energy. In southern Monterey Bay, erosion rates increased along a wave energy gradient from south to north, whereas erosion and wave energy were both focused and highest at a single location in the northern bay. Fine-scale erosion hot spots were found to occur during the 1997-98 and 2009-10 El Nino and the 1998-08 inter-El Nino period. These hot spots were found to be significantly correlated at -160 m during the 1997-98 El Nino to 1998-2009 inter-El Nino periods and 100 m during the 1998-2009 inter-El Nino to 2009-10 El Nino periods in southern Monterey Bay. Hot spots that occurred during one El Nino or inter-El Nino period shifted spatially alongshore during the subsequent El Nino or inter-El Nino period. Vessel-based LIDAR proved to be effective for detecting coastal change at high spatial resolutions and revealing fine-scale patterns of shoreline retreat.