Using vessel-based LiDAR to quantify coastal erosion during El Niño and inter-El Niño periods in Monterey, CA

Steven Quan, California State University, Monterey Bay

Thesis (M.S.) Division of Science and Environmental Policy


Vessel-based LiDAR was employed to measure shoreline geomorphology, and quantify the rates of erosion and spatial distribution of coastal retreat around Monterey Bay, California during the 2008–2009 normal (non-El Niño) winter and 2009–2010 El Niño winter. These data were compared with pre- and post- El Niño airborne topographic LiDAR data from 1997 and 1998 to assess shoreline change since 1997 and test the hypotheses that: 1) segments of the coastline exhibiting considerably higher rates of erosion than adjacent areas (erosional hotspots), exhibit a predictable alternating spatial pattern alongshore between consecutive El Niño and inter-El Niño periods, and 2) the spatial distribution of erosion rates is positively correlated with the spatial distribution of wave energy. As predicted, coastal erosion was found to be significantly higher during the 2009–2010 El Nino versus the 2008–2009 non-El Nino period (1.8 m average versus 0.1 m average in the southern bay and 0.5 m average versus 0.04 m average in the north bay). The spatial distribution of erosion rates during the 2009–2010 El Niño was positively correlated with that of wave energy. In southern Monterey Bay, these rates increased along a gradient from south to north in response to wave refraction over Monterey Submarine Canyon and the sheltering effect of the south bay by the Monterey peninsula, whereas in the northern bay, erosion was highest at the single location where wave energy was focused by a combination of wave refraction and sheltering from the bay’s northern headland from northwest waves. Erosional hotspots were found to occur along the Monterey Bay coastline during the 1997–1998 and 2009–2010 El Niño winters, as well as during the 1998–2008 inter-El Niño period. Moreover, these hotspots were found to be significantly correlated with a 100–140 m spatial lag in southern Monterey Bay. Erosion hotspots that occurred during one El Niño or inter-El Niño period shifted spatially 100–140 m alongshore during the subsequent El Niño or inter-El Niño period. Vessel-based topographic LiDAR proved to be an efficient, cost-effective method for detecting sea cliff geomorphic change. This approach revealed that over El Niño and inter-El Niño periods, the majority of the coastline exhibited fine scale retreat in the form of variable erosional hotspots and enabled the quantification of a predictable erosional hotspots spatial pattern, highly useful for coastal planning.