INITIAL ASSESSMENT OF COASTAL DUNE EROSION BEFORE AND AFTER THE CLOSURE OF SAND MINING OPERATIONS USING LIDAR AND SFM DIFFERENCING, SOUTHERN MONTEREY BAY, CA

Sandy coastlines are globally impacted by sea level rise and impacts to sand supply such as mining and damming of coastal rivers. Southern Monterey Bay is a unique example of this and among one of the fastest eroding coastlines in California. This stretch of coastline is an isolated littoral cell defined by broad sandy beaches backed by active parabolic dunes. The wide beaches and dunes aid in naturally preventing the inundation of communities and agricultural land due to sea level rise. A large sand mining plant, Cemex, was shut down in 2020 after it was determined that it was likely accelerating coastal erosion in the area. This presents us with the rare opportunity for a study aiming to monitor the recovery of coastal erosion before and after the operative shutdown of a large sand mine. Our experimental approach analyzes landscape data from 2010 to 2024 using USGS low-altitude Structure from Motion (SfM) and recent airborne lidar scans. These technologies are employed to create 3D models of the dunes to spatially and temporally compare years for volumetric differences and erosion hotspots using point cloud height differencing. Preliminary analysis shows a profound difference in erosion rates based on the phase of El Nino Southern Oscillation. During years with strong positive ENSO phases (El Niño) can see overall higher erosion rates and cyclic hotspots of dune erosion likely driven by mega cusp formation and more subdued erosion during negative ENSO phases (La Niña), supporting previous work in the area. Our study may serve as a guide for future conservation and coastal hazard mitigation efforts in the region and provide an example of the use of rapid repeat low-altitude SfM at the littoral-cell scale.