Beach recovery and sediment budget in the southern Monterey Bay National Marine Sanctuary after the 2015-16 El Niño
- Ivano Aiello
Moss Landing Marine Laboratories, California State University
Long-term (120 years) studies of the California coast show that the Monterey Bay has experienced the highest erosion rates in the state (-0.6 m/y). To create a high-resolution geomorphologic baseline of this dynamic segment of coastline we have performed multiple topographic and drone surveys along a ~17 km stretch of coastline between the mouth of Elkhorn Slough at Moss Landing and Fort Ord. Sediment transport in this region is controlled by the dynamics of the Southern Monterey Bay Littoral Cell. The cell includes the Salinas sub-cell between the mouth of the Salinas River (sediment point source) and the head of the Monterey Submarine Canyon (sediment sink). Although not natural, another known sink of sediment in the studied region is the Cemex plant where sand is extracted from the littoral system for commercial use. The surveys were done using an innovative approach combining two methods: high-resolution (<10 cm) terrestrial laser scanning (TLS) and Unmanned Aerial Vehicle (UAV) areophotogrammetry. The surveys were conducted after the 2015-16 El Niño, between Spring 2016 and Spring 2017 and Winter 2016-17, which were characterized by high-swell events and intense rain. After the main goal of the project (to produce a topographic baseline) has been achieved, the TLS surveys have been repeated for some key sites (e.g., near the Cemex plant and at Molera State Park) to measure the effects of storm activity on different beaches of the Salinas cell.
Summary to DatePreliminary results show that, overall, the elevation changes measured for the beaches just north and south of the Cemex plant and elsewhere reflected the “typical” switch from Summer/early Fall to Winter conditions characterized by the erosion of the berm and the overall flattening of the beach profiles. However, the surveys of the beach just south of Cemex showed a much larger elevation drop than for the beach to the north of the plant and other nearby beaches, possibly due to the extractive activities affecting the local littoral transport. Finally, short-lived episodes of beach accretion (with consequent formation of temporary berm-like features) were recorded during the Winter months at Molera Beach as well as in other beaches of the Salinas sub-cell. The latter indicates a deviation from the typically erosive Winter beach profile and suggests northward littoral movement of sand during westerly and south-westerly swell events. Although further studies are needed to interpret the previous results, the source of the accreted sand was probably the Salinas River and/or the erosion of the Fort Ord Dunes in southern Monterey Bay.
- Substrate characterization
- Beach elevation and volume