Sanctuary Integrated Monitoring Network
Monitoring Project

Beach recovery and sediment budget in the southern Monterey Bay National Marine Sanctuary after the 2015-16 El Niño

Principal Investigator(s)

  • Ivano Aiello
    Moss Landing Marine Laboratories, California State University
Start Date: January 01, 2016

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 Date

Preliminary 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.

Study Parameters

  • Substrate characterization
  • Beach elevation and volume

Study Methods

The surveys were done combining two methods: high- resolution (<10 cm) terrestrial laser scanning (TLS) and Unmanned Aerial Vehicle (UAV) areophotogrammetry. Global Navigation Satellite System (GNSS) receivers were used to establish project controls relative to the existing National Geodetic Survey (NGS) monuments and Continuously Operating Reference Stations (CORS). We used static differential GPS (DGPS) technology which will yield horizontal and vertical accuracies of about 5cm. Vertical controls were based upon the North American Vertical Datum of 1988 and for the elevations was used the Geoid 12A model. Permanent benchmarks were created at each of the monitored beach locations and temporary ground control points (GCPs) were created for the UAV surveys. Field surveys were carried out with a Trimble VX, a state-of-the-art Spatial Station equipped with Infrared (IR) Direct Reflex (DR) technology. Survey were done at (~5-10 cm resolution) for a total of ~7000 and 23,000 points per survey using both direct reflex and a wheel-mounted survey rod with a 360 degree laser prism. Survey data post-processing was done with Trimble's Realworks Advanced 10 software. The benchmark layout for each surveyed site includes a fore-sight (FS) form which the TLS surveys were carried out, a back-sight (BS) to establish the orientation of the TLS for the station setup prior to each survey and control points (these include also fixed element in the landscape) to assure accurate georeferencing for repeated surveys. The baseline survey of the coastline between the mouth of Elkhorn Slough and the location of the Cemex plant was produced using a fixed-wing UAV. The UAV survey of September 2016 was designed to overlap with an AUV conducted the year before in October 2015. UAV mapping occurred on September 26 and 27 of 2016 and was carried out through seven separate flights to divide the target coastline in shorter overlapping segments. Each segment includes 6 flight lines that were designed to cover the portion of the coastal area including the dune crest and the shoreface. The AUV survey results were processed using Pix4D software, and any point cloud x,y,z position has been adjusted using benchmark location information. The benchmarks (ground control points or GDPs) were temporarily created prior to the survey using a DGPS and Real Time Kinematics to establish positions with accuracies of ~5 cm.