Dr. Ivano W. Aiello, Moss Landing Marine Laboratories
July 11, 2017
Graduate student Tyler Barnes setting up the Trimble Spatial Station near Portrero Road at Moss Landing.
Along the west coast of the U.S., large storm events are the primary factor controlling beach dynamics. High-energy conditions characterized by large swell events are particularly intense during El Niño years such as the 2007-2008 and recent 2015-2016 events. Large storms associated with El Niño have dramatic effects on the coastal environments by accelerating the erosion of beaches, sea-cliffs and coastal dunes. Storms also enhance currents that can move large volumes of sand parallel to the coastline (littoral transport, often referred to as a ‘river of sand‘). If obstacles like the head of a submarine canyon intercept the ‘river of sand’ during its along-shore journey, then the sand can be deflected offshore and permanently transferred from the coastal environment to the deep seafloor where it is lost forever.
Long-term studies that track the position of the coastline through time show clear trends in coastal erosion in California dating back to the beginning of the 20th century. Of the entire California coastline, the Monterey Bay region has experienced some of the highest long-term negative net shoreline changes. Average rates of coastal retreat are ~20 cm per year, and in some parts of the bay average erosion is more than a meter per year. This significant shoreline retreat directly affects people living along the coast and the recreational, scenic, and in turn, economic opportunities of the region.
Map of the Salinas sub-cell and location of the surveys.
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littoral cell includes both a source of sand, such as a river mouth, and a sink, such as the head of a submarine canyon. In Monterey Bay, the main sources for sand are rivers such as the San Lorenzo, the Pajaro and Salinas, while the main sink is the head of the Monterey Canyon. The canyon’s head crosses the continental shelf and comes very close to shore right in front of Moss Landing Marine Laboratories (MLML). The relatively small length (~3 miles) of the beach that connects the Salinas River (main sediment source) and the head of the Monterey Canyon (main sediment sink) is called the
Salinas sub-cell and is an ideal region to monitor sediment fluxes at high spatial and temporal resolution and to monitor beach dynamics before, during and after storm events.
Although we do have a good understanding of the long-term, regional-scale picture of shoreline erosion, future predictions of rising sea level conditions combined with more frequent and larger storms and El Niño events require a better, more quantitative approach to understand sediment dynamics in littoral cells during high-energy events and storm-dominated seasons. The goal of our research is to fill this gap by monitoring changes in sand volume along the beaches of the Salinas sub-cell using advanced mapping tools.
We have baseline measurements of beach morphology at Moss Landing Beach (center of the Salinas sub-cell) that were collected during the 2007-2008 El Niño event. We also collected measurements about every season between 2009 and 2015, and we collected additional measurements before, during and after the most recent El Niño season (from October 2015 - August 2016). For our surveys we used a ground-based terrestrial laser scanner (TLS), which uses a laser beam with infrared (IR) light to measure distances. This instrumentation provides highly accurate (on the order of mm), high-resolution (on the order of cm) surface topography measurements. In early Fall 2015, we conducted a survey of the sub-cell’s coastline using an aerial unmanned autonomous vehicle (UAV) equipped with a very high-resolution camera. The UAV takes stereographic pictures of the terrain (i.e., the same area is imaged from different positions), which, just like our eyes, can be used to extract depth information. The images are converted into high-resolution digital elevation models (DEMs).
Although we are still currently analyzing the combined TLS-UAV dataset, the preliminary results can help us understand the sediment budget in the Monterey Bay and may be extrapolated to other littoral cells in California. Comparison of beach volume of Moss Landing Beach during the 2007-2008 and 2015-2016 El Niño seasons shows similar patterns:
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Initial episodes of erosion at the beginning of the storm season Normalized volume of the emerged beach at Moss Landing (i.e., the volume above mean sea level per square meter, m3/m2) in the fall before significant storm events (September-December) was ~1.2 m during both El Nino periods. In the middle of the winter season after the first large storm events (January-February), the normalized volume decreased by 0.5 m to ~0.7 m in both 2008 and 2016. To give a better sense of what these numbers mean, if we extrapolate the difference in normalized volume between the beginning and the end of an El Niño season to the entire Salinas sub-cell’s extent (~4,000 m) and we assume that the beach is ~30 m wide, we obtain a total volume of 60,000 m3 of sand lost during each El Niño.
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Episodes of beach accretion during winter conditions in March and April There were significant episodes of beach accretion following major storm events, which often included a southwesterly swell component, during both El Nino periods. These episodes of accretion were relatively short lived and were followed by beach erosion as indicated by surveys done a few days after the storm events. At Moss Landing Beach, erosion is presumably connected to sand loss to the head of the submarine canyon.
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Relatively stable summer conditions characterized by high beach volumes During summer months characterized by low-energy wave conditions, the surveys showed prevailing beach accretion and normalized volumes ranging between 1.3 m and 1.6 m.
Moss Landing Beach on 12/20/16. Photo by Tyler Barnes.
Moss Landing Beach one month later on 1/24/17 showing erosion of sand dunes. Photo by Tyler Barnes.
The beaches monitored during the 2015-2016 El Niño south of Moss Landing (Portrero and Molera) did not show any significant net volume change during the sampling period. This is not surprising since the beaches south of Moss Landing are adjacent to a larger continental shelf and away from the direct influence of the head of the Monterey Canyon.
In spite of the changes in beach volume within an individual El Niño season, overall there was no net change in normalized volume of the emerged beach at Moss Landing from 2008 to 2016. This is surprising given the high erosional rates that characterize the southern Monterey Bay (measured in previous long-term studies). The stability of Moss Landing Beach at the center of the littoral cell may be due to the accretion of sediments that erode in the southern Bay and, during heavy rain seasons, sediments from the Salinas River. Both of these sources may buffer central Monterey Bay shores against net erosion. More thorough integration of the TLS and UAV data collected for this project and continued monitoring will allow us to test this hypothesis in the future.
Overall, our results indicate that beach volume change in the Monterey Bay is spatially and temporally dynamic. When studying littoral cell dynamics in Monterey Bay or elsewhere in California, traditional long-term, low-resolution beach surveys need to be complemented with higher-frequency and higher resolution surveys to separate long-term trends from seasonal and El Niño-scale variability.