Study of wave run-up and marine cliff erosion proxies at Northern Del Mar (San Diego County, California)

Supervisors: Adam Young Robert Guza (Scripps Institution of Oceanography, San Diego, California)
The world’s coast proportion of seacliffs is high (Emery and Kuhn, 1982), and almost one quarter of the global population live in these areas (Small and Nicholls, 2003). Seacliff erosion is an important matter along the California coast since they threaten coastal structures, public property, recreational resources, public safety, and major transportation corridors (Griggs and al., 2005). Beaches have been removed and major cliff failures have occurred after storms in recent decades and to combat these problems, seawalls are increasingly used to prevent erosion. However, coarse grained seacliffs contribute sediment to beaches, and preventing seacliff erosion with seawalls reduces the beach sand input (Young and Ashford, 2006a). Beaches need to preserve as they are an important economic and cultural resource. To assess the wave impact on cliff erosion, we developed a method to process water level time series recorded by a pressure sensor located at the bottom of a seacliff northern Del Mar, California. Several kinds of data have been used during processing: water level time series, tide data, beach profiles, beach elevation, offshore wave parameters… During the process, realistic methods had been developed in order to describe natural processes. However, some linear interpolations had to be done since all parameters could not be hourly assessed (e.g beach elevation). These interpolations have always been comparing with environmental variables or common used physical and mathematical equations in order to check how coherent they were. Only higher frequency of surveying could reduce the use of these interpolations. Then, a dozen of scripts on Matlab ® software have been written along processing. The peakdet function (developed by Eli Bauer) apart, we wrote all scripts in their entirety. Once the results obtained, three different cliff erosion proxies have been studied: wave impact duration, number of waves, wave heights. Wave impact duration corresponds to the number of hours during water level is in contact with the toe of the cliff. This variable is commonly used in cliff erosion studies (Young and al., 2009) and is determined following the run-up equation developed by Stockdon and al. (2006). Then, our results allowed us to test 5 the reliability of this equation in our study area. In previous studies, only the wave impact duration had been considered to assess wave impact on the cliff. However, in addition the wave impact duration, our results gave us the opportunity to bring more details. Indeed, the run-up equation only informs if the water is in contact to the cliff for each hour, whereas our results give information about the number of waves and their respective heights for those hours. A wave analysis, based on these results, has been done to describe the wave conditions at the toe of the cliff.