STORM-RELATED CHANGES IN BEACH MORPHOLOGY AND ANALYSIS OF SEDIMENT CHARACTERISTICS, TORREY PINES STATE BEACH, SAN DIEGO COUNTY, SOUTHERN CALIFORNIA

Torrey Pines State Beach (TPSB) exemplifies the related processes of sea cliff erosion and the development of a broad beach seaward of the cliffs. TPSB is sculpted by high energy storm waves that periodically redistribute sediment in both the foreshore and backshore regions. This study documents dramatic, rapid morphological changes at TPSB in response to a storm in an El Niño year. Three profiles of the northern portion of TPSB were surveyed for this project, both before and after a March 2016 storm. Prior to the storm, exposed cobbles characteristic of a winter beach formed a substantial, well-developed berm in the backshore region. Following the storm, beach profiles were re-surveyed, documenting that the berm had been destroyed, with cobbles transported from the backshore to the foreshore and strewn across the beach. These results demonstrate the dramatic effect of heavy surf on beach morphology over very short time scales, with post-storm profile elevations up to 4 feet lower than pre-storm elevations in the backshore and up to 2.5 feet higher in the foreshore. Berm destruction has resulted in the removal of material that would otherwise dissipate wave energy, leaving the cliffs more susceptible to undercutting. In addition, exposed cobbles may be hurled at the unprotected base of the cliff in future storms, increasing erosion and causing cliff fragments, including Torrey Sandstone, to collapse onto the beach. This study also investigates contrasting sedimentological properties between the Torrey Sandstone bedrock exposed in the sea cliffs—a primary source of beach sediment—and the beach sediments themselves. One sediment sample was collected along each beach profile, together with two samples of Torrey Sandstone. Samples were dry-sieved to produce grain-size distribution curves. Analysis of grain-size distribution, together with visual investigation of rounding, sorting, and mineralogy, provides insight into the processes affecting beach sediments.