Torrey Pines Beach Nourishment Project Overview

torrey_transects.png A small (160,000 m3) beach fill was monitored at Torrey Pines State Beach, near San Diego in Southern California. The fill, constructed in April 2001, was a 600 m-long flat-topped berm, extending from a highway revetment seaward about 80 m, terminating in a 2 m-tall, near-vertical scarp. Wave conditions offshore of the nourishment were monitored with directional wave buoys. Support was provided by The California Resources Agency and the California Department of Boating & Waterways. Approximately biweekly surveys (depending on tide and wave conditions) were collected from just prior to the fill construction (April 2001) until after major storm erosion (November 2001). Less frequent surveys were collected until March 2003. Approximately locally shore-normal transect lines were established at 20 m alongshore intervals for a 700 m-long reach encompassing the fill, and at 100 m-alongshore intervals for 1000 m on both sides. The 56 survey transects extended seawards from the backing cliffs or revetment to about 8 m water depth.

Pre-storm performance of fill: During the approximately seven months following completion of the fill, the significant wave height seaward of the surf zone ranged between about 0.4 and 1.5 m, and usually was less than 1 m, typical for the summer-fall season. figure_6ab.png Water levels and wave energies were elevated sufficiently to wet the surface of the fill only once or twice, and the fill region was unchanged except for approximately 20 m shoreward retreat of the steep seawards-facing scarp of the fill near the southern end of the fill (where the blue contour overlies the dashed black curve in adjacent figure b). Changes to the bathymetry adjacent to the fill area were consistent with the seasonal cycle in Southern California with as much as 1.3 m accretion above MSL and 1.3 m erosion between 2 to 5 m depth. The accretion on the adjacent beach face reduced the super-elevation of the fill. The region directly offshore of the fill (2 to 5 m depths) eroded about 1 m, similar to the adjacent regions, but this sand was blocked from coming ashore by the fill, so moved either alongshore or offshore.

Erosion during a November storm: The incident wave significant height exceeded 2 m for about 2 days with severe erosion. The fill and the adjacent beach face both erosion in the nourished region was not uniform alongshore. figure_8ab.png Visual observations indicate that at high tide, wave uprushes overtopped the steep scarp and reached the relatively flat, elevated fill. The water on top of the fill flowed alongshore to initially small depressions that channeled the flow seawards. The offshore flow rapidly deepened and widened the channels. By the peak of the storm (~1300 on 22 November), the shoreward end of the eroded channels had retreated almost to the highway revetment. Sand peninsulas, located between the channels, were elevated roughly 1.75 m above the surrounding beach and protruded seawards roughly 50 m from the highway revetment. After the channels were well-developed, incoming waves rushed up the embayments, and the peninsulas eroded by slumping. By the following day, the cross-shore extent of the peninsulas was reduced to about 25 m. Only small remnant peninsulas remained a week later, and the bathymetry was more uniform alongshore. figure_9ab.png Although severely eroded, scarps and channels did not form in the 1 km-long adjacent unnourished regions. The maximum accretion (> 2 m relative to immediately before the storm) occurred offshore and just to the south of the original fill, consistent with transport of the fill material by waves (from the North) during the storm.

Maps of bathymetry and bathymetry change (and other products) are available in the SCBPS data section, and the storm erosion is described in detail in Seymour et al, 2002.

 

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