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Dissertation/Thesis Abstract

Wave-driven beach sand level changes in southern California
by Ludka, Bonnie Cecily, Ph.D., University of California, San Diego, 2016, 135; 10125264
Abstract (Summary)

Sand levels were monitored at five southern California beaches for periods of 3 to 15 years, spanning a total of 18 km alongshore. Every 3 months, GPS equipped vehicles measured sand elevations on cross-shore transects from the backbeach to 8 m depth, with 100 m alongshore resolution. Subaerial observations were collected monthly above the spring low-tide line. Wave buoys and a numerical model provided hourly wave estimates in 10 m depth at each site.

These observations show that beach profile shapes (depth versus cross-shore distance) evolve consistent with the equilibrium hypothesis: under steady wave conditions, evolution is toward a unique, wave condition dependent, equilibrium beach profile. Beaches far out of equilibrium change rapidly, and as equilibrium is approached they change ever more slowly. At the sandy regions, a simple equilibrium beach state model has skill >0.5 (Chapter 2, [Ludka et al., 2015]).

Repeated nourishments over multiple decades, costing hundreds of thousands of dollars, are a primary beach management strategy worldwide, but the wave-driven redistribution of nourishment sand is poorly understood. At four survey sites, 50,000-300,000 m

3 of imported sand was placed on the subaerial beach overalongshore spans between 300-1300 m. Wave conditions in the months after placement were similar at all sites, but the subaerial nourishment pads eroded and retreated landward at different rates. A pad built with native-sized sand washed offshore in the first few storms. In contrast, nourishments with coarser than native sand remained on the beach face for several years and protected shorelines during the significant wave attack of the 2015–16 El Niño (Chapter 3, [Ludka et al., 2016]). These relatively resilient and coarse subaerial pads stretched alongshore in a pattern consistent with seasonally shifting, wave-driven alongshore currents. Natural gains and losses in the total sand volume budget, integrated spatially over each site, are sometimes larger than the nourishment contributions (Chapter 4, in prep for Coastal Engineering).

Indexing (document details)
Advisor: Guza, Robert T.
Commitee: Feddersen, Falk, Gille, Sarah T., Hendershott, Myrl C., Pawlak, Geno, Terrill, Eric J.
School: University of California, San Diego
Department: Oceanography
School Location: United States -- California
Source: DAI-B 77/10(E), Dissertation Abstracts International
Subjects: Physical oceanography, Geomorphology, Marine Geology
Keywords: Beach, El Niño, Equilibrium beach profile, Nourishment, Sand transport, Waves
Publication Number: 10125264
ISBN: 978-1-339-83357-6
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