HR: 17:45h
AN: OS24A-08    [Abstracts]
TI: The Relative Effects of Wave Climatology and Tidal Currents on Beach Processes Adjacent to a Major Tidal Inlet, Ocean Beach, San Francisco, California
AU: * Barnard, P L
EM: pbarnard@usgs.gov
AF: United States Geological Survey, Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060 United States
AU: Hanes, D M
EM: dhanes@usgs.gov
AF: United States Geological Survey, Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060 United States
AU: Ruggiero, P
EM: pruggiero@usgs.gov
AF: United States Geological Survey, Pacific Science Center, 400 Natural Bridges Drive, Santa Cruz, CA 95060 United States
AB: Identifying the processes that control the morphological evolution of beaches adjacent to tidal inlets is challenging due to the complex interactions between waves, currents, and bathymetry, each with high spatial and temporal variability. In the shadow of the large ebb tidal delta at the mouth of San Francisco Bay, CA, the wave refraction patterns at Ocean Beach are complex and the effects of the offshore wave climate on beach and nearshore morphology cannot be assessed simply by analyzing data from an offshore wave buoy. Instead, the United States Geological Survey has employed a multi-faceted approach that links wave data with numerical modeling, periodic three- dimensional topographic beach surveys, cross shore bathymetric surveys using personal watercraft, onshore grain-size analysis using a bed sediment camera, and a multi-beam survey covering the entire mouth of San Francisco Bay. Initial analyses demonstrate that the spatial distribution of wave energy and direction controls short-term (i.e. days to years) beach evolution, including the location of erosional "hot spots." These conclusions are supported by topographic LIDAR surveys that covered the study area in 1997, 1998 and 2002, bracketing the last major El Ni¤o/ Southern Oscillation cycles. In this study, SWAN (Simulating WAves Nearshore) modeling is combined with high resolution bathymetry and high resolution beach surveys to quantify short-term morphological change and to provide links to nearshore processes. Initial SWAN results show a focusing of wave energy at the location of an erosional hot-spot on the southern end of Ocean Beach during the prevailing northwest swell. During El Ni¤o winters, swell out of the west and southwest dominates the region, and although the wave energy is focused further to the north on Ocean Beach, the oblique wave approach sets up a strong northerly littoral drift, thereby starving the southern end of sediment, leaving it increasingly vulnerable to wave attack when the typical northwest swell returns. Over longer time periods (i.e. decades), tidal processes emerge as the dominant control on coastal evolution is this region, as changes in sediment supply and depositional patterns exert a strong influence on the ebb tidal delta volume and morphology. The tidal delta, in turn, strongly influences wave shielding, refraction, and focusing patterns on adjacent beaches. An accurate assessment of the interaction between wave and tidal processes is crucial for evaluating coastal management options in an area that includes the annual dredging and disposal of ship channel sediment and an erosional hot spot that is posing a major threat to local infrastructure.
DE: 4255 Numerical modeling
DE: 4546 Nearshore processes
DE: 4558 Sediment transport
DE: 4560 Surface waves and tides (1255)
DE: 3020 Littoral processes
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting