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USGS Scientists Investigate Coastal Processes Affecting a Restored Tidal Wetland in the San Francisco Presidio
The ecological benefits provided by the marsh, as well as its public-education value, depend on the continuing exchange of water with San Francisco Bay through a narrow and highly dynamic inlet. Since its completion in 1999, the inlet has been closed off several times, requiring periodic maintenance to reopen it. The closures are thought to result from the marsh's limited tidal prism (the volume of water exchanged between the marsh and the bay during one complete tidal cycle) and the area's strong longshore transport (transport of sediment carried by currents moving alongshore), with a net transport from west to east. It was noted during the original design phase of wetland construction that the tidal prism of the restored marshconsiderably smaller than that of the original marshmight be insufficient to keep the longshore transport from filling in the inlet channel.
USGS scientists Patrick Barnard, Dan Hanes, and Li Erikson, in collaboration with Edwin Elias from Delft Hydraulics, have begun a scientific investigation to understand the physical processes controlling the shape and function of the inlet to Crissy Field Marsh. For several years, this group has been developing a numerical model, using the state-of-the-art software package Delft3D, to simulate the hydro-dynamic and morphodynamic processes of the San Francisco Bay region. The model domain includes the greater San Francisco Bay and the Pacific Ocean coast from approximately Point Reyes to Pacifica, with recent interest focused on the exchange of sediment between the open coast and the central bay, including the shoreline abutting Crissy Field Marsh. The related interests of the NPS (in the longshore transport responsible for closing the inlet at Crissy Field Marsh) and the USGS (in sediment exchanges between the bay and the open coast) provided an opportunity to obtain field measurements in the vicinity of the Crissy Field Marsh, extending from the Golden Gate Bridge to the San Francisco Marina. The field measurements will be used to gain further understanding of the coastal processes in the southern part of central San Francisco Bay and to calibrate and validate the numerical model. The study area features extremely powerful tidal currents that peak at more than 2.5 m/s, strong wind gusts funneled through the Golden Gate Strait that annually exceed 14 m/s in velocity, and complex wave-current interaction as Pacific swell and local seas collide with strong tidal currents.
Baseline characterization of sediments and beach and nearshore morphology were obtained in October 2007 with a suite of measurement tools. A total of 39 cross-shore transects, spaced about every 100 m in the longshore direction, and 4 longshore transects were measured for bathymetry and topography profiles. Bathymetry was collected by USGS scientist Barnard and University of California, Santa Cruz (UCSC) graduate student Jeff Hansen (with field support from USGS Marine Technician Jackson Currie) with coastal-profiling systems (CPSes). The CPSes are personal watercraft equipped with Ashtech Z-Extreme real-time kinematic (RTK) receivers linked to single-beam echosounders collecting depth measurements at 5 Hz. Tom Reiss and Gerald Hatcher of the USGS Coastal and Marine Geology Marine Facility in Santa Cruz, California, maintained the RTK and Global Positioning System (GPS) receivers. The vertical- and horizontal-positioning accuracy of this system is approximately ±10 cm in the vertical plane and ±5 cm in the horizontal plane when atmospheric conditions and satellite geometries are optimum and surface waves are small.
The dryland segments of the cross-shore transects were completed with backpack RTK GPS systems carried by Li Erikson and Kate Dallas, a former USGS employee and current graduate student at UCSC.
Sediment grain sizes were recorded with a bed-sediment camera and analyzed by using an algorithm derived by team member David Rubin. (See Sound Waves article, "Patent Awarded to USGS Scientists for Underwater Microscope System" and Journal of Sedimentary Research technical abstract, "A Simple Autocorrelation Algorithm for Determining Grain Size from Digital Images of Sediment.")
Nearshore bathymetry and beach changes over the winter season, followed by recovery rates, will be ascertained with future field efforts planned for winter and summer 2008, when waves and currents along the shore will also be measured. Additional fieldwork planned for early 2008, to complement the recently obtained data, consists of obtaining and analyzing sediment samples throughout the study area and measuring currents over spring and neap tides in the vicinity of the mouth of the bay.
in this issue:
Coastal Processes Affect a Restored Tidal Wetland
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