Abstract
Two multibeam sonar surveys of west-central San Francisco Bay, California, were conducted in 1997 and 2008. Bathymetric change analysis between the two surveys indicates a loss of 14.1 million cubic meters (-3.1 cm/yr) of sediment during this time period, representing an approximately three-fold acceleration of the rate that was observed from prior depth change analysis from 1947 to 1979 for all of Central Bay, using more spatially coarse National Ocean Service (NOS) soundings. The portions of the overlapping survey areas between 1997 and 2008 designated as aggregate mining lease sites lost sediment at five times the rate of the remainder of west-central San Francisco Bay. Despite covering only 28% of the analysis area, volume change within leasing areas accounted for 9.2 million cubic meters of sediment loss, while the rest of the area lost 4.9 million cubic meters of sediment. The uncertainty of this recent analysis is more tightly constrained due to more stringent controls on vertical and horizontal position via tightly coupled, inertially aided differential Global Positioning Systems (GPS) solutions for survey vessel trajectory that virtually eliminate inaccuracies from traditional tide modeling and vessel motion artifacts. Further, quantification of systematic depth measurement error can now be calculated through comparison of static surfaces (e.g., bedrock) between surveys using seafloor habitat maps based on acoustic backscatter measurements and ground-truthing with grab samples and underwater video. Sediment loss in the entire San Francisco Bay Coastal System during the last half-century, as estimated from a series of bathymetric change studies, is 240 million cubic meters, and most of this is believed to be coarse sediment (i.e., sand and gravel) from Central Bay and the San Francisco Bar, which is likely to limit the sand supply to adjacent, open-coast beaches. This hypothesis is supported by a calibrated numerical model in a related study that indicates that there is a potential net export of sand-sized sediment across the Golden Gate, suggesting that a reduction in the supply of sand-sized sediment within west-central San Francisco Bay will limit transport to the outer coast.
Highlights
Multibeam sonars use swaths of sound pulses focused perpendicular to the direction of a ship to measure depths of the seafloor
The goal of this paper is to report the results of bathymetric change analysis in west–central San Francisco Bay, California, using the first multibeam surveys performed with differential Global Positioning Systems (GPS) covering this region
For the total overlapping survey areas of 40.56 km2 between the 1997 and 2008 surveys, the mean vertical change was –0.13 m (–1.2 cm yr-1), which equates to a total sediment loss of 5.4 x 106 m3
Summary
Multibeam sonars use swaths of sound pulses focused perpendicular to the direction of a ship to measure depths of the seafloor. Multibeam sonar system technology has advanced to enable imaging of the seafloor with increased spatial coverage (sub-1 m grid resolution) and angular resolution, allowing a greater number of beams per swath, increased ping frequency (5 Hz in 30 m of water) and speed (3,000 soundings/sec), and the potential of measuring depths with resolution of a few centimeters. The goal of this paper is to report the results of bathymetric change analysis in west–central San Francisco Bay, California, using the first multibeam surveys performed with differential GPS covering this region. This information is critical for the sedimentmanagement community who must assess the impact of anthropogenic influences such as aggregate mining and dredging on the regional sediment supply
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