Gridded bathymetric data from repeat surveys of south San Francisco Bay, California, 2023-2025

All six bathymetric surveys performed with the SwathPlus sonar system were conducted using the same mapping systems and post-processed using the same methodology. The data were collected with a SWATHplus-M 234-kHz interferometric side-scan sonar system pole-mounted on the U.S. Geological Survey (USGS) R/V Parke Snavely survey vessel. During the mapping mission, an Applanix POS-MV (Position and Orientation System for Marine Vessels) was used to position the vessel, and it also accounted for vessel motion such as heave, pitch, and roll. The POS-MV also recorded position and vessel motion parameters for later post-processing. Sound-velocity profiles were collected throughout the survey days with a Castaway CTD. Post-processed POS-MV Smoothed Best Estimate of Trajectory (SBET) files were merged with the survey line files using BathySwath software and then the line files were imported into SonarWiz software. Soundings were cleaned for obvious errors and gridded into a 50-cm base surfaces. The base surfaces were exported as Erdas Imagine files in UTM Zone 10 North, WGS84 coordinates vertically referenced to the WGS84 ellipsoid. The data were transformed from WGS84 to NAD83 horizontal coordinates using ArcGIS Pro ArcTool raster projection command WGS_1984_(ITRF08)_To_NAD_1983_2011. NOAA's on-line VDATUM transformation tool states there is a +33.062 difference between WGS84 and NAVD88 elevations (vertical uncertainty of 0.06 m) at a point in the center of the survey area. A value of 33.062 was added to the original bathymetric grids to transform from WGS-84 elevations to NAVD88 elevations. The NAVD88 bathymetric grids were then exported as a GeoTIFF files in UTM zone 10, NAD83(2011) coordinates.

Bathymetric surveys performed with single beam sonar systems were conducted using the same mapping systems and post-processed using the same methodology. The data were collected from personal watercraft equipped with a CEEHydrosystems Ceescope single beam sonar system with a 200 kHz and 9-degree beam angle. Initially, the survey design included a total of 40 survey lines that were oriented in the cross channel direction and spaced at 90 m intervals along channel centered on the dredged sediment placement area. The survey design was adjusted for the final single-beam survey to better characterize the sediment deposits that formed as a result of the dredged sediment placements. The updated survey design included a total of 111 cross channel transects spaced at about 23 m intervals and reduced the spatial extent of the survey area. Positions of the survey platforms were recorded at 10 Hz intervals during the mapping missions with a Trimble BD990 GNSS receiver. Sound-velocity profiles were collected throughout the survey days with a Castaway CTD. Positioning data from the GNSS receivers were post-processed using Waypoint Grafnav software to apply differential corrections from the National Geodetic Survey (NGS) Continuously Operating Reference Station Site, P222, located approximately 10 km away with horizontal and vertical coordinates relative to the North American Datum of 1983 (2011 realization). Orthometric elevations relative to the NAVD88 vertical datum were computed using NGS Geoid18 offsets. Bathymetric data were merged with post-processed positioning data and spurious soundings were removed using a custom Graphical User Interface (GUI) programmed with the computer program MATLAB. The processed data were projected using the UTM Zone 10 North meters coordinate system and continuous gridded surfaces with a horizontal resolution of 0.5 m were produced using linear interpolation.

Initial inspection of the bathymetric surfaces revealed differences in elevations between the SwathPlus and single-beam surveys in regions where bathymetric change was not expected to be observed. Static vertical adjustments were therefore applied to each bathymetric surface to eliminate the mean bias based on comparisons between the initial swath survey and subsequent surveys in relatively flat areas where bathymetric change was assumed to be negligible. The mean static vertical corrections applied to the successive SwathPlus and single beam survey data were -2 cm and -13 cm, respectively.