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Historical Bathymetric Change in Suisun Bay 1867 - 1990
United States Geological Survey
Cappiella, K., Malzone, C., Smith, R. E., and Jaffe, B. E.

A continuous surface representation (surface grid) of each bathymetric survey was created using Topogrid, an Arc/Info module that utilizes sounding and contour information to create a hydrodynamically correct surface. Input data were a combination of point soundings and depth contours as described in the table below. Each historical bathymetric surface is defined by more than 200,000 grid cells (each cell is a 25-meter square). Bathymetric change grids showing erosion and deposition were generated by differencing bathymetric grids and applying vertical and horizontal corrections to bring surveys to a common datum. Creation of accurate bathymetric grids involved several steps.

The data for the earlier surveys were taken from mylar copies of original 1:10,000 scale NOS Hydrographic Sheets (12 KB), called H-Sheets. These annotated H-Sheets were scanned at 600 dpi and registered using Latitude/Longitude graticules. Point soundings and depth contours were then digitized from the registered image. NOS contours were checked for accuracy and additional contours were added in areas where point soundings were sparse. For the 1942 survey, all of the sounding information was obtained digitally from the GEOphysical DAta System for Hydrographic Survey Data (GEODAS) compiled by NOS. The shoreline and contour information for the 1942 survey was obtained from hard copies of the H-Sheets. The sounding, shoreline and contour information for the 1990 survey was obtained from GEODAS. Input data were gridded and the grids compared to the input data to check for problem areas. The final step was regridding after adding soundings and contours in order to get the best possible representation of the historical bathymetry.

Once the bathymetric grids were revised to meet an acceptable level of error, four change grids were created by taking the difference between the bathymetric grids for each set of consecutive survey years (i.e., 1887survey - 1867survey = 1867-1887change grid). These bathymetric change grids display erosion and deposition and were used to calculate actual volumes of sediment eroded or deposited once vertical and horizontal corrections were applied to bring surveys to a common datum. The grids were used to identify patterns of change as well as to calculate sedimentation rates and volume changes for each time period.

The focus of this analysis was bathymetric change, not shoreline change. In some cases the H-Sheets have incomplete shoreline and marsh front information and were supplemented with shorelines from scanned USCGS Topographic Sheets (T-Sheets). Portions of the 1922 and 1942 shorelines in the Grizzly Bay area are missing, along with bathymetric data for Grizzly Bay. In the future we hope to improve our data set by obtaining additional shoreline and marsh information for these areas.

Sounding and Contour Information for Suisun Bay Hydrographic Sheets

Year

Number of Soundings

 Contour Intervals

 1867

18,202

-4, 0, 6, 12, 18, 30, 60, 90 (feet)

1887

21,753

-4, 0, 6, 12, 18, 30, 60, 90 (feet)

1922

17,303

-4, 0, 6, 12, 18, 30, 60, 90 (feet)

1942

36,169

-4, 0, 6, 12, 18, 30, 60, 90 (feet)

1990

93,303

1, -2, -5, -10, -15, -20, -25, -30, -35, -45 (meters)

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