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Suisun Bay and Delta Bathymetry
United States Geological Survey
Foxgrover, A., Smith, R. E., and Jaffe, B. E.

Grid production is an iterative process. The first step is to adjust depth soundings to the same horizontal and vertical datum. Channel boundaries and elevation observations are then registered and modeled with an appropriate gridding algorithm. The resultant grid is compared with original soundings for verification. Data outliers are identified and removed from the source coverage and the process is repeated until differences are within the limitations of the source data and the grid resolution.

The initial data used for the Delta grid were:

  • Major streams coverage: The US Bureau of Reclamation created a single cover of the Delta hydrology based on 1:24000 quadrangle maps (Metadata)
  • Soundings: The California Department of Water Resources combined soundings collected by several agencies, including, NOAA, DWR, USGS, USBR, and COE. These soundings were corrected to a common datum (NGVD29) and combined into a single database. The database is available on the footer CSDP web site.
  • Ortho photography: Orthoquads based on a1992 aerial survey, resolution 1:12000. Most images are available on line at the USGS Bay Area Regional Database (footer BARD).

Figure 2 is an example overlay of these three source data sets. The blue and orange dots are depth and elevation observations, respectively. Red lines are channel outlines digitized from topo maps and the background image is ortho photography from a 1992 aerial survey. Note that the channel shoreline is not as accurate as the more recent aerial survey and was subsequently corrected before depth modeling.

Figure 2 - Shoreline cover over orthquad and soundings
Figure 2 - Shoreline cover over orthquad and soundings.

Data Preparation and Editing

Soundings: The soundings obtained from DWR were analyzed for accuracy. Points with vertical datum ambiguity or that were collected before 1980 were removed from the gridding process with a few exceptions. In the southern Delta and the old Sacramento River channel the only soundings available were collected prior to 1980. Figure 3 shows the channels where bathymetry modeling is supported by pre 1980 observations.

Figure 3 - Channels where the only depth soundings available were collected before 1979
Figure 3 - Channels where the only depth soundings available were collected before 1979.

Shoreline Corrections: In several areas the shoreline cover was based on surveys from the 1950's. In theses areas the shorelines were modified to conform to the soundings and more recent ortho photography. The black lines in the figure 4 below are the 'corrected' shorelines.

Figure 4 - Shoreline corrected to conform with soundings and DOQ shorelines
Figure 4 - Shoreline corrected to conform with soundings and DOQ shorelines.
Channel Criteria: Channels that were less than 30 meters (3 grid cells) wide and channels without soundings were eliminated (Figure 5). The three grid cell minimum width criterion was chosen to insure continuity of flow around bends in a meandering channel.
Figure 5 - Highlighted channels were removed or adjusted to more recent aerial photography before modeling
Figure 5 - Highlighted channels were removed or adjusted to more recent aerial photography before modeling.
  • Channel Slope: To improve grid cell estimates near the channel edge; a depth contour was added five meters from the shoreline and assigned a depth value based upon nearby soundings. The addition of this shore buffer contour ensures a channel slope that reflects the steep rip-raped levees found throughout most of the Delta. In Suisun Bay where the shoreline was natural, such as around islands, a shore buffer was not used.

  • Depth Contours: In open bays and broad channels (channels > 200m wide), contours were added at 5, 10, 20, 30, 50, 70, 90 and 110 feet to stabilize the grid interpolation. The 10 and 30-foot contours were extended into the channels less than 200 meters wide. Frequently, soundings taken in the same area a few years apart can differ by 1-3 feet. This is likely caused by sand wave migration or seasonal deposition. Since there appeared to be no systematic error between surveys, all points were retained and depth contours were used to control channel morphology. In channels with a sparse or semi-regular distribution of soundings, such as areas where we have zigzag patterns of soundings or single cross-sections (example: Cross Section), it was necessary to add a central depth contour. This contour is needed to insure that the gridding routine would follow a channel and not use soundings from adjacent channels or create unrealistic channel morphology.

  • Shoreline Elevation: To insure the initial channel slope during the gridding process was realistic; all hard shorelines were assigned an elevation of +4.0 ft (NGVD), slightly above the higher high water tidal datum. 'Soft' or tidal shorelines were set to +1.0 ft (NGVD).

    Figure 6 shows our previous example channel after adjustments to the shoreline, addition of near shore contour, and the central channel contour. Figure 7 shows the color shaded depth grid (10m grid cell) of the same section of channel.
  • Figure 6 - Same area as figure 5 with, new shorelines, shore buffers and central channel contours
    Figure 6 - Same area as figure 5 with, new shorelines, shore buffers and central channel contours.

    Figure 7 - Final color-shaded depth grid
    Figure 7 - Final color-shaded depth grid. Color grades from blue (20-40') to green (10-20') and tan (0-10'). The horizontal resolution of the grid is 10 meters.
    In special cases, soundings were added estimated based on navigation charts and unpublished data to ensure that important hydrologic features would be retained in the final bathymetry grid. Specifically, soundings were added near the Port of Stockton and Elk Slough; the 'Toe Drain' which parallels the Sacramento ship canal was assigned an estimated mean depth based on limited observations. Figure 8 shows these additions.
    Figure 8 - Channels where soundings were added or depths assigned to retain channels that would otherwise have been deleted
    Figure 8 - Channels where soundings were added or depths assigned to retain channels that would otherwise have been deleted.
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