Grayson/Murderer’s Creek Hydrology Study

In 2005, Contra Costa County Public Works Department, which staffs the Contra Costa County Flood Control and Water Conservation District, conducted a hydrology study for the Grayson and Murderer’s Creek area. The work was done in cooperation with the U.S. Army Corps of Engineers, Sacramento District.

Arc Hydro was used for the preliminary delineation of the subwatershed and channel definition. Corrections and refinement of the subwatershed boundaries were required because analysis of digital elevation models is notoriously inaccurate in low-lying areas. CAD files of the local drainage systems from three cities and the county were referenced in the project file and used to correct the subwatershed boundaries. Heads-up digitizing of scanned historic hydrology maps was used where possible. The combination of using Arc Hydro, CAD reference, and heads-up digitizing provided an accurate and timely way for delineating the subwatersheds.

The Corps of Engineers’ hydrology criteria called for existing conditions, not the county General Plan “ultimate build-out” that was available in a layer. A new land-use layer was created by identifying each parcel that had a zero-dollar structure value assigned to it in the assessor’s parcel data. Aerial orthophotos were examined to determine which of these parcels were undeveloped. If a parcel had no buildings, it was given a separate “vacant” land-use designation. The revised land-use layer was used to valuate parameters related to the urbanized watershed. In a spreadsheet, each land use was assigned an infiltration rate and watershed N-value. (The N-value is similar to Manning’s n and is an approximation of the ease with which water flows off the watershed and not specifically the friction of the channels.)

Reports of the land-use areas in each watershed were created from ArcMap, and a spreadsheet was used to calculate composite infiltration and N-values. With the undeveloped parcels being designated as open space, the resulting parameters reflected existing conditions.

Arc Hydro tools were used to measure and analyze several hydrology model inputs including area, centroid, longest flow path, total channel fall, and length from centroid. The local flood control district mean seasonal isohyets were in a GIS layer and were used to calculate composite mean seasonal rainfall depths for the hydrologic analysis. The parameters for each subwatershed were used in a unit hydrograph model and creation of design flood hydrographs.

Such detailed analysis of the watersheds and land use could not have been accomplished without Arc Hydro and GIS capabilities. Use of GIS provided a way to confidently define parameters critical for creating hydrographs for use in modeling different flood control project alternatives.