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  • When available, published flow records provide the most accurate data for designing culverts and bridge openings. This is because the values are based on actual measured flows and not calculated flows. The streamflows are measured at a gaging site for several years. A statistical analysis (typically Log Pearson Type III) is then performed on the measured flows to predict the recurrence intervals.

    The USGS maintains a large majority of the gaging sites throughout Washington State. A list of all of the USGS gages that have adequate data to develop the recurrence intervals and their corresponding flows is provided in Appendix 2-1. In addition to these values, the OSC Hydraulics Branch maintains records of daily flows and peak flows for all of the current USGS gages. Also, average daily flow values for all current and discontinued USGS gages are available through the Internet on the USGS homepage (note that these are average daily values and not peak values).

    Some local agencies also maintain streamflow gages. Typically, these are on smaller streams than the USGS gages. While the data obtained from these gages is usually of high enough quality to use for design purposes, the data is not always readily available. If the designer thinks that there is a possibility that a local agency has flow records for a particular stream then the engineering department of the local agency should be contacted. The OSC Hydraulics Branch does not maintain a list of active local agency streamflow gages.

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  • When designing flood control structures and some stormwater treatment facilities, the designer must know more than just the peak flow that will occur. Along with the peak flow, the volume of runoff must be calculated as well as the relationship between time and the rate of runoff. The only way to accomplish this is to use a method of analysis that incorporates a hydrograph. A hydrograph is a graphical representation of flow versus time.

    Of the several commonly accepted hydrograph methods, the Santa Barbara Urban Hydrograph (SBUH) method is the best suited for the types of projects that WSDOT designs. It was developed to calculate flows from small to medium sized urban basins using input data that is readily available and equations that are easily understood. While not all WSDOT projects are in urban basins, it is typically the paved surfaces (similar to urban areas) that generate the majority of the total flow.

    The SBUH method is computationally intensive. Calculations for even a single drainage area would take hours if done by hand. Because of this, the only practical way to perform an analysis is to use a computer application. The equations used are simple enough to be incorporated into a spreadsheet which would provide accurate calculations; however, it is highly recommended that one of the commercially available computer programs that includes the SBUH method be used. The advantage of using commercial software is the overall consistency of input and output formats and the reliability obtained from being tested in several different design circumstances.

    There are several commercially available computer programs that include the SBUH method. Each of these programs have certain features that make them unique from other programs but the primary calculations are performed the same way. Because of this, nearly any commercially available computer program that includes the SBUH method is acceptable for designing WSDOT projects.

    Site licenses for the computer program WaterWorksTM have been purchased by each WSDOT region and by the Washington State Ferries Division so this program and associated manual are available to all WSDOT designers. The OSC Hydraulics Branch encourages the use of WaterWorksTM whenever performing an SBUH method analysis. The OSC Hydraulics Branch is available to lend technical assistance on using WaterWorksTM.

    The SBUH method only calculates flow that will occur from surface runoff and thus is not accurate for large drainage basins where ground water flow can be a major contributor to the total flow. As a result, the SBUH method is most accurate for drainage basins smaller than 40 hectares (100 acres) and should never be used for drainage basins larger than 400 hectares (1,000 acres).

    Chapter 3 of the WSDOT Highway Runoff Manual discusses the details of performing an analysis using the SBUH method. The Highway Runoff Manual also includes information on flood control structures and stormwater treatment facilities which will be used in conjunction with almost all SBUH method analyses.

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