Various types of drainage facilities are required to protect the highway against surface and subsurface water. Drainage facilities must be designed to convey the water across, along, or away from the highway in the most economical, efficient, and safe manner without damaging the highway or adjacent property. The purpose of this manual is to provide detailed information on the subjects of hydrologic and hydraulic analysis related to highway design. This manual should be used in conjunction with the Washington State Department of Transportation (WSDOT) Highway Runoff Manual and the WSDOT Design Manual, specifically Section 1210.

  • Photo of Mean Annual Runoff

    Mean Annual Runoff

    Sometimes it is necessary to determine the mean annual flow or runoff for a given stream. When published flow records…

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  • Flood Reports

    Flood reports have been developed for many rivers in Washington State. Most of these reports, and the ones that are…

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  • USGS Regression Equations

    While measured flows provide the best data for design purposes, it is not practical to gage all rivers and streams…

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  • Published Flow Records

    When available, published flow records provide the most accurate data for designing culverts and bridge openings. This is because the…

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  • Santa Barbara Urban Hydrograph Method

    When designing flood control structures and some stormwater treatment facilities, the designer must know more than just the peak flow…

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  • Photo of The Rational Method

    The Rational Method

    General The rational method is used to predict peak flows for small drainage areas which can be either natural or…

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  • Drainage Basin

    The size of the drainage basin is one of the most important parameters regardless of which method of hydrologic analysis…

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  • Selecting hydrology analysis method

    Each of the first five methods listed above are appropriate to use for different design conditions and none of the…

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