HSPF-Based WWHM: A Tool for Stormwater Design Using Flow Duration Criteria

The Western Washington Hydrology Model (WWHM) was developed for the State of Washington Department of Ecology by AQUA TERRA Consultants to size stormwater control facilities in western Washington, based on flow duration standards. The flow duration standard is better than the design storm frequency methodology for sizing stormwater facilities because it prevents an increase in the duration of erosive flows. The design storm methodology does not do this. The erosive flows are the flows that do the most damage to the receiving streams, streambanks, and adjacent properties. The WWHM computes stormwater runoff for a site selected by the user. The WWHM uses long-term local hourly precipitation. The model runs HSPF in the background to generate 40+ years of hourly runoff data. Stormwater runoff is computed for both predevelopment and post-development land use conditions. The WWHM routes the post-development stormwater runoff through a stormwater control facility of the user's choice. The Department of Ecology standards require that post-development flow durations not exceed pre-development flow durations for the range of erosive flows. The WWHM computes the required flow duration information and reports if the facility meets Ecology's standards. The available types of stormwater control facilities include standard trapezoidal pond, infiltration pond, tank (cylindrical, arched), vault, and irregular-shaped pond. The facility can be either manually sized or the user can use the WWHM pond optimization feature. The WWHM also allows the user to select appropriate low impact development BMPs (pervious pavement, roof runoff dispersion, infiltration, etc.) and evaluate their ability to reduce the size of the required stormwater control facility. The WWHM software can be customized for a specific jurisdiction (city, county, or watershed) in western Washington or anywhere in the U.S. to better represent the hydrology of the jurisdiction's watersheds and/or to include new criteria or standards and BMPs.