AQUA TERRA Consultants
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Development of Department of Ecology Hydrology Model for Western Washington

Client: State of Washington Department of Ecology

The State of Washington Department of Ecology contracted with AQUA TERRA Consultants to develop a new hydrology software package to be used to size stormwater control facilities in western Washington. The new hydrology software is part of Ecology's revised Stormwater Management Manual.

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The new hydrology software package is based on EPA's HSPF software. A new Windows interface has been added so that the user is not required to know anything about HSPF. Local precipitation and evaporation data are included with the software. Historic (predevelopment) vegetation information is provided to the user.

The user specifies the location of the proposed development on a map of western Washington counties together with pre- and post-development land use and soils information. This allows the software to select the appropriate local precipitation station, precipitation multiplication factor, evaporation data, pan evaporation coefficient, vegetation, and corresponding HSPF PERLND and IMPLND parameter values.

HSPF computes 40+ years of 15-minute runoff data, which are stored in the Watershed Data Management (WDM) file. The user sizes the stormwater control facility to meet the State's requirements that the post-development 2-year flood cannot exceed half of the pre-development 2-year flood, the 10-year post-development flood cannot exceed the 10-year pre-development flood, and the 50-year post-development flood cannot exceed the 50-year pre-development flood. In addition post-development flow durations cannot exceed pre-development flow durations by more than 10 percent for durations between the flow range from half of the pre-development 2-year flood to 100 percent of the 50-year pre-development flood.

The user inputs a stage-storage-discharge table that represents the stormwater control facility. The software routes the post-development flow through the facility, computes the flood frequency and flow duration statistics for the flow out of the facility, and checks for compliance with the above-stated criteria.

The software summarizes the results and reports to the user whether or not the proposed stormwater control facility meets Ecology's standards.

This new continuous simulation hydrology software package replaces Ecology's previous standard based on the SCS single-event hydrology method.

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