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