Western Washington Hydrology Model 2005 AWRA Annual Conference Doug Beyerlein, P.E. Joe Brascher Shanon White Clear Creek Solutions, Inc.

Western Washington Hydrology Model Presented at the 2005 AWRA Annual Conference, Seattle, WA © Clear Creek Solutions, Inc. All Rights Reserved.

Clear Creek Solutions’ Hydrology Expertise Clear Creek Solutions, Inc., provides complete range of hydrologic and stormwater modeling services. Clear Creek specializes in continuous simulation hydrologic modeling. We have 30+ years of experience modeling complex hydrologic and stormwater problems. We created WWHM3 for Washington State Department of Ecology and are adapting it to model Bay Area watersheds and facilities. We teach WWHM and HSPF workshops.

Presentation Introduction WWHM Background and Theory WWHM and HSPF WWHM Application Adaptation to Other Regions Questions & Answers

WWHM Introduction Developed for the State of Washington Department of Ecology. Project Manager: Dr. Foroozan Labib Department of Ecology PO Box Olympia, WA (360)

WWHM Introduction Developed for the 19 counties of western Washington. Part of Ecology’s Stormwater Management Manual

WWHM Introduction Available free from the Washington State Department of Ecology web site:

WWHM Background and Theory Washington State Department of Ecology’s Stormwater Management Manual updated in 2001 to comply with:  Federal Clean Water Act  Federal Endangered Species Act  State Water Pollution Control Act

WWHM Background and Theory Ecology’s Stormwater Management Manual  Specifies a flow duration standard  Requires the use of Western Washington Hydrology Model (WWHM)

WWHM Background and Theory Flow duration standard: based on erosive flows.

WWHM Background and Theory Flow Duration Analysis: Percent of time the flow exceeds a specific value. Requires continuous simulation hydrology to compute flow duration.

WWHM Background and Theory Continuous simulation hydrology models the entire hydrologic cycle for multiple years. Stormwater runoff = surface runoff + interflow.

Where the rain goes: Forest Impervious Evapotranspiration 0% 100% Surface Runoff Interflow Groundwater/Base Flow Land use development

Ponds designed based on a single event are too small.  Single-event flow frequency standards are based on inappropriate assumptions.  The need is to NOT increase flow durations for the range of erosive flows (for Western Washington the range is from 1/2 of the 2-year to 50-year flow event).

Inappropriate assumptions used by single event methods:  Assume stormwater detention facilities are empty at start of a rain event.  Assume too large predevelopment runoff when using SCS Curve Numbers.  Assume matching flow frequencies will result in no increase in erosive flows.

Ponds designed using single- event methods fail because:  The ponds are too small to store the additional runoff.  The number of hours of erosive flows increases even if the flow frequency does not.  The downstream riparian corridor is damaged by the increase of erosive flows.

WWHM Capabilities WWHM uses continuous simulation hydrology to compute stormwater runoff for both predevelopment and post-development conditions. Predevelopment Post-development

WWHM Capabilities  WWHM continuous simulation computations from HSPF (included in WWHM software).  HSPF runs in the background with calibrated parameter values and local meteorological data.  HSPF sponsored and funded by EPA and USGS.

How HSPF works  Drainage areas are divided into pervious land segments based on soil, vegetation, and land slope and impervious land segments.  Pervious and impervious segments are linked to conveyance pathways (pipes, ditches, ponds, streams, rivers, lakes, etc.)  Historic rainfall and evaporation are used as input.

How HSPF works  The entire water cycle is modeled on an hourly or shorter time step for multiple years.  The model computes changes in soil moisture, evapotranspiration, and runoff every time step.  Three types of runoff:  surface runoff  interflow  groundwater/base flow

How HSPF works  Pervious and impervious parameter values are calibrated using observed streamflow data.  Calibration uses multiple years of observed flow data so that the model is calibrated for both dry years and wet years.  We use a number of different statistical tests to judge the accuracy of the calibration.

How WWHM works  Calibrated parameter values are built into WWHM (different calibrated parameter values are used for different climatic regions).  Local precipitation and evaporation data are included in WWHM.

How WWHM works  User locates project on county map, inputs pre- and post-development land use, and proposed mitigation measure(s).  Predevelopment and mitigated flows are compared at Point of Compliance (POC).  Mitigated flows are not allowed to exceed flow duration standards.

How WWHM works  User outputs report file and project file.  Output files can be submitted to the permitting agency for approval.  Permitting agency can load project file and rerun analysis, if needed.  Report file summarizes input data and output results.

How WWHM works  Guidance/help is provided by  User Manual  Training Workshops  CCS Project Book

WWHM Application: Example: Convert forest to suburban residential housing.

WWHM Application: Step 1 User selects land development location and places red dot on map.

WWHM Application: Step 2 User inputs pre- and post-development land use. Pre: Forest Post: roof, street, landscaping, & pond

WWHM Application: Step 2 cont’d. User inputs initial pond specifications or uses AutoPond to optimize pond size.

WWHM Application: Step 3 WWHM computes and compares predevelopment and post-development routed runoff statistics Flow frequency (2-year to 100-year):

WWHM Application: Step 3 cont’d. WWHM compares predevelopment and post-development routed runoff statistics Flow duration (50% of 2-year to 100% of 50-year):

WWHM Application: Step 3 cont’d. WWHM identifies water quality flow rates and volumes for separate treatment facilities based on Washington treatment standard of 91% of total runoff volume:

WWHM Application: Step 4 WWHM output summarizes statistics and whether or not the facility meets Washington State Department of Ecology’s standards:

WWHM Application: Step 4 WWHM designed pond meets Washington State Department of Ecology’s standards:

WWHM: LID Options WWHM includes the following LID options:  Dispersion of impervious surface runoff on adjacent pervious surface (example: roof runoff to lawn)  Infiltration of impervious surface runoff on adjacent pervious surface (soil dependent)  Pervious pavement  PSAT (Puget Sound Action Team) recommends how to represent other LIDs in WWHM

WWHM LID Analysis Tool: WWHM compares different development options selected by the user and shows the runoff distribution: POC 1: Conventional POC 2: Dispersion (20% reduction in pond volume) POC 3: Dispersion and Infiltration (84% reduction in pond volume)

WWHM Adaptation to Other Regions  HSPF model construction/calibration/parameterization  Meteorological data  Interface modification  AutoPond rules  Documentation and user manual  Training workshops  Maintenance and support  Distribution

WWHM Future Enhancements  low impact development/source BMPs  wetland modeling  hydraulic routing  HSPF water quality  3-D graphical interface  your ideas?

Questions? Call Doug Beyerlein Joe Brascher