HYDROLOGIC COMPUTER MODELING Washington Hydrology Society March 2006 Joe Brascher, President Clear Creek Solutions, Inc.

Clear Creek Solutions’ Hydrologic Modeling 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 the Western Washington Hydrology Model Version 3 (WWHM3) for Washington State Department of Ecology. We teach WWHM and HSPF workshops.

Presentation Introduction History of Hydrologic Modeling Single Event Models Continuous Simulation Models Western Washington Hydrology Model 3 Questions & Answers

History of Hydrologic Modeling In the beginning: Single event

History of Hydrologic Modeling Today: Continuous simulation

Single-event Hydrologic Models Based on flood frequency. When slide rules roamed the earth modeling had to be simple. Rational Method (Q = CIA) SCS Method (Curve Numbers) TR-20, TR-55, SBUH, HEC-1, Waterworks

Typical Synthetic Precipitation Event

Historical Precipitation Event

Single-event problems:

 Single-event frequency standards are based on inappropriate assumptions.  Single-event methods cannot compute flow durations (percent of time a flow is exceeded).

Single-event inappropriate assumptions:  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.  Assume 100-yr storm causes 100-yr flood.

Single-event designed stormwater facilities fail because:  The facilities 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.

Continuous Simulation Models Continuous simulation hydrology models the entire hydrologic cycle for multiple years.

Continuous Simulation Models  Stanford Watershed Model (original)  HSPF (EPA)  KCRTS (King Co)  MGS Flood (WSDOT)  LIFE (CH2M Hill)  WWHM (Ecology)  WWHM3 PRO (Clear Creek Solutions)

Continuous Simulation Models HSPF: DOS-based, steep learning curve, recommended only for experienced users KCRTS: Limited to King County, pre- generated runoff from HSPF, limited LID capabilities MGS Flood (WSDOT): HSPF-based, extended precip time series, proprietary pond sizing option

Continuous Simulation Models LIFE (CH2M Hill): soil measurement- based, proprietary WWHM (Ecology): HSPF-based, NOAA precip records, LID elements included in WWHM3 WWHM3 PRO (Clear Creek Solutions): WWHM3 with extra features

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

Where Are We Going?

Models that do more  Better user interfaces  Combining the best models

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

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

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

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

WWHM Flow duration standard: based on erosive flows. Erosive flow range: ½ of the 2-year to the 50-year

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

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

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

WWHM  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.

WWHM  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.

WWHM  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

WWHM  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.

WWHM  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.

WWHM  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.

WWHM WWHM-designed ponds meet 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

Changes from WWHM2 to 3: Goal: give the user greater flexibility and options. Ecology wanted:  High groundwater element  Lateral flow between basins  More elements  More output options  New LID options

Changes from WWHM2 to 3: Version 2 projects were limited to drainages of less than 320 acres because of lack of conveyance options (channels and pipes). Version 3 projects have no limitations: entire watersheds can be modeled. Version 3 can be customized to local jurisdictions (e.g., City of Bellingham)

Changes from WWHM2 to 3: 14 elements: 1.basin 2.trapezoidal pond 3.tank 4.lateral pervious basin (NEW) 5.high groundwater/wetland (NEW) 6.vault 7.open channel (NEW) 8.time series (NEW) 9.lateral impervious basin (NEW) 10.gravel trench bed (NEW) 11.sand filter 12.flow splitter (IMPROVED) 13.irregular-shaped pond 14.stage-storage-discharge table

Changes from WWHM2 to 3:  Complex stormwater systems can be modeled

WWHM3 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)

WWHM3 PRO: WWHM3 with additional enhancements:  user selected time step (5 minutes to one hour)  tagged flow routing  additional elements  culverts  green roofs  landscape swales/bioretention/ rain gardens  SWMM routing connection  GIS land use connection

City of Bellingham  Comprehensive Stormwater Plan (streams and pipe systems): Includes SWMM and GIS connections

Seattle Public Utilities Natural Drainage System Program: Green/vegetated/eco-roof modeling

Examples of Green Roofs

Green Roof Model Results Hamilton Bldg, Portland, OR

Green Roof Flow Duration Blue: Conventional Roof Red: Green Roof

Green Roof Hydrograph Change

Seattle Public Utilities Rain garden/bioretention/landscape swale modeling

Seattle Public Utilities 110 th Street Cascades: 12 swales in 3 blocks

WWHM3 PRO SWMM routing connection

WWHM3 PRO GIS connection

WWHM3 PRO WWHM3 PRO will be available Spring 2006 See for details.

Turning Hydrologic Modeling Upside Down

WWHM3 PRO Future Enhancements  additional low impact development/ source BMPs  HSPF water quality  3-D graphical interface  your ideas?

Questions?