Presentation on theme: "UW LID Workshop Bioretention Flow Control Modeling May 2008 Doug Beyerlein, P.E. Clear Creek Solutions, Inc."— Presentation transcript:
UW LID Workshop Bioretention Flow Control Modeling May 2008 Doug Beyerlein, P.E. Clear Creek Solutions, Inc.
Clear Creek Solutions’ Stormwater LID 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 WWHM: Western Washington Hydrology Model Bioretention Implicit Modeling Bioretention Explicit Modeling Seattle Bioretention Swale Modeling Modeling Results Summary Questions & Answers
Bioretention/rain garden/landscape swale:
Bioretention: Planter Box
Bioretention Reduces Runoff Volume: 1.Infiltration to native soil. 2.Evaporation and transpiration.
Flow Control Modeling Continuous simulation: WWHM (HSPF) Continuous simulation hydrology models the entire hydrologic cycle for multiple years.
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)
Western Washington Hydrology Model (WWHM) Developed for the 19 counties of western Washington. Part of Ecology’s Stormwater Management Manual
WWHM3 Available free from the Washington State Department of Ecology web site:
WWHM3 WWHM helps the user design facilities to meet the Washington State Department of Ecology’s flow control standards.
WWHM3 Ecology’s flow duration standard: based on erosive flows. Erosive flow range: ½ of the 2-year to the 50-year
Disclaimer: Bioretention by itself will not meet Ecology’s flow control standards… but bioretention will reduce the size of a flow control facility (stormwater pond, vault, etc.).
Bioretention Flow Paths Infiltration to Native Soil Weir Flow Inflow to Bioretention Facility Infiltration to Amended Soil Underdrain Flow Vertical Orifice Flow
Bioretention Flow Paths Weir Flow Underdrain Flow Vertical Orifice Flow Weir, vertical orifice, and underdrain flow all are subject to Ecology’s flow control standard (1/2 of 2-yr to 50-yr).
Bioretention Flow Paths Infiltration to Native Soil Infiltration to native soil is dependent on native soil characteristics.
WWHM: Bioretention Modeling Options Bioretention can be modeled implicitly or explicitly: PSAT (Puget Sound Action Team) recommends how to implicitly represent bioretention in WWHM2 WWHM3 explicitly represents bioretention
WWHM: Bioretention Modeling Options Implicit modeling: Represent bioretention as a pond filled with dirt. Reduce pond volume to volume of available void space. Disadvantage: Assumes pond fills from the bottom up to the surface.
WWHM: Bioretention Modeling Options Explicit modeling (current): Check infiltration rate into amended soil vs. soil moisture volume available (surface ponding). Invert the stage-volume relationship so that the soil column fills from the top down. Disadvantage: Simplifies the movement of water through the soil column.
WWHM: Bioretention Modeling Options Explicit modeling (future): dynamic hydraulic conductivity based on soil saturation levels conductivity computed based on the Van Genuchten equations discharge from a given soil layer is then computed based on conductivity, stage, and surface area
WWHM Bioretention Modeling Options cont’d: Explicit modeling (future): Soil parameters based on Rosetta parameters (Table 1).
WWHM Rosetta parameters* (Table 1). Soil Name SrSsKsn Course sand Fine sand Sandy loam Loam Silty clay loam Clay loam Peat*** Gravel loamy sand Gravel** *Values taken from Schaap and Leij (1998). **Values based on Hazen and Naval Facilities Engineering Command (NAVFAC). ***Estimation of Unsaturated Hydraulic Conductivity of Peat Soils, Schwarzela, et al.
WWHM Bioretention Modeling Options cont’d: Explicit modeling (future): The level of saturation of that soil layer is proportionate to the fraction of soil volume within a given stage: Se = Sr + [1- (H / Hm) *(Ss-Sr)] Where: Se= Saturation Sr = Residual Saturation H = Stage (within layer) Hm = Height of the layer Ss = Max saturation
WWHM Bioretention Modeling Options cont’d: Explicit modeling (future): Hydraulic conductivity K as a function of the saturation level within the soil layer is determined by the Van Genuchten equation: K = Ks *Se^(1/2) * [1-(1-Se^(1/M))^M]^2 Where: Ks = Saturated Hydraulic Conductivity Se= Saturation M = 1-(1/n) n = Van Genuchten fitting parameter
WWHM Bioretention Modeling Options cont’d: Explicit modeling (future): Flow through the porous layers is determined using Darcy’s equation: Q = As * K * S Where: Q = Flow (inches/hour) As = Surface Area K= Hydraulic Conductivity S= Stage
SPU Bioretention Modeling Results Figure 3. Rain Garden with Live Storage Depth of 5 Inches
SPU Bioretention Modeling Results Figure 4. Rain Garden with Live Storage Depth of 10 Inches
Bioretention Modeling Results For example: For an impervious area of 5000 sq ft a bioretention area of 195 sq ft with 10” of surface ponding is needed (assuming a native soil infiltration rate of 0.25 inches/hour).
Bioretention Modeling Results Summary: Bioretention works best for flow control when there is sufficient native soil infiltration. Underdrain flows must still be mitigated to flow control standards. Bioretention can reduce stormwater runoff volume, but additional mitigation will still be required to meet Ecology’s flow control standards.
Acknowledgements Seattle Public Utilities (Tracy Tackett) and Washington State University (Curtis Hinman) provided the funding for the WWHM3 bioretention modeling.
For more information on WWHM3 bioretention modeling go to:
Questions? Contact: Doug Beyerlein Joe Brascher