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: Infiltration to native soil. 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 47600 Olympia, WA 98504-7600 (360) 407-6439 email: flab461@ecy.wa.gov

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: http://www.ecy.wa.gov/programs/wq/stormwater/

WWHM3 WWHM helps the user design facilities to meet the Washington State Department of Ecology’s flow control standards.

Ecology’s flow duration standard: based on erosive flows. 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 Inflow to Bioretention Facility Weir Flow Infiltration to Amended Soil Vertical Orifice Flow Underdrain Flow Infiltration to Native Soil

Bioretention Flow Paths Weir, vertical orifice, and underdrain flow all are subject to Ecology’s flow control standard (1/2 of 2-yr to 50-yr). Weir Flow Vertical Orifice Flow Underdrain Flow

Bioretention Flow Paths Infiltration to native soil is dependent on native soil characteristics. Infiltration to Native Soil

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 Sr Ss Ks n Course sand 0.052 0.395 3.162 0.501 Fine sand 0.364 2.552 0.404 Sandy loam 0.030 0.380 1.445 0.317 Loam 0.061 0.399 1.479 0.197 Silty clay loam 0.077 .0475 1.513 .0184 Clay loam 0.087 0.445 1.412 0.133 Peat*** 0.099 0.863 3.050 0.229 Gravel loamy sand 0.1 0.45 3.5 9.37 Gravel** 0.02 0.42 10 18.4 *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

WWHM Bioretention Modeling Seattle Bioretention Swales

WWHM3 Bioretention Modeling Drainage areas A, B, and C to bioretention swales.

WWHM Bioretention Modeling Seattle Bioretention Swales

WWHM Bioretention Modeling Downstream control structure: Water infiltrates into the soil before runoff.

WWHM Bioretention Modeling Weir and vertical orifice outlet Amended Soil (1-3 layers) Native Soil Native Soil Underdrain (optional) Native Soil

WWHM3 Bioretention Modeling WWHM3 bioretention element stores and infiltrates runoff.

SPU Bioretention Calibration Model results: Seattle Swale N-2 (2004) Amended soil infiltration rate = 3.0 in/hr Native soil infiltration rate = 1.9 in/hr

SPU Bioretention Calibration Model results: Seattle Swale N-2 (2004) Amended soil infiltration rate = 3.0 in/hr Native soil infiltration rate = 1.9 in/hr

SPU Bioretention Calibration Model results: Seattle Swale N-2 (2004) 29-30 January 2004 storm event

SPU Bioretention Modeling Results Seattle Bioretention Frequency Comparison Return Period (years) Reduction 2 66% 5 63% 10 64% 25 50 67% 100 69%

Bioretention Modeling Results Stormwater volume reduction:

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: www For more information on WWHM3 bioretention modeling go to: www.clearcreeksolutions.com

Questions. Contact: Doug Beyerlein 425. 892 Questions? Contact: Doug Beyerlein 425.892.6454 beyerlein@clearcreeksolutions.com Joe Brascher 360.236.1321 brascher@clearcreeksolutions.com