1. UW LID Workshop Bioretention Flow Control Modeling May 2008
Doug Beyerlein, P.E.
Clear Creek Solutions, Inc.

2. 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.

3. Presentation Introduction WWHM: Western Washington Hydrology Model
Bioretention Implicit Modeling
Bioretention Explicit Modeling
Seattle Bioretention Swale Modeling
Modeling Results
Summary
Questions & Answers

4. Bioretention/rain garden/landscape swale:

5. Bioretention: Planter Box

6. Bioretention Reduces Runoff Volume:
Infiltration to native soil.
Evaporation and transpiration.

7. Flow Control Modeling Continuous simulation: WWHM (HSPF)
Continuous simulation hydrology models the entire hydrologic cycle for multiple years.

8. 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
(360)

9. Western Washington Hydrology Model (WWHM)
Developed for the 19 counties of western Washington.
Part of
Ecology’s
Stormwater
Management
Manual

10. WWHM3
Available free from the Washington State Department of Ecology web site:

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

12. 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

13. 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.).

14. Bioretention Flow Paths
Inflow to Bioretention Facility
Weir Flow
Infiltration to Amended Soil
Vertical Orifice Flow
Underdrain Flow
Infiltration to Native Soil

15. 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

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

17. 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

18. 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.

19. 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.

20. 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

21. WWHM Bioretention Modeling Options cont’d:
Explicit modeling (future):
Soil parameters based on Rosetta parameters (Table 1).

22. 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.

23. 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

24. 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

25. 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

26. WWHM Bioretention Modeling
Seattle Bioretention Swales

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

28. WWHM Bioretention Modeling
Seattle Bioretention Swales

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

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

31. WWHM3 Bioretention Modeling
WWHM3 bioretention element stores and infiltrates runoff.

32. 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

33. 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

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

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

36. Bioretention Modeling Results
Stormwater volume reduction:

37. SPU Bioretention Modeling Results
Figure 3. Rain Garden with Live Storage Depth of 5 Inches

38. SPU Bioretention Modeling Results
Figure 4. Rain Garden with Live Storage Depth of 10 Inches

39. 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).

40. 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.

41. Acknowledgements Seattle Public Utilities (Tracy Tackett) and Washington State University (Curtis Hinman) provided the funding for the WWHM3 bioretention modeling.

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

43. Questions. Contact: Doug Beyerlein 425. 892
Questions? Contact: Doug Beyerlein Joe Brascher