Bio-Retention Cell LID Reference Values
Excerpt from the EPA manual Storm Water Management Model Reference Manual Volume III – Water Quality (PDF) which can be found here
6.5 Parameter Estimates
The variety of LID controls modeled by SWMM introduces a significant number of design variables and parameters that must be assigned values by the user. These include sizing parameters (surface area, layer depths, and capture ratio), surface parameters (freeboard depth, outflow face width, slope, and roughness), soil parameters (moisture limits and hydraulic conductivity), pavement parameters (void ratio and permeability), storage parameters (void ratio and native soil conductivity), drain parameters (discharge coefficient and exponent, roof drain capacity, and drain mat roughness), and clogging parameter. Because of the high interest and acceptance of LID, many local and state agencies have prepared design manuals that recommend ranges for many key parameters. Table 6-1 lists a selection of these manuals, all available online. Unless otherwise noted, these manuals served as the source of the LID parameter values described in the sub-sections that follow.
6.5.1 Bio-Retention Cells and Rain Gardens Parameter Values
Table 6-2 lists ranges of parameter values for bio-retention cells and rain gardens, expressed in their typical US units of inches and hours. They are internally converted to feet and seconds for use in the governing conservation equations.
Table 6-1 Design manuals used as sources for LID parameter values
| Organization | Manual Title | Year | URL |
| Prince Georges County Maryland | Low-Impact Development Design: An Integrated Design Approach | 1999 | http://water.epa.gov/polwaste/green/upload/lidnatl.pdf |
| Denver Urban Drainage and Flood Control District | Urban Storm Drainage Criteria Manual, Volume 3 Best Management Practices | 2010 | http://udfcd.org/wp-content/uploads/uploads/vol3%20criteria%20manual/USDCM%20Volume%203.pdf |
| Toronto and Region Conservation Authority | Low Impact Development Stormwater Management Planning and Design Guide | 2010 | http://www.creditvalleyca.ca/wp-content/uploads/2014/04/LID-SWM-Guide-v1.0_2010_1_no-appendices.pdf |
| Washington State University Extension | Low Impact Development Technical Guidance Manual for Puget Sound | 2012 | http://www.psp.wa.gov/downloads/LID/20121221_LIDmanual_FINAL_secure.pdf |
| District of Columbia | Stormwater Management Guidebook | 2013 | http://doee.dc.gov/swguidebook |
| Philadelphia Water Department | Stormwater Management Guidance Manual, Version 2.1 | 2014 | http://www.pwdplanreview.org/upload/pdf/Full%20Manual%20%28Manual%20Version%202.1%29. pdf |
| University of New Hampshire Stormwater Center | UNHSC Design Specifications for Porous Asphalt Pavement and Infiltration Beds | 2014 | http://www.unh.edu/unhsc/sites/unh.edu.unhsc/files/pubs_specs_info/unhsc_pa_spec_10_09.pdf |
| NY State Department of Environmental Conservation | Stormwater Management Design Manual | 2015 | http://www.dec.ny.gov/docs/water_pdf/swdm2015entire.pdf |
Table 6-2 Typical ranges for bio-retention cell parameters
| Parameter | Range |
| Maximum Freeboard, inches (D1) | 6 – 12 |
| Surface Void Fraction (f1) | 0.8 – 1.0 |
| Soil Layer Thickness, inches (D2) | 24 – 48 |
| Soil Properties: | |
| Porosity (f2) | 0.45 – 0.6 |
| Field Capacity (qFC) | 0.15 – 0.25 |
| Wilting Point (qWP) | 0.05 – 0.15 |
| Saturated Hydraulic Conductivity, in/hr (K2S) | 2.0 – 5.5 |
| Wetting Front Suction Head, inches (y2) | 2 – 4 |
| Percolation Decay Constant (HCO) | 30 – 55 |
| Storage Layer Thickness, inches (D3) | 6 – 36 |
| Storage Void Fraction (f3) | 0.2 – 0.4 |
| Capture Ratio (RLID) | 5 – 15 |
Table 6-3 Soil characteristics for a typical bio-retention cell soil
| Soil Property | Value |
| Porosity (f2) | 0.52 |
| Field Capacity (qFC) | 0.15 |
| Wilting Point (qWP) | 0.08 |
| Saturated Hydraulic Conductivity, in/hr (K2S) | 4.7 |
| Percolation Decay Constant (HCO) | 39.3 |
| Wetting Front Suction Head, inches (y2 = 3.23(K2S)-0.328) | 1.9 |