Bioretention - Sizing

Two Excel spreadsheets are provided for download in order to help with sizing the bioretention cell. When sizing a bioretention facility, the designer has to first determine the intended purpose of the cell. For example,

1. What are the site requirements for water quality and quantity control? Will the bioretention cell be used to address both water quality and quantity or just one of the issues?

2. What design storm is required in order to meet the stormwater management criteria?

3. Is the facility being used independently of other practices or is it part of a treatment train approach?

Two tools are provided: one for infiltration-based sizing, the other for a cell used mainly as a filtration device. Using the infiltration tool, the projected reduction in annual stormwater flows due to interception, infiltration, and storage by the bioretention cell can be evaluated for a drainage area. This reduction in stormwater flows helps to alleviate demands on the storm drain system. Given site constraints, cell characteristics can be modified in the spreadsheet until the annual flows are reduced by the desired percentage. The filtration tool is applicable to highly impervious areas where most rainfall becomes runoff and the primary desire is to "biofilter" the runoff. The result in the spreadsheet is the percentage of the annual rainfall that can be treated by the cell.

Both sizing tools determine the ability of the cell to process the runoff from up to 6 inches of rainfall over a 24-hour period (Maryland and the District of Columbia's approximate 10-year storm event). The annual rainfall distribution is based on analysis of historical rainfall data for the District by Dr. Mow-Soung Cheng of Prince George's County Government. For application to an area with a noticeably different rainfall regime, a new distribution function would have to be entered. The capacity of a cell to treat runoff is considered to be a simple function of the above ground storage volume of the cell and the infiltration volume for the soil in the cell. Factors such as rainfall intensity, infiltration rate, sloping cell sides, etc. are not considered. The volume of runoff generated for each storm event is based on the NRCS Curve Number (CN) method. An area-weighted CN is used for the drainage area. In determining this value, the actual land surface covered by the proposed bioretention cell should be considered separately with a CN value of 98 to 100. In the context of this analysis, a surface’s CN value indicates what proportion of the rainfall does not infiltrate locally – thus, ultimately making it to the bioretention cell for treatment. The cell itself should have a high CN value since almost all rain falling on it receives treatment, i.e. infiltration is into the cell’s soils and any runoff generated does not leave the depressed basin.

NRCS Curve Number (CN) Method

Hydraulics and Hydrology Tools and Models - TR-55
http://www.wcc.nrcs.usda.gov/hydro/hydro-tools-models-tr55.html

TR-55 Documentation
ftp://ftp.wcc.nrcs.usda.gov/downloads/hydrology_hydraulics/tr55/tr55.pdf
http://www.ecn.purdue.edu/runoff/documentation/tr55.pdf

TR-55 Background Information
http://www.hydrocad.net/tr-55.htm

LID Hydrologic Analysis
Prince George's County Department of Environmental Resources, MD, July 1999
http://www.epa.gov/owow/nps/lid/lid_hydr.pdf

Milwaukee Metropolitan Sewerage District
Surface Water and Storm Water Rules
Technical Guidance, Volume 2
http://www.mmsd.com/stormwaterweb/Volume2E.htm

Rainfall Information

NOAA Northeast Regional Climate Center
http://met-www.cit.cornell.edu/nrcc_home.html

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