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Bioretention

Sizing

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

What are the site requirements for water qualityand quantity control? Will the bioretention cell be

used to address both water quality and quantity orjust one of the issues?

1.

What design storm is required in order to meet thestormwater management criteria?

2.

Is the facility being used independently of otherpractices or is it part of a treatment train approach?

3.

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. Thisreduction in stormwater flows helps to alleviate demands on the storm drain system. Given siteconstraints, 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 areaswhere most rainfall becomes runoff and the primary desire is to "biofilter" the runoff. The result inthe spreadsheet is the percentage of the annual rainfall that can be treated by the cell.

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To use the cell sizing tool, you will need to know the:

site's drainage area (acres)

site's area-weighted CN value

proposed cell area (ft2)

proposed cell depth (in)

proposed above-ground ponding depth in the cell (in)

volume fraction of pore space in the cell's fi llmaterial (typically 0.2)

Click below to access an MS Excelspreadsheet for determining

bioretention cell s ize.

Filtration

Infiltration

Both sizing tools determine the ability of the cell to process the runoff from up to 6 inches ofrainfall over a 24-hour period (Maryland and the District of Columbia's approximate 10-year stormevent). The annual rainfall distribution is based on analysis of historical rainfall data for theDistrict by Dr. Mow-Soung Cheng of Prince George's County Government. For application to anarea with a noticeably different rainfall regime, a new distribution function would have to beentered. The capacity of a cell to treat runoff is considered to be a simple function of the aboveground storage volume of the cell and the infiltration volume for the soil in the cell. Factors suchas rainfall intensity, infiltration rate, sloping cell sides, etc. are not considered. The volume ofrunoff generated for each storm event is based on the NRCS Curve Number (CN) method. Anarea-weighted CN is used for the drainage area. In determining this value, the actual land surfacecovered by the proposed bioretention cell should be considered separately with a CN value of 98 to

100. In the context of this analysis, a surfaces CN value indicates what proportion of the rainfalldoes 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 cells soils and any runoff generated does not leave the depressed basin.

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Urban Design Tools - Bioretention http://www.lid-stormwater.net/bio_sizing.htm

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7/27/2019 LID Urban Design Tools - Bioretention

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Urban Design Tools - Bioretention http://www.lid-stormwater.net/bio_sizing.htm

2 08/06/2009 14:16