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