|Sizing the Garden
NR 151 requires that for residential areas (one- and two- family dwellings), 1% of the site area be dedicated to active infiltration. For more dense residential and commercial properties the requirement is 2% of the site area dedicated for active infiltration. NR 151 defines the site as the area being developed before roads and other areas are dedicated to the public. Therefore, when meeting the requirements of NR 151 on a lot-by-lot basis, the following sizing criteria shall be followed:
ACTIVE AREA ON EACH LOT (X) = [(A-B) * (C/(A-(B+D)))] * E
X is the Area for Active Infiltration (acres);
A is the Total Site Area (acres);
B is the area dedicated to the public for park & stormwater managmenet (acres);
C is the percentage specified by NR 151 for active infiltration (0.01 or 0.02);
D is the area dedicated to the public for road right-of-way (acres);
E is the Individual Lot Area (acres)
Example: Given a 100-acre commercial development with 25% dedicated to open space & 8% dedicated to street right of way, how much area must be dedicated to active infiltration for a 10-acre lot?
A=100; B=25; C = 0.02; D = 8; E = 2
Given the above you can calculate the following:
X = [(100 - 25) * (0.02 * (100 - (25 + 8)))] * 2
X = 0.4477 acres (1,950 sq ft)
Note: This area is only for the active infiltration area (this is the flat area in a rain garden); it is expected that the total area for the garden with side slopes and berms will add another 1% to the total area required.