GENERAL DESIGN CRITERA FOR STORMWATER PERMITS
When proposing a building and a storm water plan is required, you can follow these steps
of general design criteria:
1. Calculate the impervious area in square feet. This includes, but is not limited to, the
roof area, driveway, loading and parking areas, patios, and sidewalks.
2. Multiply the impervious area by 4.5 inches (the amount of rain in a 25 year, 24 hour
storm).
3. Divide by 12. This gives the cubic foot volume of water that will need to be
detained and infiltrated on the property.
4. Size a basin to detain this cubic foot volume of water so it can infiltrate at that
location.
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To achieve maximum infiltration rates, attempt to keep the basins 1’ deep or less.
Multiple basins are allowed.
Basins must be maintained in vegetation and can not be rock lined.
Following are some examples. These basin sizes can be used or the formula can be used to
create one for the required storage volume.
1. 1000 ft2 X 4.5 ÷ 12 = 375 cu ft. A 10’x 38’x 1’ deep basin or 10’ x 75’ x 6” deep
or 20’ x 20’ x 1’ deep or something equivalent is needed.
2. 2000 ft2 X 4.5 ÷ 12 = 750 cu ft. Double the length in the 1st example.
3. 3000 ft2 X 4.5 ÷ 12 = 1125 cu ft. Triple the 1st example (10’ x 114’ x 1’ or 2 basins
at 10’ x 112’ x 6” )
4. 4000 ft2 - Multiple the 1st example by 4
If you have any questions with this or any other storm water concerns, please stop into the
Land Conservation office or call 715-346-1334.
DISCLAIMER: SEE WISCONSIN WATER LAW- CHAPTER 7 –WATER DRAINAGE