Small Site Stormwater Management Design Guidance
Stormwater managment permits are required for sites that exceed 25% impervious surface lot coverage (>10,000 square feet, see Large Site Guidance), for impervious surfaces that are within the Shore Impact Zone, and for projects that require stormwater mitigation as required by the City of Detroit Lakes or Becker County Zoning Ordinances.
Stormwater is required to be controlled by a stormwater management plan for all new and/or reconstructed impervious surfaces on the lot once the thresholds above have been met. The stormwater plan will be sized so that it provides a volume of stormwater storage equal to a 1.1" rainfall over the impervious surfaces that drains to the designated area (i.e. rain garden, infiltration basin, french drain, etc.) Below is a sample calculation for the construction of a new 20 ft x 30 ft structure.
20 x 30 = 600 square feet x 1.1"/12"(1.1" rain event) = 55 Cubic feet of storage required.