Section 90-148. Water quantity requirements.
All stormwater management systems will be evaluated on their ability to prevent flooding and to prevent negative impacts on the watershed basin and natural systems according to the following:
(1) Peak discharge.
a. The post-development peak rates of discharge must not exceed the pre-development rate for the 25-year, 24-hour and the five-year, 24-hour storms.
b. For retention basins with no positive outfall, the 100-year, 24-hour design storm shall be used.
c. For aggregate discharger where multiple off-site discharges to the same receiving water body are designed to occur, the total post-development peak discharge shall not exceed the total pre-development level.
(2) discharge volume. The post-development volume of direct runoff must not exceed the pre-development level for a 25-year, 24-hour storm event. For discharge to landlocked receiving water bodies, volume requirements must be met for a 25-year, 96-hour design storm.
(3) floodplain storage and conveyance.
a. floodways and floodplains and levels of flood flows or velocities of watercourses must not be altered so as to adversely impact the off-site storage or conveyance capabilities of the water resources.
b. A stormwater management system shall not cause a net reduction in flood storage capacity within the 25-year floodplain or floodplain bank. Such a system shall not encroach within the limits of a floodway adopted by ordinance or cause a reduction in the flood conveyance capabilities of the 100-year floodplain.