There shall be no net increase in peak stormwater run-off flow leaving a development from pre-development conditions for the one (1)-year design storm. New developments are required to minimize damage to subject streams caused by storm flows.
(A) Calculation of Peak Flow
Acceptable methodologies for computing pre- and post-development conditions for the one (1)-year design storm include:
(1) The Rational Method (used when the drainage area is two hundred (200) acres or less)
(2) The Peak Discharge Method as described in USDA Technical Release Number 55 (TR-55) (drainage area is two thousand (2,000) acres or less)
(3) The Putnam Method (drainage area is greater than two thousand (2,000) acres)
(4) USGS Regression Equations, where applicable
The same method must be used for both the pre- and post-development conditions.
(B) Exceptions to Peak Flow Control
Developments with less than ten (10) percent net increase in peak flow of the one (1)-year design storm are not required to control peak flow for the one (1)-year storm from the site. However, if the net increase in peak flow from the new development is greater than ten (10) percent, the entire net increase from pre-development one (1)-year design storm peak flow must be controlled.