(A) It is recognized that the smaller streams and drainage channels serving the City of Mt. Vernon may not have sufficient capacity to receive and convey storm water runoff resulting from continued urbanization. Accordingly, the storage and controlled release rate of excess storm water runoff shall be required for any development, redevelopment and new construction located within the regulated area not exempt under this ordinance. No improvement location permit shall be issued for the construction or extension of any proposed or existing building in the regulated area until the required drainage plans have been approved in writing by the Council, except for the following exemptions:
(1) Construction or extension of a single family dwelling house or an extension of a single family dwelling house or an accessory use building thereto;
(2) Construction or extension of a duplex dwelling house or an accessory use building thereto;
(3) Construction or extension in that area of the City designated Primary Business District (B-1 Zoning Classification); or
(4) Construction, extension or replacement of a building or buildings on a site of 20,000 square feet or less.
(5) Extension or replacement of any existing building that does not increase the existing rate of runoff.
(B) The exceptions (A)(1) through (A)(5) above, however, shall not be applicable to a project if located in a previously designated Impact Area as established per § 53.18 of this ordinance.
(C) The release rate of storm water from development, redevelopment, and new construction, as stipulated above, may not exceed the peak rate of runoff from the land area in its undeveloped state. The developer must submit to the Council, detailed computations of runoff before and after development, redevelopment or new construction. These computations must show that the peak runoff rate after development, redevelopment, or new construction, for the 10 year and 100 year return period of critical duration must not exceed the undeveloped peak runoff rate for either return periods. The computation method used in determining storm water runoff for land areas up to and including 5 acres may be the “Rational Method.” Other proven hydrograph techniques and/or computer drainage modeling methods may be used for determining storm water runoff for both areas smaller and larger than 100 acres.
(Prior Code, Art. 20, § 20-5)