(A) It is recognized that the smaller streams and drainage channels serving the county may not have sufficient capacity to receive and convey stormwater runoff resulting from continued urbanization. Accordingly, the storage and controlled release rate of excess stormwater runoff shall be required for any development, redevelopment, and new construction located within all territory of the county that is not located in a municipality.
(B) Possible exceptions to the requirements are residential or commercial developments that increase the impervious or semi-impervious surfaces by less than 43,560 square feet (one acre), or individual residential structures to be constructed on lots situated within existing platted additions and/or subdivisions, or on unplatted tracts. These projects may be exempt from this chapter, but must submit an engineering scaled plot plan showing the entire property, the proposed improvements, the existing drainage breaks and flow direction and the proposed drainage breaks and flow directions. The plot plan shall be submitted along with the building permit for the proposed improvement.
(C) Residential or commercial developments that increase the impervious or semi-impervious surfaces by less than 43,560 square feet (one acre) are exempt. However, a development shall be subject to this subchapter when the combined sum of the impervious or semi-impervious surfaces initiated after the effective date of this chapter is 43,560 square feet or greater.
(D) For those developments that will increase the impervious or semi-impervious surface areas by more than 43,560 square feet, the release rate of stormwater from developments, redevelopments, and new construction may not exceed the stormwater runoff from the land area in its present state of development. The developer must submit to the Board, detailed computations of runoff before and after development, redevelopment, or new construction which demonstrate that runoff will not be increased. These computations must show that the peak runoff rate after development for the 50-year return period storm of critical duration must not exceed the ten-year return period pre-development peak runoff rate. The critical duration storm is that storm duration that requires the greatest detention storage.
(E) Computations for areas up to and including 200 acres may be based on the rational method, typical runoff coefficients are listed herein. For areas larger than 200 acres, hydrograph techniques and/or computer drainage modeling methods shall be used. Hydrograph techniques and computer modeling methods used to determine stormwater runoff shall be proven methods, subject to the approval of the Board.
(Ord. 2007-10, passed - -)