Sizing of inlets, storm sewers, outfalls, culverts and drainage ditches within a subdivision/development shall be based on the following and the latest revision (as of the adoption of this ordinance) of the Montgomery County Drainage Criteria Manual:
(A) Design storm. An interior drainage system will be designed for a storm with a frequency of occurrence of once in five years, occurring under fully developed conditions.
(B) Runoff computation. The determination of runoff rates shall be in accordance with the latest revision (as of the adoption of this ordinance) of the Montgomery County Drainage Criteria Manual.
(C) Sewer. A sewer shall be sized to carry the discharge (Q) derived from the rational formula. The capacity of storm sewers shall be determined by the use of the Mannings formula on the basis of hydraulic gradients, rather than the physical slope of the pipe. The minimum storm sewer size shall be 18 inches. The minimum downstream tailwater shall be the top of pipe or the 25-year water surface elevation in the receiving water body, whichever is greater.
(D) Inlets. Inlets shall be spaced so that the maximum travel distance of water in gutters will not exceed 400 feet for residential streets and 300 feet for major thoroughfares. Inlets will be provided at all low points in gutter gradients. Inlets will be sized using the latest revision (as of the adoption of this ordinance) of the Montgomery County Drainage Criteria Manual.
(E) Ditches. Ditches shall be sized to carry the runoff, derived from the rational formula with appropriate velocities. The use of roadside ditches must be approved by the City prior to the start of design.
(Ord. 09-0721, passed 7-21-2009; Ord. 19-1022B, passed 10-22-2019)