Various parts of a drainage facility must accommodate runoff water as follows:
(A) Minor Drainage System.
(1) The minor drainage system such as inlets, catch basins, street gutters, swales, sewers and small channels which collect storm water (runoff) must accommodate peak runoff from a 10 year return frequency storm. Duration, for sizing these conveyances, using the rational method shall be equal to the time of concentration. The Rational Method is acceptable for storm sewer design, as long as the TR-55 time of concentration methodology is used. Determination of hydraulic capacity for storm sewers sized by Rational Method analysis should be done using Manning’s Equation.
(2) When using the HEC-1, TR-20 or comparable computer program, the design should use the Huff First Quartile (50%) rainfall distribution with no greater than a one hour storm duration; however, these minimum requirements must be satisfied:
(a) The allowable spread of water on Collector Streets is limited to maintaining two clear 10 foot moving lanes of traffic. One lane is to be maintained on Local Roads, while Places can have a water spread equal to one half of their width.
(b) Open channels carrying peak flows greater than 30 cubic feet per second shall be capable of accommodating peak runoff for a 50 year return period storm within the drainage easement.
(c) Culverts shall be capable of accommodating peak runoff from a 50 year return frequency storm when crossing under roads which are part of the functional classification and are classified as primary or secondary arterial streets.
(Prior Code, Art. 20, § 20-9)