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The following criteria shall control when designing storm water conveyance facilities and water quality control structures:
(A) Open channels and roadside ditches. The design storm for the design of open channels and roadside ditches shall be a storm with a recurrence frequency of ten year/24-hour duration. The time of concentration for open channel and roadside ditch design should be assumed to be 15 minutes.
(B) Storm sewers and inlets. The design storm for the design of storm sewers and inlets shall be the 25-year storm/24-hour duration (TR-55 Method and Rational Method). The duration of the peak rainfall event shall be assumed to be equal to the calculated time of concentration. Storm sewers and inlets shall be checked under 25-year storm/24-hour duration (TR-55 Method and Rational Method) loading conditions for ponding limits. The ponding limit for streets with curb and gutter shall be an eight-foot spread measured from gutter to driving lane. Spread calculations shall be based upon an intensity of four inches per hour. Pipes should be sized based upon the actual time of concentration. The minimum time of concentration should be assumed to be eight minutes.
(C) Entrance pipes and cross drains. The design storm for the design of entrance pipes and cross drains shall be the 25-year storm/24-hour duration (TR-55 Method and Rational Method). The duration of the peak rainfall event shall be assumed to be equal to the calculated time of concentration. The minimum time of concentration shall be assumed to be eight minutes. Entrance pipes and cross drains shall be checked for overtopping of roadways and flood damage to affected areas. Situations requiring pipes larger than 36 inches shall be designed using the culvert criteria in division (D) of this section, below.
(D) Culverts and cross drains. The design storm for the calculation of runoff for culvert design shall be the 25-year storm. The duration of the peak rainfall event shall be assumed to be equal to the calculated time of concentration. The recommended check storm is the 100-year storm. The maximum headwater under 100-year storm conditions should not be allowed to overtop roads or increase the flooding potential in the affected areas.
(E) Erosion control. Plans for storm water conveyance systems shall include appropriately designed temporary and permanent erosion-control measures both for the open channel conduits and all disturbed land draining to both open and closed conduits within the system. (Best Management Practices for Construction Activities prepared by the Kentucky Natural Resources and Environmental Protection Cabinet and § 156.36 of this subchapter should be used as design guides for erosion and sediment control).
(F) Water quality control structures. The design rainfall event for the design of all stormwater quality control structures shall be the surface depth of runoff produced from an 80th percentile precipitation event. Stormwater quality control structures shall be designed, built and maintained to treat filter, flocculate, infiltrate, screen, evapo-transpire, harvest and reuse stormwater runoff, or otherwise manage the stormwater runoff quality for the 80th percentile precipitation event. Green infrastructure devices may be used as water quality control structures if they meet the design criteria.
(G) Design certification. Design of all storm water conveyance and water quality facilities shall be prepared and stamped by a licensed professional engineer (Kentucky registration required). Design methods shall be in accordance with the Kentucky Department of Highways Manual of Instructions for Drainage Design, latest edition.
(Ord. 2001-1253, passed 8-23-01; Am. Ord. 2014-1636, passed 4-10-14)