The following criteria shall control when designing stormwater 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 (10) years. The time of concentration for open channel and roadside ditches shall be calculated with a minimum of ten (10) minutes. In all cases, the 100-year discharge elevation must be checked to ensure that adjacent structures do not suffer flood damage.
(B) Storm sewers and inlets. The design storm for the design of storm sewers and inlets shall be the 10-year storm. Storm sewers and inlets shall be checked under 25-year storm loading conditions for ponding limits and ensure the system does not surcharge out of any inlets creating potential structure hazards. The ponding limit for streets with curb and gutter shall be an 8-foot spread measured from gutter to driving lane. Spread calculations shall be based on the design for the ten (10) year storm with a minimum intensity of four (4) inches per hour. Pipes should be sized based upon the actual time of concentration. The minimum time of concentration used should be no less than ten (10) minutes.
(C) Driveway entrance pipes and cross drains. The design storm for the design of driveway entrance pipes and cross drains shall be the 10-year storm. The duration of the design storm shall be assumed to be equal to the calculated time of concentration. The minimum time of concentration shall be assumed to be ten (10) minutes. Entrance pipes and cross drains shall be checked under 25-year storm conditions to ensure against overtopping of roadways and flood damage to affected areas. Situations requiring pipes larger than thirty-six (36) inches shall be designed using the culvert criteria in division (D) below. Minimum pipe size shall be twelve (12) inches except that driveway entrance pipe may be 10 inches with prior approval by the City Engineer.
(D) Culverts. The design storm for the calculation of runoff for culvert design shall be the 25-year storm. The duration of the design storm 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. Headwater for the 10-year discharge should not exceed 1.2 times the structure rise. Downstream channel must receive appropriate protection or energy dissipation if the design outlet discharge would cause erosive conditions. Traffic safety must be considered in the design of culvert end treatments. This may include extending the culvert beyond the right-of-way limits, installing catch basins to intercept roadside swales, and installing guardrails. Designs must conform to meet the requirements of the agency responsible for road maintenance and safety. Follow method as described in the Kentucky Transportation Cabinet Drainage Guidance Manual.
(E) Erosion control. Plans for stormwater conveyance systems shall include the applicable provisions identified under Chapter 160, Erosion Prevention and Sediment Control.
(F) Design certification. Design of all stormwater conveyance, management and 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 and City of Madisonville Standards Guidance for Development.
(Ord. O-2019-20, passed 12-16-19)