(A) Storm event peak rates and volume. Two methods are to be used for determining peak stormwater run-offs for the design of storm sewers, open channels and culverts.
(1) (a) The rational formula method is to be used to design storm sewer systems.
(b) Information on the rational method is available in Chapter 7 of the MDT Hydrology Manual, including rainfall-intensity return periods.
(2) Use the one-hour precipitation return periods identified for station Butte - eight miles south.
(a) When it is necessary to know the volume of water and discharge in addition to the peak rate of flow, the SCS TR-20 Hydrologic Analysis Model or pre-approved hydrologic simulation model will be used together with a detailed routing analysis.
(b) For flow routing through detention facilities models such as; SWMM, StormNet and HydroCAD are also acceptable methods.
(B) Culverts and open channel design.
(1) Culverts shall be designed in accordance with the U.S. Department of Transportation, Hydraulic Engineering Circular No. 5.
(2) Open channels shall be designed using the energy concept and the Manning formula.
(3) Channels in earth shall have sides with a maximum slope of one-foot vertical rise for every two feet of horizontal distance.
(4) Steeper slopes will be permitted only if some type of bank protection is used to prevent erosion and sloughing of the sides into the bottom of the channel.
(5) Constructed and natural channels and culverts shall be designed with sufficient capacity to convey, at a minimum, the depth associated with the 100-year 24-hour design storm peak flow rate, assuming developed conditions for on-site tributary areas and existing conditions for any off-site tributary areas.
(C) Storm drain systems and inlets. The ten-year 24-hour design storm shall be used to size the enclosed storm sewer system and inlets.
(Prior Code, § 11.05.040.030) (Ord. 2019-9, passed 9-3-2019; Ord. 2021-15, passed 12-20-2021)