A. Culverts under all streets shall be designed to accommodate a 50-year, 24-hour frequency storm runoff utilizing the maximum available head. The maximum available head shall be determined by the uppermost ponding elevation chosen to prevent flood damage to upstream properties.
B. Culverts shall be located at each natural draw or watercourse as conditions warrant to prevent excessive accumulation of flow in roadside ditches or along the toe of slopes. Draws and watercourses shall be cleared of debris for a distance of one hundred feet (100') upstream from all culvert inlets and designed with provisions accounting for increases in flow velocity and erosion potential.
C. Inverts at the inlet shall be slightly elevated above the normal flow line in steep or natural draws to avoid plugging by debris. Inlets shall not be elevated in those instances where ponding or accumulation of backwater curves would be objectionable (stagnation, irrigation ditches, etc.). The culvert shall slope downward in the direction of natural flow and be designed to be self-cleaning whenever possible. The outlet shall be designed not to discharge on unprotected fills or unstable material or at adverse angles to streams or open channels. Headwall, riprap or other means of protection are required at inlets or outlets where erosion might occur.
D. Velocities of flow in culverts shall be calculated using acceptable design charts of formulas. Where the Manning equation is used, the following "n" values will apply:
TABLE 7-F
Material | Manning Equation "n" Values | |
Corrugated steel pipe | 0.024 | |
Reinforced concrete pipe | 0.013 | |
Concrete | 0.013 to 0.020 | |
Asphalt | 0.016 |
E. Minimum diameter for round pipe shall be eighteen inches (18"). The minimum rise for elliptical or arch pipes shall be twelve inches (12"). All pipes shall have end sections placed on the inlet and outlet end of the pipe.
F. When a battery of pipes is used, a clear spacing of one-half (1/2) the pipe diameter (1 foot minimum, 4 foot maximum) must be provided between the pipe end sections. Minimum and maximum cover, pipe metal gauge, and strength classification shall be as specified by the manufacturer.
G. Cleanout access shall be provided at least every two hundred feet (200') for pipes twenty four inches (24") in diameter or less and at least every four hundred feet (400') for larger pipes. Cleanout access shall also be provided at each angle point and at each change in grade.
H. Inlets and other facilities draining the street surface shall accommodate the 50-year, 24-hour frequency storm runoff. All streets shall remain free of ponding during the 50-year, 24-hour frequency storm. (Ord. 2007-06, 8-14-2007)