Proper engineering calculations shall be used to determine pipe, culvert, ditch, orifice and inlet control devices sizing.
(a) Mannings equation may be used to size pipes, culverts and ditches, as follows:
Q = (1.49/n) AR- S1/2
Where: Q = discharge (cfs)
n = Manning's roughness coefficient
A= Cross-sectional area (SF)
R = the hydraulic radius
Note: R = wetted perimeter/cross-sectional area
S = Slope of the channel or pipe (ft/ft)
(b) The size of storm lines shall be determined on the basis of the hydraulic gradient to provide adequate capacity of the computed run-off, using n=0.015 for vitrified clay and concrete pipe, bituminous paved corrugated metal pipe plan 0.024, paved invert 0.020, asphalt spun 0.013, and brick sewers and n = 0.013 for monolithic concrete sewers.
(c) All storm sewers shall be designed with a hydraulic slope sufficient to provide a mean velocity when flowing full, of not less than three feet per second. Where the velocity exceeds twelve feet per second, special provisions shall be made to protect against erosion and displacement.
(d) All storm sewers shall be properly sized, but in no case shall the size of the lines be less than twelve inches in diameter.
(1) All storm sewers shall be sized to flow approximately full for a five year storm. The size shall be determined by working downstream from the inlet of the first sewer run.
(2) The hydraulic gradient for a ten year storm shall be kept below the catch basin grates in all cases. This will eliminate storm water being detained on the pavement surface during this period.
(e) The design frequency to be used for an individual structure and/or system may be altered by the City where the health and safety of the residents would be endangered by the hazards of flood waters or increased flows. Minimum depth for storm sewers shall be planned to provide clearance for all utilities and to permit inlet leads to be laid on not less than one percent (1%) slope.
(g) When storm sewers are increased in size in the direction of flow, the invert of the larger pipe shall be lowered to maintain the same energy gradient (place the crowns of both pipes at the same elevations). A larger pipe shall not discharge into a smaller pipe unless specifically approved by the City.
(h) All drainage structures and appurtenances shall comply with applicable City specifications and/or the most current edition of ODOT's Construction and Materials Specifications and Standard Construction Drawings. Any discrepancy between the two shall be decided by the City.