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San Antonio |
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Unified Development Code |
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Appendix H. STORM WATER DESIGN CRITERIA MANUAL |
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Chapter 7. STORM DRAIN SYSTEMS |
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Appendix 7.2. Hydraulics Of Storm Drainage Systems |
§ 7.2.3. Hydraulic Capacity
The hydraulic capacity is controlled by the conduits size, shape, and frictional resistance. Use Manning's Formula for the design of all conduits.
(Equation 7.2.3a)
Q = AV
Q = flow (cfs.)
A = cross section area (sq. ft.)
V = velocity of flow (ft./sec.)(Equation 7.2.3b)
Q = flow (cfs.)
A = cross section area (sq. ft.)
n = roughness coefficient of conduit
R = hydraulic radius = A/ WP (ft.)
WP = wetted perimeter (ft.)
S f = frictional slope of conduit (ft./ft.)The "n" value to be used in Manning's Formula shall conform to Table 7.2.3 for design purposes. Any other "n" value shall be based on generally accepted engineering principles.
Table 7.2.3 - Manning's Roughness Coefficient
Type of Culvert Roughness or Corrugation Manning's n Concrete Pipe Smooth 0.013 Concrete Box Smooth 0.013 Corrugated Metal Pipe, Pipe-Arch - Unpaved 2-2/3 by 1/2 inch Annular 0.024 Corrugated Metal Pipe, Pipe-Arch - Unpaved 3 by 1 inch Annular 0.027