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4. Flow Depth <br />Target for depth of flow in pipes during the flood control design storm shall be 80% of <br />pipe diameter. A particular design may result in a deeper or shallower flow, but pipes <br />shall not be designed to flow surcharged unless outfall conditions or other hydraulic <br />conditions prevent open channel flow. <br />5. Manning's Roughness Coefficient ("n" value) <br />"n" value for PVC pipe shall be 0.010 <br />"n" value for concrete pipe shall be 0.013 <br />"W' value for metal pipe shall be 0.020 <br />6. Materials <br />Stormwater pipe may be concrete, ductile iron, PVC or metal. See Appendix I for the <br />Metal Pipe Design Policy. <br />7. Skewed Pipe Systems <br />Pipe networks in streets shall be designed parallel to street centerlines whenever <br />possible; skewed pipe alignments are discouraged. <br />8. Velocity <br />Minimum velocity permitted for design flow is 3 feet -per -second (fps). When design <br />Q causes a pipe to flow surcharged at less than 3 fps, the engineer shall demonstrate <br />that flow in the pipe will exceed 3 fps during normal low -flow, non -surcharged <br />conditions. <br />9. Buoyant Forces <br />The effect of buoyant forces shall be checked for all pipes and manholes used below <br />the normal water table. Buoyant forces shall not be greater than the weight of the <br />pipe, manholes and backfill. <br />G. Underground Injection Control (UIC) Facilities <br />New public UIC facilities are prohibited. All existing public drywells are scheduled for <br />decommissioning. Existing drywells were installed as a temporary solution to <br />stormwater management. No additional flows shall be directed to existing drywells. <br />Construction of stormwater conveyance systems in the vicinity of existing drywells <br />Public Improvement Design Standards Manual, 2016 Page 2-14 <br />