Public storm sewer pipes shall be designed with a minimum velocity of three feet per second at full flow, or a minimum velocity of two feet per second at design flow, whichever requires the greater slope. The maximum velocity for public storm sewer pipes shall not be greater than 25 feet per second. Excessive velocities may result in hydraulic grade line problems and may increase the potential for erosion where the system outfalls. Velocities shall be designed to be non-erosive at the re-entry into the natural stream or downstream channel. An outlet velocity of six feet per second or less is generally considered to be non-erosive. The downstream receiving channel or stream may still need stabilization if site soil conditions require in order to prevent scour or erosion. In cases where the outlet velocity is greater than six feet per second, the downstream receiving channel or stream must receive adequate protection against erosion through the use of erosion prevention practices or energy dissipation devices. Velocities for all pipes and structures shall be included in the drainage calculations for both public storm sewer systems and private storm sewer systems.
(Ord. 012-118, passed 10-8-2012)