§ 5.3.1. Minimum Grades

Storm drains should operate with velocities of flow sufficient to prevent deposition of solid material. The controlling velocity is near the bottom of the conduit and is considerably less than the mean velocity. Storm drains should be designed to have a minimum velocity of two and one-half (2.5) feet per second during the 25-year design storm. When backwater conditions prevent one or more portions of the system from attaining the minimum velocity during the design storm, the portion(s) of the system that fail to meet the criteria shall be checked to ensure that the minimum velocity is attained during a lesser and more frequent storm event. If the minimum velocity is still not achieved, then the storm drain shall be redesigned so that the minimum velocity is attained or a request for waiver submitted explaining why it is not feasible to meet the design criterion.