Under full pipe design conditions, pipe flows shall not be less than 2 feet per second or greater than 10 feet per second. Velocities below 2 fps tend to leave mud and other debris lodged in pipes. Velocities over 10 fps tend to erode the inside of pipes and cause erosion problems at the outfall. Pipe discharges shall not be greater than the capacity of the natural system at the outflow when added to the flows already reaching the natural system for the design storm event.