The Manning Equation Relates Stormwater Flow Velocity in a Storm Sewer to Pipe Roughness and Hydraulic Radius

The Manning equation is very useful in storm sewer hydraulic design calculations. It gives a relationship among stormwater flow velocity, pipe roughness coefficient, and storm sewer diameter and slope. For a circular storm sewer flowing full with stormwater, the hydraulic radius is equal to one fourth the diameter. This simplifies the Manning equation, making the desired calculations fairly easy. The Manning equation is useful together with the equation: Q = VA, to determine the storm sewer diameter and slope that will give full pipe flow rate greater than the design flow, and full pipe flow velocity greater than the required design minimum, typically 3 ft/sec. The Manning equation can be used because the stormwater flows under gravity.