Design of Sewers Based on Minimum Velocity

For the computation of uniform flow velocity and discharge in partially filled circular sewers Manning's equation is used. Determination of the hydraulic and geometric elements of flow requires either iterative solutions of the Manning's equation or use of nomographs prepared for this purpose. Both methods are time consuming. A procedure is described for the direct determination of the slope of a partially filled sewer when the minimum velocity of flow is specified. Alternately, the depth of flow can be calculated when the slope is given. The diameter of the sewer and the minimum flow rate expected must be known. The equations developed are valid for circular sewers for which the depth of flow is less than 40% of the sewer diameter representing a reasonable range of initial design conditions. The maximum error between the actual water depths and those computed by the approximate equations is ±2.4%. This is much less than the probable error in selecting an appropriate value for the roughness coefficient.