Run-Off Estimation in Storm Sewer System Using Equivalent Roughness

Abstract

The rational method has been often used to estimate the storm water run-off in combined sewerage system. But rigorously, this method still remains unreasonable hydraulically because the motion of flow is usually not taken into account correctly. Therefore, in this paper, the fundamental factors concerning the process of calculation are revised by the method of characteristic curves introducing the equivalent roughness of drainage area. The inlet time must be taken equal with the time which is needed for the standard characteristic curve to cover the distance from the upstream end of drainage area to its downstream end and also the flowdown period with the shortest time required for the curve in sewer samely, while, especially, the former must be related to the width of the area and the equivalent roughness. When the drainage area is comparatively larger and the retardation occurs, the maximum rate of run-off decreases proportionally with the equivalent roughness, so that the run-off coefficient may better be represented by the equivalent roughness. Moreover, it was pointed out theoretically that the maximum rate of run-off could become larger value if retardation occured, according to the type of rainfall intensity curve. The characteristics of the equivalent roughness can be explained in relation to the width of drainage area and the density of sewer net, and it may become very small value in the street area in which side gutters or sewers are fully equipped, but may become larger in the suburban undeveloped district. If the sewerage system consists of circular pipes the overall equivalent roughness in the whole drainage area can be calculated approximately by only the roughness in the smallest end area and accordingly the run off analysis can be made without the assumption of pipe diameters.