Types of Sewer Flow

The flow in a sewer is usually turbulent and changes with respect to time and location in the sewer pipe hydraulically referred to as unsteady and nonuniform flow. Subcritical flows occur more often than supercritical flows. For flows varying slowly with time such that the flow travel time through the entire length of the sewer is much smaller than the rising time of the flow hydrograph, the flow can often be treated approximately as stepwise steady without significant error.

The flow in a sewer can be divided into three regions: (1) the entrance, (2) the pipe flow, and (3) the exit. Figure 6.1 shows a classification of 10 different cases of nonuniform pipe flow, based on whether the flow at a given instant is subcritical, supercritical, or surcharge. The four cases of entrance condition are shown in Fig. 6.2 and Table 6.4. Case I is associated with downstream control of the pipe flow. Case II is associated with upstream control. In case III, the pipe flow under the air pocket may be subcritical, supercritical, or transitional. In case IV, the sewer flow is often controlled by both the upstream and downstream conditions.

Figure 6.1: Classification of flow in a sewer pipe. (After Yen, 1986)

Figure 6.2: Types of sewer entrance flow. (After Yen, 1986)