TABLE OF CONTENTS Open-channel flow in sewers and other drainage conduits are usually unsteady, nonuniform, and turbulent. Subcritical flows occur more often than supercritical. For slowly time varying flow such as the case of the flow traveling time through the entire length of the sewer 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: the entrance, the pipe flow, and the exit. Figure 14.7 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. There are four cases of pipe entrance condition, as shown in Fig. 14.8 and below:
| Case | Pipe entrance hydraulic condition |
|---|---|
| I | Nonsubmerged entrance, subcritical flow |
| II | Nonsubmerged entrance, supercritical flow |
| III | Submerged entrance, air pocket |
| IV | Submerged entrance, water pocket |
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.
Pipe exit conditions also can be grouped into four cases as shown in...
