Generally treat a storm drain conduit system as if it operates in subcritical flow. As such, entrance losses of flow into each conduit segment are mostly negligible. However, if discharge enters into the system through a conduit segment in which there must be supercritical flow, significant head losses are encountered as the discharge builds enough energy to enter the conduit. This situation is most likely where a lateral is located on a relatively steep slope. On such slopes, evaluate the type of flow (subcritical or supercritical).

With supercritical flow, the lateral may be operating under entrance control. When a lateral is operating under entrance control as described above, the headwater level is usually much higher than a projection of the hydraulic grade line.

If the entrance control headwater submerges the free fall necessary for the inlet to function properly, it may be necessary to reconfigure the lateral by increasing its size or changing its slope. Some improvement to the inlet characteristics may help to overcome any unfavorable effects of entrance control. Usually, entrance control does not affect steep units in the trunk lines because the water is already in the conduit; however, you may need to consider velocity head losses.

Use the following procedure to determine the entrance control head: