Inlet control occurs when it is harder for the flow to get through the entrance of the pipe than the remainder of the pipe barrel.
The profile below illustrates a type of inlet control flow. The control section is at the inlet end of the culvert. Depending on the tailwater and culvert slope, a hydraulic jump may occur downstream of the inlet.
The following inlet control equations are used. If Hw is above the pipe crown the submerged equation is used. Otherwise the unsubmerged equation is used.
Submerged
Unsubmerged
Where:
Hw = Headwater depth above invert
D = Culvert Rise
Q = Flow rate
A = Full cross-sectional area of pipe
K, M, c, Y = Coefficients based on inlet edge configurations
S = Culvert slope