Culverts are classified according to which end controls the discharge capacity, the inlet or outlet.

Inlet Control

If water can flow through and out of the culvert faster than it can enter, the culvert is under Inlet Control. Flow capacity is controlled at the entrance by the headwater depth, cross-sectional area and type of inlet edge. Culverts under inlet control will always flow partially full and are in a state of shallow, high velocity known as Supercritical flow. Any downstream disturbance will not be propagated upstream since the flow of water is too great. The roughness, length and outlet conditions are not factors in determining capacity. Flow is therefore controlled upstream and is limited to what can enter the culvert.  Culverts that have a drawdown at the inlet and a perch or hydraulic jump at the outlet are usually inlet control.

Photo: Inlet Drawdown

Outlet Control

If water can flow into the culvert faster than it can flow through and out, then it is under Outlet Control. Culverts under outlet control can flow either partially full or full. In this case water is relatively deep and slower, known as Subcritical flow and a disturbance propagates upstream. Therefore flow is controlled downstream and limited to what the pipe can carry. In this case friction and roughness in the culvert are significant in the flow through a culvert and the difference in headwater and tailwater depth represents the energy which conveys flow through the culvert.

Inlet and outlet control are set by the slope of the stream, it is not a designed feature. Generally speaking, when culverts are designed, calculations are made assuming both inlet and outlet control and comparing the headwater depth under both conditions. Designs for low headwater depths reduce pipe diameter and fill material, but risk overtopping and often result in undersized culverts when exposed to natural conditions. Conversely designs for higher headwater depths are more conservative and generally govern design.

Factors affecting Inlet and Outlet Control: