What must be true if a culvert is laid on a steep slope?

• A. The culvert is outlet controlled
• B. The culvert is inlet or outlet controlled
• C. The culvert is inlet controlled
• D. The flow in the culvert is subcritical through the entire length

Answer: (C)

For a culvert laid on a steep slope, it is important to recognize how the slope affects the flow conditions within the culvert. When a culvert is installed on a steep slope, it tends to promote higher velocities in the flow. Under these conditions, one of the dominant factors influencing the flow is the inlet control, as the flow rate may be limited by the conditions at the inlet of the culvert rather than the outlet.

Inlet control occurs when the entrance conditions of the culvert restrict flow, effectively determining the maximum flow rate the culvert can carry regardless of the outlet conditions. On a steep slope, if the culvert's inlet is not designed properly or is experiencing a high head difference, it could lead to situations where the entrance geometry or flow conditions (like a submerged outlet) control the capacity and flow dynamics.

Thus, for a culvert situated on a steep slope, it must be true that it is inlet controlled, indicating that the flow characteristics at the inlet dictate its performance.