In the hydraulic design of box culvert, there are two flow controls, namely inlet control and outlet control. In inlet control, the entrance characteristics of the box culvert (e.g. headwater depth and entrance configuration) determine the hydraulic capacity of the box culvert, and the culvert is actually capable of conveying a greater flow than the inlet would allow. Barrel shapes and tailwater depth are of no significance in determining the hydraulic capacity. Inlet control usually takes place for culverts lying on steep slopes.
For outlet control, the inlet could accept more flow than the box culvert could carry and the hydraulic capacity of the box culvert is dependent on all hydraulic factors upstream from outlet tailwater.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.
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