Importance of Inlet Design on Culvert Capacity

The design of a culvert inlet has a significant bearing upon the relationship of the head to the discharge of a culvert. Its relative importance hinges upon the type of flow occurring in the culvert, which in turn is governed by the location of the control section. For part-full flow the control may be either at the inlet or the outlet depending on whether the slope is hydraulically steep or mild. In the case of short culverts, control may be at the inlet even for horizontal or mild slopes. For full flow, barrel friction provides the control. The head-discharge curves of culverts having square-edge inlets have been compared with those for culverts having rounded inlets to illustrate the conditions for which a head-advantage may be obtained by using a rounded inlet. These comparisons have been made for three categories of culvert flow: long culverts on steep slopes, long culverts on mild slopes, and short culverts. Dimensionless head-discharge curves have been plotted for culvert flow in each category. For culverts on steep slopes, experimental data have been compared with the computed values and, since the agreement was reasonably good, serve as a basis for the analysis of flow in culverts operating under conditions other than those for which the tests were made. The greatest head-advantage for a particular discharge of the rounded inlet over that of a square-edge inlet was found for those cases in which the control section was located at the inlet. These were long culverts on steep slopes or short culverts where the length was negligible. for long culverts on mild slopes, the head-advantage was far less pronounced.