Effect of Inlet Design on Capacity of Culverts on Steep Slopes

The geometry of the inlet has a significant bearing on the relationship headwater elevation and discharge for culverts with a free outlet. The relative importance of inlet design depends upon the location of the control section. The primary purpose of the research reported here was to examine the effect of inlet design upon the head-discharge curve of a model culvert. Two types of flush inlets, selected to represent extreme conditions for flush inlets, were tested--a square-edged inlet and a well rounded inlet. For each inlet the head-discharge curve was measured and the two curves compared. The comparison indicated that for certain conditions an appreciable head-advantage was gained by using a rounded inlet. Observation and analysis of the flow characteristics indicated that this gain phenomenon occurred in the region where for the same head a square-edged inlet caused separation and inhibited full flow, while the rounded inlet promoted full flow in the culvert with a corresponding increase in discharge. In connection with measurements to establish friction factors and entrance losses in the model culvert for use in the analysis of the experimental, a few behavior curves were determined. An analysis of behavior curves is included in Appendix IV. The results obtained were compared with experimental curves and other published curves.Prepared for Minnesota Department of Highways and United States Bureau of Public Road