Effects of Contraction Ratio on Inlet Control Performance of Pipe Culverts

Culverts are essential hydraulic structures for conveying stormwater under roadways and other infrastructure. Their performance is often governed by inlet control conditions, where the geometry and hydraulic characteristics at the entrance determine the flow capacity. This thesis investigates the effect of the inlet contraction ratio on the head-discharge relationship of circular pipe culverts under inlet control, focusing on two flow regimes: Type 1 (partially submerged) and Type 5 (fully submerged). The study compares the USGS (Bodhaine, 1968) and FHWA (Schall et al., 2012) design frameworks, with a focus on how each method includes or ignores the influence of inlet contraction. For Type 1 flow, the USGS method directly accounts for contraction effects, while the FHWA approach uses fixed empirical coefficients. To align the two, discharge coefficients derived from USGS curves were used to calibrate the FHWA equations using regression, resulting in contraction-dependent expressions for the empirical constants K and M. These adjustments expand the applicability of the FHWA method to a wider range of inlet geometries while keeping its practical simplicity. Type 5 flow was analyzed using historical experimental data from Smith & Oak (1995), Schiller (1956), and Straub et al. (1954), which showed little sensitivity to inlet contraction, in line with USGS assumptions. However, findings from Tullis et al. (2008) suggest that inlet geometry and approach conditions can influence performance in some cases. The thesis also discusses scale effects in physical modeling and includes an appendix that outlines an analytical approach explored early in the project. Although that approach was inconclusive, it reflects a thorough investigation of possible methods. Overall, the findings emphasize the importance of inlet geometry in culvert hydraulics and present regression-based improvements to current design practices.