Culvert Hydraulics

Several computer programs are available to analyze culvert hydraulics, including HY-8, FishXing, Broken-back Culvert Analysis Program (BCAP), Hydraflow Express, CulvertMaster, Culvert, and Hydrologic Engineering Center River Analysis System (HEC-RAS). The use of slightly different methodologies to complete the analysis results in different estimates for headwater depth, flow control, and outlet velocity. This paper compares the available hydraulic features and tests the accuracy of these seven programs as part of an effort to update the HY-8 program. Four test cases were developed. The flow controls, headwater depths, and outlet velocities were compared with values obtained through calculations based on best practice as outlined in the FHWA publication, Hydraulic Design Series No. 5. Several limitations were identified. BCAP has limited ability to analyze straight barrel culverts operating under outlet control with high tailwater, CulvertMaster reports outlet control for some low discharges as a surrogate for what is referred to as “entrance control,” Culvert misidentifies the location of hydraulic control for lower discharges and overestimates headwater elevations, and HydraFlow Express incorrectly uses critical depth at the outlet for inlet control calculations. With the test cases used here, HY-8, HEC-RAS, and FishXing most consistently agreed with accepted empirical results. It is recommended that program providers improve their products to replicate better the hydraulic conditions simulated in this paper and to extend program capabilities to include more environmentally sensitive design considerations such as fish passage.