2D Modelling for Design of Large Bend Culverts

Drainage improvement structures are quite common in populated areas from small towns to mega-cities. For small structures, simple hydraulic calculations would be sufficient for design purposes (e.g., empirical or analytical relationships). Often, one-dimensional (1D) hydraulic modeling is required to solve the hydraulics of the project. However, more complex geometries and projects would require more complex solutions. A two-dimensional (2D) model was created to evaluate the complex hydraulics in a large 90° bend culvert and downstream energy dissipation structure. The flow in bends is non-uniform due to normal acceleration (i.e., perpendicular to the main flow direction), and thus, a 1D hydraulic model is not able to simulate the water depths and flow velocities that vary crosswise. In addition, cross-wave effects also exist when dealing with turbulent supercritical flows around a bend structure, which was the case in this project. Positive and negative disturbances emanate from the outer and inner walls, respectively. This pattern is well resolved in the 2D model developed for this project. The concrete box culvert modeled was approximately 2000 ft and was designed to accommodate the 1:100-year flood flow of 1041 ft3/s. After screening a few hydraulic models, the Adaptive Hydraulics (AdH) software was selected to aid with this modeling. AdH is a finite element model that can simulate 2D subcritical and supercritical flows. A lid function was used in the model to correctly model the design geometry for potential pressurized situations and headwater impact at the culvert inlet. Detailed water levels, velocities, and pressure forces, especially at the bend section, were used for the detail design of the culvert. This paper summarizes the methodology of AdH modeling and reviews the results and how they informed the design of culvert structure and energy dissipation basin at the outlet based on the Federal Highway Administration (FHWA), Hydraulic Engineering Circular No. 14. Guideline.