Depending on the modeling purpose, there are several ways to model flow resistance in 2D hydraulic models such as SRH-2D. One economical approach is to add an extra drag force term in the momentum equation or increase the Manning’s n for the computational cells influenced by the structures. This approach does not require the mesh to conform to the boundaries of hydraulic structures, which greatly reduces the burden on modelers. However, currently almost all 2D models implement this approach in such a way that the guidelines for the drag coefficient and the Manning’s n are not appropriate. Specifically, most guidelines state that the drag coefficient should take the values in fluid mechanics textbooks. Unfortunately, these textbook values are defined based on the incoming velocity at some distance before the hydraulic structure, not at the hydraulic structure. Consequently, there is a circular dependence between the velocity at the hydraulic structure’s computational cells and the drag coefficient. In this work, we provide a detailed analysis and propose a rational remedy for this problem. 3D computational fluid dynamics (CFD) simulations are conducted to simulate the flow around and forcing on various hydraulic structures. We also provide an updated version of SRH-2D to implement the new solution. Demonstration examples will be shown.
