The prediction of bridge-pier scour has been mostly aimed for a single pier of various sizes, shapes, and alignments with flow. For the case of multiple piers, widely used manuals still recommend using single-pier scour formulas with an equivalent diameter, regardless of pier separation and angle of attack. In this paper, we present the results of a new set of laboratory experiments conducted to address the local scour around a group of three piers with different sizes, spacing, and attacking angles. Based on the phenomenological theory of turbulence, we also formulate a semitheoretical framework for calculating the maximum scour depth in the multipier condition, including the single-pier counterpart as a special case. The few coefficients in the final equation were determined by experimental data from this and other studies. Our results provide an understanding of the eddies responsible for the scour process, and show that pier diameter, pier spacing, actual pier width, flow depth, Froude number, and sediment size are all important variables that need to be considered in order to obtain an accurate prediction of the maximum scour depth. Therefore, these results shed new light on current engineering procedures based on using single-pier expressions in the case of multiple piers.

A two-dimensional hydrodynamic model, named CHRE2D, is developed for simulating surface runoff in watershed. The model is based on the solution to shallow water equations (SWEs) using the Godunov-type finite volume method. The novelty of the model is a stable and accurate algorithm to calculate flow velocities in cells of very shallow flow depth. Stability analysis shows that the friction terms in the momentum equations are stiff when flow depth is very small (e.g., ${10}^{-3}\mathrm{m}$ ). In this study, the velocities in those cells are calculated by the diffusion wave approximation (DWA). This leads to a perfect balance between the surface gradient and the friction terms and prevents the occurrence of stiffness. Furthermore, the model possesses the well-balance property for both overland and channel flow. This allows the use of the original digital elevation model (DEM) data without removing topographic depressions. The model has been tested and verified by three experimental and one field cases including the rapidly varied dam-break flow and the sheet flood in arid regions. Simulated flow depth, velocity, and discharge hydrographs were compared with available measurements in two experimental testing cases. Since the resolution of the DEM significantly affects the modeling results of flow depth for field applications, only the simulated hydrograph was compared with the observation. The simulated results demonstrate that the model is highly capable of simulating both hydrodynamic and hydrological flow processes.

The paper presents the application of a two-dimensional depth-averaged numerical model to simulate the lateral migration processes of a meandering reach in the West Jordan River in the state of Utah. A new bank erosion model was developed and then integrated with a depth-averaged two-dimensional hydrodynamic model. The rate of bank erosion is determined by bed degradation, lateral erosion, and bank failure. Because bank material in the West Jordan River is stratified with layers of cohesive and noncohesive materials, a specific bank erosion model was developed to consider stratified layers in the bank surface. This bank erosion model distinguishes itself from other models by relating bank erosion rate with not only flow but also sediment transport near the bank. Additionally, bank height, slope, and thickness of two layers in the bank surface were considered when calculating the rate of bank erosion. The developed model was then applied to simulate the processes of meandering migration in the study reach ...

A one-dimensional (1D) finite-volume model is developed for simulating nonequilibrium sediment transport in unsteady flow. The governing equations are the 1D mass and momentum conservation equations for sediment-laden flow and the sediment continuity equation for both bed load and suspended-load transport. The Rouse profile is modified to consider the nonequilibrium transport of suspended sediment. The spatial lag between the instantaneous flow properties (e.g., velocity, bed shear stress) and the rate of bed load transport in unsteady flow is quantified by using an adaptation length, which is derived theoretically by applying the momentum principle in the bed load layer. This new method for calculating the adaptation length is verified using data from several experiments and yields better results than other empirical formulas for a wide range of shear stress. The nonequilibrium model is applied to simulate a series of laboratory dam-break flows over erodible beds, and the simulated results agree ...