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This paper presents a numerical investigation of multiple identical swash events to study the swash–swash interaction processes and their impacts on beachface evolution. The numerical model, based on the Nonlinear Shallow Water Equations, is first calibrated/validated against two different single-event-based data-sets. Multiple swash events are generated by identical solitary waves separated by different time intervals, to achieve weak and strong wave-backwash interactions. After a small number of weak interaction events the main feature is erosion from lower and mid swash region and deposition seaward of the swash in a bed-step, created by a backwash bore, primarily due to bed-load. As the number of waves increases, the strength of this backwash bore reduces because of the reduced beach slope caused by the growing bed-step. This eventually leads to a net quasi-equilibrium between bed- and suspended-load per period in most of the swash and surf zones. For strong interaction, initial bed evolution per event is much slower, due to interactions, and is bed load dominated. A quasi-equilibrium is also established as the influence of suspended load grows. Overall bed change per period within the domain eventually converges in both cases. Final bed profiles (i.e. after the same elapsed time, but different numbers of waves) are fairly similar, both with an offshore swash bar. Both profiles continue to evolve on the offshore side of this bar. However, this evolution is driven by suspended load for the weak interactions and bed load for strong interactions. The implication is that similar swash morphological features can emerge from different swash processes, and also be maintained distinctly. • The roles of weak and strong swash interactions in beachface evolution are studied. • Final bed profiles for both cases are similar, but evolve distinctly. • Different sediment transport modes dominate the bed evolution in the two cases. • Both bed profiles reach an quasi-equilibrium in most of the region. • An offshore swash bar develops in both cases, and continues to evolve. [ABSTRACT FROM AUTHOR]

In this study an incompressible smoothed particle hydrodynamics (ISPH) approach coupled with the sediment erosion model is developed to investigate the sediment bed scour and grain movement under the dam break flows. Two-phase formulations are used in the ISPH numerical algorithms to examine the free surface and bed evolution profiles, in which the entrained sediments are treated as a different fluid component as compared with the water. The sediment bed erosion model is based on the concept of pick-up flow velocity and the sediment is initiated when the local flow velocity exceeds a critical value. The proposed model is used to reproduce the sediment erosion and follow-on entrainment process under an instantaneous dam break flow and the results are compared with those from the weakly compressible moving particle semi-implicit (WCMPS) method as well as the experimental data. It has been demonstrated that the two-phase ISPH model performed well with the experimental data. The study shows that the ISPH modelling approach can accurately predict the dynamic sediment scouring process without the need to use empirical sediment transport formulas. [ABSTRACT FROM AUTHOR]

Highlights: [•] Hyperconcentrated flood peak is observed to increase downstream in the Yellow River. [•] Fully coupled modeling predicts peak increase with sediment effect on bed friction. [•] Bed friction decrease is more important than erosion for the peak increase. [•] Channel storage change reconciles bed friction and erosion to explain the increase. [•] The downstream flood peak increase may occur with channel storage reduction. [ABSTRACT FROM AUTHOR]

Abstract: Experiments and numerical simulations are carried out to study clear-water scour around three bridge piers with cylindrical, square, and diamond cross-sectional shape, respectively. To handle movable-bed channels with embedded hydraulic structures, the fluid–structure interaction curvilinear immersed boundary (FSI-CURVIB) method is employed. The hydrodynamic model solves the unsteady Reynolds-averaged Navier–Stokes (URANS) equations closed with the k-ω turbulence model using a second-order accurate fractional step method. Bed erosion is simulated by solving the sediment continuity equation in the bed-load layer using a second-order accurate unstructured, finite-volume formulation with a sand-slide, bed-slope-limiting algorithm. Grid sensitivity studies are carried out to investigate the effect of grid resolution on the predictive capability of the model. Comparisons of the simulations with the experimental data show that for all three cases the agreement is reasonable. A major finding of this work, however, is that the predictive capability of the URANS morphodynamic model improves dramatically for the diamond shape pier for which sediment transport is driven primarily by the shear layers shed from the pier sharp edges. For piers with blunt leading edge, on the other hand, as the circular and square shapes, the URANS model cannot resolve the energetic horseshoe vortex system at the pier/bed junction and thus significantly underpredicts both the scour depth at the nose of the pier and the rate of scour growth. It is also shown that ad hoc empirical corrections that modify the calculated critical bed shear stress to enhance scour rate in the pier leading edge need to be applied with caution as their predictive capabilities are not universal but rather depend on the pier shape and the region of the flow. [Copyright &y& Elsevier]

Abstract: This paper presents a three-dimensional (3-D) numerical model to simulate morphological changes in alluvial channels due to bank erosion. A method for the simulation of bank erosion is established. This is incorporated into a 3-D mathematical model for turbulent flow and non-uniform, non-equilibrium sediment transport. This model is applied to simulate morphological changes in the Shishou bend of the middle Yangtze River in China, where serious bank erosions occurred during the last two decades. The double-layer sediment structure of the riverbank on the middle Yangtze River is taken into account in the bank erosion module. Both cohesive and non-cohesive bank material in the different layers are considered. The bank erosion module also includes other factors affecting the rate of bank erosion, such as the longitudinal length of failed bank, the thickness of each layer in the double-layer structure, and the erosion-resisting effect of cohesive material from the top layer of failed bank. A locally-adaptive grid system is proposed to efficiently simulate the lateral migration of alluvial channel due to bank erosion. The predictive capability of the 3-D model is examined by laboratory data. Simulated processes of bank erosion agree with field observations in the Shishou bend during the period of October 1996–October 1998, and the bank erosion module plays a significant role in simulating morphological changes of the study reach. In addition, the equivalent channel-forming discharge, which is defined as a constant discharge that can create the same amount of bank erosion in an alluvial channel as that created by natural runoff processes during the same period of time, is proposed to improve calculation efficiency for feasibility studies. [Copyright &y& Elsevier]

Abstract: Turbidity currents may feature active sediment transport and rapid bed deformation, such as those responsible for the erosion of many submarine canyons. Yet previous mathematical models are built upon simplified governing equations and involve steady flow and weak sediment transport assumptions, which are not in complete accordance with rigorous conservation laws. It so far remains unknown if these could have considerable impacts on the evolution of turbidity currents. Here a fully coupled modeling study is presented to gain new insights into the evolution of turbidity currents. The recent analysis of the multiple time scales of subaerial sediment-laden flows over erodible bed [Cao Z, Li Y, Yue Z. Multiple time scales of alluvial rivers carrying suspended sediment and their implications for mathematical modeling. Adv Water Resour 2007;30(4):715–29] is extended to subaqueous turbidity currents to complement the fully coupled modeling. Results from numerical simulations show the ability of the present coupled model to reproduce self-accelerating turbidity currents. Comparison among the fully and partially coupled and decoupled models along with the analysis of the relative time scale of bed deformation explicitly demonstrate that fully coupled modeling is essential for refined resolution of those turbidity currents featuring active sediment transport and rapid bed deformation, and existing models based on simplified conservation laws need to be reformulated. [Copyright &y& Elsevier]

Abstract: An un-mixing model is formulated within a Bayesian Markov Chain Monte Carlo framework for use within land-use fingerprinting to study watershed erosion processes. The model has two new components: (1) An equation and erosion process parameter are used to weight tracer signatures from each erosion process within a land-use. (2) An extra tracer distribution and episodic erosion parameter are used to represent soil eroded throughout the sampling duration and thus include the episodic nature of erosion. To test specification of these new parameters, the un-mixing model is applied in the 15km2 Jerome Creek Watershed in the Palouse Region of Northwestern Idaho. Erosion processes include surface erosion upon mountain slopes due to logging in the forest land-use and rill/interrill erosion on cultivated slopes and headcut erosion in riparian floodplains of the agricultural land-use (winter wheat/peas rotation and hay pasture). Episodic erosion occurs for the event where the model is applied. A sensitivity analysis shows that the smallest Bayesian credible set results when the new parameters are specified using hydrologic data and process-based models. The un-mixing model predicts that 90% of the eroded-soil originated from the agricultural land-use and 10% originated from the forest land-use. A comparative study is performed that estimates 90.5% and 9.5% of eroded-soil originated from the agricultural and forest land-uses. Successful performance of the un-mixing model highlights future application as a standalone probabilistic tool to monitor watershed erosion processes that exhibit non-equilibrium conditions and provide calibration data for process-based watershed models. [Copyright &y& Elsevier]

Abstract: Flow, sediment transport and bed deformation in alluvial rivers normally exhibit multiple time scales. Enhanced knowledge of the time scales can facilitate better approaches to the understanding of the fluvial processes. Yet prior studies of the time scales are based upon the concept of sediment transport capacity at low concentrations, which however is not generally applicable. This paper presents new formulations of the time scales of fluvial flow, suspended sediment transport and bed deformation, under the framework of shallow water hydrodynamics, non-capacity sediment transport and the theory of characteristics for the hyperbolic governing equations. The time scale of bed deformation in relation to that of flow depth is demonstrated to delimit the applicability region of mathematical river models, and the time scale of suspended sediment transport relative to that of the pertinent flow information is analyzed to address if the concept of sediment transport capacity is applicable. For shallow flows with high sediment concentrations, bed deformation may considerably affect the flow and a fully coupled model is normally required. In contrast, for deep flows at low sediment concentrations, a decoupled model is mostly justified. This pilot study of the time scales delivers a new theoretical basis, on which the interaction between flow, suspended sediment transport and bed deformation can be potentially better characterized. [Copyright &y& Elsevier]