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• An improved depth-averaged two-phase continuum flow model is adopted to investigate the effects of pore-fluid pressure and bed sediment conditions on multi-size debris flow. • New mechanisms are proposed to numerically explain the phenomenon of "the larger sediment sizes of bed materials, the higher erosion rate" and front coarsen of debris flows on the erodible bed. • The non-dimension number and energetics of the debris flows are investigated to demonstrate it is necessary to incorporate water-sediment and particle-particle interactions. Debris flows are classical two-phase flows that are greatly affected by bed sediment conditions. However, the underlying mechanism remains uncertain from either a theoretical or numerical perspective. Here an existing depth-averaged two-phase continuum flow model is further improved by incorporating the effects of pore-fluid pressure and bed sediment conditions. By numerical simulation of the USGS experiments, we have numerically reproduced the mechanism that "the coarser the bed sediments and the larger the basal pore pressure, consequently the larger the erosion rates". Moreover, an additional mechanism for this phenomenon has been revealed. Specifically, debris flows on steep slopes are likely to fall into a high shear stress regime, under which conditions the sediment transport capacity always takes a maximum value and is independent of the sediment size. Therefore, the sediment settling velocity that is proportional to the sediment size affects the erosion rate directly. It is also observed that the added potential energy due to a net erosion has positive effects on the kinetic energy of the debris flow. Notably, coarse and fine particles exhibit distinct roles in energetics and the former is characterized by more intensive interactions with the water phase, which should be appropriately considered in the modeling. Finally, particle size coarsening is observed in the head region, which is attributed to the hiding/exposure mechanisms of nonuniform sediments. These findings can enhance the understanding of the mechanism between the bed conditions and debris flow. [ABSTRACT FROM AUTHOR]

Highlights • Most of sediment is entrained by coherent structures interacting with the bed. • More accurate RANS based methods with movable bed are needed. • An augmented bed shear stress can be used to account for effect of unresolved eddies. • LES can be used to find optimum model coefficient of augmented shear stress. Abstract Movable-bed Reynolds-Averaged Navier–Stokes (RANS) simulations of local scour around bridge piers and abutments without ad hoc corrections tend to underestimate scour. This is generally attributed to the inability of RANS to accurately capture the unsteady dynamics of the large-scale coherent structures forming in the bed vicinity. Moreover, the characteristics of these coherent structures and their erosive capacity are expected to change as the bed evolves toward equilibrium. In the present study, Detached Eddy Simulations (DES) of flow past circular and rectangular piers are performed with bathymetry corresponding to different stages of the scour process between initial (flat-bed) conditions and equilibrium scour conditions to better understand how the coherent structures drive scour around the pier at different stages of the scouring process and to assess the capabilities of RANS to predict the flow and the turbulence statistics. Using these results, a new methodology is proposed to account for the effects of large-scale coherent structures on sediment entrainment that is directly applicable to RANS simulations with movable bed. The main idea is to improve the predictions of the local flux of sediment entrained from the bed by considering the effect of bed friction velocity fluctuations induced by the coherent structures situated near the bed, a critical effect that is generally neglected in RANS simulations. The new methodology is based on augmenting the bed friction velocity available from the time-accurate RANS calculation by a term that is proportional to the standard deviation of the bed friction velocity. This term is estimated from the turbulent kinetic energy field predicted by RANS. The model has one free parameter. Its value is calculated such that for fixed bed simulations the total entrainment flux predicted by the time-accurate RANS matches the value predicted by DES. Based on simulations conducted for flow past circular and rectangular piers, the value of the free parameter was estimated to be between 1.5 and 4, with lower values recommended to be used for cases when the time-accurate RANS simulation captures the large-scale vortex shedding behind the pier. Movable bed simulations performed using the augmented bed friction velocity model are shown to predict much more accurately the maximum scour depth around circular and rectangular piers compared to the corresponding simulations where the entrainment flux was calculated only based on the local value of the bed friction velocity calculated from the RANS velocity field. [ABSTRACT FROM AUTHOR]

Bank erosion can be an important form of morphological adjustment in rivers. With the advances made in computational techniques, two-dimensional (2D) depth-averaged numerical models have become valuable tools for resolving many engineering problems dealing with sediment transport. The objective of this research work is to present a simple, new, bank-erosion operator that is integrated into a 2D Saint-Venant-Exner morphodynamic model. The numerical code is based on an unstructured grid of triangular elements and finite-element algorithms. The slope of each element in the grid is compared to the angle of repose of the bank material. Elements for which the slope is too steep are tilted to bring them to the angle of repose along a horizontal axis defined such that the volume loss above the axis is equal to the volume gain below, thus ensuring mass balance. The model performance is assessed using data from laboratory flume experiments and a scale model of the Old Rhine. For the flume experiment case with uniform bank material, relevant results are obtained for bank geometry changes. For the more challenging case ( i.e. scale model of the Old Rhine with non-uniform bank material), the numerical model is capable of reproducing the main features of the bank failure, induced by the newly designed groynes, as well as the transport of the mobilized sediment material downstream. Some deviations between the computed results and measured data are, however, observed. They are ascribed to the effects of three-dimensional (3D) flow structures, pore pressure and cohesion, which are not considered in the present 2D model. [ABSTRACT FROM AUTHOR]

The artificial replenishment of sediment is used as a method to re-establish sediment continuity downstream of a dam. However, the impact of this technique on the hydraulics conditions, and resulting bed morphology, is yet to be understood. Several numerical tools have been developed during last years for modeling sediment transport and morphology evolution which can be used for this application. These models range from 1D to 3D approaches: the first being over simplistic for the simulation of such a complex geometry; the latter requires often a prohibitive computational effort. However, 2D models are computationally efficient and in these cases may already provide sufficiently accurate predictions of the morphology evolution caused by the sediment replenishment in a river. Here, the 2D shallow water equations in combination with the Exner equation are solved by means of a weak-coupled strategy. The classical friction approach considered for reproducing the bed channel roughness has been modified to take into account the morphological effect of replenishment which provokes a channel bed fining. Computational outcomes are compared with four sets of experimental data obtained from several replenishment configurations studied in the laboratory. The experiments differ in terms of placement volume and configuration. A set of analysis parameters is proposed for the experimental-numerical comparison, with particular attention to the spreading, covered surface and travel distance of placed replenishment grains. The numerical tool is reliable in reproducing the overall tendency shown by the experimental data. The effect of fining roughness is better reproduced with the approach herein proposed. However, it is also highlighted that the sediment clusters found in the experiment are not well numerically reproduced in the regions of the channel with a limited number of sediment grains. [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]

Abstract: Dredging operations of navigation channels and harbours are regularly planned in order to maintain the nautical depth and to ensure the navigation safety. Depending on the quality of the dredged material (fixed by the Oslo convention, 1972), dredged sediments can be released into the sea or stored at the Earth’s surface to be cleaned up. The present study deals with release operations. We simulate the complete process (generation and settling of a sediment plume, propagation of a density current on the bottom) using a two-phase flow model. We show that in this case, the sediment (or solid) phase strongly differs from the motion of the water (or fluid) phase. Comparisons between numerical results and experiments are carried out in order to illustrate the effects of sediment diameter, initial concentration, and ambient current on the plume dynamics and on the density current. We obtain a correct agreement with experiments for the specific release case. [Copyright &y& Elsevier]

Abstract: Recent studies, which investigate the processes leading to the formation of rhythmic morphological features in the coastal region, are reviewed. Emphasis is given to morphodynamic stability analyses which consider the presence of a heterogeneous sediment. The selective transport of the different fractions of a sediment mixture leads to grain sorting phenomena, which significantly affect the dynamics of the bottom forms. Small scale bedforms (ripples), intermediate bedforms (sorted bedforms and tidal dunes) and large scale bedforms (shoreface-connected ridges and sand banks) are considered. References are given to papers describing the new aspects of the stability analyses arising from the simultaneous presence of a time-dependent forcing flow and a sediment mixture. [Copyright &y& Elsevier]

Abstract: This paper concerns the development of high-order accurate centred schemes for the numerical solution of one-dimensional hyperbolic systems containing non-conservative products and source terms. Combining the PRICE-T method developed in [Toro E, Siviglia A. PRICE: primitive centred schemes for hyperbolic system of equations. Int J Numer Methods Fluids 2003;42:1263–91] with the theoretical insights gained by the recently developed path-conservative schemes [Castro M, Gallardo J, Parés C. High-order finite volume schemes based on reconstruction of states for solving hyperbolic systems with nonconservative products applications to shallow-water systems. Math Comput 2006;75:1103–34; Parés C. Numerical methods for nonconservative hyperbolic systems: a theoretical framework. SIAM J Numer Anal 2006;44:300–21], we propose the new PRICE-C scheme that automatically reduces to a modified conservative FORCE scheme if the underlying PDE system is a conservation law. The resulting first-order accurate centred method is then extended to high order of accuracy in space and time via the ADER approach together with a WENO reconstruction technique. The well-balanced properties of the PRICE-C method are investigated for the shallow water equations. Finally, we apply the new scheme to the shallow water equations with fix bottom topography and with variable bottom solving an additional sediment transport equation. [Copyright &y& Elsevier]

The thickness of the bedload layer is a crucial parameter for evaluating sediment transport rates in open channel flow, but it is often determined empirically. Based on the concept of the hydrodynamic diffusion related to particle–particle interactions, an analytical model is proposed in this study for computing the thickness of the bedload layer. The coefficient of diffusion is assumed to be associated with the momentum transfer induced by the random particle motion and thus can be derived from the shear-induced particle stress. The analytical result shows that the ratio of the bedload thickness to the particle diameter depends on the dimensionless particle diameter and dimensionless bed shear stress. Differences are also examined between the present study and a few empirical formulas that are derived from experimental results for limited bed conditions. [Copyright &y& Elsevier]

• Low and high bar modes originate from water discharge variations. • In turn, this induces complex compound bar formation. • Sediment sorting patterns differ between unit and compound bar patterns. • Sediment sorting does not modify bar morphodynamics if sediment mobility is high. • Floods decrease the degree of sediment sorting. Bars are large sediment deposits alternating with deeper areas that arise from alluvial river bed instability and forcing. The present study aimed at investigating the combined influence of flow and longitudinal width variations on the co-evolution between bar pattern and sediment sorting in a sandy-gravel river reach. To this goal, a fully non-linear 2D numerical model was developed to reproduce the morphodynamic behavior of bars in a reach of the Loire River consisting in a typical channel expansion/contraction. Numerical results showed that varying water discharge promoted a competition between low and high bar modes: i.e., from alternate to multiple bar patterns. Low bar modes were associated with coarse sediment over bar tops and fine sediment in pools, and this sorting pattern was inverted for higher bar modes. Surface sediment was coarser and the degree of sediment sorting was greater after periods of low than high flow. Due to high sediment mobility, sediment sorting did not significantly modify bar morphodynamics. [ABSTRACT FROM AUTHOR]