Taeun Kang

Unlike general landslides, debris flow has a high water content, due to large floods and rainfall. On steep slopes, it behaves like a large specific-weight sediment–water mixture, rather than general fluid. Thus, its momentum rapidly increases, damaging human lives and properties. This study developed a numerical model of debris flow considering driftwood and entrainment erosion, and performed numerical simulations based on the 2011 observation data of Mt. Umyeon. To develop the debris-flow model, the Nays2DFlood model, which is a flooding model based on the shallow-water equation, was coupled with the advection and diffusion of the sediment–water mixture, debris-flow bottom shear-stress, and entrainment-erosion modules. The simulation better reproduced the depth, flow velocity, and debris-flow volume of Mt. Umyeon. In particular, the driftwood generation and motions, by debris flow, were well demonstrated in the numerical simulation. Moreover, the driftwood partially reduced the drag force, and the shielding effect of the forest caused a delay in the runoff duration-time. The results of this study are expected to help establish measures to reduce debris disasters that can respond to the current situation, wherein debris damage is increasing, owing to the increased rainfall because of climate change. This subsequently increases the possibility of debris flows and the consequent damage. In particular, the predictive methodology of the refined model expands the understanding of numerical treatment for debris flow and driftwood, by reflecting the detailed physical regime.

The accumulation of driftwood during heavy rainfall may block river channels and damage structures. It is necessary to mitigate such effects by periodically capturing and removing driftwood from rivers. In this study, the behavior of driftwood in open-channel flows with a relatively large wood density was modeled numerically. The water flow and driftwood motion were solved three-dimensionally, with an Euler-type flow model coupled with a Lagrange-type driftwood motion model. A piece of driftwood was modeled as a set of connected spherical elements in a straight line for easy analysis using a discrete element method. Wood with specific gravity exceeding 1 will travel along a position near the riverbed and will be affected by bed friction. In addition, friction forces for sliding and rolling motions are considerably different. Therefore, in the numerical model, a bed friction term was introduced between the bed and driftwood considering the anisotropy of the friction force. The variation in the drag force of water flow on driftwood was also considered depending on the angle between the driftwood trunkwise direction and flow direction. The model was applied under the same conditions as those used in a laboratory experiment on driftwood behavior around an inlet-type driftwood capture facility. The computational results showed that the proposed model could qualitatively reproduce the driftwood behavior around the capture facility. The secondary flow patterns at the approaching reach and the capture ratio were found to be strongly affected by the turbulence model and the Manning roughness coefficient.

This study discusses the reproducibility of a numerical model for simulating the morphodynamics involved in the transport of large pieces of wood in a braided river, considering the root wad effect and jam formation. The developed numerical model can simulate the behaviour of large pieces of wood using a two‐dimensional depth‐averaged Eulerian flow model that calculates the water flow and bed morphology. A Lagrange‐type wood transport model is used herein, and the applicability of the combined model is discussed through a comparison with obtained experimental results. From the simulation results, we calculate the total braiding index and estimate the deposition patterns of wood pieces for comparison with the experimental results. We then analyse the bed morphology responses and wood deposition patterns in terms of the root wad effect and input supply. Moreover, we discuss the advantages and limitations of the proposed model to predict the large wood dynamics considering the bed morphology. © 2019 John Wiley & Sons, Ltd.

Results from computational morphodynamics modeling of coupled flow–bed–sediment systems are described for 10 applications as a review of recent advances in the field. Each of these applications is drawn from solvers included in the public‐domain International River Interface Cooperative (iRIC) software package. For mesoscale river features such as bars, predictions of alternate and higher mode river bars are shown for flows with equilibrium sediment supply and for a single case of oversupplied sediment. For microscale bed features such as bedforms, computational results are shown for the development and evolution of two‐dimensional bedforms using a simple closure‐based two‐dimensional model, for two‐ and three‐dimensional ripples and dunes using a three‐dimensional large‐eddy simulation flow model coupled to a physics‐based particle transport model, and for the development of bed streaks using a three‐dimensional unsteady Reynolds‐averaged Navier–Stokes solver with a simple sediment‐transport treatment. Finally, macroscale or channel evolution treatments are used to examine the temporal development of meandering channels, a failure model for cantilevered banks, the effect of bank vegetation on channel width, the development of channel networks in tidal systems, and the evolution of bedrock channels. In all examples, computational morphodynamics results from iRIC solvers compare well to observations of natural bed morphology. For each of the three scales investigated here, brief suggestions for future work and potential research directions are offered. © 2019 The Authors Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd

In this study, we conducted simulations using a two-dimensional, depth-averaged river flow and river morphology model to investigate the effect of vegetation growth and degree of flow discharge on a shallow meandering channel. To consider the effects of these factors, it was assumed that vegetation growth stage is changed by water flow and bed erosion. The non-uniformity of the vegetation growth was induced by the non-uniform and unsteady profile of the water depth due to the irregular shape of the bed elevation and the unsteady flow model reliant on hydrographs to evaluate three types of peak discharges: moderate flow, annual average maximum flow, and extreme flow. To compare the effects of non-uniform growing vegetation, the change in channel patterns was quantified using the Active Braiding Index (ABI), which indicates the average number of channels with flowing water at a cross section and the Bed Relief Index (BRI), which quantifies the degree of irregularity of the cross-sectional shape. Two types of erosion were identified: local erosion (due to increased flow velocity near a vegetation area) and global erosion (due to the discharge approaching peak and the large depth of the channel). This paper demonstrated that the growth of vegetation increases both the ABI and BRI when the peak discharge is lower than the annual average discharge, whereas the growth of vegetation reduces the BRI when the peak discharge is extreme. However, under extreme discharge, the ABI decreases because global erosion is dominant. The conclusions from this study help to deepen the understanding of the interactions between curved river channels and vegetation.