Toshiki Iwasaki

Self-accelerating turbidity currents (SATCs) are hypothesized to be the primary mechanism for transporting vast amounts of sediment to the deep ocean. However, theoretical predictions of SATCs have preceded field observations by decades, leaving a critical gap in our understanding of this long-distance delivery process. Here, we present results from a four-year field survey of turbidity currents and bathymetric evolution in the Xiaolangdi Reservoir on the Yellow River (China), the world’s most sediment-laden river. We provide definitive in situ evidence of SATCs, characterized by synchronous down-channel increases in sediment mass and current momentum, alongside massive channel incision at the event scale. Notably, these SATCs occur in a low-gradient lacustrine setting, challenging the prevailing hypothesis that such phenomena are restricted to steep submarine channels or high velocity conditions. We identify a dimensionless threshold, incorporating current velocity, channel slope, and sediment settling velocity, that governs SATC formation across sublacustrine and marine environments. This threshold provides a robust framework for predicting SATC occurrence and informs engineering strategies to sustain reservoir capacity and restore sediment connectivity in dammed river systems.

Alluvial bars in straight channels generally migrate downstream or can be non‐migrating but rarely migrate upstream. On the other hand, a recent long‐term field observation suggested a possibility of the upstream migration of bars. To understand this rare morphodynamic feature of alluvial bars, we herein perform non‐linear numerical simulations of alluvial bar formation and development in a straight channel. More specifically, we simulate alluvial bar dynamics under steady discharge and unsteady discharge (repeated hydrographs), wherein the bar regime retains super‐resonance condition. The result shows that initially formed, short downstream‐migrating free bars elongate, becoming much longer upstream migrating bars. The former bar elongation can be understood as a well‐observed non‐linear response of the bars, but much longer bar formation might originate from different morphodynamic features. The theoretical interpretation for this result using temporal and spatial mode linear analysis suggests a sort of forced bar formation in the upstream channel originating from nearly non‐migrating, slow‐movement bars. In other words, nearly non‐migration bars act as forcing factors, similar to an asymmetric obstacle in the channel, leading to an upstream morphodynamic influence observed under super‐resonance conditions. Subsequently, an interaction/coexistence of free‐migrating and forced‐steady bar responses may contribute to the formation of upstream‐migrating bars. These results provide new insights into the migration features of alluvial bars, but they still require a comprehensive theoretical framework to explain them. Additionally, observing upstream‐migrating bars will be an important future research, although it will be difficult due to the considerably slow migration speed predicted by the present numerical simulation.

Sediment supply has several essential roles in downstream river morphology and grain size; however, quantifying the relationship between sediment supply effect and river morphodynamics remains difficult. This is because of our limited understanding of the spatiotemporal scale of the nonequilibrium effect caused by the imbalance between sediment supply and transport capacity on downstream river dynamics. Here, we investigate the morphology of alternate bar with mixture-sized sediment under cycled hydrographs and constant sediment supply. This upstream water and sediment supply condition results in repeated aggradation/degradation and fining/coarsening near the upstream end. Under well-sorted sediment condition, such variation of bed and grain size takes place only limited upstream reach so-called hydrograph boundary layer (HBL). On the other hand, poor-sorted sediment breaks down the concept of HBL, causing a grain-sorting wave such as bedload sheet that migrates beyond HBL. This sorting wave represents a long-distance effect of upstream sediment supply on the downstream river morphology. The amplitude of this sorting wave is generally small, but may affect downstream bar dynamics since previous experimental and numerical studies have indicated that alternate bars with mixture-sized sediment are more unstable to some disturbance than those with uniform-sized sediment. To understand this, we perform the numerical calculation by using a two-dimensional morphodynamic model, iRIC-Nays2DH, to clarify how bedload sheet migrating beyond HBL as a disturbance affects the downstream alternate bar morphology. The computational condition is determined based on the Otofuke River, Japan, a typical gravel-bed river. The river has wide grain size distribution and well-developed alternate bars, so this will be a suitable case to understand the effect of bedload sheet on the alternate bar dynamics. The numerical result shows bed elevation variation during a single hydrograph close to the upstream end corresponding to the HBL, and small-amplitude grain sorting wave migrates downstream beyond HBL. The alternate bars developed slightly downstream of HBL reach so that the upstream alternate bar is continuously affected by the migration of bedload sheet. The bedload sheets cause irregular alternate bars upstream to a certain extent, but this has a limited role in more downstream alternate bar dynamics. This will be because of the much larger sediment storage of bars than small-amplitude bedload sheet. This result may suggest that the bedload sheet can indeed migrate downstream of HBL, but, has a small effect on the downstream alternate bar dynamics.

Climate change is a critical social concern in many research and engineering fields, and one of the vital issues in this regard is its significant impact on water and sediment-related disasters. Further increases in precipitation intensity and associated river discharge increases will change the flood characteristics and associated morphological changes in rivers, causing critical damage to river training structures and residential areas along rivers. Here, we assess the risk of the river levee breach caused by significant bank erosion due to in-channel morphodynamic processes using a large ensemble hydrological dataset and a physics-based morphodynamic model. The target river reach is the upper Otofuke River, Japan, a typical steep, gravel-bed river, and some river training structures along this river were damaged due to the significant morphological change of bed and bank during several huge flood events. Our observed dataset is highly limited, making it essentially difficult to assess risks related to river disasters, especially under future climate conditions. The use of a large ensemble climate calculation dataset, including precipitation, discharge, etc., provided in the context of climate change research will overcome the aforementioned difficulties and contribute to a comprehensive understanding of possible significant flood events in current and future climate conditions. We use a large dataset of river discharge hydrographs provided from the large rainfall calculation based on the dynamically downscaling climate calculation of the d4PDF. The thousands of hydrographs are categorized by the k-shape clustering method to evaluate the several important hydrograph shapes since the morphological change of rivers is highly affected by both peak discharge and overall shape characteristics of hydrographs. By using a physics-based morphodynanic model, iRIC-Nays2DH, and characterized hydrographs, we then simulate the possible river morphodynamic processes in the current and future climate conditions. Based on the river morphodynamic calculations, the risk of river embankment caused by morphological change of the river under current and future climate conditions is analyzed. The results show that the hydrograph shape has an important role in the morphological evolution of the river, such as sandbar and meandering development, so the damage intensity of river embankment caused by such morphological change is highly dependent on the hydrographs. More specifically, under the same peak discharge, a hydrograph with a single sharp peak causes less bank erosion; on the other hand, a relatively longer hydrograph drastically increases the risk of bank erosion and river embankment breach. Such a longer hydrograph that greatly impacts the river disaster has a small possibility of occurrence, but it does take place in the future climate conditions. It indicates that climate change does increase the risk of river levee breach caused by in-channel morphodynamic processes in steep, gravel-bed rivers.

Recent increases in heavy rainfall events, which may have resulted from climate change, have caused various natural water- and sediment-related disasters. A typical sediment-related disaster in a steep gravel-bed river is extreme bank erosion and subsequent meander development, damaging residential areas and infrastructure along the river. Assessing the bank erosion rate and its future changes under climate change is essential to prevent such severe disasters. Here, we propose a simple but useful framework for this purpose using a physics-based numerical model of free meandering and a large dataset of flood hydrographs generated from climate and runoff models. First, numerical experiments on meandering development were conducted using a two-dimensional morphodynamic model of free meandering. The results indicate that meander dynamics could reach a dynamic equilibrium state under several hydraulic and channel geometry conditions, and the characteristics of the simulated meandering channel were reasonably consistent with the field data. The meander amplitude of this state was positively correlated with the steady discharge; therefore, the increase in flow discharge may have an important effect on river dynamics under climate change. To quantify this effect, we combined a simple predictor of meander amplitude derived from a numerical experiment with a flood hydrograph dataset for current and future climatic conditions. The results suggest that the meander amplitude increased significantly for the same probability of occurrence; therefore, the risk of riverbank erosion due to river meandering will increase significantly under future climate conditions.

To understand the influence of shallow-rooted trees on channel morphology and river planforms, we conducted hydraulic experiments using bentgrass as a substitute for shallow-rooted trees, such as willow trees, that commonly flourish along the banks of gravel-bed rivers. The experiment demonstrated a meandering flow progression that maintained a channel planform with curved bank lines. Vegetation did not significantly inhibit bank erosion near the apex points in the meandering channel, where the bed near the riverbank was relatively deep. Conversely, vegetation inhibited bank erosion downstream of the meandering channel, where the bed near the riverbank was shallow. The results suggest that the inhibitory effect of vegetation on bank erosion varies depending on the riverbank height and channel depth, thereby influencing the channel morphology and river planform characteristics.

Our understanding of bedform processes and their associated effect on bedform roughness is limited, and accounts for large uncertainties in hydraulic roughness computation. It is a standard practice in hydraulic modelling to consider hydraulic roughness as a roughness coefficient and to calibrate the model to this coefficient. Such an approach is empirical and does not well capture the physical processes involved in hydraulic roughness dynamics. When bedforms are present, they can account for a significant portion of hydraulic roughness. Consequently, when bedform transitions occur, an abrupt and significant disruption in the hydraulic roughness regime occurs; affecting our water management applications, such as navigability, flood risk management, sediment transport, etc. Bedform transitions are rarely captured, either in laboratory or in real-scale river channels. As such, our understanding of such transition behaviour is further constrained.In this research, we modelled a CFD physics-based bedform model for the Chiyoda channel, Japan based on previous study of Yamaguchi et al. (2019). The model configuration and results of that study had been validated. The CFD model was initialized at flat-bed condition and run till a dynamic equilibrium in dune regime was obtained. In our research, we captured the bedform in this simulation in each time step, effectively obtaining a timeseries of bed evolution from flatbed regime to dune regime.It is hoped, the use of physics-based CFD models can simulate the physical processes that invoke bedform transitions. As these have not been easily observed in the field or in lab, the simulations can provide an important insight into these complex processes. This is particularly important in the context of changing hydraulic regimes under the changing climate scenario – possibly making past calibrations of river systems incompatible in the future. An alternative to physics-based (CFD) model is the use of a data-driven model (using machine learning techniques). The use of surrogate machine learning models that capture the behaviour of these physics-based (CFD) models, provides an advantage in terms of computational cost and computational time.We also developed a proof-of-concept artificial neural network ML models to predict dune height and mean flow depth respectively based on the CFD model results as input. Several models were built using various combinations of input variables: the lagged values of dune height and mean flow depth, mean flow depth or dune height (alternatively), as well as the present and lagged values of spectral power from Fast Fourier Transform spectral analysis. The lagged values of the predicted variable were the most important input parameters compared to other variables. The use of spectral power as predictive variable did not much improve the results, owing to a strong cross-correlation of the parameter with dune height and mean flow depth.Alternative predictive variables such as stream discharge, Froude number, etc may be considered in future studies to ensure better prediction ability. Validation of these ML and physics-based CFD model results remain a challenge as bedform transition timeseries dataset is not much available. Future outlook of the research in this direction is discussed.

Sediment supply plays an essential role in river morphology. However, the specific impact of sediment supply on river morphology is not apparent. According to the hydrograph boundary layer (HBL) concept, upstream riverbed changes caused by the imbalance between sediment supply and the capacity can propagate only a limited length and have a negligible effect on the riverbed beyond such a short length. We performed a two-dimensional morphodynamic calculation to test the concept of HBL, which was proposed under a one-dimensional simulation, meaning that the concept of HBL is still valid for plane changes in river morphology. We employed an unsteady flow with equilibrium or constant sediment supply in a straight, modeled gravel-bedded channel with an unerodible bank to simulate alternate bar morphodynamics. The results show that regardless of the sediment supply condition, the alternate bar features formed downstream of the HBL are considerably similar. This suggests that sediment disturbance at the upstream end has a negligible effect on the mobile-bed dynamic processes, including alternate bar formation and development downstream of the HBL.

The quantification of driftwood deposition in rivers is important for understanding the total budget of driftwood at the watershed scale; however, it remains unclear how such driftwood storage in rivers contributes to the overall system because of the difficulties in undertaking field measurements. Herein, we perform numerical simulations of driftwood deposition within an idealized river reach with a sand-bed, to describe the role of large-scale bedforms, more specifically, alternate bars, multiple bars, and braiding, in driftwood storage in rivers. The numerical model we propose here is a coupling model involving a Lagrangian-type driftwood model and an Eulerian two-dimensional morphodynamic model for simulating large-scale bedforms (i.e., bars and braiding). The results show that the channel with a braiding pattern provides a wide area with enhanced capacity for deposition of driftwood, characterized by exposed mid-channel or in-channel bars, leading to high driftwood storage. The alternate bar is also a large bedform representing a sediment depositional element in rivers; however, because of the narrow exposed bar area and its downstream-migrating feature during floods, the alternate bars seem to contribute less to driftwood deposition in rivers. This suggests that the role of multiple bars and braiding is critically important for the driftwood deposition in rivers.

Antidunes are fluvial bedforms that form in rivers with supercritical flows. The water surface over antidunes is strongly in phase with the bed surface, and the water surface is amplified to produce large surface waves. Many experimental studies have addressed antidunes; however, the shapes of three‐dimensional antidunes in a wide channel with alternate bars have not yet been appropriately understood. In this study, we experimentally investigated the streamwise and transverse length scales of antidunes under conditions with a large width–depth ratio. Our experimental results provide evidence for the coevolution of antidunes and free alternate bars, and show for the first time that the development of free bars greatly alters the three‐dimensional shape of water surface waves over antidunes. In the absence of free bars in a wide channel, multiple longitudinal wave trains form, and the number of wave trains counted in the transverse direction increases with increases in the width–depth ratio. However, the presence of free bars affects the local flow characteristics, resulting in a decrease of the number of wave trains in the transverse direction. Therefore, we propose a simple model for predicting the reduction in the number of wave trains by combining two previous theories for antidunes and free bars. Results obtained by the model were found to largely agree with experimental observations. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd