Wei Li

Laboratory experiments measured velocity and bedload transport in vegetated channels under current‐dominated, combined wave‐current conditions. Waves and vegetation, respectively, introduced temporal and spatial heterogeneity in the velocity, which significantly impacted sediment transport rate. Compared to pure‐current, the addition of waves only moderately increased (<40%) the time‐mean turbulent kinetic energy (TKE) but introduced temporal peaks in TKE that triggered an exponential increase in sediment transport. Similarly, local regions of intense TKE, reflected in the TKE spatial variance, enhanced channel‐average sediment transport. The TKE spatial variance decreased with increasing area density of vegetation stems, such that the influence of spatial variance was strongest in sparse canopies. The impacts of spatial and temporal heterogeneity were incorporated into a new model for bedload transport, which significantly improved prediction. The enhancement in sediment transport from both temporal and spatial heterogeneity is most significant when hydrodynamic forcing is near the critical threshold for sediment motion.

The Qiantang tidal bore is globally renowned for its spectacular landscape and its strong impacts on the Qiantang riverbed erosion/deposition process. However, due to the extremely high spatial gradient of its water level along the propagation direction around the bore front, as well as the very swift movement of this front, the numerical reproduction of the bore formation and propagation has been a challenge for several decades. Here, using GPU acceleration and Local Time Stepping (LTS), we present a high-resolution simulation of the Qiantang tidal bore formation and propagation, achieving a 10 m resolution across 1169 km², which captures the bore dynamics and the full formation-to-decay processes while simulating 2-day tidal bore phenomena in 1.2 h. The formation mechanisms of three typical tidal bores (cross-shape bore, thread-shape bore, and returned bore) are revealed numerically. The cross-shape bore appears first and is generated by flow division around the mid-channel bars; further upstream, the thread-shape bore is formed due to the increasingly narrow river along a straight reach; and its reflection results in the returned bores at the YC bending reach. This study also highlights how variations in flow discharge affect the tidal bore. When the discharge increases to the annual mean discharge, the intensity of the tidal bore is increased, whereas extremely high flood peak discharge inhibits bore propagation. This study provides a scientific basis for conserving tidal bore landscapes and offers decision support for sustainable estuarine governance.

Consecutive floods combined with hyperconcentrated floods and moderate/low sediment-laden floods have always been observed in the Lower Yellow River (LYR) characterized by complex channel-floodplain systems of alternated meandering and straight segments. Interactions between those floods and sophisticated morphological segments are much more complicated than normal low-sediment laden rivers of relatively simple geometry. In this regard, we numerically investigate the 92.8 consecutive floods in the natural channel-floodplain reach of Xiaolangdi-Jiahetan in the LYR by deploying a 2-D depth-averaged fully coupled morphological model. The major focus includes (1) the unusual phenomenon of downstream peak discharge increase and (2) the different hydro-morphodynamic behaviors between meandering and straight channel-floodplain systems. For the former, the peak discharge increase of hyperconcentrated floods could be satisfactorily reproduced when the effects of bed roughness reduction and bed deformation are considered simultaneously. For the latter, the water-sediment exchange between channels and floodplains is relatively strong in hyperconcentrated floods and exhibits distinct features in meandering and straight segments. The straight one is featured by lateral channel-floodplain diffusion while the meandering one is characterized by the transition from lateral diffusion at the meander apex to streamwise advection. Consequently, the deposition at the meanders (especially on the floodplains) is much larger than that at the straight reach floodplains resulting in a remarkable uneven deposition pattern along the streamwise direction. Key words: Lower Yellow River; Hyperconcentrated floods; Channel-floodplain interactions; Morphological modelling; Sediment transport Acknowledgements: National Natural Science Foundation of China (No. 12272349, 52339005).

The modern Yellow River Delta (YRD) has witnessed frequent channel avulsions followed by the morphological processes of “wandering‐ short‐lived braiding ‐merging” in history. However, process‐based investigation and relevant physics behind these processes remain poorly constrained. The present study complements the understanding for this evolution through numerical experiments. This is achieved by applying a two‐dimensional (2‐D) depth‐averaged fully coupled morphodynamic model that considers the feedbacks of sediment‐laden flow and bed deformation to a schematized fan‐shaped YRD. Under an uneven bed of uniform sediment, the avulsed processes of “wandering‐ short‐lived braiding ‐merging” during new channel routing on the YRD are satisfactorily reproduced by the present modelling in terms of the time evolution of planar channel pattern and channel length. Moreover, the mechanisms of the avulsed evolution have been elucidated through factor analysis on delta slope, incoming flow‐sediment conditions and artificial trenching, etc. It is suggested that morphological stability could be reached when a single meandering channel is finally sustained, of which the time scale is 4–8 years echoing the documented data of abandoned channels on the YRD. In addition, quantitative criteria have been advocated to help distinguishing the sub‐stages of the avulsed evolution together with the characteristic pattern judgement. Based on the above, implications for the YRD management are discussed.

A two‐dimensional shallow water model based on the anisotropic porosity method is developed for predictions of water flows, sediment transport and bed morphological changes through rigid vegetation. Effects of the vegetation resistance and the spatial occupation are accounted for by adding an extra vegetation drag force and introducing porosity parameters. By defining a cell‐based porosity for the volumetric occupation and an edge‐based porosity for the flux exchange, the anisotropic property of the preferential flow can be well captured. Based on the finite volume method, the model is solved explicitly with a hybrid local time step/global maximum time step method and is parallelized using the Open MP techniques. The numerical grid convergence and quantitative accuracy of the model have been tested against a series of flume experiments for various configurations of rigid emergent vegetation over fixed or mobile beds. It is shown that using shallow water equations with anisotropic porosity, a constant drag coefficient can lead to numerical solutions of comparable accuracy as those complex empirical relations. Moreover, appropriate quantification of the stem‐scale turbulence effects brings significant improvement in modeling of the vegetation‐affected sediment transport.

Previous studies for numerical representation of aquatic vegetation based on the isotropic porosity shallow water models can not only consider the effects of vegetation resistance and spatial occupation in physics, but also improve the computational efficiency in large-scale modelling. This type of models provides a promising tool to numerically study the vegetated flow and the corresponding sediment transport in practice. However, the characteristics of preferential flow among complex vegetation distributions which are often observed in nature cannot be well captured due to the isotropic assumption. Thus, we make an improvement by introducing the anisotropic porosity method in the shallow water model. Unlike the isotropic porosity method which uses only one porosity parameter to describe the vegetation spatial occupation, the anisotropic porosity method defines a cell-based porosity for volumetric occupation and an edge-based porosity for flux exchange to capture the flow heterogeneity in space. Under the framework of finite volume method, the model is solved explicitly with a hybrid LTS/GMaTS method and the Open MP techniques for fast modelling. The well-balanced property and accuracy of the developed model have been tested by a series of flume experiments with different vegetation distributions over fixed or mobile beds. In general, both velocity and deposition patterns are well reproduced. It shows that a constant vegetation drag coefficient can lead to numerical solutions of comparable accuracy as those complex empirical relations in the anisotropic porosity modelling. In addition, the stem-scale turbulence could be a critical factor affecting the sediment transport inside and around vegetation patches and its appropriate quantification in the shallow water modelling deserves further research.

This paper presents a GPU (Graphics Processing Unit)-accelerated and LTS (Local-time-Step)-based finite volume Shallow Water Model (SWM). The model performance is compared against the other five model versions (Single CPU versions with/without LTS, Multi-CPU versions with/without LTS, and a GPU version) by simulating three flow scenarios: an idealized dam-break flow; an experimental dam-break flow; a field-scale scenario of tidal flows. Satisfactory agreements between simulation results and the available measured data/reference solutions (water level, flow velocity) indicate that all the six SWM versions can well simulate these challenging shallow water flows. Inter-comparisons of the computational efficiency of the six SWM versions indicate the following. First, GPU acceleration is much more efficient than multi-core CPU parallel computing. Specifically, the speed increase in the GPU can be as high as a hundred, whereas those for multi-core CPU are only 2–3. Second, implementing the LTS can bring considerable reduction: the additional maximum speed-ups can be as high as 10 for the single-core CPU/multi-core CPU versions, and as high as five for the GPU versions. Third, the GPU + LTS version is computationally the most efficient in most cases; the multi-core CPU + LTS version may run as fast as a GPU version for scenarios over some intermediate number of cells.

Typhoon-induced intense rainfall and urban flooding have endangered the city of Zhoushan every year, urging efficient and accurate flooding prediction. Here, two models (the classical shallow water model that approximates complex buildings by locally refined meshes, and the porous shallow water model that adopts the concept of porosity) are developed and compared for the city of Zhoushan. Specifically, in the porous shallow water model, the building effects on flow storage and conveyance are modeled by the volumetric and edge porosities for each grid, and those on flow resistance are considered by adding extra drag in the flow momentum. Both models are developed under the framework of finite volume method using unstructured triangular grids, along with the Harten-Lax-van Leer-Contact (HLLC) approximate Riemann solver for flux computation and a flexible dry-wet treatment that guarantee model accuracy in dealing with complex flow regimes and topography. The pluvial flooding is simulated during the Super Typhoon Lekima in a 46 km² mountain-bounded urban area, where efficient and accurate flooding prediction is challenged by local complex building geometry and mountainous topography. It is shown that the computed water depth and flow velocity of the two models agree with each other quite well. For a 2.8-day prediction, the computational cost is 120 min for the porous model using 12 cores of the Intel(R) Xeon(R) Platinum 8173M CPU @ 2.00 GHz processor, whereas it is as high as 17,154 min for the classical shallow water model. It indicates a speed-up of 143 times and sufficient pre-warning time by using the porous shallow water model, without appreciable loss in the quantitative accuracy.

In this paper, a computationally efficient shallow water model is developed for sediment transport in the Yangtze Estuary by considering mixed cohesive and non-cohesive sediment transport. It is firstly shown that the model is capable of reproducing tidal-hydrodynamics in the estuarine region. Secondly, it is demonstrated that the observed temporal variation of suspended sediment concentration (SSC) for mixed cohesive and non-cohesive sediments can be well-captured by the model with calibrated parameters (i.e., critical shear stresses for erosion/deposition, erosion coefficient). Numerical comparative studies indicate that: (1) consideration of multiple sediment fraction (both cohesive and non-cohesive sediments) is important for accurate modeling of SSC in the Yangtze Estuary; (2) the critical shear stress and the erosion coefficient is shown to be site-dependent, for which intensive calibration may be required; and (3) the Deepwater Navigation Channel (DNC) project may lead to enhanced current velocity and thus reduced sediment deposition in the North Passage of the Yangtze Estuary. Finally, the implementation of the hybrid local time step/global maximum time step (LTS/GMaTS) (using LTS to update the hydro-sediment module but using GMaTS to update the morphodynamic module) can lead to a reduction of as high as 90% in the computational cost for the Yangtze Estuary. This advantage, along with its well-demonstrated quantitative accuracy, indicates that the present model should find wide applications in estuarine regions.

In history, channel avulsion occurred frequently in the Yellow River Delta featuring by the combination of large-scale north-south shifts and small-scale evolution of “wandering-merging-meandering-diverting” patterns. However, these evolution processes are lack of quantitative investigations due to the complex interactions between riverine and tidal flows, and between sediment-laden flow and river bed as well. Since public observations are scarce, we numerically study this problem focusing on the controlling factors for reproducing the “wandering-merging-meandering” evolution patterns and the characteristics of relative morphological equilibrium under constant discharge and sediment conditions. Using a 2-D depth-averaged fully coupled morphological model, numerical experiments are carried out for a schematic Yellow River Delta. The results show that random disturbance on initial topography is the key factor to initiate wandering patterns. Moreover, the development of river patterns and the associated morphological time scales are strongly related to initial bed slopes and upstream discharge and sediment conditions. Generally, a small bed slope and a low discharge favor the formation of wandering patterns in the early stage, while a large bed slope and a high discharge may accelerate the merging and routing processes. In the case of upstream clear flow, channel formation is dominated by erosion processes. Yet with increasing sediment, it results from the combination of levee lip sedimentation and channel erosion. In addition, the flow routing may be facilitated by enhanced tidal ranges whereas decelerated when subaqueous sedimentation extends to the sea. Regarding the equilibrium state, the morphological time scales are 4~8 years in most cases and the width-depth ratio increases longitudinally following a power-law function.