Large sediment pulses can reshape mountain river morphology over years to decades and amplify hazards, yet the river response is rarely quantified due to a lack of long-term geometric records. This study applies the developed RivBW model to analyze the morphological evolution of the Lillooet River (Canada) from 1947 to 2022, a period that witnessed a catastrophic landslide in 2010—one of the largest globally since 1947. Results show that the river has experienced a four-phase evolution since 1947. During the early contraction phase from 1947 to 1990, the total bankfull area declined by 17.9

Gravel-bed microtopographies are closely connected with many aspects of river dynamics, including incipient sediment motion, sediment transport, flow structure, and flow resistance. Grain arrangement and grain size are two important parameters for quantitatively characterizing gravel-bed microtopographies. However, how grain arrangement and grain size affect gravel-bed microtopographies is still unknown. For this research, fifteen groups of uniform gravel samples were obtained by screening natural river sediment. Then, these uniform gravel samples were separately used to artificially pave gravel beds with six typical types of grain arrangements in a laboratory, and the gravel-bed elevations were measured. On this basis, the effects of grain arrangement and grain size on gravel-bed microtopographies were analyzed using statistical parameters and variograms. The experimental results showed that the elevation frequency distributions for the stacked-layer type of gravel beds established negatively skewed, leptokurtic, and unimodal shapes, but those for the other bed types exhibited bimodal shapes; therein, the main peaks for the one-layer and imbricate bed types were close to a normal distribution. The stacked-layer and one-layer types of gravel beds are approximately isotropic, while the imbricate and striped types are anisotropic. The spherical variogram model can be used as a good theoretical model to quantify the elevation variabilities for the stacked-layer, one-layer, and imbricate types of gravel beds. The gravel nuggets of elevation variograms for these three types of gravel beds are insensitive to the grain arrangement and decrease when the grain size increases, but the sills and correlation lengths linearly increase with increasing grain size.

Floodplain vegetation plays a crucial role in maintaining river ecosystems, but the presence of vegetation increases flow resistance and water levels, thus increasing flood risk. Accurately estimating stage-discharge relationships is vital for balancing the ecological benefits of vegetation and potential flood risks. However, existing models lack convenience or accuracy in predicting stage-discharge relationships in vegetated rivers with overbank floods. In this study, a practical one-dimensional (1-D) model was proposed to predict stage-discharge relationships in vegetated compound channels. Experiments were conducted in a straight vegetated compound channel to examine the influence of floodplain vegetation on stage-discharge relationships. The results revealed that the water level increased more rapidly with increasing discharge in vegetated compound channels than in nonvegetated channels. The influence of vegetation on the water level decreased with increasing vegetation density, suggesting a nonlinear relationship between the vegetation density and water level. The discharge values predicted by the proposed model agreed well with the laboratory and field measurements. The sensitivity analysis results revealed that the momentum transfer coefficient (alpha) has a negligible effect on the predicted stage-discharge relationships in large-scale rivers, primarily because the apparent force is significantly lower than the other forces in the model. Finally, the proposed model can appropriately guide the arrangement of floodplain vegetation to reduce flood risk.

Meandering channels are common in natural rivers, and the bed material in these channels is almost always non-uniform. This paper begins with a comprehensive analysis of the forces and corresponding force arms acting on particles on concave bank slope of meandering channels, considering the hiding-exposure effect and cohesion between particles. A moment balance equation is established under rolling mode. Based on this equation, and accounting for flow fluctuations and incipient probability, a new unified formula is proposed to predict incipient flow velocity of non-uniform sediment on concave bank slopes of meandering channels. By comparison with published experimental and field data covering a wide range of particle diameters from sand to gravel to cobble, the proposed unified formula demonstrates satisfactory predictive accuracy, with relative errors within 20%. Finally, the effects of several key parameters on incipient flow velocity of non-uniform sediment on concave bank slope of meandering channels are discussed.

Due to environmental changes and anthropogenic activities, a dramatically diminished upstream sediment supply can promote riverbed erosion and alter the stage–discharge relationship in a channel with overbank flows, which can impact flood control. It is important to understand how the stage–discharge relationship in an erodible compound channel is affected and how to predict this relationship. In this study, we performed laboratory experiments in a straight compound channel with a mobile main channel under clear water scour. Stage–discharge relationships were measured before the destruction of the armor layer and after the formation of a new armor layer at the riverbed surface. The results indicated that the impact of riverbed erosion on the stage–discharge relationship cannot be ignored. Under the same discharge, the flow depth in the main channel increased, while the flow depth in the floodplains decreased after the new armor layer was formed relative to the case before armor layer destruction. Considering the impacts of the erosion depth and the variation in the bed resistance, we proposed a new method for predicting the stage–discharge relationship in an erodible compound channel after riverbed erosion. The predicted stage–discharge relationships agreed with the measurements, indicating that the proposed method could be used to precisely predict the stage–discharge relationship after riverbed erosion occurrence. The comparison of the results of the proposed method with and without considering the erosion depth supported the idea that the impact of riverbed erosion on the stage–discharge relationship must be included. Finally, the proposed method was further used to examine the influence of floodplain encroachment on flood control and riverbed erosion. Floodplain encroachment produced a larger flood flow depth on floodplains and a higher mean velocity in the main channel, suggesting that floodplain encroachment should be avoided to eliminate possible increases in flood disasters and riverbed erosion risks.

Vegetation commonly exists in river confluences and alters flow structures, affecting the strength and direction of secondary flows. This study investigated the impact of vegetation on the secondary flow structures of a confluence by measuring three-dimensional instantaneous velocities under different vegetative conditions. By utilizing downstream and horizontal flow velocity characteristics, a secondary flow coefficient concept was introduced to describe secondary flow vorticity directions and strengths. The influence of vegetation was assessed in the context of Reynolds stress, the relative strength of secondary flows, and the direction of secondary flow vorticities to clarify the mechanisms by which vegetation affects secondary flows. The findings reveal opposing vertical forces in the confluent section due to incoming tributary flows, resulting in secondary flow generation. Vegetation redistributes cross-sectional water energy, increasing energy convergence within vegetated areas. This amplifies secondary flow strengths and alters vorticity directions. The impact of vegetation on secondary flows is evident in the increase in rotational intensity. Considering various factors, such as the confluence ratio, flow velocity, and distance from the confluence, is crucial, as the influence of vegetation on flows is multifaceted.

River width is a crucial parameter that correlates and reflects the hydrological, geomorphological, and ecological characteristics of the channel. However, the width data with high spatial resolution is limited owing to the difficulties in extracting channel width under complex and variable riverine surroundings. To address this issue, we aimed to develop an automatic framework specifically for delineating river channels and measuring the bankfull widths at small spatial intervals along the channel. The DeepLabV3+ Convolutional Neural Network (CNN) model was employed to accurately delineate channel boundaries and a Voronoi Diagram approach was complemented as the river width algorithm (RWA) to calculate river bankfull widths. The CNN model was trained by images across four river types and performed well with all the evaluating metrics (mIoU, Accuracy, F1-score, and Recall) higher than 0.97, referring to the accuracy over 97% in prediction. The RWA outperformed other existing river width calculation methods by showing lower errors. The application of the framework in the Lillooet River, Canada, presented the capacity of this methodology to obtain detailed distributions of hydraulic and hydrological parameters, including flow resistance, flow energy, and sediment transport capacity, based on high-resolution channel widths. Our work highlights the significant potential of the newly developed framework in acquiring high-resolution channel width information and characterizing fluvial dynamics based on these widths along river channels, which contributes to facilitating cost-effective integrated river management.

Aquatic vegetation provides ecological, hydrological, and aesthetic functions for rivers, and measuring velocity in vegetated channels is essential for river management. The paper presents a method for modeling the lateral distributions of depth-averaged velocities behind an emergent vegetation patch. Based on SKM, this approach divides the channel behind an emergent vegetation patch into pseudo-vegetation and free-flow regions, offering analytical solutions for the depth-averaged velocities in these two regions. The model incorporates several critical parameters, including the Darcy-Weisbach coefficient, lateral dimensionless eddy viscosity, drag force coefficient, and secondary flow coefficients. These coefficients are associated with bed friction, vegetationinduced resistance, secondary flow effects, and lateral momentum exchange, respectively, affecting the depthaveraged velocities. A comparison with published experimental data validates that the proposed model can predict the lateral distributions of depth-averaged velocities behind an emergent vegetation patch. A steady wake section exists behind an emergent vegetation patch for low-flow blockage. In the steady wake section, the secondary flow coefficients in pseudo-vegetation and free-flow regions remain relatively constant. Upon exiting the stable wake section, the secondary flow coefficient in the pseudo-vegetation region decreases with increasing distance from an emergent vegetation patch, and it in the free-flow region increases with increasing distance from an emergent vegetation patch. A sensitivity analysis of drag coefficient and secondary flow coefficients suggests that secondary flow coefficient in the free-flow region has a more significant effect on the lateral distributions of depth-averaged velocities compared to drag coefficient and secondary flow coefficient in the pseudo-vegetation region.

Natural rivers are basically sinuous. Many natural events and human activities can trigger sudden base-level fall in a natural river. To discuss the impacts of base-level falls on the flow motions and riverbed evolutions in sinuous channels, a generalized sinuous river model with a stepped-falling base level was built, and the cross-sectional instantaneous velocities, cross-sectional instantaneous water levels and terrain elevations were measured. The experimental results showed that for a small base-level fall of -0.50 <= Delta z(br) <0.00, the absolute lateral water surface slopes in the arc segments increase with falling base level, and the cross-sectional circulation current strengths in the downstream regions neighboring to the apex-sectional outer banks do not exhibit obvious changes. For a large base-level fall of -1.00 <= Delta z(br) <= -0.75, the riverbed erosional extents in the straight-segment mainstream belts are small, those in the downstream regions neighboring to the apex-sectional outer banks are large, and the cross-sectional circulation current strengths in the downstream regions neighboring to the apex-sectional outer banks are greatly influenced by the lateral bed surface morphologies. When the channel bottom is unexposed, the raw elevation probability density within a complete wavelength of the sinuous channel is bimodal, but the detrended elevation probability density is unimodal and positively skewed. In any region of the sinuous channel, the mean roughness and the mean influencing scale of the particles around the bed surface increase with falling base level, and those in the stream-wise orientation are apparently greater than the counterparts in the span-wise orientation.

To investigate the statistical roughness properties of gravel bed surfaces in meandering channels, a meandering channel model is constructed, the flowrate is selected as the control variable, and the cross-sectional water levels, cross-sectional velocities and bed surface elevations are measured. The experimental results show that the elevation probability distributions of the detrended water-worked bed surfaces in the meandering channel are positively skewed and leptokurtic. With the increasing flowrate, the elevation standard deviation and the elevation skewness increases, but the elevation kurtosis decreases. The armoring layer in the meandering channel can resist a certain extent of increasing flow strength, but its protective ability is very limited. Once the armoring layer is destructed, the bed surface roughness will increase significantly. The bed surface roughness and the averaged influence scale of the constituent grains are large in the main stream path, while those in the main stream separating areas are small and decrease along the longitudinal direction. For any local zone, the roughness and the averaged influence scale in the longitudinal direction are obviously larger compared with the corresponding values in the lateral direction.

为研究不同水流条件对弯曲河道河床演变的影响,文章开展连续弯道水槽试验,测量推移质输沙、水位、河床表面高程.结果表明:弯曲段床面的横向底坡存在折点,深槽随不同的横断面而异.同一流量下,横断面平均流速与弗罗德数变化趋势完全一致;随着流量增大,各个横断面的平均流速与弗罗德数也增大.在直线过渡段中间附近容易淤积,淤积程度随流量增大而加剧.床面高程阈值区间随流量增大而增大,床面高程由单一性变为多样性.研究成果对河道治理和改善航运有指导意义.

Meandering channels widely exist in natural rivers, especially in lowland alluvial plains. In this research, a meandering channel is designed, in order to explore the effect of sediment supply on flow characteristics in it. In this channel, the water level, three-dimensional instantaneous velocity, and riverbed elevation are measured by digital wave altimeter, three-dimensional acoustic Doppler velocimeter (ADV) and three-dimensional laser scanner, respectively; and the flow characteristics with (first case) and without sediment (second case) supply are compared. The results showed that under the same discharge, the transversal slope of water surface in the first case is larger than the second case, and its change is steeper; the longitudinal change degree of water surface in the upper half bend is small, and that is large in the lower half bend. By increasing the water discharge, the number of secondary flow vortices at the apex of meandering channel is increased. The trend of the two mainstream lines, with and without sediment supply, are roughly opposite, and the difference between them decreases with increasing the discharge. In addition, the lateral and vertical turbulence intensity of the flow near the outer bank are larger under the two cases. The turbulent kinetic energy of the cross-section is larger in the first case near the bed and water surface.

床面粗糙特性是河流动力学中的重要研究内容,其量化方法一直是研究的难点.在顺直水槽中采用非均匀天然沙铺制初始床面,并逐渐增大流量粗化床面,基于激光扫描获得的床面数字高程资料,应用统计理论以及变异函数模型探讨不同粗化程度的床面表层粗糙特性.结果表明:采用非均匀沙人工铺制的床面高程频率分布呈现出近似正态分布,经历过不同水流强度依次累积冲刷后形成的稳定粗化床面高程频率分布都出现了轻微正偏态的趋势;高程标准差随水流强度增加严格增大的特点让它可以成为量化床面粗糙特性的可靠指标;水流塑造试验床面是各向异性的,随着流量的增大,床面粗化层破坏再粗化,床面高程二维变异函数的基台值越大;由于水流的作用,横向剖面平均变异函数的基台值与块金值略大于纵向剖面高程平均变异函数,径向剖面的基台值与块金值远小于横向与纵向剖面.研究加深了对床面粗糙特性的统计规律的探索,对于建立基于统计学的粗糙度定量表达方式具有一定的启示.

河道中植被群的空间布局对植被群的上下游水流特性和河床演变均会产生重要影响.为研究河道中植被群的不同布局对水流结构的影响规律,设计了定床水槽试验,试验采用PVC圆柱模拟两个相同的刚性非淹没植被群,设置4组不同的上下游植被群间距,通过研究水流的流速和紊动强度分布,分析在不同的上下游植被群间距下植被群上游水流调整长度和尾流的形成与发展过程,并与单个植被对水流结构的影响进行对比.试验结果表明:上游植被群的存在会使下游植被群前端调整区长度缩短,此长度与植被群间距无关;随上游植被群的出现下游植被群的溢出流速、尾流稳定区长度、稳定区流速均会减小,其大小均与植被群间距有关,间距越小各值越小;同时下游植被群尾流形成区长度变短,随间距增大呈现先减小后增大的趋势;且下游植被群的尾流恢复速度随上游植被的出现变的更加迅速;上游植被群的存在加剧了下游植被群尾流紊动,其变化趋势也与植被群间距有关;植被群对水流既有抑制作用也有促进作用,主要取决于植被群前端紊动强度大小.试验探究了植被群不同纵向间距对河道水流结构的影响,为河道中植被群空间布局对水流特性的影响提供了补充.

A meandering channel model with variable-height sediment barriers at its outlet is designed to research the effect of base-level fall on riverbed evolution in a meandering riverway. The instantaneous flow velocity and water level data at 13 characteristic cross-sections and the digital elevations of bed surfaces were measured by an acoustic Doppler velocimeter and a digital water level altimeter and a handheld laser scanner, respectively. The experimental results showed that the static armoring layer protects the riverbed and resists the increasing flow intensity caused by a small extent of base-level fall. When the base-level falls to a larger extent, the riverbed will be eroded globally. But the erosion extent varies in local areas: the mainstream areas in the straight-line crossover segments are eroded slightly, while the mainstream head-on areas near the concave sides are eroded largely. The closer to the base level, the easier will the base-level fall enhance the flow intensity, and hence the easier the riverbed is to be eroded. When the base-level falls slightly and the flume bottom is not exposed, the elevation frequency distribution of bed surface in a complete meander is close to a normal distribution on the whole; but if we look into the details, the distribution presents a double peak. Double peak is specifically featured by the elevation frequency distribution of bed surface in a half meander; the double peak corresponding to a complete meander is merely a superposition generated by that of two half meanders. When the base level falls to the flume bottom, the flume bottoms near the mainstream head-on areas will be seriously exposed, and the elevation frequency distribution of bed surface in a complete meander also presents a double-peak characteristic, but its overall shape has obviously deviated from a normal distribution.

Braided rivers are widespread in nature, and their bed morphology is complex and variable. This paper aims to investigate and quantitatively analyze the bed surface roughness of braided rivers utilizing statistical theory. In this paper, a physical model of braided rivers is developed, and four constant discharge experiments are carried out. Based on Structure-from-Motion photogrammetry and direct measurement of bedload transport using a load cell, data on bedload transport rate, bed morphology, and bed elevation are obtained, facilitating the in-depth investigation of the correlations between these parameters. The results show that the morphological active width increases with increasing discharge. There was a significant positive correlation between the morphological active width and the bedload transport rate, although there is considerable scatter due to the inherent variability in braided river morphodynamics. The elevation probability distribution of bed surfaces shows negative skewness and leptokurtic distribution. There is a relatively significant correlation between skewness and the dimensionless bedload transport rate. The two-dimensional variogram values of bed elevation are variable, and the bed is anisotropic. Additionally, both the longitudinal sill and correlation length values exhibit an increase with the rise in stream power. Remarkably, the correlation between the dimensionless sill and correlation length, as well as the dimensionless bedload transport rate, proves to be highly significant. Consequently, this correlation can serve as a reliable general factor for predicting bedload transport rate in the reach.

This study focuses on the effects of vegetation patch density, bed condition, and incoming sediment on flow structure and bed morphology within and around a patch. The variation in upstream adjustment velocity is not well defined for low-density vegetation patches but decreases with increasing patch density in high-density patches. The length of the upstream adjustment region is greater for high-density vegetation. Incoming sediment causes a reduction in both the steady wake velocity and the length of the steady wake at a low density. The length of the recovery region increases with density when vegetation is sparse, but remains constant in a dense patch. Additionally, the length of the recovery region decreases due to incoming sediment. Turbulent kinetic energy is not affected by the bedform and incoming sediment when reaching its first peak. However, the second maximum of the turbulent kinetic energy increases when the bed is movable. The evolution of bed morphology is closely related to the flow structure and the growth of the von Karman vortex street. Both the rising length and the adjusted length decrease with increasing patch density, while the incoming sediment causes an increase in the adjusted length. Behind the patch wake, the first minimum elevation, maximum elevation, and second minimum elevation decrease as the patch density increases. These values, in turn, increase with the sediment supply upstream of the flume.

为探讨溢流坝壅水条件下的弯道水流特性,概化设计了一在出口处布设有溢流坝的连续弯道模型,利用声学多普勒流速仪(ADV)和数字波高仪测得模型特征断面的流速、水位数据.试验结果显示:弯段绝对水面横比降随相对坝高的增大而减小;弯段下半弯的水面纵比降为正,而从直线过渡段进口至下游邻近弯顶的水面纵比降为负,但其绝对水面纵比降均随相对坝高的增大而减小;连续弯道的直线过渡段也存在二次流且其旋转方向与上游弯段二次流的旋转方向相同;当相对坝高增大时,流道各断面的垂线平均纵向流速均不同程度的减小;当相对坝高增大时,各断面最大紊动能减小;纵向脉动对紊动能的贡献最大,横向脉动次之,垂向脉动最小;雷诺应力Ruv的正负能近似标示弯道直线过渡段进出口断面的横向水流运动,当雷诺应力Ruv为正时,流体将从左侧流向右侧;当雷诺应力Ruv为负时,流体将从右侧流向左侧.

Large particles strongly influence flow resistance, energy dissipation, bed stability, and channel morphology in mountain streams. We conducted flume experiments to evaluate effects of the density and spatial distribution of coarse particles (i.e., keystones) on bed stability, surface texture and sediment transport in steep channels. Keystones were placed on the channel bed surface at the beginning of each experiment, both in clustered and in uniformly spaced (i.e., anti-clustered) configurations and in different numbers (i.e., for different values of keystone density). The flow rate was increased by 20 % every hour and each experiment continued until the bed was completely scoured. Sediment transport was measured with a sediment trap located at the flume outlet. Topographic data from high-resolution surface imagery and second-order structure functions (SSF) of bed elevations, were used to characterize bed surface structuring while the Ripley's K function was used to assess the degree of clustering of keystones relative to random spatial distributions. Our results indicate that (1) the density and spatial distribution of keystones exert a minor control on the bed surface grain-size distribution (GSD), (2) for increasing flow rates, the spatial distribution of keystones naturally evolves towards a random distribution, regardless of the initial spatial arrangment, and (3) sediment transport has a higher correlation with the proportion of dislodged keystones than with flow discharge.

Sediment bed surfaces exist widely in natural rivers, and many aspects in river dynamics are closely relevant to bed surface roughness, such as flow structure, river resistance and sediment transport. As two important parameters for quantifying bed surface roughness, how average particle size and non-uniformity affect bed surface structure is unknown. Therefore, nine groups of sediment samples with different average particle sizes or different non-uniformities were firstly prepared by screening dry natural sediments. Then, the prepared sediment samples were used to manually pave nine groups of bed surfaces, and the high-precise bed surface digital elevations were obtained by a handheld 3D laser scanner. Finally, the effects exerted by the average particle size and non-uniformity on the bed surface fractal properties were discussed. The results showed that there is only a scale-free range in a profile or a two-dimensional specific direction of a bed surface with normal-distributed particle gradation. The averaged scale-free upper limit in the two-dimensional specific directions and that related to many profiles are less affected by the non-uniformity, but more affected by the average particle size. For the bed surfaces with the same non-uniformity, when the average particle size is smaller than 15 mm, the larger the average particle size is, the smaller the fractal dimension is, but the larger the scale coefficient is; when the average particle size is larger than 15 mm, the larger the average particle size is, the larger the fractal dimension and the scale coefficient are, while for the bed surfaces with the same average particle size, the non-uniformity has no significant effects on the fractal dimension and the scale coefficient. The averaged scale coefficient in the two-dimensional specific directions of an isotropic bed surface and that related to many profiles are approximately equal, but the averaged fractal dimension in the two-dimensional specific directions is obviously larger than that plus 1 related to many profiles.