Search

Channel dynamics is a significant geomorphological process that involves the channel migration, erosion-deposition and meandering of an alluvial river channel within its floodplain region. This geo...

A common phenomenon associated with the alluvial river is its meandering action which leads to lateral migration and thus geomorphic hazards. Predicting and preventing this migration is both difficult and necessary. Barak river is one of the highly meandering rivers flowing through the alluvial plains of Assam, India. It is observed that there is a regular shifting of the river banks and development of cutoffs which create uncertainty to the people residing nearby. Therefore, in this study planform geometry and migration behaviour of the Barak river is examined considering 12 representative meandering reaches using multiperiod Landsat remote sensing images, field investigations of channel bed and bank properties and riparian vegetation cover. An attempt is made to describe and evaluate the empirical approach and time sequence extrapolation method to predict channel migration. Channel wavelength to channel width ratio ranges between 5.53 to 12.9, and the bend curvature ranges between 1.1 to 3.93. Rate of river migration varies between 0.54to 85.69 m/year. Sinuosity in most of the meandering reaches is greater than 1.5. Results show that lack of significant riparian vegetative cover, high precipitation and presence of fine sands with very low clay content are probably the main elements responsible for the planform changes. The results of the prediction of meander migration obtained from selected empirical methods show that only Nanson and Hickin method shows a moderate correlation with R2 =0.50. Time sequence extrapolation method was used to predict the radius of the best-fit circle for the year 2025 and 2030. Results of the time sequence extrapolation method indicate that the maximum and minimum radius of the best circle fit for the year 2025 is 1254m at reach-8 and 462m at reach-7 respectively, and maximum and minimum radius of the best circle fit for the year 2030 is 1319m at reach-8 and 387m at reach-12 respectively. It is believed that the outcomes of this study could form a base in river training works and in understanding and predicting the future dynamics and bank migration of this alluvial river and other river of similar geomorphic setting.

Collapse sinkholes are common in areas where gypsum strata underlie rock that is incompetent to bridge the cavities created by the groundwater dissolution of gypsum. On floodplains these can be obscured due to fill by overbank deposits, and channels tend to migrate towards larger enclosed depressions. This study utilises GIS to analyse the influence exerted by small collapse sinkholes on the channel dynamics of the alluvial River Ure, UK. At Ripon Loop, a large compound meander bend, sinkhole area and channel dynamics have been tracked over >160 years from multiple datasets, including historic maps, aerial imagery, lidar and a Structure-from-Motion photogrammetry dataset acquired for this study. This indicated two distinct populations, firstly long-lived, constant-sized sinkholes and secondly dynamically growing holes aligned with the contact of bedrock and unconsolidated materials of a buried valley at depth where artesian water saturated with sulphate enters the latter. Frequent overbank flows of the River Ure across the neck of the bend have interacted with sinkholes, e.g. by deposition of bedload in the down-valley end of the holes and localised headward incision on the opposite side due to the relatively high gradient between holes and the downstream limb. The location of holes has conditioned the incision of small shallow channels over at least 7 years, before cutoff occurred over several floods in 2019. Initially overbank flow traversed the neck via the chain of sinkholes before widening and deepening of the channel captured all the surface flow of the River Ure, resulting in the mobilisation of more than 20,000 m3 of sediment. Allogenic influences on fluvial systems in relation to meander dynamics are often neglected and this study is the first to link floodplain sinkholes with detailed mechanisms of a chute cutoff, hereby indicating a potential feedback to sinkhole dissolution processes.

Alluvial rivers commonly are subject to the integrated effects of hydrodynamic and eco-geomorphologic controls and there has been a lack of suitable methods to measure the effects. By taking the alluvial reach of the Yellow River over the Yinchuan Plain as a suitable example, this study evaluates the degree of hydrodynamic control in the channel-form adjustment of different channel patterns in light of the advances in equilibrium theory governing alluvial channel flow. In response to the significant variations in flow regime and channel forms, the non-dimensional number (Formula presented.) that measures the equilibrium state of alluvial channel flow varies in the ranges of 0.23–0.65, 0.047–0.17 and 0.0012–0.0024 respectively in the anabranching, meandering and braided reaches during 1993–2015. The significant differences among the H-ranges are mainly because the individual anabranches have neither very narrow nor very deep cross-sections, while the single-thread channels in the meandering and braided reaches take moderately and significantly wider and shallower cross-sections, respectively. These results demonstrate that the (Formula presented.) number is a good discriminator of river channel patterns, and the relatively small variability of (Formula presented.) within each channel pattern implies that the channels in the study reach are resilient to significant change in flow regime but yet hydrodynamic control is only partial. While the (Formula presented.) number is capable of embodying the outcome of the complex integrated effects of multiple localized eco-geomorphic controls with flow dynamics, more studies are required to define its specific varying ranges for different river channel patterns and differing eco-geomorphic controls.

The paper presents recommendations for using the results obtained in sediment transport simulation and modeling of channel deformations in rivers. This work relates to the issues of empirical modeling of the water flow characteristics in natural riverbeds with a movable bottom (alluvial channels) which are extremely complex. The study shows that in the simulation of sediment transport and calculation of channel deformations in the rivers, it is expedient to use the calculation dependences of Chézy’s coefficient for assessing the roughness of the bottom sediment mixture, or the dependences of the form based on the field investigation data. Three models are most commonly used and based on the original formulas of Meyer-Peter and Müller (1948), Einstein (1950) and van Rijn (1984). This work deals with assessing the hydraulic resistance of the channel and improving the river sediment transport model in a simulation of riverbed transformation on the basis of previous research to verify it based on 296 field measurements on the Central-East European lowland rivers. The performed test calculations show that the modified van Rijn formula gives the best results from all the considered variants.

Recent work in the undeveloped Rio Murta Basin, located in Chilean Patagonia, identified an evolutionary trend in the fluvial system as it progresses toward and away from dynamic equilibrium. A location-for-time-substitution model employed over the longitudinal extent of a 16 km study site assessed the performance of extremal hypotheses in identifying dynamic equilibrium conditions. Numerous extremal hypotheses were successful in identifying the spatial trend, but no means were available to discern differences between them. Thus, this study uses field measurements within the evolutionary trend to propose a new metric for evaluating extremal hypotheses. A thorough review and synthesis of the extremal approach are additionally presented. The new method compares theoretical predictions against field-measured values to determine which extremal hypothesis is most effective in identifying the condition of dynamic equilibrium in a gravel-bed river. Channel width and depth are identified as the dependent stream variables that uniquely differentiate most extremal hypotheses from one another. The results indicate that extremal hypotheses based on energy metrics of the flow are most successful, with the strongest support for minimum kinetic energy and minimum specific stream power.

At the initial stage of dam operation in an alluvial river, significant vertical bed incision and transverse bank erosion processes always occur in the river reach downstream of the dam, with the latter being usually an integrated product of several interacting processes. A coupled two-dimensional (2D) model for the processes of bed evolution and bank erosion has been proposed in this study, which integrates a 2D morphodynamic module with a bank-erosion module and a groundwater flow module. In addition to the action of fluvial erosion, the effect of groundwater flow on bank erosion was considered, with the changes in pore water pressure and matrix suction being calculated. The proposed model was calibrated and validated through simulating channel evolution processes in a 30.54 km sub-reach of the Upper Jingjiang Reach of the Middle Yangtze River, with the effects of infiltration recharge on bank erosion and incoming flow on the central bar evolution being investigated. The results show that: (i) the proposed model had a good performance in reproducing flow and sediment factors, in terms of water depth, depth-averaged velocity and suspended sediment concentration, daily averaged discharge and river stage, and flow and sediment diversion ratios in local anabranching subreaches; (ii) the proposed model correctly calculated the bank erosion processes in the study reach, with the calculated bank retreat width being close to the measured value, however, it was not enough to fully reproduce the evolution of central bars, indicating that a specialized and improved module may be necessary for central bar evolution and its interaction with bank erosion; (iii) the groundwater level change lagged behind the river stage variation, and it showed a more obvious phase lag and a higher value at the same river stage (corresponding to a higher pore water pressure), when encountering with a more rapidly changing river stage; and (iv) the rate of infiltration recharge caused by rainfall process, would generally increase the groundwater level and thus the bank erosion degree in the study reach. In addition, the shifting of incoming main flow might take an obvious impact on the calculation of nearby central bar evolution.

In an alluvial river, various kinds of river regulation engineering are usually constructed along the reach for the stability of channel regime and the safety of flood control, which greatly affects the process of channel evolution. Therefore, a one-dimensional (1D) morphodynamic model has been improved to simulate the fluvial processes influenced by large-scale river regulation works. In the refined model, each node of a cross-section was labeled using a specified point code representing the zones of floodplain and main-channel with or without regulation engineering, and sediment transport and bed deformation in different zones were treated separately. In the zones without regulation engineering, both bed deposition and bed erosion were permitted to occur; in the zones with regulation works, bed deposition was allowed, while bed erosion was not permitted to occur owing to the restriction of those engineering, unless there was a deposition layer newly formed over the previous simulation period. Then the improved model was calibrated and verified using field observations through the application to a 347-km-long alluvial channel located in the Middle Yangtze River in China. The results for the year 2016 show that the channel scour volume without the effect of river regulation works was 9% greater than the measured value (34.43 × 106 t), while the outcome considering this effect (35.81 × 106 t) was in closer agreement with the measurements. Moreover, the variation in channel geometry obtained from the improved model agreed better with the observed data. River regulation works would limit the channel incision, and it is likely to scour the other unprotected bed of a cross-section or the downstream cross-sections as the flow cannot carry enough sediment from the protected bed. Simulated results indicate that part of the main-channel zone at the Jing65 section was protected, but the riverbed continued to incise by about 0.68 m in the protected area when the effect of bed-protection works was not considered in the model, which was not in accord with the observed channel geometry. Therefore, this effect of large-scale river regulation works on channel evolution needs to be considered for a higher simulation accuracy.

Sediment supplies are a fundamental component of alluvial river systems, but the importance of sustained supplies of externally derived sediments for the evolution of meandering planforms remains unclear. Here we demonstrate the importance of sediment supply in enhancing the growth of point bars that influence the rate of sinuosity increase through flow deflections in meander bends. We use an archive of Landsat images of 16 meandering reaches from across the Amazon Basin to show that rivers transporting larger sediment loads increase their sinuosity more rapidly than those carrying smaller loads. Sediment-rich rivers are dominated by downstream-rotating meanders that increase their sinuosity more rapidly than both extensional and upstream-rotating meanders. Downstream-rotating meanders appear to establish larger point bars that expand throughout the meander, in contrast to extensional meanders, which have smaller bars, and upstream rotating meanders, which are characterized by deposition over the bar head. These observations demonstrate that the size and position of point bars within meander bends influences flow routing and thus controls the dominant direction of meander growth. Rivers with low sediment supplies build smaller point bars, which reduces their capacity to increase meander curvature and the resulting sinuosity.

An alluvial river system mostly comprises of series of turns, loops or bends in its course. This study presents the experimental investigation of the turbulence structure at the centre of a sinuous...

Morphodynamic evolution in an alluvial river is usually controlled by various boundary conditions. During the past nearly 50 years, remarkable morphodynamic evolution occurred in the Jianli reach of the Middle Yangtze River owing to the combined effects of an artificial cutoff, the upstream operation of the Three Gorges Project (TGP), and the downstream confluence of the Dongting Lake. To better understand the characteristics of morphodynamic changes in the whole study reach, variations in thalweg shifting and bankfull channel geometry were quantified using a reach-averaged approach, and the effects of upstream and downstream controls were investigated on channel geometry adjustments. Calculated results indicate that: (i) events of an artificial cutoff and high flows caused the average rate of reach-scale thalweg migration to be greater than 35 m/yr, but there was a 22% reduction in the mean migration rate after the TGP operation; (ii) channel geometry adjusted mostly in the aspect of bankfull depth under various river regulation engineering, with the reach-scale bankfull depth increasing by 0.95 m from 2002 to 2016; (iii) the reach-scale bankfull dimensions are closely associated with these accumulated effects of both the altered flow-sediment regime because of upstream dam construction, and the local base-level variation owing to the downstream confluence of the Dongting Lake. Furthermore, these bankfull variables were expressed by power functions of two key parameters, covering the previous five-year average fluvial erosion intensity during flood seasons at Jianli (upstream control), and the corresponding water level difference between Jianli and Lianhuatang (downstream control). The proposed relations were calibrated by the observed data in 2002–2014, and were further verified by measurements in 2015–2016. The proposed methodology can also be applicable to estimate channel geometry adjustments of other similar alluvial rivers controlled by both upstream and downstream boundary conditions.

Based on the Late Paleozoic geological background and the latest exploration achievements of the Ordos Basin and North China platform, it is concluded that during the sedimentary period of Permian He 8 Member, the area in concern had multiple material sources, multiple river systems, flat terrain, shallow sedimentary water, widely distributed fluvial facies sand body and no continuous lake area, so alluvial river sedimentary system developed in the whole region. Based on stratigraphic correlation and division, and a large number of drilling and outcrop data, a comprehensive analysis of lithofacies and sedimentary facies types and distribution was carried out to reconstruct the ancient geographic pattern of the He 8 Member sedimentary period. The results of paleogeography restoration show that the area of Ordos Basin was the “runoff area” in the sedimentary slope in the western part of the North China platform during the sedimentary period of He 8 Member, the whole region was mainly alluvial plain sedimentation featuring alternate fluvial facies, flood plain facies and flood-plain lake facies. According to the results of flume deposition simulation experiment, a new sedimentary model of “alluvial river & flood-plain lake” is established, which reveals the genesis of large area gravel sand body in He 8 Member of this area and provides geological basis for the exploration of tight gas in the south of the basin. Key Words: Ordos Basin, He 8 Member, North China platform, paleogeographic restoration, alluvial river, flood-plain lake, sedimentary model, exploration significance

The investigation of river bed evolution is a principal part of fluvial morphology as it clarifies the procedure of bed deformation and morphological changes on alluvial river bed. Truth be told, the advancement of such bed landscape is additionally accompanied with the regular calamity like flood as it is anticipated due to immense precipitation in Himalayan foot slopes. The flood coverage area is expanding persistently while rainfall homogeneity is acceptable aside from extremely least number of years with sudden precipitation in most recent 101 years. The height of Jayanti riverbed had changed massively in most recent 101 years influencing the idea of channel planform accompanied by huge sediment aggradations and morphological changes which increases the expansion of flood water extension likelihood over bank. It has been discovered through the Fuzzy logic and PCA examination that precipitation does not assume a key job for flood water development yet dynamism of explicit river bed topography prompts bed rise that guarantees the flood propagation of the area unexpectedly.

Normal faulting acts as a forcing on the morphodynamics of alluvial rivers by changing the topographic gradient of the river valley and channel around the fault zone. Normal faulting affects river morphodynamics either instantaneously by surface rupturing earthquakes, or gradually by continuous vertical displacement. The morphodynamic responses to normal faulting range from longitudinal bed profile adjustments to channel pattern changes. However, the effect of faulting on river morphodynamics and morphology is complex, as they also depend on numerous local, non-tectonic characteristics of flow, river bed/bank composition and vegetation cover. Moreover, river response to faulting is often transient. Such time-dependent river response is important to consider when deriving relationships between faulting and river dynamics from a morphological and sedimentological record. To enhance our understanding of river response to tectonic faulting, we used the physics-based, two-dimensional morphodynamic model Nays2D to simulate the responses of a laboratory-scale alluvial river to various faulting and offset scenarios. Our model focusses on the morphodynamic responses at the scale of multiple meander bends around a normal fault zone. Channel sinuosity increases as the downstream meander bend expands as a result of the faulting-enhanced valley gradient, after which a chute cutoff reduces channel sinuosity to a new dynamic equilibrium that is generally higher than the pre-faulting sinuosity. Relative uplift of the downstream part of the river due to a fault leads to reduced fluvial activity upstream, caused by a backwater effect. The position along a meander bend at which faulting occurs has a profound influence on channel sinuosity; fault locations that enhance flow velocities over the point bar result in a faster sinuosity increase and subsequent chute cutoff than locations that cause increased flow velocity directed towards the outer floodplain. Our study shows that inclusion of process-based reasoning in the interpretation of geomorphological and sedimentological observations of fluvial response to faulting improves our understanding of the natural processes involved and, therefore, contributes to better prediction of faulting effects on river morphodynamics.

Quantifying the time scales of sediment transport and storage through river systems is fundamental for understanding weathering processes, biogeochemical cycling, and improving watershed management, but measuring sediment transit time is challenging. Here we provide the first systematic test of measuring cosmogenic meteoric Beryllium-10 (10Bem) in the sediment load of a large alluvial river to quantify sediment transit times. We take advantage of a natural experiment in the Rio Bermejo, a lowland alluvial river traversing the east Andean foreland basin in northern Argentina. This river has no tributaries along its trunk channel for nearly 1,300 km downstream from the mountain front. We sampled suspended sediment depth profiles along the channel and measured the concentrations of 10Bem in the chemically extracted grain coatings. We calculated depth-integrated 10Bem concentrations using sediment flux data and found that 10Bem concentrations increase 230% from upstream to downstream, indicating a mean total sediment transit time of 8.4 ± 2.2 kyr. Bulk sediment budget-based estimates of channel belt and fan storage times suggest that the 10Bem tracer records mixing of old and young sediment reservoirs. On a reach scale, 10Bem transit times are shorter where the channel is braided and superelevated above the floodplain, and longer where the channel is incised and meandering, suggesting that transit time is controlled by channel morphodynamics. This is the first systematic application of 10Bem as a sediment transit time tracer and highlights the method's potential for inferring sediment routing and storage dynamics in large river systems., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe; 1119

Common phenomena associated with the alluvial river is its ‘meandering’. Lateral migration of the Barak River creates geomorphic hazards in Assam, India. Predicting and preventing this migration are both difficult and necessary. In this article, we have tried to describe and evaluate the empirical approach and time sequence maps to predict meander migration. An empirical approach is based on correlations, while as, for a given meander, earlier observed movement is used in case of sequence method to predict the future migration. In this study, 12 meandering reaches of the Barak River are considered using multiperiod Landsat remote sensing images. In order to evaluate the accuracy of these methods, both predicted migrations, as well as measured migration, are compared. Results show that empirical methods are not precise and accurate, though some of the empirical methods are conservative, and some are unconservative. More accurate, precise information on the meandering movement is given by time sequence method to predict the radius of the best-fit circle of the future meander location.

The human pressure upon an alluvial river in the Mediterranean region has changed its riverine and deltaic landscapes. The river has been channelized in the last 70 years while the delta has been retreating for more than a century (a set of data unknown, so far). The paper concentrates on the fluvial component, trying to connect it to the delta evolution. Is the channelization responsible for the delta retreat? We develop a method to compute the actual bed load transport with real information of the past river morphology. The paper compares the computation with very limited measurements, among which are bulk volumes of trapped material at a modern, deep river mouth. The decrease in sediment availability in the last 30 km of the channelized river is deemed responsible for the decrease in the sediment yield to the delta. Moreover, power development and flood frequency should be responsible for a baseline delta retreat during the 20th century. The sediment trapping efficiency of dams is less important than the flow regulation by dams, in the annual sediment yield. Therefore, it is more effective to dismantle channelization than to pass sediment at dams, to provide sand to the beaches.

The width of river channels is controlled by the resistance to erosion of the most difficult to erode material lining the channel., One of the simplest questions in riverine science remains unanswered: “What determines the width of rivers?” While myriad environmental and geological factors have been proposed to control alluvial river size, no accepted theory exists to explain this fundamental characteristic of river systems. We combine analysis of a global dataset with a field study to support a simple hypothesis: River geometry adjusts to the threshold fluid entrainment stress of the most resistant material lining the channel. In addition, we demonstrate how changes in bank strength dictate planform morphology by exerting strong control on channel width. Our findings greatly extend the applicability of threshold channel theory, which was originally developed to explain straight gravel-bedded rivers with uniform grain size and stable banks. The parsimonious threshold-limiting channel model describes the average hydraulic state of natural rivers across a wide range of conditions and may find use in river management, stratigraphy, and planetary science.

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.

In-channel large wood (LW) plays an important role in the eco-morphological functionality of many river systems. This importance has been widely recognized, yet there continues to be a poor understanding of relationships between morphodynamics and locations of wood deposition within the channel, particularly in low-gradient, semi-confined rivers. This research investigates the following hypotheses: 1) LW deposition locations (DLs) occur periodically in relation to the periodic arrangement of morphological features in the Big River, Missouri, USA, a low-gradient, variably-confined, alluvial river system; 2) geomorphic controls on DLs in the Big River exert varying levels of influence at different spatial scales. A large-scale field inventory of LW DLs was performed along the Big River. A spectral analysis was then used to identify periodic patterns of DLs along the Big River and various statistical tests of association were used to investigate the relationships between DLs and morphological variables, and between periodicity (where identified) and morphological variables. The results suggest that under certain circumstances, DLs are spatially periodic, with periodicities ranging from 270 m to 1371 m, and in some cases exhibit periodicity at different spatial scales. Regression analysis was unable to statistically associate periodicity with morphological features; however, correlation and stepwise Poisson regression models suggest that channel-scale (100 m to 500 m) sinuosity, and valley width exert more influence on DLs than other variables. The lack of strong statistical associations suggests that either 1) LW dynamics in the Big River contain a high level of stochasticity or 2) controlling variables were not included in this analysis. These results support the need for better theoretical and numerical models of stochastic LW processes in order to better manage LW in complex geomorphic systems.

Previous studies have established that under natural conditions, alluvial river confluence zones experience channel scour followed by mid-channel bar development. Less attention is given to bed evolution downstream of large alluvial river confluences under engineered conditions, such as discharge regulation and levee confinement. Here we present four decades of morphologic changes along the 69-km uppermost Atchafalaya River, a downstream distributary of the confluence of the Mississippi River Outflow channel and the Red River. We aim to find the answer to one critical question: how does the channel bed downstream of an engineering-controlled confluence respond to regulated flow? We utilize single-beam bathymetry data collected in 1967, 1977, 1989, 1998, and 2006 to quantify riverbed deformation of the reach after the flow regulation began in 1963. Suspended sediment load and stream power during the four decadal periods are calculated. Results show that between 1967 and 2006, extensive bed degradation occurred and the average bed elevation reduced by 3.8 m. A total volume of 105 ± 26 × 106 m3 sediment was scoured from the uppermost Atchafalaya riverbed over this 40-year period, implying that channel erosion in this river reach has contributed to downstream coarse sediment delivery and associated deltaic growth in the Atchafalaya Bay. Bed aggradation only occurred during 1989–1998 in response to excess sediment input from the Mississippi River, likely attributable to the 1993 long-lasting Mississippi River flood. But the same amount of riverbed deposit eroded in the following eight years, demonstrating how quickly a large regulated river can diminish a perturbation owing to excess sediment input. These findings not only reveal the complexity of morphologic adjustments of a river channel in response to intensive engineering disturbances but also provide useful information for future modeling studies and management plans for the Atchafalaya and other large, engineered alluvial river confluences.

Dunes are critical for understanding riverine sediment transport, deposition, flow resistance and channel flow processes. Although previous studies have examined the riverbed micromorphology of the Lower Mississippi River in the USA, our knowledge of detailed quantification of dune morphology in this and other large alluvial rivers is still limited. It is also not well understood how dunes in a straight channel reach and a meander bend differ in their characteristics, as well as how dune morphology may have been affected by human activities (i.e., river engineering). In this study, we utilized multi-beam bathymetric measurements over four 1.6–8.0 km long reaches in the Lowermost Mississippi River (LmMR) to analyze riverbed micromorphologic features. Three of the four reaches were located in the upper part of the LmMR between river kilometers (RK) 474–477, 483.6–485.2 and 491.7–493.3, and the other reach was located in the lower part, between RK 120 and RK 128. We analyzed a total of 3258 dunes in these river reaches and found that large dunes were dominant in the LmMR. These dunes were characterized by low mean leeside slope angle (10.8°), indicating that flow resistance caused by dunes should be relatively small. When compared with dunes in the straight reaches, dunes in the meander bends were much larger (1.06 m vs. 0.81 m) and had a higher bed roughness (0.91 vs. 0.68), which may be related to the varied flow velocity. Dune size increased with increasing water depth across the river channel of a straight reach, while it decreased with increasing water depth across the river channel of a meander bend. When compared with the dunes in the lower-river reach, the dunes in the upper-river reach were significantly higher in height and shorter in wavelength, and showed much higher bed roughness (1 vs. 0.68), which may be closely related to the greater riverbed slope and grain size of bed sediment occurred in the upper-river reach, as well as a combined effect of the Old River Control Structure (RK 500) and backwater. These findings indicate the strong impact of turbulent flow, slope and sand source on dune formation and riverbed deposition.

Hydraulic geometry (HG) is an important geomorphic concept that has played an indispensable role in hydrological analyses, physical studies of streams, ecosystem and aquatic habitat studies, and sedimentology research. More than 60 years after Leopold and Maddock (1953) first introduced the concept of HG, researchers have still not uncovered the physical principles underlying HG behavior. One impediment is the complexity of the natural river cross section. The current study presents a new way to simplify the cross section, namely, the hydraulic equivalent section, which is generalized from the cross section in the “gradually varied flow of an alluvial river” (GVFAR) and features hydrodynamic properties and bed-building laws similar to those of the GVFAR. Energy balance was used to derive the stage Z-discharge Q relationship in the GVFAR. The GVFAR in the Songhua River and the Yangtze River were selected as examples. The data, including measured discharge, river width, water stage, water depth, wet area, and cross section, were collected from the hydrological yearbooks of typical hydrological stations on the Songhua River and the Yangtze River from 1955 to 1987. The relationships between stage Z-discharge Q and cross-sectional area A-stage Z at various stations were analyzed, and “at-a-station hydraulic geometry” (AHG) relationships were obtained in power-law forms. Based on derived results and observational data analysis, the Z-Q and Z-A relationships of AHG were similar to rectangular weir flows, thus the cross section of the GVFAR was generalized as a compound rectangular, hydraulic equivalent cross section. As to bed-building characteristics, the bankfull discharge method and the stage-discharge-relation method were used to calculate the dominant variables of the alluvial river. This hydraulic equivalent section has the same Z-Q relation, Z-A relation, dominant discharge, dominant river width, and dominant water depth as the cross section in the GVFAR. With the hydraulic equivalent section, the relationships between the discharge and river width, river depth, and width-to-depth ratio are easier to depict in DHG analysis. Replacement of the cross section in the GVFAR with the hydraulic equivalent section is expected to promote further development of HG.

Changes in channel morphology provide relevant insights into sediment transport and deposition in alluvial river systems. This study assessed three to four decades of morphological changes at seven...

Thalweg migration of an alluvial river plays a key role in channel evolution, which may influence the effect of existing river training works and biodiversity on floodplains, and cause losses in riparian land and property. The braided reach of the Lower Yellow River underwent continuous channel aggradation during the period from 1986 to 1999, and then remarkable channel degradation in 1999–2015 owing to the state of operation of the Xiaolangdi Reservoir in 1999. Here we quantify associated thalweg migration changes and identify the key influencing factor in the braided reach. Thalweg-migration distances and intensities at section- and reach-scales were calculated during the past 30 years from 1986 to 2015, in order to investigate the characteristics of thalweg migration in the reach. There was a 47% reduction in the reach-scale thalweg-migration distance and a 35% reduction in the corresponding migration intensity after the reservoir operation. It is also revealed that fluvial erosion intensity is a dominant factor in controlling the thalweg migration, based on the investigation into various influencing factors in the study reach. The thalweg-migration intensity of the braided reach can be expressed as a power function of the previous four-year average fluvial erosion intensity. The calculated thalweg-migration intensities in 1986–2015 using the proposed relation generally agree with the observed data. Copyright © 2017 John Wiley & Sons, Ltd.

The presence of bare or partially-covered rock in an otherwise alluvial river implies a downstream change in transport capacity relative to supply. Field investigations of this change and what causes it are lacking. We used two sets of magnet-tagged tracer clasts to investigate bedload transport during the same sequence of floods in fully alluvial, bare rock, and partial-cover reaches of an upland stream. High-flow shear stresses in different reaches were calculated using stage loggers. Tracers seeded in the upstream alluvial channel moved more slowly than elsewhere until the frontrunners reached bare rock and sped up. Tracers seeded on bare rock moved rapidly off it and accumulated just upstream from, and later in, a partial-cover zone with many boulders. The backwater effect of the boulder-rich zone is significant in reducing tracer mobility. Tracer movement over full or partial sediment cover was size selective but dispersion over bare rock was not. Along-channel changes in tracer mobility are interpreted in terms of measured differences in shear stress and estimated differences in threshold stress.

Maintenance of low flows during dry periods is critical for supporting ecosystem function in many rivers. Managed aquifer recharge is one method that can be used to augment low flows in rivers that are hydraulically connected to an alluvial groundwater system. In this study, we performed numerical modeling to evaluate a managed recharge operation designed to retime streamflow in the South Platte River, northeastern Colorado (USA). Modeling involved the simulation of spatially and temporally variable groundwater-surface water exchange, as well as streamflow routing in the river. Periodic solutions that incorporate seasonality were developed for two scenarios, a natural base case scenario and an active management scenario that included groundwater pumping and managed recharge. A framework was developed to compare the scenarios by analyzing changes in head-dependent inflows and outflows to/from the aquifer, which was used to interpret the simulated impacts on streamflow. The results clearly illustrate a retiming of streamflow. Groundwater pumping near the river during winter months causes a reduction in streamflow during those months. Delivery of the pumped water to recharge ponds, located further from the river, has the intended effect of augmenting streamflow during low-flow summer months. Higher streamflow is not limited to the target time period, however, which highlights an inefficiency of flow augmentation projects that rely on water retention in the subsurface.

This paper deals with impact of geomorphic responses such as thalweg shifting, sandbar instability, pool-riffle alteration, river bank erosion, channel incision river bed lowering and river bank erosion on riverine land cover dynamics arises by instream and floodplain sand mining in an alluvial reach as Kangsabati River from Mukutmonipur dam to Rajnagar confluence (193 km) during 2002–2016. Four segments i.e. Lalgarh, Mohanpur, Kapastikri and Rajnagar share as 554,656 m ton/year sand from 141 mining sites whereas others segment as Khatra, Raipur, Dherua and Panskura share only 33,497 m ton/year sand from 50 mining sites in the entire channel. Several maps were prepared to identify consequences of morphological responses with the help of field study and GIS technique. Friend and Sinha’s method (Geol Soc Lond 75(1):105–111, 1993) was used to detect planform change with geomorphic responses throughout the course. Pearson correlation matrix is used to establish the relation between geomorphic responses and land cover dynamics incorporates with mining intensity. The result shows that higher value of geomorphic responses decrease sandchar, riparian area but increases channel, mining, pits and clay cover in mining prone segments as Lalgarh, Mohanpur, Kapastikri, and Rajnagar. The lower value of geomorphic responses increase sandchar, riparian area but decreases channel, mining, pit sites and clay cover in sandchar segments as Khatra, Raipur, Dherua and Panskura, respectively. Therefore, sand mining greatly impact on riverine land covers dynamic following instable geomorphic responses. Moreover, this study reveals that sustainable sand mining incorporates with stable geomorphic responses maintain fluvial dynamics controlling of erosion and deposition process.

Numerous groynes had been constructed on the Dwarkeswar River to improve bank protection. Among them, groynes adjacent to the village Rautara of Khandaghosh Block, West Bengal have been studied which were constructed in the year of 2009. This study investigates the alteration of channel morphology, sedimentology and flow characteristics influenced by emerged groynes through micro-level field study. An extensive field survey has been made with a dumpy level and fifty-nine sediment samples were collected from the field. Different channel parameters such as degradation aggradation ratio, braided index, channel instability and bar occupied area indices have been used from 2003 to 2018. The simulation of channel flow has been done using HEC RAS. It has been found that width/depth (w/d) ratio, bankfull channel width, channel area and sand-bed length of the river decrease due to groyne construction. On the other hand, channel maximum bankfull channel depth, depositional rate, the difference between average depth, maximum depth and braided index drastically increased. Increasing flow velocity, flow deflection with coarse and poorly sorted multimodal sediments near the tip of the groynes as well as decreasing flow velocity, curve flow path, accelerated sedimentation with elevating river bed have been observed. Altogether result indicates that the emerged groynes are effective in protecting the river banks at the cost of channel degradation.

In this chapter, we discuss recent developments in paleoflood hydrology that are specific to the collection of high-resolution records from alluvial settings. We describe how to develop and analyze alluvial paleoflood records, including (i) the identification of suitable depositional niches in valley environments and the mechanisms of overbank deposition that lead to flood deposit accumulation at those locations, (ii) approaches for sample material collection and methods to measure the coarseness of individual flood units, (iii) data assessment and reconstruction of absolute and relative flood magnitudes from sedimentary information, and (iv) developing flood chronologies with a discussion of available dating techniques. We argue that alluvial paleoflood archives hold enormous potential for flood hazard assessments in densely populated low-lying areas despite the challenge of quantitative discharge reconstructions in dynamic floodplain settings.

Modelling stable alluvial river profile is one of the most important and challenging issues in river engineering that several studies have been dedicated to it. The main objective of this study is to evaluate the back propagation-based multilayer neural network (BP-MLNN) performance in predicting stable alluvial river profile. We used eighty-five observational datasets to train and test, three separate models to predict each of the channel width (w), flow depth (h) and longitudinal slope (s) of stable channels. The network input parameters are the flow discharge (Q), mean sediment size (d50) and affecting Shields parameter (τ*) and w, h and s parameters are the output. It is concluded from the results that the proposed models to predict the width, depth, and slope of stable channels, with a correlation coefficient (R) of 0.96, 0.886, and 0.870 respectively, perform well. The mean absolute relative error (MARE) value of 0.063 related to the width estimation model in comparison with the depth and slope estimation model with MARE value of 0.077 and 0.518 shows the superior accuracy of the BP-MLNN model. The presented BP-MLNN models in this study are therefore recommended in river engineering projects to estimate the cross-sectional dimensions of stable alluvial channels as simple and robust design tools.

An engineered alluvial river (i.e., a fixed-width channel) has constrained planform but is free to adjust channel slope and bed surface texture. These features are subject to controls: the hydrograph, sediment flux, and downstream base level. If the controls are sustained (or change slowly relative to the timescale of channel response), the channel ultimately achieves an equilibrium (or quasi-equilibrium) state. For brevity, we use the term “quasi-equilibrium” as a shorthand for both states. This quasi-equilibrium state is characterized by quasi-static and dynamic components, which define the characteristic timescale at which the dynamics of bed level average out. Although analytical models of quasi-equilibrium channel geometry in quasi-normal flow segments exist, rapid methods for determining the quasi-equilibrium geometry in backwater-dominated segments are still lacking. We show that, irrespective of its dynamics, the bed slope of a backwater or quasi-normal flow segment can be approximated as quasi-static (i.e., the static slope approximation). This approximation enables us to derive a rapid numerical space-marching solution of the quasi-static component for quasi-equilibrium channel geometry in both backwater and quasi-normal flow segments. A space-marching method means that the solution is found by stepping through space without the necessity of computing the transient phase. An additional numerical time stepping model describes the dynamic component of the quasi-equilibrium channel geometry. Tests of the two models against a backwater-Exner model confirm their validity. Our analysis validates previous studies in showing that the flow duration curve determines the quasi-static equilibrium profile, whereas the flow rate sequence governs the dynamic fluctuations.

Alluvial and transport-limited bedrock rivers constitute the majority of fluvial systems on Earth. Their long profiles hold clues to their present state and past evolution. We currently possess first-principles-based governing equations for flow, sediment transport, and channel morphodynamics in these systems, which we lack for detachment-limited bedrock rivers. Here we formally couple these equations for transport-limited gravel-bed river long-profile evolution. The result is a new predictive relationship whose functional form and parameters are grounded in theory and defined through experimental data. From this, we produce a power-law analytical solution and a finite-difference numerical solution to long-profile evolution. Steady-state channel concavity and steepness are diagnostic of external drivers: concavity decreases with increasing uplift rate, and steepness increases with an increasing sediment-to-water supply ratio. Constraining free parameters explains common observations of river form: to match observed channel concavities, gravel-sized sediments must weather and fine – typically rapidly – and valleys typically should widen gradually. To match the empirical square-root width–discharge scaling in equilibrium-width gravel-bed rivers, downstream fining must occur. The ability to assign a cause to such observations is the direct result of a deductive approach to developing equations for landscape evolution., Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe, 680

Detailed geomorphic evaluation, drill coring, and analysis of floodplain sediments, along with geophysical imaging, were used to study the evolution of the youngest terraces and palaeo-meanders in the lower reach of the Ohře River. Floodplain sediment dating was performed by optically simulated luminescence and via the examination of sediment contamination from historical mining of metals in the river catchment. Two river segments were studied. In the upper segment, the Žatec area, the trunk channel persistently incised from the Early Holocene to ca. 2 ka, and in historical times the meandering channel changed to a low sinuous channel in a narrow channel belt. The terraces and palaeo-meanders in the Žatec area are younger than indicated in the geological maps, by one to two orders of magnitude. In the lower segment, the Pisty area, 60 km downstream, approximately half of the sediments in the floodplain transect are the Early to Middle Holocene age and the other half are younger than approximately 1,000 years, in particular from the last five centuries. The deposits of the current channel belt in the Pisty area are coarser (mostly sandy in the top metre) than in the rest of the floodplain (mostly silty in the top metre), indicating that the river must have undergone a substantial transformation in the last few centuries. We hypothesise that the trigger of this change was upstream channel straightening and lateral stabilisation, which reduced the amount of river-transported materials. Although the river catchment has been subjected to anthropogenic activity (including intense agriculture in the lower reaches), the river does not show enhanced aggradation.

Many world's large rivers have experienced significant sediment reduction in the past century. However, channel aggradation near the mouth of these rivers may be still occurring or even accelerating, as evidenced by a recent study on the Lowermost Mississippi River. This study aims to investigate whether such a trend exists near the mouth of another large lowland alluvial river, the Atchafalaya River, which enters the Gulf of Mexico via two outlets — the natural mainstem and a man-made straight channel that is shorter and has a much greater slope. The overarching hypothesis is that, despite the reduction in riverine sediment delivery, lowland alluvial rivers such as the Lower Atchafalaya River in their backwater zone aggrade. We collected bathymetric survey records in 1977 and 2006 to analyze riverbed deformation. We utilized stage records at several gauging stations to determine the change in the hydraulic head over time. Geospatial techniques were used to develop channel bed Digital Elevation Models which were used to identify deformation of the 174-km channel bed of the Lower Atchafalaya River. We found a cumulative erosion of 6.34 × 107 m3 and a cumulative deposition of 8.79 × 107 m3, resulting in a net deposition of 2.45 × 107 m3 during the study period. While much of the river including the man-made channel was in a degrading state, the last 35 km of the river's natural mainstem experienced substantial aggradation. On average, riverbed elevation of the Lower Atchafalaya decreased by 1.47 m in the upper 98 km, but increased by 0.96 m in the final 35 km of the river's mainstem to the Gulf of Mexico. The channel erosion in the upstream reach will likely continue with the river discharge projected to increase, and the channel deposition in the downstream mainstem may accelerate because of sea-level rise.

We study cross-sections on the Detailed Geological Map of Poland (SMGP) to find a geologic and geomorphic pattern under river valleys in Poland. The pattern was found in 20 reaches of the largest Polish rivers (Odra, Warta, Vistula, Narew, and Bug) located in the European Lowland, in the landscape of old (Pleistocene, Saalian) glacial high plains extending between the Last Glacial Maximum (LGM) moraines on the North and the Upland on the South. The Upland was slightly folded and up-faulted during Alpine orogeny together with the thrust of Carpathian nappes and the uplift of Tatra Mts. and Sudetes. The found pattern is an alluvial river with broad Holocene floodplain and the channel developed atop the protrusion of bedrock (Jurassic, Cretaceous limestones, marlstones, sandstones) or non-alluvial, cohesive, overconsolidated sediments resistant to erosion (glacial tills, lacustrine or “ice-dammed lake” clays) of Cenozoic (Paleogene, Neogene, Quaternary—Elsterian). We regard the sub-alluvial protrusion as the limit of river incision and scour. It cannot be determined why the river flows atop these protrusions, in opposition to “differential erosion”, a geomorphology principle. We assume it is evidence of geological flood control. We propose an environmental and geomorphological framework for the hydrotechnical design of instream river training.

This paper assesses the variability and magnitude of flow regimes at ungauged sites of an Alluvial River Reach. A finite element numerical model (TELEMAC 2-D) was used to simulate the river hydrodynamics. The performance of the model is demonstrated by integrating it from January 1, 2001 to January 1, 2016 with a resolution of 0.22° × 0.18° covering the river domain. A sensitivity analysis was carried out to realise the effect of physical parameters along with the mesh resolution on the model output. The model was calibrated and validated using actual field measurements. The statistical results imply that the predictions of water depth and velocity at different sites in the river are in fine agreement with the observed values. Hence, the model succeeded in simulating the river flow for several periods for the reach.

This study analyzed bathymetric changes of the 77-km Yangtze River Estuary in China over the past ten years in order to understand the impacts of recent human activities on the estuary of a large alluvial river. Morphological changes were assessed by analyzing digitized bathymetric data of the estuarine channels from 2002 to 2013. Additionally, multi-beam bathymetric measurements made in 2012, 2014 and 2015 were utilized to investigate microtophographic bedforms of the lower reach of the estuary. Our results showed that the middle and upper reaches of the Yangtze River Estuary experienced substantial channel bed erosion in the past 10 years, and that the recent human activities have contributed to the change. These included the construction of a 70 km2 reservoir along the Yangtze River Estuary, the Qingcaosha Reservoir, for drinking water supply for the City of Shanghai, which has caused progressive bed erosion in the North Channel. The net volume of channel erosion in the Hengsha Passage from 2002 to 2013 was 0.86 × 108 m3. A large amount of the eroded sediment was trapped downstream, causing overall accretion in the upper reach of the North Passage. The middle and upper reaches of the South Passage also experienced intense erosion (0.45 × 108 m3) in the past ten years, while high accretion occurred in the lower reach because of the Deepening Waterway Project. The channel dredging left a large range of dredging marks and hollows in the North Passage. The increasing saltwater intrusion found in the Yangtze River Estuary may have been a consequence of either dredging or erosion, or both combined.

The profile of a river that conveys sediment without net deposition and net erosion is referred to as ‘graded’ with respect to vertical aggradation of the river segment. Three experimental series, designed in terms of the autostratigraphic view of alluvial grade, were conducted to clarify the diagnostic spatial behaviour of graded alluvial?deltaic rivers: an ‘R series’, which utilized a moving boundary setting with a stationary base level; an ‘F series’ in a fixed boundary setting with a stationary base level to produce ‘forced grade’; and an ‘M series’ in a moving boundary setting with constant base-level fall to produce ‘autogenic grade.’ The results of the three experimental series, combined with geometrical modelling of the effects of basin water depth and other experimental data, suggest the following: (i) in a graded alluvial?deltaic system, lateral shifting and avulsing of active distributary channels are suppressed regardless of whether the downstream boundary of the deltaic system is fixed; (ii) in a delta with a downstream-fixed boundary, the graded streams are stabilized within a valley that is incised in the axial part of the delta plain, whereby the alluvial plain outside the valley is abandoned and terraced; (iii) in moving boundary settings, the graded river simply extends basinward as a linearly elongated channel and lobe system without cutting a valley; and (iv) a modern forced-graded alluvial river is most likely to be found in a valley incised into a fan delta in front of very deep water, and the stratigraphic signal of fossil autogenic-graded rivers will be found in deltaic successions that accumulated in the outer to marginal areas of deltaic continental shelves during sea-level falls. This renewed autostratigraphic view of alluvial grade suggests a thorough reconsideration of the conventional understanding that an alluvial river feeding a progradational delta is graded with a stationary base level., Sedimentology, 63(7), pp.2158-2189; 2016

When sediment laden flow enters the broad areas from a narrow river channel, the flow velocity declines abruptly, due to the enlarging width. As a result, the sediment silts in the region. This phenomenon occurs when the sediment laden flow enters a plain from a mountainous river, or when it flows into lakes and oceans from rivers. The former condition results in the development of an alluvial fan, while the latter leads to the formation of a delta, and then again gradually evolved into an alluvial river which by the developing process of water flow and sediment transport, and it appears various pattern along with its growth. The river pattern can divide into many types, the classics types are straight channel, meandering channel, branching channel and braided channel. Model test is one of the most important technical means researching water flow and sediment transport and predicting riverbed evolution. At present, many scholars, domestic and overseas, excavate straight river in the foundation bed to research channel morphology changes, and do less research on non-straight river in the foundation channel morphological changes. The formation and development process of alluvial river is a continuous variation, the essay will mainly discuss the evolution change of river channel from nothing on the surface of rivers into the lake delta, under the constant coming water and sediment and constant erosion base level. In this study, in order to study the evolutionary changes of channel morphology in initial stage of water flow and sediment transport in shallow water, non-uniform sediment was used, for a model generalization test, under the conditions of a constant inflow water and sediment, gradient, as well as an outlet base level. The sediment test results showed that the evolutionary changes of channel morphology experienced three stages. At the first stage, straight channel and branching channel were appeared alternately. At the second stage, a single micro-bend channel morphology swing development on the surface of delta deposition occurred with the branching channel of tributary shunt gradually decrease and die. At the third stage, there’s mainly meandering channel occurred with the coefficient of twists become larger with the concave bank continuously scouring regressive in the micro-bend channel. Through analysis and experiment, results show that the channel morphology of the second stage and the third stage present as sine curve characteristics: the upper channel located in the inlet of the sine curve, the middle and lower of the bend channel are located in the peaks and troughs of sine curve. With time going on, the amplitude and direction of sine curve become large gradually, and the coefficient of channel tends to be stable. Finally, the evolutionary changes of channel morphology is analyzed from the bed load and boundary conditions. This study is of great theoretical significance, and the achievements will enrich the theory of river dynamics and riverbed evolution.

Channel morphology in an alluvial river usually varies due to the altered flow and sediment regime from upstream damming. This paper reports an evaluation of the dynamical changes of sedimentation and erosion in the middle and lower reaches of the Yangtze River after operation of the Three Gorges Dam (TGD). Here, we present the results from a case study of the Shashi Reach in the middle Yangtze River, which is the first sandy-bed and meandering reach downstream of TGD. Databases were constructed using a digital elevation model of channel topography based on the 1:10,000 topographic maps from the 1980s to 2012 and hydrological records from 1956 to 2013. Results indicate that the erosion in the Shashi Reach was mainly confined to the deeper channel and that it has increased since the construction of the TGD. No significant changes were observed above the bank-full level, resulting in the decrease of the width-to-depth ratio. These changes may be principally caused by variations of the seasonal distribution of flow and sediment due to the operation of the dam. In addition, results show that the cross-sectional shape change of the channel is related to the relative erodibility of the channel bed and bank material.

Observations from field investigations showed that flow velocity greater than 3 m/s rarely occurs in nature, and high flow velocity stresses the bio-community and causes instability to the channel. For alluvial rivers without strong human disturbance, the flow velocity varies within a limited range, generally below 3 m/s, while the discharge and wet area may vary in a range of several orders. This phenomenon was studied by analyzing hydrological data, including daily average discharge, stage, cross sections, and sediment concentration, collected from 25 stations on 20 rivers in China, including the Yangtze, Yellow, Songhua, Yalu, Daling, and Liaohe Rivers. The cross-sectional average velocity was calculated from the discharge and wet area using the continuity equation. For alluvial rivers, the wet cross section may self-adjust in accordance with the varying flow discharge so that the flow velocity does not exceed a limit value. In general, the average velocity increases with the discharge increase at low discharge. As the discharge exceeds the discharge capacity of the banks, any further increase in discharge does not result in a great increase in velocity. The average velocity approaches an upper limit as the discharge increases. This limit velocity, in most cases, is less than 3 m/s. Human activities, especially levee construction, disturb the limit velocity law for alluvial rivers. In these cases, the average velocity may be approximately equal to or higher than the limit velocity. The limit velocity law has profound morphological and ecological implications on alluvial rivers and requires further study. Rivers should be trained and managed by mimicking natural processes and meeting the limit velocity law, so as to maintain ecologically-sound and morphological stability.

There has been quite some debate on the relative importance of particle abrasion and grain size selective transport regarding the river profile form and the associated grain size trends in a graded alluvial stream. Here we present new theoretical equations for the graded alluvial river profile that account for the effects of particle abrasion and grain size selective transport in the absence of subsidence, uplift, and sea level change. Under graded conditions we find that abrasion results in a mild profile concavity and downstream fining, whereas under aggradational conditions grain size selective transport can lead to large spatial changes in channel slope and bed surface mean grain size.

Addition of large woody debris (LWD) to rivers has increasingly become a popular stream restoration strategy, particularly in river systems of the Midwestern United States. However, our knowledge of LWD dynamics is mostly limited to high gradient montane river systems, or coastal river systems. The LWD-related management of low-gradient, Midwestern river systems is thus largely based on higher gradient analogs of LWD dynamics. This research characterizes fluvial wood loads and investigates the relationships between fluvial wood, channel morphology, and sediment deposition in a relatively low-gradient, semiconfined, alluvial river. The LWD and channel morphology were surveyed at nine reaches along the Big River in southeastern Missouri to investigate those relationships in comparison to other regions. Wood loads in the Big River are low (3–114 m 3 /100 m) relative to those of higher gradient river systems of the Pacific Northwest, but high relative to lower-gradient river systems of the Eastern United States. Wood characteristics such as size and orientation suggest that the dominant LWD recruitment mechanism in the Big River is bank erosion. Also, ratios of wood geometry to channel geometry show that the Big River maintains a relatively high wood transport capacity for most of its length. Although LWD creates sites for sediment storage, the overall impact on reach-scale sediment storage in the Big River is low (

Se presenta un modelo agregado para estimar tasas medias de producción de sedimento en cuencas tropicales de montaña con información escasa, calibrado para Antioquia (Colombia); basado en la ecuación de transporte de Engelund-Hansen. Dos enfoques complementarios se proponen: (1) modelo de cauce aluvial, según caudal, pendiente y geometría hidráulica y (2) modelo de ladera, según caudal menos flujo base, pendiente, densidad de drenaje y grado de convergencia del flujo. Ambos modelos cierran con una regresión para el parámetro de Shields, en función del rendimiento sólido (t/km²/año) y un factor geológico, que se considera robusta por la diversidad de escala, morfología y clima de las 23 cuencas analizadas. Ambos enfoques del modelo predicen dentro de +/- 50% de error el rendimiento sólido del 70% de las cuencas, las de menos particularidades; corrigiéndolo con una porción por el fondo, adicional al transporte en suspensión medido, estimada analíticamente y probada con mediciones. An aggregated model to estimate mean denudation rates of tropical mountain basins with limited data is presented, which is calibrated for Antioquia (Colombia); based on the transport equation of Engelund-Hansen. Two complementary approaches are purposed: (1) alluvial river reach model, dependent on discharge, slope and hydraulic geometry and (2) hillslope model dependent on discharge minus base flow, slope, drainage density and flow convergence degree. Both models are closed with a regression for the Shields parameter, depending on sediment yield (t/km²/yr) and an erodibility factor, which is considered robust due to the diversity of scale, morphology and weather of the 23 basins analyzed. Both model approaches predicts inside a +/- 50% margin of error the sediment yields of 70% of the basins, those with less peculiarities; correcting it with a bed load portion, additional to the suspended transport measured, analytically estimated and proved with field data.

Spatial heterogeneity in the erosion-resistance properties of the channel banks and floodplains associated with sediment characteristics, vegetation, or bedrock can have a substantial influence on the morphodynamics of meandering rivers, resulting in highly variable rates of bank erosion and complex patterns of planform evolution. Although past studies have examined the spatial variability in bank erodibility within small rivers, this aspect of the erosion-resistance properties for large rivers remains poorly understood. Furthermore, with the exception of recent numerical modeling that incorporates stochastic variability of floodplain erosional resistance, most models of meandering river dynamics have assumed uniform erodibility of the bank and floodplain materials. The present paper investigates the lateral and vertical heterogeneity in bank material properties and riparian vegetation within two elongate meander loops on a large mixed bedrock-alluvial river using several geotechnical field and laboratory methods. Additionally, the bank stability and toe-erosion numerical model (BSTEM) and repeat terrestrial LiDAR surveys are used to evaluate the capacity of the bank material properties to modify the rates and mechanisms of bank retreat. Results show that the textural properties of the bank materials, soil cohesion, and critical shear stress necessary for sediment entrainment differ substantially between the two bends and are also highly variable within each bend — laterally and vertically. Trees growing along the banks increase the resistance to erosion by contributing to the shear strength of the bank materials and are capable of increasing bank stability along a large river. Locally outcropping bedrock also influences bank erodibility in both bends. The results of this study demonstrate that spatial variability in the erosion-resistance properties of the channel banks is an important factor contributing to spatial variability in the rates and mechanisms of bank erosion determined from short-term studies of bank retreat and long-term analysis of changes in channel planform.