Search

Rapid urbanization has an accelerated impact on catchment hydrology and geomorphology due to land use and land cover changes as a result of anthropogenic development activities in river catchment areas that cause increased surface runoff and higher sediment yield. The present study investigates the channel changes and sediment transport phenomena, including bed material movement from the upstream to the estuaries of the Pahang River Basin, Peninsula Malaysia. The river geometry survey associated with the spatial variation in sediment transport has confirmed that changes in river bed profile occur due to the erosion and sediment deposition along the Pahang River. The sediment size distribution for the Pahang River was very coarse sand and gravel after the December 2014 flood. The river geometry survey showed a gentle slope that can induce lower discharge typical of alluvial rivers relating to flood events. Quantitative assessment of sediment load is, therefore, of critical importance for future river basin management. [ABSTRACT FROM AUTHOR]

The battering of bedrock by bedload collisions is the primary mechanism by which bedrock rivers erode and landscapes evolve. The energy imparted via impacts acts to detach bedrock via the growth and intersection of surface fractures. We present impact experiments designed to test the influence of particle impact energy on bedrock erosion rates. We found that erosional efficiency increased with increasing impact energy. Notably, these increases in efficiency are not captured by a widely‐used mechanistic bedrock erosion model. Observed increases in erosional efficiency were linked with enhanced elastic energy dissipation captured by differences in the coefficient of restitution. We suggest that this increase in energy dissipation is indicative of enhanced crack extension for high velocity impacts. Our experiments indicate a clear energy‐dependence for bedrock detachment processes that is not yet captured by bedrock incision models but may be integrated into long‐term erosion rates and landscape evolution. Plain Language Summary: Bouncing sediment grains batter the bedrock beds of rivers during floods. As these impacts collide with the bed, they slowly chip away at the riverbed bedrock. It's perhaps intuitive that a grain with more energy when it collides with the riverbed is able to chip away more bedrock. But, how much more destructive are these high energy impacts? We designed an experiment where we hit a bedrock sample with a steel ball at a range of impact energies, over and over again, up to 30,000 times, and measured how much material was chipped away. High energy impacts eroded more material, but this increased efficiency of erosion is not fully captured by impact‐based erosion models. We hypothesize that for higher energy impacts, a larger fraction of the incoming energy is used to crack the bedrock, more easily eroding material. We liken the relative influence of these high energy impacts to a battering ram, where only a few strikes can easily break down a door, whereas lower energy impacts, similar to a simple knock, are unlikely to achieve the same result. We suggest that this effect may be important in determining the influence of extreme events on bedrock erosion over long timescales. Key Points: Impact experiments demonstrate that bedrock erosional efficiency is enhanced with increasing impact energyExperimentally‐derived coefficients of restitution and rock resistance coefficients indicate enhanced elastic energy dissipation with increasing impact energyResults suggest that more energetic impacts enhance crack extension, increasing erosional efficiency [ABSTRACT FROM AUTHOR]

River channels, riparian and floodplain forms and dynamics are all influenced strongly by biological processes. However, the influence of macroinvertebrates on entrainment and transport of river sediments remains poorly understood. We use an energy‐based approach to explore the capacity of benthic animals to move surficial, gravel‐bed particles in field and laboratory settings and use the results to assess the relative significance of biological and physical benthic processes. Our results showed that in 11 British gravel‐bed rivers, the maximum energy content (i.e., calorific content) of macroinvertebrate communities generally matched the flow energy associated with median discharges and, at multiple sites, exceeded that of the 10‐year return interval flood. A series of laboratory experiments used to estimate the minimum energy expended by signal crayfish (Pacifastacus leniusculus) when performing geomorphic work established that crayfish move gravel particles at energy levels below that expected of the flow, complicating direct comparisons of the capacity for macroinvertebrates and fluvial flows to influence bed mobility. Our findings suggest that the influence of macroinvertebrate communities in either promoting or suppressing, the mobilisation of the bed may be large compared to equivalent values of fluvial energy. Based on these findings, we conclude that in the gravel‐bed rivers studied, the macroinvertebrate community's potential to perform geomorphic work matches or exceeds the stream power during most of the year. Although our study examined biological and fluvial energy systems separately, it is important to recognise that in nature, these systems are highly interactive. It follows that utilising the energy framework presented in this paper could lead to rapid advances in both fluvial biogeomorphology and river management and restoration. [ABSTRACT FROM AUTHOR]

Natural channel design (NCD) and analytical channel design (ACD) are two competing approaches to stable channel design that share fundamental similarities in accounting for sediment transport processes with designs based on hybrid fluvial geomorphology and hydraulic engineering methods. In this paper, we highlight the linkage between ACD's capacity/supply ratio (CSR) and NCD's sediment capacity models (FLOWSED/POWERSED), illustrating how ACD and NCD have reached a point of convergent evolution within the stream restoration toolbox. We modified an existing CSR analytical spreadsheet tool which enabled us to predict relative channel stability using both conventional bed load transport equations and regional sediment regression curves. The stable channel design solutions based on measured data most closely matched the Parker (ACD) and/or Pagosa good/fair (NCD) relationships, which also showed the greatest CSR sensitivity in response to channel alterations. We found that CSR differences among the transport relationships became more extreme the further the design width deviated from the supply reach, suggesting that a stable upstream supply reach may serve as the best design analog. With this paper, we take a step toward resolving lingering controversy in the field of stream restoration, advancing the science and practice by reconciling key differences between ACD and NCD in the context of reach scale morphodynamics. [ABSTRACT FROM AUTHOR]

The Qinghai-Tibetan Plateau, known as the world's "third pole", is home to several large rivers in Asia. Its geomorphology is exceptionally vulnerable to climate change, which has had a significant impact on historical riverbed development through runoff and sedimentation processes. However, there is limited research combining climate change, sedimentology, and chronology with river dynamics to investigate riverbed evolution patterns in geological-historical time scales and their changes in overland flow capacity. In the current study, the evolution of a representative portion of the river channel in the Nangqian basin in the Lancang River headwaters was investigated to explore the reaction of the riverbed to climatic change during the geological period via field surveys, riverbed drilling, optically stimulated luminescence (OSL) dating and bankfull channel geometry parameters. The generalized channel section of the historical period was obtained by linking sedimentary layers of the same age on the distribution map of borehole sections, and the bankfull area of the river was computed accordingly. The restored bankfull areas can effectively reflect the ability of historical river channels to transport water and sediment, thus reflecting the climate change at that time. The findings showed that river morphology in the mounded river section could be successfully reconstructed using OSL dating and sedimentary records and that the conceptual sections of the historical warm periods at 2000 years (2 ka) and 0.7 ka can be recovered. Based on the reconstruction, the calculated bankfull areas during the two warm events were larger than present by factors of 1.28 and 1.9, respectively, indicating a stronger capacity for transporting water and sediments. This is the first trial in the Lancang headwaters to investigate the response of river morphology to climate change on a geological time scale. [ABSTRACT FROM AUTHOR]

Modeling network‐scale sediment (dis)connectivity and its response to anthropic pressures provides a baseline understanding of river processes and sediment dynamics that can be used to forecast future hydro‐morphological changes in river basins. However, this requires a solid understanding of how a system is currently operating, and how it operated in the past. We present the basin‐scale, dynamic sediment connectivity model D‐CASCADE, which combines concepts of network modeling with empirical sediment transport formulas to quantify spatiotemporal sediment (dis)connectivity in river networks. D‐CASCADE accounts for multiple factors affecting sediment transport, such as spatiotemporal variations in hydrological regime, different sediment grain sizes, sediment entrainment and deposition. Add‐ons are included in D‐CASCADE to model local changes in river geomorphology driven by sediment‐induced variations in features such as channel width. We apply D‐CASCADE to the well‐documented Bega River catchment, NSW, Australia, where significant geomorphic changes to rivers have occurred post European colonization (after 1850s), including widespread channel erosion and sediment mobilization. The Bega catchment provides a useful case study to test D‐CASCADE, as original source data on the historical sediment budget are available. By introducing historic drivers of change in the correct chronological sequence, the D‐CASCADE model successfully reproduced the timing and magnitude of major phases of sediment transport and associated channel adjustments over the last two centuries. With this confidence, we then ran the model to test how well it performs at estimating future trajectories of basin‐scale sediment transport and sediment budgets at the river reach scale. Key Points: We present the new dynamic D‐CASCADE model for network‐scale river sediment (dis)connectivity modelingD‐CASCADE successfully reproduced major historic geomorphological changes observed in the Bega river network after European settlementD‐CASCADE is used to test future trajectories of basin‐scale sediment transport and reach‐scale sediment budget in the Bega river system [ABSTRACT FROM AUTHOR]

The process of dam removal establishes the channel morphology that is later adjusted by high‐flow events. Generalities about process responses have been hypothesized, but broad applicability and details remain a research need. We completed laboratory experiments focused on understanding how processes occurring immediately after a sediment release upon dam removal or failure affect the downstream channel bed. Flume experiments tested three sediment mixtures at high and low flow rates. We measured changes in impounded sediment volume, downstream bed surface, and rates of deposition and erosion as the downstream bed adjusted. Results quantified the process responses and connected changes in downstream channel morphology to sediment composition, temporal variability in impounded sediment erosion, and spatial and temporal rates of bedload transport. Within gravel and sand sediments, the process response depended on sediment mobility. Dam removals at low flows created partial mobility with sands transporting as ripples over the gravel bed. In total, 37% of the reservoir eroded, and half the eroded sediment remained in the downstream reach. High flows generated full bed mobility, eroding sands and gravels into and through the downstream reach as 38% of the reservoir eroded. Although some sediment deposited, there was net erosion from the reach as a new, narrower channel eroded through the deposit. When silt was part of the sediment, the process response depended on how the flow rate influenced reservoir erosion rates. At low flows, reservoir erosion rates were initially low and the sediment partially exposed. The reduced sediment supply led to downstream bed erosion. Once reservoir erosion rates increased, sediment deposited downstream and a new channel eroded into the deposits. At high flows, eroded sediment temporarily deposited evenly over the downstream channel before eroding both the deposits and channel bed. At low flows, reservoir erosion was 17–18%, while at the high flow it was 31–41%. [ABSTRACT FROM AUTHOR]

Climate change is decreasing glacier cover and increasing the frequency and magnitude of precipitation‐driven high flows and floods in many regions of the world. Precipitation may become the dominant water source for river systems in recently deglaciated catchments, with major rainfall events driving significant changes in river channel morphology. Few studies, however, have examined river channel response to repeated precipitation‐driven high flows. In this study, we measured the geomorphological condition of four low‐order rivers in recently deglaciated catchments (70–210 years ice free) before and after a series of repeated precipitation‐driven high flows during summer 2014. High flows drove substantial initial morphological change, with up to 75% change in baseflow channel planform position and active channel form change from pre‐ to post‐high flow. Post‐high flow years were associated with increased instream wood and geomorphological complexity at all but the youngest river. Channel changes were part of an active relaxation stage at all rivers, where channels continued to migrate, and complexity varied through time. Overall, these measurements permit us to propose a conceptual model of the role of geomorphologically effective high flows in the context of paraglacial adjustment theory. Specifically, we suggest that older rivers in recently deglaciated catchments can undergo a short‐term (<10 years) increase in the rate of geomorphological development as a result of the recruitment of instream wood and channel migration during and following repeated precipitation‐driven high flows. Enhancing our knowledge of these geomorphological and paraglacial processes in response to high flows is important for the effective management of riverine water and ecosystem resources in rapidly changing environments. [ABSTRACT FROM AUTHOR]

River meander migration plays a key role in the unsteady "conveyor belt" of sediment redistribution from source to sink areas. The ubiquity of river meandering is evident from remotely sensed imagery, which has allowed for long‐term, high‐resolution studies of river channel change and form‐process relationships. Empirical, experimental, and theoretical research approaches have described two distinct relationships between channel curvature and river channel migration rates. In this study, we employ a novel application of time‐series algorithms to calculate migration rates and channel curvature at sub‐meander bend length scales using 6 decades of aerial imagery spanning 205 km of the Minnesota River and Root River, Minnesota, USA. Results from the Minnesota River provide the first empirical evidence demonstrating how migration‐curvature relations break down for rivers with low sediment supply, which is supported by the Root River data set. This not only highlights the importance of sediment supply as a driver of river migration, but also supports a simple means to detect river reaches lacking sediment supply. Furthermore, results from both rivers demonstrate that sub‐meander bend measurement scales are most appropriate for studying channel migration rates and further indicate that a quasi‐linear relationship—rather than the more commonly inferred peaked relationship—exists between channel curvature and migration rates. The highest migration rates are associated with the highest measured channel curvatures in our data set, after accounting for a spatial lag of 2.5±0.20.3 channel widths. These findings are consistent with flume experiments and empirical data across diverse geologic and climatic environments. Plain Language Summary: River meander migration plays a key role in the redistribution of sediment from uplands and headwaters to valley bottoms, lakes, reservoirs and the ocean. In this study, we measure both river migration rates and curvature at high resolution (every 10‐m) using six‐decades of aerial imagery spanning 205 river‐km of the Minnesota and Root rivers, Minnesota, USA. Our results demonstrate how differences in measurement scales led to different interpretations in previous research, and that high‐resolution measurements are most useful for understanding curvature‐migration relationships. The highest migration rates are associated with the highest measured channel curvatures, after accounting for a spatial lag of 2.5 ± 0.25 channel widths. These findings are consistent with flume experiments and empirical data across diverse geologic and climatic environments. Lastly, we demonstrate that curvature‐migration relations cease for rivers with low sediment supply. This finding highlights the importance of sediment supply as a driver of river migration and establishes a simple means of detecting river reaches lacking bedload sediment transport. Key Points: Migration rates are negligible and migration‐curvature relations break down along reaches with low sediment supplyCurvature‐migration relationships consistently peak at lag distances of ∼2.5 ± 0.25 channel‐widths regardless of climate and geologyResults support empirical, flume, and theoretical findings of a direct monotonic relation between channel curvature and migration rates [ABSTRACT FROM AUTHOR]

In this paper we use multiple field surveys spanning several decades to systematically evaluate the geomorphic consequences of a change in flow hydraulics from uniform flow to backwater flow for the lower Trinity River in east Texas, USA. Spatial changes in lateral migration rate, channel geometry, and point bar size correspond to two distinct geomorphic zones. Within the upstream uniform flow reach, the river channel is defined by fully developed point bars and a high rate of lateral channel migration. This zone transitions where the median channel bottom elevation drops below sea level. At this point flow is affected by the backwater influence of the Trinity Bay water surface elevation, as opposed to being bed slope control dominated. The change in hydraulics within the backwater zone is reflected in the channel morphology, which is characterized by smaller point bars, narrower and more symmetrical cross‐sectional channel geometry, lower channel migration rates, and little to no bend deformation or cutoffs. Studying the connection between channel geometry, river bend kinematics, sediment transport, and fluid mechanics in each zone provides a deeper understanding of the relationship between channel shape and river mechanics. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

The threshold stress for bed sediment transport exerts a primary control on the geometry and stability of coarse‐grained rivers (diameter ≥ 5 mm). Understanding how riverbed mobility couples to channel form is a key mechanistic link for predicting river response to external perturbations such as land use practices and changing climate. Unfortunately, determination of a representative threshold stress is notoriously difficult in the field. Empirical studies have observed that the critical dimensionless shear (Shields) stress (τ*c) is correlated with channel slope, a property that is substantially easier to estimate. Mechanistic models have been developed to explain the observed correlation; however, limited field data preclude the widespread application of these models. For practical reasons, the empirical regressions between slope and τ*c are utilized as predictive models. Through a large compilation of field data, we demonstrate that there are two significant problems with using the empirical regressions: (1) they are based on a partial sampling of the observed parameter space of coarse‐grained rivers, and (2) they do not capture the covariation between the bankfull Shields stress (τ*bf) and τ*c. These regressions provide spurious predictions for the bankfull transport capacity (τ*bf/τ*c) of gravel‐bed rivers. When site‐specific empirical measurements of τ*c are made, coarse‐grained rivers exhibit a remarkably constant transport capacity that is in close agreement with equilibrium channel theory (τ*bf= 1.2τ*c). From these data we advocate that, in the absence of measurements, τ*c can be reasonably estimated from the τ*bf using equilibrium channel theory. Key Points: In natural rivers critical Shields stress covaries with bankfull Shields stress in a manner predicted by theoryEmpirical relations between slope and critical Shields stress are based on a partial sample of the known parameter space for gravel riversPredictions of changes in bankfull transport capacity based on the correlation between slope and the threshold of motion can be spurious [ABSTRACT FROM AUTHOR]

Sediment in urban stormwater systems creates a significant maintenance burden, while a lack of coarse‐grained bed sediment in streams limits their ecological value and geomorphic resilience. Gravel substrates, for example, provide benthic habitat yet are often scoured from the channel bed only to end up in a detention basin or treatment wetland. This dual problem of both 'too much' and 'too little' coarse‐grained sediment reflects a watershed sediment budget that is profoundly altered. We developed a conceptual urban coarse‐grained (>0.5 mm) sediment budget across three domains: hillslopes (urban land surfaces), the built stormwater network and stream channels. We then quantified key sources, sinks and storages for a suburban case study, using a combination of hillslope and in‐channel monitoring, and interrogation of local government records. Around 36% of the sediment supplied to the stormwater network reached the catchment outlet, a level of sediment delivery much higher than observed in similar‐sized natural catchments. The remainder was deposited in the sediment cascade and either stored, or extracted and removed from the catchment (e.g. material deposited in sediment ponds and gross pollutant traps). Conventional urban drainage networks are characterized by high hillslope sediment supply and low storage, resulting in efficient sediment delivery. Channel erosion, deposition in (and extraction from) pipes and channels, and floodplain deposition are small compared to sediment transport through the cascade. An understanding of the sediment budget of urban headwater catchments can provide stormwater and waterway managers with the information they need to address specific sediment problems such as sedimentation in stormwater assets and geomorphic recovery of urban streams. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Caddisfly (Trichoptera) larvae are an abundant and widespread aquatic insect group characterized by the construction of silk structures, including nets and cases. Case‐building caddisfly have the potential to modify the sorting and mobility of sand and fine gravel via: (1) case construction, resulting in altered sediment properties; (2) transporting sediment incorporated into cases over the river bed; and (3) changing the structure of river beds via burrowing activity. To investigate these mechanisms, it is necessary to understand the mass, size distribution and spatial variability of sediment use by case‐building caddisfly larvae. We quantified the mineral sediment used by individuals and communities of case‐building caddisfly in 27 samples, from three sites on a gravel‐bed stream. The mass and size distribution of sediment in individual cases varied between taxa (mass = 0.001–0.83 g, D50 = 0.17–4 mm). The mean mass of sediment used by the caddisfly community was 38 g m−2 and varied locally. Sediment use was predominantly coarse sand (D50 = 1 mm). 64% of sediment use was attributable to Agapetus fuscipes (Glossosomatidae). Due to within‐species variability in case mass, the abundance of most taxa, including A. fuscipes, was only weakly associated with the mass of sediment used by this species, at the river scale. Whilst the caddisfly community used a small percentage of the total sediment available (average 2.99% of the 1–1.4 mm size fraction), A. fuscipes used more fine sediment in their cases at sites where it was more available. Despite variability in local habitat, all sites supported diverse case‐building caddisfly communities utilizing mineral sediment. Consequently, geomorphological effects of case‐building caddisfly are potentially widespread. The results provide novel insights into the specific grain sizes and quantities of fine sediment used by caddisfly larvae, which represents an important step towards understanding their zoogeomorphic activities. © 2019 The Authors. Earth Surface Processes and Landforms Published by John Wiley & Sons Ltd. [ABSTRACT FROM AUTHOR]

Understanding river response to sediment pulses is a fundamental problem in geomorphic process studies, with myriad implications for river management. However, because large sediment pulses are rare and usually unanticipated, they are seldom studied at field scale. We examine fluvial response to a massive (~20 Mt) sediment pulse released by the largest dam removal globally, on the Elwha River, Washington, United States, in an 11‐year before‐after/control‐impact study of channel morphology and grain size. We test the hypothesis that for a given flow magnitude, greater geomorphic change occurs under sediment‐rich conditions than under sediment‐starved conditions. Channel response to flow forcing was significantly different during the sediment‐pulse peak, 1–2 years after dam removal began, than earlier or later. During peak sediment supply our hypothesis was supported; major geomorphic change occurred under low flows and unit stream power ≤60 W/m2. However, by 4–6 years after dam removal began, rates of geomorphic change and sensitivity to stream power had decreased substantially such that our hypothesis was no longer unequivocally supported. These findings are consistent with a two‐phase conceptual model of dam‐removal response, involving a transport‐limited state followed by a more supply‐limited state. From comparisons with other dam removals and natural sediment pulses, we infer that the longevity of sediment‐pulse signals in gravel‐bed rivers depends upon gradient, river discharge, valley morphology, and sediment grain size. Stream power associated with substantial geomorphic change varies with sediment supply, such that assigning a general threshold stream power to gravel‐bed rivers may be untenable. Plain Language Summary: In this 11‐year study we investigated sedimentary and geomorphic response to a massive sediment pulse released by the largest dam removal globally, on the Elwha River, Washington, United States. In the most detailed look thus far at river response to a large dam removal, we show that the major response was brief, lasting 5 months, and that the river recovered quickly after this large disturbance. We conclude that the amount of change that happens in a river channel for a certain sized flood depends on how much sediment the river is carrying, as well as on its recent flow history preceding that flood. Key Points: We present a high‐resolution evaluation of sedimentary and geomorphic response to the largest dam removal thus farThe peak magnitude of dam‐removal response was brief (5 months), as the sediment pulse was exported efficiently from the riverStream power capable of causing major geomorphic change is sediment‐supply dependent [ABSTRACT FROM AUTHOR]

This study presents an analysis of variable sediment erosion and deposition, and their the complex mechanism in natural heavy silt-carrying rivers in the Ningxia Plain Reaches of the Yellow River (NPRYR). The aims of this study are to examine the sediment scouring and siltation laws, along with the evolution trend of fluvial facies in the NPRYR. Four small-scale typical reaches (R 1 , R 2 , R 3 , and R 4 ) were divided firstly based on river regimes and geological conditions of the river beds. And then with ground-truth data from 1993 to 2015, a cross-section morphology method was used to calculate the average fluvial facies coefficient ζ a of the typical river reaches, and also the sediment siltation weight T during the different time periods. In this study, with application of mathematical statistics and regression analysis, the main channel, beach, and whole reach were used as the basic units in order to identify the relationships of the responses among the suspended sediment concentration per unit discharge ( ξ ), sediment deposition per unit water weight ( W ), and variations of the fluvial facies coefficient (△ ζ a ) in different units. This analysis is for establishing the prediction method for the sediment erosion and deposition, along with the evolution trends of fluvial facies. Our results show a logarithmic mapping relationship between the ξ and W , and significant correlation between T and △ ζ a . Therefore, a quick analysis and prediction of the silt sedimentation, and the trends of fluvial facies variations in the small-scale river reaches under a single water and sediment condition are feasible with our approaches presented here. The critical incoming sediment coefficient ξ cs for the scouring and siltation balance in the R 1 to R 3 reaches were 0.0051 kg·s·m −6 , 0.0055 kg·s·m −6 , and 0.0049 kg·s·m −6 , respectively, while the R 4 reach shows an overall continuous siltation trend. The fluvial facies coefficient of the R 1 to R 4 reaches show an overall decreasing trend. Our results provided robust theoretical support for analyzing the sediment erosion and siltation, and the river regime evolution in the NPRYR. Our study also indicates a need for future research with an aim to further understand the evolution of erosion and siltation in heavy silt-carrying rivers. [ABSTRACT FROM AUTHOR]

Abstract: River restoration works often include measures to promote morphological diversity and enhance habitat suitability. One of these measures is the creation of macro‐roughness elements, such as lateral cavities and embayments, in the banks of channelized rivers. However, in flows that are heavily charged with fine sediments in suspension, such as glacier‐fed streams and very low‐gradient reaches of large catchment rivers, these lateral cavities may trap these sediments. Consequently, the morphological changes may be affected, and the functionality of the restoration interventions may be compromised. Herein, we analyse the influence of these macro‐roughness elements on the transport of fine sediments in the main channel. Laboratory tests with uniform flow charged with sediments in a channel with banks equipped with large‐scale rectangular roughness elements were carried out. The laboratory experiments covered a wide range of rectangular cavity geometrical configurations and shallowness ratios. The influence of key parameters such as flow shallowness, geometric ratios of the cavities and initial sediment concentration was tested. Surface particle image velocimetry, sediment samples and temporal turbidity records were collected during the experiments. The amount of sediments captured by the cavities, the temporal evolution of the concentration of sediments in suspension and the flow hydrodynamics are cross‐analysed and discussed. It is shown that the trapping efficiency of the macro‐roughness elements is a clear function of the channel geometry and the shallowness of the flow. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Quantifying sediment transport in small mountain basins is of great relevance to assess the morphological and ecological dynamics of the entire channel network and to predict flood hazards. In high-elevation, glacierized basins, seasonal variability in sediment transport is dramatic, but despite the relevance of such basins in many regions worldwide, very few investigations have tried to quantify it. Since direct methods to assess bedload transport are time consuming and practically challenging at high flows, indirect surrogate methods, allowing continuous measurements over time, are highly desirable. Yet, these methods require calibration to provide reliable estimations. The present research is focused on the calibration of an acoustic pipe sensor in the recently established (Spring 2011) monitoring station in the Saldur basin, a high-elevation glacierized watershed in the Eastern Italian Alps. The acoustic pipe signal (which is amplified through 6 channels having different gains) was calibrated against samples collected over 26 sampling periods using "Bunte" bedload traps along a cross-section 12m upstream of the pipe. Samples were collected from June to August 2011 during daily discharge fluctuations (ranging from 1.40 to 3.63m³s-1) due to snow- and glacier-melt, featuring very different bedload rates (up to 0.14kgs-1m-1). In order to calibrate the pipe sensor signal, the average number of impulses was plotted against the corresponding unit bedload rates for the associated bedload sampling periods. As expected, the signal from the two most sensitive channels of the acoustic sensor resulted dampened even at low discharges, and thus could not be used for calibration and bedload assessment. Instead, power laws (R² from 0.76 to 0.92) relating the number of impulses per minute to unit bedload rate were obtained using channels having intermediate and low sensitivities, with higher correlations associated with the less sensitive channels. [ABSTRACT FROM AUTHOR]

This paper describes an approach to assess channel changes of a long anthropogenised river (the Middle Loire River) over decadal timescales. Channel changes are evaluated along geomorphically homogeneous river reaches. The classic geomorphic parameters (active channel width, bed slope, grain size) are complemented with parameters extracted from a 1D hydraulic model: width-depth ratio, effective bed shear stress and specific stream power calculated for the biennial discharge assimilated to bankfull flow conditions. The delineation of reaches is undertaken by combining visual inspection with the implementation of simple statistical tests to corroborate discontinuities in flow and sediment transport. The 450km long study area has been divided into 167 homogeneous reaches. The general trends observed over the last fifty years are narrowing of the active channel width and incision of the river bed. However, changes in bed level and active channel width are not consistently correlated. Channel changes at the reach scale are mainly controlled by the presence of former sediment extraction sites. Significant incision is observed at the peak of the in-stream sediment mining period. This was followed by channel recovery when extractions stopped. The 1D numerical model provides a more rigorous manner to derive hydraulic parameters. The effective bed shear stress made dimensionless by its critical value and the width-depth ratio helps to explain observed channel responses more effectively by relating patterns to geological units along the Middle Loire. [ABSTRACT FROM AUTHOR]

Abstract: River bank erosion is the main process in small mountainous basins, especially those where the main channel is built of poorly resistant sedimentary rock or alluvium, exemplified by the Słomka basin. The main goal of the study, based on the field survey work and the erosion pin method, was to define the relation between river bank erosion, accumulation, and sediment transport. The investigations were carried out in the Słomka stream basin (watercourse length of 25.2km, basin area of 69.9km2) on an ~1.0-km-long stream reach, strongly anthropogenically transformed, between watercourse kilometres 5.0 and 6.0. We found that with an increase in flow the retreat bank rate increased nonlinearly. An average migration rate of the undercut banks ranged from 0.10m for the flow rate of 11.5m3 s−1 to 0.92m for the flow rate of 35.5m3 s−1. The maximum observed rate exceeded 0.8m for the flow rate of 11.5m3 s−1 and 9.0m for the flow rate of 24.8m3 s−1. With respect to the channel vertical morphology over the whole measurement section, we found that bed gradeline was raised, on average, by ~0.04m. The trend is contrary to the results of field surveys performed earlier on the Carpathian watercourses. The phenomenon is related to the abundant supply of rock rubble into the stream channel. High flow stages in the Słomka stream, which caused the stream bed material to move, had a predominant impact on stream channel morphology. Processes such as destructive activity of rain drops, aeolian erosion (sandblasting), soil expansion, and shrinkage during the rainy and dry seasons were of minor significance. [Copyright &y& Elsevier]

Abstract: In the last decade, as computing power has increased, there has been an explosion in the use of eddy-resolving numerical methods in the engineering, earth and environmental sciences. For complex geomorphic flows, where accurate field investigations are difficult to perform and where experiments may be difficult to scale, these numerical approaches are beginning to give key insights into the nature of these flows. Eddy-resolving methods such as Large and Detached Eddy Simulation (LES/DES) may be contrasted with the time-averaged, three-dimensional simulations that only really began to be applied seriously in geomorphology fifteen years ago. While the potential of LES for geomorphology has been examined previously, DES is a relatively recent method that deserves further consideration. In this paper, we explain the method and then utilise examples from meander and confluence flows, as well as flow near the bed of a gravel bed river, to highlight the improvements to both the representation of the mean flow, and to the representation of time-varying processes, that result from the use of LES/DES. Some suggestions are provided for the future use of such techniques in geomorphology. [Copyright &y& Elsevier]

Information about the grain-size distribution of the surface layer of sediment exposed on riverbeds is often critical in studies of fluvial hydraulics, geomorphology, and ecology. A variety of sampling and analysis techniques are in common usage that produce grain-size distributions that are not directly comparable. This paper seeks to explore the appropriate conversions between different types of surface grain-size sampling methods. This is particularly timely in the light of increasingly widespread use of automatic and semiautomatic image-based measurement methods, the comparability of which with conventional measurement methods is relatively poorly constrained. For conversions between area-by-number (paint-and-pick) and grid-by-number (pebble-count) samples, the empirically derived conversion factor () was found to be greater than that predicted by the Kellerhals and Bray model (), but the errors associated with using the value predicted by the model were small (3.8% in mm). For conversions between areal samples recorded by count and weight, the empirically derived conversion factor was approximately , but the use of the value predicted by the Kellerhals and Bray model () resulted in only small errors (5.2% in mm). Similarly, for conversions between image-based grain-size distributions recorded in area-by-number and grid-by number form, the emipirically derived conversion factor was , but using the model value of resulted in only small errors (4.1% in mm). Although these results are specific to the data sets analyzed, the variety of sedimentary conditions included gives the authors of this study confidence that the results are representative. [ABSTRACT FROM AUTHOR]

Substantial research on how hydraulic and géomorphologie factors control hyporheic exchange has resulted in reasonable process understanding; however, the role of fluvial islands on the transient nature of spatial flux patterns remains elusive. We used detailed field observations of the Truckee River, Nevada from 2003 to 2009 to quantify fluid flux between the river and a fluvial island, the streambed, and the adjacent stream bank. We constructed a 3-D numerical flow and heat transport model to further quantify the complex flow paths. Our study expands on previous research typically confined to less comprehensive scales and dimensions, and highlights the transient multidimensionality of the flow field. In fact, 1-D vertical streambed flux estimates indicated that the channel bar tail displayed the highest upward flux throughout the summer; however, 3-D model results indicated that the horizontal contribution was two orders of magnitude higher than the vertical contribution. The channel bar net flux is typically 1.5 orders of magnitude greater than the adjacent stream banks and an order of magnitude less than net streambed fluxes, indicating significant differences in river-aquifer interactions between each of the geomorphic units. Modeling simulations further indicated that the channel bar induces 6 times more fluid flux than an identical location without a fluvial island, consistent with flux estimates from a nearby river restoration location. Moreover, event-based and seasonal transient antecedent moisture and near-stream storage conditions contribute to multidimensional river-groundwater interactions. These results suggest that fluvial islands are a key driver and significant component of river-groundwater interactions and hyporheic flow. [ABSTRACT FROM AUTHOR]

The linear stability of a uniform flow in an infinitely wide erodible channel is investigated with respect to disturbances of the bed that are periodic in both the transverse and the longitudinal directions. A rotational flow and sediment transport model, originally developed to study the formation of two-dimensional dunes and antidunes, is straightforwardly extended to cover variations in the lateral direction. Sediment is assumed to be transported as bed load, disregarding the role of suspension. Following a standard linearization procedure, a dispersion relationship is obtained that expresses the growth rate and the celerity of the sand wave as a function of the streamwise and spanwise wavenumbers and of the relevant flow and sediment parameters. Regions of instabilities in the space of the parameters are found, which can be associated with bed forms of different kinds, spanning from dunes and antidunes to alternate bars. Therefore, the present theory allows for a unified view of the formation of two- and three-dimensional bed forms in rivers in terms of the relevant flow and sediment parameters. [ABSTRACT FROM PUBLISHER]

Abstract: Riverine assessments of channel morphology and fluvial processes typically involve a time-consuming survey of channel cross sections and longitudinal profiles, expensive aerial imagery, and/or require specialised knowledge. As a result, geomorphic input is not widely included in multiparameter river condition assessments. We were interested in determining if surface flow type observations can be used as a rapid measure of channel morphology and then subsequently be used in river condition assessments to monitor channel change over time. We examined the relationship between flow types (e.g., rippled flow, no perceptible flow) and channel morphology. Two aspects of channel morphology were assessed: (i) within-reach depth variability and (ii) cross section geometry (e.g., width, area, Froude number). In a two-part study, each of these was correlated with flow type diversity for a range of study sites from upland and lowland environments in Victoria, Australia. We found that an increased diversity of flow types correlated with greater depth diversity, based on the coefficient of variation of depth along a reach. Flow types also correlated well with the cross-sectional Froude number, but less so with other geometric parameters. The proportion of flow types present was also notably sensitive to discharge within the low flow range examined (70–99% flow exceedance). Three ‘geo-topes’ — cross section units of a dominant flow type — were found to be the most physically distinct units at this cross section scale; no perceptible flow (deadwater), smooth boundary turbulent (glide), and rippled flow (run/riffle). Rippled flow is a revision to previous run/riffle classifications and is identified by the presence of unbroken waves of any size with crests moving in a general downstream direction. Our findings indicate that flow types can provide indicative information on channel morphology along a reach. When assessed at low flow levels, the diversity of flow types reflects the variability of cross-sectional geometry and also the diversity of the channel bed (depth). On this basis, the inclusion of flow type observations in river condition assessments can provide meaningful information on channel morphology, in a rapid and repeatable way. With repeat surveys, flow types could be used as a geomorphic metric to track changes in morphologic diversity over time, including changes associated with processes of sediment transport, scour, and deposition at a reach. [Copyright &y& Elsevier]

Abstract: To improve navigability of the major access channel into Shanghai Harbor, a large-scale Deep Waterway Project was carried out in the North Passage (NP) of the Yangtze Estuary. In this paper, we investigate how the navigational works affected morphological changes of this channel, as well as those of the adjacent North Channel (NC), Hengsha East Shoal (HES), Jiuduansha Shoal (JDS), and the South Passage (SP). Morphological changes were assessed by analyzing digitized bathymetric data of this area prior to and after execution of the engineering works. The qualitative relations between these changes, along with the hydrodynamic changes as a result of the construction of engineering works, were subsequently investigated. The results reveal that the construction of training walls, groins and jetties resulted in decreased ebb transport in the upper reach of the NP and increased ebb transport in the upper reach of the SP. In turn, this led to intensive upstream erosion with large amounts of sediment transported seaward in the SP and associated sedimentation in the downstream area. At the same time, intense siltation occurred in the upper reach of the NP, while the main channel of the NP mainly experienced erosion caused by the construction of training walls and groins that concentrated ebb flow in the main channel. The waterway deepened significantly in previously shallow areas. Small tidal channels in the HES, which used to connect the NP and the NC, were cut off by the northern training wall. Consequently, residual flow in the middle reach of the NC flowed directly toward the northern bank of the HES, leading to local erosion. Flow obstruction by the southern training wall reduced the upstream propagation of the flood tide in the SP. As a result, a collection of small flood channels evolved under the flushing action of flood currents in the JDS, preventing the natural trend of horizontal extension of the JDS. However, the flow obstruction enhanced the vertical accretion of the JDS. [Copyright &y& Elsevier]

Abstract: This paper analyses the morphodynamics of the Someş River channel on a period of eleven years. The geomorphologic observations have been carried out in the field and the measurements have been done in two sectors in the longitudinal profile of the river. The characterisation of the hydrological control within the framework of the morphohydrographic system of the Someş River has been achieved on the existing database, over a period of fifty five years. The rhythm and the pattern of the processes of the river channel have been determined and eleven meanders have been measured. The obtained data have been interpreted through the control of water flow and sediment transport through the river system. The results obtained, both regarding the delimitation of the river bed sectors with pronounced morphodynamics and the tendency of aggradation-degradation in the channel section, will allow intervention regarding the channel rehabilitation. [Copyright &y& Elsevier]

ABSTRACT There is growing acknowledgement of the interaction between animals and the river bed on which they live and the implications of biological activity for geomorphic processes. It has been observed that signal crayfish ( Pacifastacus leniusculus) disturb gravel substrates, potentially promoting sediment transport and impacting ecological communities. However, the mechanisms involved and the extent of their impact remain poorly understood, especially in relation to other processes that affect grain mobility in gravel-bed rivers. A series of flume experiments, using loose and water-worked gravel beds of narrowly graded grain sizes that were exposed to 6 h of crayfish activity under low-velocity flows, showed a substantial increase in the number of grains entrained by subsequent higher-velocity flows when compared with control runs in which crayfish were never introduced. Crayfish alter the topography of their substrate by constructing pits and mounds, which affect grain protrusion. When walking and foraging, they also alter gravel fabric by reorienting and changing the friction angle of surface grains. In water-worked surfaces, this fabric rearrangement is shown to lead to a statistically significant, partial reversal of the structuring that had been achieved by antecedent flow. For these previously water-worked surfaces, the increase in entrainment arising from disturbance by crayfish was statistically significant, with grain transport nearly twice as great. This suggests that signal crayfish, an increasingly widespread invasive species in temperate latitudes beyond their native NW North America, have the potential to enhance coarse-grained bedload flux by altering the surface structure of gravel river beds and reducing the stability of surface grains. This study illustrates further the importance of acknowledging the impact of mobile organisms in conditioning the river bed when assessing sediment entrainment mechanics in the context of predicting bedload flux. Copyright © 2011 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

A sediment plug is defined as aggradation in a river that completely blocks the main channel. Information from documented cases of sediment plug development in alluvial rivers was used to develop criteria for plug formation and to identify the setup conditions for sites that are prone to plug formation. Site characteristics, processes, and associated parameters were evaluated based on a comprehensive literature review and evaluation of data. A plug formation theory was developed and tested using a unique sediment transport/movable bed numerical model that simulates the key processes considered to affect plug formation. The theory and model were calibrated and validated against field data, and then used to develop simplified criteria that can be used to predict plug formation. Findings from this study can be used to identify sites that may be prone to plug formation, and the criteria can be used to evaluate the potential for plug formation based upon field site conditions when data are not available to complete a more detailed study. [ABSTRACT FROM AUTHOR]

Abstract: We examined the role of bed sediment size, bank materials, riparian vegetation, and discharge on channel form along a 10-km reach of the Upper Tualatin River, encompassing both gravel and sand-bed reaches. Statistical correlations and analyses of downstream changes of channel morphology reveal that bank materials and bed sediment are the dominant controls on channel cross section form. A rapid downstream reduction in bed sediment size in the gravel-bed channel is followed by an abrupt transition to a narrower, deeper, and less sinuous sand-bed channel with more cohesive bank materials. The simultaneous reductions in channel slope, bed sediment size, and width-to-depth ratio indicate the channel crosses a geomorphic threshold to maintain continuity in sediment transport. This gravel-to-sand transition and associated change in channel form are induced by a break in valley slope and an increase in bank resistance from cohesive bank materials. Bank materials, measured as the average percent silt and clay in banks, are a primary influence on channel form within both subreaches but demonstrate a greater influence on channel width and vertical stability in the gravel-bed channel and on channel depth and lateral stability in the sand-bed channel. Riparian vegetation at current densities and compositions is not a significant control on contemporary channel cross section form and may be responding to the bank and channel stability provided by cohesive bank materials in the laterally dynamic gravel-bed channel. [ABSTRACT FROM AUTHOR]

We map the distribution of wood continuously along 73.4 km of second- through fifth-order streams in Yellowstone National Park and develop a Monte Carlo model of wood supply, transport, and storage to investigate: (1) spatial distributions of wood in burned and unburned basins, (2) associations between geomorphology and spatial distributions of wood, and (3) effects of flood timing and sequence on wood distributions. Wood counts in 2000 and 2001 vary by over two orders of magnitude over distances of several hundred meters. Qualitative models using catchment-scale characterizations of streams (e.g., whether a stream is third or fourth order) do not explain the degree of local variability. Reach-scale wood counts are weakly related to channel width in the extensively burned Cache Creek basin and to streamside forest cover and number of channel threads in all basins. Wood counts are not related to other reach-scale variations in channel morphology or fire effects. Monte Carlo simulations indicate that postfire flood timing and sequence drive variability in wood counts, with spatial variations in dominant processes and asynchronous timing of fire and floods creating a complex cascade of disturbance and a patchwork mosaic of wood. Existing descriptive models for postfire wood response do not predict this mosaic, although they capture the central tendency of variations. The central tendency is, however, a poor predictor of postdisturbance wood accumulations, so we develop a probabilistic model that better captures the range of potential postfire responses. We make recommendations for wood monitoring and management based on the empirical data and model results. [ABSTRACT FROM AUTHOR]

Abstract: Analysis of the changes in rates of catchment sediment storage can provide material evidence of the impact of landscape disturbance on catchment sediment flux. A number of studies have suggested increased sediment yields from the rivers draining to the Great Barrier Reef since European settlement in the mid-nineteenth century. Many of these predictions, which indicate increases between four to ten times the pre-disturbance estimates, are based on large-scale catchment modelling that make some critical assumptions about pre-disturbance erosion rates and/or sediment delivery ratios. In addition, the majority have not been validated by empirical data. This study uses single-grain OSL dating and 137Cs depth profiles to determine pre- and post-floodplain accretion rates in Theresa Creek, a subcatchment of the dry-tropical Fitzroy River basin. We demonstrate that floodplain accretion rates have increased by three to four times since European settlement (ca. A.D. 1850). Decreased rates of floodplain accretion since the mid-twentieth century at sites contributed to by gullied terrain suggest a decrease in the supply of sediment derived from gully networks. In contrast, floodplain accretion rates from areas dominated by cultivation remain high. Widespread in-channel benches deposited since European settlement are stable and appear to be important stores of large volumes of sediment. Low magnitude increases in post-disturbance floodplain sedimentation rates (3 to 4 times), in comparison to those reported from mainly temperate climates (10 to 100 times), are attributed to the naturally high sediment loads typical of dry-tropical catchments. Consequently, previous predictions of large post-disturbance increases in sediment yields from large dry-tropical catchments draining to the Great Barrier Reef are likely to be overestimates. [Copyright &y& Elsevier]

Abstract: Rivers respond to a drop in their base level by incising the topography. The upstream propagation of an incision, as usually depicted by a knickpoint migration, is thought to depend on several parameters such as the drainage area, lithology, and the amplitude of the base level drop. We first investigate the case of the Messinian Salinity Crisis that was characterized by the extreme base level fall (1500 m) of the Mediterranean Sea at the end of the Miocene. The response of drainage areas of three orders of magnitude (103 to 106 km2) highlights the dominant role of the drainage area (with a square root relationship) in controlling the knickpoint migration after a base level fall. A compilation of mean rates of knickpoint propagation for time durations ranging from 102 to 107 years displays a similar relationship indicating that successive wave trains of knickpoint can migrate in a river: first, wave trains linked to the release of the alluvial cover and then, wave trains related to the bedrock incision, which correspond to the real time response of rivers. Wave trains with very low retreat rates (long lived knickpoints >1 My) rather correspond to the response time of regional landscape. [Copyright &y& Elsevier]

We evaluate the effects of small dams (11 of 15 sites less than 4 m high) on downstream channels at 15 sites in Maryland and Pennsylvania by using a reach upstream of the reservoir at each site to represent the downstream reach before dam construction. A semi-quantitative geomorphic characterization demonstrates that upstream reaches occupy similar geomorphic settings as downstream reaches. Survey data indicate that dams have had no measurable influence on the water surface slope, width, and the percentages of exposed bedrock or boulders on the streambed. The median grain diameter (D50) is increased slightly by dam construction, but D50 remains within the pebble size class. The percentage of sand and silt and clay on the bed averages about 35% before dam construction, but typically decreases to around 20% after dam construction. The presence of the dam has therefore only influenced the fraction of finer-grained sediment on the bed, and has not caused other measurable changes in fluvial morphology. The absence of measurable geomorphic change from dam impacts is explicable given the extent of geologic control at these study sites. We speculate that potential changes that could have been induced by dam construction have been resisted by inerodible bedrock, relatively immobile boulders, well-vegetated and cohesive banks, and low rates of bed material supply and transport. If the dams of our study are removed, we argue that long-term changes (those that remain after a period of transient adjustment) will be limited to increases in the percentage of sand and silt and clay on the bed. Thus, dam removal in streams similar to those of our study area should not result in significant long-term geomorphic changes. [ABSTRACT FROM AUTHOR]

Abstract: Rapid urbanization has accelerated impact on the catchment hydrology and geomorphology. This rapid development which takes place in river catchment will result in higher sediment yield and affect river morphology and river channel stability; it also becomes the main cause for serious flooding in urban areas. Therefore, it is necessary to predict and evaluate the river channel stability due to the existing and future developments. This study proceeds at Kulim River in Kedah state, a natural stream in Kedah, Malaysia. The FLUVIAL-12 model, an erodible-boundary model which simulates inter-related changes in channel-bed profile, width variation and changes in bed topography was selected for this study. Engelund-Hansen formula and roughness coefficient n =0.030 were found to be the best combination to represent the sediment transport activity in the study reach, where good agreements were obtained for both water level and bed profiles between the measured data and predicted results by FLUVIAL-12 model. The model simulation results for existing conditions, future conditions and long-term modeling show that the sediment size and channel geometry in Kulim River changed significantly. However, modeled results show that future changes in cross sectional geometry will be limited and erosion along the reach will slow down from 2006 to 2016, thus Kulim River was predicted to be stable at most locations. [Copyright &y& Elsevier]

In this article, we describe a method for predicting floodplain locations and potential lateral channel migration across 82,900 km (491 km2 by bankfull area) of streams in the Columbia River basin. Predictions are based on channel confinement, channel slope, bankfull width, and bankfull depth derived from digital elevation and precipitation data. Half of the 367 km2 (47,900 km by length) of low-gradient channels (⩽ 4% channel slope) were classified as floodplain channels with a high likelihood of lateral channel migration (182 km2, 50%). Classification agreement between modeled and field-measured floodplain confinement was 85% (κ = 0.46, p < 0.001) with the largest source of error being the misclassification of unconfined channels as confined (55% omission error). Classification agreement between predicted channel migration and lateral migration determined from aerial photographs was 76% (κ = 0.53, p < 0.001) with the largest source of error being the misclassification of laterally migrating channels as non-migrating (35% omission error). On average, more salmon populations were associated with laterally migrating channels and floodplains than with confined or nonmigrating channels. These data are useful for many river basin planning applications, including identification of land use impacts to floodplain habitats and locations with restoration potential for listed salmonids or other species of concern. [ABSTRACT FROM AUTHOR]

Rill bank collapse is an important component in the adjustment of channel morphology to changes in discharge and sediment flux. Sediment inputs from bank collapse cause abrupt changes in flow resistance, flow patterns and downstream sediment concentrations. Generally, bank retreat involves gradual lateral erosion, caused by flow shear stress, and sudden bank collapse, triggered by complex interactions between channel flow and bank and soil water conditions. Collapse occurs when bank height exceeds the critical height where gravitational forces overcome soil shear strength. An experimental study examined conditions for collapse in eroding rill channels. Experiments with and without a deep water table were carried out on a meandering rill channel in a loamy sand and sandy loam in a laboratory flume under simulated rainfall and controlled runon. Different discharges were used to initiate knickpoint and rill incision. Soil water dynamics were monitored using microstandpipes, tensiometers and time domain reflectometer probes (TDR probes). Bank collapse occurred with newly developed or rising pre-existing water tables near rill banks, associated with knickpoint migration. Knickpoint scour increased effective bank height, caused positive pore water pressure in the bank toe and reduced negative pore pressures in the unsaturated zone to near zero. Matric tension in unsaturated parts of the bank and a surface seal on the 'interrill' zone behind the bank enhanced stability, while increased effective bank height and positive pore water pressure at the bank toe caused instability. With soil water contents >35 per cent (sandy loam) and >23 per cent (loamy sand), critical bank heights were 0.11-0.12 m and 0.06-0.07 m, respectively. Bank toe undercutting at the outside of the nil bends also triggered instability. Bank displacement was quite different on the two soils. On the loamy sand, the failed block slid to the channel bed, revealing only the upper half of the failure plane, while on the sandy loam the failed block toppled forwards, exposing the failure plane for the complete bank height. This study has shown that it is possible to predict location, frequency and magnitude of the nil bank collapse, providing a basis for incorporation into predictive models for hillslope soil loss or rill network development. [ABSTRACT FROM AUTHOR]

Identification of coarse sediment connectivity down river-channel systems is important for understanding the linkages between river reaches, the influence of sediment sources on channel morphology and the mechanisms and liability to propagation of morphological change. A conceptual framework is developed in which several types and degrees of connectivity are proposed. It is hypothesised that sediment connectivity varies with sediment sources and with stream competence to transport the coarse fractions. A method of identifying the status of reaches is developed, based on field mapping of morphology and sedimentology. The analysis is exemplified by two channels: one an ephemeral channel in SE Spain; the other a perennial channel in NW England. Questions arise over whether reaches that lack channel stores transfer coarse sediment. Three scenarios are suggested: (1) lack of flux due to lack of competence of the reach; (2) flushing through of sediment due to high competence; and (3) potential transport of coarse sediment but exhaustion or lack of availability. Changes in connectivity will take place over different time scales depending on the nature of exchange and time scales of storage. Short-term variations in connectivity can be identified by repeated mapping. [Copyright &y& Elsevier]

The inlet of the Lagoa dos Patos lagoon is located in the southwest sector of the exceptionally long barrier-lagoon system of Rio Grande do Sul, Brazil. Historical morphological changes resulting from the interaction between waves and lagoonal flow at the lagoonal mouth were evaluated using data from 26 bathymetric charts made from 1883 to 1909. Before the 3 km long jetty was constructed in 1911, the inlet was fronted by a well developed ebb-tidal delta with a half-moon shape. Here because of wave action swash bars formed and migrated towards the shoreline over the swash platform of the ebb delta. The time between formation and attachment to the mainland of a swash bar here was about 5 years. During this five year period the swash bar has enlarged significantly and migrated 600 m across the swash platform at an average rate of migration of 120 m/year. [ABSTRACT FROM AUTHOR]

A landscape's sediment grain size distribution is the product of, and an important influence on, earth surface processes and landscape evolution. Grains can be large enough that the motion of a single grain, infrequently mobile in size-selective transport systems, constitutes or triggers significant geomorphic change. We define these grains as boulders. Boulders affect landscape evolution; their dynamics and effects on landscape form have been the focus of substantial recent community effort. We review progress on five key questions related to how boulders influence the evolution of unglaciated, eroding landscapes: 1) What factors control boulder production on eroding hillslopes and the subsequent downslope evolution of the boulder size distribution? 2) How do boulders influence hillslope processes and long-term hillslope evolution? 3) How do boulders influence fluvial processes and river channel shape? 4) How do boulder-mantled channels and hillslopes interact to set the long-term form and evolution of boulder-influenced landscapes? 5) How do boulders contribute to geomorphic hazards, and how might improved understanding of boulder dynamics be used for geohazard mitigation? Boulders are produced on eroding hillslopes by landsliding, rockfall, and/or exhumation through the critical zone. On hillslopes dominated by local sediment transport, boulders affect hillslope soil production and transport processes such that the downslope boulder size distribution sets the form of steady-state hillslopes. Hillslopes dominated by nonlocal sediment transport are less likely to exhibit boulder controls on hillslope morphology as boulders are rapidly transported to the hillslope toe. Downslope transport delivers boulders to eroding rivers where the boulders act as large roughness elements that change flow hydraulics and the efficiency of erosion and sediment transport. Over longer timescales, river channels adjust their geometry to accommodate the boulders supplied from adjacent hillslopes such that rivers can erode at the baselevel fall rate given their boulder size distribution. The delivery of boulders from hillslopes to channels, paired with the channel response to boulder delivery, drives channel-hillslope feedbacks that affect the transient evolution and steady-state form of boulder-influenced landscapes. At the event scale, boulder dynamics in eroding landscapes represent a component of geomorphic hazards that can be mitigated with an improved understanding of the rates and processes associated with boulder production and mobility. Opportunities for future work primarily entail field-focused data collection across gradients in landscape boundary conditions (tectonics, climate, and lithology) with the goal of understanding boulder dynamics as one component of landscape self-organization. • We synthesize work to date exploring how boulders affect the evolution of nonglaciated, eroding hillslopes and channels. • Boulder dynamics are one facet of landscape self-organization, influencingtransient adjustment and steady-state form. • The effects of boulders on landscape shape can be quantified by combining field data with simplified theoretical models. • The key research need is for more field data on boulder dynamics spanning gradients in boundary conditions. [ABSTRACT FROM AUTHOR]

Recent surface observations have emphasised the importance of bedload transport by rivers and streams on Mars and Titan. Previous "hydraulic" analysis, however, has also shown that transport as suspended load should be possible, if not more prevalent due to lower critical shear stresses, under reduced gravity conditions compared with Earth; where the dominant mode of sediment flux by rivers is as suspended load. A new suspended-load equilibrium condition (zero net erosion and deposition) constraint is advanced, using a flux-balance model applicable to transport (capacity) limited alluvial channels. The theory unifies Gilbert's relation for slope and flow capacity model with the Shields criterion for incipient motion. Shields-stress analysis shows that sediment transport thresholds on Earth, Mars and Titan are dynamically similar. Thus, despite large differences in dimensional shear stresses, variations in the critical slopes and flow depths required for threshold and equilibrium conditions are comparatively modest. Under reduced gravity conditions, there exists a slightly higher potential for sediment transport as suspended load; except for systems involving high particle-fluid density ratios, as may apply to the transport of organic particles on Titan. Critically we show that graded suspended-load channels should develop gentler slopes under reduced gravity, and not steeper slopes as previously inferred based on generalized hydraulic geometry relationships of terrestrial river and submarine channels. We further argue that the use of submarine channels as analogues for channels formed under reduced gravity is not physically justified. Finally, compared with bedload channels, suspended-load channels should also have markedly higher discharges, with implications for mechanistic-based discharge predictions and precipitation models. • We apply new theory to understand suspended-load sediment transport on different planetary bodies • Under reduced gravity conditions flows have a slightly higher potential for suspended-load sediment transport • Graded suspended-load channels have gentler slopes with smaller discharges under reduced gravity • Submarine channel analogues do not capture the physics of river channels formed under reduced gravity. [ABSTRACT FROM AUTHOR]

Controlling the flow and bed morphology in a river confluence is important in training and navigation works. The flow in river confluence is highly complex due to crucial and rapid changes associated with flow dynamics, sediment transport, and geomorphology. The flow in Malaysia's rivers has many confluence junctions in natural drains of catchment areas. The confluence between Kurau and Ara Rivers, in Perak, Malaysia, is selected to investigate the scour hole that usually forms in the erosion zone and the bar that forms in the deposition zone. A 2D numerical model is used in simulating hydro-morpho dynamics in the rivers confluence to mitigate the erosion and deposition zones by adopting vanes as control structures. Simulation results suggest that the most effective location, dimension, and angle of vanes can be decided based on their performance in scouring and deposition zones. The distribution velocity and flow vectors can help in deciding the location of the vanes. [ABSTRACT FROM AUTHOR]