Alluvial Channels Research Articles

Prediction of scour depth and dune morphology around circular bridge piers in seepage affected alluvial channels

Prediction of scour depth is one of the most significant problems in designing of bridges. Due to complexity of scour phenomenon, available empirical equations do not always offer accurate scour depth prediction. In this work, experiments were conducted on non-uniform sands using two piers of different diameter with and without seepage conditions. It has been found that, the scour depth at upstream of piers is decreased with application of downward seepage. The observed scour depths are compared with scour depth predicted by various prediction methods and it has been observed that the available equations are not suitable for seepage conditions. The present work thus introduces a new empirical equation for prediction of scour depth at piers with inclusion of seepage. The features of migrating dune like bedforms at downstream of piers due to deposition of scoured bed material are also explored. Height of deposition is found to be increased with downward seepage. The empirical equation describing morphology of dunes behind piers is also developed by incorporating downward seepage parameter.

Wall-Porosity Effects on Sediment Deposition and Scour at a Deflector Wall in an Alluvial Channel

AbstractMany water intakes and diversions are prone to blockage by drifting material, notably vegetation and ice. A way to avert blockage is by means of a deflector-wall and floating debris boom. H...

Degradational response of engineered channels to changes in the upstream controls and channel width: Simplified 1D numerical simulations

In response to changes in the upstream controls (i.e., the water discharge, the sediment supply rate, and the calibre of the load), engineered alluvial channels adjust their bed slope and bed surface texture to establish a new equilibrium state. Here we present and discuss various causes of degradational response of engineered channels to changes in the upstream controls and channel width. For that purpose, we apply a simplified 1D numerical research code to a schematic river reach of constant width consisting of mixed-size sediment, and assess its equilibrium state and transient response. We illustrate that the following perturbation to an initially equilibrium state lead to a degradational response: an increase of the water discharge, a decrease of the sediment supply rate, an increase of the sand content of the sediment supply, an increase of the gravel content of the sediment supply, and a decrease of the channel width. Degradational response under all conditions is associated with surface coarsening. The equilibrium states of the numerical simulations agree with analytical solutions. The results provide insight into the current degradational response of engineered rivers, such as the Rhine River, the Elbe River and the Danube River.

Twentieth-century hydromorphological degradation of Polish Carpathian rivers

Case studies of the Soła, San and Dunajec rivers are used to demonstrate changes in planform geometry of the rivers draining the western and eastern parts of the flysch Polish Carpathians and those originating in the high-mountain Tatra massif since the second half of the nineteenth century. The number and width of low-flow channels and the length of river reaches are compared between the nineteenth-century and contemporary rivers, with analysis of other features of their planform geometry prevented by differences in the cartographic presentation of rivers on Austrian military maps and Polish topographic maps. In the second half of the nineteenth century, the rivers flowed in wide channels. The Soła supported a braided channel along its entire course, the Dunajec was mostly braided, with a single-thread channel within gorge sections, whereas for the San the braided pattern was limited to the reaches within intramontane and foreland basins. Over the twentieth century, the braided channel pattern was nearly completely eliminated and substituted by a single-thread pattern; multi-thread channel segments remained only in the upper course of the Dunajec River. Average low-flow-channel width significantly decreased, even by 66%, and this was accompanied by a reduction in the variability of low-flow channel width along the rivers. The change in planform channel geometry was accompanied by channel incision, which started earlier and has resulted in greater bed degradation in the foreland and foothill reaches of the rivers than in their mountain reaches. Twentieth-century river changes also encompassed a radical change in the facies pattern of channel sediments, accompanied by their coarsening and improved sorting and development of bed armour. Moreover, transformation of alluvial channel beds to bedrock ones took place in many reaches of Carpathian streams and rivers. The degradation of physical structure of Carpathian watercourses has resulted in adverse hydraulic and hydrological effects and a marked deterioration of habitat conditions for riverine biota.

Investigating the capability of two hybrid intelligence methods to predict bedform dimensions of alluvial channels

Abstract Dimensions of river bedforms have an effect on total roughness. The complexity of bedform development causes empirical methods to differentiate from each other in predicting bedform dimensions. In this paper, two novel hybrid intelligence models based on a combination of the group method of data handling (GMDH) with the harmony search (HS) algorithm and shuffled complex evolution (SCE) have been developed for predicting bedform dimensions. A data set of 446 field and laboratory measurements were used to evaluate the ability of the developed models. The results were compared to conventional GMDH models with two kinds of transfer functions and an empirical formula. Also, five different combinations of dimensionless parameters as input variables were examined for predicting bedform dimensions. Results reveal that GMDH-HS and GMDH-SCE have good performance in predicting bedform dimensions, and all artificial intelligence methods were dramatically different to the empirical formula of van Rijn showing that using these methods is a key to solving complexity in predicting bedform dimensions. Also, comparing different combinations of dimensionless parameters reveals that there is no significant difference between the accuracy of each combination in predicting bedform dimensions.

Modelling of hyperconcentrated flow in steep-sloped channels

ABSTRACTAn unsteady two-dimensional depth-averaged numerical model for hyperconcentrated flows in steep-sloped alluvial channels is developed and applied to the Zhuoshui River in Central Taiwan. In this model, the suspended load and bed load are both included to reproduce actual sediment transport characteristics of steep-sloped channels in the chosen study river reaches. A rheological relationship for Bingham fluid and the formula for the sediment carrying capacity, established through laboratory experiments, are used to describe the hyperconcentrated flow behaviour in the model. At peak discharge during Typhoon Morakot, bed shear stress from the hyperconcentrated flow model can be 150% larger than that from the clear water model. According to the simulation for Tung-Pu-Rey Creek, an upstream tributary of Zhuoshui River, the temporal evolution of bed changes obtained from the model can be used to provide plausible explanations for the reasons behind levee failure during Typhoon Toraji.

Assessment of alluvial trends toward dynamic equilibrium under chronic climatic forcing

Abstract A remote proglacial stream in Chilean Patagonia was examined at two temporal scales to evaluate the downstream spatial progression of morphodynamics in response to chronic climatic forcing. Historic aerial imagery indicates alluvial channel response to a reduction in glacigenic sediment delivery that is driving reach-scale alterations to the channel planform and affecting the extent and character of geomorphic reaches at centurial timescales. At the decadal timescale, fluvial morphodynamics show a downstream spatial convergence toward dynamic equilibrium. The attainment of dynamic equilibrium is not considered herein, but the trend toward such a condition is discussed. Metrics of flood magnitude, hydraulic energy thresholds, inter-annual energy expenditure, variability of channel dimensions, and continuity of sediment transport capacity illustrate how alluvial systems respond to chronic climatic forcing and deglaciation subject to the constraints of valley conditions. A conceptual proglacial alluvial model is proposed in order to characterize expected fluvial changes and to evaluate the downstream spatial progression. This model is based on a combination of previous observations of alluvial evolution and a location-for-time-substitution approach validated by an intensive 3 year field data collection program.

Boundary layer development over non-uniform sand rough bed channel

Experimental investigations were carried out to investigate the development of boundary layer in non-uniform sand bed channels with and without seepage. Steady flows over non-uniform sand bed channel were simulated experimentally with downward seepage applied through the boundary. Velocity measurements were made at 12 different sections along the channel mid-section for no seepage and seepage flow conditions to investigate the fully developed flow condition. High boundary layer thickness and small boundary layer development length are observed for no seepage runs as compared to seepage runs. The value of boundary layer development length varies between 56 and 65 times of the flow depth for all flow conditions. The present study develops empirical equations to predict boundary layer thickness and flow development length in seepage affected alluvial channel under present experimental conditions.

Land use and landscape pattern changes driven by land reclamation in a coastal area: the case of Volturno delta plain, Campania Region, southern Italy

The paper provides documentary evidence of the direct impacts on the coastal setting in the northern Campania (southern Italy) region, crossed by the Volturno River, that have led to dramatic alterations of alluvial channels, floodplain and the deltaic environment, aside from the coastline retreat following four centuries of land reclamation. The Volturno delta plain is characterized by an outer delta formed mainly by beach ridges, and an inner plain containing several depressions of drained marsh regions. Until the beginning of the last century, this area was the site of swamps and ponds bearing a high incidence of malaria. Historical analysis coupled with a cartographic restitution in a geographic information system environment has produced documentary evidence of the geomorphological evolution of the coastal sector and has allowed the reconstruction of relevant landscape and hydrographic changes since the end of the seventeenth century, when—during the Spanish vice-kingdom—it was subjected to major land reclamation. The availability of reclaimed lands along the coastal alluvial plain promoted the development of agriculture and farming, as well as a strong coastal urbanization. As a consequence, a landscape fragmentation increased significantly between the 1960s and the 1990s; built-up land area increased as well, overgrowing to the sea. An overall reduction in high-quality ecosystems (humid coastal setting, lacustrine/marshy back-dune area and beach-dune system) is recorded, resulting in biodiversity loss and a dramatic reduction in environmental quality. The outcomes of this research will be beneficial to society for better decision-making over these coastal area applications, in a sustainable manner, especially in these countries that are still experiencing land reclamation.

Experimental investigation on flow and scour characteristics around tandem piers in sandy channel with downward seepage

Experimental investigations have been carried out to study morpho-hydraulic characteristics such as scour geometry and turbulent flow properties around tandem piers in alluvial channels. Experiments were carried out in a plane sand bed with two circular piers of same diameter arranged in tandem manner under no seepage, 10% seepage and 20% seepage conditions. Downward seepage minimizes the scour depth around piers and restrains the development of scour depth with time. Strong reversal flow is found near the bed at upstream of piers and near free surface at downstream of piers where velocity and Reynolds shear stress are found to be negative which reduce in magnitude with downward seepage. The flow is more critical within the gap between two piers where velocity is lesser near free surface and gradually increasing towards bed. Quadrant analysis shows that contribution of each event to the total Reynolds shear stress increases with downward seepage. Sedimentation effect prevails within the scour hole whereas outside the scour hole erosive forces become more dominant. Reduced reversal flow at upstream of pier because of downward seepage results in decreasing higher order moments and turbulent kinetic energy. At downstream of piers, secondary currents are dominant due to wake vortices. Strouhal number decreases in case of seepage runs than no seepage condition.

Flow and bedform dynamics in an alluvial channel with downward seepage

In this paper, we report the findings from an experimental study on a parabolic cross-sectional sand bed channel with uniform fine sand under no seepage and downward seepage conditions. Through experiments, we observed that an alluvial channel, which remained at threshold condition of sediment movement during the no seepage experiment, started transporting sediments with the application of seepage in the downward direction. Shields stress of the threshold channel increased significantly from its critical value after the application of downward seepage, which led to the deformation of the cross-sectional shape and consequent development of the bedforms. The role of turbulence in the development of bedforms has also been analyzed. Measures of turbulent statistics show that the time-mean velocities and Reynolds stresses were increased significantly with the application of downward seepage. Under the action of seepage conditions, increase in the flux of streamwise turbulent kinetic energy in the streamwise direction was observed in the region close to the channel boundary. Also, quadrant analysis exhibited an increase in the contributions from all the bursting events and the thickness of the sweep-dominated zone in near-bed region after the application of downward seepage. A bedform tracking tool has been used to evaluate the variability in the geometry of bedforms. We have classified these developing bedforms as current ripples and linguoid ripples according to their evolution with time under the downward seepage condition. It has been further observed that the variation in Shields stress and corresponding bedform geometry reached an equilibrium state in the presence of downward seepage when the experiments were run over a longer period of time (24–31h).

The mechanism of barrier river reaches in the middle and lower Yangtze River

Alluvial channel has always adjusted itself to the equilibrium state of sediment transport after it was artificially or naturally disturbed. How to maintain the equilibrium state of sediment transport and keep the river regime stable has always been the concerns of fluvial geomorphologists. The channel in the middle and lower reaches of the Yangtze River is characterized by the staggered distribution of the bifurcated river and the single-thread river. The change of river regime is more violently in the bifurcated river than in the single-thread river. Whether the adjustment of the river regime in the bifurcated river can pass through the single-thread river and propagate to the downstream reaches affects the stabilities of the overall river regime. Studies show that the barrier river reach can block the upstream channel adjustment from propagating to the downstream reaches; therefore, it plays a key role in stabilizing the river regime. This study investigates 34 single-thread river reaches in the middle and lower reaches of the Yangtze River. On the basis of the systematic summarization of the fluvial process of the middle and lower reaches of the Yangtze River, the control factors of barrier river reach are summarized and extracted: the planar morphology of single-thread and meandering; with no flow deflecting node distributed in the upper or middle part of the river reach; the hydraulic geometric coefficient is less than 4; the longitudinal gradient is greater than 12‰, the clay content of the concave bank is greater than 9.5%, and the median diameter of the bed sediment is greater than 0.158 mm. From the Navier-Stokes equation, the calculation formula of the bending radius of flow dynamic axis is deduced, and then the roles of these control factors on restricting the migration of the flow dynamic axis and the formation of the barrier river reach are analyzed. The barrier river reach is considered as such when the ratio of the migration force of the flow dynamic axis to the constraint force of the channel boundary is less than 1 under different flow levels. The mechanism of the barrier river reach is such that even when the upstream river regime adjusts, the channel boundary of this reach can always constrain the migration amplitude of the flow dynamic axis and centralize the planar position of the main stream line under different upstream river regime conditions, providing a relatively stable incoming flow conditions for the downstream reaches, thereby blocking the upstream river regime adjustment from propagating to the downstream reaches.

Small profile concavity of a fine-bed alluvial channel

Although most natural streams have concave-upward longitudinal profiles, there are conflicting perspectives about whether an alluvial channel is less concave than a bedrock channel. Alluvial channels can be classified as coarse-bed and fine-bed channels, depending on the bed grain size. Both are transport limited, but the threshold of motion differs greatly. Whereas a coarse-bed alluvial channel can be claimed to be as concave as a bedrock channel, we claim that a fine-bed channel is distinguishably less concave. We derive the concavity index of a fine-bed alluvial channel using the power law relationships emergent at a steady-state river network. For known ranges of the scaling parameters, our formulation informs a range of concavity index as 0.07 ± 0.09 for a fine-bed alluvial channel. Our analyses of previous laboratory experiments and real fine-bed alluvial channels in the midwestern U.S. and northern Europe also support our conclusions, i.e., small profile concavity of steady-state fine-bed alluvial channels.

Changes in floodplain inundation under nonstationary hydrology for an adjustable, alluvial river channel

AbstractPredicting the frequency and aerial extent of flooding in river valleys is essential for infrastructure design, environmental management, and risk assessment. Conventional flood prediction relies on assumptions of stationary flood distributions and static channel geometries. However, nonstationary flow regimes are increasingly observed and changes in flow and/or sediment supply are known to alter the geometry and flood conveyance of alluvial channels. Systematic changes in flows and/or channel geometry may amplify or attenuate the frequency and/or extent of flood inundation in unexpected ways. We present a stochastic, reduced complexity model to investigate such dynamics. The model routes a series of annual peak discharges through a simplified reach‐averaged channel‐floodplain cross section. Channel width, depth, and slope are permitted to adjust annually by a user‐specified fraction toward equilibrium geometries predicted based on each year's peak discharge and sediment supply. Modeled channel adjustments are compared with empirical observations for two rivers in Minnesota, USA that have experienced multiple large floods over the past 6 years. The model is then run using six hypothetical scenarios simulating nonstationary flow regimes with temporal adjustments in the mean and/or variance of the governing peak‐flow distributions. Each scenario is run repeatedly while varying parameters that control the amount of fractional adjustment that channel geometries can make annually. Results indicate that the intra‐annual mean horizontal width of floodplain inundation primarily depends on the governing peak‐flow distribution's coefficient of variation, but the intra‐annual frequency of floodplain inundation (i.e., the fraction of modeled years with inundation) primarily depends on the amount of channel adjustment permitted annually.

Bed load tracer mobility in a mixed bedrock/alluvial channel

AbstractThe 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 bed load 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 by 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.

Effect of seepage on flow and bedforms dynamics

AbstractThis study, using an experimental approach, focuses on the effect of downward seepage on a threshold alluvial channel morphology and corresponding turbulent flow characteristics. In all the experiments, we observed that the streamwise time‐averaged velocities and Reynolds shear stresses were increased under the influence of downward seepage. Scales of eddy length and eddy turnover time were significantly increased with the application of downward seepage, leading to sediment transport and initiation of bedforms along the channel length. As the amount of seepage discharge increased, eddy length and turnover time were further increased, causing the development of larger bedforms. It was revealed that the geometry of bedforms was linked with the size of eddies. In this work, statistics of bedform dynamics are presented in terms of multi‐scalar bedforms in the presence of seepage. These multi‐scalar ubiquitous bedforms cast a potential impact on flow turbulence as well as stream bed morphology in channels. We used wavelet to analyse temporally lagged spatial bed elevation profiles that were obtained from a set of laboratory experiments and synchronized the wavelet coefficients with bed elevation fluctuations at different length scales. A spatial cross‐correlation analysis, based on the wavelet coefficients, was performed on these bed elevation datasets to observe the effect of downward seepage on the dynamic behaviour of bedforms at different length scales. It was found that celerity of bedforms reduced with increase in seepage percentage. Bedform celerity was best approximated by a probability density function such as Rayleigh distribution under varying downward seepage. Further, statistical analysis of physical parameters of bedforms ascertained that the reduction in bedform celerity was a result of increased bedform size. Copyright © 2017 John Wiley & Sons, Ltd.

Stream power framework for predicting geomorphic change: The 2013 Colorado Front Range flood

The Colorado Front Range flood of September 2013 induced a diverse range of geomorphic changes along numerous stream corridors, providing an opportunity to assess responses to a large flood in a semiarid landscape. We defined six classes of geomorphic change related to peak unit stream power and valley confinement for 531 stream reaches over 226km, spanning a gradient of channel scales and slope. Geomorphic change was generally driven by erosion of channel margins in confined reaches and by a combination of deposition and erosion in unconfined reaches. The magnitude of geomorphic change typically increased with unit stream power (ω), with greater responses observed in unconfined channels. Cumulative logit modeling indicated that total stream power or unit stream power, unit stream power gradient, and valley confinement are significant predictors of geomorphic response for this flood event. Based on this dataset, thresholds for geomorphic adjustment were defined. For channel slopes<3%, we noted a credible potential for substantial channel widening with ω>230W/m2 (16lb/ft-s; at least 10% of the investigated sites experienced substantial channel widening) and a credible potential for avulsions, braiding, and loss of adjacent road embankments associated with ω>480W/m2 (33lb/ft-s; at least 10% of the investigated sites experienced such geomorphic change). Infrequent to numerous eroded banks were very likely with ω>700W/m2 (48lb/ft-s), with substantial channel widening or major geomorphic change shifting from credible to likely. Importantly, in reaches where there were large reductions in ω as the valley form shifted from confined to relatively unconfined, large amounts of deposition-induced, reach-scale geomorphic change occurred in some locations at relatively low ω. Additionally, alluvial channels with slopes>3% had greater resistance to geomorphic change, likely caused by armoring by larger bed material and increased flow resistance from enhanced bedforms. Finally, we describe how these results can potentially be used by practitioners for assessing the risk of geomorphic change when evaluating current or planned conditions.

Hydrodynamics of submerged vegetated alluvial channel with downward seepage

The flow conditions in a channel covered with fully submerged Oryza sativa has been investigated considering downward seepage. The flow measurements taken at the upstream free vegetation zone, centre of the vegetation zone, and downstream free vegetation zone will provide a better understanding regarding the flow hydrodynamics in a vegetated channel. The flow velocity, Reynolds stress, and turbulent intensities are reduced at the downstream vegetated section. Downward seepage increases the occurrence of streamwise flux along the length of the channel and vertical flux in the vertically downward direction. Integral length and time scales increase as the percentage of downward seepage increases.

Flow resistance and hydraulic geometry in contrasting reaches of a bedrock channel

AbstractAssumptions about flow resistance in bedrock channels have to be made for mechanistic modeling of river incision, paleoflood estimation, flood routing, and river engineering. Field data on bedrock flow resistance are very limited and calculations generally use standard alluvial‐river assumptions such as a fixed value of Manning's n. To help inform future work, we measured how depth, velocity, and flow resistance vary with discharge in four short reaches of a small bedrock channel, one with an entirely rock bed and the others with 20–70% sediment cover, and in the alluvial channel immediately upstream. As discharge and submergence increase in each of the partly or fully alluvial reaches there is a rapid increase in velocity and a strong decline in both n and the Darcy‐Weisbach friction factor f. The bare‐rock reach follows a similar trend from low to medium discharge but has increasing resistance at higher discharges because of the macroroughness of its rock walls. Flow resistance at a given discharge differs considerably between reaches and is highest where the partial sediment cover is coarsest and most extensive. Apart from the effect of rough rock walls, the flow resistance trends are qualitatively consistent with logarithmic and variable‐power equations and with nondimensional hydraulic geometry, but quantitative agreement using sediment D84 as the roughness height is imperfect.

Influence of alluvial cover and lithology on the adjustment characteristics of semi-alluvial bedrock channels

A growing body of research has focused on evaluating the adjustment characteristics of semi-alluvial channels containing proximate bedrock, mixed, and alluvial sections. Active orogens have been the focus of most empirical field-based studies with comparatively less focus on semi-alluvial bedrock channels located in other regions. In this study, we present an inventory of channel geometry data collected from semi-alluvial bedrock channels in Ontario and Québec, Canada, which are not subject to tectonic uplift. Data were sourced from a variety of physiographic settings, permitting evaluation of the influence of alluvial cover, lithology, and gradient on cross-sectional channel form. Our results show no substantial difference in channel width or scaling behaviour amongst bedrock, mixed, and alluvial channels included in our study, except for sedimentary bedrock channels virtually bare of alluvial cover that represent a uniquely wide, distinct subgroup. Channel gradient does not appear to exhibit any observable control on channel width amongst our study rivers, suggesting that sedimentary bedrock channels form a distinct subgroup because of lithology. Comparatively, the widths of our bedrock channels formed in igneous/metamorphic bedrock are comparable to the widths of mixed channels and alluvial channels for a given discharge and drainage area. Our findings also suggest that cross-sectional adjustment of sedimentary bedrock channels is achieved through lateral erosion of the channel banks and downward erosion of the channel bed, whereas cross-sectional adjustment of igneous/metamorphic bedrock is primarily achieved through downward erosion of the bed with limited lateral erosion of the banks.