Alternate Bars Research Articles

Morphological response of gravel bed rivers near a knickpoint: Effect of bars on dynamic equilibrium river profile

AbstractGeomorphological evolution is one of the main factors that increases flood damage in small or medium rivers located in upstream river reaches. These types of flood damage have been increasing at knickpoints where the riverbed slope and river width change abruptly and are likely to cause non‐equilibrium conditions for sediment transport during floods. Therefore, it is important to understand the non‐equilibrium morphological response at the knickpoint and the resulting new dynamic equilibrium state under given external forces. The effects of two‐dimensional (2D) morphological features on the dynamic equilibrium riverbed profile, however, have not been specifically studied because the methods currently in use for calculating equilibrium profiles are based on zero‐ or one‐dimensional (0D or 1D) modeling. Here, we perform numerical calculations using the 2D morphodynamic model iRIC‐Nays2DH to clarify the dynamic equilibrium profile and the process of reaching a dynamic equilibrium state. We also use an existing 1D model to show the 2D effect in the dynamic equilibrium state. To understand this, we set up three channels: slope transition point, width transition point, and both the slope and width transition point. 1D results show a constant slope profile in channels with constant width and upward‐convex profiles in channels with width expansion at the equilibrium state, owing to the adjustment of the difference in sediment transport volume in the two reaches with different widths by changing the slopes. In contrast, the 2D results show that the alternate bars create a small autogenic knickpoint even in the straight channel and significantly dampen sediment deposition at the width expansion point, as seen in the 1D model result. This was because the bars' shape increased the volume of sediment transport because the shape of the bars concentrated flow. These results suggest that 2D morphological features, such as fluvial bars, play a significant role in the equilibrium riverbed profile.

Modelling flow regulation effects on vegetation recruitment and survival on alternate bars in channelized rivers

AbstractMany river reaches have transitioned to heavily vegetated systems following the regulation of their flow regime. Vegetation recruitment and survival are key to these transformations. We propose a novel predictive approach to detect how changes in the flow regime may affect conditions for successful vegetation recruitment and survival in channelized rivers with alternate bars. Our approach combines elements of existing vegetation recruitment models (Recruitment Box and Window of Opportunity) with simplified morphodynamic predictions of alternate bar migration. The approach is illustrated by applying it to a reach of the regulated Isère river, SE France, which has transitioned from migrating, unvegetated to steady, vegetated alternate bars over two decades following flow regulation associated with hydropower development. Our modelling approach identifies a strong impact of modified summer flows on vegetation recruitment since 1950 when major flow regulation began. An increase of up to 1 cm year−1 (after 1990) has occurred in the vertical extent of vegetation recruitment bands on the bars. This has been accompanied by an almost 20 m total increase (since 1950) in the horizontal extent of the recruitment bands, and a 1.5 times reduction in the frequency of high flows capable of promoting bar migration. Our modelling also suggests viable flow restoration options to limit widespread vegetation colonisation. Comparing outcomes from our modelling approach with those from the Bertagni et al. (2018) model suggests that the Isère may have been highly prone to vegetation colonisation even before flow regulation and close to some threshold for a shift from an unvegetated to a vegetated state.

Experimental investigation on the effect of hoop spacing and crosstie configuration on the cyclic performance of reinforced concrete beams

According to ACI 318-19(SI), in the plastic hinge zone of RC beams, the longitudinal reinforcement at each corner and every other longitudinal reinforcement must be enclosed by the corner of the closed hoop or the hooks of crossties. However, in RC construction practice, the longitudinal reinforcement of RC beams is densely arranged, making construction difficult. In this study, four large RC beam specimens were designed and fabricated to conduct experiments to verify the effect of hoop spacing and crosstie configuration on the cyclic performance of reinforced concrete beams. It is found that: (1) The crosstie configuration with seismic hook engaging the longitudinal reinforcement at the bottom is recommended and that during construction, within the range of twice the beam depth from the RC column outer face, the bottom and side formwork should be completed after the crossties are assembled, which should solve the problem of difficult construction. (2) The transverse reinforcement spacing between 4 and 6 times the smallest longitudinal reinforcement diameter of RC beams may have no significant effect on the cyclic performance of RC beams. (3) The requirement that in the plastic hinge zone of RC beams, every corner and alternate longitudinal bar on the perimeter must have lateral support provided by the corner of a tie or crosstie in ACI 318-19(SI) may not have a major impact on the cyclic performance of the RC beam. (4) The ultimate drift angle of each specimen in the negative direction was between 4.3% rad and 4.5% rad; thus, all the specimens satisfied the ACI requirement. However, the cyclic performance (plasticity rotation angle) of all specimens basically did not meet the requirement of 3.0% rad for earthquake-resistant structures.

Empirical Determination of Free Alternate Bar Length

Empirical Determination of Free Alternate Bar Length

River restoration works design based on the study of early‐stage vegetation development and alternate bar dynamics

AbstractActive geomorphological interventions, such as reprofiling of river bars, are often used to increase bar dynamics and prevent vegetation encroachment. River restoration management should be planned based on the knowledge of what processes will follow the intervention and on the anticipation of the consequences. However, in many cases, the associated physical processes are not clearly identified whereas their consequences on bar morphodynamics are still not fully understood. This study aims to bring new insights into the biomorphodynamics evolution of the riverbed after restoration works by using a 2D biomorphodynamic model developed in the TELEMAC‐MASCARET system. It seeks to compare and evaluate the performance of five bar reprofiling scenarios in which the bar elevation is lowered to just below the water level at specified design discharges. The study area is located in the channelized and regulated alpine gravel‐bed Isère River (France). Bar dynamics and early stages of vegetation establishment are analyzed for the first 2 years after each restoration scenario. The results indicate that plant colonization would occur in all cases. Overall, maximizing the reduction of bar height is the most effective way to improve the bar dynamics and limit future vegetation encroachment.

Modelling the Impact of Climate Change on the Flow Regime and Channel Planform Evolution of the Lower Drava River

Climate change is expected to have a significant impact on changing the flow regime of the lower Drava River. Four flume experiments were run to see how stabilized, increased and decreased flow and the occurrence of a series of floods affect channel planform evolution. Constant discharge produced alternate bars that subsequently merged into bigger bedforms, bedform migration, and a higher sinuosity of the channel. While merging and migration of bedforms may happen in the lower reaches of Drava, its sinuosity is unchanged due to river regulation. Reduced flow initiated transition from a braided to incised single-thread planform, with the formation of dormant channels. Drava already has a single-thread planform (because of dikes) and, in cases of flow reduction, will have the remnant of inactive channels. Increased discharge showed greater erosion and reworking of channel banks, a decrease of sinuosity ratio and active high bluff zones. On Drava sinuosity ratio is more difficult to change because of levees, but erosion, reworking of banks, and increased high bluff risks are possible. Floods simulation generated the construction of an anabranching planform alongside the incised main channel with terraces along banks (active during floods), bars, alluvial islands, and side channels (active during low flows). On the lower Drava River, this situation correlates with past floods.

Morphodynamic equilibrium of alternate bar dynamics under repeated hydrographs

Discharge unsteadiness plays a key role in the dynamics of alternate bars, however, how migrating alternate bars determine their shape under unsteady discharge conditions, especially in the long-term, is still unclear. In this study, we numerically investigate long-term behaviors of migrating alternate bars subject to repeated hydrographs. For this, the same discharge variation but different hydrograph cycles were tested. The results show that even under unsteady flow conditions, specific combinations of hydrograph cycles and discharge variations result in an equilibrium state of migrating alternate bars (i.e., non-time dependent wavelength and migration period). This may be a unique feature of the alternate bar morphodynamics, since dynamic equilibrium state arising from steady discharge counterparts is time-dependent feature. Meanwhile, it is observed that when the discharge changes abruptly, the response of alternate bar morphodynamics changes along the time. Within a limited period, the response of alternate bars is not strong enough to destroy the original wavelength, suggesting that the hydrograph cycle that achieves this equilibrium state must be moderately long such that the timescale of the hydrograph is similar but slightly smaller than the bar growth.

The Impact of Climate Change on River Alternate Bars

AbstractClimate change is expected to alter the distribution of flow discharge in rivers worldwide. We study the impact of climate‐driven flow changes on the shape of riverbed, and specifically on alternate bars, large deposits of gravel/sand that often form in rivers. We consider the illustrative example of the Alpine Rhine River, showing two nearby reaches with similar hydro‐morphological characteristics, but different channel width. Hydrological projections are obtained from literature, while the evolution of alternate bars is predicted through a novel, semi‐analytical model. Results show a remarkably different behavior of the two reaches: the upstream one, being wide enough for a full development of alternate bars, is resistant to flow alterations; the downstream reach, whose width is close to threshold conditions, is highly susceptible to future changes, showing a strong tendency to increase bar prominence. These findings reflect a general tendency of near‐threshold geomorphic systems to be vulnerable to anthropic stressors.

Insights Into the Dynamics of Vegetated Alternate Bars by Means of Flume Experiments

AbstractAlternate bars are bedforms recognizable in straight or weakly curved channels as a result of riverbed instability. The length and height of alternate bars scale with the river width and the water depth, respectively. During low water stages, alternate bars become exposed and can be colonized by riparian vegetation. The effects of established plants on the morphodynamics of alternate bars have been poorly investigated. In this work, we focus on the effects induced by rigid vegetation on the dynamics and morphology of previously developed alternate bars in a straight channel by means of flume experiments. We investigate three different spatial densities of plants to reproduce scenarios of vegetation establishment. The results illustrate that vegetation alters both the altimetric and planimetric characteristics of bar patterns. In particular, as compared to bare‐bed bars, vegetated bars have a higher wave amplitude and scour, and this effect becomes stronger with plant density. Moreover, they exhibit decreasing wavenumbers according to vegetation density. A comparison with previous fundamental work for the planimetric instability of straight channels with bare‐bed alternate bars, suggests that the established vegetated bars may promote the transition to river meandering.

Preparation and evaluation of mango oat bar fortified with chia seeds

Ready-to-eat (RTE) cereal bars have undergone numerous advancements that blend various components to produce alternative cereal bar options. In conclusion, this study sought to create a high-energy cereal bar with a satisfying texture and high consumer acceptance using regional ingredients from local market. Other ingredients, such as exotic fruits and organic residues, can be utilized to make cereal bars and are still added to the final product as enrichment. From the breakfast table to the dinner table, cereal goods have been consumed in the form of flakes, rice, chapatti, or cereal bars as a snack. Cereals are essential in the creation of ready-to-eat snacks including cereal bars, energy bars, and quick bars. Because whole grains and their products include all three components in the same ratio as found in the intact original grain, people have come to appreciate their consumption. Mango is also discovered to be a high source of antioxidants such ascorbic acid, carotenoids, and phenolic compounds, as well as dietary fibers, starch, and pectin. A good number of dietary antioxidants like ascorbic acid, carotenoids, and phenolic compounds is found in the fruit mango (Mangifera indica L.). Cereal bars made with mango pulp at concentrations of 40%, 45%, and 50% were one such product developed.

Dynamics of Dual‐Mode Bedload Transport With Three‐Dimensional Alternate Bars Migration in Subcritical Flow: Experiments and Model Analysis

AbstractBedload transport often exhibits dual‐mode behavior due to interactions of spatiotemporal controlling factors with the migrating three‐dimensional bedforms (characterized by the fully developed patterns in the bed, such as alternate bars, pools, and clusters). This study explores dual‐mode bedload transport based on experimental measurements and develops Einstein's exponential‐based model to characterize large fluctuations of bedload sediment discharge. The particle waiting time, particle flux, and bed elevation are measured in a series of well‐controlled laboratory experiments. Flume experiments show that the waiting time distribution of sediments gives a bimodal characteristic, two distinct modes can be identified from the measured data. This study encapsulates this dual‐mode bedload transport behavior in a hyperexponential distribution of sediment resting times and introduces it into the continuous time random walk (CTRW) framework. Considering the scaling limit of the thin/heavy‐tailed CTRW processes, a single‐rate mass transfer (SRMT) and fractional‐derivative SRMT (F‐SRMT) models are obtained, and the model parameters are determined from the hyperexponential distribution. Further analyses reveal that the dual‐mode bedload transport behavior is controlled by mass exchange between the mobile and immobile zones, and a dimensionless index η can quantify the intensity of dual‐mode behavior. Applications show that the dual‐mode bedload transport models are much more accurate in characterizing bedload transport in a mixed‐size gravel bed than the traditional exponential‐based model, and the nonlocal movement of bedload sediments is significant in the mixed‐size gravel bed. Further investigations will focus on the applicability test of the dual‐mode models to other flow regimes and conditions.

Migrating bars influence the formation and dynamics of their peers in large sandy‐gravel bed rivers

AbstractThe present study investigates the morphodynamic interactions between migrating bars in a sandy gravel and nearly straight channel of the Loire River (France). From a large dataset collected from field campaigns performed between 2016 and 2020, we analysed the deflection power exerted by bars on flow and sediment transport that influences the dynamics of other bars present in the channel. We also investigated the role of low flows in bar formation. To this aim, the riverbed evolution has been documented using four years of bathymetrical and aDcp surveys. Flow deflection induced by the presence of bars in the channel differs for pre‐existing (first‐order) bars and bedload sediment accumulations identified as new developing bars (second‐order bars). The latter needs to reach an equilibrium size to influence flow direction. During low flows, first‐order bars emerge and create contraction/expansion zones in the channel, inducing a temporary local geometrical forcing that leads to sediment deposition at the outlet of these zones. Sediment deposited in these specific areas can constitute a nucleus for second‐order bar formation or contribute to first‐order bar aggradation. According to the linear free bar theory, the critical discharge that contributes to expressing an alternate bar pattern is exceeded during moderate flood, leading to the observation of a transitional bar pattern between alternate and central bar pattern.

Impacts on alternate bar geometry and dynamics in a trained sand bed river

This study investigates the occurrence and dynamics of single row alternate bars forming in a particular reach of the sand bed Elbe River in Germany. Although the formation and dynamics of alternate bars have been intensively investigated in the literature, there exists only a limited number of studies focusing on the characteristics of alternate bars forming under complex field conditions. This is particular the case for bars forming in trained sand bed rivers, as most previous field studies have focused on gravel bed rivers. Moreover, little is known on the impact of river training structures on bar characteristics in anthropogenic rivers. To close this gap, we present a comprehensive bed elevation data set that was collected over a period of 10 years within a 30 km long reach of the lower Elbe River in Germany by the Federal Waterways and Shipping Administration (WSV). The reach is characterized by a sand bed, has curved as well as straight parts, and exhibits a section that is less trained by groins than the neighboring sections. For our analyses, we propose a novel approach to estimate bar characteristics based on statistically derived geometrical parameters. The outcomes of the approach are used to show that bars in Elbe River belong to the free bar type and that their origin of formation and characteristics depend on hydrological and geometrical boundary conditions. The results reveal that the active width of the river bed, defined as the distance between the groin heads, is a crucial parameter for the occurrence of alternate in the reach. We further highlight the impact of river bends on bar characteristics, as bars in the outer bend were longer and higher than their inner bend counterparts. Finally, we show that simple predictors for bar formation can be successfully applied to predict bar formation in sand bed rivers but that care needs to be taken when applying such approaches to more complex boundary conditions.

Mechanisms of microplastics trapping in river sediments: Insights from the Arno river (Tuscany, Italy)

Rivers efficiently convey microplastics to the sea, but during this transfer microplastic can be temporary stored in sediments, where they undergo further fragmentation due to biological and physical processes. Aiming at shedding light on mechanisms governing microplastic sedimentation in rivers, we analyse deposits accumulated in alternate bars of the Arno River (central Italy). Specifically, we considered microplastics associated with floating plant debris, and those trapped in clastic suspended and bedload deposits. The overall concentration of microplastic ranges between 0.44 and 5.68 items per gram, and is comparable with that of some highly-polluted rivers in the world. Fibers are prevalent among the recovered items, and composition is dominated by nylon. Our measurements reveal that microplastics can be easily trapped by floating plant debris, and stored on bar top zones and river banks. Microplastics are also trapped in gravel and sand deposits. Sand incorporates microplastics both when it is transported at the river bottom under tractional conditions and during the waning flood stage, when settling processes contribute to bed aggradation. Gravels do not entraps microplastics when they move on the river bed, but they are extremely efficient in trapping microplastics during recessional flood stages, when water infiltrates between larger clasts, where it drops suspended microplastics. Further studies based on application of principles of fluvial sedimentology will provide crucial insights to understand mechanisms controlling transport and storage of MPs in river sediments.

Mechanical Behavior of Doweled Joints in Concrete Pavements: A Review

Dowel bars in concrete pavements can effectively improve the performance of joints as well as the overall performance of pavements. This paper reviewed the mechanical behavior, structural numerical simulation analysis, testing study of doweled joints in concrete pavements, and the research work on the alternative dowel bars. The influences of dowel bar configuration, diameter, spacing, misalignment, rust, and the bonding between the surrounding concrete on the performance of doweled joints were also discussed in this paper. Limitations of theoretical research and test methods were mentioned after reviewing these aspects of dowel bars, indicating the direction for future research on doweled joints in concrete pavements.

The role of riparian vegetation flexibility in a bio‐hydro‐morphodynamic simulation

AbstractBio‐hydro‐morphodynamic simulation is an essential tool for analyzing fluvial processes under the influence of riparian vegetation, and has developed over the past decade. However, most bio‐hydro‐morphodynamic models have neglected the flexibility of vegetation. The flow‐induced deformation of flexible vegetation changes the resistance to water flow. Therefore, flexibility affects water flow surrounding vegetation patches, and alters bed shear stress below and surrounding patches. To investigate the effects of vegetation flexibility in a fluvial, bio‐hydro‐morphodynamic simulation, we incorporated a model for predicting flexible vegetation reconfiguration in a bio‐hydro‐morphodynamic model based on Delft3D. We studied effects by considering and neglecting vegetation flexibility within a gravel bed river with alternate bars. The vegetation used in the simulation was Phragmites japonica, a widely existing reedy grass in riparian environments. Our results indicate that vegetation flexibility can impact river hydrodynamics and morphology development processes. When accounting for vegetation flexibility, water depth decreases, averaged Shields stress within a vegetation patch increases, and channel width with sediment transport increases. Erosion and deposition within the channel are additionally reduced. The significance of vegetation flexibility depends on multiple factors (i.e., the stem density growth rate, rigidity, and the river bed material). Given a fast stem density growth rate or a large rigidity, differences between flexible and rigid vegetation cases could be neglected. The difference between flexible and rigid vegetation was maximized when the bed gravel size fell within a certain range. Our results suggest that the impact of vegetation flexibility on river morphology depends on the mobility of river bed material. A very active or inactive bed reduced the importance of vegetation flexibility on river morphology.

Hyporheic Exchange in a Straight Stream With Alternate Bars

AbstractHyporheic exchange (HE) is essential to riverine ecosystem, water quality, and energy cycling in a stream due to its governing role in physicochemical and biological processes in hyporheic zone (HZ). Alternate bars, characterized by a repetitive sequence of diagonal bars with a pool‐riffle bedform, are common geomorphic features that generally form in straight streams. Following similar laboratory and numerical methodology as Huang and Chui (2021, https://doi.org/10.1029/2020wr029182) who investigated the HE induced by pool‐riffle sequences, this study focused on the HE induced by alternate bars (i.e., geomorphic features on stream banks instead of streambed) where the influences of key factors were systematically examined. The results show that alternate bars strongly influence surface flow and induce significant hyporheic flows in both stream banks and streambed, whose patterns, magnitudes, and influential patterns of key factors could be similar to the HE induced by pool‐riffle sequences. The HE flowrate increases linearly with discharge, bed slope, and bar spacing. However, influential patterns of bar width on HE vary, and the bar width changes the influential patterns of discharge and bed slope on HZ scale, highlighting the influence of bar width in the complex interactions among discharge, bed slope, and bar width. All indicators of HE characteristics generally increase with bar spacing and follow exponential relationships with groundwater flow. Findings in this study lay the foundation in research on riparian ecosystems and water quality in the streams with alternate bars and help guide the design of bars during river restorations.

On the Migrating Speed of Free Alternate Bars

AbstractFree alternate bars exhibit wave properties. However, there has been a significant lack of research regarding these wave properties, including the migration speed. In this study, we quantified the migration speed M for bars and identified the dominant variable exerting control over M. Experiments were performed in a laboratory flume under controlled steady state conditions to ensure the development of alternate bars; in addition, the bed level and flow depth were continuously measured. We derive a hyperbolic partial differential equation (HPDE) for the bed level by assuming that the bed level is a continuous function and the advection velocity of the function equals M. Then, to verify the HPDE, we show that it adequately describes the temporal variation in the bed level that is measured experimentally. The advection velocity of the HPDE can be used to calculate M from the energy slope and the Shields number, enabling the calculation of the spatial and temporal variations of M in the experiments. The formula for M showed that the magnitude of M is three to four orders of magnitude smaller than the velocity of the uniform flow. Then, we show that M obtained from the proposed formula is in agreement with those obtained from an instability analysis. Furthermore, we demonstrate that the proposed formula is applicable to actual rivers, in which the scale and conditions differ from those in the experiments.

Thermal grill illusion of pain in patients with chronic pain: a clinical marker of central sensitization?

The thermal grill illusion of pain (TGIP) is a paradoxical burning pain sensation elicited by the simultaneous application of innocuous cutaneous warm and cold stimuli with a thermode ("thermal grill") consisting of interlaced heated and cooled bars. Its neurophysiological mechanisms are unclear, but TGIP may have some mechanisms in common with pathological pain, including central sensitization in particular, through the involvement of N-methyl- d -aspartate receptors. However, few studies have investigated TGIP in patients with chronic pain and its clinical relevance is uncertain. We hypothesized that the TGIP would be increased in comparison with controls in patients with fibromyalgia or irritable bowel syndrome, which are regarded as typical "nociplastic" primary pain syndromes related to changes in central pain processing. We compared the sensations elicited by a large range of combinations of temperature differentials between the warm and cold bars of a thermal grill applied to the hand between patients with fibromyalgia (n = 30) or irritable bowel syndrome (n= 30) and controls (n = 30). The percentage of TGIP responses and the intensity and unpleasantness of TGIP were significantly greater in patients than controls. Furthermore, positive correlations were found between TGIP intensity and clinical pain intensity and between TGIP intensity and the cold pain threshold measured on the hand. These results are consistent with our working hypothesis of shared mechanisms between TGIP and clinical pain mechanisms in patients with nociplastic chronic pain syndromes and suggest that TGIP might represent a clinical marker of central sensitization in these patients.

Effects of Vegetation Patch Patterns on Channel Morphology: A Numerical Study

AbstractThis study investigates the effects of vegetation patch patterns on the morphological evolution of alluvial river channels at the reach‐scale. For this, a new two‐dimensional numerical biomorphodynamic model has been developed using the Telemac‐Mascaret system. Considering the newest development in the topic, the effects of vegetation on bedload transport are included by extending Einstein's parameters for the sediment transport formula. The model was subsequently validated by published laboratory experiments reproducing alternate bar dynamics with different vegetation establishment scenarios. The validated model was then used to study the influence of vegetation patch patterns on the channel morphological evolution considering the two most observed ones: (a) the filled pattern with plants well distributed within the patch, and (b) the stripe pattern with plants established only along the patch edges. 14 scenarios were simulated in total, including sensitivity analyses on the coefficients of vegetation characteristics. The results indicate that the morphological responses of an alternate bar system to the stripe pattern consist of channel widening, steeper slope and reduced water depth, with increased sediment transport rates. The effects of the filled pattern are similar but weaker. The results also show that with the stripe pattern, the alternate bars tend to migrate toward the centerline forming center bars. Besides, the scroll bars forming downstream are shorter, corresponding to less visible chute channels, compared to the filled pattern. Despite much less vegetation coverage, the stripe pattern decreases the bar elongation rates in a way similar to the filled pattern.