Near-bank Velocity Research Articles

Assessing the modulation of outer bank erosion by slump blocks: A case study from Marqu Meadow, China

The outer bank slump blocks play a fundamental role in modulating the rate of bank erosion, and consequently, have a profound influence on many geomorphic and ecologic processes in meandering streams, e.g., channel evolution and habitat creation. To date, no adequate research has been developed regarding how these blocks, partially strengthened by roots of herbaceous plants, impact the near-bank flow velocity, bank erosion, and migration of streambanks. This paper analyzes 246 slump blocks in a meandering stream on the Marqu Meadow, China. Aerial surveys were conducted once a year from 2018 to 2021 and the lateral migration rates (M) of streambanks are estimated by superimposing the current-year drone imagery onto its previous-year counterpart. When compared to the outer bank segments lacking slumps, our research findings indicate that the bank segments exhibiting slumps demonstrate a reduction of 57 % and 43 % in the maximum and average bank erosion rates, respectively. Overall, the wider blocks provide stronger bank protection; nevertheless, they may also introduce or enhance local scouring in their vicinity, thereby increasing the heterogeneity of bank erosion rates. To elucidate this complex influence mechanism, we simulate the three-dimensional flow velocity field of a ~100 m meandering bend containing three slump blocks using the MIKE-3 model. Furthermore, we compare one-year bank migration distance with and without the blocks by correlating the lateral migration distance of outer banks to the longitudinal gradient of near-bank velocity. The paper provides valuable insights into the complex interplay between flow dynamics and streambank behavior in real-world settings, thereby enhancing the effectiveness of future management and restoration practices.

Nonlinear multi independent variables in quantifying river bank erosion using Neural Network AutoRegressive eXogenous (NNARX) model

This study proposed a novel application of Neural Network AutoRegressive eXogenous (NNARX) model in predicting nonlinear behaviour of riverbank erosion rates which is difficult to be achieved with good accuracy using conventional approaches. This model can estimate complex river bank erosion rates with flow variations. The NNARX model analysed to a set of primary data, 60% (203 data for training) and 40% (135 data for testing), which were collected from Sg. Bernam, Malaysia. A set of nondimensional parameters, known as functional relationship, used as an input to the NNARX model has been established using the method of repeating variables. The One-Step-Ahead time series prediction plots are used to assess the accuracy of all developed models. Model no. 6 (5 independent variables with 10 hidden layers) gives good predictive performance, supported by the graphical analysis with discrepancy ratio of 94% and 90% for training and testing datasets. This finding is consistent with model accuracy result, where Model no. 6 achieved R2 of 0.932 and 0.788 for training and testing datasets, respectively. Result shows that bank erosion is maximized when the near-bank velocity between 0.2 and 0.5 m/s, and the riverbank erosion is between 1.5 and 1.8 m/year. On the other hand, higher velocities ranging from 0.8 to 1.3 m/s induces erosion at a rate between 0.1 and 0.4 m/year. Sensitivity analysis shows that the highest accuracy of 91% is given by the ratio of shear velocity to near-bank velocity followed by boundary shear stress to near-bank velocity ratio (88.5%) and critical shear stress to near-bank velocity ratio (88.2%). It is concluded that the developed model has accurately predicted non-linear behaviour of riverbank erosion rates with flow variations. The study's findings provide valuable insights in advanced simulations and predictions of channel migration, encompassing both lateral and vertical movements, the repercussions on the adjacent river corridor, assessing the extent of land degradation and in formulating plans for effective riverbank protection and management measures.

Effect of Clearwater Scour on the Flow Field at a Single Bendway Weir: Two-Dimensional Numerical Modeling Supported by Flume Data

This study used two-dimensional (2D or depth-average) numerical modeling to examine the effects of clearwater scour on the flow field at a single bendway weir (BW) set at angles of 30°, 60°, and 90° pointing into the approach flow. The flow field was characterized in terms of depth-average velocity (location and magnitude) through the contraction formed around the BW and was supported by data and observations obtained from large and small flumes. In addition, a three-dimensional (3D) model was used to assess when a 2D model is of design use and when it is not. The 2D modeling showed that, when the ratio of flow depth (about four BW lengths upstream) to BW height (Y∶H) equaled 2.0, the differences (pre- and post-scour) in depth-average velocity magnitude and location of flow around the BW were fairly negligible for all orientations of the BW. However, when Y∶H equaled 1.25, the effects of the scour on the flow field were more prominent. Similar results were obtained for all three orientations of the BW. The value of Vmax obtained with the 2D model was closer to that measured in the large flume when Y∶H decreased from 2.0 to 1.25. This was due to the shallower flow’s greater effect in directing the flow around the BW than occurred for the deeper flow, in which flow had a larger vertical component of velocity when passing over the top of the BW. The practical implications of using a 2D model versus a 3D model were briefly assessed, and 2D models were found to be suitable for designing BWs to manage thalweg position but inadequate for estimating near-bank velocities.

Streambank Erosion Prediction

Bank instability as a result of flow fluctuations may lead to massive bank erosions and subsequent damage of adjacent properties. Continuous erosion process promotes change in the river morphology, sedimentation problems due to the presence of secondary currents and local scouring at piers downstream of the erosion point. Knowledge on the extent of erosion should facilitate river engineers to resolve issues on river training works and river sedimentation problems. A study has been carried out in the field to quantify the amount of eroded materials using erosion pins that were driven into the ground normal to the bank surface. The erosion pins consist of 6 mm diameter metal rods and 60 cm – 80 cm in length. A spatial variation profile for rates of erosion has been identified with units expressed in unit cm per day. The defining parameters for bank erosion rates have included near-bank velocity, Ub, water depth, Y, stream bank geometry and soil bearing capacity. Development of empirical equations had used multiple linear and nonlinear regression techniques to determine the significant erosion predictors. It takes into consideration the coefficient of determination (r-squared) and Root-mean square error (RSME) as determinants for best predictors. Accuracy of developed equations is measured using the discrepancy ratio, D.R. This is the ratio of predicted to measured erosion rate. Analysis suggest that the equation derived using polynomial function (order-2) gave better accuracy compared to the equation derived using linear and power functions. An accuracy of 75% has been obtained. Scatter plots of the predicted to the measured erosion rates have shown to be between 0.5 – 2.0 within the line of good agreement.

The effect of instream logs on bank erosion potential: a flume study with multiple logs

Riparian trees can reduce bank erosion rates, but once a tree falls into a river it can increase local bank erosion. However, the influence of multiple logs, that hydraulically interact, on near-bank velocities has not been investigated. This paper reports flume experiments of the near-bank velocity changes and water level changes produced by multiple in-stream logs with equal and unequal spacing. The results suggest that the near-bank velocity increase caused by a single log can be reduced, and even reversed, by multiple logs. This reduced near-bank velocity mainly results from wake interference between the logs, rather than from the effect of backwater, and it varies systematically with the spacing between the logs. Bank erosion potential can be reduced where logs are spaced under 17 root-plate diameters and where the root-plate is located close to the bank. By contrast, the logs are likely to increase bank erosion when they are within an intermediate distance from the bank and are closely spaced (under 3.3 root-plate diameters apart). The flume results allow us to explore the temporal changes of the potential bank erosion in a reach with various log distributions.

Effect of instream logs on bank erosion potential: a flume study with a single log

It is well established that riparian trees reduce bank erosion. However, fallen trees can increase bank erosion as the flow is deflected and accelerated between the log and its root-plate and the bank face, increasing the boundary shear stress and eroding the bank. This erosion contributes to hydraulic variability but can also be a concern in restoring wood loads in rivers. In this first quantitative study into this phenomenon, we develop a theoretical hydraulic model to estimate the near-bank velocity around a single log using principles of continuity and energy loss for a range of log characteristics (morphology, angle, distance from the bank) and flow characteristics. Flume experiments support the theoretical model and suggest that a single log can almost double the near-bank velocity. The fractional channel area taken up by the log (blockage ratio) and the distance between the log and the bank are key factors governing the near-bank velocity. The smaller the angle of the log with the bank the lower the erosion rate, but also the further downstream the erosion extends. As the morphology of the log changes after falling into the channel, bank erosion potentially develops quickly, but slows over time; and migrates downstream.

Streambank Erosion Prediction for Natural Channel using Artificial Neural Network Autoregressive Exogenous (ANNARX) Model

This study aims to develop a streambank erosion prediction model using Artificial Neural Network Autoregressive Exogenous (ANNARX) for natural channels. ANNARX is one type of ANN models and it is a supervised network that trains spasmodic data sets. Field data of 494 data extracted from two (2) rivers in Selangor, namely Sg. Bernam and Sg. Lui were used in the training and testing phases. Total of eleven (11) independent variables are used as input variables in the input layer and the ratio between erosion rates, ? to the near-bank velocity, Ub as the output variable. The functional relationships were derived using Buckingham Pi Theorem in the dimensional analysis. A supervised learning technique was employed and the target output is streambank erosion rates, ?b. The established models were validated to assess their performances in predicting the rates of streambank erosion using 176 data. Validation of the newly developed streambank erosion rates equation has been conducted using data obtained from this study. The performance of the derived model was tested using discrepancy ratio and graphical analysis. Discrepancy ratio (DR) is the ratio of predicted values to the measured values and these values are deemed accurate if the data lie between 0.5 to 2.0 limit. Total of 8 models have been developed in the predictive model. Analysis confirmed that models developed using ANNARX are capable to achieve coefficient correlations (r-squared) values above 0.9 and successfully predict the measured data at accuracy above 90%.

Erosion of collapsed riverbank and interaction with channel

After failure of a riverbank, a cohesive block of soil can temporarily lie at the toe of the bank until it is eroded away. To investigate the erosion of this collapsed block and its interaction with the river channel, a series of experiments was conducted under similar hydraulic conditions within a U-bend flume. The flow structure and channel topography were measured and analysed in detail. The presence of the collapsed block was found to cause a 1·1–1·6 times greater downstream bank retreat, while the counter-rotating circulation cell and smaller near-bank velocity protect the bank against erosion upstream of the block end. The scour percentage of the block head or end was about 1·5–2 times larger compared with that at the middle. Cohesive soil from the block fills the gap of non-cohesive riverbed material in the downstream convex bank and can result in a maximum bed deposition thickness increase of more than 300%. The results also provide a valuable experimental dataset and a theoretical basis for practical application.

Experimental study on the bank erosion and interaction with near-bank bed evolution due to fluvial hydraulic force

Bank erosion is a typical process of lateral channel migration, which is accompanied by vertical bed evolution. As a main sediment source, the failed bank soil may directly cause the increase of sediment concentration and considerable channel evolution in a short time. The paper presents an experimental study on non-cohesive and cohesive homogenous bank failure processes, influence of the failed bank soil on bank re-collapse, as well as the interaction between bank failure and near-bank bed evolution due to fluvial hydraulic force. A series of experiments were carried out in a 180° bend rectangular flume. The results reveal the iteration cycle between bank erosion and bed deformation: undercutting of the riverbank, slip failure of the submerged zone of the bank, as well as cantilever failure of the overhang, failed bank soil staying at bank toe temporarily or hydraulic transportation, exchange between the failed bank soil and bed material, bed material load being re-transported either as bed load or as suspended load, and bed deformation. Same as bank failure, the mixing of failed bank soil and bed material is more severe near the curved flow apex. Moreover, non-cohesive bank failure tends to occur near the water surface while cohesive bank failure near the bank toe. For non-cohesive dense (sandy) soil, the bank erosion amount and residual amount of failed bank soil on the bed increase with the near-bank velocity or bed erodibility. But for cohesive soil, only bank erosion amount follows the above rule. The results are expected to provide theoretical basis for river management and flood prevention.

Interaction among mean flow, turbulence, bed morphology, bank failures and channel planform in an evolving compound meander loop

The dynamic evolution of the planform of meandering rivers often leads to the development of compound loops with multiple lobes of maximum curvature. At present, the interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound loops is poorly understood. In particular, field studies of this interaction over the timescale of planform evolution are lacking. This study examines the co-evolution of flow, bed morphology, and channel planform in a compound meander loop and relates patterns of near-bank velocity and turbulence to planform change within the loop. Data consist of repeat surveys of channel change in a compound loop over an 11-year period, coupled with Acoustic Doppler Velocimeter (ADV) measurements of 3-D instantaneous velocities for similar magnitude flows at the beginning and end of this period. Results confirm that this compound loop is highly dynamic, with major changes in planform occurring over the 11-year period. Spatial patterns of near-bank velocity and turbulence correspond to patterns of bank erosion and channel migration within the loop; however, these patterns are not strictly a function of planform curvature. Instead, local factors, including deflection of the flow by point bars and failed bank blocks, enhance or inhibit the development of high near-bank velocities and turbulence kinetic energy. The loop has elongated and become more asymmetric over time—a pattern of development consistent with patterns of near-bank velocities and turbulence at the beginning of the study period. The pattern of near-bank velocities and turbulence for measurements at the end of the period indicate that the loop will continue to elongate in the near future, supporting the hypothesis that compound loops evolve continuously over time rather than developing into a stable configuration.

Numerical investigation of long-term planform dynamics and stability of river meandering on fluvial floodplains

A mathematical model for river meandering and floodplain development is presented under the assumption of constant channel width, which combined Johanneson and Parker (1989) linear theory for flows in river bends and a conceptual depositional model from Howard (1992). A new bank erosion model is also developed, which takes into consideration of the nonlinearity in near-bank velocity and the outer bank height. Simulations reproduce the long-term behavior of river meandering and floodplain evolution in a way very similar to their natural processes. Based on the simulation results, stability of river meandering dynamics is discussed and the concept of filtration effect is introduced to highlight meandering rivers’ capability to filter out arbitrarily applied planform disturbance. Simulations are also carried out to investigate the influence of the velocity nonlinearity and the outer bank height in bank erosion model, as well as their combinations. Results show that the consideration of velocity nonlinearity in bank erosion model leads to highly upstream-skewing geometry of fully developed river meanders, as well as a slower downstream migration of meander trains. The influence of nonlinearity in bank erosion model is much smaller than the influence of bank height, which reduces the bank migration rate exponentially in the newly introduced bank erosion model. The outer bank height consideration tends to decelerate the downstream migration rate of river meanders and accelerate lateral expansion of the floodplain as well. Consequently, a broader floodplain is generated compared with simulations without bank height considerations.

Spatial autoregressive structure of meander evolution revisited

Meandering rivers are intrinsically dynamic geomorphic systems that exhibit progressive change in their planforms as they migrate across their floodplains. Recent research in river meandering has focused on the development of linear mathematical models of meander migration. These models, which are based on momentum and mass conservation principles, aim to capture the basic physical mechanisms governing flow-sediment interactions in meandering rivers. A common attribute of the models is that bank erosion is represented as a linear function of near-bank velocity and near-bank velocity is expressed as a spatial convolution of channel curvature. Thus, in all of these process-based models, planform curvature, as mediated through its influence on near-bank velocity, has a strong effect on channel migration. Mathematical details of the curvature–migration relation depend on the number of curvature related convolution terms in the model; the number of such terms increases with increasing model order. Although first- through fourth-order linear models have been developed, few studies have examined rigorously the relation between planform curvature and migration in natural rivers to determine if even the simplest model-based curvature–migration relations correspond to relations derived from empirical data. This paper evaluates empirically the spatial structure of the relation between upstream planform curvature and local channel migration for bends along the Beatton River, British Columbia, Canada. In particular, we compare empirically derived curvature–migration relations to the exponential-decay form of this relation embodied in first-order linear mathematical models of meander migration. For this purpose, we use a refined empirical methodology and recently developed analytical techniques that overcome limitations of a similar analysis of the Beatton River bends conducted by Furbish (1991). The refined methodology examines the influence of upstream lags of planform curvature on migration rates. The new analytical techniques provide data on channel curvature and migration rate at a much finer spatial interval than in the analysis of Furbish (1991). The validity of the approach for revealing the exponential-decay structure of the curvature–migration relation is confirmed by applying it to a synthetic data set generated using a first-order mathematical model. In contrast to the findings of Furbish (1991), our results for the Beatton River bends show that the structure of the spatially distributed upstream curvature effect is more complex than pure exponential decay. The findings suggest that oscillatory components of the relation between curvature and migration rate are important in the planform dynamics of individual complex bends. These findings are consistent with results of recent empirical analysis of the curvature–migration relation based on discrete signal processing and with the structure of the curvature–migration relation in high-order mathematical models of meander migration.

Test of a Method to Calculate Near-Bank Velocity and Boundary Shear Stress

Detailed knowledge of the flow and boundary shear stress fields near the banks of natural channels is essential for making accurate calculations of rates of near-bank sediment transport and geomorphic adjustment. This paper presents a high-resolution laboratory data set of velocity and boundary shear stress measurements and uses it to test a relatively simple, fully predictive, numerical method for determining these distributions across the cross-section of a straight channel. The measurements are made in a flume with a fairly complex cross-section that includes a simulated, cobble-roughened floodplain. The method tested is that reported by Kean and Smith in Riparian Vegetation and Fluvial Geomorphology in 2004, which is modified here to include the effects of drag on clasts located in the channel. The calculated patterns of velocity and boundary shear stress are shown to be in reasonable agreement with the measurements. The principal differences between the measured and calculated fields are the result of secondary circulations, which are not included in the calculation. Better agreement with the structure of the measured streamwise velocity field is obtained by distorting the calculated flow field with the measured secondary flow. Calculations for a variety of narrower and wider configurations of the original flume geometry are used to show how the width-to-depth ratio affects the distribution of velocity and boundary shear stress across the channel.

Examining the physical meaning of the bank erosion coefficient used in meander migration modeling

Widely used models of meander evolution relate migration rate to vertically averaged near-bank velocity through the use of a coefficient of bank erosion ( E). In applications to floodplain management problems, E is typically determined through calibration to historical planform changes, and thus its physical meaning remains unclear. This study attempts to clarify the extent to which E depends on measurable physical characteristics of the channel boundary materials using data from the Sacramento River, California, USA. Bend-average values of E were calculated from measured long-term migration rates and computed near-bank velocities. In the field, unvegetated bank material resistance to fluvial shear ( k) was measured for four cohesive and noncohesive bank types using a jet-test device. At a small set of bends for which both E and k were obtained, we discovered that variability in k explains much of the variability in E. The form of this relationship suggests that when modeling long-term meander migration of large rivers, E depends largely on bank material properties. This finding opens up the possibility that E may be estimated directly from field data, enabling prediction of meander migration rates for systems where historical data are unavailable or controlling conditions have changed. Another implication is that vegetation plays a limited role in affecting long-term meander migration rates of large rivers like the Sacramento River. These hypotheses require further testing with data sets from other large rivers.

Simulating sine-generated meandering channel evolution with an analytical model

An analytical model to simulate the evolution of a meandering channel was developed based on the analytical solution of flow field in a sine-generated meandering channel and application of the bank erosion model by Duan et al. The flow-field solution was derived from two-dimensional, depthaveraged, steady-flowequations in channel-fitted, curvilinear orthogonal coordinates where the transverse bed slopewas treated as a variable increasing with channel sinuosity. The flow-field solution indicated that the location where the shift of maximum velocity zone from the convex to concave banks in sine-generated channels varies with the transverse bed slope and the strength of secondary current. The rate of bank erosion was calculated as a gradient function of the longitudinal sediment transport rate and strength of the secondary flow rather than being proportional to the magnitude of excessive near-bank velocity or shear stress. This model replicated the evolution of meandering channels from mildly to highly sinuous ones. Simulation results showed the migration of meandering channel is a combination of downstream translation, lateral extension, expansion, and downstream and upstream rotation. Low-sinuous free meanders migrate rapidly toward downstream. As the sinuosity increases, downstream translation diminishes, and meandering loops expand laterally with its head rotating toward downstream and then upstream. The simulated results indicated the gradient of the longitudinal sediment transport rate is essential in modeling meandering evolution.

Stream-barb installations for narrow channel bends — a laboratory study

Phase 1 of an ongoing laboratory study of a novel form of stream-bank protection structure is described. "Barbs" are dike-like stone structures designed to protect the (usually unstable) outside-bank regions of channel bends. These low-profile structures point upstream into the flow and typically extend to about 1/4-way across the channel. By disrupting near-bank velocity gradients they promote sediment deposition along the eroding outside-bank region. Their presence also modifies the "helicoidal"-type flow pattern of the bend such that secondary currents, which would otherwise attack the outside-bank, are redirected towards the center of the channel. This novel form of bank protection structure is currently undergoing field tests on selected bends on a number of shallow "wide" streams in Illinois, USA. While initial results are encouraging, additional studies are necessary to develop design criteria for their wider application. In phase 1 of this study, the effectiveness of different arrangements of barb groups, in both 90° and 135° moveable-bed bend sections of a hydraulically "narrow" rectangular channel, are investigated. For each hydraulic condition considered, the channel-bed scour profiles generated by the different barb groups are compared to corresponding "reference" profiles generated in the absence of barbs. Judging the effectiveness of the different barb groups in promoting long-term stability of the outside-bank region is based on two criteria: (i) percent reduction achieved in scouring in the vicinity of the outside-bank and (ii) degree to which the channel thalweg (deepest portion) is moved from the outside-bank region towards the center of the channel.Key words: barb, channel bends, local scouring, bank erosion, bank protection, secondary currents.

Influence of a large woody debris obstruction on three-dimensional flow structure in a meander bend

A field experiment has been conducted to assess the influence of a large woody debris (LWD) obstruction on three-dimensional flow through a meander bend of a small stream in east central Illinois. Previous studies in unobstructed meander bends have shown that flow through a curved channel should develop a coherent three-dimensional structure characterized by large-scale helical motion. Many meander bends are complicated by naturally occurring persistent obstacles, such as LWD, that have the potential to profoundly disrupt flow structure. The results of this study show that the LWD obstruction systematically influences the three-dimensionality of flow through the bend, particularly the position of the high-velocity core and the development of helicity. The high-velocity core is positioned in the center of the channel upstream of and near the bend apex, but as flow approaches the LWD, it is steered toward the inner bank by the obstruction. Evolving helicity in the upstream portion of the bend is amplified by abrupt turning of the flow induced by the LWD. As the flow moves past the LWD, helicity diminishes rapidly and may even reverse its pattern of rotation. The net effect of the LWD obstruction is to reduce near-bank velocities along the outer bank downstream of the bend apex—a critical locus for bank erosion in meander bends. Given the persistence of the LWD obstruction, it probably has an important local influence on bend migration and evolution.