Cohesive Riverbanks Research Articles

Cohesive river bank erosion mechanism under wave-current interaction: A flume study

The effect of hydrodynamic forces due to combined action of surface waves and current on the riverbank is critical to understand sediment entrainment, transport and bank line retreatment process. In understanding the temporal effect of turbulent structures under induced wave-current flow, a series of laboratory experiments were carried out. Micro-Acoustic Doppler Velocimeter (ADV) and Ultrasonic Ranging System (URS) were used simultaneously for the measurement of velocity fluctuations and bank undercut depth increment. Modulation of the turbulent flow characteristics and the benefaction of turbulent bursting structures at the initiation of erosion process and before the failure of the cohesive bank due to undercut progression are discussed. The results show that velocity and Reynolds shear stress have direct dependence on the size and rate of the entrainment of cohesive aggregates from bank face. The effect of wave-current motion leads to an increase in shear stress at the lateral bank giving rise to erosion and transportation of sediment particles/aggregates. Quadrant analysis of the random velocity fluctuation under wave-current flow at the initiation of erosion process shows strong presence of ejection and sweep events. Findings from the present study may provide a better understanding on the design of cohesive bank erosion control measures.

Investigations on undercutting processes of cohesive river bank

River bank failure is the result of prolonged and repetitive erosion processes caused by the interplay between the flow turbulence with that of bank sediment. These interactions of affecting factors become more complex when the river bank sediment is cohesive in nature. The present study attempts to understand the erosion of cohesive river banks under controlled laboratory conditions, starting from an uneroded bank to the state just before failure. Experimental runs are carried out over the cohesive river bank, fabricated in a laboratory flume. Instantaneous real-time transverse erosion depth within the bank is recorded using ultrasonic ranging system (URS) and the corresponding velocity data by acoustic Doppler velocimeter (ADV). The results from the experimental runs with varying flow characteristics and cohesive sediment properties reveal that the turbulent shear stress combined with velocity fluctuations close to the bank increases with the flow Reynolds number. The size of the dislodged cohesive aggregates from the cohesive bank depends on the shear stress fluctuations. The transverse erosion depth as a function of time and flow Reynolds number are the significant parameters on which fluctuations of shear stress depend. These fluctuations in shear stress and flow velocity are high near the upstream and downstream sides of the undercuts, resulting in a longitudinally elongated undercut zone.

Coupled Model of Bank Erosion and Meander Evolution for Cohesive Riverbanks

In this paper, a physics-based model that couples a bank erosion model with a meander evolution model is developed and evaluated. The physics-based bank erosion model considers the cantilever failure mechanism with slump blocks and decomposition effects. Moreover, bank accretion was considered using critical values of time required for landing, shear stresses and water depths. Two cases were tested. The first case consists of a hypothetical small-scale channel with cohesive riverbanks. Cross sections in the straight and curved part of the channel were compared to evaluate the curvature effect. Furthermore, the effect of the bank strength in the plan shape of the channel was tested in this case. The results show that the curvature increases the erosion rate in the outer bank and changes the cross-sectional profile by narrowing and widening the channel width. The plan shape of the channel changed as the bank strength was increased. In the second case, the model is compared with the River meander migration software (RVR meander) and the advantages and limitations of the model are discussed in terms of meander migration plan form and bank erosion processes. The results showed that the presented model is capable of simulating asymmetric bends.

Collapsing Mechanisms of the Typical Cohesive Riverbank along the Ningxia–Inner Mongolia Catchment

As one of the major sediment sources in rivers, bank collapse often occurs in the Ningxia–Inner Mongolia catchment and, to date, it caused substantial social, economic and environmental problems in both local areas and downstream locations. To provide a better understanding of this phenomenon, this study consisted of modifying the existing Bank Stability and Toe Erosion Model (BSTEM), commonly used to investigate similar phenomena, introducing new assumptions and demonstrating its applicability by comparing numerical results obtained against field data recorded at six gauging stations (Qingtongxia, Shizuishan, Bayan Gol, Sanhuhekou, Zhaojunfen, and Toudaoguai). Furthermore, the impact of multiple factors typical of flood and dry seasons on the collapse rate was investigated, and insights obtained should be taken into consideration when completing future projects of river adaptation and river restoration.

Characteristics of intermittent turbulent structures for river bank undercut depth increment

The complex interplay between the resistive forces of the bank material and Reynolds shear stress of the turbulent flow field control the fluvial bank erosion. In-depth knowledge on the interactions of the flow-form at the river bank is a prerequisite to understand erosion mechanism and quantification of the erosion rate. Measurement at 1.5 m from the bank face as reported in the literature revealed that turbulent lateral flux of stream-wise momentum propagated towards the bank face that was held responsible for the removal of material from the bank face. Further experimental results from the literature depict that the stream-wise and lateral fluctuating velocity components deploy a random forcing effect on the river bank face which may be one of the primary agents for sediment particle entrainment from the bank face. It is gestated from these previous studies that the momentum flux and turbulent structures of the flow field has a great impact on the river bank erosion. Nevertheless, the previous field and experimental studies do not focus on the flow turbulence and sediment interaction that extracts information on the detailed temporal features of sediment entrainment process. Thus, the present study was carried out to understand the coupled dynamics of the effect of intermittent flow structures on the intermittent removal of river bank materials, and for advancing the knowledge on river bank erosion process. The present experimental study focuses on the evaluation of the turbulent flow characteristics when the undercut mechanism initiates in a cohesive river bank. Based on the results it is evident that moderate scale intermittent stream-wise flow structures probably imposes a periodic loading on the cohesive aggregates of the bank face. This accelerates the entrainment of aggregates from the bank face. The study also reveals that the undercut increment is a small scale intermittent process. Findings of the present study may be helpful for the improved design of bank protection measures.

Numerical modelling of cantilever failure and effect of slump blocks on meander migration

This study proposes a new numerical model to study cantilever failures and the effect of slump blocks on meander migration in cohesive river banks. Previous models have incorporated the effect of slump blocks using different methodologies but most of them used an “indirect approach” (such as increasing the critical shear stresses or introducing armouring factors) which does not allow to evaluate the effect of slump blocks directly. The new numerical model introduces an explicit representation of slump blocks and it was obtained by coupling two models available on the literature. The model was able to represent the mechanisms of cantilever failure with slump blocks and their effect in meander migration in a hypothetical small scale channel.

Coupled studies of fluvial erosion and cantilever failure for cohesive riverbanks: Case studies in the experimental flumes and U-Tapao River

The main objective of this paper is to introduce a novel coupled method for simulating the fluvial erosion, cantilever failure and bedload sediment transport to describe the overhanging properties of experimental banks and natural riverbanks. For fluvial erosion results, the previous relationships between the critical shear stress, and the erodibility coefficient indicated that they do not follow the values of the experimental works and the U-Tapao River, Thailand. Therefore, it is necessary to determine these relationships locally. The cantilever failure results revealed that the dominant failure mechanism of the experimental works is beam-type failure, while shear-type failure is the dominant failure in the U-Tapao River. Finally, a numerical model was developed based on a triple-grid approach to simulate the behaviour of a cantilever within the framework of fluvial erosion, cantilever failure, and bedload transportation. The simulated results showed good agreement with experimental results, and the U-Tapoa River in terms of the temporal variations of spatially averaged bank width, dominant failure mechanism, and overhanging block dimensions.

Experiments and modelling of cantilever failures for cohesive riverbanks

ABSTRACTThis paper presents experiments and modelling of cantilever failures for a cohesive riverbank. Two types of cohesive materials with different bank cohesions were investigated under similar hydraulic conditions to clarify and simulate the failure phenomena. The experimental results showed that fluvial erosion undermined the banks during the initial stage of failure, and tension cracks then developed at the upper surface of the banks. Beam failure occurred thereafter. Additionally, a numerical model was developed using a triple-grid approach to simulate the behaviour of a cantilever within the framework of fluvial erosion, cantilever failure, slump block, and sediment transport. Moreover, cantilever failure was analysed using a limit equilibrium method to compute the stability of the overhanging block. The numerical model exhibited good agreement with the experimental results in terms of the spatial-averaged bank width along the banks and of the dominant failure type, which was beam failure.

Cantilever failure investigations for cohesive riverbanks

As a cantilever failure involves rapid channel widening and delivers a large volume of sediment into the channel, it is important to understand this mechanism from the river engineering point of view. Different types of riverbank failures have been investigated in several previous studies, but these works have limitations in understanding the complex mechanisms of cantilever failure regarding the coupling of fluvial erosion with that failure. Experiments were thus carried out to assess the underlying mechanisms of cantilever failure using three types of cohesive materials classified on the basis of their percentages of silt–clay content. The experiments showed that fluvial erosion of the lower part of cohesive riverbanks progressively undermines the upper part during the initial stage of a cantilever failure. Tension cracks then develop at the upper surface of the cohesive riverbanks and beam failure occurs thereafter. A mathematical model of cantilever failure was also developed. The model uses a triple-grid approach to simulate the behaviour of a cantilever within the framework of fluvial erosion and the cantilever's subsequent failure. The simulated results show good agreement with the experimental results in terms of spatial-averaged bank width and water level along cohesive riverbanks.

Wave-current induced erosion of cohesive riverbanks in northern Manitoba, Canada

Abstract. The field of cohesive soil erosion is still not fully understood, in large part due to the many soil parameters that affect cohesive soil erodibility. This study is focused on two channels, 2-Mile and 8-Mile channels in northern Manitoba, Canada, that were built to connect Lake Winnipeg with Playgreen Lake and Playgreen Lake with Kiskikittogisu Lake, respectively. The banks of the channels consist of clay rich soils and alluvial deposits of layered clay, silts and sands. The study of erosion at the sites is further complicated because the flow-induced erosion is combined with the effects of significant wave action due to the large fetch length on the adjacent lakes, particularly Lake Winnipeg that is the seventh largest lake in North America. The study included three main components: field measurements, laboratory experiments and numerical modelling. Field measurements consisted of soil sampling from the banks and bed of the channels, current measurements and water sampling. Grab soil samples were used to measure the essential physical and electrochemical properties of the riverbanks, and standard ASTM Shelby tube samples were used to estimate the critical shear stress and erodibility of the soil samples using an erosion measurement device (EMD). Water samples were taken to estimate the sediment concentration profile and also to monitor changes in sediment concentration along the channels over time. An Acoustic Doppler Current Profiler (ADCP) was used to collect bathymetry and current data, and two water level gauges have been installed to record water levels at the entrance and outlet of the channels. The MIKE 21 NSW model was used to simulate waves using historical winds and measured bathymetry of the channels and lakes. Finally, results from the wave numerical model, laboratory tests and current measurement were used to estimate the effect of each component on erodibility of the cohesive banks.

Coupled Study of Fluvial Erosion and Overhanging Failure for Cohesive Riverbanks

Coupled Study of Fluvial Erosion and Overhanging Failure for Cohesive Riverbanks

Experimental Study of Cantilever Failure in the Upper Part of Cohesive Riverbanks

Experimental Study of Cantilever Failure in the Upper Part of Cohesive Riverbanks

On the retreat of forested, cohesive riverbanks

Data from the Brandywine Creek in Pennsylvania and the South River in Virginia document spatial and temporal patterns describing the retreat of gently curving bends with forested riparian zones containing a mix of very large and smaller trees, cohesive banks, and slow rates of migration (∼ 0.1 m/yr). Field data include 2.5 years of bank profile surveys, measurements of tree morphology, and ground-based laser scanner (LiDAR) surveys repeated biannually. These banks retreat through a cycle of erosion on decadal timescales. Initially, small volumes (mean = 0.173 m 3 per bank erosion event) of soil are removed between large trees growing on the bank. Near trees, the rate of bank retreat is initially negligible, creating a scalloped bank morphology buttressed by large trees. As the trees are increasingly exposed to the flow, they become slowly undercut, gradually leaning into the channel and sliding down to the toe of the bank, eventually toppling into the river after a few years, which restarts the cycle. A magnitude–frequency analysis of monitoring data from the Brandywine Creek indicates that the effective bank erosion event only 0.11 m 3; thus, forested bank retreat is dominated by small soil failures. Large erosion events, many associated with trees toppling into the river, greatly increase the variance of time-averaged rates of bank erosion, such that almost 4 years of monitoring are needed to measure time-averaged rates of erosion to within an accuracy of 10%. Because of the limited spatial extent of our survey data and the absence of large discharges, these results are provisional. Improved process-based models to predict the retreat of forested riverbanks should account for the spatial structure of riparian forests and the interactions between the flow, the gradual removal of bank soils, and the ongoing efforts of the trees to remain attached to the bank.

Experimental Study on Critical Shear Stress of Cohesive Bed Materials for Erosion

The aim of the study is to investigate effects of sand-silt-clay content, moisture content, and salinity on critical shear stress (τc) of cohesive bed material for erosion. The authors used the non-vertical submerged jet test apparatus designed by Hanson et al.(2002) to carry out both laboratory and in situ measurements. Forty remolded samples were made with various sand-silt-clay content, moisture content and salinity. The laboratory experimental results showed that τc depended on the sand-silt-clay content, moisture content, and salinity. The τc increased when the rate of clay content increased. The value of τc decreased when the moisture content increased. The value of τc increased when the salinity increased. The results of five in situ experiments of the Soai Rap cohesive riverbanks, southern Viet Nam also showed that the value of τc also increased when the rate of clay content increased. The erodibility (kd) of cohesive bed material can also be determined when τc is measured.

Secondary current effects on cohesive river bank erosion

The emphasis of this research was placed on the development of a methodology that allows (1) an adequate representation of the distribution of the near‐bank shear stress τs when secondary currents are present and (2) estimation of the critical erosional strength τcr and other sediment erodibility parameters for fluvial erosion. These two factors combine to form the overarching objective of this investigation, which was carried out in Union Flat Creek, a stream with pronounced cross‐sectional irregularities located within the loess region of the Palouse basin of Washington State, United States. The study has shown that the presence of the secondary currents increases the magnitude of the depth‐averaged sidewall shear stress at least by a factor of 2.0. It was also found that the ratio of the maximum to the depth‐averaged sidewall shear stress is greater than 5. These findings suggest therefore that use of the may be a good approximation of the fluid shear stress magnitude for simple channel geometries but not for natural channels characterized by complex geometries. The measured critical erosional strength (τcr = 4.16 Pa) was comparable with the findings of other laboratory and field studies that also focused on Palouse sediments.

Coupled simulations of fluvial erosion and mass wasting for cohesive river banks

The erosion of sediment from riverbanks affects a range of physical and ecological issues. Bank retreat often involves combinations of fluvial erosion and mass wasting, and in recent years, bank retreat models have been developed that combine hydraulic erosion and limit equilibrium stability models. In related work, finite element seepage analyses have also been used to account for the influence of pore water pressure in controlling the onset of mass wasting. This paper builds on these previous studies by developing a simulation modeling approach in which the hydraulic erosion, finite element seepage, and limit equilibrium stability models are, for the first time, fully coupled. Application of the model is demonstrated by undertaking simulations of a single flow event at a single study site for scenarios where (1) there is no fluvial erosion and the bank geometry profile remains constant throughout, (2) there is no fluvial erosion but the bank profile is deformed by simulated mass wasting, and (3) the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass wasting. The results are limited in scope to the specific conditions encountered at the study site, but they nevertheless demonstrate the significant role that fluvial erosion plays in steepening the bank profile or creating overhangs, thereby triggering mass wasting. However, feedbacks between the various processes also lead to unexpected outcomes. Specifically, fluvial erosion also affects bank stability indirectly, as deformation of the bank profile alters the hydraulic gradients driving infiltration into the bank, thereby modulating the evolution of the pore water pressure field. Consequently, the frequency, magnitude, and mode of bank erosion events in the fully coupled scenario differ from the two scenarios in which not all the relevant bank process interactions are included.

Hydraulic erosion of cohesive riverbanks

This study identifies and assesses the controls on hydraulic erosion of cohesive riverbanks on a 600-m reach of an urban ephemeral stream with active bank erosion. We examined hydraulic bank erosion by separating estimated bank shear stress into four properties: magnitude, duration, event peak, and variability. The values of these independent variables were used as a bank erosion context at three transects. Stepwise regression showed that the event peak (maximum peak) of excess shear stress best predicts cohesive bank erosion at the two transects with moderate critical shear stresses (1.93–4.08 N/m 2), while the variability (all peaks) of excess shear stress best predicts erosion at the transect with low critical shear stress (0.95 N/m 2). These results suggest that the amount of hydraulic erosion of cohesive riverbanks is dictated by flow peak intensities. Finally, the results of this study were combined with results from previous bank erosion studies to produce a conceptual model for estimating bank erosion rates based on their silt–clay content.

Two-dimensional numerical modeling of the longitudinal and lateral channel deformations in alluvial rivers

Two kinds of bank erosion mechanisms were analyzed, including fluvial and non-fluvial controlled mechanisms, and mechanical methods of simulating the erosion processes of cohesive, non-cohesive and composite riverbanks were improved. Then a two-dimensional numerical model of the channel deformation was developed, consisting of a 2D flow and sediment transport submodel and bank-erosion submodels of different soil riverbanks. In the model, a new technique for up dating the bank geometry during the bed evolution was presented, which combines closely two kinds of submodels. The proposed model is capable of not only predicting the processes of flood routing and longitudinal channel deformation in natural rivers, but also simulating the processes of lateral channel deformation, especially the processes of lateral erosion and failure of cohesive, non-cohesive and composite river banks.

Study of Cohesive Riverbank Erosion Mechanism through Analysis of Flow Fields Near and Inside Eroded Bank

Study of Cohesive Riverbank Erosion Mechanism through Analysis of Flow Fields Near and Inside Eroded Bank

Computer program for stability analysis of steep, cohesive riverbanks

The ability to predict the stability of eroding riverbanks is a prerequisite for modelling alluvial channel width adjustment and a requirement for predicting bank erosion rates and sediment yield associated with bank erosion. Mass-wasting of bank materials under gravity occurs through a variety of specific mechanisms, with a separate analysis required for each type of failure. This paper presents a computer program for the analysis of the stability of steep, cohesive riverbanks with respect to planar-type failures. Planar-type failures are common along stream channels destabilized by severe bed degradation. Existing stability analyses for planar-type failures have a number of limitations that affect their physical basis and predictive ability. The computer program presented here is based on an analysis developed by Darby and Thorne. The software takes account of the geotechnical characteristics of the bank materials, the shape of the bank profile, and the relative elevations of the groundwater and surface water to estimate stability with respect to mass failure along a planar-type failure surface. Results can be displayed either in terms of a factor of safety (ratio of resisting to driving forces), or probability of failure. The computer analysis is able to determine the relative amounts of bed degradation and bank-toe erosion required to destabilize an initially stable bank. Data for the analysis are supplied in the form of either HEC-2 hydrographic survey data files or user-supplied bank profile data, in conjunction with user-supplied geotechnical parameter values. Some examples, using data from the Upper Missouri River in Montana, are used to demonstrate potential applications of the software. Copyright © 2000 John Wiley & Sons, Ltd.