Search Results (4)

The dominant mechanism of riverbank cantilever failure is soil erosion of the bank toe and near bank zone. This paper demonstrates that the shape of the riverbank cantilever failure depends on the properties of the soil and the fluctuation of the river water level (RWL). With a stable RWL, a riverbank with higher resistance force leads to failure with larger and deeper overhang erosion width. When RWL rises, a less cohesive soil bank will be eroded over a larger width and riverbank failure will occur earlier. With a low rate of rising RWL, riverbank failure may happen in a type of mass failure. With a high rate of rising RWL, a riverbank will fail in a type of overhang riverbank failure, with the soil erosion rate being the main affected factor. Full article

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. Full article

River morphological evolution is a challenging topic, involving hydrodynamic flow, sediment transport and bank stability. Lowland rivers are often characterized by the coexistence of granular and cohesive material, with significantly different behaviours. This paper presents a bidimensional morphological model to describe the evolution of the lower course of rivers, where there are both granular and cohesive sediments. The hydrodynamic equations are coupled with two advection–diffusion equations, which consider the transport of granular and cohesive suspended sediment concentration separately. The change of bed height is evaluated as the sum of the contributions of granular and sediment material. A bank failure criterion is developed and incorporated into the numerical simulation of the hydrodynamic flood wave and channel evolution, to describe both bed deformation and bank recession. To this aim, two particular mechanisms are considered: the former being a lateral erosion due to the current flow and consequent cantilever collapse and the latter a geostatic failure due to the submergence. The equation system is integrated by means of a finite volume scheme. The resulting model is applied to the Tagliamento River, in northern Italy, where the meander migration is documented through a sequence of aerial images. The channel evolution is simulated, imposing an equivalent hydrograph consisting of a sequence of flood waves, which represents a medium year, with reference to their effect on sediment transport. The results show that the model adequately describes the general morphological evolution of the meander. Full article

The outer banks of meadow-type meandering river bends in the source zone of the Yellow River are especially vulnerable to bank failure. This study aims to understand how vegetation affects bank stability and the mechanism of bank failure, especially via a prediction of the width of a collapsed block of small rivers through a proposed bank stability equilibrium as well as field sampling. Soil and vegetation properties were surveyed at four sites near the riverbank in 2013–2016. It was found that the failed blocks had, on average, a dimension of 0.865 m (width) by 0.817 m (thickness) by 2.228 m (length). The variability in the size of all the failed blocks was attributed predominantly to the roots of plants. Block thickness could be logarithmically predicted by root length at R² ≥ 0.76. The block width predicted from the proposed equilibrium equation deviated from in situ measurements by approximately 22.1%, a discrepancy highly subject to the overestimation of root reinforcement using Wu’s model. By reducing the coefficient of Wu’s model from 1.2 to 0.85, the proposed equilibrium equation was reliable to predict the width of bank collapse. However, its applicability to other study areas needs to be verified in further studies. Full article