1,721,009 research outputs found
Refined hydraulic geometry data for British gravel-bed rivers
No full textDatabases describing the dimensions of stable and adjusting river channels are necessary to calibrate and verify morphological models. However, existing databases do not provide quantitative descriptions of the strength and sedimentary properties of river bank materials. They are therefore incompatible with the requirements of modern models which take into account river bank erosion processes. This note describes the methods and results of a program of bank material sampling undertaken at 58 British gravel-bed rivers. Bank material data have been merged with existing hydraulic geometry data for these sites to provide a resource for those who wish to calibrate and verify analytical and numerical models of channel morphology. The merged dataset is available online from the URL: http://www.geog.soton.ac.uk/research/62rivers.<br/
A model of equilibrium bed topography for meander bends with erodible banks
No full textChannel curvature produces secondary currents and a transverse sloping channel bed, along which the depth increases towards the outer bank. As a result deep pools tend to form adjacent to the outer bank, promoting bank collapse. The interaction of sediment grains with the primary and secondary flow and the transverse sloping bed also causes meanders to move different grain sizes in different proportions and directions, resulting in a consistent sorting pattern. Several models have been developed to describe this process, but they all have the potential to over-predict pool depth because they cannot account for the influence of erodible banks. In reality, bank collapse might lead to the development of a wider, shallower cross-section and any resulting flow depth discrepancy can bias associated predictions of flow, sediment transport, and grain-size sorting. While bed topography, sediment transport and grain sorting in bends will partly be controlled by the sedimentary characteristics of the bank materials, the magnitude of this effect has not previously been explored. This paper reports the development of a model of flow, sediment transport, grain-size sorting, and bed topography for river bends with erodible banks. The model is tested via intercomparison of predicted and observed bed topography in one low-energy (5·3 W m-2 specific stream power) and one high-energy (43·4 W m-2) study reach, namely the River South Esk in Scotland and Goodwin Creek in Mississippi, respectively. Model predictions of bed topography are found to be satisfactory, at least close to the apices of bends. Finally, the model is used in sensitivity analyses that provide insight into the influence of bank erodibility on equilibrium meander morphology and associated patterns of grain-size sorting. The sensitivity of meander response to bank cohesion is found to increase as a function of the available stream power within the two study bends. <br/
The use of one- and two-dimensional hydraulic modelling to reconstruct a glacial outburst flood in a steep Alpine valley
Full text linkThe hydrologic characteristics of the 1943 outburst flood from the Glacier du Mont Miné, Switzerland, are herein reconstructed using field evidence (palaeostage indicators) in conjunction with shallow water modelling techniques. These techniques rely on accurately characterising the hydraulic roughness of the channel, the water height established as boundary conditions, and the main flow path during the former flood, but the selection of appropriate parameter values can be problematic and hence there is uncertainty in the estimated discharge. In this study, minimal flow discharge estimates derived from one-dimensional modelling were found to vary between 429 and 557 m3 s?1 as the hydraulic roughness (ks) and water height at the inlet were varied over a realistic range of values (0.8–1.4 m and 3.31–6 m, respectively), whereas flow rates derived via two-dimensional modelling were confined in a narrower, lower, range of 358–454 m3 s?1. This degree of sensitivity to bed roughness ks, boundary conditions and the spatial dimensions of the modelling approach is, for the one-dimensional modelling, higher than reported in previous studies, but the precision of flow discharge values reconstructed using the two-dimensional modelling approach appears to be acceptable, even for floods in the very steep valley (0.1 m/m) that is subject of this study
Modelling river-bank erosion processes and mass failure mechanisms: progress towards fully coupled simulations
No full textBank stability for predicting reach-scale land loss and sediment yield
No full textWhen extensive lengths of river become stabilized (e.g., by channel incision), riverbank erosion can result in considerable riparian land loss and the delivery of large volumes of sediment downstream. The ability to predict the stability and failure geometry of eroding riverbanks is therefore an important prerequisite in estimating the rate of bank erosion and sediment yield associated with bank erosion. In this paper, a new stability analysis for layered river banks is introduced. The new analysis differs from many previous analyses in that it takes into consideration the effects of positive pore water pressure in the saturated portion, and negative pore water pressure in the unsaturated portion, of the bank as well as the influence of hydrostatic confining pressure due to the water level in the river. In addition, the failure plane is not constrained to pass through the toe of the bank and the bank profile geometry is not restricted to an idealized special case. The predictive ability of the new bank stability analysis is assessed using data from two field sites. Subsequently, a methodology for applying the bank stability analysis at the scale of the river reach (0.1 to 10 km) is discussed. This method involves the use of empirical models of bed level adjustment to estimate the magnitude of incision at specific locations along the reach, with these estimates used to drive the stability analysis. Application of the new method is demonstrated with an example
EU Life Environment Project on Wise Use of Floodplains: Cherwell catchment restoration scenarios: final report
No full textIdentifying the effects of parameter uncertainty on the reliability of riverbank stability modelling
No full textBank retreat is a key process in fluvial dynamics affecting a wide range of physical, ecological and socioeconomic issues in the fluvial environment. To predict the undesirable effects of bank retreat and to inform effective measures to prevent it, a wide range of bank stability models have been presented in the literature. These models typically express bank stability by defining a factor of safety as the ratio of driving and resisting forces acting on the incipient failure block. These forces are affected by a range of controlling factors that include such aspects as the bank profile (bank height and angle), the geotechnical properties of the bank materials, as well as the hydrological status of the riverbanks. In this paper we evaluate the extent to which uncertainties in the parameterization of these controlling factors feed through to influence the reliability of the resulting bank stability estimate. This is achieved by employing a simple model of riverbank stability with respect to planar failure (which is the most common type of bank stability model) in a series of sensitivity tests and Monte Carlo analyses to identify, for each model parameter, the range of values that induce significant changes in the simulated factor of safety. These identified parameter value ranges are compared to empirically derived parameter uncertainties to determine whether they are likely to confound the reliability of the resulting bank stability calculations. Our results show that parameter uncertainties are typically high enough that the likelihood of generating unreliable predictions is typically very high (> 80% for predictions requiring a precision of < ± 15%). Because parameter uncertainties are derived primarily from the natural variability of the parameters, rather than measurement errors, much more careful attention should be paid to field sampling strategies, such that the parameter uncertainties and consequent prediction unreliabilities can be quantified more robustly
Numerical simulation of bank erosion and channel migration in meandering rivers
No full textA numerical model of river morphology for meander bends with erodible cohesive banks is herein developed and tested. The new model has three key features. First, it couples a two-dimensional depth-averaged model of flow and bed topography with a mechanistic model of bank erosion. Second, it simulates the deposition of failed bank material debris at and its subsequent removal from the toe of the bank. Finally, the governing conservation equations are implemented in a moving boundary fitted coordinate system that can be both curvilinear and nonorthogonal. This simplifies grid generation in curved channels that experience bank deformation, allowing complex planform shapes associated with irregular natural channels to be simulated. Model performance is assessed using data from two flume experiments and a natural river channel. Results are encouraging, but the model underpredicts the scour depth in pools adjacent to the outer bank and, consequently, underpredicts bank migration rates
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