Stability of composite river banks
01 Sep 1981-Earth Surface Processes and Landforms (John Wiley & Sons, Ltd)-Vol. 6, Iss: 5, pp 469-484
TL;DR: The stability of a cantilever depends on the balance of forces, motive and resistive, associated with the most critical mechanism of failure as mentioned in this paper, i.e., shear, beam and tensile failure.
Abstract: The stability of a river bank depends on the balance of forces, motive and resistive, associated with the most critical mechanism of failure. Many mechanisms are possible and the likelihood of failure occurring by any particular one depends on the size, geometry and structure of the bank, the engineering properties of the bank material, the hydraulics of flow in the adjacent channel and climatic conditions.
Rivers flowing through alluvial deposits often have a composite structure of cohesionless sand and gravel overlain by cohesive silt/clay. Bank erosion occurs by fluvial entrainment of material from the lower, cohesionless bank at a much higher rate than material from the upper, cohesive bank. This leads to undermining that produces cantilevers of cohesive material. Upper bank retreat takes place predominantly by the failure of these cantilevers. Three mechanisms of failure have been identified: shear, beam and tensile failure.
The stability of a cantilever may be analysed using static equilibrium and beam theory, and dimensionless charts for cantilever stability constructed. Application of the charts requires only a few simple measurements of cantilever geometry and soil properties. In this analysis the effects of cracks and fissures in the soil must be taken into account. These cracks seriously weaken the soil and can invalidate a stability analysis by affecting the shape of the failure surface.
Following mechanical failure, blocks of soil must be removed from the basal area by fluvial entrainment if rapid undermining and cantilever generation are to continue. Hence, the rate of bank retreat is fluvially controlled, even though the mechanism of failure of the upper bank is not directly fluvial in nature.
This cycle of bank erosion: undermining, cantilever failure and fluvial scour of the toe, operates over several flood events and has important implications for river engineering, channel changes, and the movement of sediment through fluvial systems.
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TL;DR: In this article, the authors presented regime type equations for mobile gravel-bed rivers based on data obtained from 62 stable gravel bed river reaches in the United Kingdom, and applied multiple regression techniques to derive equations relating reach average and riffle values of width, mean and maximum depth, slope, velocity, sinuosity and ruddle spacing to bank full discharge, bed and bank material characteristics, valley slope, bank vegetation type and an independent estimate of bank full bed load load transport rates.
Abstract: Regime type equations for mobile gravel-bed rivers are presented based on data obtained from 62 stable gravel-bed river reaches in the United Kingdom. Multiple regression techniques are applied to derive equations relating reach average and riffle values of width, mean and maximum depth, slope, velocity, sinuosity and riffle spacing to bankfull discharge, bed and bank material characteristics, valley slope, bank vegetation type and an independent estimate of bankfull bed load transport rates. Although discharge has a dominant control on channel geometry, these equations indicate that bed load discharge also has a significant influence, particularly with regard to channel slope. Bank vegetation has a major control on width and velocity while depth, velocity and slope are storngly affected by bed material size. Reasons for these results are considered in terms of the physical processes controlling channel adjustment. The application of these regime equations is discussed, and particular consideration is given to the design of sinuous channels with a pool-riffle bed topograph.
575 citations
TL;DR: In this article, a detailed field and laboratory studies of the geometry, flow and sedimentary processes in braided rivers of simple geometry, in single river bends, in channel confluences, and using some theoretical reasoning, it has been possible to construct fully 3D qualitative and quantitative models of braided river deposits.
Abstract: Abstract Models of braided-river deposition must be detailed, fully 3D, and preferably quantitative to be of use in understanding and predicting the nature of ancient deposits. In order to construct and validate adequate predictive models it is necessary to have information on: (1) variation and interaction of channel geometry, water flow and sediment transport in time and space in modern channel belts, as these control erosion and deposition, the formation and migration of channels and bars, and channel abandonment and filling; (2) 3D variation of bed geometry, texture, sedimentary structures and paleocurrents throughout modern channel-belt deposits, including the age and spatial arrangement of preserved parts of bars and channel fills; (3) long-term (more than hundreds of years) trends in channel and floodplain geometry, flow and sedimentary processes in order to understand channel-belt movements such as avulsions, and the spatial arrangement of channel-belt deposits relative to overbank deposits. Such information is rare because: (1) it is difficult to study modern braided-river geometry, flow and sedimentary processes throughout a range of the all-important high discharges; (2) detailed reconstructions of braided channel and bar geometry and movement are only available for the past half-century and cannot readily be linked to causative mechanisms; (3) 3D documentation of modern deposits below the water table (especially large scale features like lateral-accretion bedding) requires extensive coring and dating of the deposits, and geophysical profiling. As a result of this lack of information, and because of the quality of analysis and presentation of the information available, existing braided-river facies models are virtually useless as interpretive and predictive tools. The nature of the information available is critically reviewed. Using information from recent detailed field and laboratory studies of the geometry, flow and sedimentary processes in braided rivers of simple geometry, in single river bends, in channel confluences, and using some theoretical reasoning, it has been possible to construct fully 3D qualitative and quantitative models of braided river deposits. These models can be used to provide sophisticated quantitative interpretations of palaeochannel geometry, hydraulics and migration, as illustrated by comparison with some particularly well described examples of ancient braided river deposits.
445 citations
TL;DR: A detailed review and chronological survey of the various techniques which have been used for the measurement of river bank erosion and channel change is presented in this article, where the techniques are classified according to the time scales involved (long, intermediate and short).
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392 citations
TL;DR: A conceptual framework for alternatives that address bank erosion issues is developed that conserve riparian linkages at appropriate temporal and spatial scales, consider integral relationships between physical bank processes and ecological functions, and avoid secondary and cumulative effects that lead to the progressive channelization of rivers.
Abstract: Bank erosion is integral to the functioning of river ecosystems. It is a geomorphic process that promotes riparian vegetation succession and creates dynamic habitats crucial for aquatic and riparian plants and animals. River managers and policymakers, however, generally regard bank erosion as a process to be halted or minimized in order to create landscape and economic stability. Here, we recognize bank erosion as a desirable attribute of rivers. Recent advances in our understanding of bank erosion processes and of associated ecological functions, as well as of the effects and failure of channel bank infrastructure for erosion control, suggest that alternatives to current management approaches are greatly needed. In this article, we develop a conceptual framework for alternatives that address bank erosion issues. The alternatives conserve riparian linkages at appropriate temporal and spatial scales, consider integral relationships between physical bank processes and ecological functions, and avoi...
308 citations
References
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2,139 citations
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30 Jun 1972
TL;DR: In this paper, the interaction of force and resistance in geomorphic systems and models is discussed. But the authors do not consider the effects of wind and rain on the dynamics of these systems.
Abstract: Preface Introduction 1. Geomorphic systems and models 2. Starting points: systems of reference Part I. Force and Resistance: 3. Force: sources of energy for debris transport 4. Resistance Part II. Process: The Interaction of Force and Resistance: 5. Process: introduction 6. Instability processes in rock masses 7. Instability in soil masses 8. Surface water erosion 9. Sub-surface water erosion 10. Soil creep part III. Form: comparison of Real Forms with Process-Response Models: 11. Humid temperate areas 12. The periglacial landscape 13. Semi-arid and arid landscapes 14. Tropical landforms Part IV. Synthesis: 15. Slope profiles 16. Slopes in drainage basins Appendices.
808 citations
TL;DR: The River Klaralven a Study of Fluvial Processes as discussed by the authors is a study of fluvial processes in the literature, which is related to our work.
Abstract: (1956). The River Klaralven a Study of Fluvial Processes. Geografiska Annaler: Vol. 38, No. 2-3, pp. 125-316.
454 citations