Processes of Bank Erosion in River Channels

• Main Author: Thorne, C. R.
• Published: University of East Anglia 1978
• Subjects: 551.4
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.475148

Bank erosion processes are identified on the basis of current geomorphological and engineering literature and some preliminary field observations. Equations are developed to describe the stability of a bank in relation to the failure mechanisms associated with these processes. The terms in the equations are fundamental parameters of hydraulics and soil mechanics. The equations describe the static equilibrium of a potential failure surface in terms of a factor of safety. The lowest factor of safety for the potential failure surfaces associated with the various erosion processes, defines the case of limiting stability and the critical mode of failure. Comparison of the various factors of safety indicates the relative effectiveness of the process and allows identification of the dominant process (or processes) for a specific bank. Some of the stability analyses are standard civil engineering techniques, others are presented here for the first time. Special emphasis is placed upon consideration of low composite banks, which are ubiquitous to flood plain rivers with coarse beds. These banks are characterised by cantilever overhangs in the upper bank. Three modes of failure are identified and analysed. Beam and tensional failures dominate banks with highly cohesive top strata, shear failures are more likely in sandy soils with a low degree of cohesion. The accuracy and reliability of the new equations is tested by applying them to specific sections of bank at several experimental sites. Flow parameters are measured using an electromagnetic flowmeter and the patterns of secondary flow close to eroding banks in meander bends are closely investigated. Engineering parameters for bank materials are examined using standard civil engineering techniques. Predictions based on these data and the theoretical equations are tested against observations of bank erosion processes made using standard techniques for geomorphological process studies. The study shows that the stability of natural river banks can be described successfully using equations which are based on the basic principles of hydraulics and soil mechanics. It is apparent from the study that all river banks are fluvially controlled. This is the case because the nature of erosion processes operating on the bank is determined by the degree of fluvial activity at the base. The new equations presented here are not yet fully developed and further work is required before any of them could be adopted as standard techniques. They do however have a sound theoretical basis and their functional form is thought to be correct. In this case the equations should merit further consideration and development.