Bank‐toe processes in incised channels: the role of apparent cohesion in the entrainment of failed bank materials

Abstract

AbstractNumerous processes may instigate bank retreat and the consequent collection of failed cohesive materials at the bank toe. Cohesion between the failed material and the substrate can provide additional strength to resist direct fluvial entrainment. Failed, cohesive material can act as a form of natural bank‐toe protection by consuming and diverting flow energy that may otherwise be used to further scour the basal zone of incising channels.Investigations in Goodwin Creek, Mississippi, have revealed the existence of apparent cohesion between failed, cohesive blocks and their underlying surface. The method used to assess this cohesion involved a pulley system mounted on a tripod and supporting a load cell. Mean and maximum apparent‐cohesion values of 1·08 kPa and 2·65 kPa, respectively, were measured in this way, identifying a source that bonds blocks to the underlying surface. Cohesion values and types vary spatially and temporally. Tensiometric tests beneath blocks suggest that cohesion resulting from matric suction alone may be as much as 3·5 kPa in summer and 1·8 kPa in winter.Apparent cohesion is believed to have been sufficient to help prevent removal of the largest blocks by a peak flow of 66·4m3/s on 23 September 1997. Maximum excess shear stress required to entrain a D75 block can be augmented by as much as 97% by the presence of apparent cohesion at the block–substrate interface when compared with a condition with zero apparent cohesion at the block underside. Given these findings, it is no longer sufficient to estimate block entrainment in the basal area from block size or bed roughness alone, as in a Shields‐type approach. Copyright © 2001 John Wiley & Sons, Ltd.