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

Numerous 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 natura l bank-toe protection by consuming and diverting flow energy that may other wise be used to further scour the basal zone of incising channels. Investigations in Goodwin Creek, Mississippi, have revealed the existence o f apparent cohesion between failed, cohesive blocks and their underlying su rface. The method used to assess this cohesion involved a pulley system mou nted on a tripod and supporting a load cell. Mean and maximum apparent-cohe sion values of 1.08 kPa and 2.65 kPa, respectively, were measured in this w ay, identifying a source that bonds blocks to the underlying surface. Cohes ion values and types vary spatially and temporally. Tensiometric tests bene ath 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 remov al of the largest blocks by a peak flow of 66.4 m(3)/s on 23 September 1997 . Maximum excess shear stress required to entrain a D-75 block can be augme nted by as much as 97% by the presence of apparent cohesion at the block-su bstrate interface when compared with a condition with zero apparent cohesio n 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 roug hness alone, as in a Shields-type approach. Copyright (C) 2001 John Wiley & Sons, Ltd.