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

Abstract

Abstract: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 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 1Ð08 kPa and 2Ð65 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 3Ð5 kPa in summer and 1Ð8 kPa in winter.Apparent cohesion is believed to have been sufficient to help prevent removal of the largest blocks by a peak flow of 66Ð4 m 3 /s on 23 September 1997. Maximum excess shear stress required to entrain a D 75 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

Effects of wet meadow riparian vegetation on streambank erosion. 2. Measurements of vegetated bank strength and consequences for failure mechanics

“…The first effect reduces rates of fluvial erosion of cohesionless sediments underlying the vegetated layer, while the second and third effects result in an increased block residence time at the toe of the bank. The greater the cohesion of failed bank materials, the greater the force required to remove them (Wood et al, 2001). Once a block has failed, vegetation densities may increase due to enhanced water availability, which helps to further stabilize the block in place.…”

Coupled simulations of fluvial erosion and mass wasting for cohesive river banks

“…The basal endpoint concept emphasizes that the residence time of bank‐derived material stored in the basal zone is a critical factor controlling long‐term bank retreat rates. Neglecting the complicating effects of variations in the caliber of the failed bank material [ Wood et al , 2001], basal storage of eroded sediment is favored for riverbanks prone to large‐scale slide failure(s), that deliver large volumes of sediment, timed during the recession limb of the hydrograph (when the erosivity of the flow is declining). In contrast, bank environments favoring multiple small‐scale cantilever failures occurring before or at the peak of the event would likely substantially reduce the residence time of eroded bank material stored at the toe of the bank.…”

River meandering on Earth and Mars: A comparative study of Aeolis Dorsa meanders, Mars and possible terrestrial analogs of the Usuktuk River, AK, and the Quinn River, NV

“…Erosion of channel banks commonly occurs by slumping or toppling because of erosion of the less cohesive and largely unvegetated sediment in the lower part of channel banks from entrainment by river flow or seepage forces from ground water (e.g., Thorne and Tovey (1981), Thorne (1990), Thorne and Abt (1993), Thorne et al (1998a,b), Simon et al (1999), Wood et al (2001), and Simon and Collison (2002)). This slumping and toppling of the cohesive upper banks usually just lowers the vegetated or cohesive layers onto the lower bank, which is often protected from further erosion until the slumped material is eroded by the flow (Thorne and Tovey, 1981;Osman and Thorne, 1988;Simon et al, 1999;Wood et al, 2001;Parker et al, 2011).…”