Bank and near-bank processes in an incised channel

Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion. The interaction of these forces control streambank mechanics. Hydraulic forces exerted by flowing water on in situ bank-toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore-water pressures. Negative pore-water pressures (matric suction) have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear. A stable bank can be transformed into an unstable bank during periods of prolonged rainfall through: 1. increase in soil bulk unit (specific) weight, 2. decrease or complete loss of matric suction, and, therefore, apparent cohesion, 3. generation of positive pore-water pressures, and, therefore, reduction or loss of frictional strength, 4. entrainment of in situ and failed material at the bank toe, and 5. loss of confining pressure during recession of stormflow hydrographs. Relatively small frequent flows during the winter have the ability to erode failed bank materials, maintain oversteepened, unstable bank surfaces and promote prolonged periods of bank retreat, channel migration and high yields of fine-grained sediment. Confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe, which steepens the bank. For example, more than 2 m of bank retreat occurred during the study period at the research site on Goodwin Creek, northern Mississippi. The loss of matric suction (negative pore pressures) due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability. Apparent cohesion, friction angle, soil bulk unit weight and moisture content were measured in situ. Matric suction was measured continuously, in situ with a series of five pressure-transducer tensiometers. A bank-failure algorithm, which combines the Mohr-Coulomb approach, for saturated conditions and the Fredlund modification for unsaturated conditions was developed for layered cohesive streambanks. The resulting equation has been used successfully to investigate the role of matric suction, positive pore-water pressures and confining pressure for layered streambanks composed of cohesive materials. (C) 2000 Elsevier Science B.V. All rights reserved.