Bank and near-bank processes in an incised channel

Gravitational forces acting on in situ bank material act in concert with hy draulic forces at the bank toe to determine rates of bank erosion. The inte raction of these forces control streambank mechanics. Hydraulic forces exer ted by flowing water on in situ bank-toe material and failed cohesive mater ial at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces e xerted on in situ bank material by groundwater, downward infiltration of ra inwater and lateral seepage of streamflow into and out of the bank are crit ical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the la g time inherent in reductions in shear strength due to losses of matric suc tion and generation of positive pore-water pressures. Negative pore-water p ressures (matric suction) have also been shown to increase the resistance o f failed cohesive blocks to entrainment by fluid shear. A stable bank can b e transformed into an unstable bank during periods of prolonged rainfall th rough: 1. increase in soil bulk unit (specific) weight, 2. decrease or complete loss of matric suction, and, therefore, apparent co hesion, 3. generation of positive pore-water pressures, and, therefore, reduction o r 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 yield s of fine-grained sediment. Confining pressures provided by stormflow are n ot 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 s ite on Goodwin Creek, northern Mississippi. The loss of matric suction (neg ative pore pressures) due to infiltrating precipitation has been found to b e as significant as the development of excess pore pressures in contributin g to mass bank instability. Apparent cohesion, friction angle, soil bulk un it weight and moisture content were measured in situ. Matric suction was me asured continuously, in situ with a series of five pressure-transducer tens iometers. A bank-failure algorithm, which combines the Mohr-Coulomb approac h, for saturated conditions and the Fredlund modification for unsaturated c onditions was developed for layered cohesive streambanks. The resulting equ ation has been used successfully to investigate the role of matric suction, positive pore-water pressures and confining pressure for layered streamban ks composed of cohesive materials. (C) 2000 Elsevier Science B.V. All right s reserved.