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 Ž . 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