Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/19/2000
Publication Date: N/A
Interpretive Summary: An improved method to predict the conditions and timing of bank failures is developed. This new technology incorporates the effects of the water pressures within a streambank provided by infiltrating rainfall, and the pressures tending to hold the back in place which are provided by streamflow. In addition the predictive equation accounts for variations in soil properties and permits the user to include multiple soil layers. The bank-stability equation is tested with field data collected from an intensive monitoring site on Goodwin Creek, northern Mississippi. Data collected at this actively eroding streambank includes cross-section surveys, water levels, bank pore-water pressures, and bank-material strength. In addition, information on the role of stream processes on eroding the base of the streambank is collected for use in predicting the type of flows necessary to steepen the base of the bank and to induce bank failures. Results indicate that relatively frequent flows (those that occur several times in a year) have sufficient power to remove most of the material collected at the bank tow, thereby maintaining a steep and unstable streambank.
Technical Abstract: Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion and, therefore, bank morphology. 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 low-bank profiles. The data from Goodwin Creek, Mississippi, 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. A stable bank is transformed into an unstable bank during periods of prolonged rainfall by : (1) increases in soil unit weight, (2) decrease or complete loss of matric suction, and, therefore, apparent cohesion, (3) generation of positive pore-water pressures, and, therefore 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. A bank-failure algorithm, which combines the Mohr-Coulomb approach for saturated conditions and the Fredlund modification for unsaturated conditions is developed for layered, cohesive streambanks. The resulting equation is used successfully to investigate the role of matric suction, positive pore-water pressures, and confining pressure, for the Goodwin Creek site.