Pore water pressure and streambank stability: Results from a monitoring site on the Sieve River, Italy

To investigate the role of pore water pressures in the stability of a strea mbank, a series of tensiometers and piezometers was installed in a bank of the Sieve River, Tuscany, Italy. Fluvial entrainment at the bank toe was mo nitored by repeated cross-profiling, erosion pins and marked pebbles. Fluct uations in matric suction measured at the tensiometers reflected the overal l response of pore water pressures to rainfall, evapotranspiration, rising and drawdown of the river stage, and variations in water table. An expressi on was derived for the safety factor with respect to mass movement of the u pper bank, incorporating the failure criterion for unsaturated soils and th e normal Mohr-Coulomb criterion for saturated conditions. Variations in mat ric suction have important effects on the stability of the streambank. Duri ng low-flow periods, the shear strength term due to the matric suction allo ws the bank to remain stable at a steep angle. However, during rainfall and now events, reduction in matric suction and increase in unit weight of the material from vertical and lateral infiltration may be sufficient to trigg er a mass failure, without development of significant positive pore water p ressures. During the rising limb of high-flow events, the factor of safety increases as a consequence of the stabilizing confining pressure of the wat er in the river, despite a reduction in matric suction. During drawdown in the river, when the suction values are still low and the confining pressure in the river decreases to zero, the factor of safety falls to lower values than those experienced prior to the runoff event. Measurements of fluvial entrainment reveal that, although the processes, mechanisms and the frequen cy of retreat of basal and upper bank zones differ significantly, the amoun t of retreat at the bank toe due to fluvial erosion is comparable to that o f the upper portion of the bank due to mass failure. Copyright (C) 1999 Joh n Wiley & Sons, Ltd.