Pore‐water pressure effects on the detachment of cohesive streambeds: seepage forces and matric suction

Abstract

Erosion of cohesive channel materials is not fully understood, but is assumed to occur largely as a result of hydraulic shear stress. However, field and laboratory observations of pore-water pressures in cohesive streambed materials reveal the presence of positive and negative pore-water pressure effects that may significantly affect the erosion process, as contributing and resisting forces respectively.Measurements of pore-water pressures below cohesive streambeds in the loess area of the midwestern USA were conducted in situ and in undisturbed cores with a digital, miniature tensiometer. Results disclosed matric suction values in the range of 15-50 kPa in eastern Nebraska and northern Mississippi. Repetitive tests in soft materials verified a change from positive pore-water pressures in the upper 10-15 cm, to negative pore-water pressures to depths of at least 50 cm. In firm materials, the entire sampled profile was unsaturated.Laboratory experiments were carried out in which synthetic hydrographs were imposed on undisturbed streambed cores from the same sites. Miniature tensiometers in the cores monitored the resulting pattern of pore-water pressures, and revealed upward directed seepage forces on the recessional limb of the hydrograph. Maximum calculated values of the force ranged from 10 to 275 kN for the materials and heads tested. The maximum value obtained after application and release of a 2Ð5 m head was 119 kN, with 275 kN after a 5Ð0 m head. These results were supported independently by subsequent simulations using a finite-element hydrology model coupled with a stress-deformation model.A numerical scheme was developed to calculate the forces acting on cohesive aggregates in an idealized streambed, and to evaluate the potential for their detachment. The scheme added upward-directed seepage as an additional driving force, and matric suction as an additional resisting force, to the commonly applied factors of particle weight, fluid drag and lift force. Results demonstrate that upward-directed seepage forces of the magnitude measured in the laboratory with 5Ð0 m stages have the potential to detach particles larger than 10 cm in diameter without requiring fluid drag and lift forces. When added to these hydraulic forces, erosion thresholds are lowered, enabling erosion at lower hydraulic stresses.A hypothesis for detachment of chips or blocks of cohesive bed material is proposed: (1) large (>5 m) rises in stage increase pore-water pressures or decrease matric suction dramatically in the region just below the bed surface; (2) a relatively rapid decrease in stage causing a loss of water pressure above the bed, combined with low-rates of excess pore-water pressure dissipation just below the bed surface result in steepened hydraulic gradients; and (3) a resulting net upward seepage force is great enough to contribute to detachment of cohesive bed material, or rupture the bed by exceeding the available strength and confining stress. Published in

Streambank erosion rates and loads within a single watershed: Bridging the gap between temporal and spatial scales

“…During high flow events in incised Walnut Creek, streamflow is confined within the channel and scours the streambanks over the entire bank height. In addition to the direct hydrologic scour, the steep streambanks become saturated during channel-full events and, are subject to mass failure when stream flow recedes and they are no longer supported by the flow in channel Simon and Collison, 2001). We have observed exceptionally flashy streamflow in Walnut Creek in response to precipitation when stage increased to the top edge of the channel and then decreased approximately 2.5 m within 8 h (Schilling et al, 2006).…”

Self-Organized Criticality in Riverbank Systems

“…From a localarea process perspective, for example, research has examined the effects of hydraulic shear and pore-water pressure on the instability of river banks (Simon and Collison 2001) and the effects of flow events on cohesion loss and subsequent bank failure during the falling limb of flow events (Lawler and Leeks 1992;Lawler, Thorne, and Hooke 1997;Rinaldi and Casagli 1999). Using these models, local failures can be explained if one knows the bank characteristics and the sequence of bank wetting and discharges.…”

Bank‐collapse processes in a valley‐bottom gully, western Iowa

“…Our general goal was to determine through monitoring over a 2-year period whether positive pore pressures associated with subsurface water flow were alone sufficient to instigate bank failures. The role of subsurface water in the failure of gully and river banks has received considerable attention (e.g., Bradford and Piest, 1977;Osman and Thorne, 1988;Higgins et al, 1990;Hagerty, 1991;Darby and Thorne, 1996;Casagli et al, 1999;Simon et al, 2000;Collison, 2001;Simon and Collison, 2001;Amiri-Tokaldany et al, 2003;Dapporto et al, 2001Dapporto et al, , 2003Rinaldi et al, 2004;Darby et al, 2007;Fox et al, 2007;Wilson et al, 2007), and some authors have isolated effects of reduced matric suction in the unsaturated zone (e.g., Fredlund et al, 1978;Casagli et al, 1999;Rinaldi et al, 2004). This study, motivated by observations of bank failures seated below the water table, focuses on only the saturated zone.…”