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

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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 2·5 m head was 119 kN, with 275 kN after a 5·0 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 5·0 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 2001 by John Wiley & Sons, Ltd.