The Role of Pore-Water Pressures and Upward-Directed Seepage Forces in the Erosion of Cohesive Streambeds

The entrainment and erosion of cohesive bed material, is partly controlled by the magnitude and distribution of positive and negative pore-water pressures. Upward-directed seepage forces within cohesive streambeds provide a mechanism of entrainment for flocculated aggregates. In contrast, suction caused by negative pore-water pressures is found to increase the shear strength of unsaturated cohesive bed and bank materials. By accounting for resisting forces such as particle weight, cohesion and matric suction, and driving forces such as fluid drag and upward-directed seepage forces during the recessional limb of stormflow hydrographs, a numerical scheme for evaluating the potential for erosion of cohesive aggregates is obtained. It is the resistance to entrainment provided by cohesion (enhanced by matric suction) operating over the contact surface area of the particle that makes these materials more difficult to entrain than cohesionless particles of the same size. A hypothesis for detachment and erosion of chips or blocks of cohesive bed material is proposed: (1) Large (5 m), rapid rises in stage increase pore pressures and decrease matric suction dramatically in the region just below the bed surface; (2) A relatively rapid decrease in stage causing a loss of downward water pressure combined with low-rates of pore-pressure dissipation result in steepened hydraulic gradients just below the bed surface; and (3) A resulting net upward seepage force is great enough to contribute to detachment and entrainment of cohesive bed material.