Submitted to: American Society of Civil Engineers Water Resources Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/1/2000
Publication Date: N/A
Interpretive Summary: Erosion of fine-grained (cohesive; silts and clays) streambeds is a complex issue because of the electro-chemical bonds that hold the particles together. Predicting erosion of these materials has generally been attempted using techniques that are based on the size and weight of the particles compared to the forces exerted by flowing water. This study combines field and laboratory experiments with numerical simulations to show that erosion of these materials can occur by other forces and mechanisms. Over the course of stormflow, forces exerted downward on the streambed by the water on top of the bed relative to forces within the bed itself result in a net upward-directed force which is capable of eroding blocks or chips of cohesive material. The paper describes these forces and the conditions under which these processes are likely to occur.
Technical Abstract: 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.