UC Santa Barbara

Formulas for (i) the components of acceleration, (ii) the volumetric gravitational, pressure-dependent, and friction forces, and (iii) stability conditions for aquifer solids in groundwater flowing under steady-state conditions are derived in this work. The tangential acceleration governs the magnitude of the average pore velocity along the flow path of ground water. The normal acceleration component arises from changes in the direction of the average pore velocity. The frictional force exerted by the aquifer matrix on ground water is derived via Newton's second law of motion. It is shown that the friction force is of approximately the same magnitude (but opposite in direction) to the drag or seepage force exerted by moving ground water on the aquifer matrix. Loss of aquifer solids by ground-water motion may occur whenever the contact force that holds aquifer particles static is not sufficient to counterbalance the resultant of the drag force plus the buoyant unit weight of aquifer. The hydraulic shear force exerted by stream flow contributes to the detachment of channel-bottom particles, although this force is perpendicular to the ground-water induced drag force under uniform stream flow regime. Key variables governing the detachment of aquifer solids by moving ground water are the hydraulic gradient, flow-path geometry, and the unit weights of aquifer and ground water. © 2011 ASCE.