THE VELOCITIES AND IMPACT FORCES OF DIFFERENT SIZE PARTICLES WITHIN DEBRIS FLOWS

Estimation of the potential boulder impact forces from a debris flow is important for the design of structural mitigation elements. Equations for the estimation of boulder impact forces are most sensitive to the inputs of particle size and particle velocity. Design guidelines recommend using a design boulder equal in size to the depth of flow that is traveling at the velocity of the flow, which may be overly conservative. This study used LoggerPro software to investigate the velocities and calculated impact forces of different size particles within debris flows.

Particle velocity was found to decrease with increasing particle size on the surface of granular flows where no matrix material was present between boulders and for fluid flows where few large-particle interactions occurred. No trend was found between particle velocity and particle size for flows that contained a high concentration of boulders within a viscous matrix.

For all analyzed flows, calculated impact forces were found to increase with increasing particle size. However, maximum impact forces commonly resulted from particles whose sizes were 60 to 90 percent of the largest particle within the flow.