Part 3 TRIGGER MECHANISMS FOR DEBRIS FLOWS

Groundwater percolates through weathered bedrock ribs Insitu field instrumentation suggests that runoff seeps into the exposed bedrock ribs and collects within the bedrock depression, beneath the overlying colluviumInsitu field instrumentation suggests that runoff seeps into the exposed bedrock ribs and collects within the bedrock depression, beneath the overlying colluvium

Emergent seepage forces Four common conditions that promote development of excess uplift pressures due to elevated seepage levelsFour common conditions that promote development of excess uplift pressures due to elevated seepage levels

Pore pressures develop quickly in the pervious bedrock ribs, then dissipate quickly (upper left).Pore pressures develop quickly in the pervious bedrock ribs, then dissipate quickly (upper left). Pore pressures within the colluvium continue to rise due to areal accretion, and dissipate very slowly (middle left)Pore pressures within the colluvium continue to rise due to areal accretion, and dissipate very slowly (middle left)

Explosive Failures When pore water accumulates in these spoon-shaped bedrock depressions (upper right), explosive ‘blow-out’ failures can ensure, as shown in the photo at upper left.When pore water accumulates in these spoon-shaped bedrock depressions (upper right), explosive ‘blow-out’ failures can ensure, as shown in the photo at upper left.