Field observations of basal forces and fluid pore pressure in a debris flow

Using results from an 8 m 2 instrumented force plate we describe field measurements of normal and shear stresses, and fluid pore pressure for a debris flow. The flow depth increased from 0.1 to 1 m within the first 12 s of flow front arrival, remained relatively constant until 100 s, and then gradually decreased to 0.5 m by 600 s. Normal and shear stresses and pore fluid pressure varied in‐phase with the flow depth. Calculated bulk densities are ρ b = 2000–2250 kg m −3 for the bulk flow and ρ f = 1600–1750 kg m −3 for the fluid phase. The ratio of effective normal stress to shear stress yields a Coulomb basal friction angle of ϕ = 26° at the flow front. We did not find a strong correlation between the degree of agitation in the flow, estimated using the signal from a geophone on the force plate, and an assumed dynamic pore fluid pressure. Our data support the idea that excess pore‐fluid pressures are long lived in debris flows and therefore contribute to their unusual mobility.