Debris‐bed friction of hard‐bedded glaciers

Field measurements of debris‐bed friction on a smooth rock tablet at the bed of Engabreen, a hard‐bedded, temperate glacier in northern Norway, indicated that basal ice containing 10% debris by volume exerted local shear traction of up to 500 kPa. The corresponding bulk friction coefficient between the dirty basal ice and the tablet was between 0.05 and 0.08. A model of friction in which nonrotating spherical rock particles are held in frictional contact with the bed by bed‐normal ice flow can account for these measurements if the power law exponent for ice flowing past large clasts is 1. A small exponent ( n < 2) is likely because stresses in ice are small and flow is transient. Numerical calculations of the bed‐normal drag force on a sphere in contact with a flat bed using n = 1 show that this force can reach values several hundred times that on a sphere isolated from the bed, thus drastically increasing frictional resistance. Various estimates of basal friction are obtained from this model. For example, the shear traction at the bed of a glacier sliding at 20 m a −1 with a geothermally induced melt rate of 0.006 m a −1 and an effective pressure of 300 kPa can exceed 100 kPa. Debris‐bed friction can therefore be a major component of sliding resistance, contradicting the common assumption that debris‐bed friction is negligible.