Subglacial Water Pressures and the Shape of Subglacial Conduits

Measured subglacial water pressures are frequently higher than theoretical values calculated by assuming that subglacial conduits are straight and either circular or semi-circular in shape. While this may be attributed to errors in the estimates of conduit roughness or ice viscosity, we suggest here an alternative explanation: namely, that the conduits are actually broad and low rather than semi-circular. Good agreement between measured and calculated pressures can be obtained by assuming that the cross-sectional shape of conduits resembles the space between the arc of a circle and its chord. The angle subtended by the arc, θ, is treated as an adjustable parameter. In four cases studied, θ ranged from 2° to 36°.