Braiding under glass

Water under glaciers and ice streams often flows under pressure over an erodible substrate. Do the channel patterns produced by such flows resemble those of their free-surface counterparts? We studied pressure-driven water flow over an erodible, noncohesive bed using a physical model with a transparent, rigid lid. The dominant channel pattern produced in the model is a widely distributed, braided network of broad, shallow channels. Relative to braided networks formed under equivalent free-surface conditions, those formed under pressure show higher braiding intensity, greater channel curvature and variability of flow direction, and more sharply defined channel margins. Increasing discharge increases braiding intensity, maximum channel size, and variability of flow direction. Downstream pressure gradients are insensitive to changes in discharge, which may in part reflect a tendency to maintain constant shear stress in the channels, as observed in rivers. Lateral pressure gradients measured in the pressurized model indicate that pressure surfaces are highly variable in both magnitude and direction over time and space. When converted to equivalent topographic slope, these pressure gradients represent much larger lateral slopes than are typically produced in rivers, accounting for the wider range of channel directions in the pressurized-flow experiments.