Braiding under glass

Water under glaciers and ice streams often flows under pressure over an ero dible substrate. Do the channel patterns produced by such flows resemble th ose of their free-surface counterparts? We studied pressure-driven water fl ow over an erodible, noncohesive bed using a physical model with a transpar ent, rigid lid, The dominant channel pattern produced in the model is a wid ely distributed, braided network of broad, shallow channels, Relative to br aided networks formed under equivalent free-surface conditions, those forme d 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 insens itive to changes in discharge, which may in part reflect a tendency to main tain 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 grad ients represent much larger lateral slopes than are typically produced in r ivers, accounting for the wider range of channel directions in the pressuri zed-flow experiments.