Abstract

Since the 1991 eruption of Mount Pinatubo, the specific sediment yields from watersheds draining its slopes have been the highest ever recorded. In spite of this overwhelming sediment load, rivers inundated by pyroclastic-flow deposits delivered almost half of the initial deposits to downslope alluvial and/or debris fans by 1996. Although most of this transport occurred by hyperconcentrated flows and debris flows (lahars), very high sediment transport rates also characterize low flow. Measured flow velocities and depths, channel properties, and the size of both mobile and stable clasts in the Sacobia and Pasig-Potrero Rivers indicate median bed-surface grain sizes of 2 to 8 mm, grain-size-independent dimensionless critical shear stresses (τ*c) of 0.016 to 0.041 and Manning's n values of 0.017 to 0.024, values well below those previously reported for steep mountain channels. The dramatic bed-surface fining, bed mobility, and selective transport in these extremely sediment-rich channels indicate that changes in grain size, τ*c values, and bed roughness can increase transport capacity in response to high sediment supply. Our observations also suggest viewing the apparently contradictory concepts of an equal threshold of bed mobility and grain-size–dependent selective transport as end-member concepts that apply to channels with low (or intermittent) and high (or continuous) sediment supply, respectively.

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