Channel changes in badlands

Abstract

Stripping of the vegetation and soil from a 13-hectare site in Virginia underlain by coastal plain sediments created a rapidly evolving badland topography. Two types of channels developed: (1) sand-bed alluvial channels were graded to transport the bed material load supplied from slope erosion with available runoff, but they also generally eroded their beds slowly, and (2) steeper, bedrock-floored channels incised rapidly. In bedrock channels the erosion rate was proportional to the 4/9ths power of drainage area and the 2/3rds power of gradient. These exponents are consistent with a model in which the erosion rate is proportional to the bed shear during high flows. Due to rapid mass wasting and reduced runoff, the alluvial channels became as much as 50% steeper during the winter than the summer, with an attendant yearly cycle of winter aggradation and summer entrenchment. The gradients, their seasonal variability, and their downstream hydraulic geometry were consistent with the predictions of total load transport formulas for sand beds and high loads. The hydraulic geometry of alluvial channels in the Virginia badlands were similar to that on the Morrison Formation in the western United States.