Bedload Layer Thickness and Disturbance Depth in Gravel Bed Streams

A field investigation in ten gravel bed stream reaches determined that substrate disturbance depth associated with a moving bedload layer was a small multiple of the bed surface D90. Disturbance depth during plane bed transport of coarse, heterogeneous mixtures appeared similar in magnitude to particle exchange depth and moving layer thickness. Maximum disturbance depth was distributed approximately uniformly over the most active areas of the streambed when local scour and fill were negligible. The distribution upper bound was the smaller of approximately 1.5 times the competent grain size or twice the surface D90, and was invariant with flow strength once the largest grains present were mobilized. Disturbance depth did not scale with grain sizes smaller than D50 when larger grains were mobilized. Thicker traction carpets were not predicted to occur because much larger shear stresses then observed naturally were needed to mobilize two or more layers of the bed simultaneously. Bedload transport rate in coarse streambeds is suggested to increase primarily with mobile fraction of bed surface area and grain velocity, than with layer thickness.