ON TRACTION-CARPET SEDIMENTATION

Traction carpets are highly concentrated bedload layers that are devel oped beneath and driven by turbulent overlying flows, They have a ''co nvex-up'' velocity profile and comprise a lower frictional and an uppe r collisional region. The frictional region, having a particle concent ration more than 80% of the packed bed, is characterized by nearly con tinuous grain contacts, low strain rate, and hampered grain segregatio n, On the other hand, the collisional region has a particle concentrat ion between 15% and 80% of the packed bed and is characterized by acti ve grain collisions and higher strain rate, Deposition from the tracti on carpets occurs via progressive aggradation of the bed, rather than via en masse freezing, while the downward grain flux from the overlyin g flow maintains them, The thickness of a traction-carpet stratum is t herefore determined by the cumulative amount of sediment settled durin g the lifespan of a traction carpet and can be much larger than the th ickness of a moving traction carpet, Inverse grading can be produced i n the collisional region by the vertical gradient of dispersive pressu re, which is related to nonuniform distribution of particles, When a t hick frictional region develops, however, the inverse size distributio n in the collisional region is poorly recorded in the deposits. Deposi tional features of traction-carpet strata are therefore determined by the duration of a traction carpet and the thickness ratio of collision al to frictional regions, The thickness ratio is further controlled by the applied shear stress, sediment fallout rate, and grain size, Gene rally, a collisional region is better developed beneath a highly compe tent and coarse-grained (gravelly) flow, whereas a thick frictional re gion is developed beneath a heavily sediment-laden fine-grained (sandy ) flow, This explains the more common occurrence of inverse grading in coarse-grained deposits.