199-4 THE VON KARMAN CONSTANT, ISN’T: NEAR-BED FLOW IN MODERATELY STEEP STREAMS

Session: Advances in Fluvial Processes and Sediment Transport, Part II

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Vertical-velocity profiles were measured in uniform flow at the center of straight, moderately-steep stream channels with a channel width greater than about 5 times depth, and channel length about 10 times width. Flows were free of suspended sediment with beds essentially free of periphyton and absent significant bedforms. The d₈₄ reference k_s was determined using a Wolman procedure to sample bed-particle flow-penetration height.  Velocity profiles were numerically depth-corrected for log-linear closure of v = 0 at z = k_s/30. Various kinematic and hydraulic properties were then assessed.  These results were compared with published hydraulic assessments of similar streams for which k_s was taken as 42 percent of the reported b-axis d₈₄.   The evaluation showed that in streams having uniform, two-dimensional, open-channel flow, k will obtain at the bed as 4.42•u_*/V.  This is the defining initial slope of virtual kinematic properties.  For the stream in general, depth of flow for about 2.7 k_s adequate submergence is approached near Weisbach f = 0.7 with k being approximately 1.3.  As Froude number and consequent relative depth increase, resistance to flow declines and when f < 0.066, then k < 0.4, and near-bed conditions for bed-load transport become evident. For uniform flow in the center of straight channel segments having adequate depth, channel width and length, these various aspects may be observed consistently across the full range of flow resistance and relative depth.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11185

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