Changes In Turbulent-Stress Profiles In Response To Increasing Bed Roughness: Implications For Sediment Transport In High Gradient Streams
Pitlick, John 1 ; , GEOG 5100 students 2 ; Nelson, Jonathan 3
1 University of Colorado
2 University of Colorado
3 US Geological Survey
The bed roughness in headwater streams is generally large in comparison to the flow depths required to initiate bed load transport The roughness produced by cobbles and gravels distorts the vertical structure of the flow, to the point where a logarithmic velocity profile may no longer be present. The resulting changes in flow structure make it difficult to estimate near-bed shear stresses for the purposes of defining bed load transport thresholds. In this talk I will present results from a laboratory experiment in which a group of us (students and instructors in GEOG 5100, listed below) varied the concentration of large roughness elements in a 25-cm wide flume, and measured the changes in flow structure using a two-dimensional laser doppler velocimeter (LDV). The measurements show very clearly that, as the concentration of roughness elements increases, there is a noticeable upward shift in the near-bed Reynolds stresses, such that the stress available for transporting the bed sediment is greatly reduced. The experiments shed light on results from a previous study (Mueller et al., 2005) in which we showed that the thresholds for bed load transport in natural gravel-bed channels increase systematically with increasing channel gradient and relative roughness.
Participants: Nate Bradley, Anthony LaGreca, Nora Matell, Scott McCoy, Erich Mueller, Jonathan Nelson, John Pitlick, Catalina Segura
References: Mueller, E. R., J. Pitlick, and J.M. Nelson, 2005, Variation in the reference Shields stress for bed load transport in gravel-bed streams and rivers, Water Resources Research, v. 41, W04006, doi:10.1029/2004WR003692