Author(s): Stephen R. Mclean
Linked Author(s):
Keywords: Dunes; Sediment transport; Bedform evolution
Abstract: The key to predicting flow resistance in natural, time-varying flow is to predict the evolution of bedforms such as ripples and dunes. By employing a simple model for flow within the boundary layer that develops downstream of reattachment and specifying a relationship between sediment transport and boundary shear stress (or near-bed velocity), a bed evolution model has been developed by making use of the Exner equation. Recent measurements of transport over asymmetric dunes in a laboratory flume indicate that 1) as a result of high levels of separation-induced turbulence, transport rates over much of the stoss are significantly higher than predicted by classical transport formulae based on mean flow parameters, 2) in the vicinity of reattachment near-bed velocity fluctuations in both upstream and downstream directions regularly exceed critical conditions so net transport results even for mean flow well below threshold conditions. An empirical expression for transport as a function of mean near-bed velocity that takes these factors into account for flow over dunes has been developed. Results of the dune evolution model using this empirical dune transport equation and more traditional transport formulae are compared. The newly-developed formula produces steeper and higher dunes under steady conditions than do the more traditional relations. They also propagate significantly faster.
Year: 2010