A Study of Aspects of the Coupling of Flow and Sediment Transport for Sand and Gravel Dunes
River dunes , Hydraulics , Sediment Transport
This work investigates the coupling between flow, sediment transport and changes to geometry of river dunes. It aims to broaden the understanding of this coupling by examining: a) intrinsic differences between flow and sediment transport over equilibrium and non-equilibrium dunes; and b) whether the equations relating flow conditions to dune development and geometry developed for sand dunes are equally applicable to gravel dunes. To this end three novel laboratory experiments were carried out. Two experiments build upon a previous test by Wiebe (2007) in which equilibrium dunes were developed and then subjected to a sudden decrease in flow depth. The equilibrium dunes in the test under consideration were recreated and immobilized. For the first experiment this enabled the study of the flow field at equilibrium state, and just after the flow depth was suddenly decreased. In the second experiment a 2 cm layer of sand was placed over two of the dunes enabling the measurement of sediment transport for the aforementioned two flow conditions. For the third experiment three different flow conditions were run over a flat bed and the bed profiles were measured as gravel dunes developed and migrated. The findings of this work are as follows: 1) No differences were found in the patterns of time-averaged velocity and turbulence characteristics over the equilibrium and non-equilibrium dunes which would explain the observed changes in dune geometry. 2) An attempt was made to predict bed shear stress over the dunes. Fits were used to extend viscous stresses calculated by a RANS numerical model below the bed surface to the theoretical wall. Shear stresses calculated in this way exhibited significantly different patterns under equilibrium and non-equilibrium conditions and could be used to accurately predict bed changes. The validity of this method, however, has yet to be proven. 3) The downstream derivatives of bed surface elevation and the bed-load rate was found to be proportional to each other at equilibrium state. 4) The geometry and time of development of gravel dunes appears to be well predicted by equations previously developed for sand dunes while their wave spectra are not.