Morphology, flow and sediment transport over a natural 3D dune field

Abstract

ABSTRACT: The morphology, flow and process mechanics of river dunes has attracted much interest over many years. However, many of these studies have concentrated on investigating two-dimensional (2D) bed features and their associated flow structures and bed stress distributions. This morphological simplification imposes inherent limitations on our interpretation and understanding of dune form and flow dynamics, as natural dunes are invariably three-dimensional (3D) with an associated fully 3D flow structure. For example, studies over 2D forms neglect the significant effect that lateral flow and secondary circulation may have on the flow structure and thus dune morphology. This paper details a field investigation of the interactions be-tween the 3D morphology, 3D turbulent flow structure and suspended sediment movement over large alluvial sand dunes in the Rio Parana, NE Argentina. Fixed point and moving vessel surveys enabled the links be-tween three-dimensionality, large-scale turbulence and sediment suspension over the dunes to be investigated. 1 INTRODUCTION Dunes are ubiquitous forms in river channels, occur-ring within a wide range of bed sediment sizes. The presence of dunes on the bed significantly influences both the nature of the mean and turbulent flow struc-ture, and consequently exerts a strong control on the entrainment, transport and deposition of bed sedi-ment. As a result of their importance, the morphol-ogy, flow and mechanics of river dunes have at-tracted much interest over many years (e.g. McLean and Smith 1986, Yalin 1992, Nelson 1993, McLean et al. 1994, Bennett and Best 1995, Best 1996, Am-sler and Garcia 1997, Shimizu et al. 1999, Kostaschuk 2000, Best and Kostaschuk 2002, Car-ling et al. 2000, Yue et al. 2003) This interest in dune flow and form dynamics has enabled elucidation of the main characteristics of flow associated with dunes, which are: (a) accelerat-ing flow over the dune stoss side, (b) flow separation or deceleration (Best and Kostaschuk 2002) from the dune crest in the lee side, (c) flow reattachment at 4 to 6 dune heights downstream, (d) a shear layer be-tween the separated flow zone and streamwise flow above, which expands as it extends downstream, and (e) an internal boundary layer that grows from reat-tachment beneath the wake along the stoss slope of the next dune downstream. Most of the studies that have contributed to describing these key flow fea-tures over dunes have however concentrated on sim-plified two-dimensional (2D) bedforms, with con-stant heights and straight crestlines transverse to the flow. Given that natural bedforms are invariably three-dimensional (3D) (e.g. Allen 1982, Baas 1994) this morphological simplification has imposed in-herent limitations on our interpretation and under-standing of dune form and flow dynamics. Natural dunes often have variation in crestline planform curvature, crestline height, and crestline continuity (Allen 1982, Dalrymple and Rhodes 1995, Roden 1998), with phase differences between successive crests often producing variability in dune wavelength and steepness (Gabel 1993). This three-dimensionality has significant implications for un-derstanding the flow structure over dunes. For ex-ample simplified 2D forms do not possess lateral flows and secondary circulation present in more three dimensional forms, which will have major im-pacts on the lee side flow structure, bed shear stress and overall dune dynamics. This complexity of bed-form morphology in alluvial environments has been known for many years (e.g. Sorby 1859, Neill 1965, Allen 1968) and the considerable complications that the three-dimensionality of form can introduce into the flow structure over bedforms highlighted (e.g. Allen 1968). However, only recently have the full 3D effects of dune form been investigated in detail in the laboratory (Maddux et al. 2003a,b, Venditti 2003). Maddux et al. (2003a,b) investigated dunes in which the planform crestline was straight (i.e. or-thogonal to the mean flow at all positions), but where the dune height varied in the cross-stream di-rection in the form of a full cosine wave. Successive