Three-dimensional numerical modeling of flows over a natural dune field R.J. Hardy Department of Geography, University of Durham, Durham. DH1 3LE, UK. D.R. Parsons & J.L. Best Earth and Biosphere Institute, School of Earth and Environment, University of Leeds, Leeds. LS2 9JT, UK. S.N. Lane Department of Geography, University of Durham, Durham. DH1 3LE, UK. R. Kostaschuk Department of Geography, University of Guelph, Ontario, Canada. O. Orfeo Centro de Ecologica Aplicada del Littoral (CECOAL), Corrientes 3400, Argentina. ABSTRACT: Our ability to numerically predict the interaction between complex topography and three di- mensional flow at large scales (~1 km) has yet to be fully developed. This is the result of our inability to de- sign numerically stable meshes for complex topographies at these spatial resolutions. This paper deals with the numerical simulation of flow over a natural dune field located in the Rio Paraná, NE Argentina. The methodology is based upon the development and application of a new five term porosity algorithm that modi- fies the mass conservation equation within a regular Cartesian mesh. Bathymetric measurements were made in the field using a multibeam echo sounder which provided an unparalleled topographic dataset to test this new numerical modeling approach. Measurements of flow were made using an acoustic Doppler current pro- filer and are used as inlet boundary conditions and validation data. The results demonstrate simple flow accel- eration due to topographic forcing over the dune stoss and expansion and deceleration over the dune lee. 1 INTRODUCTION Dunes are one of the most common depositional bed forms in river channels, forming in a range of sedi- ment sizes from silt and sand through to gravel (Di- nehart, 1992; Best, 1996; Carling, 1999; Kleinhans, 2001, 2002; Carling et al. 2005). Their presence sig- nificantly influences both the nature of the mean and turbulent flow and consequently exerts a strong con- trol of the entrainment, transport and deposition of sediment (Parsons et al. 2005, Best, 2005). In recent years progress has been made in our knowledge of dune dynamics due to the significant advances in our ability to monitor flow and dune morphology both in the laboratory and field (Best, 2005). Previous re- search has determined the main flow characteristics associated with dunes dynamics, which are; (1) ac- celerating flow over the dune stoss side; (2) flow separation or deceleration (Nelson et al. 1993; McLean et al. 1994; Best & Kostachuck, 2002) from the dune crest in the lee side; (3) flow reattachment at 4 to 6 dune heights downstream (Engel, 1981); (4) a shear layer between the separated flow and stream wise flow above which expands as it extends down- stream and; (5) an internal boundary layer that grows from reattachment beneath the wake along the stoss slope of the next dune downstream. However, virtually all process understanding has been deduced from work that has dealt with morphologies that are essentially regular and two-dimensional, a situation that is rare in natural river channels. Records of bed morphology from natural channels show the fre- quent occurrence of dunes of different scales (Allen & Collinson, 1974; Allen, 1978; Rubin & McCulloch, 1980; Harbor, 1998; ten Brinke et al. 1999; Wilbers & ten Brinke, 2003; Wilbers, 2004; Parsons et al. 2005) possibly as a response to both non-uniform and unsteady flow, hysteresis effects within a flood hydrograph, or the developing inter- nal boundary layer on the stoss side of large dunes. Although, the effect of three dimensional dune mor- phology on flow structures generation has been rec- ognized for a long time (e.g. Allen, 1968), it has only recently been fully explored. Maddux et al. (2003a,b) report that the friction coefficients for 3D dunes are 50% greater compared to their 2D dunes which has the direct effect of reducing turbulence. However, the observed sinuous crest lines imply the presence of secondary circulation, which are respon- sible for a large percentage of the momentum flux over the dune (Maddux et al. 2003a, b). These form- induced stresses are secondary circulation and aug- ment the low Reynolds stresses present over the 3D dunes (Maddux et al. 2003 a,b). Recent summaries on the dynamics and role of dunes (ASCE Task Force, 2002; Bridge, 2003; Best, 2005) highlighted that in order to gain a more complete understanding of the dynamics of river dunes, a fuller appreciation is needed of the complex links between turbulence, natural bed morphology and sediment transport. One approach that has yet to be fully explored to gain an insight into the interaction between dune dy-