The morphodynamics of fluvial sand dunes in the River Rhine, near Mainz, Germany. II. Hydrodynamics and sediment transport

The dynamics of large isolated sand dunes moving across a gravel lag layer were studied in a supply-limited reach of the River Rhine. During daylong s urveys, suspended sediment concentration, bedload transport rate, water dep th, flow velocity, turbulence intensity, near-bed shear stress and water te mperature were recorded over individual isolated dunes. This paper consider s the hydrodynamic environment and sediment transport over the dunes. A com panion paper details the sedimentology and morphology of the dunes. Flow ov er the flat gravel lag upstream of large dunes is more uniform than that ov er dunes, and gravels are rarely entrained by in-bank discharges. Unsteady and non-logarithmic velocity profiles are common within the boundary layer above the stoss side of large dunes, and the near-bed flow demonstrates evi dence of large-scale, coherent low-frequency flow structures; these may ref lect stacked sequences of separated boundary layers generated by secondary dunes. However, the low-amplitude morphology of large dunes does not affect the statistical properties of turbulence production over the stoss sides. Bed roughness and near-bed shear stress commonly increase steadily over the stoss of dunes, but may decrease near the crest, especially where a cresta l platform exists that is devoid of secondary bedforms. Bed roughness scale s with the physical size of bed roughness elements. However, variability in roughness lengths is large, owing to the composite nature of bed roughness . Bedload transport over stoss slopes is spatially variable, but often show s an increase with increasing bed shear stress over the stoss. Well-formed wakes only develop downstream of lee slopes, which are close to the angle o f repose; otherwise, separation is weak, and suspension and settling of fin e sediments is of little consequence to dune evolution. Wake flow is charac terized by turbulence production one order of magnitude greater than over t he stoss side, which may be related to vortex shedding from the dune. Howev er, wake current speeds are extremely low and variable in direction; revers e flow is not sustained, and no retrogressive bedforms occur in the lee of large dunes. Near-bed shear stress and bed roughness are usually low within the wake, reflecting low current speeds. Weak wake-flow reattachment occur s at a variable distance downstream, up to several times the duneform heigh t. A two-dimensional numerical model for flow over dune topography, calibra ted using average parameter values, provided a reasonable description of th e flow upstream, and over the backs of large dunes as far as the crestal re gion. Wake flow could not be modelled. However, the temporal and spatial co mplexity of natural three-dimensional flow over the dunes resulted in varia nce in parameter estimates; this variance precluded modelling of flow and b edload transport. Consequently, it was not possible to model dune evolution in a deterministic sense.