G. H. Sambrook Smith

Numerical simulation of three-dimensional flow hydraulics in a braided channel

Results are presented from a numerical simulation of three-dimensional flow hydraulics around a mid-channel bar carried out using the FLUENT/UNS computational fluid dynamics (CFD) software package. FLUENT/UNS solves the three-dimensional Reynolds-averaged form of the Navier–Stokes equations. Turbulence closure is achieved using a RNG k–ϵ model. Simulated flow velocities are compared with measured two-dimensional velocities (downstream and cross-stream) obtained using an electromagnetic current meter (ECM). The results of the simulation are qualitatively consistent with the flow structures observed in the field. Quantitative comparison of the simulated and measured velocity magnitudes indicates a strong positive correlation between the two (r=0·88) and a mean difference of 0·09 m s−1. Deviations between simulated and measured velocities may be identified that are both random and systematic. The former may reflect a number of factors including subgrid-scale natural spatial variability in flow velocities associated with local bed structures and measurement uncertainty resulting from problems of ECM orientation. Model mesh configuration, roughness parameterization and inlet boundary condition uncertainty may each contribute to systematic differences between simulated and measured flow velocities. These results illustrate the potential for using CFD software to simulate flow hydraulics in natural channels with complex configurations. They also highlight the need for detailed spatially distributed datasets of three-dimensional flow variables to establish the accuracy and applicability of CFD software. Copyright

Measuring and defining bimodal sediments: Problems and implications

Gravel bed rivers close to the transition to a sand bed often have bimodal grain size distributions. These can be quantified in several ways, giving wide differences in derived statistics and calculations. Problems associated with measuring, summarizing, and defining bimodality are examined using evidence from flume experiments and a simple numerical simulation of sand deposition. No single index of bimodality serves all purposes; alternatives are suggested. Measured near-bed hydraulic properties relate more closely to grain size statistics calculated by area than by volume; this has implications for the abrupt downstream transition often found from gravel to sand bed.

Small-scale cyclicity in alpine proglacial fluvial sedimentation

Abstract This paper compares contemporary sedimentation in an Alpine proglacial braided river with deposits from inactive areas of the braidplain. This approach allows confidence in determining the key characteristics of Alpine proglacial braided river deposits. The diagnostic features are: (1) repeated coarse-fine couplets in fine grained sediment (