Effect of migrating bed topography on flow turbulence: implications for modeling sediment transport

Include a brief (<300 words) abstract, by itself, on p.2. Abstracts may not be published in the printed book but will be used in review and editing and may be used in web presentations of the book contents. Understanding and quantifying the co-evolution of migrating bed topography and flow turbulence is crucial for characterizing sediment transport in alluvial rivers. The self-organized bed forms at the bottom of these rivers exhibit a complex multi-scale structure which affects and is affected by the dynamics of near-bed turbulence and particle transport. Here we review and integrate some recent results related to the space-time characterization of gravel bed elevation, near-bed 3D turbulence, and sediment transport fluctuations, using simultaneous data obtained in a large-scale laboratory experiment in a 84m long, 2.75m wide flume for a range of discharges. We show that migrating bed topography imprints a distinct signature on the power spectral density of near-bed velocity fluctuations presenting the possibility of inferring bed form characteristics from long time series of turbulence measurements above the bed. A quadrant analysis of velocity fluctuations depicts the sweep-ejection nature of flow turbulence and its relation to particle transport patterns. We report that the probability density functions (PDFs) of the bed elevation increments and the instantaneous Reynolds stress exhibit heavy-tail statistics and a strong asymmetry revealing two-way feedbacks between bed form dynamics and near-bed turbulence and having implications for particle movement contribution to large fluctuations in sediment transport. Deviation of these PDFs from Gaussian form calls for looking beyond the distribution of the energy across scales (spectrum) and we present a higher order moments analysis using the standard multifractal formalism of turbulence. We show that both the roughness parameter and the intermittency parameter of the bed elevation fluctuations increase 4 with increasing discharge. Finally, we propose a simple relation to quantify sediment transport rates from bed form averaged instantaneous Reynolds stress.