Fabián López

open-channel flow through simulated vegetation: Suspended sediment transport modeling

A two-equation turbulence model, based on the k-ϵ closure scheme, was used to determine the mean flow and turbulence structure of open channels through simulated vegetation, thus providing the necessary information to estimate suspended sediment transport processes. Dimensional analysis allowed identification of the dimensionless parameters that govern suspended sediment transport in the presence of vegetation and thus helped in the design of numerical experiments to investigate the role of different flow properties, sediment characteristics, and vegetation parameters upon the transport capacity. A reduction of the averaged streamwise momentum transfer toward the bed (i.e., shear stress) induced by the vegetation was identified as the main reason for lower suspended sediment transport capacities in vegetated waterways compared to those observed in nonvegetated channels under similar flow conditions. Computed values of kinematic eddy viscosity were used to solve the sediment diffusion equation, yielding distributions of relative sediment concentration slightly in excess of the ones predicted by the Rousean formula. A power law was found to provide a very good collapse of all the numerically generated data for suspended sediment transport rates in vegetated channels.

Characterization of near-bed coherent structures in turbulent open channel flow using synchronized high-speed video and hot-film measurements

High-speed video recordings (500 Hz) of flow visualizations in the near wall region of a turbulent open channel flow were synchronized with hot-film measurements of flow velocity and bed shear stress. Analysis of the video images provided information about the main characteristics of coherent flow structures associated with the occurrence of low-speed streak ejections near the bed. These structures consisted mainly of oscillating shear layers that were converted in the downstream direction and lifted away from the bed. A visual detection criterion was developed to obtain ensemble averaged profiles of the velocity and shear stress data during ejection events, allowing for the characterization of the associated flow field during the occurrence of coherent structures. Conditional averaging suggests that the occurrence of such coherent patterns affects mainly the turbulence structure in the wall region, and that the observed events reveal a plausible mechanism by which energy is extracted from the mean flow by large scale turbulent fluctuations, and then further transferred towards smaller eddies, while the structures lose their coherence. The intermittent nature of production and dissipation of turbulent energy becomes noticeable, taking place about 21% of the time. The results obtained also provide evidence that seems to link the structures responsible for the turbulent vertical transport of momentum, and for the maintenance of the turbulent state, with the mechanism that triggers the entrainment of sediment into suspension. Comparison of present results with other experiments conducted in different types of flows strongly confirms a universal structure of coherent events in wall bounded flows.

Laboratory Experiments on Pool-Riffle Sequences Designed to Restore Channelized Low-Gradient Streams

This paper presents laboratory experiments on pool-riffle sequences designed to provide 3D velocity data of high-resolution -in space and time- for different flow conditions, in order to analyze the implications for the physical habitat in low-gradient streams. Two different configurations were studied. The first one has the pools located on the channel centerline, in an effort to minimize the possibility of scour near the banks. This design is suitable for restoring streams in heavily urbanized areas, where the channel planform alignment is constrained by the existing infrastructure and bank erosion is unacceptable. The second configuration is more representative of natural pool-riffle sequences, with alternated pools located close to the banks. Both configurations where compared with data on flat bed uniform flow.

Turbulence and Coherent Flow Structures Associated with Bedform Amalgamation: An Experimental Study of the Ripple-Dune Transition

Although the amalgamation of bedforms is central to the evolution of dunes from a rippled bedstate, relatively little is known about the fluid dynamics and changing structure of turbulence associated with bedform amalgamation. This paper presents some results on the structure of turbulence and coherent events documented in laboratory runs simulating three different stages of bedform amalgamation across the ripple-dune transition. Velocity data was gathered by a 2D LDA system on a fine spatial grid and for periods long enough to satisfy convergence of fourth-order joint moments of velocity fluctuations. Processing of the information includes the mean velocity field, classical turbulence statistics, local production, dissipation and inertial turbulent fluxes of kinetic energy, evaluation of contributions from different coherent events to the total Reynolds stresses as well as local deviations from the mean pressure caused by dynamical effects.

Sedimentation in vegetated rivers

Dimensional analysis and numerical modeling are combined to obtain a simple expression for estimating the capacity of vegetated open channels to transport suspended sediment under equilibrium conditions. The numerical model consists of a two-equation closure scheme of the k-s type, with parameters calibrated using laboratory observations in an open channel with simulated vegetation. A simple power-type expression is found to provide a very good collapse of all the experimental data involving the Rouse number, the relative submergence, the particle Reynolds number and the dimensionless plant density as independent variables.