Transversal and longitudinal mixing in compound channels

Abstract

[1] An experimental campaign, based on particle image velocimetry (PIV) measurements of free-surface velocities, forms the basis for an analysis of the mixing processes which occur in a compound-channel flow. The flow mixing is characterized in terms of Lagrangian statistics (absolute dispersion and diffusivity) and of the related mean flow characteristics. Mixing properties strongly depend on the ratio r h between the main channel flow depth (h à mc ) and the floodplain depth (h à fp ), and three flow classes can be identified, namely shallow, intermediate, and deep flows. In the present study the large time asymptotic behavior of the mixing characteristics is analyzed in terms of the absolute diffusivity in order to characterize typical values of longitudinal and transversal diffusivity coefficients. Various sets of experiments, which cover a wide range of the governing physical parameters, have been performed and the asymptotic values of the absolute diffusivity have been evaluated. The results are then compared with several studies of flow dispersion for both the longitudinal diffusivity coefficient and the transversal turbulent mixing coefficient. The present results highlight a stronger dependence of such coefficients with the flow-depth ratio than with the flow regime (Froude number).

Estimating the microbiological risks associated with inland flood events: Bridging theory and models of pathogen transport

“…The transfer of momentum across these shear layers affects flow velocity and is potentially important for understanding the transport of sediments and associated pathogens within channels (Ikeda and McEwan 2009). Furthermore, resulting large-scale turbulent structures may contribute to substantial dispersion of transported constituents into inundated areas (Besio, et al 2012). In most compound channel formulations, transfer of momentum between the channel and floodplain is neglected (Seckin, et al 2009).…”

Shallow mixing layers between non-parallel streams in a flat-bed wide channel

“…If the jump in the flow depth and the associated velocity difference are sufficiently large, topographic forcing will induce the generation of quasi-two-dimensional KH (macro) vortices inside the interfacial region (Stocchino & Brocchini 2010) that are similar to the ones generated for the type of MLs investigated in this study. Shallow MLs generated in compound channels and the associated mixing between the flows over the deeper and shallower regions were investigated by Stocchino et al (2011) and Besio et al (2012). Complex cases of shallow MLs are also relevant for geo-physical applications where both upstream differences in the velocity and orientation of the two streams and sharp changes in the bathymetry are widely encountered.…”

Mixing layer and coherent structures in compound channel flows: Effects of transverse flow, velocity ratio, and vertical confinement

“…As the relative flow depth D r (ratio of the FP flow depth to that in the MC) rises, the velocity difference between subsections and shear layer turbulence decrease, while the secondary currents cells strengthen, especially in the MC [ Tominaga and Nezu , ]. The shape of the lateral profiles of streamwise mean velocity, the number (one or two) of two‐dimensional (2‐D) coherent macrovortices at the MC/FP interfaces, and the mixing processes were also found to be a function of D r [ Nezu et al ., ; Stocchino and Brocchini , ; Stocchino et al ., ; Besio et al ., ]. The role played by an abrupt transverse change in flow depth on the generation of 2‐D macrovortices was analyzed by Soldini et al .…”