An analysis of the characteristics of rough bed turbulent shear stresses in an open channel

Abstract

Entrainment of sediment particles from channel beds into the channel flow is influenced by the characteristics of the flow turbulence which produces stochastic shear stress fluctuations at the bed. Recent studies of the structure of turbulent flow has recognized the importance of bursting processes as important mechanisms for the transfer of momentum into the laminar boundary layer. Of these processes, the sweep event has been recognized as the most important bursting event for entrainment of sediment particles as it imposes forces in the direction of the flow resulting in movement of particles by rolling, sliding and occasionally saltating. Similarly, the ejection event has been recognized as important for sediment transport since these events maintain the sediment particles in suspension.In this study, the characteristics of bursting processes and, in particular, the sweep event were investigated in a flume with a rough bed. The instantaneous velocity fluctuations of the flow were measured in two-dimensions using a small electromagnetic velocity meter and the turbulent shear stresses were determined from these velocity fluctuations. It was found that the shear stress applied to the sediment particles on the bed resulting from sweep events depends on the magnitude of the turbulent shear stress and its probability distribution. A statistical analysis of the experimental data was undertaken and it was found necessary to apply a Box-Cox transformation to transform the data into a normally disgributed sample. This enabled determination of the mean shear stress, angle of action and standard error of estimate for sweep and ejection events. These instantaneous shear stresses were found to be greater than the mean flow shear stress and for the sweep event to be approximately 40 percent greater near the channel bed.Results from this analysis suggest that the critical shear stress determined from Shield's diagram is not sufficient to predict the initiation of motion due to its use of the temporal mean shear stress~ It is suggested that initiation of particle motion, but not continuous motion, can occur earlier than suggested by Shield's diagram due to the higher shear stresses imposed on the particles by the stochastic shear stresses resulting from turbulence within the flow.

Investigation of turbulence flow and sediment entrainment around a bridge pier

“…By the same definition, a sweep is a downward movement of high-speed fluid towards the bed. Therefore, bursting events in this quadrant have a significant influence on the entrainment of particles into the flowing water (Keshavarzi and Ball 1997), while transport of suspended sediment occurs most frequently during ejection events.…”

Determinants of modelling choices for 1-D free-surface flow and morphodynamics in hydrology and hydraulics: a review

“…Other works plead for "no-slip" conditions (Panton, 1984;CasadoDiaz et al, 2003;Myers, 2003;Bucur et al, 2008Bucur et al, , 2010 or suggest the separation of flow domains within or outside bed asperities, with a complete slip condition (non-zero tangential velocity) at the interface (Gerard-Varet and Masmoudi, 2010). A wider consensus exists at the RANS level, calculating bottom friction as the local grain-scale values of the "Reynolds stresses" (Kline et al, 1967;Nezu and Nekagawa, 1993;Keshavarzy and Ball, 1997), which has proven especially relevant for flows in small streams over large asperities (Lawless and Robert, 2001;Nikora et al, 2001;Schmeeckle et al, 2007). However, he who can do more, can do less, and it is still possible to use the simplest empirical friction coefficients (Chézy, Manning) within sophisticated flow descriptions (NS: ; RANS: Métivier and Meunier, 2003).…”

Frequency pattern of turbulent flow and sediment entrainment over ripples using image processing

“…The contribution of sweep and ejection events has been found to be more important than outward and inward interactions. Additionally, sweep and ejection events occur more frequently than outward and inward interactions (Nakagwa and Nezu, 1978;Thorne et al, 1989;Keshavarzi and Ball, 1997) with the average magnitude of the shear stress during a sweep event being much higher than the time averaged shear stress (Keshavarzi and Ball, 1997). A number of studies, for example, Offen and Kline (1975) and Papanicolaou et al (2002) investigated the characteristics of the bursting process and its effect on particle motion.…”