Bridging the gap between turbulence and larger scales of flow motions in rivers

Abstract

Although flow turbulence in rivers is of critical importance to earth scientists, ecologists and engineers, its relations with larger flow scales are not well understood, thus leaving a fundamental gap in our knowledge. From an analysis of a long time series of the streamwise and vertical flow velocity fluctuations measured in a gravel-bed river, we show that the signature of the fundamental turbulent flow structures (e.g. ejections and sweeps) is embedded within increasingly larger flow scales in a self-similar manner. The imbrication of turbulent structures into large flow pulsations of flow acceleration and deceleration covers more than two-orders of magnitude from a few seconds to nearly 10 minutes. This property is explained by the clustering of turbulent events creating an emergent pattern at larger scales. The size of the larger flow pulsations scales with the spacing of the pools and riffles in the river. This implies a mutual adjustment between turbulence generation mechanisms and long pulsations of flow acceleration and deceleration controlled by the bed morphology. These results bridge a gap in our understanding of flows in rivers and offer a new perspective on the interactions between the turbulent flow with larger scales of flow motion that are critical for sediment transport, habitat selection and fish behaviour.

Three‐dimensional turbulent structures at a medium‐sized confluence with and without an ice cover

“…Here, the drone imagery clearly reveals that the dominating structures are KH (vertically-oriented) vortices which oscillate through large-scale pulsations in the tributaries. Such large-scale pulsations were noted in 20-minute velocity time series in a straight gravel-bed river by Marquis and Roy (2011).…”

Airborne Infrared Remote Sensing of Riverine Currents

“…These boils have spatial scales comparable to the water depth [36] and can be meters or larger in horizontal scale. In addition to strong disturbances from boils, surface layer straining can be caused by flows and turbulence often associated with secondary flows in the channel caused by bends [37]- [39], vegetation beds [40], and bottom roughness variations [41], bathymetry variations [42], or confluences [43], [44]. The surface skin layer is strained by these flows on horizontal scales of meters or larger, creating long-lasting surface temperature signals [45], [46].…”

Sensitivity of interfacial hydraulics to the microtopographic roughness of water‐lain gravels

“…Numerous field studies have captured information about interactions between surface roughness and roughness layer hydraulics in wadeable gravel‐bed rivers where relative submergence is low. This work has focused on characterizing time‐averaged properties and turbulent structures associated with isolated roughness elements like pebble clusters (Buffin‐Belanger and Roy, ; Tritico and Hotchkiss, ; Lacey and Roy, ; Strom and Papanicolaou, ), on the impact of relatively homogeneous roughness on turbulent properties (Papanicolaou et al ., ; Franca et al ., ), or on understanding the scales of turbulence and their association with particular roughness features (Clifford, ; Roy et al ., ; Lacey and Roy, ; Marquis and Roy, ). Most of this field work and relevant flume studies (Nowell and Church, ; Lawless and Robert, ; Canavaro et al ., ; Strom et al ., ) necessarily employed a relatively small number of closely located vertical profile measurements to accumulate information about the flow field.…”

“…Interfacial hydraulics are poorly understood both within deep flows over relatively fine gravel beds and for relatively shallow flows over coarse beds (Nikora et al ., ; Sarkar and Dey, ). This is despite a general expectation that the hydraulic forces in this region, including turbulent structures, are important for the dynamics of sediment transport and the formation of bed forms (Cleaver and Yates, ; Drake et al ., ; Nelson et al ., ; Nino and Garcia, ; Schmeeckle et al ., ; Paiement‐Paradis et al ., ; Cooper, ), it is in this region where skin friction and form drag contribute to the momentum balance and it is here that the turbulence structures of the boundary layer are generated (Kirkbride, ; Robert, ; Papanicolaou et al ., ; Hardy et al ., ; Marquis and Roy, ). This is also the region of the flow where benthic species live, so that near‐boundary hydraulics is an important element of the physical habitat template in gravel‐bed rivers (Nowell and Jumars, ; Davis and Barmuta, ; Lancaster, ; Jowett, ; Rice et al ., ).…”