Influence of collective boulder array on the surrounding time-averaged and turbulent flow fields

Abstract

Arrays of large immobile boulders, which are often encountered in steep mountain streams, affect the timing and magnitude of sediment transport events through their interactions with the approach flow. Despite their importance in the quantification of the bedload rate, the collective influence of a boulder array on the approach timeaveraged and turbulent flow field has to date been overlooked. The overarching objective is, thus, to assess the collective effects of a boulder array on the time-averaged and turbulent flow fields surrounding an individual boulder within the array, placing particular emphasis on highlighting the bed shear stress spatial variability. The objective of this study is pursued by resolving and comparing the timeaveraged and turbulent flow fields developing around a boulder, with and without an array of isolated boulders being present. The results show that the effects of an individual boulder on the time-averaged streamwise velocity and turbulence intensity were limited to the boulder's immediate vicinity in the streamwise (x/dc < 2-3) and vertical (z/dc < 1) directions. Outside of the boulder's immediate vicinity, the time-averaged streamwise velocity was found to be globally decelerated. This global deceleration was attributed to the form drag generated collectively by the boulder array. More importantly, the boulder array reduced the applied shear stress exerted on the individual boulders found within the array, by absorbing a portion of the total applied shear. Furthermore, the array was found to have a "homogenizing" effect on the near-bed turbulence thus significantly reducing the turbulence intensity in the near-bed region. The findings of this study suggest that the collective boulder array bears a portion of the total applied bed shear stress as form drag, hence reducing the available bed shear stress for transporting incoming mobile sediment. Thus, the effects of the boulder array should not be ignored in sediment transport predictions. These effects are encapsulated in this study by Equation (6). Citation: Tsakiris AG, Papanicolaou AN , Hajimirzaie SM, et al. (2014) Influence of collective boulder array on the surrounding time-averaged and turbulent flow fields. Journal of Mountain Science 11(6).

Boulder Array Effects on Bedload Pulses and Depositional Patches

“…Flow depths were verified at 2.0 m intervals along the length of the flume using rulers placed on the plexiglass flume sidewalls. The boulder array test section was placed 10.9 m downstream of the honeycomb entrance, where the flow was fully developed (Tsakiris, Papanicolaou, Hajimirzaie et al, ). The array test section was composed of 19 boulder rows (48 total boulders) distributed over a length of 5.2 m. This length ensured that fully adjusted flow conditions developed within the boulder array, which were estimated to occur within the first half of the array length for all tested flow conditions (Belcher et al, ; Papanicolaou et al, ).…”

“…Investigating the flow field characteristics around boulders, Papanicolaou and Tsakiris (2017) recently provided evidence that this reversal in depositional location corresponds with the distinct topologies of turbulent vortex structures that develop at HRS and LRS conditions. Specifically, at HRS conditions the vortex topology in the boulder wake is dominated by arch vortices, which induce circulation to promote the entrapment of bedload sediment in the boulder wake region (Papanicolaou & Tsakiris, 2017;Tsakiris, Papanicolaou, Hajimirzaie et al, 2014). In contrast, the limited available data at boulder LRS conditions suggest that in the case of Fr < 1 the necklace developed by the bimodal state of the HSV creates entrapment of sediment at the stoss; in the case of Fr > 1 it is the LWCs and potentially a von-Karman vortex street, which controls deposition and effective area of deposition (Papanicolaou et al, 2011;Shamloo et al, 2001).…”

“…The influence of boulders in high gradient gravel‐bed rivers where they are prevalent can be significant and multifaceted. Boulders have been documented to bear a portion of the total bed shear stress through form drag (Nitsche et al, ; Schneider et al, ; Yager et al, ), create spatial variability in bed shear stress (Dey et al, ; Monsalve et al, ; Papanicolaou et al, ; Shamloo et al, ), and generate turbulent eddy structures (Hardy et al, ; Papanicolaou & Tsakiris, ; Tsakiris, Papanicolaou, Hajimirzaie et al, ). Through the modification of reach‐scale hydrodynamics, boulders also induce bedload fluctuations by affecting the rate of particle entrainment and deposition in their vicinity (e.g., Heays et al, ).…”

Influence of Boulder Concentration on Turbulence and Sediment Transport in Open‐Channel Flow Over Submerged Boulders

“…In terms of the mean velocity field, boulder wakes form as a result of the recirculation zones downstream of submerged boulders. Large‐scale coherent flow structures dominate the instantaneous and local flow in the vicinity of boulders (Ferro, ; Hajimirzaie et al, ; Strom & Papanicolaou, ; Tsakiris et al, ). Such structures include tip vortices created by flow separation at the crest (Hajimirzaie et al, ), and near‐bed horseshoe vortices just upstream of the boulder (Tsutsui, ).…”

Effects of Bed Forms and Large Protruding Grains on Near‐Bed Flow Hydraulics in Low Relative Submergence Conditions

“…In particular, it is known that bed forms and large roughness elements independently cause deviations from commonly assumed velocity profiles (Afzalimehr & Rennie, 2009;Hajimirzaie et al, 2014;Lawless & Robert, 2001;Papanicolaou et al, 2012;Shamloo et al, 2001;Strom & Papanicolaou, 2007;Tsakiris et al, 2014). Velocity profiles within the roughness layer have been assumed to be linear (Nikora et al, 2001), nearly constant (Recking, 2009), or quadratic (Lamb, Dietrich, & Venditti, 2008;Wiberg & Smith, 1991) and are used to determine boundary shear stresses, which are subsequently used in sediment transport calculations.…”