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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).

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Mean dimensionless reattachment lengths on the symmetry plane measured from the obstacle center line, and nondimensionalized by obstacle height (which remains constant for all obstacles) and obstacle diameter. 4.2 Circulation of base vortex in different relative submergence in Figure 4.14. 4.3 Average dimensionless circulations of structures identified for streamwise and transverse ellipsoid at x/H = 1.5 in two different turbulence intensity, and the left/right deviation.