The goal of this study is to provide an improved understanding of the effects that a boulder array has on flow and sediment transport using a power spectral density (PSD) analysis of instantaneous velocity measurements. Existing velocity time series obtained via an Acoustic Doppler Velocimeter (ADV) in a controlled laboratory flume are analyzed. First, it is shown that the energy contained in large-scale flow structures, or eddies, is significantly reduced in the immediate boulder wake, which helps explain sediment 'hiding' in the wake. Second, it is shown that the near-bed, large-scale energy is also reduced, though not as drastically, behind each boulder at apparent 45° angles to the streamwise direction and up to a lateral offset of l.5d_c from the boulder side. This estimated geometric description of the affected flow region behind a boulder would account for 13% of the bed plan view area in the boulder array with possible implications for delaying bedload movement. Finally, the dominant flow frequencies determined via the PSD analysis within the wake of a boulder were attributed to be the combined effects of large scale flapping in the wake behind the boulders and of general bursting found in gravel bed streams.
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