Abstract:Stepped boulder arrays induced by flash flood are common riverbed forms in mountainous areas,and they can greatly affect the river flow structure and bed transformation.Although boulder arrays increase flow and river erosion resistance,they also stabilize river boundary and change the ability of the water to carry sediment,and therefore restrict the evolution of river bed.Based on the field investigation of typical riverbed and open channel experiments,the influence of stepped boulder array on flow structure with shallow depth conditions has been analyzed and discussed.All of the experiments were conducted on the fixed bed made of 4.0 cm plastic ball as model boulders.A side-view acoustic Doppler velocimeter (ADV) was equipped to record rapidly responding flow movement so as to analyze the flow turbulence.Our study showed that velocity and turbulence intensity were greatly affected by different spacing of stepped boulder arrays.Within the stepped boulder arrays,the turbulence intensities in u,v and w directions increased first and then decreased and their peak points of the turbulence intensities in v and w directions were located at boulder downstream 0<y/h≤0.2.The scale of far-wake region,which is mainly concentrated at x< 2D area in the longitudinal and h=1D in the vertical impact range of boulder,was positively correlated with the boulder size.The boulders arrays had a "uniform" effect on turbulence near the bed,and the increase of the spacing of stepped boulder arrays significantly reduced the turbulence intensity near bed.