THE EFFECTS OF A FULLY SUBMERGED BOULDER ARRAY ON THE SURROUNDING MEAN AND TURBULENT FLOW FIELDS IN MOUNTAIN STREAMS

Abstract:

Arrays of large immobile boulders in mountainous stream reaches modify the patterns of bedload movement through the controls that they exert on their surrounding flow field. Boulders within an array generate complex mean and turbulent patterns in their vicinity, resulting to a variable bed shear stress field. At the same time, boulder arrays generate form drag, which reduces the bed shear stress applied by the flow on the incoming bedload sediment. This study aims to assess the collective effects of a boulder array on its surrounding time-averaged and turbulent flow fields, as well as the collective array form drag effects accounting for the variable bed shear stress distribution around a boulder of the array. Controlled laboratory experiments are conducted with a single boulder and an array of identical, isolated boulders mounted atop a flat, rough bed and subjected to identical flow conditions. The mean and turbulent flow fields around the boulder with and without the presence of a boulder array are resolved via a Laser Doppler Velocimeter and an Acoustic Doppler Velocimeter, respectively. The results show that an individual boulder modifies the time-averaged streamwise velocity and turbulence intensity in its immediate vicinity along the streamwise (x/d_c < 2-3) and vertical (z/d_c < 1) directions. At locations further from the boulder, the time-averaged streamwise velocity was found to be globally decelerated, likely due to the form drag generated collectively by the boulder array. Our results suggested that the collective array form drag absorbs a portion of the total applied shear by the flow, hence reducing the bed shear stress exerted by the flow on the individual boulders of the array. Furthermore, the array was found to significantly reduce the turbulence intensity near the bed. Overall, our findings suggest that the collective boulder array has sizeable effects on the surrounding flow field and bed shear stress distribution, which should not be ignored in sediment transport predictions.