Experimental Investigation of Turbulent Flow Velocity Distribution Profile and Bed Shear Stress over Alluvial Beds

The laboratory experimental investigations on turbulent flow characteristics, utilizing the point velocity data measured from a 16 MHz acoustic Doppler velocimeter (ADV), are reported for non-cohesive uniform and non-uniform sediment beds of 10 cm thickness. The primary objective was to examine turbulent flow roughness’s response over uniform and non-uniform sediment beds across a wide range of roughness conditions. Sediments of varying sizes, ranging from 0.5 to 8.0 mm, were used. The raw data were post-processed to remove noise and spikes. The analysis focused on the bed shear stress (BSS), which was evaluated for uniform and non-uniform sediments under various flow conditions. The BSS derived from the Reynolds’ shear stress (RSS) distribution was of lesser magnitude than the bed shear stresses estimated using the bed slope. For sediment-laden flow with the same-sized sediment, the BSS was higher than that for clear water flow. Coarser sediments exhibited more significant resistance (BSS) than finer ones under similar clear water flow discharge conditions. Regarding the turbulent velocity distribution model, the magnitude of kappa (von Karman’s constant) was close to the standard value (0.4) for clear water conditions. However, when sediments are in motion, both uniform and non-uniform size sediments demonstrated a considerable decrease in kappa values, as a portion of the shear stress was utilized for the motion of sediments near the bed.