Examining the physical components of boundary shear stress for water-worked gravel deposits

It is argued in this commentary that, in order to understand better the physical mechanisms that generate boundary shear stress over water-worked gravel beds, flow velocity data should be re-evaluated by spatial averaging the Reynolds equations to produce time- and space-averaged (double-averaged) momentum equations. A series of laboratory experiments were conducted in which the flow velocities were measured using a PIV system over two water-worked gravel deposits. Combined with detailed data on the bed surface topography and vertical porosity, the physical components of shear stress were obtained. This enabled the various momentum transfer mechanisms present above, within and at the interface of a porous, fluvial deposit, to be quantified. This included the examination of the relevant contributions of temporal and spatial fluctuations in velocity and surface drag to the overall momentum transfer. It is demonstrated that double-averaging represents a logical framework for assessing the fluid forces responsible for sediment entrainment and for investigating intragravel flow and sediment water interface exchange mechanisms within the roughness layer in water-worked gravel deposits. By considering the physical components of shear stress and their relative sizes it was possible to provide a physically based explanation for existing observations of enhanced mobility of gravel sand mixtures and the transfer of solutes into porous, gravel deposits. This analysis reveals the importance of obtaining co-located, high quality spatial data on the flow field and bed surface topography in order to gain a physical understanding of the mechanisms which generate boundary shear stress. Copyright (C) 2010 John Wiley & Sons, Ltd.