Author(s): A. G. Roy; G. M. Marquis
Linked Author(s): André Roy
Keywords: No Keywords
Abstract: This paper addresses three questions concerning macroturbulent flow structures in gravel-bed rivers: 1. How are the macroturbulent flow structures embedded within large scale flow pulsations? 2. What is the relation between water surface fluctuations and the passage of macroturbulent and large scale pulsations? 3. Are the macroturbulent flow structures modified as water level rises? Experiments were conducted to obtain long simultaneous high resolution records of flow velocity and of water surface fluctuations in rivers with a pool-riffle structure. At one site, water surface fluctuations were observed while the river was in flood and sediments were moving as bedload. Analysis of the data shows the presence of flow structures that maintain a similar velocity signature from the turbulent (seconds) to the pulsation (several minutes) scale. This similarity across scales reveals a clustering in time of high-speed and low-speed wedges, the macroturbulent unit of flow in gravelbed rivers, thus creating periods of flow acceleration and deceleration at larger scales. As expected, water surface bulges slightly during deceleration phases and is depressed in phases of acceleration. This evidence is still noisy due to other factors that affect the water surface. When the river is in flood, the water surface is characterized by standing waves and the typical signature of macroturbulent signature is changed. The dominance of sweep and ejection events becomes less pronounced and even disappears thus raising issues on the relation between turbulent flow structures and sediment transport.
Year: 2009