CGRG Bibliography of Canadian Geomorphology
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Author : Marquis, G.A. ; and Roy, A.G.
Date : 2007.
Title : The role of mean flow velocity on turbulent structures in gravel-bed rivers at temporal scales from 1 second to 10 minutes.
Publication : CANQUA Ottawa 2007. Canadian Quaternary Association Conference, June 4-8, 2007. Carleton University, Ottawa, Ontario, Canada.
Issue :
Page(s) :
Abstract
In gravel-bed rivers, the size, shape and dynamics of large-scale turbulent flow structures are often described as a function of flow depth (Y). Occupying the entire flow depth, the turbulent structures are elongated and narrow, their typical length varying between 2Y and 5Y while their width range between 0.5Y and 1Y. They are organized as alternating high- and low-speed wedges. Most studies describing individual turbulent flow structures in natural rivers have been conducted at mean flow velocities of less than 1 ms-1. Flow pulsations lasting several minutes also exist in gravel-bed rivers. The flow pulsations were recently measured in rivers and their mean effect is an increase of the occurrence of one type of turbulent structure. This means that for several minutes, there will be a pulsation during which low-speed wedges are more frequent followed by a pulsation during which high-speed wedges are more frequent. The purpose of this study is to assess the influence of mean flow velocity on the characteristics of large-scale turbulent flow structures and of the characteristics of longer flow pulsations. The sampling design consists of measuring instantaneous flow velocities at sites where flow depth is constant while mean velocity differs. The data set is composed of fifteen velocity profiles taken at mean flow velocities ranging from 0.4 ms-1 to 1.5 ms-1. In order to control the effect of depth, five profiles were measured at three different flow depths. To investigate the flow pulsations, two very long velocity profiles were also taken at two different mean flow velocities. Results show that mean velocity has a strong effect on the length and duration of large-scale turbulent flow structures and on the flow pulsations. As mean velocity increases up to 1.5 ms-1, the mean duration of the structures doubles while the length of the structures changes from 3Y to 1Y. This finding challenges the idea of a universal scaling for the large-scale turbulent flow structures. We have also found that the length of the pulsations, controlled by mean flow velocity, corresponds to the riffle-pool spacing. Because longer flow pulsations are induced by an increase in the relative frequency of one type of macroturbulent flow structure over the other, the characteristics of the pulsations and occurrence of macroturbulent structures may be linked to the channel morphology.
Bibliography of Canadian Geomorphology