A study of large-scale horizontal turbulence in alluvial streams, with a view towards its morphological consequences

Abstract

As is well known, the origin of alternate bars and meandering remains a matter of debate in the literature. This chapter concerns the hypothesis previously raised by a number of authors that large-scale turbulence, and more specifically large-scale horizontal coherent structures (LSHCS’s), are the reason for their occurrence, or at least a contributing factor. In order to properly assess the validity of such hypothesis, it is necessary to first develop an understanding of the dynamics of LSHCS’s occurring in open-channel and river flows, and establish their characteristic scales. This work is motivated by this need. The work is to be viewed as an extension of recent experimental studies on LSHCS’s in shallow flows to the case of deep flows. Accordingly, a laboratory flow having width-to-depth ratio of 7.1, and conveyed in a 1m wide and 21m long straight channel, is investigated. The flat channel bed was formed by a coarse silica sand. Flow velocity measurements were carried out with a 2D SonTek™ Micro ADV. Several techniques are used to treat the velocity signals, including continuous wavelet transform and quadrant and spectral density analyses. Special attention is paid to the time and length scales of the LSHCS’s, the effect of superimposition of the LSHCS’s on the mean flow, and the variation of turbulence structure over the flow depth. To the best knowledge of the writers, this is the first work where LSHCS’s have been detected in a deep open-channel flow. The work is used also to shed light on why the initiation of meandering in alluvial streams is not always preceded by the occurrence of alternate bars.