A DATA BASED STUDY OF ASPECTS OF THE GEOMETRY AND MECHANICS OF MEANDERING STREAMS
Segura Serrano, Laura
meander wavelength, resistance to flow, cross-circulatory motion, convergence-divergence zones
The present study explores aspects of the geometry and mechanics of meandering streams based on a significant dataset consisting of laboratory and field observations. The work focuses on three main issues that, despite systematic research on the topic, remain poorly understood. The first aspect examined is the relationship between meander wavelength and the flow width B. A dimensional analysis of the phenomenon indicates that there is a proportionality factor in this relationship which is a function of h/D and B/h. On this basis, a new empirical equation introduced. The second issue studied was the performance of different methods to quantify the effect of bends on the resistance to flow in real meandering. The analysis conducted in this work included the methods by Rozovskii (1957), the Soil Conservation Service (SCS) (1963), Chang (1983), the linearized SCS (LSCS) by James and Wark (1992), James and Wark (1992), and Ervine and Ellis (1987). The main difficulty in evaluating the methods was the uncertainty on the selection of the original value of the resistance factor (n, c, f). In this work, this was selected by the author so as to produce the best overall results. The methods by Rozovskii (1957) and Chang (1983), which have a theoretical foundation, were found to best capture the additional losses due to bends. However, due to the uncertainty on the selection of the original value of n, it is not possible to assert how close, or how far, they are from reality. The third issue addressed in this work relates to the cross-circulatory motion in meandering rivers and its intensity depending on the values of width-to depth ratio B/h. Four laboratory tests involving the tracking of the paths of particles transported as bed load as well as flow velocity measurements, were conducted to gain insight into the threshold value of B/h beyond which cross-circulatory motion becomes of secondary importance where the bed deformation is concerned. It was demonstrated the decrease of the intensity of the cross-circulation with the increase of B/h, Factual evidence that bed material is transported downstream without crossing from the outer to the inner bank.