Abstract
Abstract. The formation of alternate bars in straightened river reaches represents a fundamental process of river morphodynamics that has received great attention in the last decades. It is well-established that migrating alternate bars arise from an autogenic instability mechanism occurring when the channel width-to-depth ratio is sufficiently large. While several empirical and theoretical relations are available for predicting how bar height and length depend on the key dimensionless parameters, there is a lack of direct, quantitative information about the dependence of bar properties on flow discharge. We performed a series of experiments in a long, mobile-bed flume with fixed and straight banks at different discharges. The self-formed bed topography was surveyed, different metrics were analyzed to obtain quantitative information about bar height and shape, and results were interpreted in the light of existing theoretical models. The analysis reveals that the shape of alternate bars highly depends on their formative discharge, with remarkable variations in the harmonic composition and a strong decreasing trend of the skewness of the bed elevation. Similarly, the height of alternate bars clearly decreases with the water discharge, in quantitative agreement with theoretical predictions. However, the disappearance of bars when discharge exceeds a critical threshold is not as sharp as expected due to the formation of so-called “diagonal bars”. This work provides basic information for modeling and interpreting short-term morphological variations during individual flood events and long-term trajectories due to alterations of the hydrological regime.
Highlights
Alternate bars are large-scale bedforms characterized by a repetitive sequence of scour holes and depositional diagonal fronts with longitudinal spacing on the order of several channel widths, which are observed in both sand and gravel bed rivers (e.g., Engels, 1914; Jaeggi, 1984; Rhoads and Welford, 1991; Church and Rice, 2009; Jaballah et al, 2015; Rodrigues et al, 2015)
We explored how the equilibrium properties of free migrating alternate bars depend on water discharge through a series of laboratory experiments, wherein width, channel slope, and bed material were kept constant
The equilibrium bar height generally decreases with increasing discharge
Summary
Alternate bars are large-scale bedforms characterized by a repetitive sequence of scour holes and depositional diagonal fronts with longitudinal spacing on the order of several channel widths, which are observed in both sand and gravel bed rivers (e.g., Engels, 1914; Jaeggi, 1984; Rhoads and Welford, 1991; Church and Rice, 2009; Jaballah et al, 2015; Rodrigues et al, 2015). They have been extensively studied in the last 50 years because of both their practical and theoretical relevance. Theoretical and laboratory experiments (Fredsoe, 1978; Jaeggi, 1984; Fujita and Muramoto, 1985; Colombini et al, 1987; Lanzoni, 2000a) identified the channel width-to-depth ratio as the key controlling parameter for the formation of free alternate bars: Published by Copernicus Publications on behalf of the European Geosciences Union
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