Morphodynamics of Bars in Gravel-bed Rivers: Bridging Analytical Models and Field Observations

The present paper aims to bridge analytical models with field observations on bar morphodynamics in alluvial gravel-bed rivers by examining the outcomes of their application to several river reaches with both single-and multiple-thread morphology. Analytical bar models have often been validated through controlled flume experiments while application to real rivers still poses several challenges. Moreover, despite evidence that alternate and mid-channel bar patterns significantly affect the morphodynamics of transitional and braided rivers, analytical bar models have never been applied to multiple thread streams because they are formulated referring to a single channel with totally sediment-transporting cross section. The paper reviews recent developments on the role of steady bars in single-thread channels and on the dynamics of migrating bars in braided rivers. A key ingredient for sound application is a suitable computation of model input parameters that must be based on the morphologically active portion of the cross section of the analyzed reach. This results in effective prediction of spatial scales, channel pattern, and processes of bedform-planform interaction in many diverse field situations, despite the simplifying assumptions of analytical models. These models can therefore be employed to improve present understanding of the dynamics of multiple-thread rivers, which has often been limited by their complexity. One of the most limiting assumptions of the analytical models is the strong separation between the timescales of bed and bank processes, which leads to a decoupled approach. This appears as one of the major challenges that shall be addressed by future research.