Rhythmic roughness elements and channel morphology of gravel bed rivers

A first aim of this paper is to investigate the existence, if any, and the parameters defining the occurrence conditions of a continuum of rhythmic, transverse roughness elements of gravel-bed rivers such as transverse ribs, step-pools and riffle and pool sequences. The investigation is carried out by the analysis of a large data set consisting of almost all the data available in the literature and unpublished field data measured by the authors on a few rivers in the eastern Alps and the northern Apennines. Open questions about the origin of these bedforms are considered and tested against the data set available. A few equations relating the main geomorphic parameters are obtained. They can be used for river restoration and paleohydrological inference. The jamming factor (ratio between the size of the coarser particle and channel width) is found as a relevant parameter in the development of the bedforms considered. The conditions and the interplays between the jamming factors and others parameters such as bedform wavelength and streambed gradient are considered in order to discriminate the fields of existence of the larger roughness transverse, cyclic elements of gravel bed rivers and to show that transverse ribs, step-pools and riffle and pool sequences are part of a coarse grained bedform continuum.