How Large Immobile Sediments in Gravel Bed Rivers Impact Sediment Transport and Bed Morphology

A common approach used to mitigate riverbank erosion and maintain watercourse alignments has been through the application of riprap or larger, more stable particles to channel boundaries along reaches of interest. However, very often, these large particles become dislodged from their intended locations (failed erosion measures), becoming part of the bed material composition. In natural systems, large immobile sediments or boulders can also be found, which are often sourced from glacial erratics or colluvial inputs with different spacing and arrangements among them. In lower gradient gravel-bed channels, the impacts that large clasts may impart on river morphologies are uncertain and are studied in this paper. This paper utilizes laboratory experiments to evaluate the effects that varying spacing of large immobile particles in a gravel-bed channel have on sediment transport and bed morphology. The laboratory experiments consist of a series of test cases with a varying spacing of large immobile particles and one base case with no large immobile particles present. In each case, the flume bed was composed of a poorly sorted gravel mixture with a bimodal distribution of sand and gravel meant to be representative of a natural gravel-bed channel. The results of the test cases demonstrated that at a low spacing of large immobile particles, the transported material and the bed material both became coarser. At a medium spacing of large immobile particles, the bed material size and erosion reached a maximum, and the coarser bed material was transported at approximately the same rate as the finer material. Finally, at a high spacing of large immobile particles, the size of the transported material and bed material sizes were similar to that of the base case, and the sediment transport also had the strongest clockwise hysteresis trend, which ultimately led to a net erosion of the gravel-bed channel.