QUANTIFYING GRAIN EXCURSION LENGTHS FOR A WIDE RANGE OF FLOW CONDITIONS

Sediment transport is a key process in the morphodynamics of alluvial rivers. Much work has been done historically and recently to understand the motions of individual grains and to integrate that knowledge into reach-scale or basin-scale flux models. A majority of these studies have focused on saltating grain motion under relatively low flow conditions (bed load) where the characteristics of suspended grain motion have not yet been directly quantified. In contrast to saltating grains that hop relatively short distances close to the bed (up to hundreds of grain diameter), grains in suspension travel much larger distances once they are picked up from the bed (typically several meters in flumes and potentially up to hundreds of kilometres in natural rivers). This makes direct measurements of suspended grain motion very challenging. Therefore empirical relationships between suspended grain travel distances (excursion lengths) and flow conditions remain largely unexplored. In the present study, using a series of 8 video cameras, we were able to quantify the exact motion of grains for a wide range of flow conditions resulting in direct measurements of the distribution of excursion lengths. In addition, a continuous model for sediment excursion length was derived based on the classical grain advection-settling model and grain saltation data from literature. Initial results show good agreement between measured and predicted particle travel distances for both modes of sediment transport.