The development of structure in coarse-grained fluvial sediment; reference to sediment flux and surface characteristics

The broad grain size distributions typical of gravel-bed rivers allow for considerable selective entrainment and dynamic textural response to local perturbations of sediment supply and transport capacity. River-bed surfaces are therefore often highly irregular and spatially heterogeneous. Consequently, evaluation of particle resistance to entrainment requires an understanding of not only the bed surface grain size composition but also the structure of that bed surface. However, our ability to measure and parameterise that structure in ways that are useful and meaningful for the prediction of bedload transport or near-bed hydraulics, remains inadequate. Fundamentally, this reflects poor understanding of the interactions between bed sediment composition, transport capacity, structural development and near bed hydraulics. The experiments reported here assess the temporal development of surface particle organisation under different sediment transport regimes. During experiments a unimodal grain size distribution (σg = 1.30, D50 = 8.8mm) was exposed to three different levels of constant bed shear that produced sediment transport conditions ranging from marginal transport to conditions approaching full mobility of all size fractions. Sediment was re-circulated during the experiments. Surface structuring characteristics, bedload transport rate, and surface grain size distribution were measured at a high temporal resolution such that the time evolution of the beds could be fully described as they approached and attained an equilibrium transport condition. Discussion concentrates on the trajectory of structural evolution under these different treatments where we examine relations between changing patterns of bedload transport and the corresponding changes in surface structure and grain size to evaluate how well grain size alone indexes transport conditions. The outcome of this research is pertinent to developing new methods of linking the development of bed surface organisation with bed load transport in gravel bed rivers. Keywords: Graded, Sediment, Structure, Sediment transport