Effects of grain arrangement and grain size on gravel-bed microtopographies

ABSTRACT Gravel-bed microtopographies are closely connected with many aspects of river dynamics, including incipient sediment motion, sediment transport, flow structure, and flow resistance. Grain arrangement and grain size are two important parameters for quantitatively characterizing gravel-bed microtopographies. However, how grain arrangement and grain size affect gravel-bed microtopographies is still unknown. For this research, fifteen groups of uniform gravel samples were obtained by screening natural river sediment. Then, these uniform gravel samples were separately used to artificially pave gravel beds with six typical types of grain arrangements in a laboratory, and the gravel-bed elevations were measured. On this basis, the effects of grain arrangement and grain size on gravel-bed microtopographies were analyzed using statistical parameters and variograms. The experimental results showed that the elevation frequency distributions for the stacked-layer type of gravel beds established negatively skewed, leptokurtic, and unimodal shapes, but those for the other bed types exhibited bimodal shapes; therein, the main peaks for the one-layer and imbricate bed types were close to a normal distribution. The stacked-layer and one-layer types of gravel beds are approximately isotropic, while the imbricate and striped types are anisotropic. The spherical variogram model can be used as a good theoretical model to quantify the elevation variabilities for the stacked-layer, one-layer, and imbricate types of gravel beds. The gravel nuggets of elevation variograms for these three types of gravel beds are insensitive to the grain arrangement and decrease when the grain size increases, but the sills and correlation lengths linearly increase with increasing grain size.