Pools, riffles and surface roughness in a particle interactions model of steep gravel-to-cobble streams

Particle interactions are important in sediment transport in mountain channels, and in all particulate surfaces. Particle interactions in poorly-sorted stream sediments arise from the constraints on particle mobility imposed on a sediment particle by the particles surrounding it. These interactions were evaluated in terms of the size and position of the top of a given particle compared to neighboring particles. Model simulation results from a particle interactions cellular automata model suggest that particle interactions lead to non-random structuring of the quasi-steady-state sediment surface, and influence the size and spacing of pools in pool-riffle channels. The modeled pool spacing ranges from one to fourteen bed widths, similar to the range from theoretical predictions and from measurements in pool-riffle channels. Pool spacing shows a strongly non-linear decrease as the particle size distribution widens to encompass a greater fraction of finer sediment. The spacing of the pools was essentially unvarying with slope and the mobility threshold. Nearest-neighbor analyses of cell-scale highs and lows in the detrended modeled bed surface indicate a regularity in the spacing of high points, and some clustering at low particle mobility. The coarse areas of the bed also show a tendency to occupy different areas than the deep areas of the bed, as is characteristics of pool-riffle channels. As in streams, the model bed surface coarsens, and the degree of coarsening differs with the model control variable, and between the areas that are deep and the areas that are coarse. Surface roughness of the model channel bed was characterized using the root-mean-square of the height deviations of the surface. The roughness of the model bed increases as a power of time, then reaches a maximum value that is stable, on average, during the remainder of the model run. The roughening also varies as the model boundary conditions vary.