The influence of coarse particle abundance and spatial distribution on sediment transport and cluster evolution in steep channels under sediment-starved conditions

Large particles strongly influence flow resistance, energy dissipation, bed stability, and channel morphology in mountain streams. We conducted flume experiments to evaluate effects of the density and spatial distribution of coarse particles (i.e., keystones) on bed stability, surface texture and sediment transport in steep channels. Keystones were placed on the channel bed surface at the beginning of each experiment, both in clustered and in uniformly spaced (i.e., anti-clustered) configurations and in different numbers (i.e., for different values of keystone density). The flow rate was increased by 20 % every hour and each experiment continued until the bed was completely scoured. Sediment transport was measured with a sediment trap located at the flume outlet. Topographic data from high-resolution surface imagery and second-order structure functions (SSF) of bed ele-vations, were used to characterize bed surface structuring while the Ripley's K function was used to assess the degree of clustering of keystones relative to random spatial distributions. Our results indicate that (1) the density and spatial distribution of keystones exert a minor control on the bed surface grain-size distribution (GSD), (2) for increasing flow rates, the spatial distribution of keystones naturally evolves towards a random distribution, regardless of the initial spatial arrangment, and (3) sediment transport has a higher correlation with the pro-portion of dislodged keystones than with flow discharge.

Automated summary

Large particles have significant effects on various aspects of mountain streams, including flow resistance, energy dissipation, bed stability, and channel morphology. Through flume experiments, we investigated the impact of the density and spatial distribution of coarse particles (keystones) on bed stability, surface texture, and sediment transport in steep channels. Our findings suggest that the density and spatial distribution of keystones have a minor influence on the grain-size distribution of the bed surface. Moreover, the spatial distribution of keystones naturally evolves towards a random distribution as flow rates increase, regardless of the initial arrangement.