Methods to measure and analyze particle organization in boulder-bed streams.

摘要

Bed flow resistance is a function of both particle organization and particle size in natural streams. Large scale organization of particles into bed forms such as dunes, riffles and step-pools is well characterized and recognized as providing significant flow resistance. Small scale particle organization, which considers the natural position of the particle on the bed into features, such as particle clusters is not well characterized.; In order to study particle organization, methods needed to be developed to collect information on the bed particles that make up the particle size distribution. Particle sampling techniques were modified to account for more uniform sampling of the bed particles along the channel width. Statistical analysis of the particle distribution was modified to account for the distribution of larger particles and provide a standard method of calculating particle statistics.; Particle organization in boulder-bed streams was found to be controlled by channel width as well as the sorting of the particle distribution. To characterize these observations two length scales were developed. The first is the horizontal roughness length scale. It is the sum of the particles in the upper tail using the 84th largest ranked particle size in the sample as the lower limit. Particles in the upper tail represent the width constriction in boulder-bed streams. The second length scale is the particle cluster. Particle clusters are defined as two or more adjacent particles with sizes in the upper tail. This length scale addresses spatial organization between particles as well as along the channel width. Particle clusters are found to be a function of both particle width and particle sorting.; To analyze the spatial organization of bed particles and contributions of particle organization to channel stability, particles were sampled using a grid of 0.25 m by 1 m. The particle's size and position were recorded. The natural spatial organization of the particles was compared with a random organization. Data from a large bankful discharge event was used to evaluate the effect of particle position on particle mobility. The results indicated that particle organization has an important effect on sediment transport and channel stability.