Flume studies of gravel bed surface response to flowing water

Almost all sediment transport equations incorporate the Shields parameter, which is a ratio of the total boundary shear stress as a driving force and the particle weight as a resisting force. Shields (1936) equated particle resistance to entrainment with particle weight, which is proportional to particle diameter, or bed texture. The present work analyses the particle resistance term in the Shields parameter. As the bed material adjusts to a given flow condition, bed stability increases. The arrangement of particles into more stable configurations is here termed geometric structure, and includes the formation of pebble clusters, and imbrication. After an initial surface coarsening, here termed textural structure, particle resistance to movement is a function primarily of geometric structure. The Shields number for entrainment is thus a measure of particle resistance due to both types of bed structure rather than the conventional notion of particle resistance due to particle weight. The response of a mobile bed surface composed of < 8 mm diameter gravels to flowing water was explored in a 6 meter by 0.5 meter flume using four different slopes and various water depths. Corrected bed shear stresses varied between 0.05 and 2.79 Pa. Step increases in discharge with a constant slope caused the bed surface to develop a structure which was more stable at the end of a run than at the beginning. Under these conditions, the Shields number for incipient motion was found to vary between 0.001 and 0.066. This variability can be explained by the degree of geometric structure present. Previous studies, including Shields' work (1936), have implicitly included the effects of geometric structure on incipient motion. Surface coarsening develops with very low flows, but subsequent coarsening in higher flows is minor, with less than 5% increase in median diameter following a 50% increase in bed shear stress. Calculations of Manning's n based on depth, slope, and velocity measurements show an increase in flow resistance as structure develops. The development of a coarse surface layer appears to be limited by flow characteristics near the bed which are in turn modified by the development of structure. Measurements of the area occupied by the largest stones show that they do not cover more than 14% of the surface during maximum coarsening. Froude scaling of the flume data indicates that the time necessary for development of maximum strength is on the order of a month for natural rivers under steady flow conditions. This suggests that gravel river beds are rarely in equilibrium with natural flow conditions.Arts, Faculty ofGeography, Department ofGraduat