STEP-POOL MORPHOLOGY IN HIGH-GRADIENT STREAMS

The focus of this study is to examine bed stability and morphology in high-gradient gravel-bed streams, and thus to improve understanding of the various parameters governing the sediment flow characteristics in mountain streams. Ultimately, this knowledge can be used to design pseudo-natural channels, as in the stream simulation method of culvert design; with this in mind, prototype conditions are evaluated in a flume with slopes ranging from 3% to 7%, and particle relative submergence varying from 0.5 to 2.5 for three bed size distributions. These experiments are designed to satisfy the conditions of dynamic similarity for flow and sediment, and they are preferred over field measurements since they allow a high degree of control over testing conditions. It is found that step-pool bedforms are the most ubiquitous features along the gravel bed. A new formula is developed that correlates step height with the gravel-bed size distribution, relative submergence of the particles, and the Froude number. The step spacing is found to be related to step height and streambed longitudinal slope. Flow resistance is also examined, and a formula is developed which accounts for the resistance due to the bedforms (form resistance), as well as the individual sediment particles (grain resistance).