An experimental study of the step-pool bed form.

摘要

The step-pool bed form is common in steep mountain streams. It consists of a series of steps and pools, which in profile resemble a staircase. The essential geometry of the step-pool bed form is described by its height and spacing. Step height is largely controlled by the size of the largest step-forming clast, making step spacing the dominant free dimension. Step spacing is set by the deposition of a large, step-forming grain, which occurs only during large floods capable of moving the step-forming clasts.; The flow conditions under which steps form are too infrequent to allow an effective study of step-forming mechanisms and their controls in the field. Determining the controls of step spacing in the field is made more difficult by the fact that the primary controlling variables are strongly interrelated. Laboratory experiments allow not only control of the flow and transport conditions, but also provide the opportunity to observe the formation and break-up of many step-pool bed forms directly, providing a data set that can be statistically summarized.; The dissertation reports on experiments in which two coarse sand/gravel mixtures were fed into a laboratory flume. One goal of these experiments was to identify the mechanisms of step formation. A second goal was to measure directly the spacing of the steps, evaluating the controls of step spacing and whether the spacing is regular, as has been reported. Few steps formed using the first sediment, whereas the second sediment, which was identical to the first except for the addition of a coarse size fraction, formed robust steps, indicating the essential role played by the availability of large step-forming grains.; Development of step spacing can be usefully considered in terms of the depositional location of a step-forming clast relative to an existing step upstream. A number of theories suggest that the interaction of the flow with an existing step sets up preferred depositional sites, leading to regularly spaced steps. Step-forming grains were observed not to deposit immediately downstream of an existing step, suggesting the existence of an exclusion zone. Downstream of this exclusion zone, step spacing was found to follow an exponential distribution, suggesting that deposition of a step-forming grain is equally likely at any location along the bed. This is consistent with the dominant step-forming mechanisms observed. The observation that steps are not located in regularly spaced locations is consistent with the large variance in step spacing observed in the field.