No abstract

Flume experiments have been carried out to study the formation processes and the bed morphology of step-pool channels. From the experiments different step types and step configurations could be distinguished depending on the stream power. These step types can be seen as an image of the generation mechanisms of step-pool systems. These results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution. In contrast to a variety of other studies, antidunes did not contribute to the generation of the step structures. However, the data of the presented study fits well into the region of antidune formation proposed by Kennedy for sandbed rivers. This observation points out that step-pool field-data located in the Kennedy region do not inevitably prove that antidunes played a role in step development. It is rather proposed that in Kennedy's region of antidune formation there exist hydraulic conditions where the flow resistance is maximized. It is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height, independently of whether these are antidunes in sand-and gravel-bed rivers or step-pool units in boulder-bed streams. The considerations of the Kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self-stabilization by modifying the step-pool structure towards a geometry that provides maximum flow resistance and maximum bed stability.steeper and flatter sections are denoted as riffle-pool sequences and are often associated with bar features. As width is often constricted in steep open channels, riffles and pools in connection with alternate bars tend to be absent from or poorly developed in boulder-bed streams. Nevertheless, bar features as well as meandering tendencies have also been described for boulder-bed channels (Leopold et al., 1964).In the present paper the results of the flume study are presented that focus on bed morphology and the generation of step-pool channels. In contrast to a number of other flume studies, the generation and configuration of steps and pools was studied with regard to its dependence on flume width. R. B. Weichert et al.

For large-particulated fluids encountered in natural debris flow, building materials, and sewage treatment, only a few rheometers exist that allow the determination of yield stress and viscosity. In the present investigation, we focus on the rheometrical analysis of the ball measuring system as a suitable tool to measure the rheology of particulated fluids up to grain sizes of 10 mm. The ball measuring system consists of a sphere that is dragged through a sample volume of approximately 0.5 l. Implemented in a standard rheometer, torques exerted on the sphere and the corresponding rotational speeds are recorded within a wide measuring range. In the second part of this investigation, six rheometric devices to determine flow curve and yield stress of fluids containing large particles with maximum grain sizes of 1 to 25 mm are compared, considering both rheological data and application in practical use. The large-scale rheometer of Coussot and Piau, the building material learning viscometer of Wallevik and Gjorv, and the ball measuring system were used for the flow curve determination and a capillary rheometer, the inclined plane test, and the slump test were used for the yield stress determination.For different coarse and concentrated sediment-water mixtures, the flow curves and the yield stresses agree well, except for the capillary rheometer, which exhibits much larger yield stress values. Differences are also noted in the measuring range of the different devices, as well as for the required sample volume that is crucial for application.

Historically, river training measures in the European Alps usually resulted in rivers being confined within relatively narrow, lined channels. Today, river management seeks, whenever the required space is available, to widen river reaches and so restore the original, braided character of Alpine rivers. Assessment of the effect of channel widening on bed-level changes, river morphology and the flood hazard requires an understanding of the bed load capacity of braided reaches. The experimental study described in this paper focuses on bed load transport and channel morphology in relatively steep braided rivers with a wide bed material grain-size distribution. Results of the study and a comparison with previous investigations show a need to improve existing approaches for describing bed load transport in braided gravel-bed rivers. An improved bed load equation is presented that allows the effects of wide grain-size distributions to be taken into account. In addition, the influence of varying discharge conditions on bed load transport and morphology in braided rivers is discussed.