Author(s): Natascha Korecky
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Keywords: River bed stabilisation measure; Step pool ramps; Boulder weight; Shape and adjustment of step boulders; Inner fraction angle; Collapse mechanisms STEP POOL RAMP STABILITY DEPENDENT ON STEP BOULDER SHAPE; ADJUSTMENT AND FILTER MATERIAL NATASCHA KORECKY
Abstract: Step pool systems are increasingly used as river bed stabilisation measures, due to their exceptional adaptation to natural surroundings, the longitudinal connectivity for fish and the hydraulic functionality. Particularly big re-naturalization projects could require boulders weighing several tons, increasing project costs. Choosing boulders heavy enough to provide stability while still keeping costs - or in this matter, weight – in relation is the issue at hand. In the course of the experiments stone fraction angles of various shapes and adjustments were examined. Since this value is of key importance in calculating the needed boulder weight, a default fraction angle of 35° is used. As the results of the detailed experiments demonstrate, the fraction angle of a step, made of three rows of boulders, can vary between 31° and 48°, depending on shape and adjustment chosen, as well as on filter material. The whole construction’s stability can be increased up to 8 % if square-edged grain instead of rounded grain is used as substratum. Experiments in a tilting flume were conducted in which models of step pool ramps were built and exposed to continuously increasing discharge until complete destruction occurred. Several different parameters were analysed, and one of the results demonstrated the most stable parameter combination in building a step pool ramp is a rooftile-pattern adjustment in combination with rather flat boulders, placed on a square-edged filter material.
Year: 2007