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Sediment transport and flow conditions in steep rivers with large immobile boulders

In steep mountain rivers, morphological features such as large mostly immobile boulders, strongly influence flow behavior and bedload transport. Without taking into account these large immobile boulders, which are acting as macro-roughness elements, classical bedload transport formula largely overestimate the sediment volume transported. The effect of randomly distributed boulders creating a cascade morphology on the sediment transport capacity and bedload fluctuations in steep mountain channels was studied for the first time by Dr. Tamara Ghilardi with systematic laboratory experiments. A wide grain size distribution was used in the experiments for the movable bedload material present between the large boulders, as it can be found in typical alpine torrents. Boulder size and density was varied in the experiments using shapes of boulders like the ones found in nature. Special attention was also given for the first time on the evolution of the sediment transport characterized by bedload pulses. This allows a new insight on the sediment transport processes in steep channels having large immobile boulders. Finally a new sediment transport formula is suggested which takes into account the presence of such large boulders.

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