For ecological and flood protection reasons, the current policy of the Swiss Federal Office for Water and Geology is to give rivers more space. In contrast to past river training measures, the spatial needs of a river are no longer defined as being a narrow straight channel. As a result, in some areas, rewidening projects are planned or have already been realized. To understand the morphologically dynamic processes of such rewidened sections, which in most cases lead to braided rivers with a limited width, a research project has been started at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) of the Swiss Federal Institute of Technology (ETH) in Zurich. This project aims to describe and quantify aggradation, degradation and channel rearrangement during floods by means of numerical and physical modelling of braided rivers. A brief overview of existing work, useful in the design of rewidened channels, is presented, and the experimental set-up at VAW as well as the planned experimental concept for the new study are introduced. The first experiments, accomplished in a laboratory flume with constant discharge, are described. Some preliminary results allow the assumption that a sediment transport formula, developed at VAW, is also applicable for widened areas in steep rivers. It is concluded that a low braiding index in the widened river section implies a high transport capacity and vice versa. The highest transport rate was always observed when a dominant single channel was moving laterally.
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