Kleine Emme - Reuss: A physical scale model of a confluence

In Lucerne, the River Reuss leaves the Lake Lucerne and is joined 3 km downstream by the River “Kleine Emme” at the so called “Reusszopf”. Because of the lake there is basically no sediment transport in the Reuss before the Reusszopf, where the Kleine Emme brings in a lot of sediment. On average 5000 m³ of sediment have to be removed in this area to guarantee sufficient discharge capacity in the “Kleine Emme”.
In case of large flood events such as the 2005 flood the sediment deposition pattern in the junction area initiate a back water effect in the River Reuss leading to a slightly reduced outflow of the Lake Lucerne. That means that during extreme flood events the peak discharge in the Reuss is somewhat reduced and is therefore advantageous for the downstream residents. (fig. 1 and 2)
To increase the flood safety in the Kleine Emme close to the junction site, it is planned to widen the Kleine Emme and to divert the Kleine Emme into two separate channels. To investigate the effects of these flood protection measures in the Kleine Emme on Reuss discharge during extreme floods, a physical scale model (1:45) was built at the VAW. In a first experimental phase, the current geometrical conditions of the Kleine Emme was modeled to reproduce the flow and sediment transport conditions with long-term experiments These experiments were used to calibrate the model and to adapt the sediment transport rates. To model the sediment transport characteristics two different sediment mixtures are used: One to reproduce the bed material and the bed load during extreme flood events, and a second (finer) one to simulate bed load during smaller flood events. After calibration the physical model has been adapted to represent the planned geometry with two channels in the Kleine Emme river. First results show that the water levels in the river Reuss is hardly effected. Further experiments have to be done to prove these findings with different boundary conditions (discharge, initial bed elevation…). The investigation will be completed in early 2012.