Author(s): Roland Faeh; Davood Farshi; Renata Mueller; Patric Rousselot; David Vetsch
Linked Author(s): David Vetsch, Davood Farshi
Keywords: River morphology; Numerical modelling; Sediment transport; Shallow water equations; Riemann solver; Finite volume method
Abstract: During the last two centuries, most of the rivers in Switzerland have been regulated to reduce the flood risks. The rivers were straightened by cutting the meanders, canalized and narrowed. As a consequence, the specific bed load capacity increased and the river beds degraded. To stabilize the river beds sequences of sills were build. Thus, the slope of the river bed between the sills was reduced and the sediment retained. A negative consequence of this measures is that sills prevent fishes to swim upstream and thus to get to their spawn places. In general, it can be stated that in earlier days the design of river training works was guided by a technical point of view. The ecological aspects were poorly considered. Nowadays, environmental aspects have become more important. In river engineering, this led to the idea to concede more space to the rivers. A particular restoration measure is to replace the sills by a local widening of the river bed. Due to the increased width, the bed load capacity in the widening is reduced and the sediment is locally accumulated. This has an analogous function as the sills. Depending on the length and width of the widening, the morphology of the river bed develops towards a more natural state like meandering or even braiding. Of course these restoration measures should not affect the flood protection. To ensure this, numerical models are applied to determine the water-surface elevations during a flood. The experience of flood events in Alpine regions shows that a meaningful simulation of such a situation must be based on a moveable bed approach. During a flood, the processes at the bed cannot be observed. To close this gap appropriate numerical models are used. Due to idealisation and simplification, such simulations are always affected by uncertainties. To investigate the capabilities and the limitations of different numerical approaches, the corresponding models were applied to well documented floods at a widening of the Thur River in Switzerland.
Year: 2007