Some aspects of the sediment transport during the 2005 floods in Switzerland

Some aspects of the sediment transport during the 2005 fl oods in Switzerland.In August 2005 a very severe fl ood hit large parts of Switzerland.Strong erosion occurred in several steep, apparently stable reaches of mountain streams, what produced heavy sediment loads.Where these were deposited, they caused channel avulsion or channel obstruction with subsequent inundations.Numerical simulation proved to be a useful tool to analyse the processes and to plan mitigation measures.In particular, it could be shown that the fl ooding of the Schächen river had its origin in a lateral overspilling in the fi nal reach of this tributary and a subsequent blocking of the fl ow section.The load eroded in the upstream reach, which is normally stable because of the presence of big boulders, was suffi cient to feed this process.A detention basin with an open slit dam located upstream did only retain a limited amount of sediment temporarily.During fl ow recession, most of these deposits were eroded.This was in agreement with the original concept, but was not very useful in this fl ood situation.On the Lütschine river, a similar massive erosion in a steep reach caused widespread deposition in an area where such phenomena did probably not occur over the last 150 years.For such rare events, it is reasonable to plan a deposition area where limited agricultural land use is allowed.Sediment deposition occurs only in function of slope reduction and channel widening; no special structures are planned.