The Poschiavo flood event of 1987

The aggradation of bed load during the flood event in July 1987 caused a reduction of the discharge capacity in the river Poschiavino. The cross-section at the bridge at the upper end of the village Poschiavo was completely filled up with bed load. As a result water and bed load flew into the village and caused severe damage. In order to analyse the situation, the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) assessed the amount of bed load to be expected during future floods. With the numerical model MORMO, which had been developed at the VAW, the aggradation problem in the Poschiavino River was investigated. As a result from this investigation, a special design of the bridge was proposed. A shield at its upstream end prevents water to spill over the bridge. Thus, the flow under the brigde can occur under pressure and the resulting scour prevents the bridge cross-section to be obstructed with bed load. This special bridge design was tested and optimised in a hydraulic model. In a second model the forces acting on the shield were determined. With these investigations it was possible to improve the protection against flood. Similar studies were made on the Saltina River in Brig, the Kelchbach in Naters, the Lonza River in Gampel/Steg and the Meerenbach in Mühlehorn.