Kelchbach, Naters (VS): Sediment retention basin with a bypass in a mountain river

 

After the devastating flood in Brig, Canton Wallis (CH) in 1993, Naters, the neighbour municipality, became aware of its insufficient flood protection. The capacity of the river Kelchbach in Naters is too small to accommodate discharges larger than the 20-year flood.

A flood control concept intended to combine a sediment retention basin with a flood bypass in a river reach upstream of Naters. A natural channel widening is used nowadays as sediment reservoir by realizing a permeable barrier at the end of the widening. Thus, during large discharges sediment and wood will be retained. The maximum discharge of water flowing through the barrier is limited in order to pass the village without damage. It was planned to imply a solution that larger quantities of water are relieved by a bypass directly into the Rhone. A fundamental component of the spillway structure is the retention of wood and sediment at the structures intake in order to prevent blockage and abrasion.
The feasibility of the project concerning the efficiency of the sediment reservoir and the retention of wood and sediment at the inlet of the spillway were tested in a hydraulic model. It could be shown that the sediment reservoir is capable to retain the sediment volume that is expected during a large flood event. In addition an appropriate design of the spillways inlet with a downflow baffle is suitable for the retention of drift wood.