Renggbach (LU): Flooding risk study on a torrent

The Renggbach is a torrent in a densely populated Swiss prealpine area. In the Middle Ages several flood events in the Renggbach caused inundations of extensively populated areas. The last big event occurred in 1738. Nowadays, the torrent is characterized by a high degree of realized river control structures.

Due to the increasing number of object of values in the area former subjected to inundations and the expensive maintenance work, the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) was commissioned to carry out a hazard assessment of the Renggbach.

The VAW investigated both, the conditions for the onset of overbank flooding and the overbank area inundated by a flood. The probability of a flood is strongly influenced by the maintenance of the existing structures as e.g. check-dams. If maintenance continues on a high level, the probability of inundation of the populated area of Lucerne and neighboring communities remains small, with an estimated return period of about 300 years or more. If the check dams maintenance decreases, then the risk increases and carries the potential to cause significant damage.