The Jonenbach River in the residential area of Affoltern am Albis was flooded several times in the past, causing damage of some million Swiss Francs, due to its insufficient carrying capacity.
The flood control reservoir reduces the maximum outflow on the lower river to 17 m3/s; a discharge which can be passed by the existing river channel without causing any damage. The retention dam has been constructed as an earthfill dam for technical and aesthetic reasons. The 16.5 metre high dam is curved and has upstream and downstream slopes of 1:3. The concrete works, culvert and spillway, have been integrated into the earthfill dam. In case of low and medium water, the Jonenbach River flows through the culvert unhindered. Only very seldom, in case of heavy rainfall and a discharge of more than 5 m3/s, the water accumulates in the flood control reservoir. The reservoir will only be full every 100 years, on average. The retention water level at the level of the overflow edge of the flood control reservoir will then be reached. The reservoir filling is mostly of short duration, since a full reservoir empties within hours (not more than one day). Vegetation in the retention area is not affected. The spillway is an important security item for the dam and was built for the maximum discharge of 116 m3/s.
Pöyry provided detailed design, tender design, final design, and technical-site supervision.