Assessing the Contribution of Karst to Flood Peaks of the Suze River, Potentially Affecting the City of Bienne (Switzerland)

The city of Bienne (BE, Switzerland) located downstream of the Suze River is exposed to flooding caused by the River overflows. Although the infrastructures were designed for a maximal discharge rate of 100 m3/s (return period of 100 years), it appears that the River threatened to flood the city more than 6 times during the past century. The frequency analysis of the River discharge rates shows an abrupt increase for discharge rates >75 m3/s and a plateau at 95 m3/s; in other words discharge rates for return period events between 30 years and 150 years appear to be almost the same… Such a plateau could be produced by a significant storage upstream of the city, which smooth extreme discharge peaks. This storage was assessed to about 17 millions of cubic meters for a T100 flood. However, the storage is not visible along the river and it was suggested that karst aquifers may be responsible for it. The questions are (1) “Does such a storage exist in the karst aquifer?”, (2) “What is its capacity and how does it work?”, (3) “What could happen if this capacity is suddenly exceeded?”, (4) “What is the real contribution of karst water to the flows peak in Bienne?” In this view, a pragmatic approach is proposed to assess the contribution of the karst systems in the River flood generation. As the work is ongoing only the conceptual approach is here presented.