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The extreme precipitation events of 19–22 August 2005 led to exceptionally high readings at the gauging stations operated by the Hydrology Division of the FOEN. In many cases, record values were observed.
The heavy rainfall was due to what is known as a Vb weather situation: a low-pressure system located over France moved into the Gulf of Genoa and then travelled northwards over the eastern Alps; moist and warm air masses from the Mediterranean were carried over the eastern Alps and returned with north‑easterly winds to the northern slope of the Alps.
According to an assessment of the provisional data, the area of the regions affected was greater than during the floods of 1999. In most cases, the regions that had suffered flood damage that year were struck once again in 2005, and in addition new regions were affected on the River Saane and in Graubünden. The flooding was less serious in the Jura region and in parts of Eastern Switzerland.
At around 80 of the 200 discharge measurement stations distributed across Switzerland, floods with a return period of more than 2 years were recorded (cf. map). The orange and red dots on the map indicate peak discharge values with a return period of more than 100 years. These extreme events occurred in the Aare basin above Lake Biel, on the Emme and in the Reuss catchment.
The 10 stations reporting a return period of more than 200 years were concentrated in the Bernese Oberland and Central Switzerland. These estimates are based on a measurement series of at least 80 years.
Existing peak values were exceeded at a total of 26 measurement stations, including sites where new records had already been established in 1999. At a number of stations, the previous maximum values were dramatically exceeded, e.g. on the Sarner Aa and Engelberger Aa, where in each case the discharge was well over double that measured in 1999.
In several lakes - e.g. those of Biel, Thun, Brienz, Sarner, Lauerzer and Aegeri - the highest-ever water levels were recorded. It is notable that in some cases the previous records, set in 1999, were exceeded by a substantial margin (Lake Brienz + 69 cm, Lake Sarner + 97 cm). Lake Lucerne only narrowly missed the record level seen in 1910 (- 2 cm).
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