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AN EARLY WARNING SYSTEM FOR THE THUNERSEE
HYDRO-METEOROLOGICAL INDICATORS FOR THE OPERATION OF THE CULVERT IN THUN
Peter Mani1, Peter Schmocker2, Erik Bollaert3, Martin Andres4
The flood events from May 1999 and August 2005 have clearly shown the problems of the hydraulic system Thunersee-Aare. Compared to the possible inflow volume, the outflow capacity of the Thunersee is insufficient. A reason for this is the reroute of the Kander into the Thunersee, which was made in the year 1714 and increased the catchment area at around 100%. The Thunersee, with its steep alpine watershed and its small retention volume between normal water level and damage limit, reacts very rapidly to events.
To improve this situation a culvert will be built. With this culvert about 100 m³/s additional water can be derived from the Thunersee. When ideal controlled, this improvement can be reached without causing an increased outflow peak and flooding of the downstream residents.
However, the culvert can only bring a significant improvement, if it is used to lower the water level of the lake before an event occurs. On behalf of the developer of the culvert, the construction and the water management authorities of the canton of Bern, extensive investigations were carried out to derive indicators for an early flood warning. Thereby we distinguish between indicators which describe short to medium term dispositions and such, which indicate a beginning event.
The different claims to the lake-level (navigation and nature protection which demand a high and constant level in contrast to flood security which demands as low as possible lake-levels) must be considered in context of new operating rules, during situations of flood risk.
INDICATORS FOR SHORT TO MEDIUM TERM DISPOSITIONS
As the event from May 1999 showed, larger snow quantities in the altitude between 1000 and 2000m can lead, due to the altimetry of the catchment area, to wide and intensive snowmelt.
For the derivation of the relevant limit values, snow heights and water equivalents were evaluated.
High soil moisture in combination with heavy rainfall causes a rapid and strong reaction in discharge. Since a spatial representative measurement of the soil moisture is still not possible or if then only with large expenses, an indicator was derived from the precipitation data.
1geo7 AG, geowissenschaftliches Büro, Neufeldstrasse 3, 3012 Bern, Switzerland, (Tel. +41 (0)31 300 44 33, Fax: +41 (0)31 302 76 11, email: peter.mani@geo7.ch)
2 bhc Projektplanung, Mühlematteweg 7, CH-3752 Wimmis, (Tel. +41 (0)33 657 24 26, fax +41 33 657 24 86, email info@bhc-projektplanung.ch)
3AquaVision Engineering, P.O. Box 73 EPFL, CH–1015 Lausanne, (Tel. +41 (0)21 691 45 13, email:
erik.bollaert@aquavision-eng.ch)
4 Kissling + Zbinden AG, Ingenieure Planer USIC, Oberlandstrasse 15, 3700 Spiez, Switzerland, (Tel. +41 (0)33 650 71 71, Fax +41 (0)33 650 71 72, email: martin.andres@kzag.ch)
– – – – INDICATORS FOR BEGINNING EVENTS
For the detection of a beginning event spatially and temporally graduated indicators are used.
First the meteorological warnings of MeteoSchweiz are analysed. The precipitation in the watershed forms the second indicator. Therefore the precipitation is measured and analysed once per hour at 11 stations spread over the Bernese Oberland. A first estimation of the
discharge reaction can be made from the discharge measurements of the watersheds situated in higher regions. The measurements of 4 discharge gauging stations are analysed for this.
Directly at the system border to the Thunersee and Brienzersee the main inflows are analysed.
The last indicator is calculated from the change of the lake-level per time unit. The nearer at the Thunersee the indicators are, the shorter the reaction time is to lower the sea level.
COMBINATIONS OF THE INDICATORS
On one hand, the definition of the threshold values for the parameters must allow the lake-level to be lowered early enough; on the other hand it has to be avoided as far as possible to lower the water level excessively many times without following event. This goal is achieved, if the indicators are combined and if these combinations cause seasonally different effects on the regulation of the lake.
EXAMINATION OF EFFICIENCY ON THE BASIS OF PAST EVENTS
The examination of the effects of the indicators and their combination took place on the basis of the last 7 years. With the extreme event in August 2005 the indicator combination would have led to preventive lowering of the lake-level 3 days before the event. With today’s available technique the lowering of the water level took place only approx. 6 hours before the event.
Fig. 1:Alarm releases due to the criteria “Soil Moisture/Antecedent Precipitation” in the period 1999 – 2005
100.0 120.0 140.0 160.0 180.0 200.0 220.0 240.0 260.0
01.01.1999 11.04.1999 20.07.1999 28.10.1999 05.02.2000 15.05.2000 23.08.2000 01.12.2000 11.03.2001 19.06.2001 27.09.2001 05.01.2002 15.04.2002 24.07.2002 01.11.2002 09.02.2003 20.05.2003 28.08.2003 06.12.2003 15.03.2004 23.06.2004 01.10.2004 09.01.2005 19.04.2005 28.07.2005
N-Summe 20 Tage [mm]
