ABSTRACTS 'Extreme Discharges' - CHR-KHR

Does the perception of extremity change? An ongoing case
study in the Sure river basin
Hugo Hellebrand
G. Drogue
P. Matgen
C. Schmitz
J. Juilleret
E. Vansuypeene
L Hoffmann
L. Pfister
Centre de Recherche Public Gabriel Lippmann
Av. de la Faiencerie 162A, L 1511 Luxembourg
hellebra@crpgl.lu
Introduction
In the last twelve years the Grand Duchy of Luxembourg experienced its three largest flood events of the
past five decades, causing considerable economic damage. From the layman’s point of view this high frequency
of floods seems unacceptable and gives the hydrologist to answer the following important question:
did the frequency of extreme flood events change? The answer to this question lies mainly in the field of two
major topics, which are presently of high interest: climate change and land use change.
Concerning climate change several studies have pointed out that there is an actual change in the climate of
the Rhine and Meuse basins. Pfister et al. (2004) demonstrated an increase of westerly atmospheric fluxes
leading to an increase in winter rainfall duration and intensity. In another study climatological observations
through the 19 th and 20 th century for the Grand Duchy of Luxembourg have shown a clear trend towards
higher rainfall totals as a consequence of changes in the dominating atmospheric circulation patterns (Pfister
et. al., 2004). An increase of average discharge rates and flood frequency in the Rhine during the last century
has been attributed to the increase of winter rainfall (Engel, 1997). Concerning micro- and mesoscale basins
in the Grand Duchy of Luxembourg the observed climate change signal of the last 50 years has had very
contrasted consequences on maximum stream flow, mainly due to the orientation of the westerly fluxes and
the influence of the topography (Pfister et al., 2004). However in the same study it was concluded that an
increase in number of days with rainfall and/or in rainfall event duration could lead to a higher overall basin
humidity, thus creating favourable conditions for high surface runoff rates.
During the last century considerable changes in land use have occurred, mainly consisting in an increase in
urbanisation, in a change in forest cover and in an increase in drained agricultural lands. However, no clear
evidence exists of the impact of land use changes on flood frequency and magnitude in the main channels of
the Rhine and Meuse basins (Pfister et al., 2004). Land use change may nevertheless have a significant impact
on the hydrological behaviour in micro- or mesoscale river basins and particularly urbanisation can have
significant local effects in small basins (headwaters) with respect to flooding, especially during heavy local
rainstorms (Pfister et al., 2004).
Another cause of discharge regime change is the straightening of the rivers by reducing the floodplain with
dykes and other structures. For the Rhine and Meuse basins Ebel and Engel (1994) have evaluated the loss in
floodplain areas to as much as 70% of the initial 1400 km 2 . The frequency of bank overtopping for the Geer
(Belgium) increased 16 fold from 1965 to 1980 and was mainly attributed to river straightening (Mabille and
Petit, 1987).
The above-mentioned natural and/or anthropogenic changes in climate, land use and river morphology and
their interaction make it difficult to predict changes in discharge regimes on every scale. This holds especially
for extreme flood events, since they are very rare and their statistical properties are extremely difficult
to determine via existing observation periods (Pfister et al., 2004). In order to point out these difficulties we
tried to assess the discharge regime of the Sure river located in the Grand Duchy of Luxembourg for two
different periods, one from 1870-1920 and the other from 1966-2003, at the village of Steinheim.
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