ABSTRACTS 'Extreme Discharges' - CHR-KHR

cm for M95++). It is important to understand that consideration of the possible retention polders upstream
of Maxau would yield an additional reduction in the range of 10 cm, in particular for the Upper Rhine
area.
- The unrealistic, extreme land-use scenario (50% increase of urban areas) would result in a water level
increase not much more than 10cm.
- The M95+ and even more the M95++ scenario results in higher reduction of flood levels in the case where
the flood polders are active. This is due to the fact that according to the operation rules, the flood polders
are to be filled only if flood discharge exceeds the 200-year value. In January/February 1995 (M95) the
flood discharge along most stretches of the Rhine was above the 100-year value, but below 200 years.
That is why the polders were used only at a few stretches resulting in small water level reduction only.
Some more general results from the whole study are summarised in the following:
(1) At the lower meso-scale level the influence of land-use on storm-runoff generation is stronger for convective
storm events with high precipitation intensities than for long advective storm events with low precipitation
intensities, because only storm events associated with high rainfall intensities are at least partially
controlled by the conditions of the land-cover and/or the soil-surface. Convective storm events, however,
are of minor relevance for the formation of floods in the large river basins of Central Europe because the
extent of convective storms is usually local.
(2) An estimated, rather dramatic, further increase of urban areas of about 50% may result in an increase of
medium-size flood peak discharge (e.g. return intervals between 2 and 8 years) in catchments of the lower
meso-scale (up to ca. 1000 km²):
⎯
Between 0% and 4% for advective rainfall events, and
⎯ up to 30% for convective rain storms.
(3) The flood impacts due to a more realistic representation of urbanisation increase are in the order of 1 to 5
cm in the main channel, while the effects are even less for extreme rainfall amounts.
(4) The decentralised storage, detention, and infiltration of urban storm water yields reduction of flood peaks
in catchments of the lower meso-scale which are about the same size as the increase due to urbanisation.
(5) The superposition of flood waves originating in different sub-basins shows that the maximum effect of
water retention in the landscape generally occurs in the rising limb of the flood wave in the main channel.
The flood mitigation effect at the peak is considerably smaller.
(6) Water retention measures in polders along the Upper and Lower Rhine, under the given boundary conditions,
yield flood peak attenuation along the Rhine all the way down to Lobith of between 1 cm and 15cm. The
optimised and co-ordinated control of the polders can result in a considerable stronger decrease of the peaks.
Final report:
Bronstert, A., Bárdossy, A:, Bismuth, C., Buiteveld, H., Busch, N., Disse, M., Engel, H., Fritsch, U., Hundecha,
Y., Lammersen, R., Niehoff, D., Ritter, N., (2003): Quantifizierung des Einflusses der Landoberfläche
und der Ausbaumaßnahmen am Gewässer auf die Hochwasserbedingungen im Rheingebiet. Reports of
the CHR, Series II, No. 18, 78pp.
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