ABSTRACTS 'Extreme Discharges' - CHR-KHR

the tributaries of the Rhine exceed the 1,250 year design discharge, i.e. 16,000 m 3 /s, near the Dutch-German
border at Lobith. So, even though at Lobith the maximum observed discharge is “only” 12.500 m 3 /s, the relevant
hydrological processes that occur during a 16,000 m 3 /s event possibly have been observed on a more local
scale. This means a relatively good estimate of model uncertainties can be obtained by comparing observed and
simulated discharges. First enquiries were executed to do this quantification with the so-called “GLUE”-method
in which uncertainties (likelihoods) in parameter estimates are based on the goodness-of-fit of observed and
simulated hydrographs. In this method a large number of optional parameter sets are then used in model predictions,
and model outcomes are weighted according to the associated likelihood. From this ensemble of weighted
model outcomes the mean and uncertainty bounds can be evaluated (see e.g. Figure 2).
Special focus in this process is needed for the hydraulic component of the model. During design flood conditions
long stretches of the river network are confronted with discharges that exceed the highest observed discharges
by far. Therefore we need to know [a] at what stage floods will occur and [b] which amount of water
will be lost. Even though no data on these type of extreme floods is available, the physical processes involved
are quite well-known. Hydraulic model simulations can therefore provide valuable information. However, this
requires detailed information on cross sections, dikes and flood plains along all stretches of the basin. This may
still take a huge effort but it will pay off eventually. The physical boundaries of the river network may well reduce
the effects of uncertainties of the rainfall generator and rainfall-runoff model on the final outcome.
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