Predicting the likelihood of floods

More and more rivers have been bursting their banks in Germany since 1993. Evidence of climate change?

Flooding in the Elbe region is by no means rare. Yet the extreme flood in 2002 hit the population largely unprepared. Houses, streets and bridges fell victim to the mass of water, the electricity supply and telecommunications services collapsed in some cases, villages were cut off from the outside world. The damage in Germany ultimately totalled 9 billion euros, some people even lost their lives in the flood.

In 2013, Central Europe was once again affected by extreme flooding, and the Elbe broke its banks again, too. Yet the damage in the upstream reach of the Elbe River in Saxony was far less than eleven years before. The State of Saxony invested in new flood protection systems following its experiences in 2002. The team led by Professor Andreas Schumann from the Bochum-based Institute of Hydrology, Water Resources Management and Environmental Engineering was involved in laying the foundations for this. “It became clear in 2002 that the statistical models for predicting extreme flooding events are no longer suitable,” he explains. Instead, extreme flooding was more likely than in the assessments done with this model. The Bochum-based hydrologists developed a new statistical model, which they have continuously further optimised ever since.

Three causes of flooding

As part of a research group funded by the German Research Foundation, the Bochum engineers are investigating the causes of extreme flooding in Germany and thus improving the statistical methods. In their model, they distinguish between three types of flood, which stem from different causes: Heavy rain, which lasts one or two days; prolonged rain over four to five days; and snow-related flooding due to melting snow.

The annual maximum values used to be analysed statistically, and so the statistical model did not distinguish between the three types of flooding. However, this is precisely what is required in order to estimate the likelihood of flooding. Brief, localised heavy rain can, for instance, cause rivers to break their banks in smaller regions, but not in larger regions. “The Rhine in Cologne can only cause flooding as a result of prolonged rain because local rain is balanced out spatially,” clarifies Schumann.

Bringing together weather data and water levels

His group painstakingly calculated the three flood types for the new model. In doing so, the researchers looked back to the start of the observation series, which in some cases dated back 150 years, although generally between 50 and 70 years. They drafted a separate statistic for each type of flooding, which calculates the likelihood of such an event. As the basis, they received records of the discharges for certain rivers from the respective State Offices and set them in relationship  with meteorological data from the Deutscher Wetterdienst from the same time. They thus obtained statistics concerning which weather events cause what effects in the rivers and are able to derive statements on the flood risk for the future on this basis. The model fundamentally works for the whole of Germany, although it must be adapted to each region.

This is because the boundary conditions for each region are also important in order to make a statement that is as apt as possible. These include soil moisture, forestation, whether and how a region is used for agriculture or developed, and the relief of the region, which, for instance, determines whether there are steep or flat flood waves and how quickly the flood ebbs away.

Flooding occurs at irregular intervals

The new statistical model is based on data for the river Mulde and small catchments in the Harz mountainous region. “We can now calculate how likely it is that a certain type of flooding will occur in any year,” summarises Schumann. However, the events are not distributed evenly over time. “There were floods on the Rhine in 1993 and 1995 that were both so severe that they should, in theory, have only occurred once in hundred years. But they were very close to each other in terms of time,” says the engineer. The Elbe floods in 2002 and 2013 are a further example of such irregularity. “Dresden was only flooded so severely once before, in 1845. Then it happened twice within twelve years,” says Schumann.

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