Rainstorms over Europe
From late May until mid-June, a persistent large-scale weather pattern with thunderstorms produced intense precipitation which caused both local flash floods and widespread flooding in central Europe. The floods struck many places with no warning.
Southern and central Germany were the first to be affected by the severe weather when violent thunderstorms and hail hit on 26 May 2016. Parts of Baden-Württemberg received over a month’s worth of rain in just one day on 29 May. Four people lost their lives in flash floods. Many houses in the town of Brauns¬bach were damaged or destroyed, and a well-known car manufacturer was forced to halt production temporarily.
Almost simultaneously, storms in France and the Benelux countries caused floods: at first only smaller rivers were affected, but the Loire and Seine later burst their banks, too. In the town of Nemours to the south of Paris, the River Loing rose to a record level. In Paris, the Louvre and the Musée d’Orsay had to be closed and works of art moved to higher storeys.
From 31 May to 1 June, further flash floods followed in Saxony, Bavaria, and in Austria. In Simbach in Lower Bavaria, the stream of the same name rose from 0.5 metres to around 5 metres within just a few hours, flooding around 5,000 households. Seven people lost their lives. Continuing this chain of events, central Europe and Germany in particular witnessed repeated cases of localised damage from severe thunderstorms throughout the first half of June.
Blocking weather pattern
The floods in central Europe stemmed from an unusual general weather pattern that persisted for an exceptionally long time, from 27 May to 9 June. A characteristic feature of the pattern was that the fast-flowing, high-altitude air current known as the jet stream formed a wave over Europe that resembled the Greek letter omega (see map).
Flash flood triggers
The starting point for flash floods are small-scale thunderstorm cells. Masses of warm, moist air rise to high altitudes and condense into towering clouds. Such thunderstorm cells can theoretically occur anywhere, and it is virtually impossible to predict where they will discharge their rain. How quickly, where, and to what extent heavy precipitation leads to flash floods and inundation depends on the catchment characteristics. Factors favouring a dangerously rapid run-off of surface water include steep terrain, low water retention capacity of the land due to a high proportion of paved and developed areas, soil that is saturated with water or clogged through mud, and little or no vegetation. If the soil is saturated after repeated cloudbursts, slopes can become unstable, resulting in landslides. Due to their high kinetic energy, the discharged masses of water sweep along debris and eroded soil. When rivers and streams become blocked, water builds up behind the obstacle. If it gives way, a surge-type flood wave forms. The dominant factor in flash floods, however, is extreme rainfall over a very short period.
In a region within this omega pattern, numerous thunderstorms formed in unstable stratified air, fostered by a deep low-pressure system over Germany and neighbouring countries. At the same time, large parts of northeastern and central France were hit by thundery rain under the influence of the accompanying surface low, Elvira. Protracted duration is a typical feature of omega blocks, which prevent weather systems from moving eastwards.
The block had devastating consequences in some regions. In Germany, storms formed on a daily basis from 28 May to 5 June, each bringing over 50 mm of rainfall. As the storms hardly moved, all of the rain fell on an area of just a few square kilometres. In some locations, daily precipitation rates were measured that statistically occur just once every 200 years. In areas with more sloping terrain and valley incisions, such as the towns of Simbach and Braunsbach, the huge amount of local rainfall led to abrupt and destructive flash floods. In contrast, early warning systems in place for the Loire and Seine made it possible to give advance notice and evacuate several thousand people.
Differences between flash floods and river floods
The discharge diagrams show two notional examples, with the typical hydrographs of a flash flood and a river flood wave. For flash floods, the maximum discharge is reached very quickly. It can exceed the normal discharge by a factor in the tens or hundreds. In large rivers, on the other hand, the flood discharge increases gradually, and seldom reaches more than ten times the normal value. In absolute terms, the discharge peak and volume (blue hatched area) of a river flood are many times higher than for a flash flood. Large areas are submerged in a river flood. Flash floods, in contrast, sweep along rubble and debris in sloping terrain.
Building damage in the billions
The overall loss from the storms in Germany is estimated at €2.6bn. Insured losses amount to €1bn in the property line, and €200m in motor insurance. Besides the inundation depth, the key factor for damage caused by the flash floods was the flow velocity and the trees, boulders, debris and sludge the waters swept along with them. However, it is extremely difficult to account for such variables in catastrophe models. In France, the insured loss from the floods came to €1.2bn. Of this amount, slightly more than half was attributed to residential buildings, almost a quarter to commercial buildings, one sixth to agriculture, and approximately one twentieth to the motor insurance class. 1,220 municipalities were affected and 175,000 claims filed.
Risk of change
Return periods considerably longer than 150 years (Seine), and of approximately 100 years (Loire), were calculated for the three-day precipitation totals that led to the floods in the Seine and Loire catchment areas. A climate model-based study showed that the probabilities of such precipitation events in the region are roughly double that of a virtual world without climate change. The intense storms in Germany broke a number of records.
It was the largest area to have ever been hit by a continuous period of thunderstorms prone to torrential rain in the observation period since 1960.
This record is due to the exceptional persistence of the weather pattern. It corresponds to a phenomenon that was discussed in Topics Geo 2014. That is, we are now observing persistent weather patterns more and more frequently during the summer half-year in the northern hemisphere. Their long duration can result in extreme outcomes.
Identify storm risks early on
The summer of 2016 demonstrated that a single weather pattern can trigger both localised intense pre¬cipitation with flash floods and large-scale precipitation with river floods. In the case of extreme rainfall and flash floods, measures such as watercourse restoration, a reduction in surface sealing, flood protection structures, and higher capacities for culverts and drainage systems are of little use in reducing potential loss consequences when bound by realistic cost-benefit analysis. For such extreme events, it would seem more expedient to develop hazard maps due to specific extreme precipitation scenarios for communities, pointing out likely run-off paths, locations where debris accumulation is expected, and inundation areas within the built-up sectors.
Evacuation plans can be drawn up based on this information, and corresponding emergency drills held with the participation of residents and emergency forces. Following the events of 2016 in Europe, it should be clear that extreme amounts of precipitation within a very short time are possible almost anywhere. Flood insurance should therefore form a central element of risk prevention, even for locations far from rivers.