VE-I-3: Heavy rain and roads

The picture shows a completely flooded street crossing in a city, into which two cars are just driving.Click to enlarge
Heavy rain puts road safety at risk and can make roads impassable.
Source: Photograph: © Animaflora PicsStock /

2019 Monitoring Report on the German Strategy for Adaptation to Climate Change

Table of Contents


VE-I-3: Heavy rain and roads

Heavy rainfalls can have serious consequences impacting on road traffic. In Germany, from 5 to more than 20% of federal highways are affected every year by heavy rain at warning level 3. For most of the road sections affected, the overall temporal extent of these events amounts to between 1 and 6 hours per year.

The graph describes the percentage of the length of federal trunk roads with an exceedance of heavy rain warning level 3 (severe weather).
VE-I-3: Heavy rain and roads

The graph describes the percentage of the length of federal trunk roads with an exceedance of heavy rain warning level 3 (severe weather). Five categories are shown: 1 to 3 hours, more than 3 to 6 hours, more than 6 to 12 hours, more than 12 to 24 hours, more than 24 hours. The highest category is only represented in a few years. The two lowest categories have the clearly highest percentage share. No trend analysis was carried out. The values fluctuate significantly between years. At 22 per cent, the overall value was highest in 2002 from the years considered, 2001 to 2017.

Source: DWD (RADOLAN-Climatology); Bundesanstalt für Straßenwesen (BISStra)

Heavy rain – brief but violent

Even though many people will remember 2018 especially for prolonged periods of heat and drought, there were, in many parts of Germany, violent tempests with substantial amounts of rainfall which caused massive damage and obstructions. In many places, road traffic was affected too. In late May, for example, a belt of road closures extended from North Rhine Westphalia to Saxony, because roads were either flooded or covered in mud.

These road closures were caused by so-called heavy rainfalls. These are events with brief intensive rainfall usually in a localised area. In summer months they are frequently associated with violent thunderstorms. These are usually referred to as convective precipitation events. These events occur when humid air at ground level warms up quickly causing a strong uplift, with the vapour rising up into the atmosphere. Rising to cooler air layers higher up, the vapour contained in the air condenses and freezes. Given the requisite combination of air currents, temperature and air humidity, convective storms (thunderstorms) and heavy rain can build up within a few hours.

Brief and violent rainfalls can have severe consequences. The nature of these consequences and the specific damage caused, depend on the location as well as the intensity and duration of rainfalls. On level ground the water cannot run off quickly, and the soil will eventually run out of capacity to absorb it. The result can be long-term flooding of roads in hollows and underpasses. That is where water will collect which can no longer be absorbed by drainage systems and conducted away. As a rule, in upland or alpine regions, depending on the incline, water tends to run off the surface quite rapidly. Very violent heavy rain can turn minor streams into torrential rivers, where rising flood waves force their way through the narrow bed of a stream. Time and again, the tail ends of these flash floods reach areas which had not actually been affected by heavy rain. In extreme cases, the flow of the water masses or high water pressure can damage infrastructures.56

More frequent are cases where bad roads and poor visibility increase the risk of accidents, e.g. from aquaplaning or from obstacles in the road, for instance when driftwood or other flotsam gets stuck in a bottleneck at bridges or underpasses causing a logjam and congesting the traffic network. Obstructions can also occur when after heavy rainfall slopes and embankments start slipping or are washed out thus covering roads in mud or damaging them in some way.

Between 2001 and 2017 heavy rainfalls have occurred practically all over Germany. It seems obvious therefore that this phenomenon is not limited to upland or mountain areas. Here you see the outcome of an evaluation of radar data57 by the DWD. In future the temporal extension of this data collection process will also enable trend analyses of the frequency of events when the warning levels used by DWD are exceeded. (see p. 25).

It is expected by climate researchers that in future the frequency of heavy rain and its intensity will in general increase. One reason for this is that at higher temperatures the air can absorb more water – approximately 7 % more water at a temperature increase by one Kelvin. Furthermore, the changing meteorological conditions give rise to intensified formation of clouds and precipitation.58 As far as Germany is concerned, it is expected that especially in the second half of the 21st century, there will be – albeit with major regional and seasonal differences – more instances of daily rainfall in quantities which are relatively infrequent at present. The comparatively strongest increase is expected for events which are currently still infrequent.59 If these developments materialise, the risk of road traffic disruptions and – in extreme cases – damage to roads and infrastructures will increase. So far, there are no data in the public domain which might enable a reliable and cause-related assessment of the extent of such disruptions and damage. Therefore, the indicator illustrated here in this context only provides an insight into the temporal extent to which the network of federal highways was affected by torrential heavy rainfalls as recorded by means of radar data for specific spatial areas.

It is not very likely that infrastructures are damaged in all heavy rain events covered by the indicator. The DWD issues warnings against torrential heavy rain whenever rain amounts of more than 25 litres per square meter (l / m²) are expected to fall in one hour, or when more than 35 l / m² are expected to fall in 6 hours. If these warning levels are exceeded in only moderate amounts, the intensity of heavy rain is usually not likely to damage traffic infrastructures.

However, the DWD ranks weather situations at warning level 3, ‘tempests’ when they are categorised as very hazardous, in which case the DWD recommends that people avoid staying outdoors. Anyone who has been overtaken by heavy rainfall – whether on foot, on a bicycle or in a car – will be able to relate to this.

56 - S. Endnote Nr. 45: 19. S. Endnote Nr. 40: 20ff
57 - S. Endnote Nr. 46.
58 - S. Endnote Nr. 44: 1.
59 - S. Endnote Nr. 45: 21.



VE-I-4: Weather-related road traffic accidents

BAU-I-4: Heavy rain in residential areas



The Federal Government will examine whether and – where appropriate – to what extent […] the dimensioning of drainage infrastructure requires adaptation to more abundant precipitation levels. Where necessary, the Federal Government […] will adapt the relevantregulations for dimensioning the drainage infrastructure. (DAS, ch. 3.2.11)

The objective of a common (joint) management of heavy-rain risks is to reduce the risks caused by adverse effects of heavy rain or flash floods on human health, buildings and infrastructure, the environment, cultural heritage and commercial activities. (LAWA-Strategie Starkregenrisikomanagement, ch. 1)