BAU-I-4: Heavy rain in residential areas

The picture shows a view of a group of older multi-storey buildings and some trees, which are barely visible due to a heavy rain that has just fallen.Click to enlarge
Flash floods due to heavy rain can present considerable threats to residential areas.
Source: Photograph: © Tom Bayer / stock.adobe.com

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

Table of Contents

 

BAU-I-4: Heavy rain in residential areas

In 2002 the high number of hours of heavy rain in the south and east of Germany was one of the causes of floodwater disasters in the river basins of Elbe and Danube. High amounts of damage can, however, be inflicted even at distinctly lower exposure to heavy rain. For 2016 the amounts of insured damage caused by heavy rainfall is estimated at just under 1 billion Euros.

The stack column graph shows the percentage of the settlement area with an exceedance of heavy rainfall warning level 3 (severe weather) according to the number of hours. Six categories are shown: 0 hours, 1 to 3 hours, more than 3 to 6 hours, more than 6 to 12 hours, more than 12 to 24 hours, over 24 hours.
BAU-I-4: Heavy rain in residential areas

The stack column graph shows the percentage of the settlement area with an exceedance of heavy rainfall warning level 3 (severe weather) according to the number of hours. Six categories are shown: 0 hours, 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 30 per cent, the overall value was highest in 2002 from the years considered, 2001 to 2017.

Source: DWD (RADOLAN-Climatology); Bundesamt für Kartographie und Geodäsie (DLM250)
 

Flash floods – high damage potential for residential areas

In recent years images such as the disaster in Berlin in July 2017 or Braunsbach in May 2016 have been featured repeatedly in the media. It can be a matter of minutes for torrential rain to make sewers overflow, flooding entire street networks. Torrential rain can cause flash floods engulfing cars and devastating streets and buildings. Such events can cause immense damage to property and, in individual cases, even cost lives.

Heavy rain is a weather event in which massive amounts of rain fall within a very brief period of time. The DWD (DWD) issues torrential rain warnings when it is expected that within one hour more than 25 litres per square metre (l / m²) or in six hours more than 35 l / m² rain will fall. Amounts of rain of more than 40 l / m² or 60 l / m² falling in these time periods are classed as extremely severe weather. Frequently these cloud bursts occur when massive clouds – formed as a result of convection – open their floodgates causing torrential rain to fall on mostly small areas. However, persistent rain falling on extensive areas can also temporarily exceed the warning thresholds for heavy rainfall.

During or after heavy rainfall, damage is caused primarily by so-called flash floods. These are extreme floodwater events as a result of heavy rainfall. They occur in lowland areas when rainwater cannot drain or seep into the ground fast enough. In those cases water accumulates on the surface or it dams up owing to overburdened sewage and drainage systems as their ultimate design loads are exceeded. Water levels can rise very rapidly then, especially in hollows and underpasses. On sloping terrain this can cause so-called ’mountain flash floods’. The quick-draining water will accumulate in gutters or stream beds and can swell extremely rapidly forming surge-like floodwater waves. These waves are able to reach areas where no rain had fallen previously.40 When such mountain flash floods sweep away material such as tree trunks or rocks, they can cause massive damage to houses or culminate in the complete loss of buildings.

Heavy rain and flash floods can, however, damage buildings in other ways too. For example standing water can reach levels which exceed the design threshold thus enabling water to enter the building e.g. through entrance doors at ground level, cellar windows or due to tailbacks in the sewage system. The ingress of water then leads to the distribution of mud and detritus which may be polluted additionally by a mixture of mineral oil, chemicals and faeces. Above and below the earth’s surface, standing water or high soil humidity can cause typical floodwater damage to the fabric of buildings, such as moisture penetration and linear water level marks, efflorescences on surfaces, detached coatings or fungus.41 In order to prevent damage, house proprietors can take a number of measures, possibly by placing building apertures at a sufficient height above ground level, the use of waterproof building materials and suitable drainage systems with backflow preventers.42

Climate researchers and meteorologists expect that the incidence of heavy rainfall will increase in future. One reason for this is that at higher temperatures the air can absorb more water – at a temperature increase by one Kelvin approximately seven per cent more water.43 Furthermore, it is to be expected that owing to changed meteorological circumstances during the formation of showers and thunderstorms, the formation of cloud and precipitation will intensify.44 During the second half of the 21st century, Germany – albeit with major regional and seasonal differences – will experience a distinct increase in the incidence of daily precipitation amounts compared to the relatively infrequent occurrence at present. The comparatively strongest increase is expected for events which are currently still infrequent.45

It is hard to forecast how frequent and how intense heavy-rain events will be and whether global warming may already be impacting on this phenomenon. In view of the fact that such events are often relatively localised, heavy-rain events are not always recorded by the nationwide network of measuring stations. This is one of the reasons why the DWD developed a radar-based service for detecting precipitation levels which has been providing precipitation data at an almost nationwide level since 2001.46 This data set contains nearly all heavy-rain events which occurred in Germany since 2001. In future the temporal extension of this data collection process will also enable trend analyses regarding the frequency of events when the warning levels used by DWD are exceeded. (see p. 25).

In the illustration for the indicator concerned, the annual data from the radar-based precipitation measurements were superimposed with the data for Germany’s residential terrain. The outcome makes the year 2002 stand out. In that year, numerous heavy rainfalls over an extended period caused the flooding disaster in the Elbe and Danube river basins. Not one of the other years examined reflected a greater extent of residential areas affected by heavy precipitation for the same amount of hours. However, those two factors are not always of crucial importance for the extent of damage. For example in 2016 – a less eye-catching year in this time-series – the amount of insured damage was just under 1 billion Euros.

40 BBK – Bundesamt für Bevölkerungsschutz und Katastrophenhilfe (Hrsg.) 2015a: Die unterschätzten Risiken „Starkregen“ und „Sturzfluten“ – Ein Handbuch für Bürger und Kommunen. Bürgerinformation, Ausgabe 1, Bonn: 27.
41 BBSR – Bundesinstitut für Bau-, Stadt- und Raumforschung im Bundesamt für Bauwesen und Raumordnung (Hrsg.) 2018: Starkregeneinflüsse auf die bauliche Infrastruktur. Bonn: 19ff.
42 BBK – Bundesamt für Bevölkerungsschutz und Katastrophenhilfe (Hrsg.) 2015b: Empfehlungen bei Unwetter – Baulicher Bevölkerungsschutz. Bürgerinformation, Ausgabe 1. Bonn: 27ff. BBSR 2018: see endnote no. 40: 25ff.
43 DWD – Deutscher Wetterdienst (Hrsg.) 2016: Starkniederschläge in Deutschland. Offenbach am Main: 1.
44 Becker P., Becker A., Dalelane C., Deutschländer T., Junghänel T., Walter A. 2016: Die Entwicklung von Starkniederschlägen in Deutschland – Plädoyer für eine differenzierte Betrachtung: 1.
www.dwd.de/DE/fachnutzer/wasserwirtschaft/entwicklung_starkniederschlag_deutschland_pdf

45 LAWA – Bund/Länder-Arbeitsgemeinschaft Wasser (Hrsg.) 2018: LAWA-Strategie für ein effektives Starkregenrisikomanagement. Erarbeitet von der Kleingruppe „Starkregen“ des Ständigen Ausschusses „Hochwasserschutz und Hydrologie“ (LAWA-AH) der LAWA. Thüringer Ministerium für Umwelt, Energie und Naturschutz, Erfurt: 21.
46 Winterrath T., Brendel C., Hafer M., Junghänel T., Klameth A., Lengfeld K., Walawender E., Weigl E., Becker A. 2018: RADKLIM Version 2017.002: Reprozessierte, mit Stationsdaten angeeichte Radarmessungen (RADOLAN), Niederschlagsstundensummen (RW).
DOI: 10.5676/DWD/RADKLIM_RW_V2017.002

 

 

Interfaces

BAU-I-5: Claims expenditure for property insurance

VE-I-3: Heavy rain and roads

VE-I-5: Impacts on roads from extraordinary weather events and disasters - case study

BS-I-1: Person hours required for dealing with damage from weather-related incidents

 

Objectives

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)