Heavy Rain

What is heavy rain and what are the lurking dangers? The page offers a brief overview of the topic, gives recommendations for heavy rain precautions and provides useful links.

What is heavy rain?

Heavy rainfall is defined as high amounts of precipitation in a short period of time, which usually falls spatially limited and is accompanied by short warning times due to its convective origin. In Germany, heavy rainfall occurs more frequently in the summer months from May to September. As the temperature rises, the air’s capacity to absorb water increases. The upward movement and resulting cooling of warm, humid air triggers heavy rain events (BBK 2015).

The very large amounts of rainfall can cause water levels in small rivers and streams to rise rapidly, leading to flooding. Flash floods occur even without bodies of water due to the enormous amount of rain, especially on slopes. The rain masses running off the surface collect in topographically low areas and can cause flooding and damage to infrastructure. Heavy rainfall and the resulting flooding are intensified by waterlogged soils, lack of retention areas, sealing and backwater in the sewage system. Due to a high degree of building and sealing, the damage potential of flash floods is high, especially in urban areas (Klimanavigator).

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Forecast

Heavy rain can occur anywhere. The short-term nature of heavy rain as well as insufficient early warning systems make it difficult to predict the location and time of the storm-like rainfall (Klimanavigator). The German Weather Service (DWD) warns of heavy rain in 3 stages:

Tabelle 1: Unwetterwarnung nach dem Deutschen Wetterdienst (DWD)

Catastrophic heavy rainfall events

Abbildung 1: J. Bogardi

2023

In August 2023, a rainy low-pressure system from Italy caused heavy rainfall in large parts of Slovenia, Austria and Croatia. Vorarlberg, Tyrol and Slazburg as well as parts of Upper Austria and Carinthia were particularly affected. Slovenia was the country most affected by this crisis, with heavy flooding, especially in the north-east and center of the country, due to the amount of rain that fell within 24 hours for an entire month. Within just a few hours, this amounted to between 60 and 330 mm of water per square meter (normal average values in August are around 126 mm per square meter).

2021

The heavy precipitation lasting for several days, caused by low pressure system “Bernd”, led to heavy flooding in large parts of Rhineland-Palatinate and North Rhine-Westphalia as well as in the underlying areas in Belgium and the Netherlands. Over a wide area, precipitation values of more than 100 l/m² were registered within 72 hours (DWD 2021). Regionally, even higher precipitation values occurred, e.g. in Cologne-Stammheim, where 165 l/m² of precipitation fell within 15 hours (CEDIM 2021). The floods claimed more than 180 lives in Germany and caused massive losses of around 33 billion euros (DKKV 2022). In a 2021 loss statistic, Munich Re ranked the event as the second most expensive natural event after Hurricane Ida due to a total loss amount of 46 billion euros (Munich Re 2022). A detailed report of the 2021 flood event was published as part of the 62nd DKKV publication series “The 2021 flood disaster in Germany: One year on and initial lessons for the future“.

2017

In 2017, heavy and continuous rain again led to a state of emergency in many places. Heavy rain from the low pressure systems “Rasmund” and “Alfred” led to flooding in Berlin and Brandenburg as well as in the cities of Cologne and Moers (DWD 2017). This series of storms alone caused damage of over half a billion euros in the period between 20 June and 2 July 2017.

2016

In May and June 2016, landslides, flash floods and floods were triggered in large parts of southern Germany after exceptionally heavy rainfall. The events claimed several lives and caused billions of dollars in damage. In Braunsbach, more than 140 buildings were severely damaged, ten had to be demolished. Property damage in Braunsbach is estimated at over 100 million euros. (More on the events: DKKV special newsletter issue on heavy rain, June 2016)

Future prospects

Not only in Germany, but worldwide, the frequency and intensity of extreme weather events is increasing. The Intergovernmental Panel on Climate Change (IPCC) assumes that extreme precipitation events are very likely to increase in Europe by the end of the century (IPPC 2021). As part of an attribution study on the flood disaster in July 2021, researchers from the DWD and other researchers from the World Weather Attributions Initiative looked at the influence of climate change on heavy precipitation. The study suggests a connection, as the probability of such events increases by a factor of 1.2 to 9 (World Weather Attribution 2021, Prognos 2022). However, the wide range shows that an exact calculation is very difficult. Nevertheless, the projections illustrate how important adaptation to climate change is – also for disaster risk reduction.

With the funding measure "Water extreme events (WaX)"

… the Federal Ministry of Education and Research (BMBF) is funding 12 research projects that develop new approaches to the management of water-related natural hazards such as heavy rain, floods and drought across disciplines and sectors. For example, monitoring, forecasting and management concepts for urban heavy rainfall events are being developed.

As a transfer and networking project, the DKKV is supporting the projects together with the University of Potsdam. The project facilitates intensive networking and exchange between the research projects, creates synergies and generates a sustainable and target group-oriented transfer of practice through a cross-project synthesis of the research results.

project website

Recommendations for heavy rain

In Germany, the key role for heavy rainfall prevention and management at the local level lies with the respective municipalities and those affected. This includes, among other things, land use and urban land use planning, but also building, risk and behavioural precautions. The Federal Office of Civil Protection and Disaster Assistance (BBK) and the Cologne Municipal Drainage Works (StEB) have published guidelines for this purpose (Die unterschätzten Risiken Starkregen und Sturzfluten, Wassersensible Stadt- und Freiraumgestaltung in Köln, Wassersensibel planen und bauen in Köln, Mehr Grün für ein besseres Klima in Köln). Personal precautions (building, risk and behavioural precautions) by those affected are also an important element in protecting against heavy rain. The following measures are therefore recommended by the BBK, the StEB Cologne and the HochwasserKompetenzCentrum e. V. (HKC):

Precaution

In Case of an Event

General precautionary measures and tips on how to behave in the event of a disaster are compiled by the BBK here: BBK Rategeber für Notfallvorsorge und richtiges Handeln in Notsituation and on the BBK Webseite.

Notfall- Apps

NINA – Notfall-Informations- und Nachrichten-App: NINA warnt deutschlandweit und bei Bedarf standortbezogen vor Gefahren, wie z. B. Hochwasser und anderen Großschadenslagen. Auf der Webseite Warnung.bund.de werden die Warninformationen online zusammengestellt.
WarnWetter-App des DWD: Fasst wichtige Hinweise zur aktuellen Warn- und Wettersituation zusammen. Videos erklären die Situation. Allgemeine Warnungen zu Gewittern, Sturmböen oder Starkregen finden Sie auf der Webseite des Deutschen Wetterdienstes (DWD).
KATWARN: Die App KATWARN wurde vom Fraunhofer-Institut FOKUS im Auftrag der öffentlichen Versicherer entwickelt und vermittelt Warnungen und Handlungsempfehlungen von autorisierten Behörden und Sicherheitsorganisationen.
Meteoalarm – Alerting Europe for Extreme Weather: Die Webseite liefert Warnung für Europa.
Starkregengefahrenkarten der StEB Köln: Einschätzung der Gefahrenlage bei Starkregen und Sturzfluten im Raum Köln.
KLiVO Portal: Der Klimavorsorgedienst unterstützt bei der notwendigen Anpassung an die Folgen des Klimawandels, z. B. Leitfäden, Werkzeuge, Webportale oder Klimadaten.