The past has shown that flooding can occur even far away from bodies of water. This is triggered by small-scale, short but intense rainfall events—known as heavy rain. The German Weather Service defines heavy rain as 15 to 25 liters per square meter in one hour or 20 to 35 liters per square meter in six hours. Unlike flooding on large rivers, the exact location and time of heavy rainfall is difficult to predict and can take those affected by surprise. In small catchment areas in particular, even smaller bodies of water such as streams can suddenly swell due to the amount of rainfall, overflow their banks and cause flooding.

During flooding caused by heavy rainfall, the ground can no longer absorb the precipitation, or cannot do so quickly enough. The water flows across the surface of the terrain toward the nearest body of water or other low points in the terrain, such as ditches, depressions, or basements. Strong currents develop in hilly terrain. Large quantities of flotsam (such as wood, hay bales, and silage bales) and eroded materials (such as soil or debris) can be swept away. This material can then accumulate at narrow points or bridges, for example, and block them. The resulting backwater floods the surrounding area and can cause further serious damage to buildings and infrastructure.

Heavy rainfall can also cause flooding in flat areas, especially on sealed surfaces in our cities. Since the large volumes of water usually exceed the design limits of the sewer networks, the water cannot drain away underground and the rainfall floods large areas. Buildings and infrastructure in low-lying areas in particular can be severely damaged.

Since heavy rainfall cannot be prevented, measures must be taken to minimize the risks and damage. Due to the short warning time, prevention is particularly important.

Cities and municipalities therefore play a key role in heavy rainfall risk management, as they can develop a concept tailored to local conditions. However, as a citizen, company, or institution, you should also take active precautions and protect yourself against damage caused by heavy rainfall. You can take particularly targeted precautions on your own property if you are aware of the hazards and risks at your location from municipal heavy rainfall hazard maps.

In Mainz, a flood and heavy rain prevention concept (HSVK) was drawn up in 2023 for the first two districts of Ebersheim and Finthen. It aims to raise awareness among those affected of the risk of flooding and to promote personal initiative and responsibility for private flood prevention. The plan opens up opportunities for community-based prevention and presents location-specific flood and heavy rain prevention solutions in a package of individual measures.

It is based on flash flood hazard maps from the Rhineland-Palatinate State Office for the Environment (see list of links), which show water depths, flow velocities, and flow directions of surface runoff water as a result of heavy rainfall. Three different rainfall events and their effects are shown for specific locations. These findings are incorporated into Mainz's flood and heavy rainfall prevention plan. At the district level, clear maps now illustrate which areas in the two districts are particularly at risk during heavy rainfall and which measures are derived from this. A catalog of measures lists the respective potential hazards and recommended preventive measures for which the city or the respective owners are responsible. Work on developing an HSVK concept for the districts of Drais, Marienborn, and Hechtsheim will begin in 2024.

Homeowners are encouraged to take appropriate precautions on their own initiative to protect themselves against damage caused by flooding. Below, we have compiled a list of the most common hazards posed by heavy rain and possible preventive measures. The need for natural hazard insurance that covers damage caused by heavy rain and flooding is often underestimated.

Is your residential area at risk of flooding? Or susceptible to flooding during heavy rainfall? Find out more and be well prepared!

The Rhineland-Palatinate Flood Prevention Information and Advice Center (IBH) offers comprehensive information on flood and heavy rain prevention for the public on its website. The Federal Office for Civil Protection and Disaster Assistance provides the NINA warning app for current weather warnings and flood warnings. Here you will also find detailed checklists for emergencies. The German Weather Service also provides information on current severe weather warnings on its website and offers its WarnWetter app. All links can be found at the end of the article.

At the Mainz Environmental Store, interested homeowners can obtain basic information on how to protect themselves from damage caused by heavy rain. You can obtain existing heavy rain prevention plans and personal advice from heavy rain consultants through Mainz Economic Affairs, see list of links.