Heavy Rainfall Risk Management

Heavy rain refers to intense rainfall over a short period. Heavy rain is usually localized. The German Weather Service issues warnings in three stages:

Heavy rain: 
15 to 25 liters per square meter in one hour or 20 to 35 liters per square meter in six hours

Severe heavy rain: 
25 to 40 liters per square meter in one hour or 35 to 60 liters per square meter in six hours

Extremely severe heavy rain:
 more than 40 liters per square meter in one hour or more than 60 liters per square meter in six hours

However, there is no uniform definition of the term "heavy rain." The term is used – depending on the context – for rain of varying intensity and severity. 

Events in recent years have shown that essentially all regions can be affected by this natural hazard. Unlike flooding of major rivers, the exact location and timing of heavy rain are difficult to predict and can be very surprising for those affected.

Warning times, during which short-term measures can still be taken, are therefore very short or even non-existent. Especially in small catchment areas, smaller water bodies like streams can suddenly swell, overflow, and cause flooding due to the rain volume.

During flooding caused by heavy rainfall, the ground can no longer absorb the precipitation or cannot absorb it quickly enough. The water flows across the terrain surface towards the nearest body of water or other low points in the terrain, such as ditches, depressions, or basement rooms.

Flooding with just a few centimeters of depth can occur almost across the board in residential areas. This is because the ground in settlements is mostly built upon, that is, sealed. However, across the area, heavy rainfall can also lead to greater flood depths in different locations.

In particular, in hilly or mountainous terrain, flash floods caused by heavy rainfall have high flow forces and can carry large amounts of debris (such as wood, hay, and silage bales) and eroded materials (like soil or rubble). This material can then accumulate at bottlenecks or bridges and block them. The resulting backwater floods the surrounding terrain and can cause further severe damage to buildings and infrastructure.

Even in flat areas, heavy rainfall can cause flooding. Since the large amounts of water often exceed the design limits of sewer networks, the water cannot drain underground, and the rainwater floods wide areas. In particular, buildings and infrastructure in depressions can be significantly damaged.

On the website Hochwasser Pass of the Flood Competence Center (HKC) e.V., you will find a wealth of information materials on the topic of self-protection.

The municipal heavy rain hazard maps show the results of simulations that reflect the current state of technology. The county-wide calculations depict surface runoff based on a digital terrain model. The public sewer system is not considered in the simulation, as it no longer has a significant influence during these extreme rainfall events.

The heavy rainfall hazard maps serve as a first orientation since details such as property walls or curbs, which can alter water flow, are not depicted in the district-wide terrain model. The flooding results must also be verified through on-site inspections. If there are entry points to buildings in the depicted flood areas, such as basement entrances or light well openings, it is advisable to implement object protection measures at these locations.

The maps thus provide owners, businesses, and those affected with the opportunity to assess the risks to their property and, if necessary, take object protection measures. They thereby form a fundamental basis for effective civil protection.

The heavy rainfall hazard maps will serve as a planning basis for all urban planning in the future.  Together with the damage potential and risk analyses as well as the river flood maps from the district government, they form the basis on which all water management planning for extreme events is assessed and aligned.

In addition, the heavy rainfall hazard maps serve as a basis for disaster protection planning by the fire department and local rescue forces.