Heavy rainfall or snowmelt raises the level of rivers and streams. This can lead to fluvial flooding. The warning time depends on the size of the river and its catchment area. Large rivers tend to rise more slowly, giving people time to take precautions. Smaller rivers tend to rise and cause flooding more abruptly, leaving little or no time for appropriate action.

Flooding can also occur in areas that are not close to water bodies. Heavy rainfall exceeds the capacity of drainage systems and natural infiltration, causing water to run off above ground. Buildings on slopes or in depressions are particularly at risk. Water can also enter the building from the roof if the guttering is overloaded.

Flooding also raises the groundwater level in the surrounding area. This changes the direction of the water flow: Instead of flowing towards the river, the groundwater flows landwards. As this usually happens after the river has flooded, it is an unintended hazard. Even weeks after a flood, the ground-water level can rise high enough to flood the basements of buildings or damage their walls from water pressure.

Heavy rainfall that overwhelms the capacity of the drainage system can cause sewage to back up into the building. This causes flooding and contamination within the building. Backflow prevention measures are a simple way to prevent wastewater from flowing back into the building.

image copyright: Rückstauprofi

Coastal areas experience high and low tides on a daily basis. Adverse weather conditions can turn a high tide into a storm surge that is pushed towards the mainland, causing flooding.

Heavy rainfall in steep areas can cause debris flows by loosening debris from the ground and transporting it downhill. The debris flow ends at the foot of the slope where it accumulates. Buildings and infrastructure in the path of a debris flow are particularly vulnerable due to the increased destructive force compared to a river flood.

image copyright: picture-alliance/dpa