Flood (pluvial)

A pluvial flood occurs when precipitation exceeds the capacity of natural or engineered water drainage systems, including streams and sewers, or when these systems fail due to poor maintenance. Pluvial floods can result from either short, intense rainfall or prolonged precipitation.

Figure shows the different types of flooding in the Netherlands.
The image displays the different types of flooding in the Netherlands. (A) Flooding of unprotected areas along the main water system. (B) Flooding of protected areas along the main water system. (C) Flooding of protected areas along the regional water system. (D) Flooding of unprotected areas from the regional water system. (E) Pluvial flooding. (F) Groundwater flooding. Source: adapted from Klopstra & Kok and Ministry of Infrastructure and Water Management (PDF).

Short, intense rainfall

Pluvial floods resulting from short, intense rainfall are most common in densely built-up areas with poor water absorption. In such cases, runoff rapidly enters drainage systems, surpassing their capacity and leading to local flooding. In the non-built-up environment, they may also occur after drought periods when soil struggles to absorb water.

How does intense rainfall form?

To understand what causes intense rainfall, it is helpful to first understand how rain forms. The Royal Netherlands Meteorological Institute (KNMI) defines rain as a form of precipitation in which water droplets fall from a cloud. If the temperature of the cloud and the air beneath the cloud are above zero degrees Celsius, the cloud consists entirely of water. Collision of water droplets can cause the droplets to grow further. If they are large enough, they fall from the clouds, and it rains.

In a shower cloud that reaches a great height in the atmosphere where it freezes, there are not only droplets but also ice crystals and supercooled water droplets. The ice crystals can grow by extracting water from (supercooled) cloud droplets (Wagener-Bergeron process). This creates larger precipitation particles, and it usually rains or snows harder from a shower.

Prolonged rainfall

Pluvial floods resulting from prolonged rainfall generally occur during winter and in the non-built-up environment. The water- and drainage systems and saturated soil cannot process the rainwater fast enough. The extent of pluvial flooding from prolonged rainfall depends on the type of land use, absorbing capacity of the soil and discharge capacity of the water system (the entirety of sewerage, ditches, canals, pumps, and so on).

Impact from pluvial flooding

Most damages from pluvial flooding stem from issues with the regional water system. Although rare, the regional extreme rainfall event in Limburg during the summer of 2021 resulted in an estimated €433 million in damages, according to the Netherlands Environmental Assessment Agency’s (PBL) report (PDF). While local flooding from heavy rainfall occurs more frequently, PBL’s assessment indicates that these events typically result in lower financial losses.