When does waterlogging occur?

The increase in precipitation volumes and in precipitation severity is adding to the probability of waterlogging. The exact effect depends on the location and on the type of precipitation. For example, severe precipitation over a short period of time will tend to affect urban environments, whilst prolonged precipitation will mainly cause problems in rural areas. In addition, some locations are more susceptible to waterlogging than other areas. The paragraphs below explain the situations in which waterlogging can occur and the potential consequences.

Waterlogging due to severe precipitation in a short period of time

Especially in summer, heavy rainfall over a short period of time will result in waterlogging. The increasing summer drought poses an additional challenge because dry soil does not easily absorb rainwater. This will add to the probability of waterlogging. Furthermore, the built environment is particularly susceptible to this type of waterlogging, as it lacks sufficient room for storing or draining large volumes of water. And due to the large proportion of pavement, water can hardly infiltrate into the soil. The sewer system is capable of draining a maximum of 20 to 30 mm of water per hour to a water purification plant or to surface water in the vicinity. Excess precipitation volumes will result in streets being submerged. In streets with insufficient precipitation storage capacity, water may flow into buildings or render key through roads impassable.

Waterlogging due to prolonged precipitation

Waterlogging due to prolonged precipitation will mainly occur during the winter months. In rural areas, prolonged precipitation will pose more of a problem than severe precipitation over a short period of time. Prolonged large volumes of precipitation may fill up the water system, as a result of which rainwater will form puddles on the soil. Furthermore, ditches and brooks may inundate the land. The severity of the waterlogging will primarily depend on land use. Built areas and farmland planted with crops will be affected by waterlogging to a greater extent than, e.g., grassland. In addition, slopes and railroad tracks may subside, once the soil is saturated with rainwater.

Excessive groundwater levels due to precipitation

It is normal for groundwater levels to rise during periods of rain. Following a rainy period, levels will slowly fall again. Groundwater will seep into deep subsoil and drain away. It will end up in ditches, or is absorbed by trees and plants, whereupon it will evaporate. Occasionally, the soil cannot keep pace with rainwater volumes, for example, if the precipitation goes on for a very long time, or in the event of very severe rainfall during a wet period. In such cases, groundwater levels will rise excessively, which may cause damage to crops and Nature. High groundwater levels may also affect urban areas, leading to, e.g., wet cellars and crawl spaces; rising damp; and soggy gardens. The Excessive Groundwater Levels map in the Climate Impact Atlas shows the locations at which the probability of excessive groundwater levels may increase between now and 2050.

What soil types are prone to waterlogging?

Areas whose soil contains peat and clay are most susceptible to waterlogging, because water does not easily subside here. Lower lying polders are also prone to waterlogging, especially during long periods of severe precipitation. If regional water systems are not up to processing large volumes of rainwater, such polders will quickly flood. Sandy soils are far less prone to waterlogging, as they easily absorb water.

What makes urban areas susceptible to waterlogging?

Many urban areas are susceptible to waterlogging, as they comprise a large proportion of buildings and pavement. This prevents rainwater from draining away in the soil. Neighbourhoods with a great deal of greenery are far less susceptible. The Basic Green and Grey Map of the Climate Impact Atlas shows the percentages of greenery (green) and buildings/pavement (grey) in each neighbourhood. Furthermore, in urban environments, the sewer system often plays a key part in rainwater drainage. Many municipalities feature a combined sewage system. As such systems have limited capacity, they tend to overflow when overloaded. Rainwater will then flood the streets, mixed with dirty water, or end up in surface water by way of a sewer overflow. This can pose health risks.