Flooding is a collective term for situations in which people experience nuisance as a result of too much water. Common forms of flooding are flooding as a result of rain and seepage (emerging groundwater), but flooding is also a form of flooding.
Flooding due to rain
Flooding as a result of rain mainly occurs in cities and other heavily built-up areas (such as industrial estates). In urban areas, water is drained through the sewage system and does not infiltrate into the soil. This is often a mixed sewerage system; wastewater and rainwater together. The construction of separate sewers, with separate processing of rainwater, is increasing.
When there is a lot of precipitation, it is possible that the sewer cannot handle the amount of water and therefore overflows. A sewer is never big enough to handle the heaviest showers, such as popcorn showers and cloudbursts. That is why the street and other above-ground space is used to absorb peak showers for a short time. Then the street is full. Much worse is that sometimes basements also get flooded.
If the mixed sewer system overflows, this water will also contain waste from the sewer. This is a public health hazard due to the presence of bacteria, viruses and other organisms. Another risk is the risk of injury or drowning if manhole covers are pushed out of their rebates by the flowing water.
Although the fire brigade is often alerted when there is water on the street, there is little they can do about it. The amount of water over the entire area is so great that pumping is of little use. Usually the nuisance is largely over within an hour after the rainfall.
Sewerage is not designed for peak showers, that would be much too expensive. That is why sewage systems contain overflows. In this way, heavy showers can still be discharged without causing damage in urban areas. The wastewater diluted with rainwater is then discharged untreated into the surface water (eg canals or canals). If the overflow frequency is high, an (environmental) problem arises, for which the municipality and water board must find a solution.
To prevent sewer overflows, a special rainwater sewer is often installed these days. As a result, rainwater and domestic wastewater no longer end up together in one sewer pipe and rainwater can be discharged more easily into the surface water. Another possibility is the construction of a mountain sedimentation facility. The waste water is temporarily stored there during heavy showers, and when that tank is also full, the pollution has been able to settle, so that the environmental impact is limited.
Another solution to rainwater nuisance is the construction of modern infiltration facilities to accelerate the infiltration of water into the soil.
Flooding as a result of groundwater
In areas with a high groundwater level, flooding can occur when there is insufficient drainage and the water ends up in crawl spaces or basements. A common problem is that spontaneous seepage occurs under houses. This is groundwater emerging as a result of precipitation or a rise in the water level in nearby surface water. Seepage is difficult to combat, except with drainage and possibly pumping.
In the context of climate change, the KNMI expects that heavier rainfall will become more frequent in the future. In order to keep Dutch water management in order and to be well prepared for the supply of large amounts of water, government parties signed the National Administrative Agreement on Water in 2003.
Grontmij about groundwater nuisance
Rainwater nuisance file RIONED