How come the groundwater level can rise so much?
The high groundwater level has two causes: a lot of rain and the location of Hilversum. It rained a lot from mid-October 2 to February 2023. It was the wettest winter since measurements began. For example, there normally is about 2024 millimeters of rain in our region in February. Now it was more than 75 millimeters. That is 120 liters of water per square meter!
In addition, Hilversum is located on an offshoot of the Utrechtse Heuvelrug. That is high sandy soil and it absorbs a lot of water easily. But with sandy soils there is often no surface water to which the water can flow, such as rivers, lakes or canals. In the west of Hilversum the water can go to the Gooise Vaart and the Hilversums Kanaal. But from two-thirds of Hilversum the water runs to the east. There is no surface water there. There is only a buffer at Anna's Hoeve, but that is already very full.
What can we do about it?
The high groundwater level as it was in 2024 is a complicated problem that has affected the whole of the Netherlands. The options we have to do something about it are unfortunately limited. Of course we understand that it is very annoying that basements are flooded. We understand the concerns of residents and find it very annoying that residents experience nuisance from the groundwater.
We want to be there for our residents, but at the same time we are also realistic: we cannot solve this problem easily in one go. There are millions of liters of water in the ground: we cannot pump that away. We do everything we can, within the limited possibilities that exist.
Measures
In places where it is urgent, we take measures. These measures were also presented at the information evening of 19 November. Where the groundwater has been just below ground level in 2024, facilities are being prepared and constructed to be able to partially drain this groundwater in the event of high groundwater.
When it rains heavily, the sewer is also full. Rainwater from the separate sewer (so without waste water) goes to the ponds, for example at Anna's Hoeve and the Riebeeckvijver. When there is a lot of rainwater discharged via the sewer, the water level of the Anna's Hoeve ponds rises. This increased level can locally affect the groundwater level. The water board is investigating the influence of the water level in the Anna's Hoeve ponds on the groundwater level in the vicinity of the ponds. In this investigation, we would also like to include which parts of Hilversum are vulnerable to groundwater nuisance if there had been even more rain.
The number of wells used to measure groundwater in Hilversum has been expanded.
What about groundwater and sewer?
The sewer system of the municipality of Hilversum is intended for rainwater (rainwater) and waste water from houses, such as from the toilet. The sewer has no function in the drainage of groundwater. The groundwater cannot therefore be drained there. The ponds in Hilversum also only have a function in the rainwater system: for collecting rainwater. The problem now is not about water in the sewer: it is about water in the ground.
Who is responsible for the groundwater?
The groundwater is therefore very complicated and does not remain within the boundaries of Hilversum. The Water Act therefore states that groundwater is an 'inter-administrative' responsibility. This means that it is the responsibility of the water board, the province and the municipality.
Water (rain) that falls on private property is the responsibility of the owner. It is therefore also the owner's responsibility to ensure that underground spaces, such as a cellar, are waterproof.
We see that some residents without a watertight basement, during a high groundwater level, pump the water out of their basement. That water ends up in the garden and on the street, and then flows into the sewer. Officially, that is not allowed, the sewer does not have sufficient capacity to drain that water.
Rain in the future
We expect more heavy rain showers in the future. We can partly absorb these peak showers with our sewer system. Then rainwater can stand on the street for a while, but not for long. And the groundwater does not rise immediately after one heavy rain shower as it did in 2024. That only happens if it rains a lot for a very long time in a row.
If the groundwater is just below ground level for several weeks per year and for several years in a row, the municipality must take measures. But these are usually expensive measures: we cannot simply install them. Then we probably still cannot remove all the groundwater. Cellars must therefore still be waterproof to prevent flooding.
