Groundwater flooding

As an owner, you are responsible for keeping your basement watertight. Water in your basement is often caused by basement walls not being waterproof, improperly installed or broken down rainwater pipes, or improperly installed patios. Waterproofing basement walls is work for professionals.

What you can do yourself

• Check that your basement is still waterproof
• Check that the stormwater drain is properly constructed and does not discharge under the house
• Lay paving around your house so that rainwater runs away from the house and not toward it
• Reconnect the downspout if it has broken off or become loose due to subsidence
• Have drainage installed around or under your home by a professional

A crawl space does not have to be and usually is not waterproof. There is no harm in having water in it from time to time.

Your garden can sag if water stays in it too often. You can tell:

• Plants and trees not growing because it is too wet
• Unfinished (foundation) parts visible above ground

What you can do yourself

• Getting rid of rocks and greening your yard can help. Did you know that greenery acts like a sponge? Grass, soil and plants absorb a lot of water. Many parts of Wassenaar have a sandy soil in which water is quickly absorbed. Therefore, it is a good idea to remove as many stones as possible from your yard and replace them with more greenery. This is not only good for water collection, but also for biodiversity. Insects and small mammals or frogs also really like a green garden. 
• Disconnect the downspout to the garden. Disconnecting the downspout to the garden or to a rain barrel can help. This way less water gets into the sewer system and the sewer system is less likely to be overloaded. A rain barrel can store water so you can use it when there are extended periods of drought. So you can use your rainwater to keep your garden green and beautiful during dry periods.
• Accept a wetter spot in your garden. Lay out your garden so that water flows to this deeper spot. Water-loving plants (ferns or rhododendrons) actually grow very well in this spot.
• Elevate your garden with sand and earth. In a very serious situation, you will have to clear the entire garden including paving and planting and rebuild it. Raising your own garden can cause flooding for your neighbors.
• Lay drainage if elevation does not have the desired effect.

Municipalities in the Netherlands try to ensure that the groundwater level in public areas averages -0.70 meters below ground (the ground level). This does not succeed everywhere because groundwater is difficult to control. In several places throughout Wassenaar there are pipes in the ground to measure groundwater levels. 

There are several things that Wassenaar municipality is doing to reduce water flooding. Especially in the layout of public spaces, this is being worked on. For example, a road can be constructed with a very small slope so that the water does not remain in the middle of the road. In addition, efforts are being made to create a greener environment and look at how and where we can replace stones for more greenery.

Development of new projects

When developing new projects, there is an increasing focus on water harvesting. For example, new housing developments must be able to absorb a downpour of 60mm on their own property. This can be done, for example, by underground storage in crates or by constructing Wadis (water drainage by infiltration). A wadi is a place where rainwater can flow to and sink into the ground. It is a kind of ditch or low spot in the garden, street or park. When there is a lot of rain, a wadi ensures that the water does not go straight to the sewer, but first sinks into the ground. This helps prevent flooding and keeps the soil moist.

For example, every new (construction) project considers how the environment can be made more climate-adaptive. 

Posted December 19, 2023

Is 2023 that much wetter than previous years?

The Water Board's gauge data shows how much precipitation has fallen per area from October 10 until now (6 weeks). In Wassenaar this is approximately 342mm. Within 6 weeks more than 40% of the normal average annual precipitation has fallen. This is extremely high.

Currently, this puts 2023 on course to become the wettest year on record. That is, since records began in 1906. KNMI figures show that January, spring, July, August, October and November were much wetter than normal. Also see Despite an extremely wet 2023, a water shortage next year is not out of the question. 

Does the municipality monitor groundwater levels?

The municipality has monitored groundwater levels for decades. Unfortunately, recent measurements are not currently available. The municipality is actively restoring groundwater monitoring. It will be reinstated in the first quarter of 2024.

Why are there differences in groundwater levels?

There are large differences in groundwater levels by neighborhood. This is due to differences in elevation from ground level (NAP elevation). If you live in the higher dune area, the groundwater is deeper than in other areas where, on the contrary, there are high groundwater levels. 

Is sewer operation related to high groundwater?

No, the drainage in the public road does not affect groundwater level rise. Drainage is constructed to drain groundwater. Constructing drainage within the lots and connecting to the drainage in the streets can help drain groundwater as it rises.

Does anyone manage groundwater?

Groundwater management is an integral part of water system management. More information on this can be found on the website of the Ministry of Infrastructure and Water Management.