Water storage and groundwater

The quays in De Onlanden are being raised so that extra water can be temporarily stored during extreme weather conditions — on average once every 25 years, for around 5 to 10 days. After that, the water flows off in a controlled manner towards the sea.
 

Research shows that this has no lasting effect on the groundwater level. In some areas, the water level may rise slightly for a short period, but the water board is taking measures to prevent any inconvenience.
 

During extreme rainfall, the water level rises everywhere, including outside De Onlanden. During droughts, it actually falls. A geohydrological analysis shows that the changes in water levels are limited and temporary, with no structural consequences for surrounding residential areas.

A higher groundwater level can be caused by:

• A lot of rain
• Poor drainage
• Soil that allows little water to pass through (such as loam)
• Land subsidence due to peat.

Drainage is the removal of water via ditches and trenches. Dehydration takes place underground, for example with drainage pipes. Both are important for proper water drainage.

The owner of the plot is responsible for drainage on their own property. The water board is responsible for the larger water system. In the event of flooding in villages or towns, the municipality is the point of contact. 

Drainage is the removal of water via ditches and trenches. In villages, water first flows into ditches or ponds, then into the main water system. In the past, sometimes too little surface water was provided for new construction, which meant that water could not drain away during prolonged rainfall. Since the mandatory water assessment was introduced in 2003, more attention has been paid to providing water drainage in new construction projects. 

 

Groundwater is found in the soil and changes due to rain and evaporation. Surface water is water that you can see, such as in ditches and rivers. Sometimes the water in ditches can affect the groundwater, depending on the soil.

The Noorderzijlvest Water Authority measures groundwater levels using monitoring wells. Anyone can view information about water levels online:

• WAM Portal:up-to-date data on groundwater, surface water, pumping stations, and weirs.
• Geoweb:map showing waterways, pumping stations, weirs, and water level areas.
• AHN viewer: elevation map of the entire Netherlands.

This information is public and accessible to everyone.

The water board is taking measures in areas where the water level may rise temporarily. These measures have been discussed with residents during home visits. Examples of measures include:

• Additional pumps to remove water
• Building quays around houses
• Raise the ground level and drain water differently
• Drain water to another area
• Widen ditches and dig deeper

No. The current network of monitoring wells is sufficient to measure groundwater in the area.