The shifting of the quay at the Valkenburgse Meer, municipality of Katwijk, is caused by settlement flow. This is evident from the interim advice of knowledge institute Deltares. The Hoogheemraadschap van Rijnland had given Deltares the task of investigating the cause of the erosion. Settlement flow occurs when a soft sand layer starts to move, increasing the amount of water between the sand grains and causing the sand to flow. Just like quicksand, the sand layer then loses all its strength. The risk of settlement flow is greater on steep slopes. The direct cause of this settlement flow is still unknown.

The Province of South Holland and the municipalities of Katwijk have decided to start a follow-up investigation in the short term. The municipality of Wassenaar and the Hoogheemraadschap van Rijnland will be involved in this. The aim of the investigation is to initially map out how safe the current situation is in and around the Valkenburgse Meer. This is to provide clarity to directly involved parties, such as entrepreneurs, but also to recreationists in the area. The cause of the erosion will also be included in this investigation. The emergency ordinance remains in effect until the follow-up investigation provides clarity about safety.

Arne Weverling, deputy Water, Soil and Climate Adaptation: “First of all, we would like to thank the Hoogheemraadschap van Rijnland for the quick start of the first investigation. In the meantime, we have also received the interim advice and will study this further. Together with the municipality of Katwijk, we have decided to start a follow-up investigation. In this investigation, we want answers to the question of how safe the current situation is. What is possible? But also the cause; how could this happen? This investigation is not done lightly. It is a careful process that cannot and should not be rushed. We understand that the current situation is very unpleasant for the affected entrepreneurs and direct stakeholders. But safety comes first.”

Measures remain in effect

The Hoogheemraadschap van Rijnland keeps the current emergency dam (row of sandbags) and the emergency dam on the north side of the Valkenburgse Meer in place and continues to monitor the situation. Given the uncertainty and the risks, Rijnland therefore advises to keep the emergency ordinance in place and to prohibit activities in and on the lake and the banks. The Province of South Holland is the owner of the sunken quay; they are responsible for the maintenance and restoration of the bank. The Valkenburgse Meer is located in the municipality of Katwijk, near the border of the municipality of Wassenaar. Safety and the prevention of water nuisance are therefore a priority for both municipalities. Each municipality is responsible for public order and safety within its own municipal boundaries. This means that the municipality of Katwijk, as the competent authority, is responsible for the safety of residents, visitors, entrepreneurs, and recreationists in and around the area of the Valkenburgse Meer. To ensure safety in the area as a whole, Katwijk and Wassenaar work closely together.

Settlement flow

As a result of the outflow of the soft sand layer, the soil above it has also shifted, causing the quay to sink. Measurements of the underwater soil indicate that the settlement flow has now stopped and the soft sand layer is no longer moving. However, settlement flow can occur again if the sand layer is set in motion again. As long as there is no clarity on how the settlement flow may develop, the other banks may not be entered.

Letter from the Provincial States about the state of affairs at Valkenburgse Meer

In early April, the Provincial States informed the Provincial States about the state of affairs regarding the Valkenburgse Meer. The letter briefly outlines what has happened, what measures have been taken, and provides a brief outlook.

Read here the letter (opens in a new window).
Read here the press release (opens in a new window) from the Hoogheemraadschap van Rijnland.