Valkenburg Lake: cause found, cause not yet found
An interim advisory report by independent knowledge institute Deltares shows that the shearing of the quay at the Valkenburg Lake in the municipality of Katwijk was caused by settlement flow. This phenomenon occurs when a weak sand layer starts to move, increasing the amount of water between the sand grains and causing the sand to 'flow'. Like quicksand, the sand layer then loses all its strength. The risk of settlement flashing is greater with steep slopes. The direct cause of this settlement flow is as yet unknown.
Dike reeve Rogier van der Sande: "The subsidence of the quay due to subsidence seems to have stopped, although it is expected that the quay will still partly erode in some places. For now, the remaining quay looks strong enough. We cannot yet guarantee the stability of the banks of Lake Valkenburg. Activities that could cause a new instability should therefore be avoided until the current situation is fully understood."
Measures remain in place
The Rijnland Water Board will leave the current emergency barrier (row of sandbags) and the emergency dam on the north side of Lake Valkenburg in place and continue to monitor the situation. Therefore, given the uncertainty and risks, Rijnland advises to keep the emergency ordinance still in place and to prohibit activities in and on the lake and its shores. The Province of South Holland is the owner of the sagging quay; they are the ones taking care of the maintenance and restoration of the shore. The Valkenburg Lake is located in the municipality of Katwijk, against the border of the municipality of Wassenaar. Safety and the prevention of flooding are therefore paramount 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 Valkenburg Lake area. In order to ensure safety in the area as a whole, Katwijk and Wassenaar work closely together in this regard.
Settlement flow
As a result of the outflow of the soft sand layer, the soil above it also shifted, causing the quay to sink. Measurements of the submerged soil indicate that settlement flow has now stopped and the soft sand layer is no longer moving. However, subsidence may occur again if the sand layer is moved again. Until there is clarity on how the settlement flow may develop, the other banks should not be entered.
Follow-up research
The province of South Holland and the municipality of Katwijk have decided to start a follow-up study in the near future. The municipality of Wassenaar and the Hoogheemraadschap van Rijnland will be involved. The purpose of the study is to initially Map how safe the current situation is in and around Lake Valkenburg. This is to provide clarity to those directly involved, such as entrepreneurs, but also to recreational users in the area. The reason for the erosion will also be included in this investigation. The emergency ordinance will remain in place until the follow-up investigation provides clarity on safety.