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Rewetted peatland for the development of reed marsh in De Blankaart (Photo VLM)
Reed marsh in Kwetshage developed since 2012. This smaller area acts as a reference for the rewetting of the entire project area (Photo VLM)
Current view on the not yet rewetted part of Kwetshage
HYDROLOGICAL ISOLATION AND PUMPING CONDITIONS
Generally, rewetting within the Flemish polders can only be achieved by means of hydrological isolation, whereby water levels suitable for restoring or conserving natural habitats can be implemented. Water levels favouring natural habitats deviate from normal water level regime in the polders, as water levels in the polders are typically kept low in winter and early spring. From April onwards water levels are raised. For conserving and restoring natural habitats water level regime should be the reverse: higher water levels in winter and spring, and somewhat lower levels during summer.
For the hydrological isolation to work, the building of suitable weirs is essential. The weirs should ideally be adjustable, with fully automated weirs, remotely controlled by registered upstream water levels, giving less variation in water levels.
Specifically in areas where water levels are managed by means of a pumping station, rewetting can be achieved by adjusting the pumping conditions. This is for instance the case in the project area De Blankaart, where in combination with a new automatic weir, water levels can be spatially differentiated within the project area. On its turn this gives the opportunity for a phased implementation of the rewetting.
Automatic weir in De Blankaart. (Photo VLM)
BUFFER COMPENSATION IN FLOODING AREAS
In case the area to be rewetted is a flooding area, compensating measures for the loss of buffering capacity are legally imposed. For reasons of space-saving and minimizing the impact on surrounding agricultural land, this compensation is preferably to be achieved within the rewetted area itself.
Flooding during March 2021 in De Blankaart. On the left the river IJzer is visible (Photo VMM 2021).
Flooding in De Blankaart (Photo VLM)
Result of shallow excavation of an entire plot for development of shallow water reed beds (Photo VLM)
Reed marsh developing in a shallow excavation (De Blankaart) (Photo VLM)
SHALLOW EXCAVATIONS
An optimal combination of maximum water levels after rewetting and lowering the ground level by shallow excavations makes it possible to combine habitat restoration and buffering compensation. These shallow excavations are only possible if the peat layer is covered by a sufficiently thick clay layer. Obviously it is of prime importance the peat layer remains intact.
REPROFILING EXISTING DITCHES
Reprofiling of extisting ditches generates additional room for the development of natural habitats, but it also provides extra space for water. In this way, under fixed maximum water levels, more water can be stored in the rewetted area. Augmenting the amount of stored water is also achieved by removing sludge from existing ponds.
Reprofiling a ditch in De Blankaart. The crane bucket was esspecially designed and made for these works (Photo VLM)
Result after partial filling of the reprofiled ditch (De Blankaart). (Photo VLM)
Final result after complete filling of the reprofiled ditch (De Blankaart) (Photo VLM)
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