Marsh island using geosynthetics
In the Dutch lakes Markermeer and IJmeer pilot projects are being conducted with the aim of creating a future-proof ecological system. One of these pilot projects is a marsh in the north-east of the Markermeer between Trintelhaven and Lelystad, which will be used to explore the technical feasibility of a marsh on silty soil. Both TenCate Geocontainer® and TenCate Geotube® technology are being used to create the sides of the marsh island.
In June of this year the contracting company Gebr. Van der Lee, commissioned by the Department of Waterways and Public Works, started the construction of a rectangular-shaped, marsh in the Markermeer, covering an area of ten hectares (24 acres). This pilot island is part of the Natuurlijk(er) Markermeer-IJmeer (NMIJ) research programme. Within the framework of NMIJ, the Department of Waterways and Public Works is investigating how it can improve the ecological quality of the Markermeer-IJmeer. Gebr. Van der Lee opted for TenCate Geocontainer® and TenCate Geotube® technology due to its proven concept. The marsh must continue to exist for at least 30 years, so risks will be kept to a minimum. This is also a relatively cost-effective way to build up the sides of an island.
An animation of the marsh in the Markermeer
The Markermeer covers an area of 70 thousand hectares (approx. 173,000 acres) and has an average depth of some 3.5 metres, making it one of the largest freshwater lakes in Europe. Winds of force 4 and upwards cause a great deal of sludge to swirl up from the bed of the lake, making the water turbid. When the wind dies down, the sludge sinks like a smothering blanket onto the bed of the lake, thus preventing water plants, fish and zebra mussels from developing properly. The number of birds that live on these organisms also decreases. The construction of the pilot marsh should determine whether it is technically feasible to build a large-scale marsh on silty soil. A large-scale marsh involves the creation of large areas of land-water transitions and contributes to a reduction in the silt content of the water. It will also have a positive effect on the flora and fauna and produce an abundance of birds, thus bringing closer the aim of making the Markermeer-IJmeer a future-proof ecological system.
The pilot marsh consists of a closed and an open compartment of two and eight hectares respectively. The open compartment will be directly connected with the Markermeer and work is taking place here to determine the best method for its construction and for the depositing of material. The closed compartment has a regulated water level, which will be used mainly to study the creation of land-water transitions.
TenCate Geocontainer® technology
The sides will be constructed with 180 units using TenCate Geocontainer® technology, each 30 metres in length. Once filled with sand from the IJsselmeer, the containers will be 7 metres wide, one-and-a-half metres high and have a capacity of 280m3. A layer of 60 units using TenCate Geotube® technology will be placed on top of the containers, so that the side of the marsh thus created will be above the surface of the water. These units are 30 metres long, have a diameter of 2 metres and are hydraulically filled with sand from the lake. Water goes through the pores to the outside, leaving behind the sand. The marsh is then filled with material dredged from the navigable channel linking Amsterdam to Lemmer. The outer side will be covered with a layer of rubble to provide additional protection against floating ice and wave impact. This layer will ultimately be covered with clay containing reed cuttings, which will help to purify the water and give the island the appearance of a marsh.
Each unit using TenCate Geocontainer® technology is mechanically filled in a split hopper: a barge the bottom of which opens lengthways like a mechanical gripper so as to enable sand or rubble to be dumped. The units are filled with sand extracted from the IJsselmeer. Once filled, the containers are 7 metres wide, one-and-a-half metres high and have a capacity of 280m3. The split hopper is sailed into position and opened, after which the containers are sunk. The positioning and sinking of the containers is work that requires great precision.
The construction of the pilot marsh is expected to be completed by the end of 2013. After this a period of measurement will start, which will continue until the end of 2015. The results of the measurements on the pilot marsh will be combined with the results from other experiments to be conducted as part of NMIJ. In 2015 all these results should lead to a recommendation to the Cabinet about possible measures to be taken in the Markermeer and IJmeer.