Floating marsh – Markermeer

Initiation

The Markermeer area was designated to be the last reclaimed area, but the plans were cancelled as the need for agricultural land decreased. By that time the dike had already been built. Since then, the lake has become ever more turbid, due to organic SPM (soluble microbial products) and re-suspended old marine clay deposits. This has led to a severe degradation of the ecosystem.

Project Objective

The Markermeer (Lake Marken) is the south western part of the IJsselmeer (Lake IJssel). Lake IJssel is the remainder of the shallow sea arm Zuiderzee after construction of a closure dam (1932) and extensive land reclamations between 1930 and 1968. The project NMIJ (“Natuurlijker Markermeer en IJmeer”) aims at the improvement of the ecological quality of the lake by stimulating experiments with innovative measures (van Geest et al. 2010). The improved ecological quality should be reached in three steps:

1. reducing suspended matter concentrations,
2. enhancing habitat diversity and dynamics and
3. increasing connectivity.

Initiation

Several ideas were launched to improve the ecological quality of the Markermeer including the construction of wetlands and the implementation of soft vegetated foreshores. Creating floating wetlands with braided brushwood mattresses was considered a quick, multifunctional and cheap option. From other studies (e.g Coops et al., 1996 and Lovstedt et al., 2010) it is known that vegetation in front of a dike is able to reduce wave impact and stabilize and raise the shore by sediment trapping. In freshwater environments, such as the IJsselmeer, this vegetation will most likely consist of reed and willows. However, many parts of the shore are too deep for the development of such a foreshore. The concept of a floating marsh may allow development of wave-attenuating vegetation without the need to raise the bed with imported sediment. The floating marsh concept is based on existing natural floating wetlands and islands in Louisiana and in the Danube delta. Additionally, floatlands with vegetation are broadly applied in urban environments, such as in canals in Amsterdam.

Pre-feasibility

To assess feasibility of the floating mattress pilot factors such as construction technique, material use, flotation, and functionality were addressed. Braiding mats of willow branches is an old traditional Dutch practice, thus, techniques for construction and transportation of braided brushwood mats are available. The mats are generally used as sinking units to stabilize beds under water in preparation of dike construction. Mats are known to remain buoyant for considerable time.

After successful establishment of reed, sinking of the mat and submergence of reed could be beneficial for reed growth, as reed needs dry periods for germination and establishment, but prefers wet conditions once established. Creation of sheltered areas below and behind the mats might enhance settling of suspended matter. At the lower submerged side of the mats filter feeders might be able to attach, and hiding- and spawning places for fish are created.

Sinking speeds were difficult to predict and depend among others on quality of material and climate conditions. If mattresses can be implemented with a sufficient floating capacity over a sufficiently long time, other possible functions of the floating marsh could be:

• storage of CO₂,
• nutrient and contaminant uptake,
• biomass production for energy or fodder production,
• sediment trapping below the mattress,
• stabilization of soft and muddy beds, and
• adaptation to varying water levels.

Feasibility

In the feasibility phase, a conceptual design was finalized of floating wetlands constructed of bundles of brushwood (‘wiepen’ in Dutch). The willow wood should be harvested at least one year before, to prevent re-growth of willow sprouts from the brushwood. A filling layer of dry reed material is created in between the bundles of brushwood for extra floating capacity and to prevent initial grazing on the reed roots and shoots by geese (van Geest et al. 2010). Reed growth is initiated by planting rhizomes of Phragmites (van Geest et al. 2010). Within the marshes, open water areas are designed to increase the length of edges (not covered) and to increase areas of quiet clear water, available for utilization by animals and vegetation. Furthermore, open areas will allow light and oxygen to reach the water underneath the mattress and prevent water quality deterioration.

The floating marsh has to be strong enough to resist wave action. The strength is determined by the strength of the connections of the bundles. Although not in accordance with the ‘cradle to cradle’ principle, the connections between the bundles of brushwood are made of metal wire in order to guarantee sufficient strength. 
The reed marsh was first tested in a mini-pilot in a small pond at the premises of Deltares in Delft.