These experiences have generated valuable lessons regarding the management and maintenance of such installations that can be used for further implementation of artificial marshes (floating or sinking).
This first-built floating marsh did not deliver sufficient long-term floating capability (van Geest et al 2010). The floating capacity of the growing Reed rhizomes (they have natural air chambers) did not compensate the gradual loss of floating capacity of the brushwood mattress. Reed growth from the rhizomes on the mattress was limited by lack of nutrients. Although this experiment may seem unsuccessful at first sight, it has been successful as application for underwater mattresses. The combination of cheap material, transportability over water, applicability in autumn and winter and an open and slowly sinking structure allowing bottom-dwelling organisms to escape turns out to be attractive for bottom mattresses.
- Brushwood in water absorbs a certain amount of water in the course of time. Consequently, brushwood mattresses as tested here keep their buoyancy for at most two to four months. If longer buoyancy is required the design should be adapted, potentially with additional floating devices or by using material with more buoyancy.
- Flume experiments showed that weakest points of the mattresses were edges and anchoring points. Edges and connections of the brushwood mattresses should be improved in order to better resist wave action.
- The shape of the floating mattresses and the size and location of the openings should be designed according to ecological requirements, preventing negative shading impacts but optimizing the wave attenuation and sediment capture impacts.
- Potential impact of geese on the establishment of reed should be assessed before creating reed areas that are partly submerged to minimize failure caused by overgrazing of geese.
- Establishment of reed by embedding rhizomes in brushwood mats is initially successful but can be improved by also adding sufficient soil so that there is no nutrient limitation for reed growth.
- Wave attenuation by brushwood mats is effective. For the wave-attenuating effect to be optimal, length, thickness and porosity of the mattresses should be designed in accordance with the local wave conditions.
- The sinking property of these mats allows application as bottom mats on shallow foreshores. The mats are a suitable method to create (reed) marshes in shallow water.
Spin-off and open end
Consortium partners are working to get floating marsh mattresses internationally applied for combating wave intrusion into opened up marshes, by reducing wind fetch, and for closing shallow and small channels without sediment additions. However, at present, there is a need for an improved design. The challenge is to design mattresses of biodegradable materials that have sufficient buoyancy and that do not break on anchoring points, while having a good anchoring mechanism. Extruded recycled plastics and biodegradable plastics are being looked into for applicability, but have limitations as their production is generally not mainstreamed.