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 Building with Nature Guideline > Projects > Shoreline protection by floating marsh - Houtribsluices, NL

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Shoreline protection by floating marsh - Houtribsluices, NL

 

Location: Houtribsluizen Markermeer side, Netherlands
Date: 2009 - 2010
Involved parties: Deltares, Waterdienst, van Schaik B.V.

Technical Readiness Level: 5 (technology validated in relevant environment)
Environment: Lakes and Rivers, Estuaries
Keywords: Wave attenuation, sediment entrapment, habitat restoration, water purification, floating wetlands/marshes

 

 

Building with Nature designTraditional design


The innovative application of braided brushwood mattresses aims to create floating foundations for emergence of reed vegetation. The floating mattresses locally reduce currents and waves thereby decreasing hydraulic loads on the dikes and creating valuable habitats above and below water. More low-energy conditions stimulate settling of suspended solids and promote the stabilization of silty soils.


Traditionally dikes in Dutch freshwater lakes directly border the water. However, historically this zone was used to be a gradual transition between land and water inhabiting freshwater marshes. To reduce wave impact on dikes, rows of poles can be placed in front of the dike, or dikes can be designed to withstand wave impacts themselves.

 

 

 

Abstract

Along the coast of large Dutch freshwater lakes, dikes often border the water directly, with relatively steep slopes. Shallow zones and the gradual slope from land to water are lacking. Consequently, species that inhabit these zones are decreasing. In addition, constant lake-water levels cause erosion of shores. To dampen waves and recreate gradual land-water transitions brushwood mattresses were constructed in front of the dike. These mattresses might facilitate development of floating reed marsh in the shallow zone in front of a dike. This marsh reduces wave impact on the dike, enhances sedimentation and creates a clear shallow water zone with (submerged) vegetation. Thereby, the initial substrate of the mattress could be suitable for establishment of filter feeders, such as zebra mussels (Dreissena polymorpha) and for other species.


Initiation Phase

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.
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Planning and design phase

The floating marsh was designed to improve ecological quality, while additionally sustaining dike safety (van Geest et al. 2010). The marsh will function as a wave attenuator, and thus reduce wave impact on the dike. Underneath the floating marsh, low-energy zones will exist that allow suspended matter to settle.

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Construction phase

The brushwood part had a total area of 2440 m 2 and a weight of 80.000 kilo. Part 1 and 2 were constructed of 100% Salix alba, part 3 was constructed of 25% Salix alba and 75% Salix viminalis due to a shortage of Salix alba. All the wood was harvested at least 3 years prior to construction. The floating marsh was completed on the 26th of June 2009. 

 

Operation and Maintenance phase

By the 1st of October parts of the mattress were starting to sink. Floating capacity of the mattress was enhanced artificially with empty plastic jerry cans. A storm on the 13th of October caused the braided willow branches to disentangle and large parts of the mattresses sank further due to wave impact. Consequently, in December the marsh was removed and the brushwood was transported to a biomass burning power plant. 

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Lessons Learned

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).

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References

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