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Shellfish reefs as shoreline protection - Eastern Scheldt, NL

Location: Eastern Scheldt (Eastern Scheldt), The Netherlands
Date: 2008 - present (BwN - projects ZW2.1 and ZW2.4)
Involved parties: IMARES, Deltares, Rijkswaterstaat (WINN, Zeeland), NIOZ (former NIOO-CEME), Van Oord, TUD, Ecoshape.

Technical Readiness Level: 6 (technology demonstrated in relevant environment)

Environments: Estuaries, Lakes & Rivers and Sandy shores
Topics: shellfish ecology, tidal flat protection, estuary protection, measures against erosion, biodiversity, ecosystem engineer, oyster.

 

 

 

Building with Nature designTraditional design


Shellfish reefs can stabilize eroding (intertidal) coastal areas. They protect sediment on the flats from erosion by currents and waves. Shellfish filter material from the water column and deposit it in the form of faecal pellets. This together with sediment is trapped between the shells. The reefs also provide a complex habitat for many other species, can provide economic benefits and contribute to a healthier ecosystem functioning.


Hard structures are common solutions when protecting shores and shoals against erosion. When constructed from rock or loose elements, they generally provide substrate on which sessile organisms can settle and mobile fauna can shelter.

 

 

Abstract

Shellfish such as oysters and mussels are reef-forming ecosystem engineers. Their reefs can protect shorelines and stabilize eroding (intertidal) areas. Apart from locally fixing sediment, they are able to influence tidal flow and wave action at larger scales, causing changes in depositional patterns. Oyster reefs are applied in many places around the world as a coastal protection method (e.g. living shorelines), reducing hydraulic forces and enhancing sediment entrapment. In this project we constructed artificial reefs in the Eastern Scheldt (SW Netherlands) to reduce the erosion of tidal flats. To be successful, the artificially placed substrate needs to develop into a living, persistent oyster reef and at the same time protect the tidal flat against erosion.


Initiation phase

The construction of a storm-surge barrier and a number of closure dams has led to severe erosion of the tidal flats in this tidal basin. The objective of the project was to test artificial shellfish reefs in the intertidal zone in front of mudflats or salt marshes as a cost-effective and sustainable measure to protect intertidal habitats in the Eastern Scheldt (SW Netherlands).

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Planning and design phase

The Building with Nature program established that oyster reefs can be used in coastal protection under three conditions:

  • Availability of substrate for oyster larvae settlement;
  • Environmental conditions that enable the survival and growth of oysters;
  • Evidence that oyster reefs do protect the shoreline and the intertidal habitats at the location considered.

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Construction phase

After a number of small-scale pilot experiments in 2009 the construction of large-scale artificial oyster reefs was started in 2010 as part of the Building with Nature program. We created three oyster reefs at places that differ in elevation, hydrodynamic exposure and degree of erosion.

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Operation and Maintenance phase

In 2009 a small-scale oyster reef experiment was set up at the mudflat of Viane in the Eastern Scheldt to investigate the feasibility of oyster reefs for shore and shoal protection. Viane is an exposed intertidal area that is subject to severe erosion. The experiment showed that living oyster reefs can contribute to shoal protection, because (1) the gabions with oyster shells proved to be a stable substrate, (2) oyster larvae settled on the oyster shells and grew fast the next year, and (3) sedimentation and reduced erosion was observed behind the reefs (see first figure on this page). In 2010, the experiments were scaled up to three large reefs of 200 x 10 m (see construction). Their effectiveness is discussed below.

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Lessons Learned

From the experiments in the Eastern Scheldt we found that iron wire cages (gabions; in Dutch: schanskorven) filled with dead oyster shells can:

  • grow out into living persistent reefs,
  • reduce erosion of intertidal flats through wave attenuation and trapping and detention of sediment,
  • increase local diversity in a sandy habitat by providing a hard substrate habitat.

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References

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