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 design||Traditional design|
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.
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).
The Ecosystem Engineers (Dutch: Biobouwers) project of the Rijkswaterstaat’s WINN research programme (2006 – 2008) was the first project in the Netherlands in which the concept of shellfish reefs for shore protection was explored. The “Biobouwers project’ was executed by Deltares, NIOO-CEME (now NIOZ) and IMARES. Within the Building with Nature program, this concept was further investigated and large-scale experimental reefs have been built and are now being monitored and evaluated. An artificial ecosystem engineer (in this case a shellfish reef) is considered successful if it becomes self-sustaining (i.e. living and persistent) and reduces erosion of the tidal flat it is supposed to protect (i.e. stabilizing element).
The main goal of the project is to maintain intertidal sandy shoals, mudflats and salt marshes by utilizing ecosystem engineers in the form of oyster reefs. Their most important functions are:
- sustainable and cost-effective protection of intertidal areas against erosion,
- reduction of hydraulic forces by 1) dissipating wave energy and 2) increasing sediment deposition and reducing erosion,
- enhancing biodiversity, biomass, productivity and functioning of the ecosystem.
Ecosystem engineers (Jones et al., 1994) are organisms that modify their abiotic environment via their own biological activity and their physical structure. They are considered an important factor in the functioning of estuarine and marine ecosystems. Especially in the intertidal and shallow subtidal zone of estuaries and coasts, several ecosystem engineering species are present that form a three-dimensional structure on an otherwise bare soft-sediment bed. Examples of such ecosystem engineers are mangroves, salt marshes, sea grass beds and shellfish reefs. As the structure of these organisms can considerably decrease hydraulic forces such as wave action, it can be used to create protective barriers in front of dikes (Borsje, van Wesenbeeck et al. 2011). The structure of a shellfish reef is such that it will dissipate wave action rather than reflect it. Building with Nature adopted the concept of using ecosystem engineers as a way of solving shore erosion problems.
Oysters are ecosystem engineers that form reefs. These reefs form a conspicuous habitat that can influence tidal flow and wave action within estuaries. Oyster reefs thereby modify patterns of sediment deposition, consolidation, and stabilization. Additionally, these filter feeders can generate an extra flux of fine sediments to the intertidal habitat through excretion of faeces and pseudo-faeces (i.e. bio-deposition). After placement of a substrate of dead oyster shells, ideal for oyster larvae to settle, natural processes should further stimulate living reef formation in order achieve long-term sustainability (Powers, Peterson et al. 2009). The newly formed reef also provides habitat to a diverse species community (Scyphers, Powers et al. 2011). When successful, the initial substrate may therefore evolve into an economically and ecologically advantageous object full of life, acting as an adaptive coastal defense mechanism. The Building with Nature program therefore studies reef-building ecosystem engineers and their potential to consolidate and stabilize eroding tidal flats in the Eastern Scheldt.
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.
Phase 1: WINN "Biobouwers" project 2006 - 2008
The WINN Biobouwers project, the precursor of this Building with Nature project, applied loose oyster shells as a substrate for artificial reefs. In the course of the project it became clear that such reefs are unstable and yield oyster shell densities primarily determined by flow direction and intensity. After a severe winter storm, nearly all oysters in the area had been dislodged, leaving the tidal flat nearly empty of oysters, except from a nearby oyster reef that had established naturally.
At the end of the two-year project, all oysters had either been buried or washed out of the area. Several additional experiments were performed to test methods to improve the resilience of the reefs to physical pressures and to enhance the settlement of oyster larvae. These experiments showed that oysters were mainly dislodged under stormy conditions with relatively high flow velocities. A further habitat analysis showed significant relationships between the natural occurrence of oyster reefs and bottom shear stress and exposure time. These findings were used to produce a habitat suitability map based on bottom shear stress and water depth.
The WINN biobouwers project led to the conclusion that self-reliant oyster reefs can establish if a suitable substrate enables the settlement of oyster larvae and oyster growth. Therefore, the sequel of this project, in the framework of Building with Nature, paid special attention to habitat suitability and the type of substrate used. Factors that may limit oyster presence were identified, as they help finding locations where oyster reefs can be used for shore and shoal protection.
Phase 2: Building with Nature 2008-2012
Availability of substrate for oyster larvae settlement
After fertilization, oyster eggs evolve in oyster larvae which live a planktonic life in the water column before settlement. In order to grow into adult oysters they must settle on a hard substrate. Which substrate is best depends on local conditions, such as waves, currents and sediment dynamics. At sheltered sites, most (natural and artificial) hard substrates, ropes or even plant materials can create the necessary substrate for settlement (Brumbaugh and Coen, 2009). Examples are shells, cement “reef balls”, crushed concrete and other waste materials. Most artificial reefs are built using oyster shells, as these attract larvae and yield high settlement, survival and growth rates of oyster larvae (Nestlerode, Luckenbach et al., 2007). Dutch oyster farmers successfully catch oyster larvae on loose lying mussel shells in sheltered areas with low sediment deposition rates. In such areas, fixing the substrate can enhance oyster settlement and survival, but this is expensive and may be unnecessary if sufficient substrate is provided (Brumbaugh and Coen, 2009). In contrast, it is impossible to use loose substrates at exposed sites like the North sea coast. Currents, wave action and sediment displacements will toss and turn or smother them, thus hampering oyster settlement and survival. Lenihan (1999) also found that flow velocity and wave action determine the delivery and settlement of oyster larvae.
Means to fix the substrate are baskets, nets, concrete, Biogrout blocks, gabions, etc. During the Building with Nature program we tested different ways to fix oyster and mussel shells in the Eastern Scheldt. As we aimed for the substrate of oyster shells to become a live and natural reef after some time, we used degradable structures to fix the shells. We choose three potential substrates: Biogrout blocks, which are composed of sand that is calcified by bacteria, chicken wire cages and the stronger gabions (Dutch: schanskorven). Gabions are large metal frames, usually filled with rock in order to create a larger construction element. Oyster shells in gabions proved to be the best combination to catch oyster larvae as the gabions were strong and able to fixate the larger oyster shells. The combination between cages and mussel shells was unsuitable as the chicken wire was too weak and flexible and mussels were lost from or moved within the cages. The Biogrout blocks tested in these experiments disintegrated within a few tides.
Environmental conditions that enable the survival and growth of oysters
Oyster reefs can be persistent if the environmental conditions favour 1) settlement of oyster larvae, 2) their survival and 3) the growth of adults. Oyster reefs are therefore best installed at sites where oyster reefs can naturally occur. However, this does not mean that the application of artificial oyster reefs is restricted to those areas. In soft sediment habitats oysters may be absent by lack of hard substrate, which makes it impossible for them to settle. The provision of a hard substrate eliminates this bottleneck to larvae settlement. In the Netherlands, existence of oyster reefs is often induced by the introduction of hard substrate, such as waste materials and artificial coastal defense structures.
Yet, substrate is not always the limiting factor in oyster growth and survival. Whether absence of hard substrate is the reason for the absence of oysters is best tested on site and should be based on knowledge of the population dynamics of the ecosystem engineer considered. Not only a suitable substrate for settlement, but also temperature, salinity, acidity, water quality, currents, predation and food availability determine oyster survival and growth. In the intertidal area, all these factors vary strongly during the tide. Hence exposure time is one of the determining factors for survival and growth of oysters (Schellekens, Wijsman et al. 2011). Experiments in the Eastern Scheldt revealed an optimal position of oyster reefs low in the intertidal. Oyster larvae there settle most and grow best at some 60 cm below the mid intertidal (Walles et al., unpublished). The survival of oysters is also strongly influenced by sediment dynamics: if too much sediment settles on the reefs, oysters will get smothered and die.
Evidence that oyster reefs do protect shorelines and intertidal habitats
Clearly, oyster reefs cannot replace primary flood defense systems such as dikes or dams. Placed in the right position, however, oyster reefs can reduce wave height and tidal currents leading to a net deposition of sediment under average weather conditions. This will reduce erosion and create a wave-attenuating barrier in front of the primary coastal defense.
The latter is shown in three different ways. Firstly, observations in the field clearly show that oyster clutches can increase sediment deposition, as illustrated by the photo to the right of oyster clutches in the Eastern Scheldt. The white lines indicate sheltered areas behind the clutches sediment deposition rates are higher (or erosion rates lower). A depth analysis around an oyster clutch in the Eastern Scheldt revealed similar patterns at a larger scale. The shallow area behind the oyster reef (extending to the north-east) clearly shows that oyster reefs can increase sediment deposition or reduce erosion (Walles et al., unpublished).
Secondly, a flume study (see figure, showing the relative wave height in a flume with sandy sediment, mussels and oysters) revealed that oyster reefs can reduce wave height, in this experiment by approximately 40% (Borsje, van Wesenbeeck et al., 2011).
Thirdly, a simple wave model can be used to determine the conditions under which oysters reefs are capable of reducing wave height. Inputs are the slope of the tidal flat, the height of the reef, the incoming wave height and the water level (TUD, Bram van Prooijen). The Area of influence figure shows the following color indications:
- yellow zone: waves are already broken in front of the reef, due to limited water depth;
- green zone: waves break on the reef, giving the strongest reduction of wave height;
- turquoise zone: waves do not break, but are dissipated by the reef;
- blue zone: no influence of reef, water depth too large compared to wave height & reef height.
During high tide or in case of a high water level set-up the effect of the reef may be minimal, whereas it may be significant during low tide.
The construction of artificial oyster reefs in the Eastern Scheldt requires a legal permit. As the basin is a national park and a protected area under Natura 2000, local authorities had to verify whether the deployment of oyster reefs would threaten the conservation goals of the area. This was done with an appropriate assessment, a formal procedure that assesses the effects of a proposed activity on any of the conservation goals of the area. Based on the appropriate assessment for the deployment of oyster reefs in the Eastern Scheldt, local authorities granted a preliminary legal permit. Stakeholders were given a few months’ time to object against the plan, but nobody did so and the licence was granted. Nonetheless, local oyster farmers did raise their concern about larger-scale application of artificial oyster reefs for shore and shoal protection, as this might exceed the carrying capacity for shellfish in the area.
Governance issues that where important in the Planning and Design phase:
- The discussion on the role of the Pacific Oyster (Ostrea gigas) in the Eastern Scheldt. Some consider the Pacific Oyster as an invasive exotic that should not be allowed to expand. Additionally, many mussel farmers fear that the Pacific oyster, with its high filter capacity, will put a strain on the (already limited) amount of food (phytoplankton) available in the Eastern Scheldt. On the other hand, the Pacific Oyster can also be claimed to fill the empty space left by the Dutch Oyster (which has been decimated by a disease) and to provide habitat to a diverse species community. The presence of the Pacific Oyster is therefore related to protected nature values in the Eastern Scheldt area. Either way, at present there are no regulations with respect to the occurrence of the Pacific Oyster in the Eastern Scheldt.
Interdisciplinary collaboration between several research institutes (Deltares, NIOZ (former NIOO-CEME, IMARES, Radboud University Nijmegen)) was beneficial to the planning and design phase.
The concept of shellfish reefs will also be used in the project Soft Sand Nourishment Oesterdam. This project involves the realization of an innovative safety buffer along the Oesterdam, a dam in the Eastern Scheldt. Sand nourishment within the area should realize a safety buffer to ensure flood safety and nature development all in one.
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.
The oyster reefs are all located between the low-water mark and 75 cm above the mid intertidal zone. From other experiments we deduced that in this zone oyster larvae will be able to colonize the dead oyster shells and form a “natural” biogenic reef before the gabions have rusted away. The gabions were provided by a company specialized in ecological embankment and slope stabilization with gabions (Nautilus – Ecociviel). The reefs were constructed by this company in collaboration with a local oyster farmer, who equipped his oyster fishery boat with a crane in order to place the oyster shells into the gabions. Each reef had a footprint of 2000 m2, a maximum length of 200 m and a maximum width of 10 m. The height is approximately 0.25 m. The reef has an elongated schape, due to logistic constraints. The gabions are made on non-galvanized steel wire, expected to fix the oyster shells for at least 2 years, although this may vary across sites.
The actual construction of the reefs is simple. During low tide empty gabions are placed on the mudflat and filled with oyster shells with a crane. The oyster shells are collected from a site near the project location provided by a local oyster farmer who fishes natural stocks from places where the oysters are considered a nuisance (e.g. mussel culture plots covered with oysters, public beaches where oyster shells can harm people). The use of indigenous substrate has the advantage that there is no risk of introducing new species or pathogens into the area (Bushek, Richardson et al., 2004; Miller, Ruiz et al., 2007). After filling the gabions, they are closed with non-galvanized steel wires.
Small-scale pilot Viane
Filling gabions with oyster shells at low tide on Viane (June, 2009).
The closed and open spaced reef constructed at Viane in 2009. Each reef covers 45 m2.
Large-scale pilots at Viane and De Val
Oysters and oyster shells being fished in the Eastern Scheldt to be used as filling material for the gabions.
Construction of the large oyster reef on Viane (2010).
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.
The artificial reefs
Similar to the small-scale experiment, the gabions proved successful in holding the oysters in place. When recently the first holes appeared in the structure, only few shells or oysters were lost, because loose elements had mostly been fixed by new recruits. In some places, especially in the most dynamic areas, the reefs are covered with shells, macroalgae and sand, which hampers oyster settlement and growth. Based on two years of experience, both settlement and growth seem to be generally sufficiently high to achieve a persistent natural reef. Recruitment and growth of the oysters on artificial reefs can be optimised by carefully positioning the artificial reefs relative to the low water line. Grazing experiments with snails (periwinkles, Littorina litorea ), revealed that these herbivores keep macroalgae to overgrow the reefs (Walles et al., unpublished). Some organisms may benefit from artificial reefs. An experiment with seagrass demonstrated that the more sheltered conditions behind the reef may be beneficial to this endangered species.
Effects on surrounding
The large-scale oyster reefs reduced the erosion rate of the intertidal flats they were meant to protect. A Comparison between the morphological changes over a period of 20 months in transects with (green line) and without (red line) an oyster reef reveals an average erosion of 5.9 cm and an average accretion of 0.1 cm, respectively (see last three figures to the right).
The oyster reefs are also effective in wave attenuation. In the graph, blue dots indicate the wave height in front of the oyster reef and green dots wave heights behind the oyster reef. The area between the red lines indicates the depths at which wave action causes the sediment to move. The figure therefore reveals that the overall wave height decreases at the oyster reef and especially at water depths at which the waves would otherwise stir sediment.
These are preliminary conclusions, drawn after the experiments have been running for two years. For final conclusions the experiments will need to continue for at least another three years. Only then we can conclude whether the settlement and growth of oysters is sufficient to ensure the survival of the reefs in a dynamic environment like the Eastern Scheldt. The rusting of the gabions may constitute a threat to the survival of the reef under severe conditions such as storms and high flow velocities.
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.
Ecology and morphology
- Wave impact and substrate availability can be limiting factors for oyster presence in intertidal areas.
- Which substrate is suitable for oyster settlement and growth is determined by local conditions (e.g. flow velocities and storms).
- Under the dynamic conditions in the Eastern Scheldt, the substrate for oyster settlement must be artificially fixed.
- Gabions (schanskorven) are suitable to hold oyster shells under the Eastern Scheldt conditions.
- A substrate of fixed oyster shells can grow into a living, self-sustaining oyster reef.
- The position of the oyster reef in the intertidal determines the survival of oyster larvae and the growth of oysters.
- The addition of grazers can reduce algal cover and may enhance oyster settlement.
- Both small- and large-scale oyster reefs can attenuate incoming waves, thus reducing erosion of the intertidal flats they protect.
- Artificial oyster reefs cannot replace existing coastal defense structures, but they can contribute by causing shoal and foreshore accretion under average weather conditions.
- It appeared to be important to involve parties that:
- are responsible for the construction, management and maintenance of the primary flood defense system
- know the local environment (e.g. ecology, morphology, hydrology, laws & regulations, policies)
- have specific knowledge on shellfish
- have specific knowledge on technical aspects of reefs
- might oppose the BwN initiative
- In the initiation phase of the BwN trajectory it is useful to carry out a stakeholder analysis to find out with whom to interact, when and how. Especially in the case of scaling up experiments, a stakeholder analysis is helpful to design a strategy for collaboration and communication with potential opponents and supporters.
- It appeared to be useful to spend time and effort to careful planning and targeting the communication on building with nature.
- A mix of communication methods (e.g. press release, movie, excursions) is required to reach different categories of stakeholders.
- A tangible experiment which can be touched and seen is attractive for policy makers to show their innovative achievements.
- Involve parties knowledgeable in quickly finding out about procedures and required licences and ask them to help processing the necessary forms.
- Start discussions with stakeholders who could oppose the initiative well before submitting the requests for the necessary permits.
- The new Dutch nature conservation law (2012) facilitates the legislative and juridical procedures.
- BwN experiments require different types of knowledge derived from different sources, such as models, measurements and experience/expert judgement. To fill knowledge gaps in the field of morphology and ecology, it appeared to be useful to organise workshops bringing together professionals, practitioners and researchers who jointly shared and co-created new knowledge.
Dealing with uncertainty
- Uncertainties typical of shellfish reefs include: the formation of a living reef, its longevity, and the increase in sediment deposition / reduction of erosion. Monitoring the ecological and morphological effects of the shellfish helps reducing these uncertainties.
- Any monitoring programme has to be preceded by a baseline study.
- Experiments like this need a flexible planning that allows regular adaptation and learning.
- Stakeholders and the general public tend to expect results of this type of experiments within one year. It has to be clear from the start that this is about long-term development. Effects on ecology, morphology and dike stability can only be made visible after a number of years. On the other hand, proponents of this kind of measures need to realize that policy makers and other decision makers do not need 99% or 95% confidence for taking decisions. Communication of preliminary results can therefore be very relevant.
Economic value of building with nature
- One needs to show how building with nature can work before its economic benefits can be demonstrated. Yet, the costs of the initiative should be made as explicit as possible from the very beginning.
- Acceptability and economic viability of shellfish reefs can be enhanced by financial and institutional arrangements that bring together water safety, aquaculture and nature conservation networks (coupled functionality).
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- Construction of a large oyster reef. – Leo Adriaanse, RWS
- Oyster reef at De Val, The Netherlands. – Anda van Riet
- Small oyster – Brenda Walles
- Settlement of small oysters.- Brenda Walles
- Sediment deposits. – Brenda Walles
- Flume study. – Walles et al., (unpublished)
- Depth analysis. - Walles et al., (unpublished)
- Area of influence. - Walles et al., (unpublished)
- Oyster transport. - Brenda Walles
- Handfilling gabions. - Brenda Walles
- Open spaced reef. - Brenda Walles
- Closed spaced reef. - Brenda Walles
- Fishing for oysters 1. - Brenda Walles
- Fishing for oysters 2. - Brenda Walles
- Construction of the large oyster reef on Viane (2010) - Brenda Walles
- Lee site of pilot oyster reef – Brenda Walles
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