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In the Netherlands, the demand for marine sand is still increasing. In 2015, a total volume of 26 million m3 of sand was extracted from the Dutch Continental Shelf for coastal nourishment. Due to the expected sea level rise, the demand for sand to maintain the Dutch coast with nourishments may increase from 12 million m3 to 40 - 85 million m3. To safeguard the supply of sand, new sand extraction strategies are needed and therefore ecosystem-based design rules, which optimise the balance between impacted surface area, sand yield, costs and ecological effects are developed.

A pilot was planned to investigate which design- and organizational procedures are needed to landscape a large-scale extraction site in this way, as well as to monitor and analyse the effects of this type of ecological site development (expressed/indicated as an increase in biodiversity and biomass). The project explored ways to design and create a seabed landscape, such that after extraction the ecosystem can optimally benefit.

Building with Nature Design Traditional Design

Ecological research on tidal sand bars and sand waves shows that there are differences in the benthic community composition present on the troughs, slopes and crests. Ecosystem-based landscaped bedforms of a natural scale may induce similar ecological differences. Due to this landscaping, a higher overall biodiversity and higher heterogeneity may develop within sand extraction sites.

 

A traditional design of a sand extraction site is characterized by a flat seabed. After extraction, biodiversity and biomass will develop in a rather homogeneous pattern.


    General Project Description

    Title: Ecosystem-based design of sand extraction sites
    Location: The Netherlands, The Dutch coastal area in front of the Port of Rotterdam (PoR)
    Date: construction 2008 – 2012; monitoring 2013 -2017.
    Companies: Boskalis and Van Oord (PUMA), Port of Rotterdam (PoR), Rijkswaterstaat, Deltares, Wageningen Marine Research (WMR, former IMARES) and DHV.
    Costs: Additional costs of ecosystem-based landscaping about € 200.000 Euro. (Note: these costs are influenced by the late addition of the pilot to sand extraction project which was already in progress; if the landscaping would have been incorporated from the beginning of the project costs could have been lower).
    Cost monitoring (first phase): € 400.000
    Abstract: Applying ecosystem-based landscaping techniques in a marine sand extraction site to develop habitat conditions that enhance habitat heterogeneity and biodiversity.
    Topics: Ecosystem based landscaping, deep sand extraction, BwN, environmental monitoring, macrozoobenthos, demersal fish, sediment characteristics, hydrodynamics.

    Project Objective

    The goal of the pilot experiment is to test whether ecosystem-based landscaping enhances biodiversity after cessation of the sand extraction activities. Present sand extraction policy aims at restoration of the original pre-dredging habitat. This is, however, a limiting approach. Flat seabeds tend to be ecologically less valuable than seabeds with meso-scale bedforms such as tidal ridges, shoreface-connected ridges and sand waves. Such bedforms provide habitat to a larger range of species assemblages.

    In a sand extraction site, seabed morphology and sediment composition is changed by removing a part of the seabed. Depending on the granted sand extraction license, this may lead to a significant increase of the water depth. Returning to the pre-dredging morphology may take decades or more. In recent years, it has become increasingly clear that seabed heterogeneity (i.e. bed forms) constitutes habitats allowing for more biodiversity and more biomass. The pilot project described herein has applied this knowledge to a large marine sand extraction area off the coast of South Holland, the Netherlands. Two large-scale bedforms were left behind after extraction, to test whether these will accelerate ecological recovery and enhance biodiversity and biomass in the dredged area.

    The project encompasses the design, organisation and realisation of two ecosystem-based landscaped sand bars in a large-scale and deep sand extraction site. Next to the ecosystem-based landscaping, ecosystem-based design rules for future sand extraction sites which optimise the balance between impacted surface area, sand yield, costs and ecological effects are developed.

    BwN dimension: To safeguard the supply of sand with sand extraction strategies based on ecosystem-based design rules which optimise the balance between impacted surface area, sand yield, costs and ecological effects. Ecosystem-based landscaping will be used to accelerate recovery and to boost habitat heterogeneity and biodiversity.

    Project Solution

    Background for pilot

    With the traditional state of development allocation of permits and related monitoring for sand extraction sites tend to focus on the rate of habitat recovery in the areas. It is often unclear, however, what is meant by the term recovery. The basic goal is re-establishment of a  species assemblage like the one existing before dredging. This can only be attained if the original habitat, especially bed topography and sediment composition, are restored (Boyd et al. 2003). This is more likely to occur in case of shallow (2 m) extractions, which give rise to limited geomorphological changes. Yet natural fluctuations may lead to the establishment of a community that differs from the original one (Van Dalfsen and Essink 2001). If the environmental conditions after dredging have changed significantly, a different stable state may develop with a different species assemblage. Seiderer and Newell (1999) suggest that recovery can be interpreted as the establishment of sufficient species diversity following cessation of dredging, which would allow the biological resources to progress towards diverse equilibrium assemblages.
    It is therefore advisable to use 'development of habitats' instead of 'restoration of habitats'. This allows the creation of (new) habitats that are tailored to the new circumstances and thus have a much higher chance of reaching a stable ecological state.
    The creation of bed forms and/or combinations of sediment characteristics yielding geomorphological gradients in a sand extraction site will enhance the conditions necessary for habitat diversity. In turn, habitat diversity may result in high biodiversity as a variety of species will be able to settle within the area.

    Planning and Design

    The Planning and Design works for the Maasvlakte Sand Extraction pilot consisted of the following phase and aspects:

    It is obvious that such pilot, set within the national coastal policy and as part of a commercially running construction project only could have been made with due attention to governance aspects.

    Construction

    Execution of the works

    After the planning phase, the actual dredging and creation of the sand bars started. The dredging activities mainly took place during slack tide. Normal dredging occurs in line with the tidal current (Fig. 5) and filling of the hopper occurred when sailing from the northern to the southern part and vice versa. Changes in the dredging direction can induce constraints with regards to lateral deviation of the trailing arms and the drag heads.

    The following steps were taken:

    1. The bathymetry and sediment characteristics in the sand extraction site were studied and together with the contractor PUMA, locations within the sand extraction site were determined.
    2. Based on expert judgement concerning hydrodynamical, morphological and ecological aspects, a choice was made for the preferred two locations. In both cases, sand bars were positioned near the edges of the sand extraction site to add an extra slope and trough in the overall design.
    3. Before the construction of the sand bars, a temporary exemption became active in the near proximity of the designated sand bar extraction site.
    4. The design was integrated into the GIS system used on board the dredging ships. This system includes bathymetry and sediment composition data and enables calculating dredged volumes. The information from this system was used to fine-tune the design for optimal working conditions.
    5. The final locations were integrated into the THSDs’ GIS systems and the captains received instructions on how to dredge around the sand bars.
    6. The temporary exemption was abolished and the trailing suction hopper dredgers were asked to create the sand bars.

    The pilot experiment took place in the sand extraction site used for the development of the Port of Rotterdam enlargement ‘Maasvlakte 2’ (MV2). Between 2008 and 2012, 220 million m3 was extracted from a 20-m deep area south of the Euromaasgeul shipping lane. A large northern extraction site is separated by an exclusion area consisting of clay and a southern smaller extraction site (Fig. 5).

    Figure 5: Bathymetry of the deep and large-scale MV2 sand extraction site with a large northern and smaller southern part separated by an exclusion area consisting of clay. In the northern part 2 ecosystem-based sand bars are visible (picture: Maarten de Jong, based on multi-beam data of PUMA).

    One sand bar, parallel to the tidal current, was dredged out the seabed in 2010 in the north-western part of the northern sand extraction site (Fig. 1 no. 1) and one sand bar oblique to the tidal current in the south-western part in 2011 (Fig. 1 no. 2). Several trailing suction hopper dredgers (TSHDs) were involved Fig. 6), systematically following each other to deepen the projected trough.

    Figure 6: Trailing suction hopper dredgers (TSHDs) of Boskalis and van Oord dredging out the ecosystem-based sand bars on the seabed of the 40-m deep Maasvlakte 2 extraction site (picture: Daan Rijks, Boskalis).

     

    The sand bars, resulting in between the two troughs, are 700 m long and the crests are 70 m wide at a water depth of 30 m. The water depth of troughs surrounding the parallel sand bar is 40 to 44 m (Fig. 7). The troughs of the oblique sand bar are at 32 m water depth (Fig. 8). These dimensions are not different to those of natural sand ridges.

         Figure 7: 3D representation of the parallel ecosystem-based sand bar (picture: Maarten de Jong).

     

          Figure 8: 3D representation of the oblique ecosystem-based sand bar from an eastern direction. (picture: Maarten de Jong).

     

    Operation and Maintenance

    Around 2009, knowledge on the relationship between ecology and the seabed was predominantly based on expert judgement and a few scattered datasets. This project offered a unique opportunity to gather field data.

    During design and preparation of the project a solid monitoring plan was drafted, making use of on data collected in a baseline study. During the recolonization phase after the sand extraction operation and the development of ecosystem-based landscaped sand bars much data from short term monitoring campaigns was gathered. Next to monitoring results about short-term effects, medium and long-term data are currently collected during the recolonization measurements of the Port of Rotterdam (PoR).

    Lessons Learned

    During the MV2 sand extraction project, many new insights were gained concerning technical and ecological aspects of the design and the organisation of large scale, deep sand extraction projects and ecosystem-based landscaping.

    The project also generated a broad discussion amongst the various stakeholders on how changing physical conditions can trigger the development of new ecological habitats. 
    An important lesson learned is that ecosystem-based landscaping in sand extraction sites only make sense if:

    • the sand extraction volume and area are large enough for ecosystem-based landscaping to have maximum added value;
    • Ecosystem-based landscaping can be used to increase ecological value (de Jong et al., 2015; De Jong et al., 2016, 2014) ; and
    • Ecosystem-based landscaping can be carried out during the extraction process without additional equipment mobilisation and with minimal interference to the overall sand extraction production process.

    Overall, it became clear that it is still too early to prescribe landscaping to other sand extraction projects, even if they meet the above conditions. The present pilot experiment is still on-going and, although the concept appears to be positive, its added ecological value remains scientifically to be proven.

    The following lessons have been learned:

    References

    Literature

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