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 bed forms such as tidal ridges, shore face connected ridges and sand waves. Such bed forms 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 bed forms 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 realization 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 optimize the balance between impacted surface area, sand yield, costs and ecological effects are developed.
To safeguard the supply of sand with sand extraction strategies based on ecosystem-based design rules which optimize 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.
Restoration of habitats
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 (2m) 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).
Development of habitats
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.
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.
The increased demand for marine aggregates can lead to a growing impact on the seabed and the organisms that depend on it (macrozoobenthos and demersal fish). Traditionally, these impacts are considered to be negative, though inevitable. Therefore, permitting authorities prescribe that impacts need to be minimized in time and space (traditionally, in the Netherlands permits did not allow for extraction deeper than 2 m below the undisturbed seabed; recently, based on new understanding permits allow for deeper sand extraction, up to 20 m below the seabed).
In the pre-feasibility phase, a literature study supported the idea that an extraction site can be designed and operated in such a way that a good ecological status (species assemblage, high biomass and species richness) can be achieved after extraction. By taking the potential ecological response into account both during the early stages of the design phase and when determining the operational methods, enriched ecological conditions of the sand extraction site could be reached. This is especially the case if the seabed in the sand extraction site is landscaped, e.g. by creating bedforms such as large-scale sand waves or tidal ridges.
Fig. 2. Relation between numbers of species and the location on tidal ridges (tops/crests, swales or troughs or the slopes (van Dijk et at. 2012).
A typical species for the crests of sand waves is the crustacean Urothoe elegans and for troughs the polychaete Spiophanes bombyx . A typical species for the shoreface-connected ridge area is the polychaete Nephtys cirrosa . The swale of the shoreface-connected ridge is typified by the polychaete Eteone longa (Baptist et al. 2006). The study of van Dijk et al. (2012) shows that marine habitats over two tidal ridges in the North Sea vary from low-density/low-diversity communities on the well-sorted sandy crests of the ridges to high-density/high-diversity communities in the poorly sorted muddy, gravelly sediments in the adjacent troughs (see adjacent figure Van Dijk et al. 2012).
Fig. 3. Hypothetical benthic fauna distribution at sand waves.
The troughs of sand waves which are characterised by fine sediment and higher mud content, biodiversity often appeared to harbour more species and a higher biomass (Fig. 3). In turn, the macrozoobenthic communities are an important food source to other species such as flatfish. Clearly, natural physical gradients due to the presence of bedforms have a positive effect on the overall biodiversity and biomass. Landscaping an extraction site to increase the variation in physical parameters such as morphology and hydrodynamic conditions is therefore expected to have a similar effect on the ecosystem in the area.
A desk study was carried out to determine if seabed landscaping was possible within the Dutch legislation. The study identified and analysed the opportunities and obstacles in the existing judicial and licensing framework related to sand extraction in Dutch coastal waters. Policy and legal requirements were assessed by the extent to which, and the way in which, they stimulate or impede the development of seabed landscaping, considering technical dredging aspects as well as ecological opportunities.
For the Maasvlakte 2 sand extraction site a license for sand extraction was already granted. Discussions with permitting authorities and stakeholders led to the conclusion that the landscaping could be integrated in the contractor’s working plan. If the design of the landscape remained within the general requirements and boundaries of the permit, no objections were to be expected (Lulofs, 2010).
The next step was to prepare the design of the landscaping. This was done according to the principles of Eco-Dynamic Development and Design (EDD):
After defining the ecological and physical baseline conditions , the planning and design phase concentrated on optimising the landscaping design for the sand extraction site. It followed the relatively simple approach described below (Rijks, 2011):
The following stakeholders were asked for their input, based on their role in the project:
Choice of Pilot Location
The Maasvlakte 2 sand extraction area lies about 20 km offshore from the Port of Rotterdam (PoR) (Fig. 4, no. 1). The site was chosen in consultation with the project owner, the permit authority and the contractor.
Fig. 4. Location of the pilot site Maasvlakte 2 in front of the Port of Rotterdam (PoR) denoted with 1.
The project team consisted of marine ecologists, seabed morphologists, hydrodynamical and contracting experts from consultancy agencies, contractors, applied research institutes, universities and governmental authorities. Their expertise was used during regular brainstorming sessions and an international group of experts were asked for their verdict concerning the project. Furthermore, the early involvement of stakeholders in the project team was essential for the success and the swiftness of the design-phase. The stakeholders represented all aspects of the project, from technical aspects to contracting and governance. They contributed to the project through regular meetings with the project team throughout the project development.
Trailing suction hopper dredgers
(picture: Daan Rijks, Boskalis)
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 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.
Execution of the works
The following steps were taken:
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.
3D representation of the parallel ecosystem-based sand bar (left) and from the oblique ecosystem-based sand bar from an eastern direction (right) (picture: Maarten de Jong).
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 and one sand bar oblique to the tidal current in the south-western part in 2011. Several trailing suction hopper dredgers (TSHDs) were involved, systematically following each other to deepen the projected trough. 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. The troughs of the oblique sand bar are at 32 m water depth. These dimensions are not different to those of natural sand ridges.
Box core sampling for monitoring
Around 2009, knowledge on the relationship between ecology and the seabed was predominantly based on expert judgement and a few scattered data sets. 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).
Monitoring in the Maasvlakte 2 (MV2) sand extraction sited focused on the general effects and the landscaped sand bars. General effects were studied in a deep area without landscaping and the effects of landscaping were studied at two landscaped sand bars .
The monitoring was meant to provide answers to the following questions:
To assess the situation prior to the large-scale and deep sand extraction for the construction of MV2, data were collected by NIOZ and IMARES in the framework of the baseline study for the Environmental Impact Assessment (EIA). The EIA was commissioned by the Port of Rotterdam (PoR) and investigates (ecological) effects of the Maasvlakte 2 project. A set of 470 box corer and bottom sledge samples from the baseline monitoring campaign of PoR was selected from data collected in April-June 2006 and 2008 at 235 locations in a 2500 km2 study area off Rotterdam.
235 Sampling locations of the Port of Rotterdam (PoR) baseline study in 2006 and 2008.
Read more about the baseline study
A monitoring campaign was designed and aligned with the survey campaign of PoR to maximize inter compatibility and amount of field data output and to reduce costs. During the campaign focus was placed on the short-term effects in the sand extraction site on macrozoobenthos (infauna and epifauna), demersal fish, sediment characteristics, sedimentation-erosion and hydrodynamics.
Used monitoring techniques: top from left to right: a box corer for infauna sampling, sediment sampling from a box corer sample, sieving of sediment to extract infauna: bottom from left to right: bottom sledge to sample epifauna, 4.5 m wide beam trawl to sample demersal fish and multibeam measurements for bathymetry and changes in sedimentation-erosion.
Sediment and modelled hydrodynamical variables
An increase in mud content was observed after the cessation of sand extraction. Locations with a high mud value also have a high sediment organic matter content (SOM). Modelled time-averaged bed shear stress is high at the crests of natural occurring sand waves in the reference areas. The northern edge of the northern sand extraction site shows the highest bed shear stress values. The southern part of the northern sand extraction site has the lowest bed shear stress values. Differences in bed shear stress are also visible at the ecosystem-based sand bars.
Read more about the monitoring results
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:
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.
Lessons on project development
The most important lessons learned are:
Lessons on physical and technical aspects
Several lessons were learned concerning physical parameters and the technical realization:
Lessons on the ecological response
Lessons on governance
The project team worked closely together with the stakeholders, thus learning several lessons concerning governance issues:
Lessons on realization
Several important lessons were learned during the execution of the dredging works, concerning the workability of the design and the consequences of creating a specific landscape as compared with a case without predefined bedforms:
Lessons on monitoring
EcoShape pilot project