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Ecosystem-Based Design Rules for Sand Extraction Sites
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 (DCS) 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.
Next to sand extraction, many other facilities and services are provided on the DCS such as wind parks, oil and gas platforms, cables and pipelines, shipping and fishing and to keep the harbours accessible also maintenance of shipping lanes and the disposal of dredged harbour sediments (Fig. 1). Specific areas are also designated as Natura2000 areas.
To safeguard the supply of sand, new sand extraction strategies are needed to the balance between impacted surface area, sand yield, costs and ecological effects.
Figure 1: The Netherlands (dark grey) with the Dutch Continental Shelf (DCS) depicted in light grey and all human activities to indicate the large-scale spatial extent. (photo: Maarten de Jong).
On the DCS, sand extraction takes place in areas with water depths over 20 m (Fig. 2). Until recently, extraction depths were restricted to 2 m below the seabed. In 2000, possibilities of extraction depths larger than 2 m below the seabed were explored (Boers et al. 2000). It became clear that in water depths of less than 40 m, the chance of reduced seawater oxygen content is rather small and that re-establishment of macrozoobenthos (organisms living in and on the seabed) on the seabed is possible.
Figure 2: Bathymetry of the Dutch Continental Shelf (DCS) with 4 sand extraction case studies, 1: shallow sand extraction (-2 m) near barrier island Terschelling, 2: intermediate deep sand extraction (- 8 m) in the turning basin of the Euromaasgeul shipping lane, 3: deep sand extraction (- 20 m) and 4: ecosystem-based seabed landscaped sand bars. The other areas are shallow sand extraction sites (photo: Maarten de Jong).
Currently larger extraction depths are allowed, for the development of harbour enlargement Maasvlakte 2 of the Port of Rotterdam, 220 million m3 of sand was extracted (Fig. 2, no. 3). Extraction depths up to 20 m below the seabed were employed mainly to decrease impacted surface area. All these activities can be considered as ecosystem services (Boerema et al., 2016).
To further exploit these opportunities studies were made to better understanding of the system, the
One needs to have knowledge of marine ecology (macrozoobenthos and demersal fish), sediment characteristics, hydrodynamics and morphodynamics. These different fields of expertise needs to be combined into ecosystem-based design rules in such a way that policy makers and the sand extraction companies know how to successfully apply these design rules.
Due to the worldwide increase in the demand of marine sand and increase of other human activities space is becoming scarce. New sand extraction strategies are needed to optimise the use of space and safeguard the global supply of marine sand. Strategies which prevent against negative effects or even improve the productivity of an area is of interest.
How to Use
Ecosystem-based design (EBD) rules for sand extraction sites
The short-term effects of deep sand extraction (20 – 24 m) are compared with other case studies such as, regular shallow sand extraction on the Dutch Continental Shelf (DCS) (2 m) and an 8-m deepened shipping lane (Fig. 10). For intercomparing between case studies, we used tide-averaged bed shear stress as a generic proxy for environmental and related ecological effects. Bed shear stress ( ) can be estimated with a two-dimensional quadratic friction law (eq. 1) and showed a decrease from 0.50 to 0.04 N m−2 in a sand extraction site in 20 m deep water and extraction depths up to 24 m.
Figure 10: Bathymetry of the Dutch Continental Shelf (DCS) with 4 sand extraction case studies, 1: shallow sand extraction (-2 m) near barrier island Terschelling, 2: intermediate deep sand extraction (- 8 m) in the turning basin of the Euromaasgeul shipping lane, 3: deep sand extraction (- 20 m) and 4: ecosystem-based seabed landscaped sand bars. The other areas are shallow sand extraction sites (picture: Maarten de Jong).
ρseawater = density of seawater
Macrozoobenthos in a sand extraction site with a tide-averaged bed shear stress of around 0.41 N m–2 on the DCS is expected to return to pre-extraction conditions within 4–6 year (De Jong et al., 2016; van Dalfsen et al., 2000; van Dalfsen and Essink, 2001). When tide-averaged bed shear stress decreases below 0.17 N m−2 on the DCS enhanced macrozoobenthic species richness and biomass may occur. Below a tide-averaged bed shear stress of 0.08 N m−2 on the DCS increasing abundance and biomass of brittle stars, white furrow shell (Abra alba) and plaice (Platessa platessa) can be expected. Below 0.04 N m−2, an overdominance and high biomass of brittle stars can be expected whereas demersal fish biomass and species composition may return to reference conditions. Next to changes in faunal composition, a high sedimentation rate can be expected.
Ecological data and bed shear stress values were transformed into ecosystem-based design (EBD) rules for the DCS (Fig. 11). At higher flow velocities and larger water depths, larger extraction depths can be applied to achieve desired tide-averaged bed shear stresses for related ecological effects (De Jong et al., 2016).
Figure 11: Left panel: Data on infauna (biomass in g m-2, number of species and dominant species), epifauna (biomass in g m-2, number of species and dominant species), fish (biomass in g m-2, number of species and dominant species), sediment (grain size in µm, volume percentage of fines) and extraction depth (0, 2, 8, 20, 24 m below the seabed). The red line is the estimated bed shear stress of the case studies 1: 20 m deep regions without sand extraction, 2: shallow sand extraction (2 m) near barrier island Terschelling, 3: intermediate deep sand extraction (8 m) in the turning basin of the Euromaasgeul shipping lane, 4: deep sand extraction (20 and 24 m).
Right panel: Calculated extraction depths needed to reach bed shear stress values of each case study (2 m: 0.41, 8 m: 0.17, 20 m: 0.08 and 24 m: 0.04 N m-2), for areas with different depth averaged flow velocity (solid lines: 0.65, large-dash lines: 0.7, intermediate- dash lines: 0.75 and small dash lines: 0.8 m s-1, see Fig. 16) and initial water depth at the x-axes (15, 25, 30, 35, 40 m).
Figure 12: Depth averaged flow velocity (U) derived with Delft 3D (van Duren et al., 2016).
The Ecosystem Based Design (EBD) rules can be used in the early-design phases of future sand extraction sites to simultaneously maximise sand yields and decrease the surface area of direct impact. The EBD rules and ecological landscaping can also help in implementing the European Union’s Marine Strategy Framework Directive (MSFD) guidelines and moving to or maintaining Good Environmental Status (GES).
Table 1: The ecological effects of the different extraction depths and ecological landscaping in view of the criteria of the MSFD descriptors for the Dutch coastal area with 20 m pre-extraction water depth and a flow velocity of 0.65 m s-1. Green: positive effects, yellow: minor effects, red: negative aspects and brown intermediate effects (positive and negative)
Lessons Learned and Recommendations
The development of Ecosystem-based Design Rules for Sand Extraction Sites, being based on studies on short- and medium term effects of sand extraction sites on the Dutch Coastal Shelf, has provided the following lessons.
Considering the context of the studies, on which the design rules are based, further confirmation of the validity and applicability of these rules in general can be obtained through
Pilot Ecological sand extraction site
A Building with Nature pilot has been executed in the sand extraction site used for the construction of the Port of Rotterdam harbour enlargement Maasvlakte 2 (MV2). (Ecosystem-oriented Landscaping of Sand Extraction Sites).
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. 13).
Figure 13: 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 (photo: Maarten de Jong, based on multibeam data of PUMA).
The Building with Nature pilot ecological sand mining pit involved the realization of two sand bars dredged out the seabed (Fig. 14) in the 20-m deep sand extraction site which was used for the construction of harbour enlargement Maasvlakte 2 of the Port of Rotterdam (PoR). The effects of deep sand extraction and two ecosystem-based sand bars were studied using a suite of research tools and aligned with the monitoring campaign of PoR to maximise comparable field survey data and to reduce costs.
The effects of deep sand extraction and ecosystem-based landscaped sand bars are monitored after completion for 2-3 years. For scientific output see
Focus has been on direct and short-term effects and inter-comparison of effects in the sand extraction pit, on ecosystem-based sand bars and reference areas and investigated infauna (organisms living in the sediment), on epifauna (organisms living on the seabed), on demersal fish (fish living near the seabed) and on sediment characteristics and changes in bathymetry.
Figure 14: Trailing suction hopper dredgers (TSHDs) of Boskalis and van Oord dredging out the ecosystem-based sand bars on the 40-m deep seabed of the Maasvlakte 2 extraction site (picture: Daan Rijks, Boskalis).
Outside this Building with Nature application, plenty of examples of similar projects are present.
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