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The expected worldwide increase the demand of marine sand extraction due to economic growth and urbanisation and the long-term threats of climate change call for innovative approaches for sand extraction activities. Marine sand extraction traditionally focuses on minimizing environmental impact and quick recovery of seabed sediment composition and benthic assemblages. With large-scale extraction, this conservative approach can lead to constraining mitigation measures. Moreover, the potential of ecological development, cost reduction and a more efficient use of scarce space are not recognized.
Ecological landscaping of sand extraction sites involves the realisation of seabed level gradients and other morphological features in newly dredged sites. The overall aim is to make the sand extraction site attractive for a variety of macrozoobenthic species that in turn attract demersal fish, mammals and birds. This is done by creating different bedforms and/or combinations of sediment characteristics which provides ideal settlement and habitat circumstances for a larger variety of species. Due to this landscaping, the re-colonization of the mined pit is presumably faster and results in a higher biodiversity than in a traditionally dredged sand extraction site.
General Building Block Description
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 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.
Ecological research on tidal sand ridges and sand waves showed that there are differences in the macrozoobenthic species composition present on the troughs, slopes and crests. Ecosystem-based sand extraction can create bedforms of a similar scale as the naturally occurring sand ridges in the area. This approach may result in a higher biodiversity and biomass in the sand extraction site after dredging. An ecological design of an extraction area may help to increase the potential post-dredging value of the area and opportunities can be taken to improve and add to the overall sustainability of the sand extraction project.
Fig. 1. Hypothetical benthic fauna distribution at sand waves.
This Building Block provides guidance for design, organisation and realisation of ecosystem-based landscaped sand extraction sites, based on research in the Maasvlakte 2 sand extraction site in the North Sea. Ecosystem-based design rules for future sand extraction sites are developed which to optimise the balance between impacted surface area, sand yield, costs and ecological effects.
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 boost habitat heterogeneity and biodiversity.
- Case – Ecosystem-based design of sand extraction sites – Pilot MV2 Sand Extraction
- Tool - Ecosystem-based Design Rules for Sand Extraction Sites
To properly design a sand extraction scheme knowledge is required of both the hydrological and morphological behaviour of the area under consideration as well of the ecological response of marine organism to the changes in the seabed landscape. For this purpose, data of the local system and from comparable regions need to be collected and statistically analysed.
When specific bedforms are desired, knowledge of construction options and possible complications are required during the preparation phase of the project, combined with monitoring and adaptive management options.
- It has been found that ecosystem-based landscaping will help to promote higher biodiversity compared to the traditional dredging approach.
- Enhancing biodiversity and overall sustainability of the sand extraction project may facilitate social and political acceptance and thus accelerating licensing procedures and project realization.
- The practical feasibility of the design sand extraction sites will be a prerequisite for success.
- The ecologically most favourable set-up and orientation of ecosystem-based sand bars may imply risks to the sand extraction project and productivity.
- Ecosystem-based landscaping might introduce additional costs to the sand extraction project, although it has been shown that with proper design and execution this will be kept to a minimum.
- Application of ecosystem-based landscaping in sand extraction sites implies the transformation of existing habitat into a new habitat which is not embedded in current legislation.
How to Use
This section describes and gives guidance to the steps to be taken to design landscaping of the seabed on basis of ecosystem approach. The steps are described following 4 of the 5 BwN design steps:
1.. Understanding the system
- Effects of sand extraction
- Shallow sand extraction
- Deep sand extraction
2 . Identify realistic alternatives
- Assumptions in the approach of landscaping
- Guidelines and boundary conditions
- Determine the feasibility of an ecosystem-based sand extraction site
- Technical parameters
- Physical parameters
- Ecological parameters
3. Valuate the qualities of alternatives and pre-select an integral solution
- Determine a basic design for a landscaped ecological mining pit
- Ecosystem-based design rules for the minimum excavation depth ecological mining sites
4. Elaborate selected alternatives.
In recent years, it has become increasingly clear that higher seabed heterogeneity (i.e. meso-scale bedforms such as sand waves and sand ridges) is linked to higher overall biodiversity. The Building with Nature pilot with the ecosystem-based sand bars was executed in the deep sand extraction site used for the construction Maasvlakte 2 of the Port of Rotterdam (PoR). The ecological response due to the deep sand extraction and the presence of the sand bars was monitored.
The project encompassed the design, organisation and realisation of two ecosystem-based landscaped sandbars (Fig. 6) 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.
During the project, new insights were gained concerning technical and ecological aspects of the design and the organisation of a large-scale and deep sand extraction project with ecosystem-based sand bars.
The project also generated a broad discussion amongst the various stakeholders on how changing physical conditions can trigger ecological development of new habitats.
The most important lesson learned is that seabed landscaping in sand extraction sites only make sense if:
- the sand extraction volume and sand extraction site is large enough for landscaping to have added value;
- landscaping is indeed expected to yield added ecological value; and
- landscaping can be carried out during the extraction process without additional equipment mobilisation and with minimal hindrance to the production process.
Overall, it became clear that it is still too early to prescribe or promote landscaping to other sand extraction projects, even if they meet the above described conditions. The seabed and assemblages are not yet in a stable state so the added ecological value remains to be proven.
Few of the other learned on project realisation are:
- Take a joint approach involving all stakeholders, from initiator, consultant (technical) experts and contractor to permitting authority.
- Make sure the decision to include ecosystem-based landscaping is taken early in the (design) process.
- Use expert knowledge and numerical models to predict the behaviour of the bedforms in the sand extraction site to make sure that they are relatively stable and allow sufficient time for ecological development.
- Make sure that there is enough space around the bedforms for the dredging equipment to manoeuvre.
- The extraction depth influences the potential of the landscaped area. If the extraction is too deep, the effect of morphological landscaping may be dominated by the hydrodynamic effects of the deep pit (e.g. high sedimentation rate).
- Discuss and investigate the potential of landscaping in a certain extraction area within the prevailing permit limitations before the design is made (input for design);
- Ensure frequent updates of the bathymetry of the created bedforms, in order to be able to monitor their development and steer expectations among stakeholders. Morphological monitoring will enable evaluation of the design, predict the longevity and assess the effectiveness of reaching the original goals (e.g., increasing biodiversity or biomass).
- Apply adaptive processes to the ecological monitoring strategy: tune next monitoring to developments found at previous efforts, while still maintaining consistency in data collection for good comparison objectives.
Outside this Building with Nature application, plenty of examples of similar projects are present:
- Gravel-seeding techniques to restore the seabed to pre-dredge conditions after gravel extraction in the English part of the North Sea (Cooper et al., 2011). Changes in bed shear stress values after sand extraction also the main drivers of ecological changes although not yet fully recognised. Optimisations in bed shear stress values, by fine-tuning extraction depths and orientations of the sand extraction sites with respect to the tidal current are possible (e.g. to prevent against sedimentation which may be harmful to hatching herring larvae on the gravelly seabed).
- Seine estuary cooperation between dredging and fishing industries (Desprez, 2000; Marchal et al., 2014). Optimisations in bed shear stress values, by fine-tuning extraction depths and orientations of the sand extraction sites with respect to the tidal current are possible.
- Maximum allowable changes in seabed level and bed shear stress values after sand extraction to maintain original macrozoobenthic characteristics (poster and oral sessions of Koen Degrendele and Dries van den Eynde at the ICES Annual Science Conference 2016) http://www.ices.dk/news-and-events/asc/ASC2016/Pages/Theme-session-K.aspx
- Maintenance dredging in river and estuarine systems (Yuill et al., 2016).
- Rijke riffen (van Duren et al., 2016), Building with North Sea Nature: eco-friendly scour protection (Lengkeek et al., 2017) and construction of artificial reefs in Japan (Thierry, 1988).
- Rejuvenation dredging of tidal creeks in a mangrove systems (Bonaire and Curaçao). https://publicwiki.deltares.nl/display/BWN1/Building+Block+-+Habitat+requirements+for+mangroves#Generalbuildingblock-praticalApplications, http://www.wur.nl/nl/project/Ecologisch-herstel-mangroven-Lacbaai-Bonaire-van-kennis-naar-pro-actief-beheer.htm
- Baptist, M.J., Van Dalfsen, J., Weber, A., Passchier, S. and Van Heteren, S., 2006. The distribution of macrozoobenthos in the southern North Sea in relation to meso-scale bedforms. Estuarine, Coastal and Shelf Science, Volume 68, Issues 3--4, July 2006, Pages 538-546.
- Cooper, K., Ware, S., Vanstaen, K., Barry, J., 2011. Gravel seeding - A suitable technique for restoring the seabed following marine aggregate dredging? Estuar. Coast. Shelf Sci. 91, 121–132. doi:10.1016/j.ecss.2010.10.011
- Degraer, S., Verfaillie, E., Willems, W., Adriaens, E., Vincx, M. Van Lancker, V., 2008. Habitat suitability modelling as a mapping tool for macrobenthic communities: An example from the Belgian part of the North Sea. Continental Shelf Research 28 (2008) 369—379
- De Jong, M., Baptist, M., Lindeboom, H., Hoekstra, P., 2015. Short-term impact of deep sand extraction and ecosystem-based landscaping on macrozoobenthos and sediment characteristics. Mar. Pollut. Bull. 97, 294–308. doi:10.1016/j.marpolbul.2015.06.002
- De Jong, M.F., Baptist, M.J., Borsje, B.W., Aarninkhof, S.G., n.d. Applicability of ecosystem-based design rules for sand extraction sites in the North Sea, Baltic Sea and Mediterranean Sea. Hydrobiologia.
- De Jong, M.F., Baptist, M.J., Lindeboom, H.J., Hoekstra, P., 2015. Relationships between macrozoobenthos and habitat characteristics in an intensively used area of the Dutch coastal zone. ICES J. Mar. Sci. 72, 2409–2422. doi:10.1093/icesjms/fsv060
- De Jong, M.F., Borsje, B.W., Baptist, M.J., van der Wal, J.T., Lindeboom, H.J., Hoekstra, P., 2016. Ecosystem-based design rules for marine sand extraction sites. Ecol. Eng. 87, 271–280. doi:10.1016/j.ecoleng.2015.11.053
- Lengkeek, W., Didderen, K., Teunis, M., Driessen, F., Coolen, J.W.P., Bos, O.G., Vergouwen, S.A., Raaijmakers, T., Vries, M.B. de, van Koningsveld, M., 2017. Building with North Sea Nature: eco-friendly scour protection.
- Marchal, P., Desprez, M., Vermard, Y., Tidd, A., 2014. How do demersal fishing fleets interact with aggregate extraction in a congested sea? Estuar. Coast. Shelf Sci. 149, 168–177. doi:10.1016/j.ecss.2014.08.005
- Newell, R.C., Seiderer, L.J., Hitchcock, D.R., 1998. The impact of dredging works in coastal waters: a review of the sensitivity to disturbance and subsequent recovery of biological resources on the sea bed. Oceanogr. Mar. Biol. an Annual Rev. 36, 127–78.
- Rijks, D. 2011. Ecological landscaping of sand extraction sites (HK2.1). Final report: design and creation of a landscaped pilot extraction site in the North Sea. Document number 08001-3-R-01-1-DCRI.
- Thierry, J.-M., 1988. Artificial reefs in Japan — A general outline. Aquac. Eng. 7, 321–348. doi:http://dx.doi.org/10.1016/0144-8609(88)90014-3