|Sandy Shores||Estuaries||Delta Lakes||Tropical Shelf Seas and Shores||Coastal Seas|
Coastal seas are relatively accessible, easy to exploit and highly productive. They are of great economical and societal importance to mankind as they deliver a wide variety of goods and services. This has resulted in the development of different human activities such as fishing, transport, oil and gas exploitation, aggregate extraction, navigation and wind farming. Combining such activities with each other and with other interests, such as nature protection, may be complicated. The challenge lies in combining them in a sustainable manner, with little to no impact on the marine environment, its species and habitats.
Coastal seas extend from the lower shoreface, where wave action has only limited influence on the sea bed, to the edge of the continental shelf. The dominant substrate consists of sand and mud, but other substrates do occur, including rocky areas and coral reefs.
Coastal seas are the most biologically productive parts of the ocean. They are complex physical systems, in which water, sediment and the seabed interact, often in a nonlinear way. This creates a diverse spectrum of habitats with a wide variety of organisms (Cox & Moore, 2005). The ecology of coastal seas is strongly linked to the water column, as planktonic life in the water column is the basis of the marine food chain. The mixing of water masses and the high nutrient influx provide ideal circumstances for a high productivity. Physical processes continuously shape the seabed, causing bedforms at various scale levels, such as megaripples, sand waves and tidal ridges. Sediment characteristics in coastal seas also vary according to the source. In temperate zones, continental shelves tend to be covered by terrigenous particles brought there by rivers at lower sea-stands. Polar shelves are covered by a variety of glacially derived sediments, dumped by glaciers or melting icebergs, whereas in the tropical reef region sediments include biogenic material composed of calcium carbonate, derived from the hard parts of organisms, including mollusks, echinoderms, algae and corals (Pinet, 1992).
As there is a strong relation between the sea bed characteristics and the local faunal community (Baptist et al., 2006), sea bed morphology in combination with the diversity in sea bed sediments and food production in the water column give rise to habitat gradients. While the coastal seas experience gradual changes (climate change and sea level rise), human activities affecting the geomorphological structure and hydrodynamic conditions will add to those changes and result in faster and more local adaptations of faunal assemblages.
In the context of Building with Nature we focus on the shallow sandy parts of the coastal sea where activities such as sand and gravel extraction take place, with the Dutch Exclusive Economical Zone as example. Aiming at interventions that optimize the use and provision of ecosystem services, we explore opportunities and alternatives for traditional sand extraction. Other existing coastal sea bed habitats, such as reefs and seagrass meadows, and the role they can play in infrastructure development are treated in the section Estuaries.
Coastal seas are highly productive, due to the input of nutrients and light that favour the growth of phytoplankton, the basis of the food web. The diversity of the biological community is related to habitat complexity. The dominant habitats types in coastal seas are sand and mud based and therefore geometrically not very complex. Habitat types on reef or rocky substrates have a more complex 3D geometry and support more diverse communities.
Coastal seas are characterized by highly variable physical processes, both in time and space (e.g. storms, currents, particle movement, climate change, sea level rise). Organisms adapt to this dynamic environment and the types of flora and fauna occurring at a certain place can be considered as a response to the prevailing biophysical interactions. The ecological value of coastal seas lies in the vertical (in the water column and at the sea bed) as well as in the horizontal (longshore, cross-shore). BwN-type interventions, however, will often take place on and in the seabed, influencing sea bed morphology and sediment characteristics. Such environmental changes may create particular habitat conditions.
When designing an intervention, it is essential to take into account the different scales and interactions involved. In the following chapters the principal system processes will be discussed, focusing on those related to sea bed characteristic. Understanding these processes and their response to human interventions is crucial to Building with Nature projects.
Ecology of coastal seas
As stated above, light availability and nutrient input from land into coastal seas lead to a high phytoplankton productivity. The phytoplankton, particularly algae, are a food source to many zooplankton species, which themselves serve as a food source to higher organisms. This continues stepwise through the food web by hundreds of fish species, a suite of benthic species, a small number of marine mammals and seabird species. Bird species however can occur in high numbers of individuals. Phytoplankton distribution seems to be higher near hydrographic fronts (areas where different water bodies meet) and in along-shelf currents, while high zooplankton abundance is often found over the slopes of tidal sandbanks (Pedersen et al., 2005 as mentioned in Simonsen et al., 2006). The distribution of fish larvae seems to exhibit patterns related to topography and water mass characteristics. Frontal areas usually have a high primary production, which result into a rich benthic community on the seafloor. Fish, seabirds and marine mammals are attracted to these areas to feed. Coastal seas are important for sea birds for e.g. foraging, wintering and moulting habitat. (Tens of) thousands of migratory birds, such as Common Scoter (Melanitta nigra), Grebe (Podiceps cristatus), Red-throated Diver (Gavia stellata) and Great Cormorant (Phalocrocorax carbo), coming from their breeding areas in the north, winter on the Dutch coastal sea (Leopold et al., 2011).
The organic material produced in the food web, once it has died, is broken down into nutrients by micro-organisms, such as bacteria. Abundance and diversity of the pelagic (i.e. living in the water column) flora and fauna depends on factors such as currents, wave action, salinity, temperature, light, nutrients and food available. Near and in the sea bed sediment composition and bed morphology are important factors controlling the suitability of an area for benthic organisms to settle and survive.
Coastal sea bed habitats
In coastal seas, a number of habitat types occur, including sandy to muddy plains, sandbanks, reefs, seagrass beds and rocks (Lindeboom et al., 2008, Salomidi et al. 2010). The differences in seabed characteristics have a direct influence on the density, biomass, distribution and diversity of benthic communities (Heip & Craeymeersch, 1995). For European waters, a habitat classification system is developed, named EUNIS. For further information look here.
Sandy habitats and sandbanks
In the North Sea most of the seabed habitats have a sandy substrate. Sandbanks in the North Sea are shaped by currents, but the timescale of their evolution is such, that they may also be relics from the ice ages. The grain size of the sand is largest in coastal areas and decreases towards the deeper parts due to the natural sorting of sediments coming from the rivers and entering areas with deeper water and lower current velocities. In the southern part of the North Sea large sandbanks are present, tens of kilometres in length and thousands of years old. They are covered with migrating sand waves upto ten metres high and megaripples up to a few metres high, with a grain size of typically 350 μm (Goudswaard et al., 2011). More to the north, further away from the coast, the sandy bottom flattens and the grain size decreases to fine sand (<125 µm). Sandy habitats are home to shellfish, worms, anemones, starfish, and other benthic species. The sand allows benthic species to burrow in the sediment and many feed on the organic material captured in the sediment or filtered from the passing water. Flatfish species such as plaice and sole are typical of sandy environments and seek their prey amongst the sand, preferring benthic species. Sand eels (Ammodytes sp.) are also typical for sandy habitats and are an important prey species for marine mammals and some seabirds.
Muddy habitats occur at places where small particles settle on the seafloor. In the central part of the Dutch North Sea, the combination of a relatively great depth (50 m) and weak currents make that waves do not impact the bottom. The 'Oyster Grounds' are an example of a muddy habitat. Muddy habitats accommodate a typical benthic community, often with large densities of filter feeders. After a disturbance, recovery of communities from muddy sand habitats is slower than that of sand communities (Dernie et al., 2003). One of the typical species of fine sediment is the burrowing shrimp ( Callianassa subterranea ), which creates complex burrows in the seafloor.
Biogenic reefs are structures built by marine organisms, vulnerable to destruction and hard to restore once they have disappeared. Only a few invertebrate species are able to build reefs, which are therefore restricted in distribution and size (Davies et al., 2001). Biogenic reefs in the temperate North Sea include biogenic structures formed by oysters, mussels or tubeworms. These organisms are known to add or alter physical, chemical and biological factors influencing their own habitat and are therefore often referred to as ecosystem engineers (Jones et al., 1994). Biogenetic reefs represent important habitats to a variety of marine organisms.
Seagrasses are mostly found in sheltered to very sheltered environments with variable salinity conditions (10-45 ppt). On sandy to muddy substrates, they may form extensive meadows. Both salinity and temperature tolerance differ between species. The depths at wich seagrasses occur (reportedly 0-60 m) may vary significantly between different species and regions, with the upper limit mostly determined by the exposure to wave action and the lower limit by light penetration. In addition to nutrient uptake from the water column, seagrasses can also take up nutrients from the sediment bed, a fact that contributes much to their ability to survive in nutrient-starved environments, more so than other primary producers (Salomidi et al., 2010).
Physical characteristics and drivers
Shelf seas are usually have a rather large cross-shore dimension (70-100 km on average) and represent the submerged edge of continent. Water depths are relatively shallow, up to 120-200 m. Energy for pick-up and transportation of solid particles is provided by tides and wind-generated waves and currents. In the North Sea, where the nearshore tidal range amounts to typically 2-4 m, tidal and storm-driven currents are about equally important. At a large spatial scale, the mean grain size of the sediment can serve as an indicative measure of the near-bed energy of the system. A fine-grained bed indicates low energy-conditions, a coarse bed high-energy conditions. The larger the particle, the stronger the water motion must be to transport the material. Generally speaking, the grain size changes from coarse sand nearshore, via muddy sand and sandy mud to muddy sediments further offshore.
Besides relics from the latest ice age, during which large mounds and ridges were formed that have survived millennia, sandy coastal sea beds exhibit a wide range of ‘live’ bedforms created by currents and wave action. The occurrence, size and dynamics of these’ bedforms depends on factors such as current velocity, wave conditions, water depth and sediment properties (e.g. Flemming, 2000). Offshore bedforms can be classified according to the flow mechanism that forms them: ripples and mega-ripples formed by separating near-bed flow, sand waves formed by acceleration-induced residual near-bed currents, sand banks and long bed waves formed by depth-averaged residual currents (Reineck et al., 1971; Knaapen et al., 2001). Such bedforms tend to migrate, some very slowly due to long-term residual sediment motion, others faster under the influence of instantaneous tidal currents. Tidal residual transport enable bedforms to migrate over significant distances without disintegrating.
Wind, Waves and Tides (short to medium drivers)
In the shallow coastal seas wind-, density- and tide-driven flows are usually controlled by bottom friction. In deeper waters, currents are controlled by pressure gradients, inertial forces (including the Coriolis-effect) and external forcings (tide-generating force, wind action).In the North Sea tide- and storm-driven currents can be equally important.
Depending on the latitude, the Coriolis effect also forces low-salinity river outflows to follow the coastline, on the Northern Hemisphere to the right, on the Southern Hemisphere to the left. This gives rise to secondary circulations which sustain a cross-shore salinity gradient. Strong winds, on the other hand, may spread low-saline surface water over large distances down-wind, thus giving rise to salinity stratification . Stratification, either salinity- or temperature-induced, will be breached by severe storms. This explains why in summer coastal seas are often stratified, whereas in winter they are not. Storminess is also the reason why in summer the bed is often covered by a veneer of soft fine material, whereas in winter the bed is clear and the fine material is in suspension.
Shallowness and friction give rise to deformation of the tidal wave and storm surges. This may lead to residual currents that are important transport agents for sediment, nutrients, larvae, contaminants, etc. Long-term variations, such as the 18.6 yearly nodal cycle in the tidal forcing, or the North-Atlantic circulation determining the storminess in the North Sea area, or El-Niño events in the Pacific, will give rise to long-term variability in tidal and storm surge effects.
Bottom disturbing fishery, especially beam trawling, is an important agent for sediment mobilisation. In the unprotected parts of the North Sea, every square metre is ploughed several times per year, thus mobilising the top layer of sediment and making especially the finer particles available for transport. Thus this activity disturbs the benthic community, decreases the food availability for sea birds in general and temporarily disturbs the habitat of sensitive species (e.g. divers, Scoter; Leopold et al., 2010). It also temporarily decreases the transparency of the water, resulting in a decrease in prey capture success of sea birds that use eye-sight for food location (Baptist & Leopold, 2010). Further, beam trawling influences the composition of the bed and the transport of fines and the nutrient and contaminants adhering to them.
On top of this, incidental activities, such as sand mining or other dredging works, may release a certain amount of fines, which are subsequently transported in plumes, resulting in a decreased prey capture success of diving sea birds.
In shallow waters, shipping may also bring sediment into suspension via their propeller wash. One example is the turbidity of the Singapore coastal waters, where sediment from a fluffy layer on the bottom is repeatedly brought into suspension by the many ships sailing to and from Singapore harbour.
Sea level rise, temperature rise (long term drivers)
Sea level rise itself does probably not impact the coastal seas, since the sea level rise is relatively small compared to the average water depth. In contrast, temperature rise may result in shifting habitat zones. They may also may give rise to changes in the ocean current pattern, which will have profound impacts on the ecosystem by affecting the distribution of species and with that also the food chain. Changes in zooplankton and fish communities due to temperature rise have already been noticed in the North Sea (Ter Hofstede et al., 2010). Another effect of climate change, ocean acidification, resulting from the uptake of CO2, may hinder the growth of shellfish and other species that use calcium for their shells.
For further reading on physical characteristics and drivers of coastal seas look here.
Main issues in coastal seas are economic issues and international conventions, often on ecological protection. Working in coastal seas means that laws, regulations, policies, treaties and stakeholders have to be considered on a national and international level. Reference is made to ‘Current international policies in European coastal areas’, which describes the 'policy landscape' that is relevant to working in European coastal seas, including the North Sea.
The spatial management of a coastal sea (including the exclusive economic zone) is the responsibility of the national government involved, though international laws and conventions have to be respected. The government develops management plans, issues permits for offshore activities (wind farms, gas/oil exploitation) etc. The European Directives are implemented through national legislation. In the implementation of measures other stakeholders are involved, including private parties (wind/oil/gas/other maritime industry, nature conservation organisations) and fisheries organisations.
For more information on governance processes see Governance.
Natural ecosystems in general provide a multitude of resources and processes from which man benefits. Collectively, these benefits are known as ecosystem services. Ecosystem services have been discussed for decades. In 2004 the United Nations Millennium Ecosystem Assessment (MEA) divided the ecosystem services in four categories; production services, such as the production of food and water; regulating services, such as the control of climate and disease; cultural services, such as spiritual and recreational benefits and supporting services, such as nutrient cycles (TEEB). For further reading on ecosystem services see here.
The ecosystem services provided by coastal seas are the following:
These are natural resources, including products that a society gets from ecosystems. Examples of products that are obtained from coastal seas are:
- food provision, e.g. fish, shellfish and other seafood for human consumption;
- raw materials, e.g. sand and gravel for construction and shore nourishment, marine organisms or their residues for other purposes than human consumption;
- transportation routes, e.g. shipping lanes;
- renewable energy, e.g. off shore wind farms, tidal energy and wave energy.
These are benefits that arise from how a system regulates individual processes, resources and entire ecosystems. Benefits that are obtained from the regulation of coastal seas are:
- air quality and climate regulation, e.g. regulation of carbon fluxes (CO2), regulation of volatile organic halides, ozone, oxygen and dimethyl sulphide;
- coastal protection against flooding and erosion, if sufficient sediment is transported from the coastal sea to the coast;
- water quality regulation, e.g. removal of pollutants through storage, burial and recycling.
These are non-material benefits people enjoy. Examples of non-material benefits obtained from coastal seas are:
- tourism and recreational activities, e.g. sailing;
- research and education, e.g. as subjects of scientific research;
- aesthetic value, e.g. the appreciation of coastal seas make them an attractive environment to live on and the aesthetic value may manifest itself in higher prices of real estate close to the coast;
- reflection and spiritual enrichment.
These are processes that are required to provide the other three categories to society and other life on earth. Supporting services are necessary for the production of all other ecosystem services, but do not yield direct benefits to humans (e.g. to sustain critical ecosystem processes that determine the vitality and resilience of ecosystems). Examples are:
- sediment transport, e.g. currents may cause the relocation of sandbanks;
- photosynthesis, chemo-synthesis and primary production, e.g. the production of oxygen by photosynthesis of organic compounds;
- transportation of nutrients, fish larvae and temperature/water masses, resulting in e.g. zonation in productive processes and in the ecological exchange between different habitats;
- reproduction, spawning, nursery, foraging and resting habitats for different species such as flatfish, seabirds and marine mammals;
- resilience to climate variability and incidents, climate change, pests and also human interventions and impacts, mainly vested in the complex interaction between the various species and their environment.
Building with Nature
Traditionally, human activities in the coastal seas use only a few of the ecosystem services provided - more specifically one or more of the production-, regulating- or cultural services -, paying little attention to the supporting services. These activities therefore often undermine the sustainability of the system. BwN adopts a new approach, implying a socio-economically profitable and environmentally sustainable use of the services the system provides. The basic idea is that natural processes can be used and stimulated to achieve an optimal and sustainable fit of a man-made project into its environment.
Exploration of BwN opportunities for coastal seas focuses on the potential of sand extraction areas and offshore wind farms to enhance biodiversity and biomass. In this way multiple ecosystem services will be produced. In the following section we explain the practical implementation of these concepts in further detail.
In this part, we describe two types of Building with Nature opportunities for coastal areas:
- Ecological landscaping of dredging areas / sand extraction sites
- Creation of artificial reefs on off shore windfarms
1. Ecological landscaping of dredging areas / sand extraction sites
Context in the Netherlands
In 1990 the Dutch Government adopted the national policy of Dynamic Preservation (MIN V&W,1990) which aimed at a sustainable preservation of safety against flooding, as well as values and functions in the dune area (Aarninkhof et al 2010). Acknowledging sand as the carrier of all functions (NSS, 2006), the principal intervention procedure is nourishment of sand, making optimal use of natural processes and leaving room for natural dynamics. This strategy of coastal management fits in well with nature and dynamics of the sandy coast of the Netherlands. The strategy is a form of adaptive management: monitoring makes it possible to respond to local erosion, sea level rise and infrastructural developments.
The sand needed is mostly dredged/extracted at sites in the North Sea, as Dutch legislation does not allow extraction within the -20 m depth contour. It is expected that the present yearly volume of 12 million m3 of sand needed to maintain the coast will rise in the near future. For every millimetre of sea level rise, an extra 7 million m3/yr has to be supplied. On top of the sand extraction for coastal maintenance purposes another 13 million m3 of sand is extracted in the Netherlands for industrial use and infrastructural development. Moreover, incidental projects may require significant amounts of sand. For the extension of Rotterdam Harbour, for instance, 200 million m3 was extracted from the North Sea bed.
Marine dredging is widely considered as an activity that is stressful to the environment and many studies have described its impacts (Newell at al., 1998; Van Dalfsen et al., 2000; Boyd et al., 2005; De Backer et al., 2011). This has resulted in environmental policies that set restrictions and limitations to dredging operations. When the volumes extracted increase a larger impact in space and time is to be expected. As there is a strong relation between the local sea bed characteristics and the associated community (Baptist et al., 2006), changes in the geomorphological structure are expected to result in adaptations of the faunal assemblages. So far, sand mining policies aimed at recovery or restoration of a flat seabed habitat.
Depending on the amount of material extracted Dutch legislation distinguishes between small-scale or regular extractions (less than 10 million m3) and large-scale extractions (more than 10 million m3) (RON2). Only for large-scale extractions dredging is allowed to lower the original seabed more than 2 m.
The expected increase in the amounts of sand extracted, combined with the legislation on the dimensions of the extraction sites, offers unique opportunities to develop these sites for more than just extraction of sand.
Building with Nature approach
Taking an ecosystem-based approach, one may turn the supposed ecological threats of sand mining into sustainable opportunities. BwN is investigating ways to landscape the seabed in such a way that the ecosystem can optimally benefit when the sand extraction is finished,.
The concept of ecological landscaping of sand borrow areas is inspired by terrestrial infrastructure projects, where ecological engineering has almost become a standard component of licensing procedures for sand and gravel mining operations (Aarninkhof et al. 2010). The potential post-dredging value of a marine extraction site is seldom considered. As a result, opportunities that could improve or add to the overall sustainability of the dredging project are missed. Recovery and recolonization of a dredged area after dredging are the result of complex interactions between the dimensions of the extraction and the capability of organisms to reach and adapt to the new environment (Robinson, 2005).
The overall aim of ecological landscaping is to make the extraction site attractive to a variety of benthos and flora that in turn attract fish, mammals and birds. This is done by creating ideal settlement and habitat conditions by way of different bed forms and/or combinations of sediment characteristics. It involves the realisation of bed level gradients and other morphological features in newly dredged areas (Van Dalfsen and Aarninkhof, 2009). This will enhance the speed of recolonisation and restoration and increase the biodiversity (volume and type) in the borrow area. Analysis of the ecological samples taken from a landscaped extraction site showed that inside the dredged area 4 to 5 times as much fish was found as in the vicinity outside (Aarninkhof et al., in prep). Although this observations requires further analysis and interpretation, ecological landscaping of extraction sites with such positive environmental effects can facilitate social and political acceptance of future dredging works in the marine environment, thus also accelerating licensing procedures and project realisation.
2. Creation subsea habitat near structures
Within coastal seas the construction of artificial reefs is often used to enhance the availability of several ecosystem services such as habitat provision (for nature development), fisheries and/or recreational possibilities (e.g. angling, diving). The materials used in constructing artificial reefs differ across the globe (e.g. concrete, steel, fiberglass, recycled materials, tyres, airplanes, ships) (Pickering et al., 1999). See examples of artificial reefs.
The use of artificial reefs, however, is debated. On the one hand artificial reefs may enhance the occurrence of species as they can provide in refuge and feeding habitat. Which in turn may result in fishery enhancement and attractive areas for recreational diving. These activities may generate millions of euros/dollars in revenue. On the other hand there are concerns about the potential effects of such a reef on the surrounding environment (e.g. leaking of pollutants, roaming of broken-off debris etc.). Few countries have explicit legal provisions governing artificial reefs (Pickering et al., 1999). The application and design of artificial reefs has been focus of research for several years (Baine, 2001; Sherman et al. 2002; Miller, 2002).
Building with Nature opportunities lie in designing artificial reefs without the presently known disadvantages such as leaking pollutants etc. For more information, see Artificial reefs.
The number of off-shore wind farms within coastal seas is increasing rapidly. Recent research showed that the turbine foundations and surrounding rocks may provide a new habitat for organisms living on the sea bed such as anemones, mussels, etc. Thus they contribute to increased biodiversity (Lindeboom et al. 2011). Building with Nature opportunities lie in optimal designing the turbine foundation and surrounding rocks to enhance biodiversity.
Within the Building with Nature program the Holland Coast case focuses on the development of sustainable strategies for coastal maintenance by making optimal use of natural processes, exploring the opportunities for nature development and other functions next to coastal protection and to create space for natural system dynamics. In order to meet the needs for coastal protection via sand nourishment, the part of the case concerning the coastal sea focuses on the possibilities of Building with Nature strategies for large-scale sand mining.
Maasvlakte 2 sand mining
Deeper sand extraction, up to 20 m below the original sea bed, would roughly result in a tenfold smaller affected area, which was expected to have less negative impacts on the ecosystem. Sand extraction involves the direct removal of benthos from the borrow site and burial and smothering of benthic fauna in the vicinity or even further away. Also the sediment characteristics in the area may be altered. After extensive research, taking into account sediment characteristics, hydrodynamics, shipping intensity and positions of cables and pipelines, extraction sites were designated at some 11 km distance from the construction site, which is relatively close (website Maasvlakte 2, De Jong, 2009)(Dutch).
Building with Nature type solutions aim to create added value through integral consideration of all functions and values of the system considered. In order to apply this concept to this sand mining project, it was decided that the borrow pit would not simply be a hole of 20 m deep, but that it would be shaped in such a way that a variety of slopes and depths would be created, providing a diverse environment that would not suffer from oxygen depletion and that would provide for a variety of habitats.
The Case 'Ecological Mining Pit' in the BwN-programme focuses on reducing the effect of sand extraction by ecological landscaping of the borrow area. It is hypothesised that higher habitat heterogeneity will result in a faster recolonisation, higher biodiversity and productivity of benthic and demersal fauna and overall economic benefits.
Within the Building with Nature program, the following aspects of the Maasvlakte 2 borrow area are being studied:
- case Ecological Mining Pit aiming to explain the morphological and ecological evolution of the Maasvlakte 2 mining pit in more detail;
- monitoring data from the ecological mining pit and silt dynamics North Sea (ADD LINK);
- modelling the ecological potential of sand extraction; (ADD LINK)
- eco-morphological design of landscaped mining pits (ADD LINK);
- eco-morphological design of mega nourishments (ADD LINK).
The demand of offshore wind farms is rapidly increasing worldwide, as they provide a source of renewable energy which is highly in demand, both economically as politically (Stenberg et al., 2011). Offshore wind farms consist of multiple wind turbines often placed in relatively shallow waters (20-40 m depth) because of engineering and economic constraints (Stenberg et al., 2011). The turbines are sunk into the seabed and their foundation is protected from erosion by a filter structure, often consisting of rock. Moreover, the underwater part of the turbine itself turns out to be another hard substrate for a variety of species. Thus the placement of offshore wind farms introduces hard substrate and higher substrate heterogeneity, sometimes resulting in a higher biodiversity.
A Danish study on the wind farm Horns Rev 1 (Denmark) showed that wind farms have a positive impact on fish abundance and fish community structures by increasing habitat heterogeneity and through the exclusion of trawling activities within the wind farm (Stenberg et al., 2011). The introduction of hard bottom substrate (the turbine itself and the bottom protection structure around it) resulted in higher fish species diversity close to each turbine. The stone structures resemble artificial reefs and attract fish species that prefer them.
Similar results were found from recent research in the Netherlands (Lindeboom et al., 2011) which focused on the short-term impacts of the first offshore wind farm in The Netherlands (Offshore Windfarm Egmond aan Zee, OWEZ) on the surrounding environment. The benefits of the introduction of new, hard substrate were studied, together with possible adverse impacts (Lindeboom et al., 2011). The study concluded that the offshore wind farm acts as a new type of habitat, resulting in a higher biodiversity of benthic organisms, with possibly an increased use of the area by the benthos, fish, marine mammals and some bird species. The increase of fish and marine mammals within the wind farm area are expected to be related to the turbines, the bed protection and the absence of fisheries. Investigations into the effect of the moving rotor blades resulted in the conclusion that several bird species seemed to avoid the wind farm, while others were indifferent or even attracted (Lindeboom et al, 2011).
The full scale experiments and historic cases that are part of the Dutch Coast sub-programme of building with Nature show that the following conditions on the seabed are favourable for biodiversity and biomass in coastal seas:
- a variety in grain size of the sediment of the seabed
- variety in shear stress originating from variation in the sea bed level
- stable structures (e.g. sand ridges)
- oxygenated water. In deep sand mining pits with steep slopes, the water becomes stagnant and anoxic.
Based on these ecological conditions, we could formulate the following BwN design guidelines for landscaping dredging areas / sand extraction sites.
- When investigating the sediment properties in the top 20 m of the seabed at the site, also consider the grain size variation. Trim habitat and biodiversity expectations to this variation.
- Create variation in the seabed level. Design a hilly underwater landscape.
- By maintaining or creating stable structures (e.g. sand ridges) in the underwater landscape. Herewith take into account:
- if the environmental conditions after extraction are favourable to the formation of stable bedforms, either sandwaves or tidal ridges;
- the naturally stable crest direction, perpendicular to the main current direction if they have the size of sandwaves, under a sharp angle with the main tidal current axis if they have the size of tidal ridges;
- the distance from crest to crest, which should corresponds with that of the naturally stable bedforms of the relevant category (sandwaves or tidal ridges);
- that (creating) variety in the level of stable structures is beneficial, as it creates variety in shear stress;
- the size of the extraction site, which should be large enough to realize a number of stable structures.
- By designing the slopes of sand borrow areas in such way (typically 1:7 or milder) that the current stays attached to the bed and refreshes the water, so as to avoid anoxic conditions.
- By maintaining or creating stable structures (e.g. sand ridges) in the underwater landscape. Herewith take into account:
Also some design guidelines can be derived from limitations of dredging operation practices:
3. While dredging, sail preferably parallel to the current. This favours tidal ridges as stable bedforms, but their size only fits into very large borrow areas.
4. Chose a dredging / extraction area that is large enough to be able to manoeuvre the dredgers. This guideline is in accordance with the guideline 2a-v.
5. Tune the excavation depths in the seabed landscape to the characteristics of the dredging fleet available at the site.
- Use numerical models to predict the behaviour of the bedforms in the extraction area and make sure that they are relatively stable allowing sufficient time for the ecosystems to develop. Simple stability models can be used for a first-order assessment of the appropriate bedform dimensions.
- Natural tidal ridges or sand waves are usually quite stable, as they migrate only approximately 1 to 10 m per year (under typical North sea conditions).
From a dredging operation point of view, the following was learned.
- Include knowledge on bathymetry, sediment properties, local currents and tidal windows as an integral component in the preparation and planning of the project. This allows contractors to design a cost-effective dredging plan.
- Carry out the ecological landscaping during the extraction process, thus avoiding extra equipment mobilisation and minimizing extra work and costs.
- Involve ecologists familiar with the situation at the site in an early stage, in order to determine what habitat development is desired, how it can be stimulated and which physical parameters are of importance to achieve ecologically optimal landscaping.
- Type, spacing and height of the bedforms should be tailor-made to the area and should consider the naturally occurring bedforms/morphology (taking into account the new situation, excavation depth and preferred bed form height) and the ecosystem thereon.
- Make the description of the project’s nature objectives as SMART as possible. Which added ecological value is seabed landscaping at this particular location expected to yield?
- Actively involve NGO’s in the project preparation and design. It was experienced that NGO’s are interested to be involved in these type of projects and that their involvement can yield a broad societal support.
- Involve contractors early on in the design phase so that the final decision fits easily into the dredging plans and methods, and can therefore be created cost-efficiently.
- If you position the project as a pilot or showcase, assure in advance the required finances and conditions to be able to carry out a monitoring scheme before, during and after construction.
- Continuously inform stakeholders of the progress and new ideas etc. (permanent liaison).
- Aarninkhof S.G.J., J.A. van Dalfsen, J.P.M. Mulder & D. Rijks, 2010. Sustainable development of nourished shorelines, Innovations in project design and realization. PIANC MMX Congress Liverpool, UK.
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- The North Sea, photo: Robbert Jak/IMARES.
- Sea anemone, photo: Oscar Bos/IMARES.
- Sandy habitat in the Mediterranean, Italy. Photo: Oscar Bos/IMARES
- American razorshell Ensis directus. Photo: Peter van der Kamp/IMARES
- Great skua. Photo: Oscar Bos/IMARES
- Horse mussel can form abiotic reefs. Photo: Henk Heessen/IMARES
- Former oyster reefs in the Dutch North Sea. Olsen, O.T., 1883. The piscatorial atlas of the North Sea, English and St. George's Channels, illustrating the fishing ports, boats, gear, species of fish (how, where, and when caught), and other information concerning fish and fisheries, Taylor and Francis, London, UK.
- Exaggerated 3-D depth profile of the Dutch North Sea. Lindeboom HJ, Dijkman EM, Bos OG, Meesters EH, Cremer JSM, De Raad I, Van Hal R, Bosma A (2008) Ecologische Atlas Noordzee ten behoeve van gebiedsbescherming, Wageningen IMARES
- Sediment thickness, NGDC/NOAA.
- Shipping and wind energy. Photo: Robbert Jak/IMARES
- A variety of benthic life. Photo: Michel Trommelen/IMARES
- A tallship in the Dutch coastal waters.Photo: Oscar Bos/IMARES
- Maasvlakte 2. Photo: Oscar Bos/IMARES