Horns Rev 2 Offshore Wind Farm
Published: 31. July 2006
Prepared: Simon B. Leonhard Editing: Gitte Spanggaard
Checked: Bjarne Moeslund Artwork: Kirsten Nygaard
Approved: Simon B. Leonhard Cover photos: ©Elsam Engineering/Bio/consult
Christian B. Hvidt Jens Christensen Simon B. Leonhard
Christian B. Hvidt Jens Christensen Maks Klaustrup Simon B. Leonhard
English review: Matthew Cochran
© No part of this publication may be reproduced by any means without clear reference to the source.
List of contents
Sammenfatning (in Danish) ...3
1 Introduction ...5
1.1 Background ...5
1.2 Introduction ...5
2 Horns Rev...7
2.1 Topography and sediment ...7
2.1.1 Geology and geomorphology ...7
2.2 Hydrography ...7
3 The wind farm area ...9
3.1 Description of the wind farm area...9
3.2 The turbines...10
126.96.36.199 Gravitation principle ...11
188.8.131.52 The monopile principle ...11
3.2.2 The scour protection...11
3.2.3 Subsea cables ...11
184.108.40.206 Electromagnetic fields...11
4.1 Assessment methodology...13
4.2 Screening survey ...14
4.2.1 Sample handling...15
4.2.2 Data analyses...16
220.127.116.11 Sediment characteristics...17
18.104.22.168 Species composition...17
4.3 Assessment of cumulative impacts ...18
4.4 Designation of reference sites and areas ...18
5 Existing benthic communities ...19
5.1 Sediment characteristics...19
5.2 Benthic communities...20
5.2.1 Population ecology and distribution at Horns Rev ...20
5.2.2 Species distribution pattern in the wind farm area...22
5.3 Influence of fishing activity ...28
6 Sources of impacts ...30
6.1 Main impacts ...30
6.1.1 Noise and vibrations...30
7 Assessments of effects ...32
7.1 Phases 1 & 2 – pre-construction and construction ...32
7.1.1 Noise and vibrations...32
7.1.2 Suspension and redistribution of sediments ...33
7.1.3 Loss of seabed area and change in substrate type ...34
7.2 Phase 3 – Operation ...34
7.2.1 Introduction of hard substrate ...35
7.2.2 Physical presence of the wind turbines ...37
7.2.3 Noise and vibrations...38
7.2.4 Electromagnetic fields...39
7.4 Cumulative effects ...40
7.4.1 Preconstruction and construction ...41
7.5 Mitigation ...42
8 Conclusions ...43
9 References ...45
Appendix 1. List of positions ...50
Appendix 2. Species list. ...51
As part of the Danish Governmental Plan for Renewable Energy, permission was given in 2005 to carry out an Environmental Impact Assessment for the establishment of a new offshore wind farm at Horns Rev.
Knowledge about potential impacts on the marine benthic communities from the establishment and operation of offshore wind farms is available due to the demonstration projects carried out at Horns Rev 1 Offshore Wind Farm. However, provision of supplementary information was found necessary regarding the existing habitats and benthic communities, which include benthic vegetation and invertebrate communities, in the designated wind farm area.
Two alternative sites are designated for the wind farm at Horns Rev 40 km west of Blåvands Huk. Both sites cover an area of approximately 35 km2 and the water depths range from 5-15 m. The sediment in the wind farm areas display large variability and surface sediments consist of pure medium to coarse sand that is constantly reworked by waves and currents. Along the top of the reef and in shallower parts that are strongly exposed to waves, the sediment is more sorted compared to deeper parts where the sediment is coarser due to exposure to strong currents. Bedforms of small and large sand riffles caused by wave action and evidence of sand transport are found all over in the area. In the northern part of the designated sites, the sediment is generally finer closer to the reef. No unambiguous relationship between the depth regimes and the sediment structure is found at the different sampling sites in the wind farm area.
No vegetation, and no rare and endangered species, is found within the designated wind farm areas. The variations of the benthic infauna composition and community structure reflect the heterogeneous sediment in the area. In general, the benthic infaunal community in the Horns Rev area can be characterised as the Goniadella-Spisula or the shallow Venus community. These two communities are commonly found at sandbank where the seabed consists of relatively coarse sand and hydrographical conditions are turbulent. In the northern part of the designated wind farm area, the sediment generally shows a more uniform character with finer sand. In such areas, a more typical Venus community is found. Even within short distances, differences can be found in the community structure resulting in subdivisions of the main communities inside the designated wind farm areas, which reflects the character species’ preferences for different sediment characteristics.
In the Horns Rev area and the wind farm areas, more epifaunal species can be found including the brown shrimp (Crangon crangon), which is object of commercial fishing.
The benthic communities in the Horns Rev area are generally influenced by trawling and dredging activities. Dredging for the character clam species (Spisula solida) and trawling for sandeels are the main fishing activities in the area.
The wind turbines will be founded by use of either monopiles or gravitation foundations.
The main impacts on benthic communities from the activities in the pre-construction, construction, operation and decommissioning phases are considered equal for the two foundation types. The sources of impact that are similar to both types of foundations include noise generated from piling activities. However, additional sources of impact
from dredging activities related to the establishment of gravitation foundations include increased smothering and suspended sediments.
In the pre-construction and construction phases, it is expected that noise and vibrations from pile driving activities may have a temporary and negligible local impact on the benthic communities and a very local and negligible destructive effect on infaunal species.
Smothering and increased suspended sediment from dredging activities is expected to have a temporary local negligible effect on benthic communities due to the general loss of fine sand. Benthic communities generally show a high tolerance to smothering with a presumed high recovery rate.
Loss of seabed with native benthic communities and change in substrate type during construction and operation is less than 0.2% of the total wind farm area. The change of habitat type and change from sandy infauna communities to epifouling communities are expected to be local and of minor significance. The deployed hard substrate will rapidly be colonised with algae and invertebrates, which is known to increase the biodiversity in the wind farm area. The succession will increase the diversity over a period of 5-6 years after deployment of the hard substrates, at which time the communities are expected to reach climax.
The physical presence of the wind turbine foundations will have a very local, minor, but permanent effect on the benthic community structure due to changes of the hydrodynamics near the turbines. During operation, significant effects from noise and vibration are not expected. Effects from electromagnetic fields are considered negligible, although migrating crabs, believed to be sensitive to the Earth’s magnetic fields, may be affected.
Effects during decommissioning are generally considered as the same during construction but in the reverse order.
In the operation phase, cumulative impacts are be expected as a consequence of reduced trawling activities inside the wind farm sites, which will be beneficial to benthic communities by enabling very sensitive species to establish and all species to mature more undisturbed. The introduction of more consolidated substrates from more offshore wind farms may generate a cumulative effect by introducing higher species richness and faster colonisation of specific and potentially vulnerable species to newly deployed foundations. No cumulative effects on benthic communities are expected from simultaneous sand and aggregation activities and construction activities.
No specific mitigation measures are necessary because rare or endangered species are not found and only minor impacts from construction, operation and decommissioning activities are expected on the benthic communities inside the designated wind farm areas.
Sammenfatning (in Danish)
Som del af den danske regerings plan for vedvarende energi, blev der i 2005 givet tilladelse til at udføre en VVM-undersøgelse (Vurdering af Virkningen på Miljøet) af etableringen af en ny havvindmøllepark på Horns Rev.
Viden om effekter på bunddyrsamfundene af etableringen af havvindmølleparker er tilgængelige fra demonstrationsprojekter gennemført i forbindelse med anlæggelsen af Horns Rev 1 Havvindmøllepark. Det var imidlertid nødvendigt at indhente supplerende information om de eksisterende habitater og bentiske samfund, omfattende vegetation og invertebrat-samfund, i det planlagte mølleparkområde.
Der er udpeget to alternative områder til placering af vindmølleparken. Begge de udvalgte områder dækker 35 km2, og vanddybderne varierer mellem 4-15 m. Der er stor variation i sedimentets sammensætning, med overfladesedimenter bestående af rent mellemkornet til groft sand, som konstant omfordeles af bølger og strømme. Langs den øverste del af revet og i mere lavvandede områder, der er stærkt påvirkede af bølger, er sedimentet bedre sorteret end i dybere områder, hvor sedimentet er grovere som følge af stærke strømme. En bundstruktur med små og store sandribber, dannet af bølgebevægelser og som tegn på sandtransport, findes overalt i området. I den nordlige del af de planlagte mølleområder er sedimentet generelt finest tæt på revet. Der er ikke fundet nogen klar sammenhæng mellem dybdeforholdene og sedimentstrukturen i de forskellige prøvetagningsområder i mølleparkområdet.
Der findes ingen vegetation eller truede arter i de udpegede mølleområder. Variationerne inden for de bentiske faunasamfund reflekterer generelle forskelle i områdets sedimentforhold. Generelt kan det bentiske infaunasamfund karakteriseres som Goniadella-Spisula samfundet – eller Venus samfundet på lavt vand. Disse to samfund er almindelige på sandbanker, hvor havbunden består af relativt groft sand, og forholdene er turbulente. I den nordlige del af det udpegede mølleparkområde har sedimentet generelt en mere ensartet karakter med finere sand. I sådanne områder forekommer et mere typisk Venus samfund. Selv inden for korte afstande findes der forskelle i samfundsstrukturen, resulterende i underinddelinger af hovedsamfundene i de planlagte mølleparkområder, hvilket afspejler artsspecifikke præferencer for forskellige sedimentforhold.
Ved Horns Rev og i mølleparkområderne forekommer der flere arter tilhørende epifaunaen, herunder hesterejen (Crangon crangon), som fiskes kommercielt i området.
De bentiske samfund i Horns Rev-området er generelt påvirkede af trawl- og skrabeaktiviteter. Skrab efter muslinger, især karakterarten (Spisula solida), og trawling efter tobis er de væsentligste fiskeri-aktiviteter i området.
Møllerne vil blive funderet på enten monopæle eller gravitationsfundamenter. De mest betydende påvirkninger af de bentiske samfund fra aktiviteter i præ-konstruktionsfasen, konstruktionsfasen, driftsfasen og nedbrydningsfasen forventes at være af samme omfang for begge typer af fundamenter. De påvirkninger, der er ens for de to fundamenttyper, inkluderer støj ved ramningsaktiviteter. Andre påvirkninger fra graveaktiviteter, herunder øget sedimentering af suspenderet sediment og suspenderet sediment, er i højere grad forbundet med etableringen af gravitationsfundamenter end med etableringen af monopæle.
I præ-konstruktionsfasen og konstruktionsfasen forventes det, at støj og vibrationer i forbindelse med ramningsaktiviteterne kan medføre forbigående og ubetydelige lokale påvirkninger af de bentiske samfund og en meget lokal og ubetydelig ødelæggende effekt på infauna-arterne.
Aflejring af suspenderet sediment og øget suspension af sediment pga. graveaktiviteter forventes at få forbigående, lokale og ubetydelige effekter på bunddyrssamfundene som følge af et generelt tab af fint sand. Bentiske samfund har normalt en høj tolerance over for aflejringer af sediment, og en hurtig normalisering af forholdene kan derfor forventes.
Tabet af havbund med naturligt forekommende bentiske samfund og forandringer i substrattype under konstruktionen og i driftsfasen udgør mindre end 0,2% af det samlede mølleparkområde. Ændringen af habitattypen og ændringen fra infaunasamfund på sandbund til påvækstsamfund på sten forventes at være lokal og af mindre betydning. Det udlagte hårde substrat vil hurtigt blive koloniseret med alger og invertebrater, hvilket generelt vil øge biodiversiteten i mølleparkområdet. Der forventes at succession i de introducerede påvækstsamfund vil øge diversiteten over en periode på 5-6 år efter, at det hårde substrat er blevet udlagt, hvorefter et klimakssamfund vil udvikles.
Den fysiske tilstedeværelse af vindmøllefundamenter vil have en meget lokal, ubetydelig, men permanent effekt på de hydrodynamiske forhold tæt på fundamenterne, hvilket vil påvirke den bentiske samfundsstruktur. I driftsfasen forventes ingen betydelige effekter af støj og vibrationer. Effekter af elektromagnetiske felter vurderes at være uden betydning for bunddyrsamfundene, selvom migrerende krabber, der menes at være følsomme over for Jordens magnetiske felter, potentielt kan blive påvirket.
Effekter i nedbrydningsfasen anses generelt for at være de samme som i konstruktions- fasen, men i omvendt rækkefølge.
I driftsfasen kan kumulative effekter muligvis forekomme som konsekvens af reduceret trawl-aktivitet i og mellem mølleparkerne på Horns Rev. Dette kan være til gavn for bunddyrsamfundene derved, at meget følsomme arter får mulighed for at etablere sig, og alle arter vil uforstyrret kunne opnå reproduktionsdygtig alder. Udlægning af faste substrater i flere havvindmølleparker kan muligvis forårsage kumulative effekter ved at medføre højere artsrigdom og hurtigere kolonisering af specielle og potentielt sårbare arter til nyanlagte fundamenter. Der forventes ingen kumulative effekter på bentiske samfund som følge af konstruktionsaktiviteterne og samtidige sand- og ralindvindings- aktiviteter i området.
Særlige afværgeforanstaltninger vil ikke være påkrævet, fordi sjældne eller truede arter ikke findes i området, og fordi der kun forventes mindre påvirkninger af bunddyrsamfundene i de udpegede mølleparkområder i forbindelse med konstruktion, drift og nedbrydningsaktiviteter.
In 1996, the Danish Government passed a new energy plan, ”Energy 21”, that states the need to reduce the emission of the greenhouse gas CO2 by 20% in 2005 compared to the 1988 emissions. Energy 21 also sets the scene for further reductions after 2005 (Danish Environmental Agency, 1996). The means to achieve this goal is to increase the use of wind power and other renewable energy sources from 1% of the total energy consumption in 2005 to approximately 35% by 2030.Offshore wind farms are planned to generate up to 4,000 MW of energy by the year 2030. In comparison, the energy generated from offshore wind farms in January 2004 was 426 MW (www.offshorecenter.dk).
In 1998, an agreement was signed between the Danish Government and the energy companies to establish a large-scale demonstration programme. The development of Horns Rev and Nysted Offshore Wind Farms was the result of this action plan (Elsam Engineering and ENERGI E2, 2005). The aim of this programme was to investigate the effects on the environment before, during and after the completion of the wind farms. A series of studies on the environmental conditions and possible impacts from an offshore wind farm were undertaken for the purpose of ensuring that offshore wind power does not have damaging effects on the naturally occurring ecosystems. These environmental studies are of major importance for the establishment of new wind farms and extensions of existing offshore wind farms like Nysted and Horns Rev.
Prior to the construction of the demonstration wind farms at Nysted and Horns Rev, a number of baseline studies were carried out in order to describe the environment before the construction. The studies were followed by investigations during and after the construction of the wind farms and assessments were made of the eventual environmental impacts from the wind farms.
On August 25, 2005, the Danish Energy Authorities gave permission to carry out an Environmental Impact Assessment (EIA) at Horns Rev with particular reference to constructing a new offshore wind farm at the site, Horns Rev 2 Offshore Wind Farm. The wind farm is planned to operate in 2009 and the installed capacity of this wind farm will be 200-215 MW, equivalent to 2% of the Danish consumption of electricity.
During the demonstration programme at Horns Rev 1 Offshore Wind Farm, monitoring was performed on the impact to benthic communities from introducing artificial hard substrates into the pre-existing habitats of pure sand.
The present report comprises an assessment for the potential impacts from the establishment of Horns Rev 1 Offshore Wind Farm on benthic communities. The assessment will be carried out by describing the basic conditions of the area and experiences from the demonstration projects in Horns Rev 1 Offshore Wind Farm.
A provision of supplementary information on pre-existing habitats and benthic communities in the wind farm area was necessary in order to assess the impact from the establishment of the wind farm. For this report, benthic communities include benthic vegetation and benthic invertebrates. Therefore, samples providing information of benthic communities and sediment parameters were collected during a field survey in January 2006.
Impacts during the pre-construction phase, the construction phase, the operational phase, and the decommissioning phase of the turbines will be assessed, including the cumulative or the combined impacts from already established and further developed offshore wind farms at Horns Rev.
Only impacts from wind farm construction and establishment inside the wind farm area are considered. The effects from laying the cable in the cable trace are excluded from this assessment.
2 Horns Rev
Horns Rev is an extension of Blåvands Huk extending more than 40 km to the west into the North Sea. Horns Rev is considered to be a stable landform that has not changed position since it was formed (Danish Hydraulic Institute, 1999). The width of the reef varies between 1 km and 5 km.
Blåvands Huk, which is Denmark’s most western point, forms the northern extremity of the European Wadden Sea, which covers the area within the Wadden Sea islands from Den Helder in Holland to Blåvands Huk.
2.1 Topography and sediment 2.1.1 Geology and geomorphology
Horns Rev was formed from deposits of sand and gravel on top of deposits created during the Eem geological period and glacio-fluvial sediment deposited during the Saale glaciation. The constituents of the reef are therefore not the typical mixed sediment of a moraine but rather well sorted sediments in the form of gravel, grit and sand. Huge accumulations of Holocene marine sand deposits, up to 20 m in depth, formed the Horns Rev area that is known today with continuous accumulations (Larsen, 2003). Horns Rev can be characterised as a huge natural blocking sand ridge, which blocks the sand volume transported along the Jutland coast. The yearly transport of sand is in a magnitude of 500,000 m3 (Danish Hydraulic Institute, 1999).
Horns Rev is constantly subject to variations in hydrography and sea level changes but it is considered a quasi-stable formation that will continue to adjust to minor changes in the local conditions.
The seabed surface sediments at Horns Rev are constantly reworked by waves and currents. The sediments in the Horns Rev area consists generally of pure medium fine sand to coarse sand with no or very low organic content (<1%) (Leonhard & Pedersen, 2006). Coarser sediments can be found towards slopes facing greater depths where currents are stronger. Bedforms of small sand ripples can generally be found that are caused by the wave impact on the seabed. Great variability in the sediment grain size distribution can also be found within short distances,
The North Sea is a complex resonant tidal system caused by the rectangular form of the basin. The mean tidal range in the area is about 1.2 m (Danish Hydraulic Institute, 1999).
Within the wind farm area, the water depths vary from about 4m to 14 m. The depth conditions in the area result in the waves breaking in the wind farm area. The average wave-height is about 0.6-1.8 m.
The hydrographic conditions in the Horns Rev area are mainly a result of the intrusion of Atlantic water into the southern part of the North Sea. The water moves erratically towards the Skagerak. The flow continues northwards as the Jutland coastal current and follows the Danish west coast towards the Skagerak under the effect of prevailing winds.
The tidal current is mainly in a north south direction. Current speeds above 0.7 m/s up to 1.5 m/s are not unusual at Horns Rev (Leonhard & Pedersen, 2006).
Due to the tidal currents, rough waves and mixing of water, stratification does not develop in the Horns Rev area and therefore oxygen deficiency is not likely to occur (Danish Hydraulic Institute, 1999).
The salinity in the area is 30-34 psu and is determined by the inflow of freshwater from the German rivers to the German Bight and the inflow of relatively high-saline water from the North Sea.
Low transparency due to high amounts of re-suspended material in the water column is characteristic for the Horns Rev area. High temporal variability is found in the water transparency due to the influence of tidal current, wind induced current, current speed and seasonal plankton dynamics. In general, the water transparency is low in spring, 1.8- 6.0 in adjusted Secchi depth [Adjusted Secchi depth = estimated Secchi depth x (1+0.4) x wave height] and higher during autumn, 2.5-8.8. Pronounced diel variability in transparency is found within a few hours and can be associated with changes in the prevailing current directions (Leonhard & Pedersen, 2006).
3 The wind farm area
3.1 Description of the wind farm area
The Horns Rev 2 Offshore Wind Farm will be located approximately 30 km west of Blåvands Huk. The distance to the north-western point of Horns Rev 1 Offshore Wind Farm will be approximately 14 km, depending on the exact placement of the wind farm.
The area selected by the Danish Energy Authority for the preliminary study is shown in Figure 3.1. The establishment of the wind farm is expected to be in one of two different appointed sites. The exact position of the individual turbines has not yet been decided and there may be some minor adjustments regarding the positioning of both sites.
However, the final placement will be inside the selected area of the preliminary studies referring to the two alternative sites, the northern or the southern site. The northern site extends northwards from the reef. The southern site extents east towards west and partly covers the reef. Both the selected areas cover 35 km2, which is the maximum size of the Horns Rev 2 Offshore Wind Farm. The water depths at the two sites range from 4-14 m, Figure 3.1.
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Figure 3.1. Sites selected for the Horns Rev 2 Offshore Wind Farm.
3.2 The turbines
The exact type of turbine and type of foundation has not yet been decided.
The wind turbine technology is undergoing rapid development with regard to the layout and efficiency of wind turbines as well as their size. In order to take advantage of this development, the final selection of the wind turbine type will take place later. The basis scenario for this EIA is a setup comprising 95 turbines plus possibly 1-3 experimental turbines. The expected distance between the turbines in this setup will be approximately 600 m. However, with an installed total capacity of 200-215 MW for the wind farm, the factual number of turbines may be reduced if larger units are selected.
The experimental turbines are included in this EIA although they will not be part of the wind farm established by ENERGI E2. The maximum total capacity of the experimental turbines will be 15 MW. The maximum height will be 200 metres and the type of foundation will be selected and decided by the developer, independently of what type of foundations will be decided for the wind farm.
The probable row patterns of the turbines at the two sites are shown in Figure 3.2.
However, the position is subject for adjustment in the final park layout.
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Southern wind farm location Wind turbines South Turbines for experimentel use Transformer platform Area for preliminary studies Horns Rev 1 Offshore Wind Farm
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Figure 3.2. The proposed turbine positions at the northern site (a) and the southern site (b), the cable connecting the turbines and the transformer platform. Horns Rev 2 Offshore Wind Farm.
The foundations of the turbines will either be monopiles or gravitation foundations. For both types, a scour protection is necessary to minimize erosion due to strong currents at the site. The foundations including protection will occupy an area less than 0..3% of the entire wind farm area.
22.214.171.124 Gravitation principle
The gravitation foundation consists of a flat base to support the basis of the turbine tower. The size of the base is determined by the size of the turbine with the weight of the basal disc being typically >1000 tones. The gravitation foundation is made of concrete or a steel case filed with heavy weight material such as boulders and rocks. This type of foundation is typically used at water depths in the range of 4-10 metres.
The establishment of a gravitation foundation requires preparation of the seabed. This preparation includes removal of the top layer of sediment and construction of a horizontal layer of gravel. Additionally, the gravitation foundation requires scour protection to prevent wave erosion.
126.96.36.199 The monopile principle
The foundations of the existing wind turbines at Horns Rev 1 Offshore Wind Farm are so-called monopile foundations. The monopile foundation is a steel pile driven into the seabed. The pile is normally driven 10–20 m into the seafloor and has a diameter in the range of 4-7 metres. The pile diameter and the depth of penetration are determined by the size of the turbine and the sediment characteristics. Contrary to the gravitation foundation, no preparation of the seafloor is needed.
On the seabed, establishment of scour protection might be necessary around the foundation to minimise erosion due to strong currents at the site.
3.2.2 The scour protection
The scour protection has a diameter of approximately 25-35 m in total, varying between sites and depending on the type of wind turbine chosen. The scour protection is approximately 1-2 m in height above the original seabed and consists of a protective stone mattress of large stones with a subjacent layer of smaller stones.
3.2.3 Subsea cables
The wind turbines will be interconnected by 36 kV cables sluiced down to a depth of one metre into the seabed. The cables will connect the turbines to a transformer platform with each string consisting of up to 14 turbines. From the transformer station, the power is transmitted via a subsea 150 kV cable to shore. Assessment of impact attributable to this cable is not included in this EIA.
The power cables are expected to be tri-phased, PEX-composite cables carrying 50 Hz alternating current. The cables have a steel armament and contain optical fibres for communication.
188.8.131.52 Electromagnetic fields
An electric current passing through a cable creates an electromagnetic field around the cable. The strength of the field is proportional to the size of the electric current.
Magnetic fields are measured in microtesla (µT). One µT = 10 mG (milligauss) = 0.8 A/m. The Earth’s magnetic field is about 50 µT (www.electricity.org.uk). The maximum intensity of a magnetic field immediately below an 11 kV overhead power line at ground level is approximately 7 µT (www.electricity.org.uk).
From German wind farms, magnetic fields have been measured around the power cable connecting the wind farm with the mainland. The magnetic field around a tri-phased composite power cable covered by a steel armament was less than 10 µT (microtesla).
The magnetic field is reduced to less than 3 µT one metre from the power cable. The natural magnetic field in the ground in Germany is approximately 45 µT. (ABB Power Technologies, 2003).
Eltra (2000) has calculated the size of the magnetic field from the power cable connecting the wind farm at Nysted to the mainland when the wind farm is at maximum production. The magnetic field is approximately 5 µT on the sea bottom one metre above the power cable when the wind farm produces up to 600 ampere (Eltra, 2000). The corresponding induced electrical field generated is greater than 1000 µV/m at a distance of 4 metres from the cable. Additionally, the electrical field extends approximately 100 m before dissipating.
4.1 Assessment methodology
The main effects from the establishment of Horns Rev 2 Offshore Wind Farm on the benthic communities are identified and assessed according to certain criteria shown in Table 1.
The impact significance on marine benthos ecology has been evaluated by ranking the status and level of importance of marine benthos target species and communities or issues and the magnitude of any potential impacts.
Magnitude is determined on the basis of species vulnerability, spatial and temporal incidence of impacts and the ability of a species or community to recover.
In determining the significance of an impact, ‘magnitude’ is assessed against
‘importance’ to provide a range of significance from ‘negligible’ to ‘major’ as shown in Table 2.
Table 1. Criteria for the assessment of impacts (after DONG, 2006).
Criteria Factor Note
Importance of the issue International interests National interest Regional interest
Local areas and areas immediately outside the condition Only to the local area
Negligible to no importance
In physical and biological environment local area is defined as wind farm area
Magnitude of the impact or change Major Moderate Minor
Negligible or no change
The levels of magnitude may apply to both beneficial/positive and adverse/negative impacts Persistence Permanent –for the lifetime of the
project or longer
Temporary – long term – more than 5 years
Temporary –medium-term- 1-5 years
Temporary –short term- less than 1 year
Likelihood of occurring High (>75%) Medium (25-75%) Low (<25%)
Other Direct/indirect impact – caused directly by the activity or indirectly by affecting other issues as an effect of the direct impact;
Cumulative –combined impacts of more than one source of impact