Department of Energy and Climate Change
Review of Round 1 sediment process monitoring data – lessons learnt
A report for the Research Advisory Group
Final Report 2008
Summary
Environmental monitoring around the initial phase of offshore wind farm development has generated important evidence related to sediment processes. It has now been possible to review the available data and provide consideration to how the information develops our present understanding and to inform further and potentially larger developments.
Three areas of information presently exist and relate to:
(i) Suspended sediment concentrations monitoring the short-term disturbance during construction
(ii) Local detailed surveys around individual turbine foundations to monitor scour
(iii) Broader scale surveys to consider the general impact on seabed morphology from the wind farm as a whole
In general, the available sediment monitoring evidence appears to support considerations made as part of the environmental impact assessment process. It is noted however, that the evidence base remains exclusive to mono-pile type developments and that initial conclusions are mainly formed from short-term monitoring.
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Acknowledgements
This research project has been prepared by senior scientists from ABP Marine Environmental Research Ltd (ABPmer), the Centre for Environment, Fisheries and Aquaculture Science (CEFAS) and HR Wallingford Ltd. The principal authors were Bill Cooper (ABPmer), Jon Rees (CEFAS) and Tom Coates (HR Wallingford).
The research has only been possible from the support received from offshore wind farm developers whose data and information has been kindly shared with the project team.
The research was funded through the pan-Government Research Advisory Group (RAG), which facilitates a co-ordinated approach among the regulatory and funding bodies to address the key impact issues of Round 2 wind farms. The research falls within the subject area of Seabed and Coastal Processes.
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Acronyms and Abbreviations
ABPmer ABP Marine Environmental Research ABS Acoustic Backscatter BERR Department for Business and Regulatory Reform CD Chart Datum CEFAS Centre for Fisheries and Aquaculture Science COWRIE Collaborative Offshore Wind Research into the Environment CPA Coast Protection Act DIMP Data and Information Management Plan DTI Department for Trade and Industry EIA Environmental Impact Assessment FEPA Food and Environmental Protection Act ICES International Council for the Exploration of the Sea MCA Maritime and Coastguard Agency MCEU Marine Consents & Environment Unit MDM Marine Data Management MFA Marine Fisheries Agency NASA National Aeronautics Space Administration NMMP National Marine Monitoring Programme OBS Optical Backscatter PML Plymouth Marine Laboratory PSA Particle Size Analysis RAG Research Advisory Group SEA Strategic Environmental Assessment SSC Suspended sediment concentration UK United Kingdom UKMMAS United Kingdom Marine Monitoring and Assessment Strategy
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Contents
Summary...... i Acknowledgements ...... ii Distribution ...... ii Acronyms and Abbreviations...... iii
1. Introduction...... 1 1.1 Background...... 1 1.2 Research Co-ordination ...... 1 1.3 Project Aims...... 2
2. Approach ...... 3 2.1 Overview ...... 3 2.2 Consent Conditions...... 4 2.3 Evidence Base ...... 6 2.4 Structure of Data Review ...... 7 2.5 Data Review Issues ...... 8
3. Lessons Learnt...... 9 3.1 Data management...... 9 3.2 Evidence Base ...... 9 3.3 Suspended Sediment Concentrations...... 10 3.4 Morphology ...... 13 3.5 Scour...... 15
4. Recommendations...... 16 4.1 Recommendations for Appropriate Monitoring Strategies...... 16 4.2 Recommendations for Further Research ...... 17
5. Consideration of Broader Scale Issues ...... 18 5.1 Related Strategic Monitoring Initiatives...... 20
6. References ...... 22
Appendices
A. Standard Data Request Letter B. Project Database C. Review of Suspended Sediment Concentrations D. Review of Seabed Morphology E. Review of Scour
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Figures
Figure 1. Schematic for offshore wind farm monitoring and review (after MCEU) ...... 5 Figure 2. Standard folder structure for project database...... 6 Figure 3. Example of seabed morphology at Scroby Sands...... 14 Figure 4. Satellite images of reflectance at 555nm during 1998 in the North Sea, closely related to SPM concentrations...... 22
Tables
Table 1. Built offshore wind farms ...... 6 Table 2. Status of data collation for the four operational Round 1 sites (date order) available at time of review...... 7 Table 3. Present gaps in evidence base...... 10 Table 4. Site-by-Site Summary of SSC monitoring evidence ...... 12 Table 5. Site-by-Site Summary of scour monitoring ...... 15 Table 6. Assessment of Impacts on Coastal Processes (after BMT, 2002)...... 19 Table 7. NMMP in-situ monitoring ...... 21
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1. Introduction
1.1 Background
The UK has committed to challenging targets for energy production from renewable sources. In order to reach these targets it is expected that an increasing emphasis will be placed on production from sites located offshore. Offshore wind leads the way and has commenced with a first phase of size limited projects (Round 1) to provide experience to the industry and to develop an evidence base of understanding to support the succeeding phase of larger scale commercial projects (Round 2).
The Department of Trade and Industry (DTI)1, established a Research Advisory Group (RAG) to consider research priorities in relation to the potential environmental impacts of offshore wind energy developments, and consequential impacts on other users of the sea. To assess the need for further research related to coastal processes, RAG established a sedimentation theme and organised a workshop in June 2005 to examine the issues. From this initial workshop three priority research projects were taken forward:
• Review of Round 1 sediment process monitoring data – lessons learnt (SED01)
• Dynamics of scour pits and scour protection (SED02)
• Review of channel migration (SED06)
1.2 Research Co-ordination
A consortium of research partners comprising ABPmer, CEFAS and HR Wallingford was commissioned to carry out both SED01 and SED02 projects, and with interim results from this work presented at a RAG Sedimentation Technical Review Workshop held on 17th January 2007, along with SED06 and two further related studies:
• Review of cabling techniques and effects applicable to the offshore wind farm industry; and
• COWRIE Data Management and Co-ordination initiative.
These projects represent a focus of ongoing research into coastal processes and data management issues that are intended to inform key nature conservation groups, developers and their consultants, and to assist the regulatory process respond to further marine renewable determinations.
1 The Department of Energy and Climate Change (DECC) brings together the energy group from the Department for Business, Enterprise and Regulatory Reform (formerly the Department of Trade and Industry), with much of the climate change functions previously housed within the Department for Environment, Food and Rural Affairs.
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Active co-ordination has taken place between these related studies to ensure efficient engagement with key data providers and to facilitate data exchange between studies.
1.3 Project Aims
The aim of SED01 is to draw together the sediment process monitoring work carried out on Round 1 developments and review the methods, data, results and impacts in order to identify lessons learnt and to provide relevant recommendations for monitoring of Round 2 developments. A further aim for the project is to consider if the Round 1 monitoring assists in any way the consideration of broader scale effects relevant to Strategic Environmental Assessment (SEA) review requirements.
In delivering these project aims the following activities have been undertaken:
a. Identification and collation of available field evidence from built Round 1 projects, and, in addition, any further data available from other built European projects.
b. Management of the information resource to enable the onward supply of approved data in line with associated research interests.
c. Review of the available data and reports to determine lessons learnt.
d. Assessment of the present scope of sediment process monitoring placed on Round 1 developers to determine the appropriateness of monitoring as might be required for Round 2, and with special regard to differences in scale from Round 1 to Round 2 projects, and lessons learnt to date.
e. Close liaison with Client and Theme Leader for Seabed and Coastal Processes throughout the project and facilitate a technical review workshop.
f. Preparation and dissemination of an authoritative technical report suited for application by regulator, developer and consultant.
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2. Approach
2.1 Overview
The primary data source for this research relates to sediment process monitoring applied to all Round 1 projects that have progressed through their construction phase. At the time of advancing the data collation activity the list of projects that have advanced to this position is limited to:
• North Hoyle; • Scroby Sands; • Kentish Flats; and • Barrow.
It is also recognised that at the time of conducting this review ongoing construction is occurring for two further Round 1 projects; Burbo and Lynn & Inner Dowsing.
Alongside these UK projects further monitoring data has been sought from other relevant European offshore wind farms as well as relevant data from the Blyth Offshore demonstration site.
Project developers responsible for each site were contacted at the commencement of the study and requested to participate through provision of their available monitoring data. The Marine Consents & Environment Unit (MCEU) was also contacted for information, given their regulatory function in setting of the respective monitoring requirements for Round 1 schemes as part of the FEPA licence.
The process of requesting data has been carefully co-ordinated between other RAG project teams so as to minimise the number of requests received by developers and to identify linkages between related research areas. A copy of the generic letter sent to developers for this purpose is provided in Appendix A.
Relevant data holdings requested from project developers include pre-construction, construction and post-construction surveys and primary themes of information including:
• Seabed levels (inc. localised scour development); • Seabed features (e.g. bedforms); • Surficial sediment coverage (e.g. particle size analysis); • Suspended sediment loads; • Shoreline profiles (where relevant); • Tidal parameters (water levels and currents); and • Wave parameters (height, direction, amplitude and period).
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2.2 Consent Conditions
The MCEU was established in April 2001 as a jointly managed cross-departmental unit of the Marine Environment Branch of the Department for Environment, Food and Rural Affairs (Defra) and the Ports Division of the Department for Transport (DfT). As of 1st April 2007 the function of MCEU has become integrated into the Marine and Fisheries Agency (MFA) and this service is now provided by their Marine Environment Team.
MFA is responsible for the administration of a range of applications for statutory licences and consents to undertake works in tidal waters around England and Wales; including marine developments, offshore energy, coast defences, dredging and waste disposal. The primary legislation presently relevant to offshore wind is the Coast Protection Act 1949 (CPA) and the Food and Environment Protection Act 1985 (FEPA). The unit also administers certain applications on behalf of the Welsh Assembly Government for which it is the licensing authority in Welsh territorial waters.
At the time of conducting this review consents have been issued for seven Round 1 sites, and the Scottish Executive has also consented the Robin Rigg project in the Solway Firth.
To date, monitoring requirements have been considered on a case-by-case basis, with the FEPA licence (and occasionally Section 36 of the Electricity Act) providing the means of instructing the developer with relevant consent conditions, including appropriate monitoring. In relation to sediment process monitoring data, the following categories are of immediate relevance:
• Suspended Sediment Concentrations • Seabed Morphology and Scour
The key concern in relation to these issues is to minimise the risk of significant impact through smothering sensitive receptors (e.g. benthic communities, Sabelleria, etc) and from water quality.
In addition, it is also noted that a range of other site-specific coastal process requirements are referred to in the supplementary consent conditions, such as contaminants and current monitoring, etc.
An initial schedule of monitoring requirements is also detailed in the FEPA licence, with a general expectation for surveys in late summer / autumn and at the following phases through project development:
• Pre-construction • Construction • Post-construction
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Post-construction surveys are generally requested annually and initially for period of three years following construction. The licensing authority may then choose to extend this period, subject to review of the information.
It is the responsibility of the project developer to deliver monitoring reports to MFA for review, a process which is normally carried out by CEFAS and the Environment Agency.
Further monitoring requirements may be imposed on the developer at the discretion of the licensing authority in light of the results of the monitoring work previously undertaken.
It is the sediment process data and information generated from these monitoring programmes that is the primary interest to the present research, with the objective to identify lessons learnt and to provide recommendations for monitoring applicable to new projects. It is important to note that this research has no mandate to revise monitoring applied to existing projects. This process remains the responsibility of MCEU.
The generic path for offshore wind farm monitoring is presented in Figure 1.
DRAFT ROUND 1 OFFSHORE WIND FARM MONITORING – TIMELINE
Baseline Monitoring CEFAS Review Baseline as fit for MCEU confirm baseline meets licence Baseline reports made Reports sent to MCEU purpose within 6 weeks requirements and makes available to available on the internet. wider audience with 2 weeks EN/CCW Review Baseline as fit for purpose within 6 weeks or MCEU confirm baseline does not meet licence requirements and asks for additional information from Licence holder
1st Year post construction CEFAS review and evaluate MCEU confirm findings Reports made available monitoring reports sent against baseline within 6 weeks acceptable and licence on the internet. to MCEU requirements have been met within 2 weeks EN/CCW review and evaluate
against baseline within 6 weeks or
MCEU confirm findings are not MCEU write to Licensee to Insufficient data acceptable due to: request additional data be supplied Adverse findings require additional mitigation measures to comply with licence conditions
Agree additional mitigation measures with the Licensee and stakeholders and vary licence
Figure 1. Schematic for offshore wind farm monitoring and review (after MCEU)
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2.3 Evidence Base
The available digital output, obtained through the data collation efforts, has been organised into a project database in the form of a sequence of standard project folders (Figure 2). The folders contain copies of each project’s available data, and in the formats supplied. Appendix B provides a catalogue of the present digital data holdings within the folder structure. It is noted that there has been a diverse range of file formats required to record the data and information.
Figure 2. Standard folder structure for project database
The following sites are presently included in the evidence base (Table 1):
Table 1. Built offshore wind farms (valid at time of requesting data)
Site Country Commissioning Date Scheme Turbines Arklow Bank Ireland 2003 - 7 Barrow England 2006 Round 1 30 Blyth England 2000 - 2 Burbo England Due 2007 Round 1 30 Horns Rev Denmark 2002 - 80 Kentish Flats England 2005 Round 1 30 North Hoyle Wales 2003 Round 1 30 Nysted Denmark 2003 - 72 Scroby Sands England 2004 Round 1 30
Of particular note is that mono-pile foundations have been for every project with the exception of Nysted which uses a concrete gravity caisson.
Three levels of information have been sought from each of these projects:
(a) Pre-consent – includes baseline surveys and Environmental Statements (ES). This information is used to describe a project baseline and offers predictions of ‘presumed’ effects attributable to the wind farm.
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(b) Consent conditions – e.g. FEPA licence. Generally applied by MFA to mitigate uncertainties in impacts.
(c) Monitoring data – pre-construction, construction and post-construction surveys aimed at obtaining field evidence of ‘actual’ effects and to fulfil requirements of consent conditions.
It is to be noted that in some cases extra monitoring has been obtained outside of any requirements for environmental monitoring. Such monitoring has normally been undertaken as a risk management practice, for example to ensure integrity of buried cables.
Table 2 summarises the outcome of the data collation efforts relating to the target Round 1 projects.
Table 2. Status of data collation for the four operational Round 1 sites (date order) available at time of review Monitoring Data Site ES FEPA Suspended Morphology Scour Sediments North Hoyle Yes Yes Yes Yes Yes Scroby Sands No Yes Yes Yes Yes Kentish Flats Yes Yes Yes Year 1 Year 1 Barrow Yes Yes Yes Pre-construction Year 1
2.4 Structure of Data Review
The process of data review undertaken in this research is structured to be complimentary to the project level reviews being advanced separately by MFA. This is achieved by consideration of a set of technical subjects relevant to all present UK projects. The following data categories are adopted for this purpose for consistency with topic headings referenced from FEPA licence conditions applied to all presently built Round 1 offshore wind farms:
a. Suspended Sediment Concentrations (SSC)
Focused on measuring the potential short-term construction impacts relating to sediment disturbance (e.g. foundation and cable installation), and considered relative to baseline levels. Data types tend to represent temporal variations over several tidal cycles and typically during benign conditions suited to construction operations.
Primary data types:
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• In-situ Optical Backscatter Sensor (OBS) (either bed mounted or towed by a vessel); and • Mass concentrations determined from water samples.
Appendix C provides further details of the data review for SSC.
b. Seabed Morphology
Assessment of generalised changes in seabed levels across the footprint of the development and along the export cable route, including changes in sediment regime (e.g. shift in deposition/erosion trends observed from variability in particle size data) and the wider significance of any secondary scour (where secondary scour is a measurable effect represented by a lowered seabed profile which is not immediately in contact with the foundation). These surveys form the basis for assessing any cumulative wind farm effects.
Primary data types:
• General bathymetry pre & post-construction; and • Particle Size Analysis (PSA).
Appendix D provides further details of the data review for seabed morphology.
c. Scour
Assessment of near-field changes in seabed levels around individual foundation units, including cable spanning at j-tubes. Extent of evidence verses foundation types and sediment regimes (non-cohesive & cohesive), scour protection and secondary scour effects.
Primary data types:
• High-resolution bathymetry from a ‘sample’ of foundations from the wind farm area and sufficient to be representative of the spread of sediment types present across the site.
Appendix E provides further details of the data review for scour.
2.5 Data Review Issues
For each category of data a review has been provided based on addressing the following issues:
• What reliability and confidence can be placed on the field data and how might practices be improved?
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• How do the observations compare to statements made in the ES (i.e. is the measured data in line with the assessment of effect, or is there a ‘surprise')?
• Has the data addressed the FEPA requirement to provide the additional understanding required to reduce apparent uncertainties?
• Are the methods of survey sufficient and what approaches demonstrate best practice (e.g. if various approaches to monitoring have been applied, then identify which has worked best)?
• Summary of lessons learnt to advise on future requirements (inc. Environmental Impact Assessment (EIA) guidance, regulatory requirements, monitoring provisions, etc).
3. Lessons Learnt
3.1 Data management
A key lesson learnt from the process of data collation from Round 1 projects is the need for improved data management. The present research has identified that the assembled data for any particular project has required access from multiple sources which generally include the developer, their environmental consultant(s), their survey contractor(s) and their build contractor(s), and with no centralised inventory of data and information.
In addition, in the time elapsed between conducting the original surveys and requesting the information, some projects have changed ownership and some of the staff involved on the projects have moved companies. The combination of all these issue has further complicated access to original information.
It is recognised that improved data and information management is an issue which is being addressed through COWRIE and in relation to lease requirements for Round 2 projects. It is recommend that this practice is also quickly adopted through remaining Round 1 sites, although there is no lease obligation related to this.
3.2 Evidence Base
This project has achieved an important and valuable evidence base of sediment process monitoring data from the four completed Round 1 projects available at the time of publication. The evidence base has also been supplemented with further data from other built offshore wind farms from Europe, where information is available. It is strongly recommended that this evidence base is maintained, developed and expanded on with further sediment process data when this becomes available from
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further completed offshore wind farms, especially from those sites which provide the opportunity to broaden the evidence base and fill important data gaps in scientific understanding (Table 3).
Table 3. Present gaps in evidence base Issue SSC Morphology Scour Alternate foundation Short-term effects Effects of larger structures on Effects of larger types to mono-piles during construction (e.g. secondary scour (e.g. interaction structures (e.g. gravity base) seabed levelling) with bedforms) Drilling out substrata Short-term effects Effects of drill cutting piles (e.g. and disposal of drill during construction interaction with bedforms) cuttings Alternate scour Effectiveness protection measures compared to conventional methods Larger scale projects Cumulative array effects of larger projects, especially across mobile sea beds and over the longer-term Cumulative impact with In-combination issues, especially adjacent projects across mobile sea beds and over the longer term
It remains for developers to consider on a case-by-case basis if their site presents a significant risk to any environmental receptor. If the available evidence is suitable to their specific application then it is reasonable to expect that further monitoring requirements can be avoided.
It remains for regulators to decide if the present evidence base provides the level of certainty required to consider revised monitoring conditions of new projects. Where monitoring requirements remain then the present ‘default’ scope may need reconsideration in light of lessons learnt. When a suitable survey scope has been agreed with the developer then this needs to be formally documented and made available as part of any future evidence base.
It also remains for Round 2 developers to act on both their future licence and lease requirements and contribute any relevant environmental data in accordance with the Data Management and Information Plan (DMIP) being developed through the Collaborative Offshore Wind Research into the Environment (COWRIE) project (COWRIE, 2005).
3.3 Suspended Sediment Concentrations
The review of SSC monitoring data (Appendix C) has revealed that the assumptions made through the environmental impact assessment process are generally upheld by the available evidence, with short-term localised impacts (i.e. events that continue over comparable time-scales to the construction process) occurring around construction
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activities which disturb the seabed, in particular cable laying and foundation installation (drilling).
Optical backscatter (OBS) instruments have been used to collect the available SSC evidence, and are considered to be the preferred option when finer sediments (tens of microns and lower, i.e. silts and clays) are in suspension. Acoustic backscatter (ABS) devices offer a further means to monitor SSC but are inherently more suited to larger sediment fractions (ideally tens to hundreds of microns, i.e. coarse silts to sands). Both approaches are single frequency instruments which can not differentiate between a change in concentration and a change in particle size, with a change in particle size being interpreted as a change in concentration. The research community is presently investigating the development of multiple frequency ABS systems capable of differentiating between particle size and concentration fluctuations, however these devices are yet to be introduced into mainstream application.. Both approaches measure surrogate properties of SSC which need thorough calibration through analysis of water samples. In mixed sediment loads this analysis needs to include both mass concentration and particle size measures. To date, only one project has achieved this during baseline monitoring.
The use of water samples to convert into natural SSC units (i.e. mg/l) has generally been very limited and tended to deliver poor instrument calibrations over limited concentration ranges. The consequence of this has led to large amounts of extrapolation to higher concentrations from generally weakly correlated data. The issue of reliability in this means of conversion is typically un-stated.
The effects of different cable laying methods appears to indicate that jetting is not a major concern, and with sediment plumes tending to remain close to the seabed (up to 2m displacement above the seabed). Knowledge of the relative position of any sediment plume should assist further monitoring strategies.
Despite any apparent weaknesses in present monitoring arrangements, the general interpretation of relative changes in turbidity concentrations above background levels shows that the majority of effects fall within natural variations due to waves and tides for the shallow water sites, concluding that there is unlikely to have been any significant impact due to offshore wind farm construction. This outcome is consistent to the predictions offered from the Environmental Statements of each project.
Table 4 provides a site-by-site summary of the available SSC monitoring evidence.
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Table 4. Site-by-Site Summary of SSC monitoring evidence Site ES Prediction Observed Impacts North Hoyle <10% increase to SSCs <5% detectable increase in SSCs as a result of construction works Kentish Flats SSC levels could increase to double that of Installation of the first cable saw no significant background levels alterations to SSCs Final deposition site unknown Installation of the second and third indicated a 9% increase on background levels Scroby Sands Impacts unlikely due to dynamic nature of the SSC seen to increase 9 to 11% during site construction works (may have been due to period of increased wave heights) Barrow No significant impact predicted as naturally Increases found to be small and relatively high levels of SSCs at site localised, whilst remaining between 1 to 2m above the seabed Nysted Small increases predicted Small increases observed but both temporary and localised Horns Rev No significant impact due to naturally high Impacts minimal SSC levels
Key recommendations from lessons learnt are:
• Preferred use of OBS devices calibrated against sufficient water samples spanning the range of monitoring conditions (i.e. peak flow events), ideally a minimum of 20 samples to provide a more robust statistical correlation;
• Deployment of sensor at a fixed height above the seabed (notionally at 1m) with an additional vessel deployed sensor sampling through the water column at times of equipment deployment, servicing and recovery;
• Water samples analysed for mass concentration, particle size (laser diffraction method), inorganic and organic content;
• Consideration for use of sediment traps to monitor fate of drill cuttings;
• Associated metocean data and local seabed sediment samples to assess natural sediment disturbance; and
• Near-field sampling at no more than 500m from the sediment source.
It remains prudent for EIA guidance to continue to recommend developers to undertake appraisal of SSC issues, especially as Round 2 projects may seek to develop larger schemes using alternative foundation options (i.e. sites and techniques that fall outside of the present evidence base). As additional evidence is provided from these new sites then further consideration can be given to updating present guidance.
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3.4 Morphology
A detailed review of available morphological monitoring evidence is given in Appendix D. Of this evidence the most complete record of seabed morphology is provided by the Scroby Sands offshore wind farm with 3½ years of post-construction monitoring with bi-annual surveys. Other sites have also monitored the seabed over pre- and post-construction phases, but generally these sites appear to exist in quieter sediment regimes absent of any major morphological features and with data that indicates no major change in general seabed profile, e.g. North Hoyle.
What has been shown from the Scroby Sands project is that the natural dynamics of the sandbank remain very high. Previous research, CEFAS (2006), estimate that sediment transport activity for modal medium sands occurs for around 80% of the time in summer conditions increasing to 94% during winter. This level of activity leads to continual changes in the sandbank form as well as general bedform movement across the bank (e.g. sandwaves).
One surprise from the detailed monitoring conducted on Scroby Sands was the appearance of secondary sour in certain locations over the period of available surveys, and in particular on the eastern side of the array. These features are described as scour ‘tails’ or ‘wakes’ and appear in the direction of the dominant flood tide and for distances of around 400m. The pattern on the seabed at these locations resembles a shallow depression overlain with smaller sandwaves which appear to be common across the surrounding area. The appearance of scour tails is also transitory over the period of surveys and is most evident after scour protection is laid (post-March 2004 survey).
Further analysis of the data undertaken as part of this review process also identifies that where a scour tail has extended towards an existing surface wreck there is apparent development of group scour. Future siting arrangements should consider avoiding the risk of scour wakes extending over such features.
Scour wakes had not been anticipated in any part of the EIA or engineering design process and are considered to be a ‘surprise’. It is possible that similar patterns may be revealed in the future for other projects, especially for sites with highly mobile sea beds and with active bedform features.
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(a) Array scale description of seabed. (b) Local view of seabed from eastern side illustrating scour tails. Figure 3. Example of seabed morphology at Scroby Sands (red lines representing monopiles, purple and green lines representing installed cables)
Key recommendations from lessons learnt are:
• The continued use of multi-beam (swath) bathymetry equipment is identified as the preferred survey method to reveal the detailed form and features of the seabed which has not always been practical or possible using single beam methods;
• For ease of comparison between sequences of surveys it is preferred that as much consistency remains in the execution of surveys and processing of data as possible;
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• An understanding of relative sediment mobility can be gained from consideration of the exceedence threshold of bed shear stress and be used as a guide for determining monitoring requirements; and
• Further investigation is made in relation to the potential risks of secondary scour (e.g. on inter-array cable burial), especially where new developments are located in areas of high sediment mobility.
3.5 Scour
A detailed review of available scour monitoring is given in Appendix E. The existing evidence base for scour spans a range of site conditions from mobile sandbanks, gravely mixed sediments to clay geology. These sites are exposed to differing degrees of metocean conditions but are all generally in shallow water where combined wave and tidal influences may lead to scour. The extent and rate of scour development for each site has generally been as predicted. Critically the present evidence base remains limited to the mono-pile case foundation, which can generally be regarded as conforming to slender pile theory.
It is noted that the monitoring undertaken to assess scour has inherently been conducted during calm conditions which may bias the interpretation of maximum observed scour towards a potential “recovery” condition.
Table 5 provides a site-by-site summary of the available scour monitoring.
Table 5. Site-by-Site Summary of scour monitoring Site ES Prediction Scour Protection Installed Observed Impacts North Hoyle Minimal scour due to boulder No Results confirm predictions clay Kentish Scour predicted but limited No Unexpected deep pits around Flats due to London Clay mono-piles and location of jack-up legs Scroby Deep scour predicted Yes (quantified after first Depth of scour generally as Sands survey) predicted Extent of scour greater than predicted Secondary Scour formed Barrow Scour predicted in areas of No Initial results confirm fine sand (and limited by sub- predictions strata) Arklow Deep scour predicted Yes (rock protection) Minimal secondary scour Bank recorded
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Key recommendations from lessons learnt are:
• All local scour surveys from Round 1 sites remain related to mono-pile foundations and do not yet include cable routes or cable crossings;
• Present swathe surveys have not resolved small scale features which may be attributable to J-tubes;
• Scour features are resolved well by the use of high resolution swathe systems, but the post-processing of the data often looses the location of the mono-pile through interpolation of ‘holes’;
• The footprint of the scour survey remains local to the foundation and generally extends up to 50m around each structure, which is sufficient to encompass the anticipated scour width dimension for monopiles. The presence of any secondary scour is unlikely to be revealed from this process and must depend on the more general morphological survey which extends over larger distances;
• Further reporting of scour monitoring needs consideration of the metocean conditions in the lead up to the survey to enable a view of any potential “recovery” phase which may contribute to partial in-filling of a scour hole;
• Future monitoring is most important around new foundation types that differ in scale to mono-piles; and
• The time period for data collection may depend on site specific circumstances, but to understand better the time evolution of scour then an initial survey immediately after construction (e.g. within the first two weeks) and then soon after (e.g. within the next 3-months) would expand the scientific understanding of scour development. This subject is being considered in further detail by SED02.
4. Recommendations
4.1 Recommendations for Appropriate Monitoring Strategies
Present requirements for sediment process monitoring have been achieved by a combination of standard vessel based bathymetric surveys for scour and morphology, and separate SSC measurements using devices deployed at fixed stations or towed from vessels.
At the time of responding to Round 1 monitoring requirements there was no direct guidance available for developers of how to conduct appropriate monitoring strategies bespoke to the offshore wind industry, and the combined expertise of marine survey contractors and environmental consultants formed the basis of delivering the
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specifications for this work. However, guidance is available for the marine aggregate industry (CEFAS, 2002) for similar monitoring requirements and is considered to be immediately transferable. This guidance provides a general description for bathymetric surveys and monitoring suspended sediment concentrations, as well as laboratory analysis for grab samples.
In addition, the International Council for Exploration of the Sea (ICES) Working Group on Marine Data Management (MDM) has developed guidance to assist those involved in the collection, processing, quality control and exchange of various types of (mainly) physical oceanographic data, for example; water samples, moored and shipborne sensors and multibeam echosounder data. These guidance documents are available from:
http://www.ices.dk/datacentre/guidelines/MDMguidelines/DataTypeGuidelines.asp.
In terms of the present study, a clear recommendation is for the adoption of multi-beam surveys as the preferred means to resolve and quantify small-scale features of bedforms (e.g. sandwaves), scour around structures and other associated effects such as scour tails.
In addition, where multiple repeat surveys have been specified as part of the FEPA licence then a clear recommendation is to maintain as much consistency between surveys as is reasonably practical, and with specific regard to the following:
• Operate the same equipment on each survey; • Use the same survey contractor; • Remain consistent in any post-processing methods; and • Document fully the process of any inter-comparisons.
A final comment on present monitoring strategies relates to data management. A project funded by the Collaborative Offshore Wind Research into the Environment (COWRIE) is developing guidance for the industry to respond to the obligation of developers to submit environmental data to The Crown Estate as part of their Round 2 lease requirements (COWRIE, 2005). It is the intention of this guidance to ensure that best practice in data management is introduced into the project development process at the initial stage and to avoid many of the difficulties experienced under this contract in collating data from Round 1 offshore wind farms.
4.2 Recommendations for Further Research
The current evidence base has secured sediment monitoring information from the four operational Round 1 projects completed at the time of study, along with comparable data from other completed European projects. The present summary of lessons learnt has been provided from this evidence alone and may not yet reflect the longer-term effects or the entirety of issues from all Round 1 sites and issues related to larger
17 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Round 2 projects which are likely to also consider alternative foundation options to mono-piles.
The research has established an important evidence base that has extended present understanding and can offer an informed view on issues related to comparable sites. It remains important to maintain this database and include new data and other projects when such information becomes available.
The review of new information should be considered important to develop the understanding of environmental impacts and for developers to become more aware of possible issues related to onward operational and maintenance requirements.
5. Consideration of Broader Scale Issues
To date monitoring activity has been limited to discrete interests associated with the Round 1 sites which are in locations dispersed around the UK coast. The monitoring has focused on the near-field issues such as local scour or short-term localised seabed disturbance stemming from foundation installation and cable laying operations. The restricted scale of Round 1 projects (generally limited to 10km2) limits the potential risk for effects occurring at the broader scale, with the present evidence base upholding the general hypothesis that the main physical changes occurring around each structure do lead to any greater interaction over the scale of the array. Consequently, the potential risk of any broader scale effects arising from Round 1 projects are considered to be minimal.
In comparison, Round 2 will develop larger projects within three strategic areas, and consequently cluster new projects more closely together. The planned duration for these larger projects is likely to be at least 50-years and is a relevant timescale for consideration of longer-term and potential cumulative effects. It is noted that the three strategic areas were defined through the Round 2 Wind Farm Strategic Environmental Assessment (SEA) (BMT, 2003).
The SEA Directive (2001) requires that:
“Member States shall monitor the significant environmental effects of the implementation of plans and programmes in order, inter alia, to identify at an early stage unforeseen adverse effects, and to be able to undertake appropriate remedial action” (Article 10.1).
Table 6 summarises the perceived levels of risk associated with significant environmental effects related to coastal process issues, as identified through the Offshore Wind SEA process (BMT, 2002).
18 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Table 6. Assessment of Impacts on Coastal Processes (after BMT, 2002) Impact Likelihood Consequence Risk 1 = unlikely + = positive 0 to 8 = low 3 = likely 0 = none 9 to 14 = medium 5 = certain 1 = minor 15 to 25 = significant 3 = moderate 5 = serious Physical Processes Sandbank mobility 3 3 9 (medium). Tentative conclusion Sediment redistribution 5 1 or 3 5 or 15 (low or significant). Latter if local contaminated areas of great consequence. Seabed morphology – small 3 1 3 (low) sites Seabed morphology – large 3 Unknown Unknown sites Scouring 5 1 5 (low). Affected area small. Flow regime and wave 5 1 5 (low). Only occurs within and near site. climate – local effects Flow regime and wave Unknown Unknown Unknown climate – far-field effects Coastal sediment budgets – 1 5 5 (low) small sites Coastal sediment budgets – Unknown Unknown Unknown large sites Benthic Environment Scouring and scour 5 1 5 (low) protection Scour and scour protection 1 5 5 (low). Unless potential for regional – rare benthic species extinction, then significant. Redistribution of sediments 1 – 3 3 3 - 9 (low to medium). Localised and mostly applicable to cabling. Large-scale changes to Unknown Unknown Unknown. Effects of large developments sedimentation and near- need to be assessed. bottom conditions.
From Table 6, it can be seen that the perceived significant impacts (a risk score of 15 or above) may exist for sediment redistribution during construction activities and from subsequent scouring of sediments, especially if these sediments are associated with chemical contaminants. In addition, at the time of providing the SEA review, a number of unknowns (i.e. gaps in scientific understanding) remained, particularly in relation to large sites and the risk for consequential changes in sediment morphology. The issues in Table 6 clearly need further critical review for the present SEA process in light of present understanding.
19 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
For example, subsequent research undertaken on Scroby Sands (CEFAS, 2006) has concluded that the overall sandbank form has not changed since the construction of the Round 1 wind farm and that natural change dominates. Only localised effects have appeared so far, as seen from the evidence secured from the associated monitoring programme. This outcome can be accepted as indicative of similar sandbank sites where the level of sediment mobility is high and where mono-piles are used.
At the present time it remains unclear what formal provisions have been made to implement any strategic level monitoring programmes in regards to sediment processes as a consequence of Round 2 projects and to respond to either presumed significant environmental effects or the identified gaps in present scientific understanding.
Elsewhere in Europe the Dutch Government has established the ‘The Near Shore Wind Farm Monitoring and Evaluation Programme’ (NSW-MEP) to monitor effects around their first commercial scale demonstration 100MW project (SenterNovem, 2001). It is noted that this programme identifies similar issues and includes:
• The impact of foundations on morphology. Forecasting the erosion (time- dependent) around (mono-pile) foundations as well the effectiveness of protection technologies; and
• The morphological changes. Determining whether there is a large-scale, measurable impact. Estimating impact on current and suspension; local impact (erosion pits near turbine piles) extrapolation to higher scale. Supplementation at erosion pits? If yes, which methods minimise turbidity? Also applies to cable laying phase.
5.1 Related Strategic Monitoring Initiatives
At the present time there are a number of active marine monitoring programmes in UK waters collecting a variety of data types and for a number of purposes, although the co-ordination of efforts between programmes is often not clear.
For seabed mapping the Maritime and Coastguard Agency (MCA) is responsible for administering the Civil Hydrography Programme which has a primary aim to deliver up- to-date charting across UK waters for safe navigation (excluding port authority areas). It is a core function of the United Kingdom Hydrographic Office to publish these charts which may be supplemented by data from other parties, including the Ministry of Defence.
The scale of the territorial seas means that for some areas charting has not been repeated for several decades, however, in areas of shipping where the seabed is known to change more rapidly the MCA also conducts Routine Re-surveys as part of the same programme. The areas included in this process are generally associated with channels running adjacent to mobile sandbanks, such as the Yarmouth Banks
20 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
(relative to Scroby Sands) and the Outer Thames Banks (relative to London Array) with the data analysed for long-term morphological changes. SED06 provides further consideration of this subject.
For suspended sediments there appears to be very little strategic monitoring. One survey programme with operational data collection is the UK National Marine Monitoring Programme (NNMP). NMMP was established to provide a co-ordinated approach to environmental of monitoring long-term trends in coastal and estuarine areas. The programme brings together the statutory marine monitoring agencies throughout the UK with the shared objective to provide reliable and harmonised information for the UK coastal area. At the present time NNMP includes five locations where CEFAS maintains SmartBuoys collecting near-surface measurements of temperature, salinity, fluorescence, irradiance and turbidity. The location of these operational sites is given in Table 7.
Table 7. NMMP in-situ monitoring Deployment Group Description Location Position Depth From (Lat/Long) (m) West Gabbard Southern North Sea 51°59'.0N 002°05'.2E 32 28/08/02 Liverpool Bay Liverpool Bay 53°32'.0N 003°21'.8W 25 13/11/02 Warp Anchorage Outer Thames 51°31'.5N 001°01'.9E 18 30/11/00 Oyster Ground Central North Sea 54°25'.0N 004°02'.0E 45 14/03/06 North Dogger Central North Sea 55°41'.0N, 002°16'.8E 85 24/02/07
It is noted that the NMMP programme combines some activities with the Liverpool Bay Coastal Observatory.
The use of satellite imagery, when combined with the in-situ NMMP measurements for ground truthing, may provide one useful means of mapping the broader scale distributions of suspended sediment concentrations (noting that both measures are relative to properties of the surface water).
Figure 4 shows interpretation of sea colour which is related here to the sediment particulate matter (SPM) load. The images are NASA SeaWifs composites at a resolution of 1.1km. Courtesy of NASA and PML Remote Sensing Group.
21 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Figure 4. Satellite images of reflectance at 555nm during 1998 in the North Sea, closely related to SPM concentrations.
Finally, the UK is striving to improve co-ordination across various monitoring programmes (MARG, 2007). This work aims to establish the United Kingdom Marine Monitoring and Assessment Strategy (UKMMAS) to shape the UK’s capability, within National and International Waters, to:
Provide and respond, within a changing climate, to, the evidence required for sustainable development within a clean, healthy, safe, productive and biologically diverse marine ecosystem and within one generation to make a real difference.
UKMMAS identifies future requirements for marine monitoring over a number of thematic areas, including:
• seabed mapping (habitat types, geology and bathymetry); and
• physical damage (wind farms/dredging).
It is recommended that any future monitoring strategy taken forward for the SEA process becomes an integral part of this wider strategy.
6. References
BMT, 2003. Offshore Wind Energy Generation: Phase 1 Proposals and Environmental Report. For consideration by the Department of Trade and Industry. Cordah/DTI.009.04.01.06/2003.
CEFAS, 2002. Guidelines for the conduct of benthic studies at aggregate dredging sites. Produced for Department for Transport, Local Government and the Regions. May 2002.
CEFAS, 2006. AE0262. Scroby Sands Offshore Wind Farm – Coastal Processes Monitoring. Final Report.
22 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
COWRIE, 2005. Data Management and Coordination Data and Information Management Plan. Version 1.3: Consultation Draft.
MARG, 2007. UKMMAS: A strategy for UK Marine Monitoring and Assessment. UKMMAS Paper – Revised 12th March 2007.
SenterNovem, 2001. Near Shore Wind Farm Monitoring and Evaluation Programme (NSW- MEP). October 2001.
The SEA Directive (2001). Directive 2001/42/EC Of The European Parliament And Of The Council of 27 June 2001 on the assessment of the effects of certain plans and programmes on the environment.
23
Appendix A
Standard Data Request Letter
ABP Marine Environmental Research Ltd Suite B Our ref: R/3613/WSC Waterside House Town Quay Southampton SO14 2AQ Address Tel: +44 (0)23 8071 1840 Fax: +44 (0)23 8071 1841
www.abpmer.co.uk
e-mail: [email protected]
26 May 2006
Dear Sir/Madam
REVIEW OF ROUND 1 SEDIMENT MONITORING DATA - LESSONS LEARNT
We write to request your support in supplying available data from your built offshore wind farm to assist our efforts in completing a research study being conducted on behalf of the UK Department of Trade and Industry (DTI).
Background
DTI has recently commissioned a number of research projects on behalf of the Research Advisory Group (RAG) to advance the generic level understanding of key environmental issues relating of offshore wind farms and to respond to policy level requirements, with the intent of using the outcomes of this research to assist the review process of Round 2 applications. The range of studies now underway includes:
Review of Round 1 Sediment Process Monitoring Data - Lessons Learnt Dynamics of scour pits and scour protection Review of channel migration Review of cabling techniques and effects applicable to the offshore wind farm industry Reef effects
Review of Round 1 Sediment Monitoring Data - Lessons Learnt
The project relating to the Review of Round 1 Sediment Monitoring Data has been awarded to a consortium of technical specialist comprising ABPmer, CEFAS and HR Wallingford. The focus interest for this research project is sediment monitoring data provided to built Round 1 sites, and including information collected from pre-consenting, construction and post-construction phases. For completeness, we also wish to include built projects elsewhere across Europe.
A copy of the project specification is enclosed for reference.
1
Request for Data
The immediate target interest for this project relates to monitoring information and any of the following data types which may be available to your project:
Seabed levels (inc. localised scour development); Seabed features (e.g. bedforms); Surficial sediment coverage (e.g. particle size analysis); Suspended sediment loads; Shoreline profiles (where relevant); Tidal parameters (water levels and currents); and Wave parameters (height, direction, amplitude and period).
We recognise that the nature of such requests may involve time and effort on your part to respond to, but we also trust that you recognise the wider benefits to the offshore wind farm industry of delivering successful outcomes to these research themes. To minimise this effort we wish you to identify initially the extent of any data holdings available through your organisation by way listing the information. The intention would be to review this list and seek to arrange follow-up meetings to obtain the primary data identified as most relevant.
We further wish to identify that ownership of any data supplied for this purpose rests with your organisation and will be treated without prejudice to your project site.
I trust that this introductory letter sufficiently sets out the request for data and the purpose of the research which depends on this information. We will endeavour to contact you directly over the course of the next two weeks to discuss this issue further, and with the intent of arranging dates for visiting your organisation. In the meantime, should you require clarification on any aspect of this request and research then I would be delighted to help.
Yours sincerely for ABP Marine Environmental Research Ltd
Bill Cooper Project Manager
Pan-Government Research Advisory Group on Offshore Renewable Installations - Project Specification
Project Title: Review of Round 1 Sediment Process Monitoring Data - Lessons Learnt (SED01)
Background
Following the Round 2 wind farm Strategic Environmental Assessment, the DTI established a Research Advisory Group (RAG) tasked to establish research priorities in support of offshore wind energy developments. In particular, RAG was tasked to look at the potential environmental impacts of such developments, and consequential impacts on other users of the sea.
To assess the need for further research related to coastal processes, RAG established a sedimentation “theme” and organised a workshop in June 2005 to examine the issues. One of the conclusions to that workshop was that this project should be promoted for potential funding by RAG.
At the RAG meeting held on 11 October 2005, the RAG committee approved the proposal.
Project Aim
The Round 2 wind farm Strategic Environmental Assessment (Offshore Wind Energy Generation: Phase 1 Proposals and Environmental Report for consideration by the Department of Trade and Industry April 2003) recommended that monitoring be carried out at all future developments. In addition to this the SEA recognised the need to use lessons learned in order to design appropriate, achievable and efficient monitoring programmes.
DTI and Defra require the results of Round 1 wind farm site monitoring to be reviewed so that lessons learned can be gleaned and enhanced recommendations on monitoring requirements given for Round 2 offshore wind farm developments. This proposed project addresses the results of sediment process monitoring only.
Project Scope
Sediment process monitoring work carried out on Round 1 developments will be drawn together and reviewed. The review will assess the requirements, methods, data, results and impacts in order to make recommendations for monitoring of R2 developments (as below).
Element 1 - To review the requirement for sediment process monitoring currently placed on individual R1 and R2 developers, together with approaches taken by developers to establish baseline conditions, and such results as are available. It is noted that while compiled reports of R1 monitoring studies are readily available, the data upon which they are based may not be so. For this reason, at an early stage, an assessment shall be made as to what data are available for analysis within the timeframe of the project. The review is to concentrate on the
appropriateness of the strategies and methods adopted and their statistical robustness to detect change over time in areas of high natural variability.
Element 2 - Based on the results of Element 1, to recommend appropriate monitoring strategies for an offshore wind farm or to signpost them if they already exist in published papers or guidance. In meeting this objective consultation should be undertaken with selected wind farm developers, seabed survey contractors, government department advisers and academics to ensure broad input of perspectives.
Element 3 - To assess whether individual wind farm development sediment process monitoring surveys can adequately characterise broad scale effects such as gross changes in sediment budgets over multiple banks and gross changes in sediment budgets. In particular to consider if wider field sampling is necessary to achieve this. This consideration is to assess potential synergies with regional monitoring being undertaken as a result of other initiatives and drivers e.g. under the MCA navigation survey programme.
Element 4 - Reporting and preparation of recommendations. The results of the work should be reported in two documents:
A technical project report outlining the methods, datasets reviewed, results and recommendations; Recommendations (based on the results of the work) to wind farm developers on applicable approaches to sediment process monitoring.
Initial drafts of the technical report and guidance document will be submitted for review to the RAG Theme Leader. Following incorporation of comments a second draft of the technical report and guidance document will be circulated to selected stakeholders for further review. Comments will be collated, reconciled and incorporated into the final issue.
Appendix B
Project Database
Arklow Bank
Arklow Subfolder Filename File Short description Date Pages Comments type Consent empty ES\Vol 1\ AB_EIS-RPT004-1.PDF pdf “EIA Arklow Bank Wind Park. Non June 2001 22 technical summary” ES\Vol 2\ AB-EIS_rpt003-1.pdf pdf “EIA Arklow Bank Wind Park. Final June 2001 396 report.” ES\Vol 3\ AB-EIS_1 - List of pdf List and addresses of consultees February 2 AB-EIS_Appendix001 Consultees.pdf 2001
AB-EIS_ 1-Copy of letter.PDF pdf Letter to consultees March 2 2001 Appendix 1 - Fly Sheet.pdf pdf Fly sheet March 1 2001 Copy of Letter & List of pdf Letter to consultees March 2 Consultees.pdf 2001 Location of Proposed Wind pdf Map of proposed wind park March 1 Park.pdf 2001 Stat list..pdf pdf Same as list of consultees March 4 2001 ES\Vol 3\ Appendix 2 - Flysheet.pdf pdf Fly sheet March 1 AB-EIS_Appendix002 2001 Attendance list.pdf pdf List of attendees to public meetings October 2000 Eitricity invites.pdf pdf Thank you letter May 2001 Let 02.03.01.pdf pdf Letter March 2001 Photograph.pdf pdf Photograph ES\Vol 3\ A3 Doc.pdf pdf Map of proposed turbines and cable March 1 AB-EIS_Appendix003\ routes 2001 Appendix 3 fly sheet.pdf pdf Fly sheet 1 Extra print.pdf pdf Irish Times 11Oct2000 press cut October 1 2000 Information day + speakers.pdf pdf Information day programme 10 October 11 2000 ES\Vol 3\ Media Section 1.pdf pdf Report with press cuttings 19 April 26 AB-EIS_Appendix003\Media 2001 Coverage\ Media Section 2.pdf pdf Press cuttings Oct 2000-Jan 2001 Jan 2001 4 Media Section 3.pdf pdf Press cuttings Feb 2001 Feb 2001 4 ES\Vol 3\ A3 drawing.pdf pdf Map of proposed turbines and cable March 1 AB-EIS_Appendix004\ routes 2001 App 4.pdf pdf Replies from statutory bodies, nog 24 and public. ES\Vol 3\ AB-EIS_5-Preliminary word Outline design June 2001 90 3.Design AB-EIS_Appendix005\ Engineering Design(including 6.4 Scour Append~513.doc
AB-EIS_Appendix 5.PDF pdf Outline design June 2001 90 AB-EIS_Design_included.pdf pdf Outline design June 2001 90 Appendix A&B-Layout Drawings pdf Drawings and sketches 26 & Sketches of Construction ~3D1.pdf Appendix A-Coloured Layout pdf Layout drawing 1 Drawing 2.pdf Appendix A-Coloured Layout pdf Layout drawing (other view) 1 Drawing.pdf ES\Vol 3\ AB-EIS_Appendix 6 - Turbine eml Email with excel attachments AB-EIS_Appendix006\ Layouts.eml ES\Vol 3\ Turbine Layout figures - 3.xls xls AB-EIS_Appendix006\Charts\
Arklow Subfolder Filename File Short description Date Pages Comments type Turbine Layout figures - option pdf Data 3.PDF Turbine Layout figures.PDF pdf Data Turbine Layout figures.xls xls Turbine Layouts - option 4.xls xls Turbine Layouts - option 5.xls xls Turbine Layouts.xls xls ES\Vol 3\ AB-EIS_Appendix 7.PDF Pdf Ship Collision Frequencies and June 2001 54 AB-EIS_Appendix007\ Potential Oil Spillage Trawl and Anchor damage frequencies for Cables ES\Vol 3\ AB-EIS_Appendix 8.PDF pdf Socio-economic Appraisal June 2001 54 AB-EIS_Appendix008\ ES\Vol 3\ AB-EIS_Appendix 9.PDF pdf Prepared by Fugro June 2001 1 AB-EIS_Appendix009\ ES\Vol 3\ AB-EIS_Appendix 10 & 11.PDF pdf Appendix 10- Effect of Wind Farm June 2001 96 2.Existing AB-EIS_Appendix010&011\ Structures- Murphy Dollard report environme Appendix 11- Baseline nt Archaeological report 3.Scour
ES\Vol 3\ Appendix 12 - Assessment pdf Submarine wreck June 2001 32 AB-EIS_Appendix012\ Report Submarine Wreck on Arklo~1F9.pdf ES\Vol 3\ AB-EIS_Appendix 13- pdf Cultural Heritage/ Archaeology June 2001 4 AB-EIS_Appendix013\ Geophysical Report.PDF ES\Vol 3\ AB-EIS_Appendix 14 - pdf Review of commercial fisheries June 2001 29 AB-EIS_Appendix014\ Commerical Fisheries Report.PDF ES\Vol 3\ AB-EIS_Appendix 15.PDF pdf Baseline survey June 2001 51 AB-EIS_Appendix015\ \Monitoring Data\Autumn Autumn Dive Survey Report doc Dive report 2004 35 2004\Survey Reports\ 04.doc Autumn Interim Report.doc doc Hydro graphic & Environmental 2004 53 2.1.3 Monitoring Survey Seabed features Chart1-Aut2004.pdf pdf Cable route swath bathymetry Sep 2004 1 Chart2-Aut2004.pdf pdf Cable route swath bathymetry Sep 2004 1 Chart3-Aut2004.pdf pdf Cable route swath bathymetry Sep 2004 1 Chart4-Aut2004.pdf pdf Cable route swath bathymetry Sep 2004 1 Chart 5 -Turbines-Aut-2004.pdf pdf Swath bathymetry centred on Sep 2004 1 turbines Chart 6 -Turbines-AccDep-Aut- pdf Graphic representation of Accretion 2004 1 2004.pdf / deposition between spring and autumn surveys on turbines Hydroserv- Autumn Interim doc Hydro graphic & Environmental 2004 54 4.4 Seabed Report.doc Monitoring Survey. Interim report scour Monitoring Data\Autumn Autumn Dive Survey Report doc Dive report 2004 35 2004\Survey Reports\Autumn 04.doc Dive Report 2004\ Monitoring Data\Autumn Thumbs.db 2004\Survey Reports\Autumn Dive Report 2004\Autumn Digital Stills 04 Monitoring Data\Spring Spring 2004 Environmental Doc Benthic Ecology Survey Report Dec 2004 9 2004\Benthic ReportS\ Rep.doc Spring 2004 Monitoring Data\Spring 1-#.jpg Jpg Photos with comments 2004\Photos-Spring2004\ # = 1 to 18 Monitoring Data\Spring Hydroserv- Interim report.doc doc Hydro graphic & Environmental 2004 35 2004\Survey Report\ Monitoring Survey. nterim Report – Spring 2004
Hydroserv- Spring04_ Hydro graphic & Environmental 2004 77
Arklow Subfolder Filename File Short description Date Pages Comments type Report.doc Monitoring Survey. nterim Report.
Monitoring Data\Spring empty 2005\Benthic Reports Monitoring Data\Spring Arklow-Spring_2005-Interim doc Hydro graphic Survey. Iterim Report 2005 42 2005\Survey Reports\ Report_v4-210705.doc – Spring 2005
Monitoring Data\Spring Chart5_of_turbines_Sprng_05.pd Pdf Swath bathymetry centred on Apr 2005 1 2005\Survey f turbines Reports\Arklw_Charts_Spring _05\ Spring05_Ch01_Bathy.pdf pdf Cable route swath bathymetry Apr 2005 1 Spring05_Ch02_Bathy.pdf pdf Cable route swath bathymetry Apr 2005 1 Spring05_Ch03_Bathy.pdf pdf Cable route swath bathymetry Apr 2005 1 Spring05_Ch04_Bathy.pdf pdf Cable route swath bathymetry Apr 2005 1 Monitoring Data\Spring *. Turbine #.jpg jpg Photos with comments 2005\Survey Reports\Digital * = 1 to 28 Pics File & Index_Sprng_05\ # = 1 to 7 Hydroserv Digital Film Log pdf Index of photos Jun 2005 1 Index.pdf Other\ Arklow%20modelling%20new1.p pdf Publicity doc 2 df Arklow_infosheet_Sept.pdf pf Info sheet 2 Marine%20Notice%2029- pdf Marine notice 2003 4 2003%20Arklow%20Bank%20Of fshore%20Wind%20Project.pdf
Barrow
Barrow Subfolder Filename File Short description Date Pages Comments type ES\ Non_Technical_Summary.pdf Pdf Non technical summary May 2002 8 FEPA\ Barrow_licence2.pdf pdf License Jul 2003 20 Monitoring 3 Tides.pdf pdf Plot with data for 3 tides (March05- Data\Morphology\bowbathym Apr05) etry\ Area_final.ecw ecw Area_final.ers ers erviewer20e.exe View mosiac readme.txt txt Instructions to see the mosaic Monitoring Daily Log #.xls xls Geophysical and bathymetric Apr 2005 1 Data\Morphology\bowbathym # = 010405, 020405, 290305, surveys log etry\Daily Logs\ 300305, 310305
Monitoring C5002-#.pdf pdf Trackplot for monitoring surveys Apr 2005 1 Data\Morphology\bowbathym # = 01-01, 01-02, 01-03, 01-04, etry\Drawings\pdf versions\ 02-01, 02-02, 02-03, 02-04, 03- 01, 03-02, 03-02 navcheck.pdf pdf Navigation verification Mar 2005 1 vesseloffset-LIA.pdf pdf Vessel offset diagram Mar 2005 1 Monitoring C5002 report-rev01.doc doc Pre-construction geophysical survey May 2005 29 Data\Morphology\bowbathym etry\Final Report text\ \Monitoring Barrow Benthic Sampling xls Benthic survey logs Dec 2004 Data\Morphology\BOWpsa\ Logsheet.XLS Barrow PSA Charts_1.xls xls Particle size analysis Dec 2004 Barrow PSA Charts_2.xls xls Particle size analysis Dec 2004 Figure 1 Sample locations Model pdf Benthic survey sampling locations 1
Barrow Subfolder Filename File Short description Date Pages Comments type (1).pdf Monitoring Data\Scour zip pdf All the files below BOWF-115-SVY-1026-01 BOWF zip Scour Pit survey Final report Jul 2005 23 + Scour Pit Survey.pdf Pile No.T-#.pdf Bathymetry of pile scour (25m 1 + # = 01, 04,08,18,20, 21, radius) 24,25,26,27, 28,29,30 Monitoring Data\Suspended C5031-BoWreport-rev01.zip zip Contains: Sediments\ C5031-BoWreport-rev01.pdf BOW-Navcheck.pdf C5031-01.pdf Vesseloffset-Barinthus.pdf Nephelometric Turbidity.pdf pdf Chart with nephelometric Turbidity 1 Other\ Barrow Offshore Wind Shortcut to bowind web page Barrow scour report.htm Htm Emails between Tom Coates, Bill July 2006 Cooper and Jon Rees FW Barrow.htm htm Tom Coates email with contact details Positions and establishing xls Turbines positions data dates.xls
Blyth
Blyth Subfolder Filename File Short description Date Pages Comments type ES\ empty FEPA\ Burbo_licence.pdf pdf License for wind farm 19 June 2003 Monitoring Data\Morphology empty Monitoring Data\Scour empty Monitoring Data\Suspended empty Sediments Other\ Shortcut to report_020.pdf Shortcut to report “Design methods Nov 2003 4. Analysis for offshore wind turbines at of exposed sites” Final report of the environme OWTES project which is under ntal conds general\report_020.pdf 8 Recommen dations for design (inc. scour)
Burbo
Burbo Subfolder Filename File Short description Date Pages Comments type ES\ empty FEPA\ Burbo_licence.pdf pdf License for wind farm 19 June 2003 Monitoring Data\Morphology empty Monitoring Data\Scour empty Monitoring Data\Suspended empty Sediments Other\ Burbo Bank monitoring data.txt Txt Email from Jesper Lykke Pedersen 23 June To: Andrew Symonds; Bill S Cooper 2006
Burbo Subfolder Filename File Short description Date Pages Comments type Cc: Kim Ahle Subject: Burbo Bank monitoring data Burbo Offshore Shortcut to The Burbo project May 2005 website in Elsam Environmental condition Burbo pdf Environmental conditions, Burbo 3.Global Design Basis. ver 4.pdf Offshore Wind Farm, Background seabed report for Design Basis changes Version 4 4. Sea level and water depth 5.Wind 6.Waves 10.Scour SV Burbo Bank monitoring htm Email between Jesper Lykke July 2006 data.htm Pedersen, Bill S Cooper and Kim Ahle. Subject: SV: Burbo Bank monitoring data
General
General Subfolder Filename File Short description Date Pages Comments type RCEM_2003.pdf pdf Poster on “Local scour in a tidal 2003 1 + environment: a case study from the Otzumer Balje tidal inlet, southern North Sea” by Noormets et al report_002.pdf pdf “Potential Effects of offshore wind 2002 127 2.Definition developments on coastal of study processes” parameters
report_003.pdf pdf “An assessment of the 2000 76 environmental effects of offshore wind farms” report_020.pdf pdf “Design methods for offshore wind Nov 2003 4. Analysis turbines at exposed sites” Final of report of the OWTES project environment al conds 8 Recommend ations for design (inc. scour) report_040.pdf pdf “Guidance note for environmental Jun 2004 48 3. Coastal Impact assessment in respect of and FEPA and CPA requirements. Ver sedimentary 2” processes
Report_Windguard_V24.pdf pdf “Case study: European offshore 158 wind farms. A survey for the analysis of the experience and lessons learnt by developers of offshore wind farms” standard_environmental.pdf pdf “Standards for EIA of offshore wind Feb 2003 55 turbines in the marine environment” COD database\ COD_Env_Issues%20Report_17 pdf “Concerted Action for Offshore Wind 2005 118 _11.pdf Energy Deployment. Work Package
General Subfolder Filename File Short description Date Pages Comments type 4: Environmental Issues” COD_InfoSheet_DatabaseEnviro pdf 4 nment_July2005.pdf Information sheet on the COD environmental database
COD-Final_Rept.pdf pdf “Concerted Action for Offshore Wind 2005 44 Energy Deployment. Principal Findings 2003-2005” Offshore Wind Energy - htm Offshore Wind Energy - Information Information for professionals.htm for professionals report_029.mdb Acce ss report_029_97.mdb acce ss COD database\Offshore Wind keuze.html Energy - Information for professionals_files\ keuze.html Txt2html.html COD database\Offshore Wind logo_ca-owe-1.jpg All these files are supporting the htm document “Offshore Wind Energy - Energy - Information for Information for professionals” professionals_files\keuze_file s logo_ca-owe-2.jpg Wew.css COD database\Offshore Wind collision_risks.gif Energy - Information for professionals_files\txt2html_fil es\ visualisation.jpg wew.css
Horns Rev
Horns Rev Subfolder Filename File Short description Date Pages Comment type \ES\ horns rev eia (part).pdf pdf “EIA of sea bottom and marine biology” Mar 43 2000 horns rev wq.pdf pdf “EIA on WQ” May 24 2005 Resume_eng.pdf pdf “Summary of EIA report” May 17 2005 Monitoring Data\Morphology empty Monitoring Data\Scour empty \Monitoring Data\Suspended empty Sediments Other\ Engelsk hjemmeside - frameset htm Shortcut to Elsam webpage on Horns Rev Hard Bottom Status Report pdf Hard bottom substrate monitoring. 2004 79 2004-R2438-03-005-rev3.pdf Annual status report 2004 hard_bottem_substrate_monitori pdf Hard bottom substrate monitoring. 2004 15 ng_march_2004.pdf Survey report n1 Introducing_Hard_Bottom_Subst pdf Introducing Hard bottom substrate sea 2002 67 3.Sed rate_Data-2001_rev2.pdf bottom and marine biology. Data report Annex 2001 4.Particle size characterist ics Introducing_Hard_Bottom_Subst pdf Introducing Hard bottom substrate sea 2002 73 3.Sed rate_Status-2001.pdf bottom and marine biology. Status report 2001 Memorandum_Baseline_surveye pdf Memorandum Re: Summary Baseline 2002 9 s_2001.pdf surveys 2001 POST-CONSTRUCTION-Annual pdf Hard bottom substrate monitoring. 2003 62 Report-2003-Hardbottom.pdf Annual status report 2003 review rapport 2004 version0.pdf pdf Review report 2004 2004 135 The Danish Offshore Wind Farm Demonstration Project: Horns Rev and Nysted Offshore Wind Farms Environmental impact assessment and monitoring.
Kentish Flats
Kentish Flats Subfolder Filename File Short description date Pages Comments type \ES\ 0394 Final Report.pdf pdf Environmental Statement Aug 297 + 2002 kentish_flats_nts.pdf pdf Non-technical summary Aug 8 2002 \ES\Chapter# These directories contain the text in pdf and the figures in pdf of each of the chapter of the ES above, including the contents and the executive summary \FEPA\ KFlats_licence.pdf pdf License 7 Mar 19 2003 Monitoring Data\Morphology\ Debris Post-con.zip zip Contains: J1020869_101.pdf J1020869_102.pdf J1020869_103.pdf 0869_debris final report.pdf Debris Pre-con.zip zip Contains: J1020708a.102.pdf
Kentish Flats Subfolder Filename File Short description date Pages Comments type J1020708a.101.pdf 0708a Final Report Debris.pdf Monitoring Data\Morphology READ ME TO VIEW FILES!.TXT txt Instructions to view autocad files 1 \Hydrographics and Geophysics\Figures J_1_02_0387_001&002.dwg dwg Bathymetric contour sheet- area C&D Jul 2002 J_1_02_0387_003.dwg dwg Sidescan sonar classification chart Jul 2002 J_1_02_0387_004A.dwg dwg Seismic profiles- Sheet A Jul 2002 J_1_02_0387_004B.dwg dwg Seismic profiles- Sheet B Jul 2002 J_1_02_0387_004C.dwg dwg Seismic profiles- Sheet C Jul 2002 J_1_02_0387_004D.dwg dwg Seismic profiles- Sheet D Jul 2002 J_1_02_0387_004E.dwg dwg Seismic profiles- Sheet E Jul 2002 J_1_02_0387_005.dwg dwg Rockhead Isopach Jul 2002 J_1_02_0387_006.dwg dwg Topographic survey Cable landing, Jul 2002 Horne Bay Monitoring Data\Morphology Lots of different files in directory Files to install Volvo View Express to \Hydrographics and view the dwg files Geophysics\Figures\VVE Monitoring Data\Morphology Kentish Flats Hydrographic & pdf Hydrographic and geophysical survey Jul 2002 21 \Hydrographics and Geophysics.pdf Geophysics\Text\ Monitoring Data\Scour Swath post-con1.zip Mar + Contains: 2005 J1020758.101 Overview of Swath Data – Cable Corridor J1020758.102 Bathymetric Plot – Turbines E2 J1020758.103 Bathymetric Plot – Turbines F2 J1020758.104 Bathymetric Plot – Turbines F3 J1020758.105 Bathymetric Plot – Turbines F4 J1020758.101a Comparison Plot of Pre-Construction and Post-Construction Bathymetry J1020758.102a Bathymetric Comparison Plot – Turbine E2 J1020758.103a Bathymetric Comparison Plot – Turbine F2 J1020758.104a Bathymetric Comparison Plot – Turbine F3 J1020758.105a Bathymetric Comparison Plot – Turbine F4 0758 Final Report.pdf “Post construction swath survey” Swath post-con2.zip Dec + Contains: 2005 J.1.02.0869.201 Overview of Swath Data – Cable Corridors J.1.02.0869.202 Bathymetric Plot – Turbine E2 J.1.02.0869.203 Bathymetric Plot – Turbine F2 J.1.02.0869.204 Bathymetric Plot – Turbine F3 J.1.02.0869.205 Bathymetric Plot – Turbine F4 J.1.02.0869.201a Comparison Plot of Pre-Construction and Post- Construction Survey 2 J.1.02.0869.202a Bathymetric Comparison Plot – Turbine E2 J.1.02.0869.203a Bathymetric Comparison Plot – Turbine F2 J.1.02.0869.204a Bathymetric Comparison Plot – Turbine F3 J.1.02.0869.205a Bathymetric Comparison Plot – Turbine F4 0869_scour final report.pdf “Post-Construction Swath Survey2” Swath post-con3.zip Apr + Contains: 2006 J.1.02.0942.301 Overview of Swath Data – Cable Corridors J.1.02.0942.302 Bathymetric Plot – Turbine E2 J.1.02.0942.303 Bathymetric Plot – Turbine F2 J.1.02.0942.304 Bathymetric Plot – Turbine F3 J.1.02.0942.305 Bathymetric Plot – Turbine F4 J.1.02.0942.301a Comparison Plot of Pre-Construction and Post- Construction Survey 3 J.1.02.0942.302a Bathymetric Comparison Plot – Turbine E2 J.1.02.0942.303a Bathymetric Comparison Plot – Turbine F2 J.1.02.0942.304a Bathymetric Comparison Plot – Turbine F3 J.1.02.0942.305a Bathymetric Comparison Plot – Turbine F4 0869_scour final report.pdf “Post-Construction Swath Survey3”
Kentish Flats Subfolder Filename File Short description date Pages Comments type Swath pre-con1.zip Jan + Contains: 2005 J.1.02.0708b.101 Overview of Swath Data – Cable Corridors J.1.02.0708b.102 Contour Plot – Turbine E2 J.1.02.0708b.103 Contour Plot – Turbine F2 J.1.02.0708b.104 Contour Plot – Turbine F3 J.1.02.0708b.105 Contour Plot – Turbine F4 0708b_ Final Report Swath.pdf “Pre-Construction Swath Survey3” \Monitoring Data\Suspended Turb monitoring rep.doc doc “Turbidity Monitoring” Apr 13 Sediments 2005 Other\ Kentish Flats offshore wind farm Htm Shortcut to elsam webpage with Kentish Flats info Kentish Flats offshore wind htm Time schedule webpage saved farm.htm other\Kentish Flats offshore Lots of different files Files necessary to open Kentish wind farm_files Flats offshore wind farm.htm
North Hoyle
North Hoyle Subfolder Filename File Short description date Pages Comments type \ES\ NH ES Chp 1.pdf pdf Environmental Statement Ch1- 2002 4 Introduction NH ES Chp 2.pdf pdf Environmental Statement Ch2- 2002 18 + Project description NH ES Chp 3.pdf pdf Environmental Statement Ch3 - 2002 6 Regulatory and Policy context NH ES Chp 4.pdf pdf Environmental Statement Ch4 – 2002 59 + Existing Environment NH ES Chp 5.pdf pdf Environmental Statement Ch5 – 2002 70 + Assessment of EI NH ES Chp 6.pdf pdf Environmental Statement Ch6 – 2002 9 Mitigation Measures NH ES Chp 7.pdf pdf Environmental Statement Ch7 - 2002 3 Monitoring NH ES Chp 8.pdf pdf Environmental Statement Ch8 - 2002 4 + Conclusions NH ES Front Cover & Contents.pdf pdf Environmental Statement cover and 2002 4 contents NH ES NTS English.pdf pdf ES non-technical summary in 2002 13 english NH ES NTS Welsh.pdf pdf ES non-technical summary in welsh 2002 13 NH ES References.pdf pdf ES references 2002 4 \FEPA\ NHoyle-new_licence.pdf pdf License 19 Mar 16 2003 Monitoring Fugro_pre-construction.pdf pdf “North Hoyle Geophysical Survey” Sep 66 Data\Morphology\2001 (same report as below but scanned 2001 Fugro_pre_construction in) NH Geophysical survey - survey pdf “North Hoyle Geophysical Survey” Sep 66 service 04Jul to 25Aug 01.pdf 2001 f-cht01-trk.dgn ? ? f-cht02-trk.dgn ? ? f-cht03-bathy.dgn ? ? f-cht04-bathy.dgn ? ? f-cht05-sfeat.dgn ? ? f-cht06-sfeat.dgn ? ? f-cht07-isop.dgn ? ? f-cht08-isop.dgn ? ?
North Hoyle Subfolder Filename File Short description date Pages Comments type f-cht09-profs.dgn ? ? f-cht10-rte.dgn ? ? Monitoring hydrographic & geophysical site pdf Hydrographic and geophysical site Aug 50 Data\Morphology\2003 survey - 382127, Aug02, vol1.pdf survey 2002 Exploration_Associates_Pre_ construction\ Base_grids.dwg dwg 2127_03A.dwg dwg Seabed features chart 2002 2127_03B.dwg dwg Seabed features chart 2002 Sss.dwg dwg Monitoring L2127_P2_rev1.pdf pdf “Phase 2 Bathymetric Survey” Feb 37 Data\Morphology\2003 2003 Exploration_Associates_Pre_ construction\Report Monitoring 2127_01c.dwg dwg Vessel trackplot Feb Data\Morphology\2003 2003 Exploration_Associates_Pre_ construction\Report\enclosure C 2127_02c.dwg dwg Vessel trackplot Feb 2003 2127_02d.dwg dwg Vessel trackplot Feb 2003 Monitoring base_grids.dwg dwg Data\Morphology\2003 Exploration_Associates_Pre_ construction\Report\enclosure C\XREFS Bathy_Feb03.dwg dwg Bathy_July02.dwg dwg trackplot_Feb03.dwg dwg Monitoring hoyle_feb03.xyz txt Data file Feb Data\Morphology\2003 2003 Exploration_Associates_Pre_ construction\Xyz\ Monitoring C4014 Bathy All.tif tif Data\Morphology\2004 OSIRIS_Post_Construction\D rawings C4014 Bathy All.tfw tscale.tif tif Scale C4014b-01-ACAD.dwg dwg Trackplot Jan 2005 C4014b-02a-ACAD.dwg dwg Pseudo color bathymetry Jan 2005 C4014b-02b-ACAD.dwg dwg Swath bathymetry (relief) Jan 2005 C4014b-03-ACAD.dwg dwg Sonar mosaic including contacts Jan 2005 \Monitoring 4014_Mosaic.ecw ? ? Data\Morphology\2004 OSIRIS_Post_Construction\M osaics 4014_Mosaic.aux ? ? Monitoring C4014b report-rev01.doc “Post construction Jan 22 Data\Morphology\2004 Geophysical survey 2005 OSIRIS_Post_Construction\R eport Data collection period - 12th august
– 12th october2004”
Monitoring Final XYZ 20mBIN Cleaned.xyz txt Xyz data
North Hoyle Subfolder Filename File Short description date Pages Comments type Data\Morphology\2004 OSIRIS_Post_Construction\X YZ Final XYZ 20mBIN Cleaned txt Xyz data negative values.xyz Final XYZ 20mBIN Cleaned txt Xyz data negative values.txt Final XYZ 20mBIN Cleaned txt Xyz data negative values.dbf \Monitoring Data\Scour\2004 Same files as in pdf versions directory but in dgn format Scour Survey\Drawings\Microstation \Monitoring Data\Scour\2004 C4014a-01-v8.pdf pdf Trackplot Scour Survey\Drawings\pdf versions Figure#.pdf pdf Detail of Turbine WTG#. Swath 1 + # = 1 to 33 image with bathymetric contours and grab sample locations Monitoring Data\Scour\2004 T3.zip Cannot open it! Scour Survey\Images\2D images Monitoring Data\Scour\2004 3D images.zip Tif Images + Scour Survey\Images\3D Containing: and images tt#.tif & tt#.tfw tfw # = 1 to 30 met m se 3d.tfw & .tif met mast3 3d.tfw & .tif met m nw 3d.tfw & .tif \Monitoring Data\Scour\2004 CompleteLog.xls xls Post construction survey logs for 26 Scour Survey\Report\Daily days Logs \Monitoring Data\Scour\2004 C4014a report-rev01.doc “Scour monitoring Mar 24 + Scour Survey\Report\Report Surveys. Data collection period - 2005 text 12th august – 12th october 2004”
\Monitoring Data\Scour\2004 metmast 1 0-5mbin.xyz xyz Xyz data + Scour Survey\XYZ metmast 2 0-5mbin.xyz xyz Xyz data + metmast 3 0-5mbin.xyz xyz Xyz data + Turb# 0-5mbin.xyz xyz Xyz data for turbine # + # = 01 to 33 \Monitoring Data\Scour\2005 C5011 report-rev00.doc doc “Scour monitoring Jul 2005 30 + Scour Survey Surveys. Data collection period - 26th april – 2nd may 2005”
Figure#.pdf pdf Swath image with bathymetric Aug 1 + # = 1 to 33 contours for 2004 and 2005 2005 surveysand 3d representation of volume comparison of turbine location 2004 against 2005. \Monitoring Data\Suspended Shortcut to chapter4.pdf pdf Shortcut to “Chapter 4 Marine Seds” 19 Sediments under other\chapter4.pdf Turbidity position.doc doc Doc about turbidity position 1 Monitoring Data\Suspended 371_020.csv csv Turbidity data Sediments\Turbwavejune04 371_031.csv csv Turbidity data 371_032.csv csv Turbidity data Monitoring Data\Suspended 3p.csv csv Turbidity data Sediments\Turbwavejune04\3 points 3p.xls xls Turbidity data and plots with waves \Monitoring Data\Suspended turbwave3.xls xls Turbidity data and plots with waves
North Hoyle Subfolder Filename File Short description date Pages Comments type Sediments\turbwavepost \Monitoring Data\Suspended turb2.xls xls Turbidity data and plots Sediments\turbwave-pre turbwave2.xls xls Turbidity data and plots with waves \other\ baseline_monitoring.pdf pdf Baseline monitoring report Jun 41 2003 chapter1.pdf pdf Annual FEPA monitoring report Jun 10 2005 Chapter4.pdf pdf Chapter 4. Marine Sediments 19 chapter11.pdf pdf Chapter 11. Additional agreements 2 on detail and reporting npower renewables htm Npower website on North Hoyle
Appendix C
Review of Suspended Sediment Concentrations
Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Appendix C Review of Suspended Sediment Concentrations
C.1 Background
The primary requirement placed on developers to measure suspended sediment concentrations (SSC) arises from the need to quantify potential environmental impacts that may arise due to the placement of wind farm structures into the seabed, notably cables and foundations, and to assess if these impacts are significant to any environmental receptors.
The general assumption in the environmental impact assessment is that the major impact occurs during the construction phase where the mechanical disturbance of the seabed, through the installation of structures, heightens the risk for increased sediment disturbance and elevated levels of suspended sediment. The interpretation of impact is associated with the relative increase in suspended sediment concentrations over background levels and the further risk of sensitive receptors becoming smothered as the material settles out. In general terms, it is assumed that such impacts remain localised, of moderate to high intensity and are short-lived.
In most studies it has been assumed that the disturbed materials are entirely of inorganic origin and can be assessed in terms of physical properties of sediments. In real terms material in suspension and materials on the seabed may each comprise of organic and inorganic matter. Materials in suspension can also be express as ‘Total Suspended Solids’ (TSS), a term which encompasses both inorganic solids such as clay, silt, and sand, and organic solids such as algae and detritus; and is a measure of the dry weight of suspended solids per unit volume of water.
Disruption to the seabed also has the potential to release sediment bound contaminants, such as heavy metals and PAH/PCBs, into the water, particularly in areas that have historically been used for industrial, sewage or ammunition disposal. These contaminants could then be responsible for water quality issues both in and outside of the license area. Visual impacts of large sediment plumes may also be an issue.
C.2 EIA Guidance
In the early phase of wind farm development the assessment of impacts has generally been based on theoretical cases which have lacked corroborating field evidence. The Round 1 offshore wind farms were deliberately aimed at providing experience to the industry and to develop an evidence base of understanding to support the succeeding phase of larger scale commercial projects. The regulatory process recognised these limits and provided a combination of generic guidance for the EIA process (CEFAS, 2001 and 2004) along with site-specific mitigation considerations incorporated into the
C.1 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
conditions of the FEPA licence as a series of monitoring requirements to generate the further evidence to validate the general hypothesis of localised, high-intensity and short-term impacts.
The issue of suspended sediment concentrations is recognised within these considerations and has required the developer to include the following within the scope of the EIA:
• Establish the baseline sedimentary processes, including quantification of SSC’s.
• Assess magnitude and significance of any changes caused directly over both the near-field and far-field related to:
o scour around any supply cables overlying the surface and the resulting potential for higher SSC’s; and
o effect of localised sediment dumping and higher SSC’s resulting from any cable burial process.
C.3 FEPA Requirements
In the case of consented Round 1 projects, Defra has tended to ask for the following monitoring requirements, on a consistent basis, and as a condition of their FEPA licence:
The following monitoring is required in order to validate and confirm predictions. Monitoring will use Optical Backscatter Sensor (OBS) instrumentation, and will comprise three fixed moorings (locations to be agreed) plus selected water column profiling, during construction, of any sediment plume. Full calibration of the sensors will be required (see below for details), and should be included in a report and presented to the Licensing Authority. Instrument deployment will be for a period of at least four weeks, and will be provided concurrently at each site during the pre-construction, construction and post- construction periods.
Calibration of Sensors
• Using water samples collected during each of the three deployment periods and during the selected sampling; • Covering a full tidal cycle, at hourly intervals and during spring tides (provided construction occurs over this period), during each of the three deployment periods, and at hourly intervals for two monopiles, for the duration of any sediment plume associated with construction activities; • And using subsequent standard techniques for gravimetric and particle size distribution analysis to enable accurate conversion of results to SSC.
These would need to be deployed as follows:
C.2 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
• At a representative point identified by the modelling and within the sediment plume to measure near-field effects of sediment release. • At a representative point identified by the modelling and within the sediment plume to measure far- field effects • At a point outside the predicted area of the sediment plume to provide a 'control' measure of natural suspended sediment levels over the respective monitoring periods.
Alternative approaches may be acceptable but the methodologies would have to be submitted to the Licensing Authority for review and agreement at least one month prior to the proposed commencement of the monitoring work.
In line with condition 9.4 of the Licence should suspended sediment levels associated with the construction works be shown to be at unacceptable levels (i.e. above threshold) works may need to be suspended while a less disruptive methodology is investigated. Background levels from the monitoring programme will be used to set suitable threshold levels.
C.4 Measuring Suspended Sediment
Conventional techniques to measure suspended sediment, such as OBS or ABS devices, rely on surrogate properties of the water to determine levels of turbidity which do not necessarily represent direct measures of suspended sediment.
For the OBS approach, turbidity is commonly defined as an "expression of the optical property that causes light to be scattered and absorbed rather than transmitted in straight lines through the sample." Hence, turbidity measurements provide a reading of the amount of scattered light and cannot be directly related to a gravimetric equivalent unless a working curve for the specific sample is created. The intensity of scattered light is affected by many variables including wavelength, particle size, colour, and shape, but also by any materials of organic origin. The unit of measure adopted by the ISO Standard is the FNU (Formazine Nephelometric Unit), however, it is frequently the case that the alternatives of NTU (Nephelometric Turbidity Unit) and FTU (Formazin Turbidity Units) are quoted. Critically, these all equate to the same unit i.e. 1 FNU = 1 NTU = 1 FTU.
Acoustic backscatter sensors (ABS) are a more recent development and rely on similar backscatter principles to OBS devices. The technique has developed from interpretation of the sound intensity signal and transmission losses from acoustic current Doppler profiler (ADCP) devices which generally provide velocity profiles over depth. Here reductions in sound intensity are attributed to particulate backscatter and a full vertical profile can be inferred. In comparison, an OBS device only provides a single point measure.
There are, however, two practical limitations to the method of predicting SSC from OBS and ABS measurements. The first is a limitation common to any single frequency instrument that cannot differentiate between changes in concentration and changes in
C.3 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
particle size distribution, with a risk that a change in size distribution being interpreted as a change in concentration unless independent particle size distribution measurements indicate need for additional calibrations. Secondly, acoustic and optical methods respond differently to particle size with acoustic sensors more sensitive to large particles (proportional to volume) and optical sensors more sensitive to small particles (proportional to cross sectional area (USGS, 2002).
Turbidity in seawater is caused by the presence of un-dissolved but finely dispersed matter which may have organic and/or inorganic origins. Neither OBS nor ABS can discriminate between these two, and the only proven method to quantify a sediment concentration is through direct sampling. Water samples can then be analysed to determine both a sediment load (normally expressed in mg/l) and a description of the particle size(s). Of course, water sampling has its own limitations in that the technique remains as a series of discrete samples in time and space.
Good practice is to combine water sampling with either OBS or ABS methods to develop a means of correlation. Difficulties arise in ensuring sufficient water samples are recovered to account for the range in SSC levels under different metocean conditions, over depth and in regards to varying particle size. During severe conditions of strong flows and high waves a mobile seabed is likely to yield the maximum levels of SSC into the water column, with highest concentrations closest to the seabed. Water sampling during such severe conditions is likely to be problematic, so extrapolation of the correlation is often required on such occasions, which is likely to introduce further error.
Dredging Operations and Environmental Research (DOER) provides a short technical note describing the practice of calibration of turbidity against water samples (Thackston and Palermo, 2000).
C.5 Factors affecting Suspended Sediment Concentrations
Background suspended sediment concentrations measured at any location have the potential to comprise of (typically very fine) sediments that may have been advected into the area from a distant source and/or (typically coarser) materials mobilised from the local sea bed at times when the force exerted on the sea bed through the activity of waves and currents exceeds the threshold for transport and vertical mixing due to increased turbulence brings the sediment into suspension. The coarser sediments may then only remain in suspension whilst these forces persist, thereafter falling back to the seabed at the rate of their settling velocity.
Whether sediment is cohesive or non-cohesive also has a large influence over the length of time taken for suspended sediments to settle out of a water column. Cohesive sediment particles, such as clays and silts, tend to stick together due to
C.4 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
flocculation to form larger particles. These flocs may then have a faster settling velocity than that of the individual particles, up until the point when hindered settling begins.
Suspended sediment concentrations through the water column may be inherently variable as a combination of processes, with near-bed concentrations highest at times when the local bed material remains mobilised. Variation in SSC may also exhibit strong seasonal changes between summer and winter periods responding to increased storm activity and river discharges.
Alongside these natural variations there may also be a number of other activities which contribute at times to elevated SSC by either working the seabed or disposing materials onto the seabed, such as:
• Aggregate dredging, inc overspill; • Harbour dredging, inc spoil disposal; • Seabed installations, inc offshore wind farm foundations, oil & gas platforms; and • Pipelines and cables.
Should a combination of this activities take place in close proximity to one another and coincidently then a consideration of in-combination effects may become necessary.
C.6 Factors affecting Sediment Disturbance
The degree to which any area of seabed might respond to construction activity is also dependent on a number of factors, these include:
• Geotechnical properties of the seabed and sub-soils (e.g. rock, gravel, sand or mud); • Mobility of the seabed to prevailing hydrodynamic conditions; • Method of construction (e.g. piling, drilling, ploughing, jetting, trenching, levelling, etc); • Type of construction (e.g. foundation type, burial requirements for cables, etc); and • Rate of construction (e.g. volume of sediment released over time).
Installation techniques for mono-piles involve either piling or drilling, or combinations of the two depending on the soil conditions. The piling process is assumed to lead to no elevated concentrations of suspended sediment. In comparison, drilling creates arisings that are brought above the seabed and need to be disposed of in some way. The risk to elevating suspended sediment concentrations then relates to the composition of the arisings (sediment type(s) and grade(s)) and the method of disposing of this excess material.
C.5 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Installation techniques for gravity base foundations are likely to involve a process of seabed levelling, noting that the process of levelling has not yet occurred for any UK offshore wind farm. Levelling methods are expected to involve either dredging or raking with the disposal of any material either to barge or simply being cast aside. The volume of material disturbed depends on the depth of levelling and the scale of the gravity base, which in some cases is quoted as being between 30 to 60m in diameter.
Installation techniques for cables have involved combinations of jetting, ploughing and trenching. The selection of which method is applied is normally decided by the cable contractor and depends on soil conditions and logistical constraints. A detailed review of cabling techniques and effects applicable to the offshore wind farm industry has been reported in a separate research study (DTI, 2006).
The construction process may well disturb materials and introduce new sediments which are not normally present on the seabed, especially if drilling is required into rocks and this material is disposed of locally.
The EIA investigations have, in general, included consideration of the potential impacts arising from sediment disturbance and commonly assessed the consequence in terms of ‘sediment spill’ analysis and predicting the fate of material using modelling tools which can simulate the behaviour of a ‘sediment plume’.
Once sediment has been disturbed it is likely to be loosened (less consolidated) and therefore be more prone to be taken into suspension. Once material becomes suspended then areas with strong tidal flows have the potential to advect the sediment away from source. This distance depends on the rate of flow and the time over which sediments may remain in suspension. Typically, coarser sediment may fall out of suspension far earlier than fine sediments. Finer particles such as clays and silts have a lower threshold for suspension and as such are likely to suspend higher within the water column and take longer to settle out. As a consequence, coarse sediments are likely to settle close to the site of construction whilst finer sediments could become more widely dispersed and transported a significant distance before being finally re- deposited. This also means that whilst fine sediments are likely to be carried further away from source, the thickness of the layer finally deposited will be significantly less than resulting from a coarser material.
C.7 Available Evidence from Offshore Wind Farms
Suspended sediment monitoring forms part of the available evidence base which has been collated for this study. Measurements have generally been taken during the planning and construction stages of each project. Each monitoring period is short-term
C.6 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
and generally limited to a small number of discrete locations close to each wind farm location.
The availability of SSC field evidence obtained for this review is shown in Table C.1.
Table C.1 Collated data on suspended sediment concentrations Site Pre-construction Construction Post-construction Cable route a. Arklow Bank b. Barrow ● c. Blyth d. Burbo ● e. Horns Rev f. Kentish Flats ● g. North Hoyle ● ● ● h. Nysted ● i. Scroby Sands ● ● ●
For Arklow, Blyth and Horns Rev, the EIA considerations have each concluded that the significance of raised suspended sediment concentrations is negligible in the context of background ambient loads.
For Burbo, monitoring is understood to be progressing according to the FEPA requirements, but as yet the full data has not been formally submitted to Defra.
C.8 Site by Site Review
a. Arklow Bank (Ireland)
Site Description
Completed in 2003, Aklow Bank is a small offshore wind farm located around 13km off the east coast of Ireland (hence outside of UK waters) and on a large linear sandbank of the same name. Water depths across the site are quite shallow at between 2 to 5m CD and with a seabed comprising well-sorted sands and gravels. The project currently comprises 7 turbines mounted on mono-pile foundations. These mono-piles did not require any drilling into sub-soils to achieve design depths.
C.7 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Monitoring Requirements
There has been no suspended sediment monitoring reported for Arklow Bank. The EIA considered that the opportunity for elevated suspended sediment concentrations during construction was a low risk due to the rapid deposition of any disturbed materials. Several post-construction diver surveys have been completed due to concerns about cable burial and spanning at J-tubes. These surveys generally encountered good visibility in the water column and observed large amounts of biological growth on structures.
Key Documents
Sure Partners, 2001. Environmental Impact Assessment - Arklow Bank Wind Park. Final Report. June 2001.
b. Barrow (UK)
Site Description
Barrow is the fourth Round 1 project and came into operation in 2006. The wind farm is located around 7km south-west of Walney Island in an area where the seabed shelves gently from around 12 to 18m CD. The site is exposed to severe wave conditions generated within the Irish Sea and macro tidal range and with current speeds up to around 0.8m/s. The seabed sediments are generally sandy with some muds and gravels. As with the majority of other Round 1 sites, Barrow comprises of 30 mono-pile foundations. The soil conditions at this site have required a combination of piling and drilling to achieve full installation depth.
Monitoring Requirements
The project has been subject to a FEPA licence and associated monitoring conditions.
Studies to support the Environmental Statement highlighted the likelihood for short- term elevated suspended sediment concentrations during the construction period and related to cable laying and drilling activities, in particular. Baseline (pre-construction) suspended sediment concentrations were developed from earlier fieldwork dating from 1970 and values of 0.3 to 0.5kg/m3 (equivalent to 300 to 500mg/l) are quoted for areas around Heysham and Middleton Sands, within Morecambe Bay.
The original FEPA SSC monitoring requirements were prescribed with a standard arrangement based on the use of OBS instrumentation for a period of 4 weeks at 3 fixed moorings, plus selected water column profiling during the pre-construction, construction and post-construction phases.
C.8 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
It appears that the actual surveys undertaken deviate from the stated FEPA requirement and are based on monitoring the track of a sediment plume during drilling operations on 6th October 2005. A vessel was used to tow an OBS device (Hydrolab Datasonde 4A water quality monitor) in a zig-zag pattern and in the direction of anticipated sediment plume dispersion over the period of a flood tide. Observations were made from within 100m of the pile location to around 700m down-tide. The OBS sensor was heavily weighted to maintain a constant depth equivalent to around the lower 25% of the total depth (notionally 4 to 5m above the seabed). A series of water samples were also recovered at equivalent depths to assist cross-correlation of the recorded turbidity measured in Nephelometric Turbidity Units (NTU). A limited amount of data was taken deliberately upstream of the pile to obtain background readings.
Recorded Evidence
Results from the towed OBS monitoring appear to be fairly unconvincing. The correlation between the turbidity readings (NTU) and measured concentrations (mg/l) was reported as inconclusive, as apparently the range of results was not broad enough.
The presentation of results is given as a series of discrete turbidity levels plotted along the transect lines. The data does not appear to show any distinct sediment plume either from the OBS or water samples.
The survey report suggests that the natural seabed composition of mainly medium sand, shells and some gravel would drop out of the water column fairly rapidly and not be supported in suspension above around 2m from the bed level. Whilst this comment may be true for ambient seabed sediments it does not identify the composition of any sub-strata material which is being drilled out. Furthermore, it is also interesting to note that the measurements were all apparently taken at 4 to 5m above bed level and may have been above any sediment plume.
Finally, the water samples have been analysed for mass concentration only and do not describe the type or size of sediment material in suspension.
Key Documents
HR Wallingford, 2002. Barrow Offshore Wind Farm Sedimentary Processes Study, EX4554, May 2002.
MCEU, 2003. Barrow FEPA licence. 31744/05/05. December 2005.
Osiris, 2006. Barrow Offshore Wind Farm Suspended Sediment Monitoring, Monitoring Results, C5031, March 2006.
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Warwick Energy, 2002. Barrow Offshore Wind farm. Environmental Impact Statement. May 2002.
c. Blyth (UK)
Site Description
Blyth is the first offshore wind farm to be constructed in the UK, becoming operational in 2000. The project is located 1km off the Northumberland coast and adjacent to the port of Blyth. The development comprises of two 2MW turbines mounted on mono-pile foundations drilled 15m into the seabed and grouted into a submerged rocky outcrop in water depths of about 6m CD (plus a tidal range of 5m). The site is exposed to North Sea conditions with occasional breaking waves across the outcrop.
Monitoring Requirements
Environmental studies recognised that the drilling process presented a risk of releasing additional volumes of sediment into the surrounding marine environment and with a potential impact of material blocking lobster holes in the rocky outcrop. As such, efforts were made to minimise the effects of any local sediment plume created by the drilling. Sediment was collected in a buffer container positioned directly underneath the spoil exhaust outlet and then pumped along a floating spoil pipe for near-bed disposal around 200m away outside an exclusion zone (Figure C.1).
Whilst no formal records of spoil disposal were kept a license and approval had to be obtained from the Ministry of Agriculture Fisheries and Food to authorise the deposit, which suggests the disposal site was chosen to minimise any adverse impacts. Additionally the end of the pipe was moved at regular intervals to maintain an even dispersion of the spoil. Whilst no quantitative data regarding increases in SSCs are available from the Blyth site no significant impacts at the construction or disposal site have been recorded either.
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Figure C.1: Spoil pipe in use at Blyth during drilling operations and associated sediment plume.
Key Documents
ETSU, 2001. Monitoring & Evaluation of Blyth Offshore Wind Farm. Installation & Commissioning. ETSU W/35/00563/REP/1. DTI/Pub URN 01/86.
d. Burbo (UK)
Site Description
Burbo Offshore Wind Farm gained consent in 2003 and is expected to become fully operational late in 2007, becoming the fifth completed Round 1 project. The project is located on Burbo Flats in Liverpool Bay, approximately 6.4km from the Sefton coastline and 7.2km from North Wirral. The development comprises of 25 turbines mounted on mono-pile foundations which have been piled into the seabed. In some areas drilling was anticipated, but in fact all piles were successfully driven to required depths over the period June to July 2006. Prior to piling a layer of small slate pieces was placed around each turbine position to prevent initial scouring around the piles.
C.11 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Water depths across the site vary between 2.1 to 7.5m LAT. The seabed is characterised by highly mobile sand and sandbank in the north-east of the site (Little Burbo). There appears to be little variation in seabed sediment along the cable route, which comprises coarse to medium, shelly sand with some pebbles and cobbles. The development is exposed to macro tides (with a spring range of 7.4m), currents up to 0.7m/s and strong winds from the Irish Sea.
Monitoring Requirements
At the time of preparing this report the monitoring data available for review relates to baseline surveys which were commissioned to support the EIA phase of the project. It is understood that the FEPA licence includes a standard arrangement for SSC monitoring, however, the outputs from this exercise are presently not available.
EIA baseline surveys have included the deployment of two bed-mounted frames (referenced as BB1 and BB2) within the project site for a period of 6-months. Observations taken at these sites include waves, water levels and near-bed currents, salinity and temperature at heights around 0.6m above the bed. A turbidity sensor was only included at the central site, BB2, with measurements logged in Formazin Turbidity Units (FTU).
Water samples were also taken during deployment, servicing and recovery of the frame at BB2. In total, 36 water samples were recovered from near the bed over five occasions which phased with periods of good weather. These samples were then analysed for mass concentration and sediment type.
Results of the water sampling indicate a near-bed SSC ranging between 21.6 to 250.4mg/l, but with mean concentrations over each visit of: 147.9, 194.3, 117.3, 78.1 and 62.5mg/l. It is noted that the material type observed in the samples was generally silts (10 to 12μm), but on occasion a second distinct peak was recorded at around 300μm, equivalent to a medium sand. The interpretation of this information suggests a general background silt load in general circulation around Liverpool Bay, probably originating from the rivers Dee and Mersey, and an occasional pulse of mobile sands from locally derived sediment.
The OBS readings remain quoted in FTU as it was suggested that the range in measured SSC from water samples was not broad enough to provide convincing correlation to units of mg/l. However, the OBS readings do show a variation with peak values occurring with phases of higher flows and larger waves, confirming the mobility of sediments at the measurement location.
The EIA recognised the potential for localised and short-term increases in SSC during the construction of the mono-pile foundations, especially if drilling was used. Overall
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these effects were considered to be minimal and less than the higher volumes of sediment released under a no scour protection scenario.
Key Documents
ABPmer, 2002. Burbo Offshore Wind Farm. Coastal Process Study. R.962, September 2002.
ABPmer, 2002. Water Sample Analysis for Burbo Offshore Wind farm. R.968. September 2002.
Gardline Surveys, 2002. Burbo Bank Offshore Wind Farm – Wave, Tide and Current Monitoring. Final Report. Job No. 5779.
MCEU, 2005. Burbo FEPA licence. 31864/05/0. 18 February 2005.
Seascape Energy Ltd., 2002. Burbo Offshore Wind Farm. Volume 2: Environmental Statement. September 2002.
e. Horns Rev (Denmark)
Site Description
Horns Rev is presently the world’s largest offshore wind farm consisting of 80 turbines on mono-pile foundations, providing a total installed capacity of 160MW. The site is located in the North Sea off the west coast of Denmark and is around 14 to 20km off the coast of Jutland, and became operational late in 2003. The wind farm covers an area of 27.5km² (including the 200m exclusion zone around the wind farm) on a shallow reef. Depths across the site vary from 5.8m to 17.5m, where the seabed is primarily composed of well-sorted sediments of sand, gravel, pebble and boulders.
The site is exposed to the prevailing westerly winds with an average wind speed of 10m/s, a rough wave climate during both the summer and winter, and strong currents leading, the combination of which leads to sediment transport. Background suspended sediment concentrations are quoted in the range 2 to 10mg/l, rising to several hundred mg/l during storm conditions (DHI, 1999).
The EIA identifies the construction phase providing the greatest risk of sediment spills, both in relation to cable laying and seabed preparation for a potential gravity base case. Modelling of a 80,000m3 spill scenario for a potential gravity base option (worst case) suggested that the spill concentrations would be the same order of magnitude as the normal variation in suspended matter concentrations in the area.
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Monitoring
The EIA studies concluded that the influence on the environment from spill during dredging would be negligible and hence no environmental criteria (or monitoring) were deemed necessary.
Key Documents
DHI, 1999. Horns Rev Wind Power Plant. Environmental Impact Assessment of Hydrography. 50396-01.
f. Kentish Flats (UK)
Site Description
Kentish Flats became operational in September 2005, being the third Round 1 project to be completed. The site is located around 10km offshore from the end of the East Quay in Whitstable, North Kent. The development comprises of 30 turbines installed on mono-pile foundations, providing a maximum installed capacity of 90MW. Installation of foundations occurred between August and October 2004, and three 33kV export cables were installed in January 2005 despite apparent poor weather conditions.
The site water depth is on average 5m with a variable thickness of seabed sand, underlain by soft to firm clays, on top of the London Clay formation, which favoured a simple, single driven mono-pile foundation design. Along the cable corridor the thickness of sand reduces towards Herne Bay where the sediment composition has increased silt content.
The wind farm is relatively sheltered, being positioned close to the coast and protected from offshore waves by sandbanks across the Outer Thames Estuary.
Monitoring Requirements
The project has been subject to a FEPA licence and associated monitoring conditions.
The ES states that ‘climate meaned’ suspended sediment concentrations are low over Kentish Flats, being only 8 to 64mg/l in summer and 32 to 64mg/l in winter (CEFAS archives and unpublished data Southern North Sea Sediment Transport Study, 2002). The ES identifies the risk for these concentrations to be raised locally and temporarily during construction and decommissioning but these effects rapidly diminish as shallow water mixing occurs and tidal, residual flow activity removes them from the area.
C.14 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
The original FEPA SSC monitoring requirements were prescribed with a standard arrangement based on the use of OBS instrumentation for a period of 4 weeks at 3 fixed moorings, plus selected water column profiling during the pre-construction, construction and post-construction phases.
The actual surveys undertaken appear to have been adapted through agreement with CEFAS and became limited to monitoring effects during the period of laying the export cables. The revised surveys comprised of a fixed mooring to monitor background SSC and mobile monitoring down-tide of the cable plough to detect any sediment plume.
The fixed station included a twin OBS array using YSI 6000 turbidity detectors mounted at 1m off the seabed and at a location over 6km west of the site. Monitoring commenced in October 2004 and was completed in February 2005. A Casella water sampler was also used to recover 57 water samples on initial deployment and recovery, with samples taken 1m above the seabed every 30 minutes for a period of 13 hours. These samples were analysed for total dry solids and for use in calibration of both sets of OBS readings from FTU to mg/l.
The mobile monitoring also used a YSI 6000 device which was deployed every 10 minutes during the installation of the export cable at a distance no closer than 500m down-tide of the installation vessel. Monitoring occurred over the duration of each cable lay for periods 17 to 20 January, 27 to 29 January and 30 January to 1 February 2005. Readings were taken every 1m above the seabed and at 1m intervals to the sea surface. During the survey a total of 39 water samples were also recovered using a Casella water sampler and for use in calibration of the near bed OBS readings from NTU to mg/l.
Operational Thresholds
The cable laying process was also subject to remaining within pre-determined threshold values of suspended sediment which were established from the initial period of fixed station monitoring (October to November 2004) and agreed with CEFAS. The threshold conditions stated the following:
• A peak value of >1000 mg/l should not be exceeded. If it is exceeded, confirmation of value through the deployment of a second device will be required as verification.
• Raised values of >300 mg/l should not be exceeded for > 30minutes. If they are exceeded, confirmation of values through the deployment of a second device will be required as verification, as well as measurement of existing background levels taken, at the end of the 30 minute period, from a site 2km north of the cable laying activity. If natural background levels are at, or in excess of, 250 mg/l, a further 30 minute period of monitoring will be required before any action is required.
C.15 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
If one or both values were exceeded at any time during the cable laying then the cable laying vessels would have been instructed to slow or cease activities until levels were reduced to below the thresholds specified. These threshold levels appear to substantially higher than ‘climate meaned’ values offered in the ES.
The outcome of the survey demonstrated that no exceedances of thresholds were detected during the course of the cable laying.
Recorded Evidence
It is initially noted that the calibration of OBS devices relied only on correlation to measures of total solids obtained from a number of water samples, and that the material type(s) in suspension was not considered (e.g. organic content, inorganic content & particle size).
The calibration from NTU to mg/l is based on linear correlation between paired water sample and OBS measurements. The correlation fit is then applied to all NTU readings, and in some cases this requires extrapolation to higher readings beyond those measured from the field.
In the case of the fixed station OBS devices, the 57 water samples record direct SSC values in the range 17 to 84mg/l. This data was then used to obtain correlation fits with both sets of equivalent OBS readings. A ‘measure of fit’ in correlation can be inferred from the R2, correlation coefficient with a value of 1 demonstrating a perfect fit (positive correlation) and a value of 0 demonstrating no correlation. In the case of the fixed station measurements, R2 values of 0.44 and 0.38 are obtained. It is also noted that the range of native OBS readings used in correlation are 20 to 56 NTU and 34 to 81 NTU for the two devices, hence conversions above or below this range become a function of extrapolation. Both application of correlation and extrapolation introduce error into the analysis which is not discussed. It is also noted that many peak values (generally in excess of 200 NTU) have been removed, without explanation, from subsequent conversion to mg/l.
The fixed station data also provides a rare opportunity to compare the performance of measuring turbidity from independent OBS instruments, presuming that both devices have been calibrated to the same standard, are deployed in a similar manner and are aiming at measuring the same properties. Ideally, a comparison between measured turbidity readings from the two devices would indicate a perfect match.
Figure C.2 illustrates the comparison in measured NTU for three successive deployment periods spanning the period October 2004 to February 2005, noting that high values over 200 NTU have been removed from the data.
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200 200 200
150 150 150
100 100 100 OBS (NTU) 4 OBS 4 (NTU) OBS 4 (NTU) 4 OBS
50 50 50
0 0 0 0 50 100 150 200 0 50 100 150 200 0 50 100 150 200 OBS 5 (NTU) OBS 5 (NTU) OBS 5 (NTU)
13 October to 2 December 2004 2 December to 22 January 22 January to 8 February 2005
Figure C.2 Comparison in fixed station OBS readings.
For each deployment period there appears to be a consistent bias between instruments plus a scatter in the data which is widest during the first deployment period. A trend line through the data identifies that instrument OBS 4 provides results that are generally 20% higher than instrument OBS 5. This issue is partly resolved when separate correction factors have been applied to convert to mg/l, but it is noted that even this step introduces further error related to the robustness in correlation and the requirement to extrapolate outside of the range of correlated values.
Evidence collected during cable laying provides a measure of turbidity variation over time, through the water column and along the cable route. The correlation fit between near bed water samples and OBS readings is generally very good (R2 = 0.78). Measured concentrations span the range 13 to 146mg/l, which is wide enough to incorporate the majority of data and limit major amounts of extrapolation. Data also shows no marked variation in turbidity over depth. All measurements along the cable route appear to remain well below set thresholds.
Key Documents
GREP, 2002. Kentish Flats Environmental Statement. August 2002
Emu Ltd. 2005. Kentish Flats Monitoring Programme. Turbidity Monitoring. Report No. 05/J/1/01/0733/0500. April 2005
MCEU, 2003. Kentish Flats FEPA licence. 31780/03/1. 7 March 2003.
C.17 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
North Hoyle (UK)
Site Description
North Hoyle is the first Round 1 project to become operational and was commissioned in late 2003. The wind farm is located around 7.5km from Prestatyn, off the North Wales Coast. A total of 30 mono-pile foundations were installed in the period April through to July 2003. The export cables were ploughed into the seabed over the period August to September 2003, and the array cables jetted into the seabed over the period September to December 2003. On this basis per-construction is the period pre- April 2003, construction the period April to December 2003 and post-construction the period after December 2003.
The site experiences macro tidal conditions, with a neap range of 4.4m and a spring range of 7.2m, with water depths across the site in the range 7 to 11m CD. Current speeds normally achieve around 0.8m/s on spring tides, with a maximum recorded flow speed of 1.17m/s. The site is also exposed to winds and waves from across the Irish Sea. The surface sediment material appears to be a dominated by gravels with occasional sands arranged into megaripple areas. To date, monitoring evidence indicates no major scouring around foundations.
The foundation units each comprise of 4m diameter mono-piles, which were installed by Seacore Ltd. using a combined drive and drill technique. Each pile was initially driven through upper sand and clay layers (nominally 10m thick) using a large hydraulic hammer to reach underlying bedrock. A slightly undersized hole was then drilled into the bedrock (layers of sandstone or mudstone) and the pile finally driven into this hole to the nominal design penetration of 33m (maximum) below seabed.
As a consequence of drilling into bedrock an amount of drill arisings were produced and disposed off adjacent to each mono-pile. It appears the coarser fraction (assumed to be around 4mm) has remained on the seabed in the form of small mounds.
The EIA predicted little change in suspended sediments as a result of the proposed works, with a net drift of material towards the Dee and North Wirral coast.
Monitoring Requirements
The project has been subject to a FEPA licence and associated monitoring conditions, with Annex 1 offering a fairly prescriptive monitoring requirement for SSC which appears to have been taken as the standard requirement in later licences at other sites. The FEPA monitoring requirements asked for the deployment of 3 fixed suspended sediment meters over a period of at least 4 weeks during the pre- construction, construction and post construction phases. The licence also states that
C.18 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
the deployments would be at the following locations:
• Adjacent to the existing anemometer mast at the south-eastern corner of the North Hoyle wind farm licence area, to measure near field effects of sediment release.
• Towards the Point of Ayr, and within the predicted sediment plume to determine far field effects.
• At a point outside the predicted area of the sediment plume, to provide a ‘control’ measure of natural suspended sediment levels over the respective monitoring periods.
Recorded Evidence
It appears that the requested monitoring was delivered in full, as per the FEPA licence, and based on the following sites:
SS2: Adjacent to the existing anemometer mast at the south-eastern corner of the North Hoyle Wind Farm licence area to measure near-field effects of sediment release. Co-ordinate: 306398E 392044N. Charted depth: 7m CD approx.
SS1: Towards the Point of Ayr (near mouth of Dee estuary), at a distance of one tidal excursion from the boundary of the licence area and within the predicted suspended sediment plume to determine far-field levels of SSC. Co-ordinate: 312998E 387079N. Charted depth: 0m CD approx.
SS3: At a point outside the predicted area of the sediment plume west of the licence area, and off-axis north of the dominant flood-ebb direction to provide a ‘control’ measure of natural levels of SSC over the monitoring period. Co-ordinate: 295800E 395655N. Charted depth: 20m CD approx.
Pre-construction Monitoring
Baseline monitoring was recorded at each of the three prescribed sites over a 1-month period from 15 February to 17 March 2003. Hydrolab DataSonde combined water quality recorders with shuttered optical turbidity sensors (i.e. OBS devices) were deployed on bottom mounted frames and positioned to record at approximately 1m above sea bed with readings every 5 minutes.
It is reported that each turbidity sensor was calibrated against a limited number of water samples to enable conversion from FTU to mg/l. It appears that three water samples were taken on 28 February at each site and a further two samples per site on 6 March, thus providing five samples in total for calibration of the baseline data. The resulting correlation coefficients, R2, quoted for the period of baseline monitoring are
C.19 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
given as 0.98 for SS2 (near-field), 0.77 for SS3 (‘control’) and <0.1 for SS1 (far-field). The limitations on satisfactory correlation at SS1 were recognised and with refinements anticipated as further calibration samples became available during later periods of monitoring. It is probable that insufficient water samples were used to provide statistically reliable correlation at this point. In addition, analysis of water samples appears to be limited to mass concentration and not material type and particle size.
Results of the baseline SSC suggest that levels at site SS2, nearest the wind farm, quite closely reflect those at site SS1, near Point of Ayr. Suspended sediment levels at the control site, SS3 were consistently lower than at the other two sites. As anticipated, suspended sediment levels appear to increase with proximity to the coast and the Dee estuary (Figure C.3).
400 SS1 350 SS2 300 SS3 250
200
150 SSC (mg/l) SSC 100
50
0 15/02/2003 20/02/2003 25/02/2003 02/03/2003 07/03/2003 12/03/2003 17/03/2003 Date
3.0 10 Wave Height 2.5 Water levels 8
2.0 6
1.5 4
1.0 2 Wave heightWave (m) Water Levels (m)
0.5 0
0.0 -2 15/02/2003 20/02/2003 25/02/2003 02/03/2003 07/03/2003 12/03/2003 17/03/2003 Date Figure C.3 Pre-construction SSC measurements compared to local wave and water level measurements.
Peak suspended sediment levels at SS1 were noticeably higher than at SS2 (max 199.10 mg/l and 155.00 mg/l respectively), the following summarises maximum and mean concentrations for each site:
SS1: max 199.10 mg/l mean 50.10 mg/l (n.b. estimated values only) SS2: max 155.00 mg/l mean 39.76 mg/l SS3: max 65.28 mg/l mean 15.20 mg/l
At all sites a clear relationship is evident with suspended sediment being remobilised during flood and ebb tides and reducing during slack tide periods around high and low water, and with a further longer period modulation related to a lunar period with increased concentrations during periods of stronger spring tides. From around 8 March
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2003 (a period with reducing tidal range towards neaps) wave conditions monitored at the Liverpool Bay WaveNet site exceed 2m in height (bearing of around 260ºN) and appear to indicate that such larger waves exert a further influence on the seabed to mobilise bed sediments and raise SSC to their peak values (SS1 and SS2) shortly afterwards. This effect is not so strongly represented at SS3 until around 12 March when the wave direction is from around 325ºN.
Construction Monitoring
The period of construction monitoring with valid data is reported to extended from 17 March to 23 April 2003, thereafter it is commented the confidence in the data quality is markedly reduced due to bio-fouling. In fact the full dataset actually extends through to 17 May 2003 and any apparent anomaly in measured SSC appears limited to SS2 only.
It is noted that construction activities during this period were limited to foundation installation and did not coincide with cable laying activities. During this monitoring period drilling and pilling occurred at 3 locations:
Turbine Number Mono-pile drill and pile dates 1 6 and 12 April 2 13 and 16 April 3 22 and 25 April
Drilling and piling for turbine number 6 occurred on 22 and 25 April and so the initial drilling activity is included in the monitoring period.
Figure C.4 illustrates the proximity of the three monitoring locations to turbine locations 1, 2, 3, and 6. Turbine 3 is approximately 2.6km west of monitoring location SS2. SS3 is around 10km to the north west of this cluster and SS1 8km to the south east.
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Figure C.4 SSC monitoring locations at North Hoyle.
Five calibration samples per site were taken over a full tidal cycle on 5 April from 1m above seabed, the approximate depth of the sensors. This yielded R2 values for SS1 of 0.81; SS2, 0.71; and SS3, 0.78. The resulting data were also ‘de-spiked’ prior to further analysis in order to remove what appeared to be unreliable data points that showed suspended sediments increasing from very low levels to values in excess of 300mg/l for single (5 minute) time intervals. Similarly, for several hours on 25 March the sensor at SS3 recorded suspended sediment concentrations in excess of 1000mg/l. This was attributed to be the result of temporary fouling as readings soon normalised. No de-spiking of SS2 data was required after commencement of mono-pile installation works.
Figure C.5 illustrates the full sequence of measured data and presented against local waves and water levels covering the same period of construction.
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400 SS1 350 SS2 300 SS3 250
200
SSC (mg/l)SSC 150
100
50
0 17/03/2003 27/03/2003 06/04/2003 16/04/2003 26/04/2003 06/05/2003 16/05/2003 Date
3.0 10 Wave Height 2.5 Water Levels 8
2.0 6
1.5 4
1.0 2 Wave HeightWave (m) Water Levels (m) Levels Water
0.5 0
0.0 -2 17/03/2003 27/03/2003 06/04/2003 16/04/2003 26/04/2003 06/05/2003 16/05/2003 Date
Figure C.5 Construction SSC measurements compared to local wave and water level observations (full measurement period).
In general, a similar response in SSC variation is observed as with the pre-construction period with the tidal amplitude appearing to be the dominant control in stirring up the bed sediments and with peak flows during periods of spring tide leading to increased concentrations of suspended sediment. The influence of wave activity is less obvious, this may be due to the occurrence of peak waves during the construction monitoring being coincident with spring tides where the tidal signal remains the dominant mechanism. The influence of construction activity, which commenced from 6 April, is uncertain and with no discernable effect in the short-term.
It is noted, however, that measurements have previously been stated as suspect from 23 April onwards due to an apparent reduction in data quality attributed to fouling from organisms. Whilst this phenomenon is plausible it is highly unlikely that all three devices deteriorated at the same time. Inspecting the data further suggests that the more erratic behaviour at this time appears limited to SS2 only, with this device appearing to offer more reliable information after 15 May. It is unusual for a failed instrument to behave in this way, which may suggest an alternative explanation is needed for measurements over this period. In addition, SS1 appears not to respond to a rising range of spring tides and strong waves from after around 10 May.
Variation in the measured data at SS2 is summarised in Table C.2, with the construction period split into four periods:
Construction – 1: the period where valid data is reported to 23 April
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Construction – 2: the period where results begin to climb so that SS2 > SS1 & SS3 Construction – 3: the period where measurements remain with raised concentrations Construction – 4: the period where measurements appear to recover
Table C.2 Variation in measured suspended sediment concentrations at SS2. Period Date Suspended Sediment Concentrations (mg/l) Min Max Average SD Pre-construction 15-Feb to 17-Mar 0 155 40 26 Construction – 1 17-Mar to 23-Apr 3 104 19 12 Construction – 2 23-Apr to 9-May 8 245 73 38 Construction – 3 9-May to 15-May 45 242 128 36 Construction – 4 15-May to 17-May 11 108 45 19
Post-construction Monitoring
At the present time there has been no formal post-construction monitoring report, with only raw data (FTU) provided to indicate that the period of post-construction monitoring extends from 15 April to 26 June 2004. It is presumed that the timing of the post- construction survey is deliberate to measure conditions after the project became operational at the end of 2003 and to span a similar calendar period as the construction monitoring.
Key Documents
Innogy plc, 2002. North Hoyle Offshore Wind Farm. Environmental Statement. February 2002.
MCEU, 2003. North Hoyle FEPA licence. 31579/03/1. 19 March 2003.
National Wind Power, 2003. North Hoyle Offshore Wind Farm. Baseline Monitoring Report. June 2003.
Npower renewables, 2005. North Hoyle Offshore Wind Farm. Annual FEPA Monitoring Report. June 2005.
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g. Nysted (Denmark)
Site Description
Nysted is presently the world’s second largest offshore wind farm and is located within the Baltic Sea where the coastal process conditions are relatively benign. Wind farm construction took place between June 2002 and December 2003 resulting in 72 turbines mounted on gravity base foundations and extending over an area of 28km2.
The offshore wind farm is located in the Femer Belt separating Germany and Denmark, 10km south of the town Nysted on Lolland, and 11 to 17km west of the town Gedser on the south tip of Falster. Two barrier islands, western Rødsand and eastern Rødsand, separate the Rødsand Lagoon from Femer Belt and from the wind farm. The distance from the barrier islands to the nearest row of wind turbines is approximately 2km.
Water depths across the site are generally shallow and vary from around 6 to 9.5m, with very little tidal influence.
The surface sediments within the wind farm area are characterized mostly as medium sand with very low silt/clay content. The thickness of this overlaying sand layer varies between 0.5 and 3 m. The underlying glacial deposits are mostly clay with no organic content. Concentrations of suspended sediment are generally very low for this location, with background values recorded in the range 0 to 27.6mg/l, with an average of 3.3mg/l (4.1mg/l, standard deviation).
Environmental Impact Assessment
The EIA studies recognised that excavation activities for foundations and laying cables had the potential for locally elevating suspended sediment concentrations for a short period with risk of impact on fish, birds and mussels. However, the conclusion from the EIA was that there would be no measurable environmental impact as a result of the sediment waste in connection with the excavation work.
Monitoring Requirements
Despite the views provided in the EIA, elevated suspended sediment concentrations were still seen to present a threat to the biological environment and low threshold values were set to minimise potential impacts. In addition, pre-, during and post- construction surveys have monitored the benthic fauna to assess any residual impacts.
The Danish Energy Agency (DEA) granted permission for the wind farm in July 2001 with the following threshold conditions regarding sediment spillage from seabed operations:
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• In connection with dredging operations for cables, foundations and the offshore transformer platform a work zone with a radius of 200 m around these works is defined. Outside this work zone, the turbidity must not exceed 15 mg/l in average value, and 45 mg/l in total value.
• The spreading of sediment due to construction works must not lead to a total accumulation of sediment larger than 2 kg/m² anywhere outside the work zones.
The majority of monitoring information is related to the cable laying operations. A total of 48km of interconnecting cables were installed during the period 14 January to 29 November 2003. The cables were buried to a minimum depth of 1m below seabed, with a design depth set to 1.3m. Approximately 40% of the material excavated was coarse sand with the remaining 60% comprising glacial clay.
Documentation of the cable laying operations is fairly comprehensive and identifies the variety of cable laying methods employed, including:
• Pre-trenching was required where hard substrate limited the effectiveness of jetting. The majority of pre-trenching occurred in the period 17 February to 7 July 2003. The bulk of trenching was completed using back-hoe dredgers (Castor and Rocky), achievable due to the shallow depths. Most material was cast aside at a distance of between 5 to 10m from the trench for use in back-filling, with larger rocks placed onto a barge for disposal elsewhere. Average progress for trenching was between 280 to 411m per day, depending on the plant used, averaging at 330m per day and for a trench unit volume estimated at 2.3m3/m, giving a volume of worked sediment of 759m3/day.
Figure C.6 Pre-trenching using “Castor”.
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Figure C.7 Pre-trenching using “Rocky”
• Jetting was used when substrates allowed fluidizing the sediment. Jetting occurred from 14 April to 13 September 2003. Average progress of jetting was 160m per day and for a trench unit volume estimated at 0.9m3/m, giving a volume of worked sediment of 144m3/day.
Figure C.8 Jetting from the vessel “Mika”.
• Back-filling of the pre-trenched materials using a range of back-hoe dredgers. Material was lifted out of the water for inspection prior to filling the trench. The rate of backfilling average at around 150m per day and for a trench unit volume estimated at 2.0m3/m, giving a volume of worked sediment of 300m3/day.
C.27 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Figure C.9 Back-filling operations using “Rocky”
Recorded Evidence
Turbidity measurements were made using an IQ Sensor Net System 2020, calibrated against water samples from the offshore site prior to the monitoring. The device was deployed from a small vessel that ran continuous circles around the source of potential sediment spillage and taking measurements at water depths of 1, 3 and 5m.
For each depth, measurements representing the spillage plume were identified, and maximum and average values calculated. The minimum value at the opposite (upstream) side of the plume was used as background value, and subtracted from the average and maximum plume values to find the increase due to sediment spillage, i.e. the elevated concentration over the background level. In contrast, the DEA requirements appear to relate to absolute concentrations and not excess over background.
• Pre-trenching
Figure C.10 illustrates the calculated excess suspended sediment concentrations occurring during the pre-trenching activities. It is clear that both average and maximum thresholds were exceeded on a number of occasions, although the longer- term average excess concentration remained below 15mg/l.
C.28 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
80 Pre-trenching monitoring - excess sediment concentrations 75
70 Spill Max Spill average 65 15mg/l threshold 45mg/l threshold 60 Trenching average
55
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Spill (mg/l) 35
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0 15/02/2003 02/03/2003 17/03/2003 01/04/2003 16/04/2003 01/05/2003 16/05/2003 31/05/2003 15/06/2003 30/06/2003 Date Figure C.10 Pre-trenching monitoring results
• Jetting
Figure C.11 illustrates the calculated excess suspended sediment concentrations occurring during the jetting activities. In comparison, to pre-trenching concentrations the excess sediment levels during jetting appear to remain below all requisite thresholds. In fact, all absolute concentrations also remained within DEA thresholds.
80 Jetting monitoring - excess sediment concentrations 75
70 Spill Max Spill average 65 15mg/l threshold 45mg/l threshold 60 Jetting average
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Spill (mg/l) 35
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0 12/04/2003 27/04/2003 12/05/2003 27/05/2003 11/06/2003 26/06/2003 11/07/2003 26/07/2003 10/08/2003 25/08/2003 Date Figure C.11 Jetting monitoring results
C.29 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
• Back-filling
Figure C.12 illustrates the calculated excess suspended sediment concentrations occurring during the back-filling activities. Excess sediment levels generally remained below thresholds, with only a few exceptions over the 15mg/l daily average. It is further commented that the back-filling process reworked sediments already disturbed during pre-trenching activities materials, and it is presumed that the majority of fines would have been released previously during the initial pre-trenching excavation.
80 Backfill monitoring - excess sediment concentrations 75
70 Spill Max Spill average 65 15mg/l threshold 45mg/l threshold 60 Backfill average
55
50
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Spill (mg/l) 35
30
25
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0 12/04/2003 27/04/2003 12/05/2003 27/05/2003 11/06/2003 26/06/2003 11/07/2003 26/07/2003 10/08/2003 25/08/2003 09/09/2003 Date Figure C.12 Backfilling monitoring results
Summary comments from this monitoring exercise note that jetting created far less spillage than either pre-trenching or back-filling activities with the amount of sediment released per day also being substantially less (n.b. pre-trenching and back-filling are effectively two parts of the same operation for laying the cable). In addition, both pre- trenching and back-filling operations involved lifting sediments out of the water column whereas jetting remained as a near-bed operation.
Key Documents
Energi E2, 2000. Rødsand Offshore Wind Farm. Environmental Impact Assessment. EIA - Summary Report. July 2000.
Seacon, 2005. Sediment spillage during array cable installation at Nysted Offshore Wind Farm. January 2005.
C.30 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
h. Scroby Sands (UK)
Site Description
Scroby Sands became the second Round 1 wind farm to be completed, and was commissioned in March 2004. The wind farm is located on a sandbank known as Scroby Sands, with the nearest turbine to shore around 2.3km off Caister on the Norfolk coast.
The site is very shallow, with depths generally between 2 to 10m CD. The seabed is predominantly medium sands, with some gravels and shells and a low proportion of fines. Mobile bedforms on the bank range from sand ripples, mega ripples to sand waves.
A total of 30 turbines were piled into the seabed over the period October 2003 to January 2004, with cable laying activities taking place between May to August 2004.
Monitoring Requirements
The project has been subject to a FEPA licence and associated monitoring conditions, with Annex 1 providing details of the required monitoring strategy which included monitoring of suspended sediments during wind farm construction due to piling and ship movements.
Two instrument moorings were specified in the monitoring strategy, each comprising of a directional wave gauge, OBS device and a current meter. One instrument was specified on the bank in a central location to the wind farm, and the second instrument deployed midway to the coastline in Caister Road. Water samples were also required to calibrate OBS readings, with samples to be recovered hourly over a tidal cycle.
The monitoring strategy also requested providing measurements over the pre- construction, construction and post-construction periods, and including variation between mid-summer and winter seasons.
Recorded Evidence
Prior research into potential effects of offshore wind farm developments on coastal process (ETSU, 2002) sought to identify a ‘reasonable worst case’ from the range of early interest projects. This exercise highlighted that Scroby Sands demonstrated a number of attributes which exemplified potentially the most severe (and hence most measurable) interactions with coastal processes, such as scour and effects on the adjacent coastline.
C.31 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Subsequent Defra-funded research deliberately focused on Scroby Sands to provide evidence for the magnitude and significance of the potential impacts on sediment transport and sandbank stability, and to use the findings from this additional research to develop generic guidance for sediment transport monitoring programmes for offshore wind farms and aid future licensing decisions. It appears that the field studies undertaken for this research by CEFAS encompassed the monitoring strategy described in the FEPA licence. The outputs from this work are available from http://www.cefas.co.uk/renewables/AE0262.htm.
The suite of data collected by CEFAS appears to include four deployment locations of their Mini Lander devices, two of which comply with the FEPA requirements; Scroby Sands and Caister Road (Figure C.13 and Table C.3).
Table C.3 Monitoring Locations for Scroby Sands Location Target Position Description Mean Water Height of OBS Depth Sensor above (Lat, Long) Seabed (m) (m) 1. Offshore 52° 38.58′N, 1° 49.08′E Seaward of the bank to 19 0.4 measure incoming waves 2. Scroby Bank 52° 38.98′N, 1° 47.56′E On the bank and within the 7 0.6 wind farm 3. Caister Road 52° 38.85′N, 1° 45.46′E Between the bank and the 20 0.6 Caister shoreline 4. Yarmouth Road 52° 37.14′N, 1° 45.12′E To provide information on 10 0.4 conditions that might reach the Great Yarmouth shoreline
The CEFAS MiniLander device is a seabed mounted platform equipped with a data logger and an array of devices to measure salinity, temperature, water depth, current speed, waves and turbidity. The turbidity sensor used was a Seapoint OBS sensor with an automatic gain control that enables a large range of suspended sediment concentrations to be recorded (notionally from a few mg/l to 800mg/l). The height of the sensor above the seabed varies between sites from around 0.4 to 0.6m. In addition, passive sediment traps were also mounted on the MiniLanders for post- deployment calibration of raw OBS readings.
C.32 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
Figure C.13. Location of deployment sites for MiniLanders
Table C.4 summarises the data recorded from the various monitoring locations and deployment periods. It is noted that no construction monitoring was undertaken within the wind farm site itself, which severely limits delivery of the survey rationale and achieving the FEPA monitoring strategy.
Table C.4 Monitoring Periods for Scroby Sands Survey Dates Monitoring Location Pre-construction April to June 2003 Offshore, Scroby, Caister Road, Yarmouth Road Construction February to March 2004 Caister Road, Yarmouth Road Post-construction February to March 2005 Scroby, Caister Road
It is further noted that the turbidity data presently available has remained in raw FTU units and with no further attempt reported to make use of the results from sediment traps to calibrate readings to standard units of mg/l. Consequently, further consideration of this data cannot support any conclusions about effects the wind farm construction process may have had on elevated background suspended sediment concentrations.
The findings of the Defra-funded research do, however, highlight that regulators still need the EIA process to estimate the disturbance caused by the construction of the
C.33 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
wind farm, caused by, for example, jetting, ploughing, ‘grouting-in’ or from seabed levelling for gravity-based structures. It is further stated, that best practice approaches during construction and operation of an offshore wind farm will need to consider monitoring suspended sediments during pile-driving, grouting or cabling operations, especially if the surface sediments or the immediate subsurface have a high proportion of sediment grains that might be easily mobilised, have elevated levels of contaminants or the operations take place near a conservation site (e.g. eelgrass beds, Zostera marina) or within a Special Area of Conservation (SAC).
Key Documents
CEFAS, 2006. Scroby Sands Offshore Wind Farm. Coastal Processes Monitoring. AE0262. Final Report. 12 April 2006.
ETSU, 2002. Potential effects of offshore wind developments on coastal processes. W/35/00596/00/REP.
MCEU, 2003. Scroby Sands FEPA licence. 31272/03/0. 5 September 2003.
C.9 Other Related Evidence
Further to the evidence base collated from completed offshore wind farms, a small amount of related evidence has been compiled from other related activities, in particular cable laying.
In 2005, a replacement electrical supply cable was installed between Clam Cove in Rockland Maine to Wooster Cover on North Haven Island in Penobscot Bay, USA. Prior to installation the environmental regulator thought it necessary to impose limits on levels of raised total suspended solids (TSS) from concerns related to use of jetting tools. A combination of water samples and OBS monitoring took place up-current and down-current of the jet plough and for different heights through the water column. The monitoring identifies that the peak TSS concentrations remained in the near bottom samples taken at around 1.5m off the seabed. There was no recorded exceedance of the environmental thresholds set for the upper 5m of the water column. Water depths along the cable route varied from shallow locations to depths greater than 80m. The behaviour of the sediment plume was explained by relatively coarse particles found in the substrate along the cable route which would have quickly fallen out of suspension (Leeper, 2005).
Further anecdotal remarks on cable laying across Long Island Sound using jetting tools suggests that between 10 to 35% of the fluidised sediments enters into the water column, and that the height of the sediment plume extends to no more than 2m above the seabed under conditions of low or no current velocity (Anon, 2002).
C.34 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
C.10 Summary Remarks
a. How do the observations compare to statements made in the ES (i.e. is the measured data in line with the assessment of effect, or is there a ‘surprise')?
The environmental studies appear to highlight correctly the potential for elevated levels of suspended sediment through the construction phase, with the field evidence now able to confirm and quantify more precisely the actual effects in terms of magnitude, distribution and duration. To date there has been no reported evidence of significant impact related to sediment disturbance and elevated levels of suspended sediment (i.e. ‘no surprises’).
b. Has the data addressed the FEPA requirement to provide the additional understanding required to reduce apparent uncertainties?
The available evidence for suspended sediment concentrations presently spans a variety of shallow marine environments which offer a variety of sedimentary conditions from rocky platforms, mobile sands, gravely sea beds to clay sub-soils. However, it remains that the majority of evidence for foundation installation relates to mono-piles.
The assessment of whether any raised levels of suspended sediment present a further risk of significant impact on the marine environment at new sites remains for each project to consider on a case-by-case basis, however, for areas where background concentrations are normally high and construction practices fall within those presently demonstrated within existing evidence, then it is suggested that future monitoring requirements would add nothing to present understanding.
Exceptions where uncertainties remain relate to larger projects adopting gravity base or drilled mono-pile foundations which might generate large volumes of arisings that are distinct in sediment composition to ambient seabed sediments.
c. Are the methods of survey sufficient and what approaches demonstrate best practice (e.g. if various approaches to monitoring have been applied, then identify which has worked best)?
Within the evidence base there remain some inconsistencies and weaknesses in the manner in which suspended sediment data has been collected and analysed which limits a complete understanding of the issues for all sites. Nevertheless, large-scale significant impact from elevated suspended sediments at the existing sites was never considered to be a significant risk, so these shortfalls have not become issues for any present projects.
C.35 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
A variety of survey and analytical methods have been used to respond to FEPA and other environmental monitoring requirements. Principally, these can be divided between fixed station and vessel based deployments.
Fixed station monitoring performed for the present wind farm sites has revealed a variety of differing practises with measuring devices tending to be located at range of heights above the seabed, generally between 0.5 to 1m, and also at various distances relative to the area of construction (i.e. source of seabed disturbance). The key advantage of using fixed station deployments is that the duration of measurements generally spans a sufficient period to establish a good description of natural variations to the effects of waves and tides. The key disadvantage is that the data are not immediately available and cannot therefore be used in operational decisions to respond to environmental thresholds. In addition, measurements are limited to one height above the seabed which tends to be within the sediment boundary layer and to one site which may easily miss any sediment plume. If the seabed is highly mobile then the relative position of the measuring device to the seabed might also change unnoticed (e.g. the passage of sandwaves) which could further influence SSC levels.
The vessel based deployments provide the means to track sediment plumes with the flexibility to take measurements over a variety of depths from near surface to near bed. The survey vessels have either circled a point source or swept the plume from side-to- side. In all cases optical backscatter sensors (OBS) have been favoured over acoustic backscatter sensors (ABS), which is correct when the primary interest is the finer sediment fractions. It is also interesting to note that the clearest evidence for raised SSC comes from vessel based deployments tracking sediment plumes from cable laying operations. The main disadvantage of vessel based deployments is that the surveys are inherently short-term and therefore do not easily determine the level of natural variation. Measurements over depth provide the advantage of quantifying the heights of any sediment plume in the water column, which for cable laying activity appears to be limited to around 2m. Importantly, data returns are fairly immediate and hence provide the most suitable means to respond to environmental thresholds.
The use of water samples to convert ‘turbidity’ readings into SSC units appears to have been applied with a variety of success, and in general there appears to be insufficient samples taken over an adequate range of concentrations to provide robust correlations. Consequently, this may result in weak calibration and the need to extrapolate beyond the measured limits of correlation to higher values. Importantly, the use of water samples to identify the size of the sediment fractions(s) in suspension appears to have been used in only one case.
Bio-fouling of sensors at fixed station deployments appears to be a common issue and the general explanation given to the occurrence of random data spikes. In addition, progressive deterioration of the sensor over long deployment periods is also likely to
C.36 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
be due to the same issue. The method for identifying such spikes and points of degradation in readings is not always clearly explained or documented.
The most complete analysis of the variation of suspended sediment concentrations has been achieved when associated metocean conditions (winds, waves and tides) have been considered. In most cases it is the variation in these metocean parameters that has tended to provide the explanation of primary long-term controls on raised SSC.
Finally, in terms of collating the available evidence the lesson learnt from across the industry is for improved data management.
A best practice approach for further monitoring would include the following:
• OBS device calibrated against water samples spanning the range of monitoring conditions, ideally a minimum of 20 samples to provide a robust statistical correlation; • Deployment at a fixed height above the seabed, notionally 1m or less together with a vessel deployed sensor sampling through the water column; • Water samples analysed for mass concentration, particle size, inorganic and organic content; • Consideration for use of sediment; • Associated metocean data and local seabed sediment sample to judge natural sediment disturbance; • Near-field sampling at no more than 500m from the sediment source; and • Complete data management of the monitoring programme so the information is more readily available.
d. Summary of lessons learnt to advise on future requirements (inc. Environmental Impact Assessment (EIA) guidance, regulatory requirements, monitoring requirements, etc)
The results from existing monitoring appear to substantiate the majority of the EIA presumptions about raised SSC during the construction phase of offshore wind projects and in relation to foundation installation and cable laying activities. In most cases the risk of impact from this issue has been low, short-term and localised. However, it is important to note that the available evidence presently remains to early size-limited Round 1 projects adopting mono-pile construction in a range of marine environments including rock, gravels, sands and muds and constructed in a linear sequence.
It is to be noted that the present evidence does not extend to:
• Larger projects where multiple plant may be active on site or where in-combination effects from adjacent activities may arise;
C.37 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
• Gravity base installations requiring seabed preparation (e.g. levelling); and • Mono-piles which require drilling out atypical substrates (e.g. introduction of chalk fines).
It remains prudent for EIA guidance to continue to recommend developers to undertake appraisal of SSC issues, especially as Round 2 projects may seek to develop larger schemes using alternative foundation options (i.e. sites and techniques that fall outside of the present evidence base). As additional evidence is provided from these new sites then further consideration can be given to updating present guidance in terms of further monitoring requirements.
It remains for developers to consider on a case-by-case basis if their site presents a significant risk to any environmental receptor. If the available evidence is suitable to their specific application then it is reasonable to expect that further monitoring requirements can be avoided.
It remains for regulators to decide if the evidence base provides the level of certainty required to deal with the licensing of projects. Where monitoring requirements remain then the present ‘default’ scope may need reconsideration in light of lessons learnt. When a suitable survey scope has been agreed with the developer then this needs to be formally documented and made available as part of any future evidence base.
It also remains for developers to act on both their future licence and lease requirements and contribute any relevant environmental data in accordance with the Data Management and Information Plan (DMIP) being developed through the Collaborative Offshore Wind Research into the Environment (COWRIE) project (COWRIE, 2005).
C.11 References
Anon, 2002. Resuspension of sediment by the Jet Plow during Submarine Cable Installation. May 22, 2002.
CEFAS, 2001. Offshore Wind Farms: Guidance note for Environmental Impact Assessment in respect of FEPA and CPA requirements. October 2001.
CEFAS, 2004. Offshore Wind Farms: Guidance note for Environmental Impact Assessment in respect of FEPA and CPA requirements. Version 2. June 2004.
COWRIE, 2005. Data Management and Coordination Data and Information Management Plan. Version 1.3: Consultation Draft.
C.38 Review of Round 1 Sediment Process Monitoring Data Lessons Learnt
DTI, 2006. Review of cabling techniques and effects applicable to the offshore wind farm industry. Technical Report prepared by Royal Haskoning. Draft Report Version 2. October 2006.
Leeper, P.C. 2005. Monitoring of Total Suspended Solids. Fox Islands Electric Cooperative Inc. Submarine Cable Installation. Clam Cove to North Haven Island. Penobscot Bay, Maine. August 2005.
USGS, 2002. Estimation of suspended solids concentrations based on acoustic backscatter intensity: Theoretical Background. Turbidity and other sediment Surrogates Workshop. April 30 – May 2, 2002, Reno, NV.
Thackston, E. L., and Palermo, M. R., 2000. “Improved methods for correlating turbidity and suspended solids for monitoring,” DOER Technical Notes Collection (ERDC TN-DOER-E8), U.S. Army Engineer Research and Development Center, Vicksburg, MS.
C.39
Appendix D
Review of Seabed Morphology
SED01 – Morphological changes around Round 1 Offshore wind farms
Jon Rees Cefas April 2008
Executive Summary
The most comprehensive morphological data, in the present evidence base, relates to the Scroby Sands offshore wind farm. This site has the benefit of one pre-construction and six post-construction seabed surveys. Notable issues determined from the 3½ years of post-construction monitoring are:
• The formation of scour wakes which can be directly related to the mono-pile foundations.
• A possible morphological impact expressed by a linear depression along the line of mono-piles in the SW corner of the wind farm.
Swath (multibeam) surveys offer the most realistic means to monitor, resolve and quantify the local scale effects of wind farms on seabed morphology.
Where it is intended to make comparisons between a number of surveys then consistent methods of survey execution, processing and presentation of data is required at each site.
At present, datasets remain limited to the short-term (up to 3½ years of monitoring at most) and remain unable to distinguish large-scale and longer-term morphological changes (e.g. movement of a bank or ridge of a bank) that could be attributed to the construction of the wind farm from changes due to natural processes.
Large changes in morphology, from either natural processes or the presence of the wind farm have the potential to lead to increased project risks (e.g. on the operation and maintenance of the wind farm as well as the integrity of cable burial).
The issue of morphology remains different for sites in different sediment regimes. High levels of morphological change can be expected in sand environments exposed to strong wave and tidal action, with active sandbank systems being typical examples of a dynamic seabed. Lower levels of morphology can be expected where the metocean conditions are less aggressive and/or the sedimentology is less prone to transport (i.e. coarser material or consolidated cohesive muds and clays). The use of a seabed mobility index, developed from site-specific considerations, may assist in establishing the likelihood for seabed response and assist in determining future requirements for monitoring.
Sediment particle size sampling and analysis has the potential to be a useful tool in detecting changes in sediment transport patterns and pathways. Although some sampling protocols exist, they have not been formulated for this purpose. Similarly, some analytical methods do exist to determine patterns and pathways of sediment transport but they are only used in the academic community at present. If particle size surveys are used as a licence condition then guidance must be offered on the required methods of survey, analysis and reporting.
1
CONTENTS
1. Aims...... 3 2. Site-by-Site Review ...... 3 2.1 North Hoyle ...... 5 2.2 Scroby Sands...... 8 2.3 Nysted ...... 18 2.4 Arklow ...... 19 3. Related Research...... 19 4. Recommended Practice for Further Surveys ...... 20 5. Conclusions...... 21 6. References...... 22
© Crown Copyright 2007
LIST OF FIGURES
Figure 1. North Hoyle, showing post-construction multi-beam data...... 5 Figure 2. Comparison between 2004 and 2001 bathymetric surveys at North Hoyle...... 6 Figure 3 . Observed bed shear stress exceedence curves for North Hoyle ...... 8 Figure 4. Area EA5, East Anglia - Caister Road (UKHO, 2006)...... 9 Figure 5. Comparison in 5m contour between 2005, 2002 and 1999 surveys (UKHO, 2006)...... 10 Figure 6. Change in bed levels determined between November 2006 and March 2004. (Reds indicate gain, blues indicate loss.)...... 12 Figure 7. Recent movement of the Northern Spine onto WTG-37. (The red cylinder shows the location of the mono-pile)...... 14 Figure 8. Time evolution of scour wakes at sites on the eastern flank of Middle Scroby...... 15 Figure 9. Time evolution along depth profiles...... 16 Figure 10. Observed bed shear stress exceedence curves for Scroby Sands (from CEFAS (2006))...... 18
LIST OF TABLES
Table 1. Summary of morphological data sets from the completed offshore wind farms (as available at the time of review)...... 4 Table 2. Summary of available morphological evidence for North Hoyle...... 6 Table 3. Summary of available morphological evidence for Scroby Sands...... 11 Table 4. Summary of impacts at Scroby Sands (CEFAS, 2006)...... 12
2 1. Aims
This technical appendix provides a review of seabed morphology surveys undertaken at the completed Round 1 offshore wind farms with the aim of identifying lessons learnt.
These surveys fulfil a FEPA licence condition with the intention of providing supporting evidence to underpin the Environmental Statement and to assess sediment movements in relation to cable burial depth and the long-term integrity of the cable. Where available, evidence has also been considered from constructed wind farm projects elsewhere across Europe.
The licensing authority currently places a general requirement on developers to undertake post-construction surveys at 6-month intervals and initially for a period of three years following construction. The licensing authority may then choose to extend this period, subject to review of the information.
For the purpose of this review morphological changes are considered to be quantifiable changes in seabed levels as determined by comparison between successive surveys, and including the wider significance of secondary scour effects on issues such as inter-array cable burial. Consideration is also given to evidence which provides any indication of a step change in the sediment regime (e.g. a shift in deposition/erosion trends) as observed from particle size distributions.
The changes in morphology assessed here are those associated within the immediate area of the wind farm, which are in many cases are situated on sandbanks. The research project SED06 - Review of Channel Migration, is concerned with morphological issues of channel features which may run adjacent to some of the wind farm projects.
2. Site-by-Site Review
The primary data types available to this review include general bathymetric surveys at pre- and post-construction periods. In addition, several sites have also collected surface sediment samples for particle size analysis as part of a wider requirement to examine impacts on benthic receptors.
Table 1 provides a summary of the available morphological data for the range of completed projects, including both Round 1 and European wind farms.
3 Table 1. Summary of morphological data sets from the completed offshore wind farms (as available at the time of review). Wind Morphological FEPA Further Comments farm data sets Requirement analysis possible Blyth No No No Built on rocky shoal North Yes Yes Yes Three bathymetric surveys Hoyle 2001, 2003 and 2004 Scroby Yes Yes Yes Six high quality post- Sands construction swath surveys, plus one pre-construction single beam survey Kentish Yes Yes No Pre and post-construction Flats sidescan sonar surveys Barrow Yes Yes No Only one sidescan sonar, plus PSA survey April 2005 Burbo Yes Yes No Mono-piles only recently installed, first years monitoring only. Horns Rev, No N/A No Considered to be Denmark insignificant in ES Nysted, Limited N/A No Monitoring report only Denmark Arklow, Limited N/A No Survey along cable route Ireland and turbine rows
For the available data a review has been provided based on addressing the following issues:
• What reliability and confidence can be placed on the field data and how might practices be improved?
• How do the observations compare to statements made in the ES (i.e. is the measured data in line with the assessment of effect, or are there any ‘surprises')?
• Has the data addressed the FEPA requirement to provide the additional understanding required to reduce apparent uncertainties?
• Are the methods of survey sufficient and what approaches demonstrate best practice (e.g. if various approaches to monitoring have been applied, then identify which has worked best)?
• Summary of lessons learnt to advise on future requirements (inc. Environmental Impact Assessment (EIA) guidance, regulatory requirements, monitoring provisions, etc).
4 2.1 North Hoyle
The North Hoyle offshore wind farm was the first completed Round 1 project in the UK. The site is located some 8 to 11km offshore of Rhyl, North Wales and in water depths from 6 to 12m CD. Inshore and to the east of the development site sediments are a mixture of moderately to well sorted sands. In the region of the site itself sediments are generally coarser but also more heterogeneous with sands, gravels and varying amounts of pebbles (npower renewables, 2003). The site does not appear to span any major bedform, such as a named sandbank feature, but the southern part of the site is on the periphery of sand accumulations resident around the entrance to the Dee Estuary which exhibit both sandwave and mega-ripple features (Figure 1).
Figure 1. North Hoyle, showing post-construction multi-beam data.
5 Table 2 provides a summary of the available morphological evidence for North Hoyle, which includes a combination of bathymetric surveys and seabed sediment monitoring through pre- and post-construction periods.
Table 2. Summary of available morphological evidence for North Hoyle Date Period Sediments Bathymetry August 2001 Pre-construction 51 samples Single-beam September 2002 Pre-construction 17 samples - February 2003 Post-construction - Single-beam September 2004 Post-construction - Multi-beam October 2004 Post-construction 68 samples -
There are notable differences between each survey limit a consistent comparison over time. Notably, bathymetry has been surveyed by three different contractors, using different line spacing and post-processing methods for tidal reduction. These differences introduce error and uncertainties on horizontal and vertical positioning of the seabed levels which have the potential to mask any small-scale effect due to the wind farm
On the basis of the available data a brief comparison has been provided between 2004 post-construction and 2001 pre-construction surveys. The analysis is presented in Figure 2 in terms of a difference in levels (2004 subtracted from 2001). Positive values indicate a deepening and negative values a shallowing. The mismatch in survey configurations is also evident as “banding” on approximately E-W and N-S axes.
Figure 2. Comparison between 2004 and 2001 bathymetric surveys at North Hoyle.
6 This analysis suggests that there is no systematic morphological change in seabed level between 2001 and 2004. Reported changes are generally small (tending to be within +/- 0.5m) and are most likely to be explained by differences between survey methods and data interpolations rather than any wind farm influence.
Seabed sediments have been sampled and analysed on a more consistent basis, using the same type of grab and the same laboratory methods of particle size analysis (dry sieving). Important differences remain in the number and location of samples between surveys and the use of different statistical descriptors in 2004. Local sediments are generally multi-modal and described as very poorly sorted sandy gravel. The sand fraction is typically fine to medium sand, and the more dominant gravel content as granule to pebble gravel (Wentworth Scale).
It is important to recognise that the use of such technique in heterogeneous (multi- model) sediment environments will inherently result in higher levels of variability in sediment gradings at any time and location than sampling in homogeneous (uni-modal) sediments. Furthermore, if variations in sediment particle size between surveys were to be used as an indicator for morphological change over time, then there must be a clear methodology for determining a step change in distribution and recognising that this change must always be larger than any natural variability resulting from the sampling methodology itself (i.e. setting a high signal to noise ratio). At the present time no such consistent method has been established.
In terms of sediment grading evidence obtained from North Hoyle then there presently remains no overall pattern of change, either temporally or spatially, and therefore no impact from the wind farm has been observed.
The Environmental Statement (ES) for North Hoyle (Innogy, 2002) refers to a review of historic charts (using seven charts spanning the period 1839 to 1999) with the aim of determining natural long-term changes in seabed levels for the general area. The analysis suggests a long-term trend of –5mm reduction in bed levels per annum, +/- 20mm. The ES did not conclude any additional changes in morphology due to the wind farm.
Sediment, wave and tidal measurements undertaken as part of the baseline monitoring have provided the means to evaluate levels of bed shear stress and the likelihood for sediments becoming mobile. Figure 3 illustrates an exceedence of bed shear stress against thresholds for the main sediment types recorded across the wind farm site. This analysis identifies that the more dominant gravel fraction is unlikely to be mobilised by either waves or tides and it is only the sand fraction that is partly mobile (fine sands for around 67% of the time and medium sands for around 59% of the time).
7 100%
Currents only - Raw Data 90% Currents & Waves - Raw Data
80% Fine Sand Medium Sand 70% Granule Gravel
Pebble Gravel 60%
50% Exceedence 40% currents and waves
30%
20%
10% currents only
0% 0.1 1 10 Bed Shear Stress (N/m2) Figure 3 . Observed bed shear stress exceedence curves for North Hoyle
In summary, North Hoyle appears to be located on a relatively level seabed composed of sands and gravels and with only a few minor sediment transport features. With the evidence collected so far, no impact on these morphological features has been observed in the short-term (2 years of post-construction monitoring). It is further noted that this evidence has been used successfully by the developer to agree with the regulating authority that further FEPA monitoring requirements can now be discontinued.
2.2 Scroby Sands
In 2004, the Scroby Sands became the second operational Round 1 offshore wind farm project in the UK. The site is located on a shallow sandbank feature known as Middle Scroby and is around 2.3km offshore from Caister on the Norfolk coast. The site is separated from the coast by Caister Road, a navigable channel providing access into Great Yarmouth and Lowestoft from the north.
The banks and channels off the Norfolk coast are known to change frequently and the Maritime and Coastguard Agency (MCA) include parts of this area in their Routine Resurvey Programme. These surveys are typically conducted over mobile seabed areas which require re-survey on a regular basis to ensure up-to-date charting for safe navigation. Area EA5 presently includes parts of Caister Road, Middle Scroby and also overlaps with the wind farm (Figure 4). EA5 is currently fully surveyed every 6 years with intervening 3-year check line surveys. It is further noted that the boundaries for any re-survey area remain flexible to maintain coincidence with the bank or channel features which they monitor. The further surveys proposed for this area will target EA5 across the entire seaward length of Middle Scroby and with EA4 (Caister Shoal) targeting Caister Road and the western flank of the bank. The next full surveys are scheduled for 2011 for EA5 and 2008 for EA4.
8 Figure 4. Area EA5, East Anglia - Caister Road (UKHO, 2006).
The last comparison between these surveys (2005, 2002 and 1999) identified that the general position of Middle Scroby remained unchanged, although the western side of the bank (as bounded by the 5m CD contour, Figure 5) appears to have receded (UKHO, 2006). This recession appears to be coincident with the position of the wind farm which would have featured only in 2005 data. In comparison, the eastern flank of the wind farm, where heavy wave breaking is noted, remains unchanged.
9 Figure 5. Comparison in 5m contour between 2005, 2002 and 1999 surveys (UKHO, 2006).
Initial studies to support the environmental and engineering considerations for the Scroby Sands offshore wind farm (Halcrow, 1996) also offered some consideration to the morphology of the sandbank system, but did not comment on the potential morphological impact of the wind farm other than on local scour. Related analysis has also been reported in Reeve, et al (2001) which considered analytical methods to quantify larger scale changes in the system of sandbanks and channels off Great Yarmouth between the period 1846 to 1992. One general conclusion from their analysis is that over the longer term there has been an upward trend in the volume of the
10 upper portion of the sandbank system, but also strong oscillatory behaviour between the channels and banks.
Alongside surveys provided as part of the Routine Resurvey Programme the developer has also monitored seabed levels since pre-construction and to fulfil their FEPA licence conditions. Table 3 provides a summary of the presently available evidence.
Table 3. Summary of available morphological evidence for Scroby Sands Date Period Bathymetry Comment August 2002 Pre-construction Single beam March 2004 Post-construction / Multi-beam 1m centres Pre-scour protection July 2004 Post-construction / Multi-beam 2m centres Post-scour protection February 2005 Post-construction / Multi-beam 1m centres Post-scour protection September 2005 Post-construction / Multi-beam 1m centres Post-scour protection April 2006 Post-construction / Multi-beam 1m centres Post-scour protection November 2006 Post-construction / Multi-beam 1m centres Post-scour protection
Post-construction surveys have made use of multi-beam techniques and have generally provided near 100% coverage across the wind farm site apart from areas of very shallow water towards the central and southern part of the sandbank. It is suggested that absence of data in this region is entirely due to shallow water restrictions which severely limit vessel access and reduce swath widths. It is noted that the survey coverage does not extend as far south as EA5 from the Routine Resurvey Programme.
Previous analysis of the post-construction monitoring, up to February 2005, has been undertaken by CEFAS as part of a funded research project referred to as AE0262 Scroby Sands Offshore Wind Farm – Coastal Process Monitoring.
The main recommendations from this research which relate to Scroby Sands were that FEPA licence conditions are used to require the developer to continue 6-monthly multi- beam surveys to provide further evidence of the longer-term dynamics of scour pits and wakes, scour protection and wider-scale changes in bed elevation and patterns of net sediment transport. It is considered that this data is important to support the further assessment of the equilibrium of the scour pits and any changes in the overall bed elevations, particularly any creation of cross-bank channels. Swath bathymetry remains as the primary basis to monitor such changes. It is further commented that FEPA licence conditions were justified in this instance.
Table 4 provides a summary of impacts determined from the review of swath surveys available to the research project (three surveys from March 2004 to February 2005).
11 Table 4. Summary of impacts at Scroby Sands (CEFAS, 2006) Scale (m) Type of impact Significant Impact? 0 – 100 Scour pits Yes, as predicted by the EIA 100 – 1000 Scour tails Tails present, but not significant with respect to bank volume change > 1000 Sandbank morphology No evidence
The present study has now extended the analysis of the multi-beam surveys by considering the entire data sequence of surveys up to and including November 2006.
Initial considerations are offered by comparing changes in seabed levels observed between the earliest multi-beam survey (March 2004) and the latest survey from November 2006, an elapsed period of two years and eight months. Figure 6 presents the quantified change in bed levels between these two surveys, constructed from the 1m gridded data.
Northern Spine
Sandwave field
Central spine
Scour wakes
Depression along axis of turbines
Change in depth (m)
Ridge building
Figure 6. Change in bed levels determined between November 2006 and March 2004. (Reds indicate gain, blues indicate loss.)
12 It is noted that this simple comparison is more immediately suited to determining a net change in bed levels which are represented as either gains (positive values in red) or losses (negative values in blue) in the vertical, but does not readily account for morphological features which move across the area (in the horizontal), such as major sandwave formations.
Despite this comment, the following observations can still be offered:
• The Northern Spine appears to have grown in height and extended eastward and slightly south, impinging on the foundations for wind turbine, WTG-37. Figure 7a reveals the form of the scour pit associated with the mono-pile, the scour protection and the secondary scour pits as recorded in April 2006. A slight “cut” is visible in the crest of the spine to the left of the mono-pile. By November 2006 (Figure 7b), the spine has moved eastward and is encroaching the mono-pile. The scour pit has partly in-filled with sediment and only a small scour pit remains. The spine is not as linear as before and shows an indentation caused by the mono-pile.
• The seabed has deepened on the northeast and northwest flanks, leading to a reduction in sandbank width. The reductions along the northeast flank may, in part, be due to the wider influence of channel migration from Barley Picle.
• A Central Spine, with an associated mega-ripple field, has grown in elevation, width and extent.
• There is good evidence for the sandwave field in the northwest sector to have moved a few metres south. No direct interaction between the sandwave field and the mono-piles is evident (i.e. no increased bifurcation of sandwaves or preferred orientations).
• A linear loss of sediment can be observed along the north/south axis of several turbines leading to a shallow depression in the southwest sector of the array.
• Narrow scour wakes can be observed as shallow depression to the southeast of many of the mono-piles. The direction of the scour wakes tends to be in alignment with the axis of the flood dominant tide which is measured at around 160ºN at the centre of Middle Scroby.
• A ridge has grown in volume at the extreme southwest corner of the wind farm by over 5m in the period between the two surveys. This feature is encroaching on Turbine No 1. leading to constraints in access. At present, the process controlling this development is uncertain.
13
a. April 2006
b. November 2006 Figure 7. Recent movement of the Northern Spine onto WTG-37. (The red cylinder shows the location of the mono-pile).
Additional consideration of the multi-beam data has been provided by examining the time evolution of example scour wakes at sites along the eastern part of Middle Scroby over the period March 2004 to November 2006 (Figure 8).
WTG-30 lies around 400m north of WTG-27 but in closer proximity to a charted wreck which is around 150m southeast of the mono-pile foundation. From March 2004 a scour wake appears to develop from WTG-30 to the southeast and in a consistent form to an established wake from the adjacent wreck. Overtime these two wakes appear to join and at the same time a general reduction in levels moves in from the north. Similar wakes are observed to develop at WTG-27, WTG-23 and WTG-19, although none is coincident with any wreck. Interestingly, the seabed around WTG15 appears to be more stable with no large wake, and in fact the scour hole appears to actually infill over time. It remains uncertain if such wake features have reached any equilibrium position against natural variations and therefore further monitoring of this situation is recommended.
14
Figure 8. Time evolution of scour wakes at sites on the eastern flank of Middle Scroby.
15 Figure 9 illustrates the time variation in seabed levels along two transects. Transect 1 (green) is between WTG-30 and the adjacent wreck, and Transect 2 (red) from WTG- 23.
Transect 1 (green)
WTG-30 wreck
Transect 2 (red) WTG-23
Figure 9. Time evolution along depth profiles.
Transect 1 - In March 2004 (pre-scour protection) the initial scour profile is identified around WTG-30 with a further separate shallow depression around the adjacent wreck. The July 2004 survey indicates initial seabed profiles soon after placement of scour
16 protection and a partly in-filled scour pit. Later surveys indicate a displaced scour pit which has developed around the scour protection and to a comparable depth on the southeast side to the original scour depth. The seabed profile along the scour wake indicates a general lowering over time which is in the order of 2m from March 2004 to November 2006. In November 2006 there appears to be substantial scouring around the wreck and deepening by over 6m since March 2004. It is noted that to the northwest of WTG-30 the seabed levels have generally been more stable with periods of accretion and erosion, and net lowering of less than 1m.
Transect 2 – The pattern of morphological variations is similar here to Transect 1 but without the superposition of any wreck. The scour wake has tended to lengthen and deepen over time, and may not yet be in equilibrium with the seabed. To the northeast (up-drift) the trend in seabed elevations indicates progressive accretion by around 2m.
Overall, the comparisons in seabed bathymetry indicate clear and measurable differences over the period March 2004 to November 2006 which appear to include marked effects due to the presence of wind farm structures. The dynamic nature of the Great Yarmouth sandbank system means that the seabed is continually evolving at the same time and the equilibrium position of wind farm effects may always seek to adjust accordingly.
The area around WTG-30 needs to be monitored closely. It is not certain if group scour is occurring here between the adjacent wreck and WTG-27 or if the area is adjusting more generally under the influence of channel migration from Barley Picle. The deepening seabed levels around these locations place an obvious risk on cable burial depths.
In contrast, WTG-01 is presently in an area which has shallowed considerably leading to difficulties in accessibility for operation and maintenance activities.
Further demonstration of the high sediment mobility acting across the wind farm area can be gained by considering bed shear stress exceedence levels, as determined from wave and tidal monitoring evidence (Figure 10).
17 Total Bed shear Stress Exceedance - Scroby Bank
100
90 Winter Scroby Winter Summer 80 4 micron 62.5 micron 70 125 micron Scroby Summer 250 micron 60 500 micron 1mm 50 2mm 4mm Naze
% exceedance % 40
30 Naze 20
10
0 0.1 1 10 Total Bed shear Stress (N/m2) Figure 10. Observed bed shear stress exceedence curves for Scroby Sands (from CEFAS (2006)).
The modal sediment type recorded across Middle Scroby is medium sands (grain size in the range 250 to 500 micron). Figure 10 includes threshold exceedence values for transport for a range of sediment sizes, including medium sands. The analysis suggests that for 500 micron material the likelihood for mobility is around 80% of the time in the summer (i.e. during period of weaker wave activity), increasing to 92% of the time during winter conditions (i.e. during periods of stronger wave activity).
It is instructive to compare Figure 10 for Scroby Sands with Figure 3 for North Hoyle at this point. Scroby Sands experiences higher values of bed shear stress and for longer periods of time, mostly due to the fact that water depths are much shallower than at North Hoyle, as both sites are exposed to relatively similar metocean conditions. Local sediments are North Hoyle are relatively coarse (sandy gravels) and are less prone to transport (higher threshold for transport) than the dominant medium sands found at Scroby Sands. The combination of high bed shear stress levels and mobile sediments leads Scroby Sands to be highly dynamic in seabed morphology.
2.3 Nysted
Morphological monitoring for Nysted appears to be related to use of satellite imagery to detect visible changes in the form of the Rødsand barrier islands. The wind farm itself comprises of 72 turbines (8 turbines in each row, with 9 rows aligned east to west) mounted on gravity base foundations with a diameter of 15.5m. The nearest row of turbines is around 2.5km south of the barrier islands.
DHI (2004) provided a study of the effects of the offshore wind farm on local morphology. The wind farm was predicted to reduce wave heights on the eastern island by a maximum of 10%, and flow rates by 5 to 10%, with sediment transport reduced correspondingly.
18
The EIA assessed that the barrier islands move approximately 15m each year before the erection of the wind farm as opposed to approximately 12m with the wind farm in place. Satellite images are seen to be sufficiently accurate for analysis of such nearshore morphological changes following significant morphological events (storms).
2.4 Arklow
The Arklow project comprises of a single row of seven mono-pile turbines (5.2m diameter) installed along the crest of the Arklow Bank. This sandbank is a narrow linear feature located around 13km off the east coast of Ireland. The scheme was installed in 2003 and presently there has been a series of one pre-construction (August 2002) and three post-construction monitoring surveys (June 2004, September 2004 and April 2005). These surveys have been targeted on monitoring the integrity of cable burial and local scour and do not offer a wider view of the sandbank morphology.
To achieve repeatability in data and allow for more direct comparison between surveys, the same swath bathymetry system and tide gauge arrangement has been used throughout and by the same contractor.
Volume change calculations have formed the basis of considering morphological changes in the local area. Initial problems in applying such methods occurred with an unspecified quantity of rock and stone used for scour protection. Between June and September 2004 it is suggested that a large volume of sediment shifted across the bank as part of the natural process of depletion and accretion which takes place on a seasonal basis in response to tidal and wave action. The anecdotal information from service vessels supports a general trend of progressively increasing depths over this six-month period.
There is presently no evidence that the foundations have introduced any additional affect onto the local bank sediment movements. This conclusion corresponds to initial modelling that also suggests that wind farm would have no effect on bank stability (Murphy, 2001). However, it is noted again that the evidence from available surveys remains short-term and highly localised to the row of turbines.
3. Related Research
Methods to predict long-term morphological development of the seabed in response to offshore developments remain relatively unproven. At present, it remains appropriate to develop our understanding from the monitoring evidence. The key limitation at this time is that the post-construction period currently only extends over a few years at most and may not yet represent an equilibrium condition in some areas.
Van der Veen et al (2007) have recently published an analytical method to assess the potential effects of offshore wind farms on seabed development. The method considers the consequence of additional drag forces that may occur around the foundations and how these may affect the tidally induced sediment transport. Importantly, the method
19 presumes the additional drag does not just modify the local flows around structures but is applied as a spatially averaged effect over the footprint of the array. In addition, the method does not appear to consider sediment supply, waves, local scouring or the increased turbulence induced by flows moving past structures. Two case studies are quoted to illustrate the method, which includes the Round 2 Humber Gateway project (based on mono-pile foundations only) and a site off the Dutch Coast. For the Humber site their method predicts that the seabed will take 91 years to evolve to an equilibrium state and at this point will reflect gains in the array of up to 0.67m and losses around the windfarm of up to 0.37m. If it were assumed that the rate of change in seabed remains constant over this period then the largest annual rates of change would be around +7mm/yr and –4mm/yr, respectively. It is interesting to reflect here that these very small annual variations would be outside of the accuracy of present surveys.
4. Recommended Practice for Further Surveys
It is apparent from the review of present evidence that surveys designed to monitor morphological variations of the seabed in areas of offshore wind farms need to be able to resolve both the general detail of the local seabed, including macro bedforms, and also the small scale effects which may occur around each structure placed on the seabed.
The present interest is to develop an improved understanding of how natural seabed variations may affect the wind farm (e.g. the risk of sandwave migration uncovering cables) and also how the wind farm itself may effect these variations (e.g. enhancing deposition or erosion). This interest is likely to remain to sites where the likelihood for such morphological development is high and constitutes a potential risk to the project, and may be considered unnecessary for sites which have a good history of a stable seabed.
A succession of high-resolution surveys is required to deliver this understanding that appears to be best described using multi-beam techniques only. This technique should be applied to capture near 100% coverage of an array. On this basis data interpolation methods become unnecessary and the risk of missing any relevant and local / small- scale features is minimised.
The following recommendations are offered for such morphological surveys, when required:
• Apply same survey practice throughout, including; horizontal and vertical geo- referencing, survey equipment, data processing methods, contractor, etc
• Schedule survey for consistent time of year, e.g. spring to record post-winter events, autumn to record post-summer events (n.b. No consistent inter-annual variations have yet been described from present monitoring)
• Consistent method of comparison and presentation which retain the required levels of detail to ensure quantification of small-scale effects
20
In addition, particle size surveys may prove helpful to inform on the likely thresholds of local sediment mobility. Recommended best practice is to:
• Sample in sufficient areas to account for local variability in sediment types
• Analyses materials of sand size and less using laser diffraction methods
• Provide sufficient statistical description of the grain size variation to identify modal classes
It is noted here that assessment of sediment mobility also requires information on the variability in water depth, current velocity and waves and over representative conditions, including large wave events. Such data may be available from site monitoring or a well-calibrated model.
5. Conclusions
The following site-by-site conclusions can be made from the review of available morphological evidence.
North Hoyle: A relatively stable seabed providing no evidence for large-scale morphological changes over the short-term and with an expectation of no further change in the future.
Scroby Sands: A relatively dynamic seabed with high natural mobility. Multi-beam surveys have revealed additional small-scale morphological features superimposed on areas of seabed that demonstrate a high level of natural change remains. Further monitoring of seabed levels using multi-beam remains important to the onward operational and maintenance issues of the wind farm. Secondary scour features are most prominent across this site.
Nysted: Morphological concerns remain limited to the natural spits, the relative positions of which can most effectively be observed using satellite monitoring. Local wave and tidal conditions are relatively benign in comparison to UK sites.
Arlow: Effective monitoring campaign over the immediate area of cable and turbines only. The limited survey extent impedes developing a complete understanding of the natural sandbank morphology.
In general, morphological surveys have begun to provide valuable field evidence which has added to the understanding of the wider interactions between offshore wind farms and the seabed. The present evidence base remains limited to a few years of post- construction monitoring which may not always be fully representative of the longer- term equilibrium position.
The coverage, spacing and frequency of bathymetric surveys should be assessed on a case-by-case basis. Those sites with large bathymetric features or features rapidly
21 transiting the area (based on historic data sets) should be surveyed at 100% coverage and at regular intervals (down to six-monthly). However, stable, featureless and well- documented areas may only require relatively in-frequent and only partial coverage surveys. After a defined period, typically three years, the bathymetric datasets need to be reviewed to agree further monitoring requirements. This review may lead to changes in coverage or frequency of survey or may cease the requirement for further surveys.
Natural variability in seabed levels remains important to projects and should be assessed during site selection. This practice should include reviews of available historic charts and an initial assessment of the likely level of sediment mobility.
Related monitoring programmes, such as the MCA Routine Resurvey Programme, may provide helpful information related to the wider changes in seabed morphology.
Where good information is available on sediment type and metocean conditions then the assessment of sediment mobility can be effectively gauged through consideration of threshold exceedence of bed shear stress to provide an index of likely mobility and hence morphology.
6. References
CEFAS (2006). AE0262 Scroby Sands Offshore Wind Farm – Coastal Process Monitoring. Final Report. 12 April 2006.
DHI (2004). Nysted Offshore Wind Farm at Rødsand. Morphology Study. Energi E2. Final Report. November 2004.
Halcrow (1996). Wave Climate / Scour Study. Sarah Jane Offshore Wind Monitoring Station. Study Report. Powergen plc. September 1996.
Innogy (2002). North Hoyle Offshore Wind Farm. Environmental Statement.
Murphy J (2001). On the effects of wind farm structures on the Arklow Bank seabed. Hydraulics 7 Maritime Research Centre, University of Cork.
Reeve D and Thurston N (2001). Eigenfunction analysis of decadal fluctuations in sandbank morphology at Gtreat Yarmouth. Journal of Coastal Research, 17(2), 371 – 382.
UKHO (2006). East Anglia - Caister Road. Assessment on the analysis of Routine Resurvey Area EA5 from the 2005 Survey.
Van der Veen, H.H., Hulscher, S.J.M.H. and Lepeña, B.P, Seabed Morphodynamics due to offshore wind farms, River, Coastal and Estuarine Morphodynamics, RCEM 2007.
22
Appendix E
Review of Scour
SED01
Review of Round 1 scour monitoring data - lessons learnt
Technical Note DKR3990/01
Address and Registered Office: HR Wallingford Ltd. Howbery Park, Wallingford, OXON OX10 8BA Tel: +44 (0) 1491 835381 Fax: +44 (0) 1491 832233
Registered in England No. 2562099. HR Wallingford is a wholly owned subsidiary of HR Wallingford Group Ltd.
SED01 Review of Round 1 scour monitoring data- lessons learnt
Contents
1. Introduction...... 1 1.1 Project background...... 1 1.2 Project Objectives...... 1 1.3 Report outline ...... 2
2. Site assessment...... 4
3. Discussion ...... 17 3.1 Site summaries...... 17 3.2 Monitoring approaches...... 18 3.3 Presentation of results...... 19 3.4 Future monitoring...... 20
4. Conclusions and recommendations...... 21
Tables Table 1 Barrow offshore wind farm scour review...... 4 Table 2 Kentish Flats wind farm scour review ...... 7 Table 3 Scroby Sands wind farm scour review ...... 9 Table 4 North Hoyle wind farm scour review ...... 12 Table 5 Arklow Bank wind farm scour review...... 14
Figures Figure 1 Barrow offshore wind farm, post construction survey: No visible scour in glacial till exposure, but jack-up barge spud can holes still visible ...... 23 Figure 2 Barrow offshore wind farm, post construction survey: Significant scour in silty sand and jack-up barge spud can holes still visible ...... 23 Figure 3 Kentish Flats offshore wind farm, post construction survey: Significant “scour” in exposed London Clay and jack-up barge spud can holes still visible...... 24 Figure 4 Scroby Sands offshore wind farm, post construction survey: Significant scour in fine sand prior to placement of scour protection. Cable exposed in scour pit...... 24 Figure 5 North Hoyle wind farm, comparison of post construction and 1 year surveys: No visible scour...... 25 Figure 6 Arklow Bank offshore wind farm, post construction survey: Scour limited by placed rock protection...... 25
TN DKR3990/01 iii R. 5.0 SED01 Review of Round 1 scour monitoring data- lessons learnt
TN DKR3990/01 iv R. 5.0 SED01 Review of Round 1 scour monitoring data- lessons learnt
1. Introduction
1.1 PROJECT BACKGROUND The aim of the SED 01 project is to bring coastal process field monitoring data together and to determine if the scale and magnitude of effects identified through the EIA process are in line with the monitoring evidence. The project deliverable will advise on the regulatory requirement and appropriate methods for future monitoring requirements for UK wind farm schemes. This Technical Note is the final project team report dealing specifically with the monitoring of scour associated with wind turbine foundations and cabling.
The potential for scour around the bases of the wind turbine foundations and along cable routes has been noted as an uncertainty for all of the Round 1 wind farms due to the lack of specific research and site experience. International research based on physical modelling and experience from other marine engineering works indicates that scour should be anticipated to a maximum depth of about 1.8 times the mono-pile diameter for sites with fine, homogenous sands and currents above the threshold for sediment motion (i.e. about 0.4m/s) for a significant part of the tidal cycle. There is no standard guidance for gravity bases or multi-leg structures, although some work has been undertaken for specific sites. There is also no standard guidance for sites with cohesive or widely graded soils. It is well acknowledged that the available engineering based guidelines are uncertain, and actual scour will depend on local conditions. Factors influencing scour depths include:
• Surface sediment type, depth and availability • Sub- surface soil profile (sub-strata type) • Depth of water • Current regime • Wave regime • Foundation type and dimensions • Exposure of cables at or above the seabed surface • Presence of scour protection
1.2 PROJECT OBJECTIVES The stated objectives of the project are to:
(a) identify and collate all available field evidence from built Round 1 projects, and, in addition, any further data available from other built European projects
(b) manage the information resource to enable the onward supply of approved data in line with associated research interests (e.g. SED02), and in line with the COWRIE Data Management Plan
(c) review the available data and reports to determine lessons learnt
(d) assess the present scope of sediment process monitoring placed on Round 1 developers and determine the appropriateness of monitoring as might be required for Round 2, and with special regard to differences in scale from Round 1 to Round 2 projects, and lessons learnt to date
TN DKR3990/01 1 R. 5.0 SED01 Review of Round 1 scour monitoring data- lessons learnt
(e) liaise closely with Client and Theme Leader for Seabed and Coastal Processes throughout the project and facilitate a technical review workshop
(f) prepare and disseminate an authoritative report suited for application by regulator, developer and consultant.
In addition to these main objectives the project team addressed the following issues:
• How do the observations compare to the statements in the Environmental Statements for each site (i.e. are the measured data in line with the assessment of effect, or is there a ‘surprise')? • Have the data addressed the FEPA requirement to provide the additional understanding required to reduce apparent uncertainties? • Are the methods of survey sufficient and what approaches demonstrate best practice (e.g. if various approaches to monitoring have been applied, then identify which has worked best)? • Summary of lessons learnt to advise on future requirements including EIA guidance, regulatory requirements, monitoring requirements, etc.
With specific reference to scour monitoring the following issues were noted:
• Assessment of high-resolution near-field changes in seabed levels around foundation units, including cable spanning at J-tubes • Extent of evidence versus foundation types and sediment regimes (non-cohesive & cohesive), scour protection and secondary scour effects • Need to form a strong linkage with research undertaken within the SED02 project, investigating scour prediction methods in detail.
1.3 REPORT OUTLINE This Technical Note sets outs site summaries of information relating to scour monitoring of the Round 1 offshore wind farm sites. The data and reports reviewed to prepare these summaries were kindly provided by the owners, consultants and survey contractors involved with each site.
Results are available for each of the four UK sites built to September 2006, plus the Arklow Bank site in Ireland. The information for each site is tabulated to present the relevant characteristics of each site, the FEPA monitoring requirements, the actual monitoring completed and a discussion of the results and the methods. In addition, limited information from Burbo Bank in Liverpool Bay (recently under-construction) plus Horns Rev and Nysted wind farms in Denmark was reviewed to provide a broad assessment of international experience.
The wind farm site summaries include:
• Barrow • Kentish Flats • North Hoyle • Scroby Sands • Arklow Bank
Conclusions and recommendations are then presented to guide future monitoring requirements and assessment of results. Figures 1 – 6 illustrate a sample of the survey results assessed.
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A number of figures have been included to illustrate the range of scour measured on site and the variety of presentation formats.
Appendix B presents an inventory of the digital data and reports collated by the project team.
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2. Site assessment
The following tables and text set out the information collated for each site, plus comments.
Table 1 Barrow offshore wind farm scour review Layout 30 x 4.75m diameter mono-pile foundations with cable connection to shore at Heysham. Water depths and • Site: 12mLAT to 18mLAT. Gently sloping seabed, apparently general morphology stable with no significant mobile bed forms. Exposed tillite and clay to the east. • Cable route: crosses a variety of terrain, including exposed glacial till, sandwaves and mobile sandbanks. Surface sediment • Site: Fine sand to muddy sand, coarsening to the east and north types with some gravels, to depths from 0m to 10m over tillite and clays • Cable route: Sand to boulder clay on cable route. Sub-strata types Tillite, exposed in E and dipping generally WNW Tide regime • Neap range: 4.1m • Spring range: 8.2m Current regime • Peak speed: 0.8m/s offshore and up to 2m/s along cable route. • Dominant directions: WNW – ESE rectilinear, with very weak easterly residual. Currents on cable route are strongest within channels. Wave regime Southwesterly wind-sea dominated conditions with severe storms. Secondary wind waves from the northwest. • 1:1 year: Hs=4.9m • 1:10 year: Hs=5.8m Breaking waves common across shallows on cable route. EIA scour assessment HR Wallingford assessment based on available research and experience. Extreme scour to 0.5D vertically and 1D horizontally (2.5m and 5m). Less in areas limited by substrate.
More detailed assessment by HRW for engineering design based on borehole logs indicated that 1.55D maximum depth should be assumed for loading calculations (7.4m).
Scour protection left as option by EIA. Scour protection built Not built or planned at time of review MCEU /FEPA Six-monthly swath bathymetry for three years at representative sites monitoring (to include at least four turbines) and along cable route conditions Surveys completed to • EIA (single beam, summer 2001) September 2006 • Pre-construction (single beam and side scan, spring 2005) • Immediate post-construction (swath, summer 2005) • Autumn 2006 (swath) (no cable surveys obtained) Date of foundation Summer 2005 construction Date of cable lay Summer - autumn 2005 Summary of survey Baseline side-scan and first post-construction surveys completed as results required for main site. 13 foundations monitored for scour pit
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formation. Locations considered to be representative of whole site.
Autumn 2006 survey included all turbines.
Scour pits around the mono-piles are clearly visible at many of the sites for the first survey, undertaken less than 2 months after installation, and in some cases within a few weeks.
Pits from jack-up leg penetration close to the mono-piles are also recorded; pits at some turbines are as deep or deeper than the mono- pile scour, ranging from 0.1m to 1.4m for the post-construction survey. Sediment transport between installation and the first survey was insufficient to re-fill the pits, with pits created two months before the survey still at a depth of 0.6m while the most recent pits showing depths up to 1.4m. After one year many of the pits are still visible, to a maximum depth of 1.25m. Most pits had disappeared or at least partially filled, but one pit was slightly deeper. There appears to be new jack-up marks at some turbines, indicating a re-visit for construction or maintenance – there are no records available to confirm.
Scour footprints appear to show very slight elongation along the WNW-ESE axis (dominant tidal direction), although this is not conclusive.
Scour is recorded to depths ranging from negligible to more than 1.9m for the post construction survey. The variation is well correlated with the depth and type of sub-strata below the fine sand. The maximum depth of 1.9m indicates the unrestrained depth of scour under the summer time conditions occurring over the period of less than two months from installation to survey.
The scour footprints for the first survey range from negligible to about 11m from the pile wall (total length up to about 20m on long axis). There is no evidence of deposition or erosion in the form of scour tails, nor any other broader scale impacts.
The second survey was completed after one year and therefore includes a full range of wave and tide conditions. The maximum scour depth is about 5.75m (1.2D) and the maximum footprint diameter is about 50m (10.5D, or a side slope of about 1:5). There is no visible scour around foundations set in the tillite beds.
The measured scour depth for the second survey is greater than predicted for the EIA, but less than predicted for engineering design.
It is noted that the cables are exposed on several turbines where scour extends below 4m depth. Assessment of Monitoring not completed as required by the FEPA conditions, as the monitoring and Spring 2006 survey was missed. The work appears to be to a high presentation. standard and provides the range of information required when combined with the original site investigation work for EIA (geophysical, soil samples, etc).
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There is some doubt regarding tidal elevation corrections as two different methods were applied by the two survey companies. A discrepancy of about 0.5m is evident for some turbines.
Diver inspection would be useful to determine the need for protection of the cables and J-tubes within pits suffering significant scour.
Plots do not show survey dates, but otherwise good with contours at 0.1m for 2005 and 0.25m for 2006. Referenced • FEPA licence 31744/03/3 documents • Barrow OWF: Sedimentary study. HR Wallingford report EX4554, May 2002 • Barrow OWF: MBES Scour pit survey. Titan Environment Surveys CS0137/R1/V1, July 2005. • Barrow OWF: Pre-construction geophysical survey. Osiris Report C5002, May 2005 • Barrow OWF: Scour monitoring geophysical survey. Osiris Report C6023B, December 2006.
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Table 2 Kentish Flats wind farm scour review Site 30 x 5m (approx.) diameter mono-pile foundations with cable connection to shore at Herne Bay. Water depths and • Site: 3mLAT to 5mLAT. Apparently stable and shallow plateau, general morphology with only superficial bed forms. • Cable route: shallows to the shoreline with increasing surface sediment. Surface sediment Fine sand and shells as superficial cover offshore and nearshore. types Sub-strata types Firm London Clay near or at surface, except the palaeo channel of the Swale which cuts across S and SE of site and is infilled by recent clays and sands. Tide regime • Neap range: 2.9m • Spring range: 4.7m Current regime • Peak speed: 0.9m/s offshore and up to 0.5m/s along cable route. • Dominant directions: WSW – ENE rectilinear. Weak rotational residual current. Wave regime North-easterly wind sea dominated conditions with severe storms. Secondary wind waves from the northwest. • 1:1 year: Hs=3.3m (depth limited at lower water levels) • 1:10 year: Hs=5.8m (depth limited) Breaking waves during severe conditions offshore and common across shallows on cable route. EIA scour assessment HR Wallingford preliminary assessment based on available research and experience. Extreme scour to 1.5D vertically in areas of unrestricted fine sand. Less in areas limited by substrate. Detailed field information on sub-strata not available to the preliminary work.
Later work for the EIA indicated competent clay near surface with little mobile material, except in the Palaeo Swale channel. Scour therefore expected to be minimal over the site, with some limited scour likely in the palaeo channel infill material.
Scour protection left as option by EIA. Scour protection built Not built or planned at time of review MCEU /FEPA • Post construction bathymetric survey of at least four foundations monitoring representative of the soil types. conditions • Six-monthly repeat bathymetric surveys over three years to assess the need for scour protection. Surveys to include the cable route. Surveys completed to • EIA (single beam, summer 2002), September 2006 • Pre-construction (swath, August 2004) • Three post-construction (swath, January 2005, November 2005, April 2006) Date of foundation Autumn 2004 – summer 2005 construction Date of cable lay Autumn 2005 Summary of survey Baseline side-scan and first three post-construction surveys results completed as required for main site. Four foundations monitored for scour pit formation, but only from one area of the site.
Scour pits around the mono-piles are clearly visible. The scour
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increased slightly between post-construction Surveys 1 and 2, reaching a maximum depth of 2.3m over a footprint up to 10m total diameter. The slope angles are very steep at close to 1:1. Between Surveys 2 and 3 the scour depths diminished indicating that some transport and deposition occurs across the site.
These results contradict the expectations for minimal scour based on the assumed firmness of the London Clay beds and expected resistance of the Palaeo Swale beds. The surveys are only representative of one part of the site, so other areas may show different results.
Scour depths are slightly greater on W / SW side, but the footprints show no apparent elongation along any axis (apart from the footprint of the inter-turbine cable routes which run in a NW – SE direction in the area of the four monitored foundations).
The penetration pits created by the jack-up barge legs partially filled over the period of three surveys, but are still clearly visible, indicating low sediment transport/deposition across the site.
The inter-turbine cable routes are visible in the surveys as shallow depressions and mounds. Assessment of Monitoring completed as required by the FEPA conditions. The work monitoring and appears to be to a high standard and provides the range of information presentation required when combined with the original site investigation work for EIA (geophysical, soil samples, etc).
The four foundations monitored are all in one area of the site, selected as an area where scour might be expected according to information available at the time of planning the monitoring programme (personal comm. from the site manager). A wider distribution, including the Paleo-Swale channel would have provided a higher level of confidence in the potential for scour across the site. Diver inspections are needed to establish whether the mono-pile pits are indeed caused by scour or whether they are largely a result of surface deformation during pile penetration.
Diver inspection would also be useful to determine the need for protection of the cables and J-tubes.
Presentation of results at 0.5m contour intervals would make for easier assessment. Referenced • FEPA Licence 31780/03/0 documents • Kentish Flats Environmental Statement. Emu Ltd, August 2002. • Pre and Post Construction Swath Survey Reports (4 No), Emu Ltd.
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Table 3 Scroby Sands wind farm scour review Site 30 x 4.2m diameter mono-pile foundations with cable connection to shore at Great Yarmouth. Water depths and • Site: 3mLAT to 12mLAT. Bank is known to be mobile, with general morphology changing depths, sand waves and mega-ripples. • Cable route: shallows to the shoreline over mainly mobile sand. Surface sediment Medium sand and some gravels/shells offshore and nearshore. types Sub-strata types Sand / gravel beds above glacial till, assumed to be at least 5m below the surface within the wind farm site. Tide regime • Neap range: 1.1m • Spring range: 1.9m Current regime • Peak speed: 1.65m/s over the bank, less along bank margins and nearshore • Dominant directions: NNW - SSE rectilinear. Wave regime North-easterly and south-easterly wind sea dominated conditions with severe storms. • 1:1 year: Hs=1m to 3.5m (depth limited over shallows at Low Water) • 1:10 year: Hs=1m to 5m (depth limited over shallows at Low Water) Frequent breaking waves over bank and across shallows on cable route. EIA scour assessment LIC Engineering assessed expected scour to be 1.3 times the cylinder diameter giving about 5.5m depth (DNV standard guidance) with a range of 4-6m depth. The scour holes were expected to develop within a few tidal cycles.
Original EIA predictions were 8.4m based on Johnston and Erasito (1994) assuming a smaller 3m diameter pile.
Scour protection left as optional by EIA. Scour protection built Rock added after construction by side dumping from barge in to scour pit. Intended to fill pit to form level surface, but actual operation resulted in three or four separate piles of material, partially filling the pit. Secondary scour extended the pits after the protection was added.
Pre and post protection surveys carried out to assess protection placement. A second rock dump was used to improve some sites, followed by second survey of specific foundations. MCEU /FEPA Pre-construction (summer and winter) bathymetric and side scan monitoring surveys of the whole site. conditions Six-monthly repeat bathymetric/side scan surveys of whole site for three years to assess the need for and performance of the scour protection. Surveys completed to • EIA (single beam, spring 2002) September 2006 • Pre-construction (swath, autumn 2003) • Post-construction swath (February 2004, June 2004, July 2004, February 2005, autumn 2005) Date of foundation Autumn / winter 2003/4
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construction Date of cable lay Spring 2004 Summary of survey Baseline side-scan and first four post-construction surveys completed results as required for main site. All foundations monitored for scour pit formation. Post construction export cable route survey not available at time of review.
Scour depths of 3.5m to 6m (about 0.9D to 1.5D) recorded at every turbine site, with least depths associated with shallow areas across the bank, and greatest depths along the inshore margin of the bank (this observation may not be representative of the worst case scour – surveys are done in calm periods when scour pits may have refilled due to low wave activity across the bank). Scour pits tended to be elongated north to south, with a footprint ranging from 35m to 70m diameter (about 8D to 17D). The whole bank is subject to bed mobility with vertical changes of up to about 1m during the survey period.
The post construction surveys showed no evidence of jack-up scour pits, indicating that these were filled by active bed load transport.
Scour protection placed during spring/summer 2004 using a side dump barge. Engineers design called for smooth placement to a level at or just above the surrounding bed. Actual placement was in 3 or 4 mounds around each mono-pile, leaving much of the original scour pits unfilled.
The CEFAS report assessing bed mobility over the full site notes the following based on the full swath surveys (not reviewed by HRW):
• “Scour wakes” on some mono-piles, extending from one mono- pile to the nearest downstream neighbour. The scour wakes are orientated at ~30° to the normal N-S tides, in line with the surge current direction. • “Scour pans” with a U- shaped profile in the NW corner within the sand wave field (contrasting with the “v-shaped” scour pits in the remainder of the array). Dimensions uncertain.
The original EIA prediction of 8.4m scour depth (for 3m diameter piles, therefore 2.8D) is greater than the maximum measured scour, and the prediction method was not in line with best practice. The design engineer’s predictions were less than the maximum scour and do not allow for potential extreme short term scour.
Predictions did not allow for scour “tails” or “pans”. The tails are only found in an area of the bank that is naturally subject to extensive mega-ripple fields, and may be a site specific phenomenon. Further investigations are ongoing. Assessment of Monitoring completed in excess of the FEPA conditions. The work monitoring and appears to be to a high standard and provides the range of information presentation required when combined with the original site investigation work for EIA (geophysical, soil samples, etc).
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Diver inspection would be useful to determine the need for further protection of the cables and J-tubes, and to assess the secondary scour.
Survey plots vary in standard from different contractors. Some do not show survey date or depth scales. Need consistent contouring to 0.5m intervals at least. Referenced • FEPA Licence31272/02/0 documents • Scroby Sands wind farm: Scour survey. Gardline Report, February 2004. • Design of scour protection. LIC Engineering Memo 0144-42, May 2004. • Potential effects of offshore wind developments on coastal process. ETSU ReportW/35/00596/00/REP, 2002.
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Table 4 North Hoyle wind farm scour review Site 30 x 4.0m diameter mono-pile foundations with cable connection to shore at Rhyl. Water depths and • Site: 6mLAT to 12mLAT. Plateau with mobile bed features general morphology including mega-ripples and sand ribbons. • Cable route: shallows to the shoreline Surface sediment Gravely medium sand or sandy gravel across main site. Some mobile types sand bed features to about 1m depth. Increasing sand to shore. Sub-strata types Boulder clay Tide regime • Neap range: 4.1m • Spring range: 6.1m Current regime • Peak speed: 0.7m/s offshore reducing along cable route. • Dominant directions: W - ESE rectilinear. Weak residual current to the ESE. Wave regime North-westerly and westerly wind sea dominated conditions with severe storms. • 1:1 year: Hs=3.6m • 1:10 year: Hs=5.4m (depth limited at lower water levels) Breaking waves common across shallows on cable route. EIA scour assessment ES maximum scour to be 1.4 times the cylinder diameter in mobile sands (DNV guidance with small conservative allowance) with side slopes to 1:3. Scour expected to be limited by gravels and underlying boulder clay across most of site.
Scour protection left as option by EIA. “Coarse and fine grained quarried material” proposed. Scour protection built Not built or planned at time of review, but some rock dumped to protect J-tubes and cables near each foundation. Drill cuttings were left in place where mono-piles required drill / drive installation. MCEU /FEPA Pre-construction (autumn) bathymetric and side scan surveys of monitoring whole site and surrounds conditions Bathymetric survey around a sample of adjacent turbines (minimum of four) within three months of completion of construction. Repeated at six-monthly intervals for a period of three years to assess the need for and performance of the scour protection. Surveys completed to • EIA (single beam and geophys, summer 2001) September 2006 • Pre-construction (single beam, summer 2003) • Post-construction swath survey of all 30 turbines (autumn 2004, spring 2005) Date of foundation Summer 2003 construction Date of cable lay Autumn 2003 Summary of survey Baseline and first two post-construction surveys completed as results required for main site – autumn 2005 and spring 2006 surveys not provided. All foundations monitored for scour pit formation.
Very little scour observed, with depressions of no more than 0.5m at a few turbines. Mounds recorded close to the foundations, mainly on the SE or E side, resulting from rock dumping at J-tubes and cables. Some apparent movement of these piles is noted, possibly due to
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small survey errors or secondary dumping.
Assessment of Monitoring completed as required by the FEPA conditions. The work monitoring and appears to be to a high standard and provides the range of information presentation required when combined with the original site investigation work for EIA (geophysical, soil samples, etc).
Plots do not include survey dates. Final two surveys not received or reviewed, but no further scour expected.
Records of rock dumping operations to protect the J-tubes and cables would be useful to support the survey data. Diver inspection would be useful to determine success of cable and J-tube protection, and to assess any secondary scour. Referenced • FEPA Licence 31579/03/1 documents • North Hoyle OWF: Environmental Statement, National Power, 2002 • North Hoyle OWF: Scour monitoring survey. Osiris Report C4014a, March 2005 • North Hoyle OWF: Scour monitoring survey. Osiris Report C5011, July 2005
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Table 5 Arklow Bank wind farm scour review Site 7 x 5.2m diameter mono-pile foundations with cable connection to shore. Turbines numbered 1 – 7 from south. Water depths and • Site: 2mLAT to 6mLAT over the bank. Shifting over time. Some general morphology evidence of sand waves moving in a NE - SW direction. • Cable route: Maximum 32mLAT on route to shore with variable bed conditions. Surface sediment • Site: Loose to medium density sand to ~2m depth over the bank. types • Cable route: Sand to cobbles. Sub-strata types Sandy gravel, reworked moraine deposits Tide regime • Neap range: ~ 1m • Spring range: ~2m Some uncertainty due to discrepancy between measurements and predictions Current regime • Peak speed: 2m/s over bank, less along margins and nearshore • Dominant directions: Regionally N-S, but locally WSW-ENE across the bank. Rotational residual currents clockwise around bank. Wave regime Southerly swell dominated conditions with severe storms. Secondary wind waves from the north. • 1:1 year: Hs=5.6m • 1:10 year: Hs=7m Breaking waves common across bank. EIA scour assessment Scour predicted using desk assessment but large uncertainty as to expected depth and footprint. Protection recommended.
HR Wallingford undertook physical modelling to assess scour potential and protection methods as part of the engineering design process. Extreme scour to about 1.5D was predicted. Rock protection was recommended, to be placed after development of the initial scour holes. Scour protection built Scour protection using rock to about 0.5m diameter was placed around all mono-piles following development of the initial scour holes. The finished level is believed to have been up to 0.5m above the existing seabed for some of the piles. It is not known if the rock was topped with gravel.
The cable interface with the J-tubes was not consistently protected, resulting in some damage to the cables. Inter-turbine and export cables were buried.
Post-construction work has been completed to improve protection to cables at the J-tube interface by attaching split-duct sheaths.
There has been some settling and movement of the rock protection. It is uncertain as to the quality and extent of protection at Turbine 5, as no rock is visible according to diver observations. MCEU monitoring NA – no information on any Irish licence conditions applied conditions Surveys completed to • Pre-construction (August 2002) September 2006 • Four six-monthly swath-bathymetry surveys from spring 2004 to
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autumn 2005 including the full site and the cable route to shore • Diver photo surveys of foundations with each post construction swath survey Date of foundation Summer 2003 construction Date of cable lay Summer - autumn 2003 Summary of survey Diver inspection indicates that rock protection was not laid evenly but results did fill the original scour pits. Sand and gravel have covered over much of the rock (gravel may have been laid?). There is some scour between the rocks and the mono-piles on some sites. It is unclear as to the extent of scour protection laid at Turbine 5, as sand/gravel has apparently built up generally in this area with the result that the protection is buried.
The seabed around each turbine comprises sand/gravel over the scour protection. Turbine 5 has a 2.5m scour hole, but this may be above the original protection as the general bank levels may have increased at this location post construction.
The cables and J-tubes are intermittently exposed or covered at the edge of the scour protection indicating an active transport regime.
There is evidence of general bank mobility with wide scale vertical changes of up to 1.5m, possibly in response to seasonal variations in wave activity.
There is no evidence of scour to the export or inter-array cables apart from local to the turbines. Assessment of Monitoring completed for engineering assessment. Irish Licence monitoring conditions are presently unknown.
Quality of survey work, including diver survey, appears excellent and provides a high quality record of scour protection performance. The dive record is particularly useful as a means of providing detailed assessment of structure – sediment interactions, while the swath survey gives a broad scale view and allows quantitative assessment of bed change. Referenced • Arklow Bank Environmental Impact Statement. Fehily Timoney documents & Co, June 2001 • Arklow Bank Wind Farm project: Hydrographic Survey, Interim Reports. Hydroserv Ltd, Spring 2004, Autumn 2004 and Spring 2005
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In addition to the four Round 1 sites and Arklow Bank, information was obtained for the Horns Rev and Nysted wind farms in Denmark, plus preliminary information for the Burbo Bank site in Liverpool Bay. The information from these sites is summarised as follows:
Burbo Bank – scour predicted to a depth of 1.5D using standard guidelines conservatively interpreted and protection is proposed. Water depths vary from 2m to 8mLAT, with an 8m spring tide range, moderate currents and moderately severe wave conditions. Foundations have been successfully installed over summer 2006 but no data on scour is yet available.
The FEPA consent licence assumes the use of rock dump scour protection, but also requires consideration of frond mattresses. The reason for requiring consideration of frond mattresses is not specified in the licence. This approach is untried with mono-pile foundations in shallow water conditions, and may be considered as a risk if scour is likely to impact on structural stability.
Horns Rev – scour was predicted and rock protection was installed to this exposed wind farm site on the North Sea coast of Denmark. No monitoring was required for consent and no field information is available. Anecdotal evidence suggests that scour has not been a problem.
Nysted - this sheltered site is on the Danish south coast within the Baltic. Gravity bases have been installed which normally require scour protection. No monitoring information is available, but there is no evidence that scour has been considered as a problem.
HR Wallingford also made contact with Talisman, the developers of the Beatrice project in Scotland. Multi-leg jacket foundations have been installed in about 40m water depth. Talisman note that past monitoring of the gas platforms in the Beatrice Field have revealed little if any scour, nor other mobile bed features. They have no consent conditions for monitoring so will not be generating any further data apart from their own monitoring of the cable route and foundations to ensure engineering stability.
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3. Discussion
The results from the sites assessed indicate the diversity of the wind farm locations, and the consequent range of scour responses. Scour was predicted for all sites during the EIA process, but the engineering assessment determined that scour protection was not definitely required at any of the four UK Round 1 sites, while at Arklow Bank, Horns Rev and Nysted protection was placed as part of the initial construction procedure. At the four UK sites the FEPA consent licences required bi-annual monitoring over three years to determine the need for protection, and the performance if protection was installed. Protection was subsequently placed at Scroby Sands following the initial post-construction monitoring.
The following sections summarise the site results, review the monitoring and data presentation approaches, and suggest improvements for future policies and procedures.
3.1 SITE SUMMARIES The four UK sites represent a good range of site situations. They include soil types from mobile sand to stiff boulder clays, water depths from 3m at low tide to 27m at high tide, peak tidal flow speeds from 0.7m/s to 1.65m/s (or about 2m/s if Arklow Bank is included) and 1 year return period wave conditions from 3.3m Hs to 5.6m Hs. Scour ranges from near zero to about 1.5D in depth and to a footprint extending from the mono-pile to a distance from less than 1D to about 8D (or a maximum footprint dimension of 17D, where D is the pile diameter).
At Scroby Sands extensive scour was anticipated. The first post-construction survey revealed scour to 6m depth, and protection was installed. Unfortunately the scour protection was not placed according to the design specification of “evenly distributed around the foundation” and “over-filling (the existing scour hole) a little above the existing seabed level” (LIC, 2004) with the result that secondary scour occurred giving an extensive footprint of bed disturbance at each turbine.
At Barrow moderate scour was predicted, and initial post-construction monitoring has shown that the EIA predictions were appropriate; the one year survey shows considerably more scour in line with the engineering design predictions completed after the EIA. No scour protection has been placed, but may still be required for some parts of the site depending on future monitoring results and exposure of cables. The jack-up spud can marks are clearly visible at many foundations after one year, in some cases to a depth similar to the main scour pit. Most holes filled at least partially between 2005 and 2006, but not in every case; it is possible that there have been additional jack-up operations that have left new or deepened marks.
At Kentish Flats limited scour was predicted and no protection was planned. Post-construction surveying indicated that scour had occurred to a much greater depth than anticipated over at least part of the site. It is not clear as to why scour was so much deeper than expected given the soil conditions. Initial interpretation was that the London Clay beds had deformed in response to the piling operations including penetration by the jack-up barge legs. This interpretation is questioned as the mono-pile scour pits appear to deepen between the first and second post- construction surveys during a time when they would be expected to infill and rebound if the cause had been strata deformation. The jack-up spud can holes have become shallower over time, but again it is not clear whether this change is a result of infill by mobile bed material or rebound of the clay soil. Scour protection is not planned as the scour is not considered to be a risk to foundation stability.
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Finally, at North Hoyle very limited scour was predicted and no protection was planned for the monopiles. Post-construction surveying confirms the prediction.
The Arklow Bank swath survey results show that well designed and placed scour protection can be successful. However, the supporting diver observations indicate that additional protection may be required to avoid scour and consequent damage to the J-tube / cable interfaces.
It should be noted that the scour results are all recorded during “recovery” conditions due to the need for reasonably calm sea states for vessel based survey operations. Scour is a dynamic process, so worst case conditions will be short lived during the period of greatest forcing by currents and /or waves. As soon as the forcing conditions relax, the scour holes will begin to refill to a new equilibrium depth. The recovery process for the total scour footprint will be much longer, so the recorded footprint area is likely to represent the worst case. Only continuous monitoring of individual scour pits using fixed instrumentation will reveal the peak scour depths.
There is no evidence of scour extending beyond the local area around each turbine foundation, except at Scroby Sands where scour “tails” have developed to the southeast of some of the structures. This analysis has not considered broad scale change across the site, but this has been considered within the morphological aspect of this research project (Appendix D).
3.2 MONITORING APPROACHES The consent conditions for each of the Round 1 sites require that scour and scour protection are monitored at representative sites and along the cable routes every six months for three years after construction. This is considered to have been a sensible approach for the four sites built to September 2006, but the monitoring requirement should now be reviewed in light of the findings of this study. Consideration has been given to the consented sites where licence conditions may be retrospectively modified, and to future Round 1 and 2 sites which may involve a range of foundation types and deeper / more exposed locations.
The method of scour monitoring adopted by all developers has been swath bathymetric surveying, allowing a high-resolution xyz image to be produced. The available supporting software allows easy comparison of surveys from different dates, defining areas and volumes of change and contouring to any specified interval. Cable survey results have not been obtained and reviewed for the UK sites, and it is uncertain as to whether these surveys have been completed, although this was a FEPA requirement for Barrow and Kentish Flats. Further work is being undertaken within a complementary DTI (now BERR) project on cable impacts. A single survey of the Arklow Bank cable route was obtained and reviewed.
Survey reports have not clearly addressed the confidence limits on the survey accuracy, so interpretation must allow for potential errors of position and elevation. Reports should be required to make a clear statement as to expected accuracy and resolution in the area of the foundation scour.
The surveyed areas around each mono-pile varied in extent between sites and surveys. As measurable scour appears to extend no more than 8 pile diameters in any direction, then a survey area with a radius of 70m from the foundation should always be sufficient to define the area of disturbance as well as any background variation to the sea bed beyond the immediate influence of the foundation.
One source of error is the correction for tide. Different surveyor contractors appear to have adopted different approaches with on-board GPS corrections, onshore tide gauges and simple
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use of predicted levels for the nearest coastal location. All of these approaches introduce potential errors for comparison of bathymetries recorded at different times and with differing equipment if contractors working methods change between surveys. A solution would be to install a tide gauge on each site at one or more of the turbines to provide an ongoing and consistent record.
A source of survey inconsistency is the varying wave and tidal conditions in the period immediately before each survey. To a large extent this is not controllable as surveyors must take advantage of whatever weather windows become available, particularly during the winter when wave conditions are more difficult. However, inconsistency would be reduced if surveys were specified to occur within, say, four days of peak spring tides wherever possible. This restriction would increase consistency of survey conditions as the peak spring tide period is expected to produce the greatest current related scour.
The swath surveys do not adequately resolve small areas of scour related to J-tube and cable interfaces with the sea bed. In addition, the data returns may be subject to further signal error in the immediate area of the foundations due to reflections from the structure and local turbulence. The survey accuracy and resolution will also be affected by post-processing and grid definition; standard procedures will be appropriate to surveying of typical bathymetry, but may be insufficient to resolve small-scale details or provide confidence in comparisons between successive surveys. Diver (or ROV) inspection in combination with swath surveying is likely to be the only method of assessing detailed bed changes close to the foundations, with the additional benefit of direct assessment of any damage to structures or cables, observation of scour protection performance and recording of benthic community development. Diver inspections were undertaken at Arklow Bank in association with the bathymetric surveying, providing detailed assessment of scour, damage and marine growth not available for the four UK sites.
To improve survey interpretation, contractor’s reports should include records of wave conditions at the site (preferably measured, but otherwise observed and supported by wind data) and tidal elevations (preferably measured) for a period of one week before the survey. This supporting information would allow variations in successive surveys to be assessed.
As a generic investigation of scour it would be useful to the industry as a whole if continuous monitoring of scour were undertaken at one or more foundations. This work would allow variations over time to be better understood, helping to place the snapshot surveys already available in to context. It would not be productive to propose this detailed investigation as a licence condition as the monitoring and assessment of the results is a research activity.
3.3 PRESENTATION OF RESULTS The standards of presentation for the four UK sites were variable, with several specific points needing to be standardised:
• Data presentation relies on gridded information which is typically provided at either 0.5 or 1m centres. The choice of grid resolution should recognise the need to resolve any small scale features but also remain consistent between surveys • Plots should clearly define the survey date and an inset plan showing the location of the survey area in relation to the total site • The survey area presented should extend at least 70m in all directions from the walls of the foundations or periphery of the scour protection where placed • Plots should be contoured to a 0.25m interval
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• Plots should clearly indicate the location and as-built position of the foundation within the contoured bathymetry • Plots of bathymetric change should be presented where repeat surveys have been undertaken, again contoured to at least 0.25m intervals • All survey reports and plots should provide clear guidance as to the expected survey errors, with specific reference to the confidence limits in the immediate area of the foundations.
The survey results should be provided as processed xyz files as well as in report presentation format. This approach will allow the potential for more detailed analysis at a later date.
3.4 FUTURE MONITORING The need for a blanket scour monitoring requirement for all wind farm developments in future depends on the objective of the FEPA licence conditions. If the purpose is to assess the impact of sediment disturbance on the natural environment, then ongoing monitoring will provide little useful information for sites that are either protected by well placed scour protection (i.e. Arklow Bank) or subject to modest scour (i.e. North Hoyle). Similarly, if the purpose is to provide assurance of engineering stability then bi-annual swath surveys do not provide the critical information on short-term extreme scour during periods on high loading forces.
Assuming that the main objective is to assess impacts on the natural environment, the most useful information will be gained from swath surveys of a representative group of foundations, undertaken between 2 weeks and 3 months after foundation installation and again after six months to one year, preferably in a period immediately following a peak spring tide. The surveys should include a sufficient number of foundations to represent the different sedimentary or hydrodynamic regimes. A basic target of at least 25% of the total number of foundations is suggested, with a spread across the site including any known variations in soil conditions, bathymetry or exposure to waves / currents. Each individual survey should cover an area extending at least 70m in all directions from the edge of structure or periphery of the scour protection where placed.
Further surveys spread over a number of years would only be required if the initial surveys reveal that unacceptable or unpredicted scour has occurred. Any such further swath surveys and diver inspections should be requested to monitor the results of any actions taken to remedy the scour. These surveys should cover a period sufficient to show that an equilibrium position has been reached, with a minimum of two surveys separated by at least three months.
This approach should reduce imposed costs to the developer, while still providing evidence of successful scour prediction and, where required, monitoring of scour protection. It may transpire that the guidelines for prediction of scour reach a level of certainty in the future that will obviate the need for monitoring, but as yet that situation has not been reached.
It should be noted that the developers may require additional surveys to confirm that engineering design and placement of protection is satisfactory over a longer time period. The example of the Arklow Bank combination of swath surveying and diver observations is instructive for other developers working in areas of known risk.
Cable surveys for the Round 1 sites have not been made available or reviewed for this report. A survey was available from Arklow, but it showed no evidence of cable exposure and therefore no potential for scour. The surveys from all sites show some evidence of the location of the cable routes as a result of the installation method near to each turbine. Future monitoring of cables should not be defined specifically in relation to scour. Exposure of the cables is a problem, irrespective of any scour that may result, with possible consequences for fishing gear
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as well as dangers to the cable from dropped objects, snags and cable movement particularly at the interface with the turbine foundations.
Diver inspections should be required at the J-tube / cable interface and in areas where rock dumping or other protection has been applied. In other areas, comparison of post-construction swath surveys with the installation surveys will indicate changes to the bed depth. When coupled with knowledge of the cable installation depth, locations where cable exposure is, or may become, a problem will be revealed.
4. Conclusions and recommendations
The measured and predicted scour features were in good agreement for North Hoyle (and Arklow Bank), but surprises were found at Barrow, Scroby Sands and Kentish Flats. The extent of the footprint at Scroby Sands and the depth of scour at Barrow and Kentish Flats were larger than expected. There is some question as to the actual causes, with poorly placed scour protection being a possible complicating factor at Scroby Sands and soil deformation being a possible issue at Kentish Flats. At Barrow the EIA prediction was optimistic, while a subsequent engineering design prediction was based on more conservative assumptions and greater geotechnical information, with the result that the measured and predicted depths were similar.
A further surprise has been the extent and duration of sediment disturbance due to jack-up operations. This issue was not addressed by the ES documents. Where multiple operations have occurred the amount of disturbance due to spud can penetration has been much greater than the disturbance due to foundation scour. From an EIA perspective this may not be significant as the penetration marks will refill over time, with the only damage being caused to benthic communities in the immediate area of the jack-up legs. Significance can only be attributed to rare communities or those that are unlikely to recover within a reasonable time frame.
Guidance and research relating to scour is acceptable for EIA purposes with regard to the maximum depth of scour in areas with unrestricted depths of mobile, non-cohesive sediment and strong currents; consultants appear able to interpret this guidance for other situations. There are significant uncertainties with respect to scour around any placed protection and this uncertainty can be extended to non-mono-pile foundation types (gravity bases and multiple leg structures). However, these are essentially engineering issues and are of limited importance with regard to the environment unless it is considered that the placement of scour protection may be damaging.
Results to date indicate that swath bathymetric surveys provide excellent results, but there is some uncertainty as to the accuracy and repeatability of results close to structures, specifically in relation to the maximum depths of scour.
Swath surveys of the cable routes were specified by the FEPA licence conditions at two of the four sites reviewed, and at the recently installed Burbo Bank site. The only information available for cable scour at the UK sites is for the area around each monitored mono-pile, which apparently indicates that the cables have remained buried apart from the areas of the scour pit footprints. The only site with a full survey of the export cable route is Arklow Bank where the swath bathymetry for all cable routes shows that the cables have remained buried except at the J-tube entry.
The following conclusions and recommendations are offered:
• Survey results submitted as part of the FEPA or other licence conditions should be reviewed by the regulators within a short, specified time period to ensure that the surveys
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are fit for purpose and that the results do not show any surprises; the monitored scour pits at Kentish Flats are a case in point where more intensive monitoring plus diver inspections could have been requested at an early stage to assess the reason for unexpected scour. • Where there are no surprises or anticipated problems, the requirement for ongoing surveys should be reduced or dropped completely. Where problems are revealed, diver inspection should be considered to supplement the information derived from the swath bathymetry, and a forward programme of monitoring should be agreed with the site owner. • All monitoring data and reports should be archived centrally according to agreed protocols, as being considered through COWRIE, allowing for potential future reviews and research. • The design and placement of scour protection in future wind-farm construction should be considered in more detail by the engineers and by the regulators, as poorly placed scour protection has been shown to cause secondary scour effects. • Instantaneous scour needs to be assessed through a research project to ensure that engineering design is sufficient with regard to extreme loads; this work will require concurrent measurement of scour depths, tidal elevation, currents, wind and waves. • The effectiveness of frond mattresses has not been proven for mono-pile structures in shallow water, and research is required before making recommendations for use; if fronds are installed then diver inspection of performance will need to be specified. • The potential impacts of scour protection systems on the environment need to be considered. Diver records from Arklow Bank indicate heavy marine growth on the mono- piles, rocks and cables, with associated mobile invertebrates and fish. • Further work should be undertaken to assess the magnitude and nature of the impacts of other types of wind-farm foundations (gravity bases, multi-leg structures, etc) as well as of the potential foundations structures for other marine renewable energy projects (e.g. tidal and wave devices).
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Figure 1 Barrow offshore wind farm, post construction survey: No visible scour in glacial till exposure, but jack-up barge spud can holes still visible
Figure 2 Barrow offshore wind farm, post construction survey: Significant scour in silty sand and jack-up barge spud can holes still visible
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Figure 3 Kentish Flats offshore wind farm, post construction survey: Significant “scour” in exposed London Clay and jack-up barge spud can holes still visible
Figure 4 Scroby Sands offshore wind farm, post construction survey: Significant scour in fine sand prior to placement of scour protection. Cable exposed in scour pit.
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Figure 5 North Hoyle wind farm, comparison of post construction and 1 year surveys: No visible scour
Figure 6 Arklow Bank offshore wind farm, post construction survey: Scour limited by placed rock protection
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Printed in the UK on recycled paper. Department of Energy and Climate Change. www.decc.gov.uk First published December 2008. © Crown Copyright. URN 09/508.