AE1227 Assessment of the Significance of Changes to the Inshore Wave Regime as a Consequence of an Offshore Wind Array

Abstract

This research will assess the significance of changes to the nearshore wave regime as a result of the construction of offshore wind farms, based primarily on unique field measurements, but including scenario-testing using numerical modelling techniques. The research is concentrated at a specific site (i.e. Scroby Sands), which was identified as the “worst-case scenario” for impact on coastal processes, from the 1st round of applicants for wind farm development to the Crown Estate. It is anticipated that this work will assist in ensuring that the sustainable development of offshore wind energy is maintained. Presently there is a lack of field evidence in support of numerical modelling predictions, and a lack of consensus between models, and as a consequence, judgments of the stability of the adjacent coastline to erosion may be inadequately informed.

However, the government is presently promoting energy production from renewable sources, with a target to increase production from this source to 10% by the year 2010. The development of offshore wind farms has consequently received significant attention, and indeed, the decision to grant FEPA license to developers of an offshore wind farm within coastal waters on Scroby Sands was recently made (March 2002). However, during this licensing process, the issue of wave focussing on the coastline surfaced, and the magnitude and significance of its impact remained without consensus. The issue has potentially significant consequences for coastal defence planning, particularly for vulnerable coastlines that may be susceptible to coastal erosion. Indeed, the mechanisms controlling this focussing effect are such that under the specification imminent for the next generation of wind farms, the impacts may take on a yet greater significance. Consequently, there is an urgent requirement to improve scientific understanding on this issue in time to provide informed advice to the FEPA licensing process.

The project will primarily aid licensing decisions and UK policy and advice on offshore wind farm development (FEPA 1985), thereby supporting Defra’s remit for responsibility for the sustainable development of the marine and coastal environment, and of coastal defence. Researchers and consultants would also benefit from the enhanced scientific understanding gained from the provision of unique field measurements. Although beyond the scope of this project, the wind farm community (i.e. industry, government, Crown Estate, consultancies, government agencies) would then be in a position to confirm the validity of various industry-standard coastal wave models through additional focussed research contracts.

Purpose

The provision of "coastal process" advice relating to the Scroby Sands ES highlighted areas where our present level of scientific understanding was deficient (particularly wave diffraction effects, and sediment transport pathways). Improved knowledge in these areas would improve confidence for predictions relating to future developments. Indeed, with the second round of wind farm developments imminent, it is anticipated that future developments will involve increased numbers of turbines, of larger dimension, in more tightly packed arrays, and covering larger areas of the coastal zone. It is apparent that such large offshore wind arrays are already in the early stages of the application process. It is therefore vital that our understanding of the impacts of these developments on coastal processes, and ultimately on the configuration of the coastline are well understood before significant demands are again placed on the consenting process.

Presently there is no wave data available to validate the predictions of the numerical wave models that have been used to investigate the wave diffraction effects which occur as a direct result of the construction of offshore wind arrays. As a consequence, it is not readily apparent that the suite of models presently available to developers and their consultants, are adequately representing the underlying physics of wave diffraction, particularly when issues of scale and model resolution may be in opposition. This research will address these deficiencies.

Defra support for this research will aid licensing decisions (FEPA 1985) in relation to wind farm developments, and will more importantly mean that such advice will demonstrably be based on sound scientific understanding. There is an urgent need to supply, through this R&D, high quality advice to Defra relating to imminent future developments, but also to use measurements relating to construction of the first round wind farms (i.e. through the Scroby Sands site) to feed into the numerical modelling studies, and thus provide timely results for use in the provision of advice for the second round developments. This research addresses anticipated future demands, in an attempt to improve future efficiencies in the provision of advice. Defra support for this work will maintain the department’s high profile in the UK, but more importantly with global focus on renewable energy sources, will enhance it’s profile and reputation within the European Union and worldwide.

This research will provide the field wave measurements necessary to determine whether wave diffraction effects are of detectable magnitude, and along what stretches of the coastline this impact could be important. It will thereby validate the wholly theoretical results of modelling studies carried out by Halcrow for the Scroby Sands site, but will also start to address the significance of these changes in terms of coastal defence issues [Once their importance has been verified, these issues will be developed in subsequent research – henceforth referred to as "Phase 2" of the study]. These models will be utilised in the identification of the parameters that control wave focussing, and will thus enable sensitivity analysis to be performed to account for anticipated wave regimes as a result of changing global climate. Provided these measurements and validation prove to be positive, then in Phase 2, tests will be carried out for a range of industry standard models in order to validate model output and subsequently provide the consensus, which is presently lacking, between outputs from the various models.

Science context

At present, advice to Defra on "coastal processes" for FEPA (1985) licensing is provided by specialists that rely on the knowledge base provided by the scientific literature, to inform opinion on developments and advances in the various spheres of interest. In this case, during review of an Environmental Statement (ES), an informed judgment enables confidence to be placed in any decision. However, when information availability and scientific understanding are low, this level of confidence is also low, such that ultimately, a balance exists between "knowledge", "judgment", "confidence", "significance of impact", and "environmental risk". It may be prudent to improve this knowledge base, particularly in instances when additional pressures from coastal development could threaten the sustainability of our coastline and its defences.

The UK has been committed to the production of energy from renewable sources since 1992, when it signed the United Nations Framework Convention on Climate Change, and has promoted options for energy from these sources over more recent years ⁽¹⁾. Generic guidance for onshore wind farms soon followed ⁽²⁾. However, specific guidance notes concerning the environmental impacts of offshore wind farms have only, within the last few years, reached the public domain ^(3,4,5), and more importantly, much of the related Research & Development has been ongoing concurrently with the licensing process. The scope of information, and the subsequent guidance, has steadily evolved from that provided within generic documents with broad scope of environmental issues ^(4,5), to that provided within focussed studies (e.g. coastal processes ⁽⁶⁾). However, although this recent research has clarified many coastal process issues, and certainly may be useful (as a first approximation) for highlighting fundamental parameters to consider at any given offshore site, it was not tasked with specifically addressing each of Crown Estates’ proposed first round sites. This level of information is only provided within each site-specific ES.

Recent wave modelling work by Halcrow ⁽⁷⁾, as part of the ES for Scroby Sands (i.e. Vol VIII), indicated that wave diffraction effects could result in constructive/destructive interference of up to 5% in the region immediately inshore of the turbine grid. However, the impact of these local effects at the coastline was not investigated, although it was suggested that at "far-field" distances they could be too small to be measurable, and the interference patterns would likely be complicated by the realistic situation involving natural waves as opposed to monochromatic wave trains (as used in the model). Different wave models run by ABP Marine Environmental Research Ltd (ABP MER) and HR Wallingford, applied to other potential wind farm sites ^(6,8), have indicated smaller local changes to the wave regime, and concluded similar negligible effects in the far-field. Although this may indeed be the case, it is apparent that this conclusion is reliant solely on the accuracy of the individual wave models. Although each of these models will have been calibrated and validated against baseline field data (i.e. data at the pre-construction stage), as detailed within guidance notes ⁽⁵⁾, there is at present no (post-construction) field data with which to validate the models. Indeed, the study by ABP MER suggested that "…when wind farm schemes are built and monitoring commences, such information (as provided by model predictions) is reviewed…" in the light of measurements taken in order to inform future confidence in numerical modelling techniques. These measurements will thus form an important link in the requirement to improve confidence in the output of numerical models. The changes to "coastal processes" have been predicted by modelling techniques to be small, such that those changes to wave direction (<2° , ^(6,7)), wave height (<5%, ⁽⁷⁾; <2%, ^(6,8)), current velocity (<10%, ⁽⁸⁾) may be individually insignificant, but considered cumulatively, they may result in changes to geomorphology that may have significant direct impact on the nearshore region, and in addition, as feedback, in turn on coastal processes in that region. It is the dynamic nature of these interactions over relatively long (i.e. decadal) timescales that require attention for future observations and modelling strategy, and which the latter may be presently unable to accurately predict. The datasets provided by this proposal, in combination with those provided by other research and monitoring work (e.g. AE0262 ⁽⁹⁾; Southern North Sea Sediment Transport Study ⁽¹⁰⁾), will begin to address this problem.

It is the regional impact of changes to wave patterns that may be of significant concern, rather than the local effects, and it is these that this study proposes to address. The anticipated wave interference patterns require measurement resolution to be of the order of 10’s of metres, but the potentially large region of impact/concern requires that this resolution be provided over a scale of the order of several kilometres. This requirement for wave data may be achieved using radar techniques, and which are presently the focus of considerable research and development effort in the UK (e.g. Proudman Oceanographic Laboratory) and abroad (e.g. James Cook University, Australia). However, despite this research status, the technique has been utilised successfully during many high-profile studies of coastal processes ^{(11,12,13,14)}. In addition to the provision of long-term timeseries of "images" of wave patterns ^(12,15), the technique can also provide detailed shallow-water bathymetry ^(16,17), identification of nearshore geomorphic features such as bars and sandbanks ^(18,19), and quantification of spatial variations in wave-breaking. Recent progress in the methods of data analysis and interpretation of X-band radar datasets may soon enable detailed quantification of wave height to be extracted from the measurements (Bell, pers. comm.).

This proposal will provide unique wave measurements at an offshore wind farm site, and will as a consequence, enhance our understanding of wave diffraction effects as they relate to engineering structures, and it will extend this understanding to include the effect of multiple structures. Used in conjunction with results from AE0262, this proposal will enable the significance of changes to coastal processes to be quantified, and subsequent interpretations of environmental impact and impact on coastal defence to be better informed. As such, there will be a significant increase in the confidence with which future scientific advice on wind farm development may be provided to Defra. The datasets could also be used to advance techniques in the wave height determination from X-band radar measurements, and to validate various coastal process models and consequently provide consensus on their applicability for use in predicting the environmental impact of future wind farm developments.

Objectives

1. To provide detailed field measurements of waves relating to an offshore wind farm at Scroby Sands.
2. To carry out sensitivity analysis on a numerical model for the Scroby Sands "wind array" and "wave regime" parameters that control wave diffraction.
3. To use the results of (1) in comparison with those from the numerical model, and assess the extent of agreement.
4. To extend the results of (1) and (2) for a more broad application of wave diffraction effects, to generic future windfarm developments.
5. To assess the significance of changes to the nearshore wave regime as a result of the construction.
6. To provide timely and quantitative results to enable the provision of sound generic scientific advice for the second round of windfarm developments. It is anticipated that the sensitivity analysis will test the importance of various parameters (e.g. wave height, wave period, wave direction, water depth) associated with the wave climate, in relation to wave diffraction effects. Thus although the study is site-specific, it is anticipated that it will identify any significant wave diffraction effects as they relate to wind farm arrays, and, as importantly, will identify the extent of variation of these effects. In this way, interpretation of the results of this site-specific study will be used directly to highlight developments under which more thorough investigation of wave diffraction effects may need to be undertaken. Thus guidance on the importance of the various parameters (above) will be included within this research. The present level of funding being made available for this research prohibits the inclusion of data from other sites. However, the present timetable for Scroby Sands (i.e. construction in Autumn 2003 - Spring 2004) is along similar timescales to other developments for which the ES have been reviewed, and therefore, at present there is no reason to change the proposed field site. For reasons of dynamic sediments, proximity to shore, shallow water depth, vulnerable coastline, etc, the Scroby Sands site has been preferred (and indeed acknowledged as the "worst-case scenario" for impact on changes to coastal processes in a recent study by ABPMer, and funded by ETSU). However, should the construction timetable again be changed, then several other alternative sites could be utilised (e.g. second choice Gunfleet Sands, although the ES has only recently been submitted; third choice Lynn/Inner Dowsing). The sites are largely limited by the low range (i.e. approx. 2 km) of the X-band radar, and the use of other sites would necessitate the use of 2 X-band radars (one land-based, the other in close proximity to the OWF) during each deployment. It should be noted that within the licensing conditions for each OWF, is presently being included a potential requirement for monitoring of wave diffraction effects, dependent upon the outcome of AE1227 project. However, pre-construction deployment of the X-band radar would have to be ensured for these measurements to be most effective.

Note: Validation and consensus on a range of industry-standard numerical models in relation to wave diffraction and focussing effects is only to be provided through subsequent research contracts (e.g. Phase 2 submitted previously to Defra as a Pre-CSG7.)

Approaches

1. This tender complements the work of AE0262. As such, field campaigns have been timed to necessarily coincide, and start dates for each project are similar.
2. Using the development site at Scroby Sands as the example site, it is proposed to adopt a phased research plan to investigate the wave regime (this tender covers Phase 1 only); it is proposed that each phase will adopt combined numerical modelling and observational methodologies to quantify the change to the wave regime over the region inshore of the turbine array.
3. Construction on Scroby Sands is scheduled to commence in Autumn 2003 - Spring 2004. It should therefore be emphasised that post-construction field campaigns and associated objectives detailed below are necessarily subject to any alterations/extensions to this timetable.
4. Phase 1 (cost £123,563) will comprise mostly an observational approach, incorporating the following: POL to provide X-band radar over the inshore approx. 1 km, in order to provide good spatial coverage, at the coastline and adjacent nearshore region, of wave diffraction effects; Cefas to provide calibration data for the radar at an inshore site using a directional wave gauge; Cefas to provide measurements of offshore wave climate using a directional wave gauge; Halcrow to provide some modelling of sensitivity analyses (to include simulations for wave height, wave period, incident wave direction, turbine spacing, and bathymetry) using models previously set up for input into the Scroby Sands ES produced by Powergen. Halcrow will use their MWAV_LOC model, which includes the effects of refraction, diffraction, wave reflections and wave breaking. Thus, until a comparison of the various wave models from industry and research organisations has been undertaken, importantly with reference to the same field observations, it is not possible to assess which model may be the most suitable for these coastal process predictions. However at this stage, Halcrow’s expertise in the field of engineering construction and wave modelling, and particularly their knowledge of the coastal processes operating on this region of the East Anglian coast (through their involvement with the Shoreline Management Plan) makes them highly competent to provide a vital contribution to this work. The subsequent model inter-comparison exercise will doubtless make an important contribution to future work in this area, and should identify any limitations in the present suite of wave modelling software used for these environmental assessments.
5. Three field campaigns for Phase 1 are planned, each of approx. 1-month duration. Measurements are presently scheduled for Mar/Apr 2003 (i.e. pre-construction), for Dec 2003/Jan 2004 (i.e. pre-construction), and for Dec 2004/Jan 2005. [It is anticipated that the interpretation of the sensitivity analyses will indicate that measurable wave diffraction effects are most likely under winter conditions].
6. POL and Halcrow will each provide the necessary data and a written report to Terry Oakes Consulting (TOC) and Cefas for subsequent interpretation. The environmental interpretation and recommendations for further work required will make use of the above reports, but also the Scroby Sands ES, the ABP MER report that is presently undergoing minor revision prior to its’ imminent publication, and any relevant technical and licensing information on forthcoming windfarm developments.
7. A project meeting involving TOC, Cefas (Lowestoft), Cefas (Burnham), POL and Halcrow, that will enable dissemination and discussion of ideas, will be held at Cefas (Lowestoft) once the (above) written reports have been made available. A subsequent meeting of TOC and Cefas (Lowestoft) will be held once a draft report has been produced. It is not anticipated that more frequent meetings are necessary, as the long duration of the project largely results from the infrequent deployments – it is only following the final deployment that scientific discussion would be most-productive.

Note: Should the results from this preliminary research (Phase 1) indicate that additional work on wave diffraction be required, the following is a general framework for a possible experimental approach:

8. Dependent on the significance of Phase 1 results, Phase 2 (additional cost approx. £160,000) will be proposed.
9. Phase 2 will incorporate X-band radar over the inshore 2 km with 3 additional wave moorings (i.e. offshore, on the sandbank, and inshore) to validate and provide consensus on numerical wave transformation models as they relate to windfarm structures, over a broad range of wave conditions.
10. POL will be used as a sub-contractor to provide the X-band data. 
11. The remaining observational requirement will be met largely using Cefas instrumentation and moorings, with some additional instrumentation hired from external sources. 
12. Cefas (Burnham) will provide updates on windfarm licensing information, particularly regarding scales and locations of future developments as these become available. 
13. Numerical models that have previously largely been set up and validated by Halcrow, will be utilised in the first instance for the modelling requirement. It is envisaged that the Halcrow involvement will be simply to run model scenarios specified by Cefas, with much of the subsequent interpretation of model results being performed also by Cefas. The involvement of ABP MER and HR Wallingford will be to provide comparatory model results from other suitable, and more importantly, commercially available numerical models (i.e. MIKE21, DELFT, and SWAN). Using this approach, it is anticipated that commercially available models for wave diffraction will be validated against unique field observations, and for the first time, wave diffraction effects resulting from wind turbine arrays will be reliably quantified. 

Results

• Final Report (PDF, 672 KB)