AE0262 Development of Generic Guidance for Sediment Transport Monitoring Programmes in Response to Construction of Offshore Wind farms

Abstract

This research will assess the magnitude and significance of changes to the nearshore sediment transport and sediment transport pathways as a result of the construction of an offshore wind farm on Scroby Sands. Using field measurements of coastal processes, the work will contribute to the formation of generic guidance to Defra as part of the FEPA (1985) licensing process, and thereby maintain the sustainable development of offshore wind energy.

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, two issues of potentially major importance to the development of the adjacent inshore region became apparent: those of wave focusing on the adjacent coastline (see additional proposal submitted in mini-CSG7 form as an additional study), and the change to regional sediment transport and its consequent effect on large-scale geomorphic features. This proposal details research to resolve the latter issue. The magnitude and significance of its anticipated impact remained without consensus and hence a study was made as a condition of the FEPA license. The issue has potentially significant consequences for coastline management, particularly for dynamic and vulnerable coastlines that may be susceptible to coastal erosion. Through theoretical calculations and modelling, the confidence with which it is presently possible to predict the magnitude of this sediment transport is low, and, more importantly, is such that under the specification imminent for the next generation of wind farms, the potential changes to geomorphic features may take on a yet greater significance. Consequently, through observation and subsequent validation of theoretical calculations, there is an urgent requirement to improve scientific understanding on this issue in time to provide informed advice to the FEPA licensing process, which may be relevant to generic wind farm development.

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. Researchers and consultants may also use these unique field measurements, and the enhanced scientific understanding gained from the provision of the generic information, in order to confirm the validity of sediment transport models.

Scientific Context

There are insufficient funds within this project and AE1227 to monitor sediment and wave conditions at even a short list of the proposed OWFs. Therefore, a single site has been chosen in which to concentrate resources and effort. The selection of the Scroby OWF site is also supported by the findings of the ABPmer/ETSU report identifying Scroby as the worst case scenario in the first round of developments, in terms of potential impacts on coastal processes. This project and AE1227 will hopefully act in a "pump-priming" manner and help increase confidence in numerical models which are severely lacking in terms of calibration in these shallow water, high energy environments where many large scale OWFs schemes are proposed eg. Outer Wash and Thames estuaries. Industry through their consultants can use these datasets to help increase the confidence in numerical models and thus make the information flow from this site more generic.

The calibration of current numerical sediment transport models rely on a small set of observations in relatively deep water (>20m) and moderate tides. In this proposal we aim to extend the range of conditions under which these numerical models have been validated. Specially, we aim to collect wave, current and suspended sediment data pre and post construction of a wind farm in winter and summer seasons from the "top" of Scroby Sand bank, offshore at a control site and inshore in the deeper channel. Additional sidescan and bathymetric surveys will be undertaken during the five field campaigns. These data sets will be of high quality with adequate water, sediment and depth records for calibration and validation of the appropriate datasets. For instance, the platforms for measuring current and suspended sediment will have roll, pitch and compass sensors to check for movement of the frame as well as suspended sediment traps (Bonner tubes) for post deployment calibration of OBS sensors in a turbidity tank (as well as in-situ water samples taken at the beginning and end of each deployment).

If take up of these potentially unique datasets in this and AE1227 projects is low, we will be looking for further resources to fund an intercomparison exercise of sediment and wave numerical models.

Objectives

1. To collect a dataset of waves and currents over a spring/neap cycle on sandbanks for use in calibration and validation of numerical models for potential impacts of wind farms 
2. To assess gross changes in sediment transport during winter and summer seasons pre- and post- construction to compare any effects due to wind farm construction 
3. To undertake suspended sediment monitoring during windfarm construction using a combination of OBS profiles and water samples (with "in-kind" help from the developers – e.g. access to piling vessels during construction), to monitor potential effects of piling and ship movements 
4. To liaise with numerical modellers on the implementation of the validation and to help develop further models if results indicate poor performance. There is no specific modelling effort in this proposal. Developers and users of numerical models (HR, ABPMer, Posfords, Halcrow amongst others) will be invited to test their own models against these new datasets. These datasets will help "benchmark" the models and identify strengths and weaknesses inherent in each modelling approach. This will provide "in-kind" support from industry through their consultants. 
5. To produce a GIS showing the sedimentological and hydrographic distributions for use in interpretation and licensing procedures. Maps of individual bedforms (megaripples, sandwaves etc) will be created from the interpreted side-scan sonar records as in the Southern North Sea Sediment Transport Study (see www.sns2.org for full details). Five snap-shots of the bedforms should enable detection of any gross changes of sediment transport regime and make a comparison with natural seasonal and interannual variations. This information may also help in zone management of OWFs in relation to monitoring and assessing the impact of scour protection around each monopile. 
6. To interpret the significance of findings for the management of windfarm licensing.

Approaches

Five fieldwork campaigns are proposed:

1. Pre Construction – Winter 02/03
2. Pre Construction – Summer 03
3. Pre Construction – Winter 03/04
4. Post Construction – Summer 04
5. Post Construction – Winter 04/05

(each survey is planned to be in the same month as the previous equivalent campaign)

During each fieldwork campaign the following will be undertaken:

1. Two instrument mooring sites with directional wave gauge, Optical Backscatter Sensor (OBS) and current meter (except Summer 03). 
2. At offshore site, a directional wave gauge if sufficient resource available. 
3. One bathymetric (line) and side scan surveys over "near-field" (turbine grid –100 % coverage) and "far-field" (one tidal ellipse either side of the grid). 
4. Water samples for suspended sediment concentration to enable calibration of OBS. These to be taken at both sites, once an hour, over a tidal cycle. This will be combined with suspended sediment samples collected from a sediment trap mounted on the lander. These samples will be used in a turbidity tank to calibrate the OBS sensors over a wide range of concentrations. The lithology of sediment layers within the sediment traps will be analysed for tidal rhytmites and storms events.

Within each campaign rigorous quality control procedures will be applied to each data set to check for internal consistency in each deployment and between deployments.

Milestones

Secondary Milestones

Results

Scroby Windfarm MiniLander deployments (Ae0262)

1) Scroby deployment 1

• Scroby Bank (Excel, 566 KB)
• Caister Bank (Excel, 588 KB)
• Offshore data (Excel, 303 KB)
• Inshore data (Excel, 714 KB)

2) Scroby deployment 2

• Caister data (Excel, 3.16 MB)
• Inshore data (Excel, 520 KB)

3) Scroby deployment 3

• Package (Excel, 1.37 MB)

Final Report for the Department of Trade and Industry (PDF, 3.2 MB)