Emu Ltd was commissioned by Centrica Renewable Energy Limited to undertake a post construction hydrographic monitoring survey of the Lynn and Inner Dowsing wind farm sites and export power cables to shore.
The purpose of the survey was to determine the bathymetry, seabed types and distribution of benthic habitats in the wind farm and associated surrounding areas. The relevant data were collected through a combination of swath bathymetry, sidescan sonar, multibeam acoustic backscatter data, drop down video and grab sample surveys.
The hydrographic survey was carried out using the vessel FPV Morven on a 24 hour operational basis between the 30th June and the 26th July 2009. Survey lines were run using a line plan which amounted to approximately 1542 line kilometres. The drop down video and grab survey was conducted between the 9th and the 12th of September 2009 using the survey vessel Arie Dirk.
The bathymetric data collected for this report were of very high quality. Bathymetric data were reduced to Chart Datum (CD) using post processed kinematic (PPK) GPS tidal heights recorded onboard the survey vessel. These data were cross-checked with observed tides retrieved from the British Oceanographic Data Centre (BODC). Depths ranged from approximately 4.0m to 16.5m below CD at Skegness.
The sidescan data collected were also of a high quality and full insonification of the seabed was obtained even though data collection was hindered by the acute shallowness of the site in some areas. The data were processed using manual laybacks and cross checked against the bathymetric survey data.
Multibeam acoustic backscatter data were collected with the Reson 8101 in addition to the collection of bathymetric data. Once collected, the data were processed using QTC MULTIVIEW software which uses multivariate statistical techniques to analyse the backscatter intensity. The data were then cross checked against the sidescan imagery and the bathymetric survey data.
Video footage was acquired at 38 predetermined sites to ground truth sidescan sonar and bathymetric data collected prior to the ecological survey. Grab samples were obtained from 10 predetermined grab sites, to ground truth acoustic sediment type descriptions and from a further 8 sites, to assess Sabellaria spinulosa and Ampelisca diadema aggregations.
Sabellaria spinulosa was identified in the survey area in a range of forms including tubes, crusts, clumps and potential low-lying reef. Dense seed mussel beds, Ampelisca diadema reef and old mussel patches were also found within the survey area.
The ground truth data provided by the video imagery and grabs, allowed the sidescan interpretation to be modified to show the likely extent of the Sabellaria reefs, seed mussel beds and old mussel patches.
Results showed that the potential low lying Sabellaria reef was located in four main areas within the Lynn and Inner Dowsing wind farms. Dense seed mussel beds were found at three sites one of which was in conjunction with Sabellaria reef.
Data obtained from this survey were consistent with previous data collected at this site and within survey specification. As a result, all contractual requirements were deemed to have been met.