In 2008 the first six wind mills of the C-Power farm were installed on the Thornton Bank using gravity based foundations (GBF). The use of GBFs implies important dredging works to prepare the sea bed, whereby sand piles were stored in the concession area. The construction of 110 turbines on monopiles at the Bligh Bank for the Belwind farm started in 2009. Dynamic erosion protection was chosen around the monopiles, allowing the development of an erosion pit around the monopiles; later this pit is refilled with material ensuring protection against erosion. The effects of the installation of the wind turbines, especially of the GBFs and of the dynamic erosion protection, on the turbidity and the morphodynamics of the sand banks is not well known; therefore assessment of possible impacts is necessary, based on a sound monitoring programme. The monitoring includes: (1) measurements of currents, waves and turbidity, near the wind mill parks and at a reference site, before the works, during the works and after the works; (2) the control of the possible erosion and generation of erosion pits around the foundations of the turbines; (3) monitoring of the coverage of the cables from the farms to the shore; and (4) monitoring of the movement and evolution of the sand piles or pits, which were generated, during the construction of the GBFs. Measurements of the turbidity, currents and waves were executed on the Thornton Bank before the works, during the works and after the works by International Marine and Dredging Consultants (IMDC) and at a reference site, located at the Goote Bank. Although biofouling of the OBS sensors disturbed some of the measurements, the analysis of the results showed that no significant increase in turbidity could be demonstrated. Measurements on the Bligh Bank were executed before the works by MUMM. The measurements during and after the works are foreseen in 2010 and 2011. The sea bed around the GBFs was intensively monitored. In the final survey, the scour protection is clearly visible. No indication of secondary scour is apparent. The monitoring of the dynamic erosion protection at the Bligh Bank was executed around six monopiles. In the north of the farm, an erosion pit of 6.5 m was developed. The depth of burial of the cable of the C-Power farm to the shore was monitored during the jetting and the ploughing of the cable. The cable lies most of the time 2 m below the sea bed, although at some sections with clay layers, only 1 m was obtained. Finally, the monitoring showed that, during the installation of the GBFs, an important amount of sand was dredged at the concession area for the backfill of the foundation pits and the fair channel, and that some sand pits were created. It appeared that more material was dredged and used than was expected. During backfill, most of the sediment was lost during disposal. Monitoring of these sand pits, during several months, showed that the sand pits are relatively stable and that no natural filling of the sand pits occurs.
Monitoring of Hydrodynamic and Morphological Changes at the C-Power and Belwind Offshore Windfarm Sites - A Synthesis
Title: Monitoring of Hydrodynamic and Morphological Changes at the C-Power and Belwind Offshore Windfarm Sites - A Synthesis
January 01, 2010
Published City: Belgium
Publisher: Flanders Marine Institute
Van den Eynde, D.; Brabant, R.; Fettweis, M.; Francken, F.; Van Lancker, V.; Sas, M.; Melotte, J. (2010). Monitoring of Hydrodynamic and Morphological Changes at the C-Power and Belwind Offshore Windfarm Sites - A Synthesis. (pp. 19-36). Belgium: Flanders Marine Institute.