36.
Scour assessments have been performed using metocean data derived from earlier modelling studies (GL Noble Denton 2011) and both zone-wide and project-specific (Appendix 7.5) field surveys (water depth, soil type and soil strength), to enable first order estimates of scour hole formation to be made for the full range of different foundation types and sizes which could ultimately be considered within the East Anglia THREE site. The scour assessment methods are described in detail in Appendix 7.3 (with appropriate references made to the full scientific evidence base that has underpinned the methods used). A summary overview of the approach is provided below.
37.
The earlier metocean modelling studies (GL Noble Denton 2011 – described in Appendix 7.2) provided input conditions at seven locations across the East Anglia Zone, ranging in water depth from 26.7 to 39.6m (the locations are shown in Appendix 7.2 Figure 7.2.2). These data were provided in the form of significant wave heights, peak wave periods and depth-averaged current speeds under storm events with return periods of occurrence of 1 in 1 year and 1 in 50 years. At each of the seven locations, scour hole depth calculations were performed for different foundation types, using an appropriate foundation size range for the specific water depth at each location.
38. Modelling point 3 (shown in Appendix 7.2 Figure 7.2.2) is located within the East Anglia THREE site, in 30.8m of water. At this location, the scour hole depth predictions were translated into scour hole plan areas and scour volumes (using the methods presented in Appendix 7.3) for different foundation types and size ranges appropriate to water depths up to 35m. The scour hole development under a 1 in 1 year return period condition and a 1 in 50 year return period condition were assessed.
39.
These assessments identified that, for the range of foundation types and sizes considered at modelling point 3, the greatest potential for scour hole formation is associated with the largest conical gravity base structure within the appropriate size range for its water depth under the 1 in 50 year return period condition.
40.
The calculations were performed at modelling point 3 within the East Anglia THREE site (in 30.8m water depth) because of the availability of a reliable suite of metocean conditions under different return period events at this location. This provides a valid prediction of scour hole development around foundation types and size ranges typical of water depths of less than 35m. However, this only represents around 16% of the area of the East Anglia THREE site and many other parts of the site exhibit greater water depths, typically in the range 35 to 45m (across 80% of the site) but, in limited extreme cases, in the range 45 to 49m (across 4% of the site).
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm Chapter 7 Marine Geology, Oceanography and Physical Processes Page 10
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