79.
It is notable from Tables B.2 and B.3 that the greatest scour depths are predicted to occur at Deltares model output point A9 (which is approximately coincident with Noble Denton model output point 3), where the water depth is 30.8m LAT, and that scour depths reduce in areas of deeper water. This is in keeping with the findings of the scour assessments presented in Appendix 7.3 which incorporated assessments directly at Noble Denton model output point 3 (in water depth of 30.8m LAT) and a ‘deeper water’ sensitivity test.
80.
The Noble Denton 1-year return period conditions at their model output point 3 are more severe than those obtained by Deltares at the same return period, for their model output point A9. The two modelling points are however, approximately coincident. This indicates that at the 1-year return period, the previous scour assessments at Noble Denton model output point 3 represent a worse case than would be obtained using the metocean data from Detares model output point A9.
81. At the 50-year return period, the results at Deltares model output point A9 are more severe than those applying to the Noble Denton model output point 3. This explains why the granular scour depths calculated at the 1-year return period, obtained using the Deltares data, are less severe than those obtained using the Noble Denton metocean predictions, but the situation is reversed under the 50-year scenario.
B.2.2 Soil strength scour case 82.
To investigate the effects of the more severe metocean conditions under 50-year conditions on predictions of scour volumes, predictions have been made here of the equilibrium scour depth, taking account of sea bed soil strength, using the Deltares metocean predictions at their model output point A9.
83.
The resulting scour depth, by applying the Annandale soil strength method described in Appendix 7.3, is of the order of 4.9 to 5.0m. This is slightly greater than the 4.62m obtained using the Noble Denton metocean data at model output point 3, but the difference is deemed insufficient to warrant a full re-appraisal of scour assessments, especially as Deltares model output point A9 represents a worst case in terms of scour depths compared with the other model output points.
84.
The predicted scour volume at this location is 5,194m3 (based upon a scour hole depth of 4.9m and using the 50 year metocean data from Deltares model output point A9), compared to the 4,580m3 that was reported previously and which was based upon the metocean data from Noble Denton model output point 3.
Preliminary Environmental Information May 2014
East Anglia THREE Offshore Windfarm
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