10


The collection and analysis of this bathymetric survey data should follow fully consistent methodologies involving (for instance) the same surveying equipment, the same measurement protocols and the same data processing approach.


11 MBES survey data should be accompanied with side-scan survey data which will provide acoustic interpretation of seabed texture. This supplementary information is considered necessary to help distinguish between changes in the elevation of London Clay and those associated with the movement of surficial sediments (which may be present as a very thin (< ~10cm) veneer in this region). These seabed surveys should cover the full width of the trenches including a 5m corridor either side.


5 12 Frequency of Measurement


The frequency with which measurement is required is primarily influenced by the anticipated rate of change. Maximum historic rates of seabed lowering in this area are found to be in the range 1–3 cm/yr (Burningham and French (2009), Environment Agency (2013)). However, if trenching is used to install the cable within the nearshore area, the displaced material which will be used to backfill the trench will not be as compacted as the surrounding undisturbed London Clay and will therefore potentially be more susceptible to erosion. It is therefore reasonable to expect that the rate of future seabed lowering along the line of the trenches may well be higher than for surrounding areas of seabed.


13 On the basis of the above it is suggested that initially, surveys are undertaken and reported on an annual basis. Surveys should be carried out in spring/summer months with rates of change calculated by comparison with preceding surveys. (N.B. The measured bathymetric data should be considered alongside available hydrodynamic data to better understand potential drivers of change).


6 14 Establishment of Review Periods


After a period of two years following cable installation, the requirement for continued annual monitoring should be re-assessed in light of the observed rates of change and the proximity to identified sediment depth thresholds (see below). At the two year period there will be a total of three monitoring surveys to consider;


• Post-installation survey • Year 1 survey • Year 2 survey


Outline Bawdsey bed level monitoring and remediation strategy (V1)


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