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In view of the dynamic conditions characterising the shallow nearshore environment, it is considered that the greatest risk of cable exposure will be in those seabed areas encountered landward of the depth of closure, which is defined as the seaward limit of significant profile change affected by waves. Along this stretch of coast, the depth of closure is conservatively assessed to be approximately 8m LAT1. Seaward of the depth of closure, there is a general increase in surficial (Holocene) sediment thickness allowing greater depth of cable burial, reducing the risk of cable exposure (Figure 1).


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In regards to a monitoring programme, good practice (e.g. BERR, 2008) involves precise definition of the following:


• The parameters to be examined; • The means by which the parameter will be measured; • The frequency with which the parameter will be measured; • Establishment of review periods providing the ability to stop or modify the monitoring exercise if the measurements suggest no change; and


• The identification of appropriate thresholds of change which trigger remedial action.


7 3 8 These issues are considered under separate headings below. Parameters to be Examined


The key parameter to be monitored is the future lowering of the London Clay material within and adjacent to the cable trenches relative to a baseline level, from the short HDD punch-out location to 8m LAT (Figure 1). (N.B. This baseline level is defined as the vertical elevation of the seabed along the cable trenches shortly after trenching activities have been completed).


4 9 Measurement of Parameters


Detailed information on temporal changes in seabed elevation can best be determined from repeat multibeam echo sounder (MBES) surveys with net changes in elevation calculated by comparison of successive surveys. Given that vertical changes to the London Clay seabed are expected to be no greater than a few cm’s per year, high resolution vertical and horizontal accuracies (sub-decimetre level) are required throughout the survey operations to ensure the consistency of the data between different survey periods. This level of accuracy could be achieved through establishing a real time kinematic (RTK) differential Global Positioning System (DGPS) base station over a known survey control point on the shore.


Outline Bawdsey bed level monitoring and remediation strategy (V1) Page 4


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