Annual Report for 2010/11 and Forward Programme - Sellafield Ltd

SSEM/2011/4730 June 2011A full copy of the Cefas report is provided to the Environment Agency as part of thesubmission package that accompanies this report.3.3.3 Progress on measurements of bed shear stress in the vicinity of the Sellafielddischarge pipelineA number of packages of work were included under this task, the major piece of work beingthe deployment of an Aquadopp current profiler. Other supporting packages included anevaluation of the feasibility to deploy Woodhead drifters and a review of historical currentmeter records.As reported in the 2010 report, Cefas were able to confirm that the required numbers ofWoodhead drifters were available and that a supplier had been identified to produce the tagsthat would provide details of what information would need to be returned by the finder of thedrifter. Unfortunately the collapse of the contract with WSC led to the decision not to pursuethe Woodhead drifter release at this stage. Whilst the release of the drifters is relativelystraightforward (released as a package by hand from a vessel at a recorded location) thefollow-up work of waiting for the drifters to come ashore and logging and interpreting theresulting data would have taken several months. The deployment of Woodhead drifters willremain an option for future consideration.Current meter data from the Sellafield pipeline (water depth approximately 15 mbelow chart datum), were recorded by Cefas (then MAFF) at various time periods betweenMay 1981 and March 1988. These data have been reviewed to help improve the descriptionof the hydrodynamics immediately off the Cumbrian coast at Sellafield, with the aim ofreducing uncertainty in numerical model predictions. Of particular interest are the indicationsof the direction of near bed currents during episodic events (e.g., storm waves, storm surge,and wind-driven currents) when the Sellafield-derived particles are most likely to be movingalong the sea floor.The current meter data were extracted from the Cefas data archive system and subjected toa data quality analysis. The currents were measured using Plessey M021 propeller-typecurrent meters (Figure 3.33). The direction is determined from an internal compass thatregisters the change in instrument heading as the current meter vane reorients to the flow.Current directions were corrected for magnetic declination. The data were originally stored inbinary form on magnetic tapes and have now been converted into computer data files.© Nuclear Decommissioning Authority 2011. 67