Severn Seabed Survey shows heavy Hinkley materials

Seabed surveys before and after dumping were required under a Licence condition. The report  was issued quietly in July 2020 following an April 2019 survey and much revision (8th Edition). Released after a Freedom-of-Information request by Barry&Vale FoE    https://publicregister.naturalresources.wales/Search/Download?RecordId=34343 

Titan Environmental Surveys Ltd conducted a bathymetric survey and collected some grab samples between 3rd and 12th April 2019.  The 12 samples were analysed for sizes, from clay to gravel. compared with pre-dumping levels the samples showed both coarse gravel and clay remained from the dumped material. 

Titan Explorer surveyed the seabed of Cardiff Grounds dump-site in April 2019

The bathymetry – depth measurements from the vessel - mapped the heights of the seabed compared with pre-dumping heights. Titan found problems in that the total increase was greater than the volume dumped.  They found an antenna offset and corrected for it. But this was not enough, so CEFAS invented a further fudge (3 times as big) which reduced the total amount remaining on the seabed to 103 cu m (compared with 58 000  cu m dumped).  Incredible!

The bathymetry differences pre/post-dumping are shown below.  The red spots show mounds about a metre high and 30 metres across, each might contain 50-100 cu m.  The Titan survey links the pattern to trails of the dumping boats.  The distribution of the spots shows the dumpers avoided going close to the eastern limit of the triangle, but also avoided the top of the triangle and the western apex.  The licence prescribed that EDF must dump evenly over the dumping ground – to avoid building up banks – which they failed to do.

Difference map of seabed pre/post-dumping
North is to right, West at top.  The lower left (blue) corner shows erosion.
The spots (red) around 1-metre high are identified in more detailed maps as dump trails (east-west).

Thus even after 6 months of the stormy winter weather 2018-19, the claim by NRW that dumped materials would simply disperse in the strong currents is shown to be untrue by both the sampling and the bathymetry.  NRW’s error stems from their equating ‘capital’ dredge with port maintenance dredge, because the Cardiff Grounds site was classed as a dispersive dump-site to take port and shipping channel dredging. 

Much of Hinkley’s capital dredge removes consolidated or hard stuff (clay, pebbles, cobbles etc) whereas as maintenance dredge removes short term, mobile soft sediments.  International rules (OSPAR) say solid material should be separated and not dumped at sea, but NRW insists on classifying all as port dredgings and reports them to OSPAR as this.