Severn Seabed mapping of humped material - NRW in denial

The Cardiff Grounds dump-site (LU110) normally disperses dumped dredgings in the strong currents, but the Titan survey identified a lot of mounds on the seabed around a metre high and 30 metres across. Their radar survey was carried out 6 months post-dumping from the huge Pagadder and Shloeber barges of Hinkley material. As the site is supposed to be a dispersal site where material is swept away in the strong currents, NRW asserts the site is “sustainable” as the material would “disperse over time”. They also claim the sea bed has not been raised on the average, though Titan’s mapping shows it’s generally higher.
A third claim by NRW is that it’s impossible to attribute the left-over sediment to Hinkley.

Penarth Times report 10 September 2020

Here is one of Titan's detailed images - seabed height differences between the April 2019 and 2018 surveys

The images show a series of four discrete disposal events along a transect (“Transect 01”), with each disposal identified as two parallel lines of deposited material (i.e., accretion), consistent with material being released through the hopper doors of a dredging vessel.

To the left is Titan’s result that implies accretion (orange) through much of the area as well as the mounds showing recent dumping. to the right is NRW’s favoured adjustment that implies the area has generally eroded (white to blue), except for the humps. This 'adjustment' comes from moving up the zero by 24cm. This picture shows that though an adjustment be a few cm is possible, 24cm is implausible. The mounds would be expected to spread and raise the adjacent levels.

Elevations were also depicted by Titan's Figure 7:


It appears that these parallel mounds did not spread much laterally in the 6 months post dumping, except for the right-most trail where the left side slumped into the depression.

Grab-samples of the seabed were also collected by Titan. One sample taken happened to be quite close to the lowest of the 4 trails (upper picture); it has an extreme composition of 95% mud and is characterised as “very poorly sorted”. The sample point (S4) happens to be along the parallel trail and within about 30m of the metre-high mounded material. It’s highly likely to come from Hinkley, not from a shipping channel dredger.

The Titan report is in no doubt that they see deposition in the April 2019 survey compared with the pre-dumping survey in 2018. They looked carefully into the zero-ing uncertainty, settling on 8.5cm. The NRW's 24cm on top of this implies erosion of the seabed in the area of the mounds. They are in denial not just that the mounds are Hinkley material, but also that much of the dumped material could have remained in the dump site.

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.