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.