Orsted has been accused of contributing to an “ecological disaster” with the onshore works for its largest offshore wind project underway globally, the £8bn ($9.8bn) Hornsea 3 off eastern England.

Furious local campaigners claim silt displaced by heavy rain in October from cabling work underway for the 2.9GW North Sea wind farm has “destroyed” the nearby River Glaven, clogging the 17km waterway and damaging natural habitats including those of rare crayfish species.

Carl Sayer, a professor of geography at University College London, was among those accusing Orsted and its contractors of failing to prepare for the extreme rainfall and resulting runoff that they say has devasted the local waterway, causing damage that will take decades to repair and "cancelling out" 20 years of conservation efforts.

Labelling the silt in the Glaven an “ecological disaster”, Sayer said on social media: “I used to fish here as a little boy. I am broken by this” as he asked local environmental regulators to intervene.

Danish giant Orsted is overseeing the installation of 240km of cabling to connect Hornsea 3 from its cable landfall at Weybourne on the Norfolk coast to the UK National Grid.

Henry Crawley, chairman of the River Glaven Conservation Group, told Recharge the developer should have foreseen the potential for displaced soil to cause environmental damage.

“Silt from runoff is deposited on the bottom and margins in the slower reaches, as is apparent all down the river [Glaven],” said Crawley.

While runoff from farmland and roads has also played a part, Crawley said the “contribution to silt runoff by the cable route crossing has been very significant” during extreme rainfall seen recently.

“I think it’s fair to say that Orsted [and its contractors] were not prepared for this scenario” despite expressions of environmental concern during the planning stage.

Orsted said in a statement sent to Recharge: “Recent extreme weather, including Storm Babet, has caused widespread disruption across the region. We are aware that there are pre-existing flooding issues locally with a number of contributing factors. As a responsible developer, we take potential impacts of any works we carry out very seriously.

“We are working closely with our contractors, the Environment Agency and other local stakeholders to thoroughly investigate the events and are taking active steps to ensure that mitigation measures are in place on our site can handle extreme weather.”

The impact of onshore works for giant North Sea wind farms is a live issue in East Anglia, where campaigners are also mounting a vigorous legal campaign against part of Iberdrola's East Anglia Hub.

Hornsea 3 is Orsted’s largest single offshore wind project underway globally and among the word’s biggest.

The offshore wind developer earlier this year raised a question over the viability of the project, which won a power deal in the UK’s 2022 renewable energy auction, citing an “extraordinary combination” of supply chain pressures and rising interest rates.

Orsted, which is due to report latest financial results tomorrow (Wednesday) has said it aims to make a final investment decision on the project this year.

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