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Silting up and downstream potentially a massive problem

Silting  up and downstream potentially a massive problem
LIKE Merry, Haslett believes some of the major threats posed by any Severn Barrage would be "siltation" - silting-up both up and downstream of any estuary barrage because of an inevitable reduction in tidal flow velocity.

LIKE Merry, Haslett believes some of the major threats posed by any Severn Barrage would be “siltation” – silting-up both up and downstream of any estuary barrage because of an inevitable reduction in tidal flow velocity.

Haslett identifies at least four further implications.

He says: “Raising of mean sea levels behind the barrage would reduce the area of tidal wetlands, which would impact on wildlife feeding. It would raise water tables and may lead to waterlogging of soils.”

He further suggests that a barrage could increase river flooding – but not sea flooding – and put pressure on communities not necessarily by the Severn. Also, he says present outfalls, such as sewage, would have to be raised as they would be submerged post-barrage.

On mitigation measures, Haslett says: “Offsetting tidal wetland loss will be difficult on a like-for-like basis as mudflats will be lost, but it is very difficult to re-create these elsewhere. Relocating sewage outfalls so they drain higher in the inter-tidal zone would be necessary.”

Even the BERR concedes that 65% of existing inter-tidal areas on the Severn estuary would be lost.

Roger Falconer, Halcrow professor of water management and director of the hydro-environmental research centre at Cardiff School of Engineering, Cardiff University, has a generally favourable view of the proposal. However, he dismisses caisson-type structures suggested by Loveless as “non-starters”.

“They are far too expensive and will silt up too fast.”

Falconer would welcome a barrage.

“Our work shows that the environmental aspects will not be as bad as portrayed. The main downside is that the barrage will restrict the scope for tidal-stream turbines to be located in the estuary as the tidal currents are likely to be reduced.”

Asked about the potential impacts of the barrage, Falconer says it would improve the water quality and biodiversity of the estuary.

“The estuary currently has a very high suspended sediment level and minimal light penetration to the bed,” he says.

Reduced currents would lead to a significant reduction in suspended sediment levels, which would increase light penetration and hence lead to enhanced biodiversity along parts of the estuary. The barrage would also generally reduce flood risk.

“However, there will be significant losses of the inter-tidal mudflats, which will reduce the feeding grounds for a bird population which is already declining prior to any barrage being considered.”

Pressed about barrage alternatives, Falconer suggests that tidal turbines would be better, but he cautions: “It should not be put in this context. It is not a matter of either the barrage or tidal-stream turbines. In my view, we badly need both.

“These projects are complementary and both provide much needed renewable energy. There is no competition apart from a small part of the Bristol Channel.”

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