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London’s bridges could be damaged by floating walkway

A 1km floating platform which is planned to run along the River Thames could alter its flow and damage the capital’s bridges, sources close to the scheme told NCE this week.

Speed and divert river flow

Concerns have been raised that the proposed £60M London River Park project could speed up and divert the river flow, which in turn poses the risk of scour to bridge abutments along the route.

Promoter London River Parks − comprising architect Gensler, project manager Mace and private investor Venus − submitted its detailed planning application on 18 July for the 1.6km long walkway along the north bank of the river. The walkway runs for almost 1km on floating pontoons.

8._St_Paul_s_at_twilight

The walkway’s route from the Millennium Bridge east to Custom House is in a section described by river regulator Port of London Authority (PLA) as the “busiest part of the river, characterised by a number of bridges in close proximity and strong tidal flows”.

Key infrastructure along the route includes London Bridge, Cannon Street railway bridge and the Millennium Bridge.

Plans include two 10m wide floating pontoon sections − a 420m long western section between Blackfriars Bridge and Southwark Bridge and a 540m eastern section between Cannon Street Railway Bridge and Custom House. These are divided by a central inland river bank section about 600m long.

The floating sections are to be secured by a series of 800mm to 2.6m wide precast concrete piles up to 16m deep.

Displacement “could lead to scour”

It is these two pontoon sections that pose the greatest risk to the river infrastructure, said the source. The water displacement caused by the pontoons could divert the river flow so that it causes scour on the abutments.

It should be properly mitigated before the scheme gets the full go-ahead, said a source close to the project: “Scour can build up quickly.”

The threat of scour could also be exacerbated as the reduced width of the river will increase the speed of the current, he added.

“We’re working with PLA to ensure that there is no adverse impact”

Mace project manager Jon Pettifer

HR Wallingford principal scientist John Baugh said that neither the river flow nor speed will greatly alter as a result of the pontoons.

“Our tests show that the river speed changes by just 0.2 knots in extreme conditions, against an existing flow of between 2.5 and 4 knots,” he said.

Baugh added that the testing, which includes a 1 in 100 scale mock-up, backs-up the theory.

However, sources close to the scheme believe the effects of the river flow should be assessed in a permanent rather than temporary condition.

Fears for permanent fixture

While London River Parks has applied for a five year temporary planning permission, the source said there are concerns that if successful it could become a permanent fixture on the river in a similar way as the London Eye on the city’s South Bank. It was also built with temporary planning permission.

Mace project manager Jon Pettifer said that the scheme will be up for only five years but acknowledged that there have been concerns with the reduced river navigation.

He said that there would not be a great impact. He added that the project team should work closely with PLA to ensure that river traffic is not disturbed. However, river traffic could create a further problem for bridges, said the source.

By reducing the navigable width of an already very narrow part of the river there could be a greater likelihood of vessels striking the bridges.

“We’re working with PLA to ensure that there is no adverse impact,” said Pettifer. He said that PLA conditions state that the pontoons must be between 8m to 10m away from the main navigation channel, and the current configuration meets the requirements except in two small areas.

The City of London Corporation will decide whether to approve the scheme on 14 November.

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