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Shoreham raises flood defences

E1 jack up and piling

Shoreham-by-Sea, on the south coast between Brighton and Worthing, has become vulnerable to tidal flooding from the River Adur, as last seen in December 2013.

Then, a number of homes and businesses were damaged, and the airport resembled something like  a lake.

As existing defences reach the end of their useful lives, Shoreham’s population is growing. Currently about 20,550 call Shoreham home, with about 2,450 residential and commercial properties at risk of flooding. And, left unchecked, this number is likely to double in the next 100 years as climate change leads to sea level rises and heavier, more frequent heavy rain events.

In response, a £32M scheme to strengthen and improve flood defences on both sides of the river is being led by the Environment Agency, which is contributing £23.8M to the project. Additional funding is from partnership members: Adur District Council, Coast to Capital Local Enterprise Partnership and West Sussex County Council.

Screen shot 2017 09 13 at 12.44.16

Screen shot 2017 09 13 at 12.44.16

Flood defence location map

The Environment Agency has employed framework joint venture Team Van Oord whose member firm Mackley is working as lead contractor.

The project covers the 4km of river that splits the town before reaching the sea.

In general, the inland sections of work involve the construction of large new embankments to protect assets such as the airport and major roads.

As the river reaches the High Street there is less room for such solutions and instead a mixture of sheet piles, reinforced concrete walls and flood glass is used.

On average, the town’s new defences will be raised by 1.1m. It is hoped that this will be enough to safeguard the town against 1 in 300 year flood events and push back 50 years of expected sea level rise.

W4 before

W4 before

Ferry Bridge: Before

With such a complex range of solutions and work timetables, the project has been split into 10 sections. Each section is overseen by its own management team, which leads the work and meets regularly with the public.

Construction began in October 2016, and is expected to finish by the end of 2018.

Mackley project manager Terry Gretton says liaising with the public in such a small, tight knit town has been one of the most challenging, and rewarding, parts of the project.

“Sometimes the buildability is not that challenging – piles, concrete, brickwork. Really it’s about access: looking at the size of our kit, and the size of footpath and driveways… while still allowing cars in and out. There’s lots of discussion with residents upfront on design, about how it’s all going to work,” he says.

W4 after

W4 after

Ferry Bridge: After

And Gretton says residents’ memories of major flood events are fading fast following a few dry winters. “When we were out talking to the residents recently they said: ‘Oh it doesn’t flood in Shoreham’.”

One of the key catastrophes during 2013’s flood was the overtopping of an earth embankment by the river at Shoreham airport, about 3km upriver from the mouth. Water and material swept over the embankment, filling the site.

To fix this, a new embankment –  900m long with a 2m wide crest, 5m above ordnance datum – on a new alignment will be installed behind existing defences, allowing the existing structure to remain in place, providing protection during the works.

Following completion of the new flood defence, the existing embankment will be re-graded to allow a 1.4ha saltmarsh habitat to form.

Transporting materials to site was a major issue with trucks having to cross the airport. “We’ve got about 4,000 lorry loads of chalk coming in, so then you have to think about the airport and industrial estates – how’s that going to affect their day to day operations?” says Gretton. “Already we’re in discussions with airport management… down to how many minutes it takes for a lorry to cross the airport.”

Picture3

Picture3

Computer rendering of airport embankment

Across the river from the airport is the main road route into town – the A283, Old Shoreham Rd, which branches off the A27.

The current plan is for a 12 week closure in spring next year to raise the road route by as much as 1.8m. Diverting the traffic is contentious. “It’s not going to be a short diversion, we’re talking miles here,” says Gretton. “The concern on the High Street is that if it’s too difficult for motorists… they will continue on to Hove or Brighton.” Discussions with the police are ongoing.

As the river flows further towards the centre of town, housing density increases and the amount of access for construction crews decreases.

In one section of work, apartments are squeezed to the east and west by the town’s road and pedestrian bridges, and to the north and south by the High Street and the river.

Limited foreshore access

Only narrow pedestrian paths between apartment blocks provide access to the foreshore. So to gain access to the flood defences here, a jack up barge was needed to host a crane and piling rig. Arriving in November and leaving in February, crews were piling just metres away from residential properties and there was extensive vibration and noise monitoring. “Their plaster already had signs of cracking – it was very high risk for us,” says Gretton.

Another property in this section is the Bridge Inn, a local icon. As a tourism gem it has unrestricted views of the waterway with a beer garden running right into the river. Mackley had originally planned to build a 1.5m high wall here, which would have blocked off the view.

Alternatives were sought. “What we came up with in the end was a render around the exterior wall of the pub,” says Gretton. The beer garden will be allowed to flood, with the maximum flood level still below the floor of the building. Discussion with landowners and residents often result in these improved solutions, says Gretton.

Meanwhile, directly across the river are the houseboats of Beach Green – about 50 residencies permanently moored along a 600m stretch of riverside. Each is attached to an earth embankment via jetties. The embankment has a narrow public concrete footpath running along the top – another scenic spot.

Embankment evolution

An early design here to raise the embankment called for full excavation down to high-tide level, then new piling and construction of a footpath and flood wall. Needless to say, there was some local opposition. “You would undermine all of the jetties, there would be massive disruption, people would move off their boats, there’d be scaffolding everywhere,” says Gretton. “So we asked ‘how can we make this better?’”

The new design minimises the amount of earthworks and construction, with the old footpath and much of the embankment kept in place. A Giken ECO 700S Silent Piler which specialises in embankment slopes and confined sites will be required. The rig is hydraulically powered and pushes the piles into the ground – rather than using a hammer or vibration.

Another tricky spot on the eastern end of the railway bridge, about 2.7km up river, featured an insitu concrete L-shaped wall along the boundary of Network Rail land. Following vegetation clearance it became apparent it would be impractical to excavate the foundation for the wall into the railway embankment as this would involve extensive temporary works and Network Rail approvals, including track monitoring to ensure the stability of the track.

Secant pile solution

The revised design involves installing a secant pile wall along the same alignment. This has the additional advantage of locating construction away from an existing gas main. The secant piles will also act as a seepage cut off and negate the need for new foreshore protection. The revised design also means an existing timber walkway can remain in place during the works; and a historic shipwreck can remain undisturbed.

Gretton stresses that the project is being developed with an environmental focus with work including the relocation of local wildlife, the creation of new habitats and the protection of rare wild flowers. One flower, the extremely rare Childing Pink, grows in only two known patches in Shoreham and Pagham and its presence has forced construction teams to work around it.

 

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