The Van Oord team working on the £30M Broomhill Sands tidal defence project has had to work around an active kite surfing centre, a live firing range and one of the South East’s busiest beaches.
When planning a tidal defence project, you would normally anticipate that the incoming tide would represent the greatest constraint. That holds true for the Broomhill Sands coastal defence scheme now underway in southern Kent. But in addition to the insistent battering of the waves, the £30M project has also had to devise a way of working around a live Ministry of Defence (MoD) firing range, a kite surfing centre, and one of the South East’s busiest beaches.
Located between Camber Sands and Jury’s Gap in Sussex, the project forms part of the Environment Agency’s Folkestone to Cliff End sea defence strategy. It is designed to protect 1,300 homes, the Dungeness nuclear power station, significant areas of designated habitat, and the firing range from tidal surges and predicted sea level rises.
“The project will increase the standard of defence from one in 20 years to one in 200 years with a 100 year design life,” says Jasper Blaauw, site manager with the Team Van Oord joint venture delivering the project. The team brings together the expertise of Van Oord, Mackley, Royal Haskoning DHV and Kier.
The project will protect 2.5km of coastline, firstly by building a 1.84km rock revetment along the easternmost stretch of the designated area and then by recharging the beach along the remainder of the frontage to the west (see box).
Originally the plan was to build a 1.7km long revetment, but Environment Agency project manager John Hornig says the project team took the opportunity to extend the scheme by 140m, while all of the construction teams and plant were in place. This will have the advantage of protecting a further 32 properties.
“It was going to happen at some point, but delivering it the way we have, we’ve realised significant efficiencies,” he says.
“The idea was that the revetment would start at the edge of the Lydd firing range and head towards Camber, but what we managed to do was get the approval to build an extra 140m towards the east where the scheme would fall within the boundaries of this firing range,” he says.
This was far from straightforward – not least because the boundary for the firing range also happens to be the boundary for the Dungeness Special Area of Conservation (SAC).
Broomhill Sands Coastal Defence
“Because of its environmental designations, it’s much more difficult to construct in that area, and the complexities added by the firing range don’t help either,” he says.
The principle reason for the Dungeness SAC’s protected status is the fact that it has a natural coastline. This consists of a series of ridges and shingle banks that have built up over centuries and on which the area’s unique bio-diversity depends.
“By putting a hard rock defence within part of that designated area it was obvious we were preventing some of those natural coastal processes from happening,” says Hornig. “So we needed to find a way of mitigating that.”
The Environment Agency team, with technical input, design and support from CH2M Hill, identified an MoD lookout structure – known as the Galloways lookout – 3km away from the site. It had a similar footprint and a similar impact on the coastline as the proposed sea defence. The Agency’s National Environmental Advisory Service then went about persuading Natural England and the MoD that by removing this it could counteract the effects if the proposed 140m of revetment. An added bonus was that the rock armour for the lookout could be incorporated into the new sea defence.
The considerable task of persuading a panel of academic experts from Natural England of the value of this mitigation work, and the job of gaining the necessary planning consent and permissions from the MoD was completed as a side project, as the rest of the works proceeded as planned. The fact that an additional 25,000t of rocks would need to be barged in from Norway before the winter delivery season began added additional pressure to the planning process.
Delivery of armour stone
Although Hornig commends the MoD for co-operating and helping to compress an estimated two years of pre-construction phases into a six month window, he emphasises that the construction phase for this section was conducted very much on the defence department’s terms.
“They gave us three days to remove the lookout structure and construct our revetment,” he says.
“There are safe firing arcs from the guns and there’s a short buffer zone on the edge and we were working in that and obviously the guys had to be extremely careful to stay within that. They’re not firing blanks.”
However, Hornig thinks the end result justifies the aggravation. “By doing this work early we’ll save a fortune on maintenance,” he says.
The kite surfing centre that sits roughly in the middle of the sea defence construction site represented a another health and safety challenge. The Environment Agency was keen to avoid damaging the local economy by shutting the centre during construction. But it was also keen to prevent surfers from being swept into one of the project’s rock stockpiles along the shore.
Blaauw, a kite surfer himself, explains how Team Van Oord engaged with the surfing community to keep the centre open and surfers safe during construction.
Kitesurfer Broomhill Sands
“At some point we thought: ‘why are we inducting the workers and not inducting the people that come onto our site as a visitor?’” he says.
“We’ve employed a kite surfing instructor to be our gate man on the public section of beach, so if you want to do kite surfing, you need a vest and to get the vest you need an induction and anyone surfing without a vest can be told to get off the water.”
The surfers are also segregated according to their levels of experience – something that Team Van Oord borrowed from the World Pro-surfing championships.
“The demarcation of beginners, instructors, students and advanced surfers by their different coloured vests allows more control by, for example, instructors,” says Hornig. “They can ensure that their groups are the ‘right side of the wind’ which is important in terms of ensuring that surfers don’t stray outside the safe areas.”
He adds that the contracting team’s response to this challenge was one of the principle reasons the Environment Agency awarded them the scheme.
“The Environment Agency is very hot on health and safety,” he says. “The whole approach of engaging the community and coming up with something that works but is also pretty cost effective really impressed us.”
Other efficiencies and cost savings were generated on site, thanks to the contract management involvement of consultants Arcadis, the Agency’s in-house National Capital Project Management Service and its procurement team in the shape of CH2M Hill and Team Van Oord.
Arcadis engineering construction contract project manager Zanna Jeary explains how all Environment Agency projects are “expected” to deliver efficiencies of 10% on original project budgets. But she estimates that roughly 30% has actually been saved against the project cost.
One of the principle efficiencies was achieved when the project successfully sold the 30,000t of clay that has been excavated to make way for the rock revetments. The excavated material was sold to a property development project in nearby Hythe. Originally, the salt-contaminated clay would have been expensive to dispose of safely, but because the Hythe project was also in a sea environment, the high salt content was not a problem.
“It saved something staggering like £3.46M,” says Jeary. “It was going to cost £4.2M; it actually cost £740,000.”
In addition, the same stockpiles that present such a hazard to the kite surfers were put to more productive use as a result of a change in construction methodology for the two sea defences maintenance ramps. These ramps are below beach level, to the same depth as the bottom of the revetment rock, because the EA will need them to gain access to the beach in the event of major storm event which would be expected to create considerable scour.
“Rather than install a narrow cofferdam, introducing health and safety risks to the workforce and also risking shingle filling the cofferdam, we decided to install a large “horseshoe” of rock around the ramps to create a bund against shingle, and created a large, shallow excavation to the required depth,” says Hornig.
Additional time was saved by the decision to use pre-cast units in the construction of the ramps.
“These obviously have less flexibility regarding tolerances, but were significantly faster to install,” says Hornig.
When New Civil Engineer visited the site in November, Team Van Oord was confident that it would meet its original December completion deadline, in spite of the additional work it has taken on with the extended 140m of revetment. However, recent storms on the south coast have subsequently slowed progress. At the time of writing, Hornig said the planned completion date would be 29 January.
- Click here to watch a video of the Broomhill Sands Coastal Defence scheme.
Rocks and sand
In total, 270,000t of rock and 70,000m3 of shingle will be used in the Broomhill Sands scheme.
The rock comes to Broomhill from Norway on a barge measuring roughly the size of a football pitch. This barge is moored around 4.8km from the coast and the 10t rocks are then transferred into a barge with a shallower draft. From these barges it is relayed to the beach by bulldozers at high tide and stockpiled before specialist excavators fitted with rock grabs position the rocks in the revetment.
The 2.58m deep revetment is built from the toe back up the beach stretching between 28m to 32m to the promenade.
Parts of the existing sea defence are retained and the new revetment is built on top of them, saving a substantial amount of material. A heavy duty geotextile membrane is laid over the site where the rocks will be placed before specialist plant operators move in and build 10m long sections of the revetment every tide.
Blaauw says the revetment is “not just a pile of rocks” but an engineered structure. The skill of the plant operators is paramount to the success of the project. “It’s like playing Tetris,” he says.
The revetment slopes down beneath the beach so that, like an iceberg, roughly two thirds will sit below ground. “Below that there’s a little dog leg that prevents scour,” says Hornig.
Towards the top of the revetment, the rocks sit against a mass concrete blinding, cast on top of the old defence to prevent storm waves from driving them onto the promenade. This blinding also acts as the foundation for the new precast wave wall and promenade. The back of the wave wall unit interlocks with the back of the blinding block on which it rests to help it resist storm waves. The precast wave wall units have reinforcement bars that are tied into the promenade slab reinforcement cage to make one solid structure.
Beach recharge and groynes
The 1.7km of rock revetment ends before it reaches the busy Camber Sands beach. Here the decision was taken to carry out 900m of beach recharge with a 30 year design life rather than build an intrusive sea defence.
“This is a beach that gets 20,000 visitors a year – a staggering amount for an amenity beach,” says Environment Agency project manager John Hornig. “If they had a rock revetment, it would have a massive impact on trade and on the community of Camber.
“The Environment Agency has knowingly taken on more maintenance responsibility in this area to meet the public’s needs and maintain the status quo.”
The 70,000m3 of shingle that is excavated to make way for the rock revetments is screened on site and then re-used to recharge the beach. “We have historically been very careful at the Environment Agency to ensure that the shingle stays in place, hence the large amount of maintenance work that goes into moving it all back again,” says Hornig.
To help keep the shingle in place in the 900m Camber Sands section, the project includes eight new timber groynes stretching 54m into the sea. For these, 10m long wooden piles measuring 305mm by 305mm are manually topped and tailed with a chainsaw before being fitted with a circular steel hoop and a pointed steel shoe so they can be driven without splitting.
Most of these are then clad with horizontally with timber to counteract long shore drift. However, in a first for this type of project, the Environment Agency is also trialling a new material on three of the groynes.
“We’re trying a material called Reluma, which is a wood substitute,” he says. “Usually timber in such a harsh marine environment tends to need to be hardwood rather than softwood which is difficult to source and adds cost.”