Long Rock Sea Defences

Mounts Bay

Major works to reinforce a section of concrete sea wall, and add rock armour sea defences to a large section of sand dunes on the south west coast of Cornwall – protecting the coastal path and the main railway line from Penzance to London.

Standing Up to the Storms

Repairing the damage of twenty foot waves

The beach at Long Rock is backed by sea defences that carry the South West Coast path and cycle route between Penzance and Marazion. Just beyond it is the mainline railway that connects Penzance to London.

After major storms in 2016, the foundations of the concrete sea wall at Long Rock were on the verge of collapse. The Environment Agency and Cornwall Council consulted with Cormac to design a sheet pile and rock armour defence solution that would protect against the complete breakdown of the sea wall.

During the storms a three hundred and sixty metre stretch of sand dunes had been eroded and threatened the stability of the nearby coast path and railway line. Cormac, along with their partners designed a rock armour solution to protect this section of the coastline. Thirteen thousand tonnes of three-to-five tonne boulders, had to be placed on top of smaller stones, completing rock armour sea defences that now run all the way along the shoreline into Penzance.

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Boulder Challenge

Rock armour construction

The giant boulders were placed on top of a base layer of smaller rocks. Each boulder was then carefully seated on the rock below, maintaining three points of contact, in order to provide maximum strength against the battering of waves. It’s a skilled task – using a thirty five tonne excavator with rock grab and great precision.   

The team had originally considered delivering the boulders by barge, but due to the area being of Special Scientific Interest (SSI), this was not possible and the armour stone was delivered by road. Tidal shifts and daylight hours also impacted the project, as well as working above the mean-high-water mark, which restricted things further.  Being so close to the railway line, access was through just one point– over the level crossing at Long Rock car park. To reduce pressure on the roads, we set up a one-way system for lorries bringing rock from Cormac’s Castle Quarry.

Protecting Nature

Flora, fauna and the landscape

In order to protect the wildlife in the sea and the natural coastline, all works were carried out above the mean high water line – carefully marked out by a line of scaffolding poles in the ground.

The rock armour doesn’t just serve to protect the coastal path and railway, but prevents erosion of the natural environment of the dunes that sit at the back of the beach. In some sections, dunes had to be removed and reinstated further along the coast path to minimise loss of biodiversity and habitats.

During the works, layers of contaminated soil were discovered. Our team quickly responded to ensure that these layers remained undisturbed. ‘We had to put in a membrane to prevent the contamination from migrating,’ says Zoe. ‘It was an encapsulation method, rather than having to remove the soil. It did mean that Cormac had to redesign this section of the rock armour protection, re-structure the programme, and the sequencing of the works to do that.’

Wetland Works

Managing marshes and preserving wildlife

Part of the Long Rock sea defence project included excavating silted up areas of Marazion Marsh – a significant wildlife habitat whose river outlet flows through the dunes at the easterly end of the Long Rock project. Working with the Environment Agency, Natural England and the RSPB, our in-house designers and ecologists planned a scheme to protect the reedbed habitat, create open water channels, refurbish the existing outlet weir, translocate wildlife such as slow worms, and build eel-passes on the nearby Chyandour Brook (little ramps to enable eels, sea trout and salmon to swim up and down the river freely).

Cormac’s heritage arm took care of dismantling and rebuilding a traditional stone ‘Cornish hedgerow’, carefully ensuring that all habitats were protected and reinstated.

Using an amphibious excavator (that dredges while afloat soft terrain), we cleared areas of silt and reedbeds that were causing an increased flood risk, creating open water channels that will encourage spawning fish, and we installed sluice gates which will allow water to flow out of the marsh, when levels get too high.

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    Average Weight of Boulders in Tonnes

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    Metres of Rock Armour Defence Installed by Cormac

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    Tonnes of Rock Strategically Positioned

  • In spite of the challenges of delivery by lorry, tidal, working within SSI and MMO constraints, and dealing with contamination in the soil, the project was completed within budget and within the scheduled timeframe.

    Andrew Berryman

    Senior Contracts Manager, Cormac

  • With over twelve thousand tonnes of rock, logistics were our biggest challenge. Each boulder was between two and five tonnes, so we could only fit about three boulders per lorry load.

    Andrew Berryman

    Senior Contracts Manager, Cormac

  • An ecologist survey identified that there was an ecologically beneficial dune graft present along the strip. It was lifted mechanically, stored specifically and the idea was to ensure that it was reinstated in the correct way.

    Zoe Hilditch

    Environmental Technical Officer

News (10)


Cormac began as a business with a remit to service all the civil engineering needs of Cornwall. As a consequence we have an extensive range of specialist skillsets, and can offer a true end-to-end service to our clients in Cornwall and beyond.

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