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What was needed?

The regeneration of an historic dock to an exclusive marina in the heart of a contemporary housing development required protection from storm waves and extreme high tides.

Sutton Harbour Holdings plc, who own and operate King Point Marina, were looking for a robust and reliable solution that would fully protect Millbay’s historic inner basin in the sometimes-extreme weather conditions.

Land & Water were tasked with developing an innovative and cost-effective solution that would protect against these events. Although the marina entrance is largely protected by the outer Plymouth breakwater, the wave pontoon would need to be deployed for critical wind directions to offer protection to King Point Marina berth holders as well as the Millbay quarter.

Land & Water's Solution

With the need for something robust, Land & Water began extensive and complex testing and modelling to find the optimum solution. This testing took place at the world-renowned Hydraulics Research Wallingford’s wave tank, where the solution was successfully modelled, developed and fabricated.

The comprehensive testing also included extensive consultations with the Environment Agency and Plymouth City planners, resulting in a proposal for an innovative wave pontoon. As a result of this stakeholder engagement, the advanced wave pontoon received the approval of all parties including the promotors of the quayside development.

The result was a 58-tonne floating steel pontoon that absorbs energy from waves, maintaining a calm environment within the marina.

The 26-metre long and 4-metre wide pontoon features an underwater steel skirt p32 | | July 2018 Marine, Civil Engineering & Construction that encapsulates a significant mass of water to counter the effects of the storm waves and is secured by a twin-pile guide system which means there are no underwater moving parts (unlike any traditional defensive gate). The pontoon is of skeletal/tubular construction which provides the required strength, buoyancy and ensures a long design life.

As well as innovation in the design, the solution also brought some additional and even unexpected benefits such as:

  • With no moving parts, the pontoon is very low maintenance compared to traditional lock gates

  • Practical application; because the pontoon is stored in the marina, remote from the navigation channel, it does not impede the passage of vessels

  • It can be deployed quickly and simply using a traditional workboat or tug

  • It can be easily and simply translocated to a nearby slipway for maintenance and inspections, unlike traditional gates which require heavy marine craneage for removal

The steel fabrication was undertaken by Land & Water’s preferred supplier fabricators, Roltech, based in the Midlands, and transported as a fully built, single piece of infrastructure to Plymouth where it was craned into the water at the city’s Victoria Wharf and towed to its destination in King Point Marina.

“We are delighted to see this new wavebreak pontoon launch at King Point Marina on February 12th, following a number of months of carefully managed construction works to install the system. We are confident the new pontoon will provide reliable and effective storm protection for the marina and for the harbour at Millbay, offering greater reassurance to our berth holders at King Point that their vessels will be well-protected in the event of storms and extreme weather conditions.” Mark Brimacombe, Marina Manager.

In preparation for the installation of the wave pontoon, Land & Water removed the existing wave gates using a 300-tonne crane. Land & Water installed new restraint/guide piles within the marina entrance which the pontoon will lock onto when in use, as well as new piles within the marina that will enable the pontoon to be stored when not in use.

Rather than using a traditional jack-up pontoon for the rock socketing of the piles, Land & Water’s temporary works team installed the new piles by devising an alternative piling methodology, cantilevering 30 tonne piling rigs over the quay wall on a purpose-built platform to allow the rotary cutter rigs to drill into the rock bed of the lock channel up to 7 metres below and 8 metres deep.

The piles in the marina were installed from a floating platform which relied on a tightly managed marine operation to accommodate changing tide levels and currents.

The Results

  • An effective and reliable solution to protect against flood and storm conditions

  • All stakeholders, including berthholders and potential home owners reassured by the background research and development undertaken by Land and Water to prove the concept

  • A solution requiring minimal maintenance and management over the life of the pontoon

“The pontoon has been modelled, developed, fabricated and installed by Land & Water and our team of specialists. In line with our drive to find creative solutions, we are delighted to have brought a different and inspired solution to this project.” James Maclean, CEO of Land & Water Services Ltd.

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