Breaking out of the mould
Today's port breakwater construction ties placement, innovation and ecology together, explains Stevie Knight.
These days there are a variety of methods aimed at making breakwater construction smarter. One is simply how the blocks are placed.
So it is with the 3 km breakwater at the new development of Safi in Morocco, where waves of 12 to 15 m come in that can lift a five-tonne rock. While placing of the “armour” is usually determined by a GPS signal, “this doesn’t tell you about the orientation of the block on the end of the crane’s rope or the fine detail of the slope underneath it.... it just doesn’t give you the kind of accuracy in places like this, where you have to get the blocks to sit with a predetermined gap in order to disperse a certain amount of the wave energy”, according to Richard Adams of Coda Octopus.
“In terms of challenges, [ports] are energetic sites so a lot of sediment can be stirred up around the shallows just where you are trying to build; a traditional camera can’t see that much and you can also be limited when it comes to putting divers down in these kinds of waters,” he adds.
The answer in this case is a sonar-based visualisation tool, the Echoscope, which the SGTM-STFA construction crews have mounted in a frame just above the hook of their construction cranes. However, Mr Adams is clear the magic isn’t just created by the sonar alone: “It’s the way that the software meshes a number of different signals to give a real-time picture,” he explains, adding that the programme can import a visualisation of the project and overlay a target for the crane operators so there’s no doubt as to the placing. It’s proved effective during the Safi build: rather than relying on divers limited to daylight hours for guidance, the Echoscope has allowed round-the-clock shifts and the construction team showed it was possible to lay down over 90, 22 metres cubed Antifer blocks in 24 hours – a single crew accomplishing as much as three cranes had previously managed, according to SGTM-STFA.
Meanwhile, a 3.9 kilometre breakwater in Chile is proving there’s more to construction planning than computer analysis. This breakwater is the longest the country has ever seen and will eventually embrace the new San Antonio port expansion, but the site combines big waves, a river mouth discharge and a 160 metre deep oceanic trench sitting right next to the entrance, explains Maria di Leo of HR Wallingford.
With all this complexity, issues can lurk in “secondary effects... which can be significant”, she points out. While traditional mathematical representation can be useful, even the most advanced numerical models can’t completely reproduce the complexity of the port system. For example, it won’t capture the dynamic interaction of real sea states with vessels moored inside the port or the structural elements of the breakwater “with the level of accuracy required and within the project timescale”, she says, adding evolving computational models of realistic sea states is particularly time consuming.
By contrast, “once you have added in the right ingredients to a 3D model, like the properly scaled waves and physical structures, a physical model may reveal less predictable phenomena, which is why we keep running physical models and learning from them”, says Ms di Leo. In fact, a 3D model proved invaluable at steering another project away from trouble when it revealed cumulative energy resulting from an unusual combination of incoming waves with those reflected by the internal port structures.
Therefore Chile’s San Antonio breakwater is not just being represented in virtual space: a physical model of the whole port and adjacent stretches of coast along with the sea floor – and a portion of the oceanic trench - is also being recreated in minute detail inside HR Wallingford’s massive 75 metre by 32 metre basin in the Froude Modelling Hall. It will be repeatedly tested to allow a detailed investigation of long period wave agitation, ship movements and any effects that could impact breakwater stability and overtopping performance.
Working with nature
However, smart can also mean working with nature rather than against it. According to some, the humble bivalve could be key.
Before their decline in the 20th Century, the oyster beds common around Manhattan, US, acted as a natural breakwater, absorbing underlying wave energy before it hit the shore. So, the idea to restore the oyster beds and with them, some of the coast’s resilience against extreme weather events.
Therefore, the Billion Oyster Project (BOP) in New York Harbour is trying to reintroduce a hundred acres of reef by 2035. Part of a series of rock-and-oyster-colony breakwaters off the south shore of Staten Island, BOP is getting the whole community onboard as local restaurants are providing the shells for the substrate and schools are helping seed and monitor the reefs. While the initial idea is to reduce storm damage, the project also promises side benefits including providing diversified marine habitats and cleaning up the water.
So, could the idea be used in other locations? Could it take some of the energy out of the extreme weather events hitting headlines, coastlines and port facilities recently? BOP executive director Peter Malinowski warns against getting too excited: while historically oyster reefs did play a role in armouring the shoreline outside NY Harbour “those reefs were massive and took many hundreds of years to grow to that size”. Instead of an immediate payoff, it’s a long game with climate change in mind: he explains the BOP reefs (which are based on stone and cement) will build-up over time through biogenic action and continue to grow, rising with the sea levels.
There is another practical problem inherent in introducing ‘grey-green’ infrastructure directly into ports. Tony Rodriguez of Carolina University explains that while oyster reefs are resilient enough to regrow after a single large ‘pulse’ event such as a hurricane, “trying to put them right in the wake of big ships simply won’t work... these are living creatures that can’t cope with the constant wash and the sediment transported by very large vessels”. In fact, he explains that his team tried to introduce an oyster breakwater at Moorhead, upstream of the Port of North Carolina (not a major facility like neighbouring Wilmington and Norfolk) but, he says “there was still just too much energy in the system for the oysters to thrive”.
This doesn’t mean ports shouldn’t get involved. In fact, Dr Rodriguez says that work outside the immediate fairway might yield benefits “as introducing framework species... like salt marsh grasses, oyster reefs and in tropical climates, mangroves... will probably stop some of the erosion of the coastline caused by passing ships as well as providing an ecologically diverse habitat”.
MITIGATION ON THE SPOT
Ecological mitigation measures can run into the tens, if not hundreds of thousands of dollars, but it’s an uncomfortable subject as the alternative is to simply give up on the marine life in the direct vicinity. Not great for the area in question and not so great for the bottom line either: “As a rule of thumb, putting the environment mitigation outside the original area doubles the costs,” says Shimrit Perkol-Finkel of ECOncrete.
But there might just be a way to combine marine construction and the necessary environmental balances.
While standard concrete is one of the most common materials used in breakwater armour as well as many other marine constructions, it’s a poor substrate for biological recruitment. To start with it has an extremely alkaline surface measuring around 13.5pH and exhibits a troublesome tendency to leach the additives which help it set, making it toxic to many marine organisms. More, ordinary concrete tends to provide nothing more than smooth, homogenous surfaces that don’t provide many opportunities for larvae to attach.
There are alternatives “which aim to change the way the industry works without compromising strength”, says Ms Perkol-Finkel. ECOncrete (part of the SCAPE Team’s winning project for Staten Island’s ‘Living Breakwaters’ competition) “has a modified chemistry and surface that promotes the settlement of the larval stages of marine life", she explains. However, it’s not a ‘one-size-fits-all’ solution: there is the mix of location, vessel movements and both the resilience and balance of the different types of flora and fauna to think about.
“We need to put our heads into the water at each site as the ECOncrete should be tailored to provide different hollows suited to the kind of life you want to encourage,” she concludes. “To do it right you need to accommodate a number of biological niches.”
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