Faster, deeper, cheaper
Taking root: breakwaters need to stand the test of time. Credit: Grant MacDonald
Underwater, floating, sand-filled or rock-strengthened; breakwaters come in all shapes and sizes finds Felicity Landon
Breakwaters: clients want them faster and they want them cheaper. They want them in deeper water and they want them in more remote locations. Pulling all those 'wants' together into the 'optimum' structure can be challenging.
“It is always about cost,” says Marco Pluijm, port and marine infrastructure sector manager at Bechtel Corporation. “Breakwaters are expensive and nobody wants to pay for them. They are always a heavy burden and can make or break the feasibility or economic viability of a project.
“So you try to reduce the cost of the breakwater in any port project. We adopt what we call the 'smart ports' approach - be as clever as you can be in the planning and design of the infrastructure, not only in reducing costs but in being prepared for the future with a flexible layout that can be adapted.”
That, of course, assumes that clients have allowed enough time to study and decide on the best option. Lieven Durt, area director for Africa at Dredging International, says that the opportunity to reduce costs is sometimes lost because of time constraints.
“In general, clients don't do a lot of investigation in advance - they just ask for a quote. With breakwaters, you really need to have all the information - tidal and other data, sedimentary transport, soil investigations, etc. Often clients don't have that data and don't allow the time to gather it, so you are limited in what you can come up with. We could save clients a lot of money with more time, but when it's urgent you have to deliver.”
An exception in his experience is South Africa: “That's an example of a country where traditionally they do investigations in advance and provide a nice file with all the data you could dream of, so that you can work on alternative proposals.”
When it comes to alternative breakwater designs where budgets are limited, he says Dredging International has designed and implemented underwater breakwaters to very good effect - including a number of projects in West Africa.
“You don't see the breakwater at high tide at all. The benefit is that you need a lot less rock and, as that normally has to be transported a long way, there is a good reason to use less.”
A temporary jetty was built for the construction of the underwater breakwater, and although it has to be removed when the project is completed, the overall result is still cost-effective, he says. “The underwater jetty was tested in the laboratory and came out to be exactly as effective as a traditional one, with a lot less material.”
Mr Pluijm says another ongoing challenge is the way that longer swell from bigger ships affects ports. “The bad news is that breakwaters don't really affect those incoming waves. Port operators are experiencing more and more problems with extremely long swell, including in West Africa, Australia and South America, and breakwaters don't really help to reduce the effect.”
As a result, other solutions are being found in dynamic-response mooring and fender systems, he says. “This big issue of long swell might open up opportunities to look at things from a completely different angle. If you manage to have reliable and sufficiently effective mooring and fendering systems, there might be occasions where you don't need a breakwater any more - and that would help certain port and terminal projects to become more feasible than they would have been in a more traditional approach.
“Various institutions have been working on this for several years. We have reached a point where we understand enough of the systems and response systems so that a breakwater-less port could be within reach, or at least we can reduce the length of a breakwater.”
Ronald Stive, director projects, maritime and waterways at Royal HaskoningDHV, says: “What we have noticed in the past ten years in our port and breakwater projects is that all around the world, people are in a hurry. They don't accept that these types of structures will take years to design and to construct, so they are always looking for alternative ways to do it faster.
“Secondly, there is this trend to go into deeper water, and into areas where you don't have quarries nearby and you may have problems where the seabed isn't stable enough or there's seismic activity. In these remote areas, you have to look for alternative designs to the traditional rubble mound type of breakwater, because of the fact that the materials are too far away or too costly to bring in.
“So overall, people want quicker and cheaper, but in more remote and difficult areas where there's a lack of material.”
He reports a trend towards using sand packed into geocontainers or geotextile 'sausages' at the core of the breakwater, a solution which could be ten times cheaper than traditional rock.
“Of course, you have to be very careful if you build the core of sand, paying particular attention to how you design the filter layer between the core and the outer rock layer,” he says. “But we are seeing sand fill and geocontainers more and more.”
He also reports on continuing research into the use of artificial concrete armour units as the main primary protection in breakwaters. The use of natural rock is generally preferred but where this is not possible or practical, artificial armour units can provide an alternative - albeit a costly one. “Each block would cost about €3,000, so clearly when you need about 20,000 blocks that's an enormous cost. So there's a trend to go for single-layer armour units. In that case you have to be very careful to do model testing and make sure that the interlocking of the single-layer units is suitable and well infilled. But this method could save up to 50% of the cost.”
Often, however, there's a requirement to build breakwaters that are judged sustainable and environmentally friendly - hence using only natural materials, i.e. sand and rock. For breakwaters up to 2 km long in water depths of 10 to 20 metres, construction costs can be anything from $100m to $400m, he says. “If you can only use sand and rock, you have to go for a much flatter structure - a wide footprint and flatter slopes, and that requires a lot of material. That's often not considered an economic solution because it's costly, but if you do it right it can be cheaper than artificial, depending on the location. Also, flatter slopes and a bigger footprint give a more stable base and that does make it easy to adapt. Indeed, we are often involved in modifying, improving or reinforcing breakwaters to cope with climate change.
As for new directions, Mr Stive predicts an increasing trend towards floating structures and breakwaters, particularly where there is a lot of development at the deep water's edge. These could double up - providing protection but also providing new areas for development both above and below water, in cities pressured for space.
As he puts it: “Everything is possible in civil, maritime and coastal engineering, as long as you have enough money.”
Taking the long view on design
While sea level rise is an important issue in costal engineering, including sea walls, promenades and dykes, overall its effect is not very large on existing breakwaters, according to Professor Jentsje van der Meer, principal of Van der Meer Consulting.
The wave height attacking the breakwater may be a little larger, leading to a little more wave overtopping and more severe wave attack, possibly resulting in damage to the breakwater, he says.
However, a breakwater is often designed for a functional life of about 50 years. “In 50 years, a lot may happen to a harbour, like expansion for example. Look back 50 years and compare our harbours with the existing situation. Some breakwaters have already been removed or do not function any more as expansion brought larger and bigger breakwaters and harbour areas. For this reason, there are no actual plans to improve breakwaters for expected sea level rise only.”
However, he says, sea level rise is taken into account in the design of new breakwaters or improvement of existing ones, leading to higher and stronger structures. Prof van der Meer says new technology is the main driver in terms of innovations in breakwater projects, “technology to look under water at what you are doing, even if the visibility is zero”.
“Divers were used before but more and more techniques show the driver of a crane or excavator what and where he or she is placing under water in real time. This leads to faster and more accurate construction of large rock and concrete units.”
For an example of improved technology, he points to the use of the Blockbuster crane for the construction of the low-crested breakwater at the Maasvlakte II extension. “This crane was able to place 40-tonne concrete cubes within 30 cm accuracy and in a pre-defined pattern which saved on the number of blocks.