Jenny Brown is hoping for really bad weather. She is consulting tide tables, watching forecasts and rooting for strong westerly winds that would push the spring tide over the sea wall at Crosby, on Liverpool Bay, in the days around 21 March.
Brown, a physical oceanographer at the National Oceanography Centre (NOC) in Liverpool, wants to help local officials understand how much the ocean is breaching the sea wall — and how much they need to strengthen their flood defences.
To do that, she needs the waves to splash onto a device her team will bolt into the concrete sea wall. It’s a boxlike frame built of pipes, with wires strung between them like strings on a harp. When seawater slops over the top of the barrier, the wires will measure the volume and speed of the spray.
“We don’t have a good understanding of the impact of storms,” says Brown. “Offshore, we have wave measurements and tide gauges, but what we don’t have is measurements of water coming on land.”
Such data are important to make sure that people strolling by the sea aren’t swept off their feet by big waves, and to help communities prepare for coastal flooding during storms. In the United Kingdom alone, at least £150 billion (US$197 billion) of property and 4 million people are at risk from coastal flooding. “Nobody’s going to make the call to shut a four-lane highway unless they’re really sure the conditions are likely to be hazardous,” says Tim Pullen, a coastal engineer at HR Wallingford, a civil-engineering company in Wallingford, UK.
Brown’s team’s device, called WireWall, has collected data at Crosby over three particularly high tides since October. Mid-March is their last chance to catch the ocean crashing over the sea wall before they have to start writing up their results. The researchers want to use the information to improve oceanographic models — and perhaps deploy WireWalls in other places to study topics such as dam safety and the effectiveness of mangrove trees as coastal barriers.
Wave of the future
There are hardly any field data on how often seawater splashes over coastal defences, says Pullen. In the early 2000s, he helped put large tanks behind the sea wall at a coastal park in Kent, next to the tunnel under the English Channel. They captured water slopping over the wall and, by measuring the depth of water in the tanks after each slop, the scientists could take some of the only such field measurements captured so far1.
But tanks are heavy and expensive to deploy, and they don’t gather data on how fast water is moving. So Brown got in touch with Margaret Yelland, an oceanographer at the NOC site in Southampton, who has used capacitance wires, which sense contact with salt water, on buoys in the open ocean2. Working with Pullen and others, they designed and tested the WireWall before taking it to Crosby, where local officials plan to replace the ageing sea wall and are looking for information on how best to do that. (Part of the design research involved going through photos from a Facebook group of people who walk on Crosby beach to see how often the car park flooded.)
When tides are high, winds are strong and it looks like the sea might break over the wall at Crosby, the WireWall team sets up its rig. It’s about the height of a person and has 18 wires arrayed in a grid. When seawater hits the wires, it generates an electrical contact that reveals how much of the wire is wet. By measuring how the wires get drenched as the wave passes through them, the researchers can calculate the water’s speed and volume.
“We’ve had a good range of lovely big waves coming over,” says Yelland, who has been looking through the data gathered at high tides in October, November and January. One unexpected factor is that when the waves arrive nearly parallel to the sea wall, the water splashes up in a sideways squirt that can be hard for WireWall to detect.
Eventually, the researchers aim to use the data to help improve oceanographic models and other tools, such as the European-led industry guide on overtopping known as EurOtop. Brown has already spoken to officials about taking WireWall elsewhere. One option is the Fylde peninsula in Lancashire where there are three new sea walls, each with a slightly different design at which WireWall could compare sloshing.
“We’d like to be able to go anywhere,” says Brown. In Australia, ecologists Rebecca Morris and Beth Strain of the University of Melbourne plan to use WireWall to compare flooding that breaches sea walls with flooding over natural coastal defences such as mangrove forests. In Norway, researchers may use WireWall to study how winds blowing across reservoirs can push water over the tops of dams and destabilize them, says Fjóla Gudrún Sigtryggsdóttir, a civil engineer at the Norwegian University of Science and Technology in Trondheim.
WireWall could become even more relevant as sea levels continue to rise, increasing the risk of waves breaching walls. In the United Kingdom, coastal planners are required to take the effects of sea-level rise into account when building new sea defences. At Crosby, sea level is rising by 1.6 millimetres per year.
Nature 567, 294-295 (2019)