When dikes burst, salt marshes might lessen deadly flooding


On 25 December 1717, a massive storm blew into the Netherlands. Powerful waves eroded the dikes that kept back the North Sea and an estimated 14,000 people drowned. After the flooding subsided, engineers took stock of the damage. Now, an analysis of the breaches combined with assessments from another deadly flood in 1953 show that salt marshes can protect dikes during storms—and how they may reduce flooding if the dikes fail.

The results are “really relevant” for communities below sea level, says Michael Beck, a University of California, Santa Cruz, marine scientist who studies coastal resilience, but was not involved in the new work.

It’s well known that salt marshes and other coastal wetlands dampen incoming waves. So it makes sense that vegetation along dikes or other coastal structures could prevent storm damage. But although evidence from wave machines and computer simulations supports that idea, there is less evidence from field studies, especially on a large scale.

Dikes are typically built out of sand, which is covered with compacted clay and stone to help the surface resist waves. The danger is that strong waves will climb up the slope of the dike, causing erosion on the backside. As water pours through new crevices, they widen and deepen. “If you get that, then you run the risk of a breach,” says Tjeerd Bouma, a coastal ecologist at the Royal Netherlands Institute for Sea Research, who helped lead the project.

To get the big picture, the team examined a historic map of damage inflicted by the 1717 storm. Dikes located behind larger salt marshes had fewer and smaller breaches, they report today in Nature Sustainability. Dikes sheltered by salt marshes more than 700 meters wide only had 1.2 breaches per kilometer, compared with six breaches per kilometer for dikes behind salt marshes that were less than 200 meters wide. To make sure the salt marsh was the key factor in reducing wave height, the team needed to confirm that the marshes had been hit by equally strong waves. So they used a computer model, which factored in water depth and the shapes of sand bars on another old map, to reconstruct the storm waves. Their assumption appeared justified.

A more recent event provided additional details on dike failures. A large storm that hit in January 1953 caused dikes to burst in 520 places, killing 1836 people. Reviewing extensive reports on the failures, Bouma and colleagues discovered that salt marshes lessened damage to the dikes. And because salt marshes are higher than sea level, they should reduce the amount of water that enters through a breached dike, compared with breached dikes built on mudflats, which are lower than sea level, the researchers found. “That’s a very interesting and novel conclusion,” says Richard Luettich, a storm modeler at the University of North Carolina, Chapel Hill, who was not involved in the research. “It seems well founded.”

The stakes are high. “If we had had more marshes, we would have had much more evacuation time, lower water levels, so less damage and less fatalities,” Bouma says. He and colleagues plan to test these predictions with an experimental breach of a dike, which was built on land that has since been converted into a salt marsh.

Meanwhile, the researchers say the coast could be better defended by creating more salt marshes. To prevent their erosion, these marshes could be established on the landward side of an existing dike by building a second dike farther back. As plants grow and collect sediment brought in by ocean tides, the marsh would build up, and the rising land would keep pace with rising sea level. Then, if the first dike fails, the higher land created by the salt marsh would reduce the risk of flooding.

sciencemag.org, 29 June 2020
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