Coastal wetlands and salt marshes are receiving increasing recognition as natural coastal defenses against flooding and strong waves. A research team led by scientists from the Royal Netherland Institute for Sea Research (NIOZ), Delft University of Technology, Deltares, and Antwerp University has examined salt marshes performance during extreme real-world storms by digging into major historic records of flood disasters. The study, published in Nature Sustainability, reveals how much safety and value natural wetlands can add to engineered structures during severe storms.

By examining historic data from the well-documented 1717 and 1953 flood disasters in Northwestern Europe, the researchers showed that salt marshes reduced the number of breaches of engineered defenses. The data also showed that salt marshes lower flood magnitude by limiting the size of breaches when engineered defenses like dikes fail during severe storms. 

"Evidence from two notorious flood disasters that killed thousands of people after dike breaching: 1717 Christmas flood and 1953 North Sea flood, however, show that salt marshes have already displayed their role of 'flood fighter' for hundreds of years,'" says Zhenchang Zhu, the leading author of the paper, who conducted this research at NIOZ, but is currently working at Guangdong University of Technology, China. "Salt marshes not only reduced the number and total width of dike breaches during the 1717 Christmas flood but were also found to confine the breach depth during the 1953 North Sea flood. Especially the latter, previously unknown function of natural defenses, can greatly reduce flood damage by lowing inundation depth," Zhu continues.

In the study's abstract, the researchers say, "These findings provide new insights into the mechanisms and benefits of nature-based mitigation of flood hazards, and should stimulate the development of novel safety designs that smartly harness different natural coastal defense functions." Supplementing engineered defensive structures with coastal wetlands and salt marshes could offer better protection during storms and help safeguard coastal regions from the most extreme effects of sea-level rise. 

"Flood defenses combining green and gray features are actually more beneficial than considered earlier. Beyond wave attenuation, salt marshes can lower flood impacts simply by limiting the size of dike breach, and continue to do so under sea-level rise," Zhu adds.

This generally overlooked function of salt marshes is actually more applicable than wave dissipation, as it is not limited to wave-exposed locations. To harness natural defense, marshes ideally have to be preserved or developed on the ocean-side of the dike in order to buffer the waves. This may, however, not always be possible. The study implies that it may still be possible to enhance coastal safety by creating salt marshes in between double dikes, where a secondary, landward dike is present and the primary, seaward dike is opened to allow natural processes to ensure marsh development. Despite not protecting the outward dike from waves, such marshes would be helpful for flood protection by making the landward dike more stable during extreme storms and buffering the effects of the rising sea in the long run.

Source: NIOZ