Historic floods reveal how salt marshes can protect us

Date:
June 29, 2020
Source:
Royal Netherlands Institute for Sea Research
Summary:
By digging into major historic records of flood disasters, a
research team reveals that the value of nature for flood defense
has actually been evident for hundreds of years.



FULL STORY ========================================================================== Coastal wetlands like salt marshes are increasingly recognized as valuable natural defenses that protect coasts against strong wave attacks. Yet
their performance during real-world, extreme storms has rarely been
told. By digging into major historic records of flood disasters, a
research team led by scientists from the Royal Netherland Institute for
Sea Research (NIOZ), Delft University of Technology, Deltares and Antwerp University, reveal in a publication this week in Nature Sustainability
that the value of nature for flood defense has actually been evident
for hundreds of years.


==========================================================================
Salt marshes have reduced the number of dike breaches during the
well-known 1717 historic flood disaster. More interestingly, the 1953
flood disaster also tells us that salt marshes are not only 'wave
absorbers' that ease wave attacks on the dike, but are also 'flood
fighters' that lower the flood depth by limiting the size of breaches
when the dike would fail during severe storms.

And having smaller and shallower breaches because of salt marsh protection
can save many lives.

Salt marshes have made dikes more stable during severe historic storms
Rising sea levels and stronger storms raise coastal flood risks and
inspire development of new strategy of flood dense: supplementing
engineered structures with coastal wetlands like salt marshes. Although
we have learnt from experiments and models that these natural buffers are
'wave absorbers' that reduce storm impact, it is unclear whether and how
they can indeed add considerable safety to engineered defenses during
severe, real world 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 this 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 was 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.

Hidden value of natural defense inspires novel flood protection designs
What can we learn from historic lessons? '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 continues 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 at the seaside of the dike to
buffer the waves. This may, however, not always be possible. The study
implies that even in this situation, it may still be possible to enhance coastal safety by creating salt marshes in between double dikes, where
a secondary more landward dike is present and the most seaward primary
dike is opened to allow natural processes to ensure marsh development.

Despite no longer useful for wave reduction, such marshes are still very helpful for flood protection by making the landward dike more stable
during extreme storms and buffer the effects of the rising sea in the
long run.

'Overall this research enables novel designs of nature-based coastal
defenses by smartly harnessing different natural flood defense functions',
says Zhenchang Zhu.


========================================================================== Story Source: Materials provided by Royal_Netherlands_Institute_for_Sea_Research. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Zhenchang Zhu, Vincent Vuik, Paul J. Visser, Tim Soens, Bregje van
Wesenbeeck, Johan van de Koppel, Sebastiaan N. Jonkman, Stijn
Temmerman, Tjeerd J. Bouma. Historic storms and the hidden
value of coastal wetlands for nature-based flood defence. Nature
Sustainability, 2020; DOI: 10.1038/s41893-020-0556-z ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200629120248.htm

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