How marsh grass protects shorelines

Date:
October 18, 2021
Source:
Massachusetts Institute of Technology
Summary:
Marsh plants can play a major role in mitigating coastal damage as
sea levels rise and storm surges increase. A new study provides
greater detail about how these protective benefits work under
real-world conditions shaped by waves and currents.



FULL STORY ========================================================================== Marsh plants, which are ubiquitous along the world's shorelines, can play
a major role in mitigating the damage to coastlines as sea levels rise
and storm surges increase. Now, a new MIT study provides greater detail
about how these protective benefits work under real-world conditions
shaped by waves and currents.


==========================================================================
The study combined laboratory experiments using simulated plants in
a large wave tank along with mathematical modeling. It appears in the
journal Physical Review -- Fluids, in a paper by former MIT visiting
doctoral student Xiaoxia Zhang, now a postdoc at Dalian University
of Technology, and professor of civil and environmental engineering
Heidi Nepf.

It's already clear that coastal marsh plants provide significant
protection from surges and devastating storms. For example, it has been estimated that the damage caused by Hurricane Sandy was reduced by $625
million thanks to the damping of wave energy provided by extensive areas
of marsh along the affected coasts. But the new MIT analysis incorporates details of plant morphology, such as the number and spacing of flexible
leaves versus stiffer stems, and the complex interactions of currents
and waves that may be coming from different directions.

This level of detail could enable coastal restoration planners to
determine the area of marsh needed to mitigate expected amounts of storm
surge or sea-level rise, and to decide which types of plants to introduce
to maximize protection.

"When you go to a marsh, you often will see that the plants are arranged
in zones," says Nepf, who is the Donald and Martha Harleman Professor of
Civil and Environmental Engineering. "Along the edge, you tend to have
plants that are more flexible, because they are using their flexibility
to reduce the wave forces they feel. In the next zone, the plants are a
little more rigid and have a bit more leaves." As the zones progress,
the plants become stiffer, leafier, and more effective at absorbing
wave energy thanks to their greater leaf area. The new modeling done in
this research, which incorporated work with simulated plants in the 24- meter-long wave tank at MIT's Parsons Lab, can enable coastal planners
to take these kinds of details into account when planning protection, mitigation, or restoration projects.



==========================================================================
"If you put the stiffest plants at the edge, they might not survive,
because they're feeling very high wave forces. By describing why Mother
Nature organizes plants in this way, we can hopefully design a more
sustainable restoration," Nepf says.

Once established, the marsh plants provide a positive feedback cycle that
helps to not only stabilize but also build up these delicate coastal
lands, Zhang says. "After a few years, the marsh grasses start to trap
and hold the sediment, and the elevation gets higher and higher, which
might keep up with sea level rise," she says.

Awareness of the protective effects of marshland has been growing,
Nepf says.

For example, the Netherlands has been restoring lost marshland outside
the dikes that surround much of the nation's agricultural land, finding
that the marsh can protect the dikes from erosion; the marsh and dikes
work together much more effectively than the dikes alone at preventing flooding.

But most such efforts so far have been largely empirical, trial-and-error plans, Nepf says. Now, they could take advantage of this modeling to
know just how much marshland with what types of plants would be needed
to provide the desired level of protection.

It also provides a more quantitative way to estimate the value provided by marshes, she says. "It could allow you to more accurately say, '40 meters
of marsh will reduce waves this much and therefore will reduce overtopping
of your levee by this much.' Someone could use that to say, 'I'm going
to save this much money over the next 10 years if I reduce flooding by maintaining this marsh.' It might help generate some political motivation
for restoration efforts." Nepf herself is already trying to get some of
these findings included in coastal planning processes. She serves on a practitioner panel led by Chris Esposito of the Water Institute of the
Gulf, which serves the storm-battered Louisiana coastline. "We'd like to
get this work into the coatal simulations that are used for large-scale restoration and coastal planning," she says.

The work was partly supported by the National Science Foundation and
the China Scholarship Council.

========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by David
L. Chandler. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Xiaoxia Zhang, Heidi Nepf. Wave damping by flexible marsh plants
influenced by current. Physical Review Fluids, 2021; 6 (10) DOI:
10.1103/ PhysRevFluids.6.100502 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211018154233.htm

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