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Plants Can Stand Up to Harsh Waves Better Than You Think

A living shoreline at Longwood University's Hull Springs Farm has a low stone sill and wide planted marsh. ©Karen Duhring/VIMS

A living shoreline at Longwood University's Hull Springs Farm has a low stone sill and wide planted marsh. ©Karen Duhring/VIMS

A living shoreline at Longwood University’s Hull Springs Farm has a low stone sill and wide planted marsh. ©Karen Duhring/CCRM-VIMS

By Chris Patrick, staff writer

If you talk to Yongqian Yang about living shorelines, at some point you’ll probably hear him say, “let the vegetation do its job.” He adopted this refrain after discovering that marsh plants in living shorelines can protect coastlines better than some engineers think they can.

Yang is actually an engineer himself, but his research has helped him see the amazing power of plants. As a Virginia Sea Grant graduate research fellow at Virginia Tech, Yang ran computer models looking at how good living shorelines are at absorbing wave energy under different conditions.

A living shoreline is basically a marsh planted next to the water’s edge. It uses wetland plants to absorb the energy of waves bashing the shore. It’s a more environmentally friendly way to prevent coastline erosion threatening land, houses, or roads than the frequently employed hard structures like bulkhead, which is a vertical wall, or rock revetment.

Yang was looking at one component of living shorelines in particular: sills. Not window sills. These sills are made of stones. Most living shorelines in Chesapeake Bay need something to keep the marsh plants in place. That’s where the sills, which are essentially loosely piled stone walls between the marsh and the water, come in. The plants protect the coast; the sills protect the plants.

He found that for living shorelines with sills of different heights: “the wave energy dissipated by the whole system doesn’t change that much.”

Before Yang’s research, there wasn’t a lot of information available on how big sills should be. To be safe, engineers tend to design living shorelines with higher sills, sometimes more than a foot above average high water levels.

Yongqian Yang's model simulated waves hitting a living shoreline. ©Yongqian Yang/VT

Yongqian Yang’s model simulated waves hitting a living shoreline. ©Yongqian Yang/VT

“Currently there is no clear guideline about what height of sill is enough, so engineers prefer to design sills higher,” Yang says. “They think the higher sill will be more effective for intercepting and reducing wave energy.”

But high sills pose problems similar to traditional hard structures like bulkheads and revetments. These hard structures interrupt important habitats and processes. The edge between a marsh and water is where animals, organic matter, and energy are exchanged. If a sill is too high, smaller fish may be able to swim in between the stones to move between the wetland and water, but larger animals like horseshoe crabs, blue crabs, and terrapin turtles can’t get over the wall.

Hard structures also don’t dissipate wave energy the same way plants do. Instead of absorbing energy, hard structures—including sills—deflect some of it. They can send energy up or downstream to pummel an unprotected stretch of coastline, push it upward above the structure, or push it out into shallow water, where it disrupts habitat. They can also reflect energy on the very plants they’re supposed to protect. Preventing these potentially adverse effects on the ecology and environment means using lower sills in living shorelines.

To help raise engineers’ confidence in plants’ abilities to absorb wave energy, Yang simulated a living shoreline using a model other researchers developed over the past two decades. His modeled living shoreline had sills of different heights, ranging from below average high water level to above it.

Computer models are essentially a bunch of really complicated, really long equations. A modeler, like Yang, puts these equations into a computer and tells it to solve them at different points on a grid. The output is a visual simulation composed of lots and lots of data points. This efficiency is one of Yang’s favorite things about models.

“If you do an experiment in the field or lab, you measure data from very limited points,” Yang says. “Using modeling will provide you more data. It’s 20 versus 10,000 data points.”

Yang’s modeled living shoreline amounts to bands of green, blue, and silver conjured by pages and pages of equations. It produces thousands of data points, which Yang used to figure out how much energy the system was absorbing with different sized sills.

Karen Duhring (left) and Yongqian Yang chat at Virginia Sea Grant's Project Participants' Symposium.

Karen Duhring (left) and Yongqian Yang chat about his research. ©Zoe Jakovenko/VASG

It turns out that as long as there’s some kind of sill to hold a marsh in place, sill height doesn’t seem to matter. In both day-to-day conditions and storm conditions, living shorelines absorbed about the same amount of wave energy regardless of sill height.

This means that engineers should be able to design living shorelines with lower sills, allowing edge habitats to thrive and plants to take the brunt of wave energy. They can handle it.

“Yang’s research will increase confidence and decrease skepticism in the effectiveness of planted marshes,” says Karen Duhring, coastal scientist at Virginia Institute of Marine Science’s Center for Coastal Resources Management and Yang’s outreach mentor for this project.

Once Yang summarizes his results in a report and publishes it online, Duhring will help him share it with the living shorelines community, especially the engineers designing living shorelines. She hopes this will encourage the engineers to make living shorelines with lower sills.

“We need to make sure the way we design living shorelines supports the ecosystem as they are intended to do” Duhring says.

Or in Yang’s words, “let the vegetation do its job.”