Maintaining Delaware's sea level rise defenses

The Great Marsh near Lewes has all the signs of a healthy ecosystem: two types of native grasses, a diversity of wildlife and the tidal Canary Creek that meanders through it.

During storms and floods, wetlands absorb water to reduce flooding. And as the effects of sea level rise and climate change continue to affect Delaware's coastline, the health of its marshes become more important.

Wetlands like The Great Marsh can be a great barometer of sea level rise, so scientists want to make sure they are building fast enough to keep pace with rising waters.

Twice a year, researchers from the partnership visit the marsh and take readings to see if the elevation is rising, falling or staying the same.

"The marsh surface can move up and down," said LeeAnn Haaf, a wetland specialist with the Partnership for the Delaware Estuary.

State environmental scientists are doing the same thing at the Blackbird and St. Jones Coastal Reserves and the U.S. Fish & Wildlife Service is monitoring marsh elevation at Prime Hook and Bombay Hook national wildlife refuges. The Center for the Inland Bays monitors wetlands along Rehoboth, Indian River and Little Assawoman bays.

Sea level along the Delaware coast is rising at a mean average rate of 4.31 millimeters a year at Lewes Breakwater Harbor, and slightly less at Reedy Point in the Delaware River. That's about as high as four nickels stacked on top of each other.

It may not seem like much but over the last 100 years the cumulative rise at both stations has been just over 13 inches. Consider what that means during a storm. A high tide that is more than a foot above normal means significantly more flooding than our ancestors would have seen 100 years ago during the same storm.

As air and ocean temperatures rise, sea level could rise another 1.6 feet to 4.9 feet by the end of this century according to projections from the state's Sea Level Rise Technical Working Group.

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For Delaware and other low-lying coastal states, that could mean more coastal flooding, greater storm surge and loss of wetlands. The marshes also provide valuable habitats for wildlife and a nursery area for fish. Already, low-lying areas flood more frequently during minor storms and full and new moons.

Robert Scarborough, with Delaware Coastal Programs, has studied wetlands for years. He said they grow in two ways. Sediment from higher land is swept over the marsh during storm and flood events. In addition, as wetland plants go through seasonal growth and die back, the root mass builds up. That also helps to raise the height of the marsh, he said.

"Each marsh accretes (or builds up) at a different rate," he said.

In this latest study, Haaf takes regular measurements of wetland height from established benchmarks within the the marsh. The pre-selected sites have markers that were driven deep into the marsh. She and her team started out in 2014 by sprinkling these benchmark areas with feldspar, a white, naturally occurring mineral.

Besides taking measurements of the marsh from the surface to see if the elevation has changed, they also dig out a small plug to see how much marsh has built up over the feldspar layer, she said. The feldspar is a good marker because it is heavy and "really doesn't wash away."

They replace the plug once they take the measurement.

Sometimes when the marshes are sampled, the changes are slight.

"We're talking millimeters," she said.

The good news with the Great Marsh is that it "sits reasonably high," Haaf said. "We call this elevation capital. ... It's got some resiliency to it."

Besides measuring the marsh, Haaf also collects water samples. Scientists want to monitor nitrogen and phosphorous levels because changes in these nutrients can impact wetland plant growth, she said.

And they measure for total suspended solids. These are the sediments that, if captured by the marsh plants, help the wetland build higher even as sea level rises.

Not all coastal marshes in the state are the same.

Scarborough said the plants in the marsh, the volume of sediment and the amount of flooding all play a role in how a wetland keeps pace.

At the state's Blackbird monitoring area, for instance, the waters surrounding the marsh have high sediment loads. Other areas lose sediment before it can ever make it to the marsh, he said.

And if one examines core samples from marshes "you can tell when practices changed" on the landscape, Scarborough said. For instance, you can see when colonial settlers converted forest to farmland because the sediment loads increased.

With the measurements now underway "we can see if the wetland is keeping pace with sea level rise," he said.

Reach Molly Murray at 463-3334 or mmurray@delawareonline.com. Follow her on Twitter @MollyMurraytnj.