36 Scientific American, April 2019low need seawalls, even if such
structures are not cost-effective.
Still, Beck says, certain populated
areas can benefit from a hybrid
ap proach: “Even if you’re build-
ing levees, they can be shorter if
they have marshes in front.”SHORELINE TRIALS
AND ERRORS
one reAson living shorelines are
becoming an economically viable
approach for coastal defense is
that researchers and municipali-
ties are getting better at rebuild-
ing them. Early marsh-restoration
designs, which followed forestry
science and gave each plant plenty
of space to avoid competition,
were actually counterproductive.
It turns out that in bare mudflats,
“when marsh plants are together,
they share oxygen, so their growth
rate is twice as high,” says Brian
Silliman, an ecologist at Duke Uni-
versity. Root them in large clumps,
and the growth rate of each indi-
vidual plant can triple. Add blue
crabs, which eat the snails that eat
the salt-marsh grasses, and the
plants do even better.
Scientists are also finding that
marshes do best when they have
a protective sill—a linear berm that fronts the seaside edge of
the grass and stands. Made of hard material such as shell, stone
or concrete, its height and position are typically chosen so that
water covers it at high tide, but it is exposed during low tide. The
sill takes the brunt of wave energy but also traps sediment be -
hind it, allowing the grass to thrive and marsh floor to retain its
elevation or even rise.
Almost any hard material can make a successful sill. Large
shoreline-stabilization projects use big boulders or stackable con-
crete blocks, a practice that has been criticized by some experts
who say that these structures are living shorelines in name only.
But many lower-profile restorations integrate sills more seam-
lessly into the natural habitat. In the Southeast and Gulf Coast
regions, marshes historically possessed a natural sill in the form
of an intertidal oyster reef. Many of those reefs were overharvest-
ed long ago, ruining the sill and exposing the marshes to erosion.
In these warm, oyster-friendly waters, new sills can be formed
by placing a hard substrate along the low-tide line at the front edge
of the marsh for baby oysters to set on. Some sites with lots of wave
action have used small, hollow concrete structures or plastic mesh
“onion bags” stuffed with shell and lashed together. When success-
ful, these artificial materials are quickly covered by oysters and
disappear into the interstices of the growing reef. But the concrete
often remains visible for years, and the bags have been criticized
for breaking and scattering plastic through the environment.
Gittman, now at East Carolina University, is testing an alter-
native material called Oyster
Catcher that is made of jute cloth
dipped in Portland cement and
rolled into various hollow configu-
rations. It hardens with extensive
surface area to recruit larval oys-
ters. In addition to being light and
flexible, it holds together just long
enough to get a reef established,
then disintegrates. The product
re ceived its first big test when
Hurricanes Florence and Michael
struck North Carolina last fall.
Michael tossed shell bags up into
the marshes, but the Oyster Catch-
er reefs did not budge. The show-
ing was encouraging, but Gitt-
man worries that conservation
groups may oversell the potential.
“A living shoreline can’t save your
house from a Category 5 storm.
Although neither can a bulkhead.”
Gittman and Beck both stress
the need to tailor living shore-
lines to local conditions. One rea-
son oyster restoration is so cost-
effective in the Gulf and the
South east is because there have
been plentiful wild oysters to
seed new reefs with babies. That
is not the case in most of the
country. Chesapeake Bay, for ex-
ample, was long the poster child
for futile oyster restoration. Oyster populations in the bay had
fallen to less than 1 percent of historical norms, and decades of
effort and tens of millions of dollars barely budged the needle.
“Conceptually, Chesapeake Bay was not our best model,”
Beck says. “It put oyster-reef restoration back because it made it
look so difficult and expensive. Well, when you’re working in a
system where you’ve only got 1 percent left, guess what? It ain’t
easy. When you’re in the Gulf of Mexico and you’ve still got
50 percent of your reefs left, it’s a different story. If you build it,
the oysters will come.”
Beck extends that lesson to coral reefs, the most underappre-
ciated of natural defenses. “Coral reefs are the single most effec-
tive ecosystem for flood-risk reduction,” he says. Corals, which
have evolved to take a daily pounding that would destroy most
other living things, form natural seawalls exactly where you
want them—just offshore, in front of resorts, beach towns, coast-
al roads and other pricey assets. When healthy, they make re -
markably effective breakwaters, reducing wave energy up to
97 percent. They are also affordable: reef restoration averages
about $1,300 per meter versus $20,000 for artificial breakwater
construction. The insurance industry’s assessment for mitigat-
ing risk from climate change in the Caribbean found that reviv-
ing reefs and mangroves was an order of magnitude more cost-
effective than seawalls or breakwaters.
Even though reefs do not line a lot of shorefronts, the annu-
al expected benefits they generate are significant—more than $100ECOLOGISTS, including Rachel Gittman ( in white ),
measure water levels and grasses at Carrot Island, N.C.