Dunes: Frontline for Storm Protection

When Hurricane Bertha hit Jenny Godwin’s Emerald Isle beach house in 1996, it destroyed half of a sand dune and several small cedar trees.

Not long after that, Hurricane Fran destroyed the other half of the dune. To rebuild the dune and add the proper vegetation, Godwin turned to Spencer Rogers of North Carolina Sea Grant and David Nash of N.C. Cooperative Extension for advice.

Both Rogers, a coastal construction and erosion specialist, and Nash, an extension agent in coastal management and commercial horticulture, provided information about dune planting and dune management practices. “We started planting the dune six years ago,” says Godwin. “Now we have 14 species of native plants on our dune — from sea oats and American beachgrass to bitter panicum and seaside goldenrod. The dune is now twice its original size. During Hurricane Isabel, we gained sand.”

Along North Carolina’s coast, the dune system provides many benefits — from serving as an animal habitat to storm protection.

In The Dune Book, Rogers and Nash describe the benefits of dunes as well as the best dune management practices along developed shorelines where people, buildings and roads are already in place. The new book was a collaborative effort between North Carolina Sea Grant and N.C. Cooperative Extension.

“A principal benefit for anyone living near the shoreline is that a dune acts as a storage reservoir for sand,” says Rogers. “The larger the dune, the more time it takes to be eroded by the waves, and the more protection it provides to areas further landward. However, dunes do not provide protection from seasonal beach fluctuations, long-term erosion or inlet erosion — no matter how large the dune.”

HURRICANE ISABEL DAMAGE

When Hurricane Isabel hit the Outer Banks last fall, it caused widespread dune erosion.

Although detailed dune erosion analysis is incomplete, by eye about 20 to 50 feet of dune was lost north of Avon in Dare County, according to Rogers.

“The damage was light in areas where the buildings remained behind the dunes,” adds Rogers. “Damage in this area was primarily caused by wave-induced erosion undermining piling-supported buildings and wave damage to underhouse enclosures.”

“Dune erosion south of Avon was probably comparable to areas further north, but most buildings were located well landward of the wave-induced erosion,” says Rogers. “With a few exceptions, most areas have dune grasses remaining seaward of the buildings. Buildings were more likely to be buried by ovenvash than undermined by wave-induced erosion.”

In recent months, North Carolina Sea Grant and Cooperative Extension have been helping coastal municipalities and property owners to successfully restore dunes along the coast.

Workshops focus on the dynamics of the beach — dune systems, formation of dunes using native vegetation, and how to conserve and protect the dune ecosystem.

“Dunes are the first line of defense for homes, highways and other infrastructure during hurricanes and other storms along the North Carolina coast,” says Nash. “Maintaining a healthy dune system is critical to the overall health of our coastal communities.”

Before Isabel, Rogers and Nash provided dune expertise to a number of coastal towns, including Emerald Isle where town officials and citizens planted 1,000 sea oats at the western and eastern access.

“The western access was particularly in need of dune plantings,” says Alesia Sanderson, director of the Emerald Isle Parks and Recreation Department. “Now the dunes are more established and have more vegetation.”

Since Oak Island began its dune vegetation program in 1998, town officials have planted more than one million dune plants.

The plants — including sea oats, bitter panicum, seabeach amaranth and seashore elders — are grown in the town’s dune plant production greenhouse. The plants and new sand pumped on the beach in 2001 for the Sea Turtle Habitat Restoration Project and sand from the Wilmington dredging project in 2002 have helped to restore Oak Island’s beaches and stabilize the dunes.

“We started with 90,000 plantings in the dune house,” says Russ Morrison, a former Oak Island town employee. “We have had seven crops since then.”

Every plant that was used in the turtle restoration project and other sand projects since Hurricane Floyd was grown in the greenhouse, adds Morrison.

Nash helped the town set up the greenhouse and secure seeds. A float germination system — similar to what is used for raising tobacco — was established to raise sea oats and other dune-building beach plants.

He praises the “town spirit” that spurred the innovative, cost-saving and sand-saving plant program. ‘The beach is a public trust, the only right thing to do is to preserve it,” Nash adds.

DUNE FEATURES

Ocean sand dunes are geologic features that are in a constant state of change — somewhere between building in elevation with wind-trapped sand and getting flattened in an extreme storm or hurricane.

Dunes are defined as an area landward of the active beach where dune grasses are dominant plants. It might be a classic dune shape that rises 40 feet in height and is covered with sea oats, a recent overwash terrace flattened by a hurricane where the buried sand grasses have yet to pop up through the new sand, or anywhere in between.

In addition to their aesthetic benefits, sand dunes and dune vegetation can provide substantial protection from storm-induced erosion. The larger the dune, the more time it takes to be eroded by the waves, and the more protection it provides to areas further landward.

If the dunes are large enough, the waves and storm surge are prevented from washing across the barrier island. Flooding may occur from the backside of the island but not directly from the ocean. “Even if the dune is breached and the shoreline is overwashed, the sand stored in the dunes and eroded by the storm reduces the incoming wave heights compared to areas without dunes,” says Rogers.

For these reasons, protecting the existing dunes and building larger dunes with dune vegetation are useful shoreline management practices.

“Remember that erosion has many causes,” says Rogers. “Determine what is causing your erosion before planting or building dunes.”

Areas subject to seasonal fluctuations are never good places for new dunes. In areas experiencing long-term or inlet erosion, planting vegetation or installing sand fencing is seldom recommended.

“Dune-building efforts should concentrate on areas well landward of the prestorm vegetation system,” says Rogers. “If you start too close to the water, you may lose your storm protection to chronic erosion before the big storm hits. However, if you have recently had a direct hit by a major hurricane and lost 50 to 100 feet of dune, you are likely to have a wide, recovering area on which to work.”

Faced with recent dune erosion, most people have a strong desire to return the dune to its previous location.

“However, that can be a mistake,” says Nash. “The two most common errors when building dunes are trying to stop long-term erosion and starting the dune too far seaward. If you avoid these mistakes, you will end up with a better dune. It is better to build a dune as far landward as possible.”

DUNE VEGETATION

Only a few species of plants can adapt to the dunes closest to the ocean and beach — where there are high levels of salt spray, continuous wind, large amounts of wind-blown sand and other environmental factors.

Coastal dune plants must be able to survive in soils that are low in nutrients and moisture and have extreme fluctuations in temperature and ocean overwash. The vegetation aids in forming the dune and plays an important role in the coastal dune ecosystem.

“Dune vegetation is nature’s way to build and stabilize sand dunes,” says Nash. “Plants provide food and habitat for animals and birds while adding beauty to the coastal environment. Planting and protecting dune vegetation is one of the easiest and most important things we can do to conserve this valuable coastal resource.”

The typical vegetation zones from the ocean to the sound are: pioneer dune plants, grassland species, shrub thicket and maritime forest.

Climate is the primary factor limiting the geographic range of pioneer zone coastal plants species. Along the mid-Atlantic coast, the dunes between the Chesapeake Bay and Cape Lookout are the approximate transition zone for several species.

For example, sea oats prefer the warmer climate found south of this area and appear to be limited in their northern range by cold temperatures.

American beachgrass is the dominant pioneer zone species north of the transition zone, tending to die back when stressed by the hot, dry conditions found farther south. Both American beachgrass and sea oats are excellent sand trappers and dune stabilizers.

Since local plants take years to evolve, they are usually best adapted to the climate where they were first grown. For example, South Florida sea oats do not adapt as well in the cooler climate of North Carolina as they do in Florida, and American beachgrass from New Jersey is not well suited to North Carolina’s warmer climate.

“Therefore, it is always best to obtain dune plants from seeds or parent material originating as close as possible to the beach where they are planted,” says Nash. “When possible, acquire seedlings or transplants that were grown from seeds or cuttings originating within a 100-mile radius of your beach.”

PLANT SPECIES

The “signature” plants on North Carolina’s coastal dunes are sea oats (Uniolaponiculata). As the name implies, the seed head has an oat-like appearance. The plant’s tall stems, with their seed heads blowing in the ocean breeze, are a favorite sight for coastal visitors and residents. Sea oats are aesthetically pleasing and also are important food sources for wildlife, including birds and other creatures that depend upon the dune ecosystem for their survival.

“Sea oats provide the best long-term stability for coastal dunes when planted in its native range, which includes the entire North Carolina coast,” says Nash.

American beachgrass (Ammophila brevilgulata), which is a cool-season perennial grass native to the north and mid-Atlantic coasts along the eastern seaboard, is also frequently seen along the North Carolina coast. Cape Hatteras is considered to be the approximate southern limit of its native range. Although American beachgrass is best adapted to the northern region of the mid-Atlantic coast, it has been planted extensively throughout the coastal areas of North and South Carolina to stabilize dunes.

A third type of dune plant is bitter panicum (Panicum amarum), which occurs naturally from New England to Mexico. Its stout stem and wide, blue-green leaves add unique color and texture to the dune environment.

“This plant works well in combination with other dune species and should be included in dune vegetation projects,” says Nash.

In back dune areas that have less windblown sand, saltmeadow cordgrass (Spartina patens) is highly adaptable. The slender-stemmed grass prefers moist sites but will grow in drier areas.

Seashore elder (Iva imbricata) is the only nongrass species recommended for trapping sand and stabilizing dunes in the pioneer zone along the North Carolina coast This warm-season perennial has succulent leaves and woody stems. It is a very effective sand trapper.

“When planting dune grasses, be certain to plant at the correct depths,” says Nash. “Failure to plant deep enough is the main cause of the new plant death on the dunes.”

DUNE PLANT COMMUNITIES

Native grasses and broadleaf plants stabilize pristine dune ecosystems, such as those found on the Cape Lookout National Seashore. Unfortunately, the dunes and plants in developed areas along the North Carolina coast are often destroyed by human impacts.

“Planting three or more of the pioneer zone species in a revegetation project will increase the long-term stabilization of the blowing sand and help the ecosystem recover more quickly,” says Nash.

“Once the foundation of the pioneer zone species is established, other annual and perennial plants adapted to the dune environment will establish naturally,” he adds. “As the dune system stabilizes and provides food and shelter, birds, animals and reptiles will return to the recreated habitat.”

Sand fences are usually not needed for dune building. Dune plants are just as effective and need little or no maintenance. However, sand fences can be useful in managing pedestrian and vehicular damage. When pedestrian traffic is heavy, try rope fences as a lower-cost and lower-maintenance alternative.

“Homeowners can protect dunes from pedestrian damage by choosing the appropriate accessway materials for local conditions,” says Rogers. “If the accessways are more than 3 teet lower than the adjacent dunes, look at modifying the accessways. Consider the walkways expendable but necessary to protect the dunes. Plan for future maintenance when the inevitable erosion occurs.”

Also don’t forget to check with your town hall about permits or the N.C. Division of Coastal Management at 919/733-2293,888/4RCOAST; or www.nccoastalmanagement.net.

“Planting dune vegetation does not require a permit,” says Rogers. “However, a permit may be required for other dune construction projects, including sand fences, dune walkways, vehicular ramps, removing vegetation, lowering dune vehicles, or moving any sand with heavy equipment.”

This article was published in the Winter 2004 issue of Coastwatch.

For contact information and reprint requests, visit ncseagrant.ncsu.edu/coastwatch/contact/.