Coastal Erosion

Coastal Erosion

The beaches of North Carolina's coastline face an ongoing threat: coastal erosion. Though mostly gradual and relatively unnoticeable over the course of a year or two, the rising sea level combined with a season of storms or hurricanes can cause anywhere from a few feet to hundreds of feet of this delicate shoreline being stripped away. Solutions are constantly being studied and discussed, but often, the solution to erosion can be just as damaging as erosion itself. While vacationers are all but guaranteed to enjoy decades of happy beach days in the future, the issue of coastal erosion and its potentially devastating effects on our shorelines remains on the minds of locals and visitors.

Coastal erosion is the wear2ing away of land or the removal of beach or dune sediments by wave action, tidal currents, wave currents or drainage. Waves generated by storms, wind or even fast-moving motorcraft traveling close to shore cause coastal erosion. Erosion may take the form of long-term losses of sediment and rocks or merely the temporary redistribution of coastal sediments. In other words, erosion in one location may result in a larger beach nearby, as the sand is veritably "moved" from one stretch of beach to another.

On rocky coasts, coastal erosion results in dramatic rock formations in areas where the coastline contains rock layers or fracture zones with different resistances to erosion. Softer rocky areas become eroded much faster than harder ones, and erosion on rocky coasts can literally take centuries of battering waves to make a noticeable impression.

On sedimentary coasts, however, like the beaches of North Carolina, coastal erosion typically poses more of a danger to human settlements like coastal towns and oceanfront structures than it does to nature itself, and human interference can also increase coastal erosion. For example, a town called Hallsands in Devon, England was a coastal village that was washed away overnight, an event thought to be caused by the dredging of shingle (beach gravel) from the bay in front of the village.

Erosion is not necessarily a bad thing. In fact, throughout the course of history, erosion has directly led to the opening of new inlets and harbors and has created new feeding grounds for countless coastal species. In the long term and at a gradual pace determined by nature, erosion has gently changed maritime ecosystems in a positive way.

However, human interference, combined with global warming, can speed up the process of erosion with effects that may not have been intended by Mother Nature, and this is when erosion can be a concern.

Causes of Coastal Erosion

As a general rule, North Carolina's beaches erode more in the stormy fall and winter months than in the calm summer months. It is not unusual for the mean high water line to move landward temporarily by 75 to 100 feet during the stormy season. Of course, when an ocean shoreline is hit directly by a hurricane, beachfront erosion can be even more dramatic. Inlets are also affected by seasonal storms and can change configuration rapidly and severely as tremendous amounts of water and sand flow through them. In severe storms, it is even possible for new inlets to form and existing inlets to close. Erosion associated with storms is often severe because large quantities of sand can be moved quickly offshore from the beach and dunes. This type of erosion is usually called "short-term" because the shoreline can return to its original profile as the stormy conditions return calm.

Erosion can be caused by a number of factors, some short-term and some long-term, like the changes in coastline shape that are caused by the ever-present currents that barrage our shore. A current is defined as a large mass of continuously moving ocean water, and surface ocean currents are mainly wind-driven and occur in all of the world's oceans.

One of the largest and fastest moving surface currents in the world, The Gulf Stream, runs parallel to North Carolina's coast, and is as close as 15 miles away in some locales like Hatteras Inlet on the Outer Banks.

Vertical and ocean-bottom currents are two other currents which can affect the coastline, and are mainly driven by density differences caused by changes in temperature and salinity. Originating in Polar Regions, cold, salty waters sink to the ocean bottom and move toward the opposite poles where they again surface. Vertical upwelling currents can also be caused by winds blowing off a coastline. These displaced waters are then replaced by underlying bottom waters, creating a current.

All offshore currents along the East Coast contribute to erosion and tend to remove sand from the northern ends of barrier islands and deposit it at the southern ends. Currents, however, are definitely more of a benefit than a detriment to the coastline. The Gulf Stream helps keep the coastal waters warm and provides feeding grounds for thousands of species of migrating fish, a fact that is not lost on avid fishermen.

Ocean waves are another constant on the shoreline that cause erosion, both on a short-term and long-term scale. Ocean waves are caused by winds that are sometimes thousands of miles out to sea. In fact, the great majority of large breakers one observes on an ocean beach result from very distant winds.

While waves can remove sediment gradually as they pound along the shoreline, picking up small sand particles and leaving them floating in the ocean waters, the majority of severe erosion is caused by large ocean waves, generated by coastal storms like hurricanes and Nor'easters.

The tumultuous winds found by the ocean can also attribute to erosion, particularly high-speed winds generated from storms. Wind can literally remove a sandy beach area by collecting and displacing the sand and making the shoreline vulnerable to rising tides and large waves. Tides, too, play a role in erosion as a severe high tide coupled with wind or larger than normal waves can remove a significant portion of beach in a matter of hours.

But one of the largest global affects on erosion is overall sea level change. Scientists have found that North Carolina's coast has undergone erosion due to years of a rising sea level. Evidence of the changing sea levels is found in the series of old shorelines that can be discovered across our coastal plain. These shorelines control the courses of our rivers, the topography and soil types of our land and the types of natural communities that exist upon them.

Sea level is directly related to global climate. As the earth warms, massive glaciers on the poles melt and deposit into the oceans. Over a long period of time this causes the overall level of ocean waters to rise, and over the past few decades the sea level has risen at an alarming rate. North Carolina has experienced a rise even larger than the worldwide average, because our coastal lands have been sinking as the glaciers melted. This elevation change occurred when land that had been pushed up by the weight of the thick glaciers sank back down as the glaciers disappeared. The current rate of sea level rise along the North Carolina coast is about twice the worldwide average.

Storms and Erosion

Hurricanes play a key role in massive erosion, as these storms generate a dangerous combination of high winds, currents and large waves that can be detrimental to a beach. In fact, several Hurricanes have been directly responsible for major changes in the coastal landscape.

In 1954, Hurricane Hazel made landfall near Calabash, North Carolina, halfway between Myrtle Beach, South Carolina and Wilmington, North Carolina, and it destroyed every pier along a 170-mile (270-kilometer) stretch of coastline. Hazel wiped out much of Garden City, South Carolina, especially its business district, where only three of the 275 buildings escaped damage and only two houses out of 275 remained habitable.

At landfall, Hazel brought a storm surge of 14.5 feet (4.4 meters) to a large area of coastline. Coastal damage was severe along the southeastern coast of North Carolina and the highest storm surge was recorded at Calabash, coincidentally arriving at the highest lunar tide of the year and reaching 18 feet (5.5 meters) above mean low tide. Southport and Wrightsville Beach were hit hard, with large portions of the landscape, homes, buildings and trees that were in this coastal area literally being wiped away to sea.

A more recent example happened in 2003, when Hurricane Isabel, a low category 2 hurricane, made landfall in Hatteras Village on the Outer Banks. With the strong winds and waves, the hurricane also brought what locals described as a "wall of water" that crushed the northern part of Hatteras Village and the neighboring southern village of Frisco. A number of homes were ruined and smaller beach cottages were dislodged from their pilings and pushed into the Pamlico Sound.

In between the towns, where a few miles of highway once connected the two villages, a new inlet formed, dubbed "Izzy's Inlet." The Federal Government spent weeks refilling the inlet, which was very deep with swift moving water, and this stretch of Hatteras Island once again appears to be back to normal, though locals remember just how delicate this barrier island truly is when it comes to hurricanes.

A good Nor'easter can also cause severe erosion to a coastline, and many folks believe a Nor'easter can be even more dangerous than a hurricane. While a hurricane only blows for a few hours before moving north, a Nor'easter can settle in for days, exasperating the damage to a coastline.

One area that is constantly hit by Nor'easters is the section of beach just north of Rodanthe on the Outer Banks called the "S-Curves." Once home to a wide area of beach that comfortably ran from the ocean to the Pamlico Sound, the highway has been moved further west twice over the past 20 years due to erosion caused by a series of Nor'easters.

Today, every time a Nor'easter hits the Outer Banks, this narrow stretch of highway floods severely, closing the road that connects Hatteras Island with the rest of the world. Though after each storm the North Carolina Department of Transportation (NCDOT) pushes sand back onto the dunes, locals know it is only a matter of time before the next Nor'easter will tear it down again and close the road.

As these examples show, the overall impact of storms on barrier islands depend upon qualities of the storm (the storm surge, the waves and the wind speed) and the elevation of the barrier island at landfall. To quantify the impact of storm damage, the United States Geological Survey (USGS) devised the following "hazard scale" which is divided into 4 "impacts," and accelerates in degree of damage caused by erosion:

• Impact 1 - Wave erosion is confined to the beach area. The eroded sands will be replenished in a few weeks to a few months and no significant change occurs in the system.
• Impact 2 - Waves erode the dune and cause the dune to retreat. This is a semi-permanent or permanent change to the system.
• Impact 3 - Wave action exceeds the dune's elevation, destroys the dune and pushes sediment from the dune landward (approximately 300 yards/100 meters), thereby creating overwash. This change in the system pushes the barrier island landward.
• Impact 4 - The storm surge completely covers the barrier island, destroys the dune system and pushes sediments landward (approximately 0.6 miles/1 kilometers). This is a permanent change to the barrier island or portions of it.

Effects of Excessive Erosion

There are a wide variety of effects stemming from erosion, from narrower beaches to severe damage loss, and these can have both minor and far-reaching consequences.

One of the most devastating effects to coastal residents is property loss. Erosion into residential areas usually occurs when something happens to increase the local rate of sea level rise, like a Nor'easter. The chances of losing a home increases when the house was built in a tumultuous locale, like bordering a wide flat beach backed by a modest dune.

Oceanfront property is continually being lost or is in danger of being lost on North Carolina beaches. The situation is so precarious that statewide insurance companies are undergoing a massive change in coastal coverage policies, causing up to a 50% increase in coastal property premiums. Many storms can wipe out an entire coastal neighborhood, destroying homes or simply leaving them desolate and right in the ocean's wash. At this point, the properties are usually condemned. In an effort to protect their properties, coastal homeowners use sand bags, sand fences and even beach nourishment to protect their properties from the threat of erosion.

The mainland areas along eastern North Carolina can also be affected by coastal erosion, as the saltwater and sand can mingle with freshwater, changing an entire ecosystem. If a low-lying area floods with sound or ocean water, saltwater can steep into creeks, ponds, streams and rivers, and kill the freshwater population of fish and other species that cannot live in a saltwater environment. The saltwater can also be detrimental to soil, and can have long lasting affects even after saltwater flooding has subsided. Many shrubs, bushes and even trees are not salt-tolerant and can die after one oceanside or soundside flooding. It is also difficult to grow new plants in the flooded area, as the salt is now mixed in with the soil. Some mainland that borders a sound or inlet, like that found in low-lying areas of Carteret Country, is irregularly flooded with sea water and can only grow species of salt marsh plants that can tolerate these immersions.

It is difficult to protect one's home from a detrimental sound or ocean flooding, although some homeowners try to keep their property above sea level by adding "fill," or giant loads of dirt or sand to literally raise the height of their property and protect their trees and plants.

Types of Coastline

The "type" of beach also plays a role in determining if and how the shoreline will be affected by erosion, and North Carolina has a variety of different coastal areas.

Barrier Islands are found on coastlines all over the world, but are most noticeable along the eastern coast of North America, where they extend from New England down the Atlantic Coast, around the Gulf of Mexico and south to Mexico. North Carolina is home to one of the longest strings of barrier islands on the East Coast, beginning with the Northern Outer Banks, extending through Cape Lookout National Seashore, and culminating at the Southern Cape Fear region at Wilmington. Most barrier islands are popular vacation spots and are home to a number of vacation homes and properties. However, these islands are fragile, constantly changing ecosystems that are important for coastal geology and ecology. Development has posed dangers to these ecosystems and has also increased the risk of property damage every year from hurricanes and Nor'easters.

Barrier islands are long, narrow, offshore deposits of sand or sediments that lie parallel to the coastline. Some barrier islands can extend for 100 miles (160 kilometers) or more, and the islands are separated from the mainland by a shallow sound, bay or lagoon. Barrier islands are often found in chains along the coastline and are separated from each other by narrow tidal inlets.

The formation of barrier islands is complex and not completely known. The current theory is that barrier islands were formed about 18,000 years ago when the last Ice Age ended. As the glaciers melted and receded, the sea levels began to rise and flooded areas behind the beach ridges at that time. The rising waters carried sediments from those beach ridges and deposited them along shallow areas just off the new coastlines. Waves and currents continued to bring in sediments that built up, forming the barrier islands. In addition to this process, rivers washed sediments from the mainland that settled behind the islands and helped build them up.

Barrier islands consist of three parts, including the beach, the dune, which protects the rest of the island from the beach and is naturally stabilized with plants, and the barrier flat, or the backbone, which is the remaining part of the island. Formed by sediments that get pushed through the dune system by storms, such as hurricanes, grasses grow and stabilize these areas. Barrier islands also serve two main functions. First, they protect the coastlines and mainland from severe storm damage. Second, they harbor several habitats that are refuges for wildlife.

Some wave-dominated coasts do not contain estuaries and have no barrier island system. These coasts, however, do have beaches and dunes and may even have coastal marshes. The term "strand plain" has been applied to coasts of this sort. Examples include parts of western Louisiana and eastern Texas. In most respects, they are similar in morphology to barrier islands, but simply lack inlets.

North Carolina is also known for its miles of wetlands, from the sound surrounded areas off Roanoke Island and Manteo, to the miles of wetlands that play a key role in attracting birds to the Pea Island National Wildlife Refuge north of Rodanthe, to the miles of scenic wetlands just south of Cedar Island. Because the North Carolina coast is dominated with barrier islands and inlets, wetlands thrive in this coastal environment.

A wetland is an area between dry land and water that is regularly saturated with surface or ground water. In fact, it is inundated with this water so consistently that vegetation and animals that thrive in wet conditions take up residence there. Wetlands host a wide variety of plant and animal life. They also provide water storage and filtration, and give mainland areas protection from floods.

Many of North Carolina's coastal wetlands are flooded on a regular basis from storm surges from sounds, rivers and inlets, but this helps the wetlands thrive. Composing of species that don't mind the salt and freshwater mix, wetlands perform very well with a salty marsh base and a fresh rainfall. Because of our climate and the nature of our coastline, wetlands are common and do very well in North Carolina.

Unfortunately, the main threat to our wetlands has nothing to do with natural erosion, but with human interference. Up until the mid-1980s, wetlands were viewed as wild areas that needed to be controlled. The U.S. Army Corps of Engineers spent a lot of time draining wetlands for future development or agriculture, and even today, new coastal developments are filling in wetlands to make room for new communities. This is treacherous to our coastal environment for a number of reasons. These new communities displace countless species of wildlife that formerly lived in the wetland area, and because these areas have a history of being flooded, homes in these communities are in a precarious position from the day they are built. In addition, flood waters that used to simply seep into the wetlands and intermingle with the marshy areas are displaced as well, and may affect and flood other neighboring communities.

During the 1970s, people did begin to recognize the benefits of wetlands, and beginning with The Clean Water Act of 1972, the U.S. Federal Government began instituting a series of laws and acts that would protect wetlands from further degradation. Unfortunately, the majority of wetlands in this country are located on private land, so the government can only do so much, and North Carolina's wetlands continue to deteriorate as new coastal communities are built.

Coastal Defenses

There are a number of steps property owners and communities can take to try to stop the damages caused by encroaching erosion, only some of which are successful.

Some coastal areas use an approach called "beach nourishment." Usually paid for by a local town or community and sometimes assisted with funds by State or Federal Governments, beach nourishment literally piles on new sand to an existing beach, making it wider and higher above sea level. The process can take many months, and the results are mixed. Beach nourishment has worked in several communities. In North Carolina, it has been applied to beaches in Emerald Isle and is being considered by the town of Nags Head as a way to protect property and keep nice, wide beaches.

One approach to extending the life of beach nourishment projects, or to protect an existing beach or coastal area, is through the use of stabilization structures like groins, jetties and sea walls.

The major concern with the use of stabilization structures is their potential adverse affects on the adjacent shorelines and coastal areas. For example, for a long time groins were the preferred approach to controlling beach erosion in most coastal areas. However, since groins work by trapping sand within the littoral system, they have an adverse effect on the neighboring areas that lose sand to these groins.

While beach nourishment alone is currently the preferred method of shoreline stabilization, groins and jetties are still the most common of the structures that are built perpendicular to the shoreline. Groins, usually constructed in sets called groin fields, extend like fingers away from the shore. Not all groins are built straight and perpendicular to the shoreline; some are Y-shaped or T-shaped, some are built at an angle other than perpendicular and some even zigzag. Groin fields are designed to trap and retain sand, nourishing the beach compartments between them. Sand usually accumulates on one side of the groin and erodes on the other, protecting one designated area of the shore.

A good example of a successful groin is in Folly Beach, South Carolina. There, the groins extend along about 1/2-mile of the nearly five miles of recently added beach nourishment. The area where they were installed was more rapidly eroding than the adjacent beaches. After the nourishment, it was apparent that this "hot spot" had been largely controlled by the presence of the groins added at the time of the beach fill.

Jetties are usually built to reduce shoaling in navigation channels and function like groins, interrupting the movement of sand and other material in the littoral zone, where it can be swept away with the waves. Jetties usually extend far enough from the shoreline to completely block the movement of sand in the littoral zone and therefore have a significant impact on adjacent beaches, both updrift and downdrift. In fact, the presence of jetties can affect dozens of miles of surrounding shoreline.

Jetties can work for a long period of time, though like most structures built to block erosion, nothing is guaranteed or permanent. For example, for decades, the Cape Hatteras Lighthouse was protected by a fortress of jetties. For many years, these jetties halted erosion, but after awhile, it was obvious that nothing could prevent the onslaught of erosion. As a result, the lighthouse was moved 2,900 feet southwest, putting the lighthouse 1,600 feet away from the edge of the ocean.

Breakwaters and sills are the most common structures that are built offshore. Breakwaters are structures placed well offshore to slow down the energy of incoming waves. The displacement of wave energy also allows drift material to be deposited behind the breakwater. This protects the shore and can also extend the beach itself. Because breakwaters are located well beyond the surf zone, they are exposed to large wave conditions and are therefore usually massive structures. They are frequently constructed of rock with a stone or concrete protective layer covering it, like a coat of armor.

Breakwaters may be either fixed or floating and may rise above the surface of the water or be completely submerged. When a breakwater is constructed above water, it is typically composed of several different segments that are placed side by side. Underwater breakwaters are typically not segmented but are continuous, much like an underwater reef. Breakwaters are not restricted to ocean waters and can be placed in lakes or large bodies of water near cities. For example, there is a breakwater located in Lakeview Park, Ohio.

Sills are underwater structures that are designed to hold the beach at a higher level than it would otherwise take. The vertical face of the sill holds the sand on the landward side at a higher elevation than on the seaward side, creating a perched beach that extends the shoreline seaward. To prevent the sand from leaking seaward, sills are continuous and not segmented. Unlike breakwaters, they are always constructed below low water and are essentially out of sight.

Seawalls, bulkheads and revetments are constructed onshore parallel to the beach to stabilize the position of the shoreline and protect property near, but not directly on the shore. These hard structures are designed to reflect and absorb wave energy rather than to preserve or restore the beach. In fact, visitors to the beach are likely to have observed that the beach is very narrow or non-existent in front of many of these types of structures. This happens because, when waves strike a hardened shoreline, a portion of the wave energy is reflected from the structure. This reflected energy causes sand at the base of the structure to be lifted into the water column and carried away, thus increasing the rate of erosion.

The effects of this accelerated erosion can be controlled by beach nourishment or beach-protecting structures like groins and breakwaters. Because scouring at the base of these structures can eventually result in their collapse, additional stone, called "toe stone," frequently is added at their seaward base to dissipate some of the reflected wave energy and prevent destructive scouring.

Seawalls are the largest of these three structure types, and are massive enough to withstand the onslaught of major storm waves. These giant seawalls are essentially vertical reinforced concrete structures that are built along the seaward edge of upland areas to prevent erosion and other damage from waves resulting from hurricanes and other large storm events. Seawalls are often built to protect vital infrastructure, such as the Great Ocean Highway in San Francisco, or entire cities, like Galveston, Texas. The seawall in Galveston was constructed shortly after the hurricane of September 1900, whose flooding caused approximately 6,000 deaths, to protect the city from such flooding in the future.

Bulkheads and revetments, on the other hand, are usually designed to protect property from only minor storms. Overtopping of bulkheads and revetments during major storm events can damage these structures or wash out material retained behind them, undermining their integrity and causing them to fail. Bulkheads are usually constructed of concrete and are smaller than seawalls. Revetments generally follow the natural slope of the shoreline and are often composed of large rocks.

Revetments are constructed on the shoreline to absorb the energy of incoming waves. They can be built from a wide range of materials and generally mimic the natural slope of the shoreline, dissipating wave energy as waves are directed up the slope. Like seawalls, revetments armor and protect the land behind them. They may be either watertight, covering the slope completely, or porous, to allow water to filter through after the wave energy has dissipated.

When Defenses Backfire

For many decades, the aforementioned approaches to reduce coastal erosion have been used across the country, and for the most part, these defenses have been successful. However, no matter how strong the structure, there are always examples of how the power of the sea and waves can overcome human efforts to protect the coast.

In some areas, smaller structures that are built to protect a minimal amount of property, like groins, bulkheads and jetties, have been blamed for the rise in erosion rates further down the coast. In addition, a groin or jetty may lead to the development of a sand formation away from the coastline and an unwanted lagoon in the shelter of the sand spit may develop. Basically, these structures can cause uncontrolled and undesired development or erosion in other areas of the shoreline.

These affects are not limited to oceanfront property, either. For example, a relatively new community on the Outer Banks in Avon bordered a saltwater canal that flowed into the Pamlico Sound. The community widened the canal and added a bulkhead to allow boat access and also to protect the homes that bordered the water. The bulkhead, however, stopped at the property line of the community, and outside the subdivision the canal remained narrow with grassy banks.

In 2004, Hurricane Alex surprised the Outer Banks by approaching as a tropical storm and making landfall as a Category 1 hurricane. With the arrival of Alex came soundside flooding and the water poured from the Pamlico Sound into the canal. When the water reached the end of the large bulkheaded portion of the canal, and ended up in the narrow section, it simply overflowed into both the new community and the properties that were right behind it. This area of Avon ended up being one of the hardest hit locations, due in significant part to the water rising from the canal.

Seawalls are also temporarily successful at protecting buildings or roads, but most seawalls fail sooner than their designers expect and the disintegrating walls often litter the beach with debris and place buildings at greater risk than before. Although they may sometimes achieve success as building protectors, research has shown that seawalls are commonly detrimental to beaches. For example, seawalls can have an adverse affect on beaches, causing significant erosion to the beach directly in front of the wall.

Seawalls influence the beach in front of them in various ways. Observations of a seawalled beach at Aptos, south of Santa Cruz in Monterey Bay documented that "swash that is reflected by a seawall is directed seaward several seconds earlier than swash on an adjacent natural beach, increasing backwash duration and velocity." This means that at high tide, when waves wash up against the walls, waves reflect back towards the ocean with much more energy than if the wall wasn't there.

These reflected waves often cause the sand beach in front of a seawall to erode twice as fast as an adjacent beach without a seawall. As the beach continues to erode, the seawall may also block natural replenishment of sand from the dunes or cliffs behind the wall. Reflected waves and the diminished sand supply may also degrade the sand bars and destroy the surf. The underwater part of the beach profile will become steep and deep, with a large drop off, sometimes allowing waves to reach the seawall without even breaking.

Coastal erosion will always be a concern for local residents and beach lovers, and the methods to protect the shoreline will always have some controversy. However, visitors and residents can take heart that the beaches off North Carolina's coastline have been accessible for thousands of years, and with the curbing of development and addition projects designed to protect the beaches from erosion, hopefully the beaches will be enjoyed for thousands of years to come.