Learn about coastal hazards

Chapter 7: The In-Between Case

*Inlets are located between islands, connecting backwaters and ocean. Inlets require special consideration in applying principles of property damage mitigation because a special set of natural processes are at work. Communities and property owners close to existing and potential inlets face a "special" set of hazards.

*Tidal inlets are not fixed features. Inlets can migrate in one direction or another, reverse direction of migration, open and close, narrow or widen, or follow any combination of these patterns. Because of these dynamics, some states (e.g., the Carolinas) designate inlet hazard zones or inlet hazard areas as special "areas of environmental concern" which are subject to more rigorous management requirements. You might be surprised to learn that the bulk of the sediment volume underlying most barrier islands was deposited in former inlets, indicating just how wide an area is influenced by inlet processes.

*Each inlet is unique and should be studied carefully before attempts are made at stabilization with jetties or "improvement" (i.e., dredging). Some examples of problems associated with modern inlet dynamics are examined in the following sections. Table 7.1 summarizes the range of inlet types. Relict and historic inlets are now closed, however, their location and recognition is important because inlets can reform in previous positions, as happens on Texas islands (fig. 7.1), or they may leave behind a characteristic geomorphic "signal" that may influence whether or not an inlet will reopen in a nearby location, as happens in the Carolinas. Even locations of artificially-closed inlets may be re-excavated in the next big storm and the inlet reestablished.

*Modern inlets, whether natural or artificial, often migrate over considerable distances eroding not only the adjacent shoreline, but erasing the entire downdrift island as new updrift island forms. Even so-called "stable" inlets, which may not be migrating, often show significant widening in adjustment to increased volumes of flow in storms, and stable inlets may switch to a migratory mode.
For property-damage mitigation studies, each type of inlet must be evaluated in terms of past, present, and potential future behavior. In most cases, historic behavior has proven to be a good measure of future behavior. And while the location and habits of modern inlets are very important, relict and historic inlets should also be considered as "inlet hazard areas" when evaluating island-wide risk and planning mitigation.

Inlet Dynamics

*Inlets form during storms. Most probably form when a storm system passes over an island and the water piled inside the lagoon by landward-directed winds is suddenly forced seaward by the reversed winds. This phenomenon is illustrated in Figure 3.12. The water forced across the island catastrophically seeks out low and narrow island stretches to carve new inlets. Sometimes existing inlets can handle the seaward flow of water but are greatly widened during a big storm. When Pamlico Sound water pushed seaward after the 1962 Ash Wednesday winter storm, the one-half mile wide Oregon Inlet widened to two miles in just a few hours. A number of inlets, especially in Florida, have been created artificially. In 1918 lake Worth Inlet at the northern end of Palm Beach was cut, probably by the use of mule and drag pan. The lagoon behind is filled with salt water but retains its fresh water lake designation from the pre-inlet days. The inlet at the southern end of Palm Beach (South Lake Worth Inlet) was cut in 1927. Further north, Fort Pierce inlet was cut by blasting through a hard, well-compacted limestone.

*Much is known about the behavior of individual inlets, though little of that information seems to be getting to coastal planners and decision makers. Inlets show two common types of behavior that may be categorized as "stable" and "migrating" (Table 7.1). Geologic studies have shown that tidal range is a major factor controlling the types of inlets present and the migratory behavior which they exhibit. Whether or not an inlet migrates is a function of longshore drift of beach sand. If there is a strong preferred direction of sand movement on adjacent beaches, the inlet will migrate in the same direction as sand pours into the inlet, forcing it to migrate. Some inlets are grounded in rock and are more or less permanent in their location, regardless of the tidal range or wave direction.

*All inlets have associated sand shoals called tidal deltas, formed when sand moving along shore is interrupted by the flow of water in and out of the inlet. A flood tidal delta in the estuary forms by the flooding or rising tide carrying water and sand into the inlet and the estuary. A corresponding ebb tidal delta forms in the ocean by sediment reworked in the ebbing flow of the tidal waters. The size of the flood and ebb tidal deltas is controlled by tidal range and wave energy. The greater the tidal range, in general, the larger the tidal deltas, and the greater the wave energy, the smaller the ebb tidal delta will be (fig. 7.2).

*Within the inlet is a channel that may occupy anywhere from 50% to 5% of the actual inlet width. In most inlets the channels change location depending on the flow of sand into the inlet (fig. 7.3). The location of the channel may be a factor in erosion of inlet margins. After storms, inlet channels tend to be "straightened out" as the result of large water flows exiting the lagoon during storm-surge ebb. Incoming and outgoing tidal flows tend to follow different paths across the tidal delta, resulting in a complex pattern of bars and small channels on both the ebb and flood deltas (fig. 7.4)

*Sand shoals that form part of the ebb tidal deltas where tidal range is large and waves are small, may extend for quite a distance on either side of the inlet (fig. 7.5). The ebb-tidal deltas off Georgia islands sometimes extend seaward more than two miles. The shoals actually act to protect some of the island shoreline by blocking or refracting wave energy. One result of the shoals' interaction with waves is a characteristic bulge in the island shoreline near the inlet. These bulges of sand form on either side of the inlet as a result of the interaction of ocean currents, tidal inlet flow, and sand supply in the nearshore system. The bulge of sand builds a wide beach, but one that moves as the inlet moves. Once the bulge moves on due to inlet migration, rapid "erosion" again occurs. As the channel migrates within the inlet, the location of the sand bulge changes accordingly.

*Some communities obtain sand from ebb tidal deltas for beach replenishment (Hilton Head Island) but this is not a wise move. Changing the shape of the tidal delta will change the nature of the waves striking the island, which will effect patterns and rates of erosion.

Relict Inlets

*The term relict inlet refers to any inlet that was open in the past but which is older than, and does not appear on any reliable map or chart. Relict inlets are inferred from geomorphic and geologic criteria. An excellent study of relict inlet locations along North Carolina's Outer Banks barrier island chain was provided by Fisher (1962). He investigated the geomorphic expression of former inlets and established the criteria by which relict inlets can be readily identified in the field. Distinctive geomorphic features remain as remnants on the island after the inlet is closed and also can be used along with historical and geographic information (charts, maps, ship's logs) to help locate historic inlets. Common geomorphic criteria include: old flood tidal deltas on the backside of an island where no inlet exists (the delta is constructed from sand moved into the lagoon by tidal flushing while the inlet was open), transected by old tidal channels (fig. 7.6). Fisher's study indicated up to 30 tidal inlets have existed for at least some period of time over the last 400 years along the Outer Banks (fig. 7.7). David Stick, Outer Banks historian, notes that more than 20 inlets have formed here, been around long enough to be named, and have then disappeared. Only three of those, Oregon, Hatteras, and Ocracoke are presently open and considered important waterways.

*Small islands can form in lagoons, landward of the inlet and tidal delta, and are preserved after the inlet closes. These features form as waves rolling through the inlet, washing sand and oyster shells up onto salt marsh banks. Such features are a sure sign that the adjacent barrier island area was once an inlet and that it should be examined for evidence of potential new inlet formation. The old relict inlet may have been stable, occupying a fixed position until closed, or they may have migrated. If they migrated, they may have left behind multiple incorporated flood tidal deltas and/or lines of small islands.

*The town of Atlantic Beach on Bogue Banks (fig. 6.2) illustrates the concept of locating development in a high-risk, relict inlet zone (fig. 7.8). The central part of the community is built on the natural fill of a historic tidal inlet which persisted from about the mid-1700's until at least 1800 (Fisher, 1962). Massive dune ridges characterize the island away from the low-elevation historic inlet area. Unfortunately, the area of most intense development had no such protective ridges and whatever sand dunes were present were excavated for siting of the dense development of Atlantic Beach. A great deal of property is at risk that otherwise would have been safe behind several rows of high dunes. Morehead Avenue, the main road leading to the causeway and the mainland, is located right in the this low elevation, historic, inlet location! In a big storm penetration of overwash waters will flood the evacuation route.

*Recommendations for this area are to build interior dunes (as discussed in Chapter 6) behind the front row of buildings and to build Morehead Avenue (the road leading to the causeway) up over a rebuilt dune. A drawback to replacing interior dunes, especially for the businesses right on the waterfront, is that the dunes will need to be constructed in these establishment's parking lots. This plan would be unpopular, but consider that the initial removal of sand for the siting of these establishments put all of the real estate located behind them at risk. The rule is to conserve sand. Another similar method of adding volume to the island interior was illustrated on Galveston Island, TX after the devastating 1900 hurricane that killed 6,000 people. The island elevation was raised by bringing new sand onto the island which helped greatly to reduce the risk of flooding (fig. 6.3).

Historic Inlets

*The term historic inlet refers to open inlets observed since the time of the first reliable charts and maps, but which are now closed. These inlets may be as old as Roanoke Inlet, through which Sir Francis Drake sailed in June, 1586, with supplies for the Roanoke Island Colony and of which only geomorphic remnants now remain, or as young as "New Pawleys Inlet" opened on the southern end of Pawleys Island, South Carolina by Hurricane Hugo in 1989, which was artificially closed immediately after the storm. Even younger is Little Pikes Inlet, opened across Westhampton Beach, NY in 1992 and closed in 1993 by the Army Corps of Engineers.

*In terms of storm damage mitigation, this area of the island should be treated as if it were a present day inlet and should be designated as an inlet hazard area. Several options exist for any site on a barrier island where inlet formation has occurred historically or can be predicted to occur in the future. These include: (1) add elevation with off-island sand to make it a less-likely location for a new inlet (i.e., build an interior dune field); (2) stabilize the margins of the area with vegetated sand ridges as if it were an inlet giving a degree of predictability of where a new inlet might open; (3) prohibit development in the area; and (4) preserve and even expand back- island marsh areas of the former tidal delta.

*Inlet Hazard Area--Historic Inlet, Folly Island, SC.--The pattern of erosion on Folly Island is such that there is a bend in the island outline near its northern end. The inflection point marks the area of an inlet that was open during the Civil War separating Folly Island from Little Folly Island to the north. Figure 7.10 shows how the islands looked during the Civil War. The inlet filled in naturally and its former location remains very low in elevation frequently overwashed by storms. Appropriately, this vulnerable zone is now referred to locally as "The Washout" and is said to be the second most popular surfing location on the east coast behind Cape Hatteras, NC.

*Arctic Avenue, the oceanfront road in the center of town, is gone--fallen prey to the high erosion rate in this zone. Ashley Avenue is the oceanfront road here. Houses and part of Ashley Avenue were destroyed in another large washout immediately north of Folly's inflection point. The houses will not be rebuilt because of relatively new South Carolina coastal regulations. Ashley Avenue has been rebuilt, however, and is located partially in the salt marsh because of the narrowness of the island in this area. A large revetment was emplaced to protect the road; replacing a smaller revetment that was destroyed by Hugo.

*South Carolina seems committed to saving Ashley Avenue through the washout zone to allow access to northern portions of the island for a small number of residents and for a satellite navigation station on Coast Guard property at the very northern end of the island. Relocation of the road is not possible because the island is so narrow that there is no room for the road to be moved back. This truly is an area that should be abandoned. The Coast Guard station is not active and the satellite navigation equipment certainly could be moved to a safer location. Since the structures and property are not essential or could easily be relocated, then the economically wisest option would be to abandon the buildings and leave the area for public recreation. Real estate interests often prevail over wisdom, however, and the purchasers of property in such locations aren't likely to be informed of how ephemeral the access to their new property may be.

Modern Inlets

*Modern inlets are, in a sense, also historic inlets. The distinction, as we refer to them, is that these have remained open. In other words, these are "today's" inlets, the ones we have to deal with and worry about in terms of their active processes with respect to potential property damage.

*A Stable Modern Inlet, Bogue Inlet.--The western end of Bogue Banks, NC at Bogue Inlet is a high-hazard zone. Here the State of North Carolina Division of Coastal Management has designated an "Inlet Hazard Area", recognizing the dynamic habits of the Inlet. Historically, Bogue Inlet expands and contracts depending on the channel location within the inlet, but the inlet itself does not migrate. When the channel is close to one margin or another, sand from the margin pushed into the channel is swept away. Structures in this low elevation portion of the island are subject to overwash and flooding. Areas within the known zone of expansion and contraction of stable inlets should be avoided. The study of historical records may establish an approximate set of limits for the inlet hazard zone.

*A Migrating Modern Inlet, New Topsail Inlet.--From studies of individual inlet behavior, more comprehensive and informed zoning and land use planning decisions can be made. New Topsail Inlet, the southern boundary of Topsail Island, North Carolina, is a good example of the "individuality" of inlet behavior. New Topsail Inlet historically has migrated consistently to the south. Professor William Cleary of the University of North Carolina-Wilmington, has studied the historical habits of North Carolina inlets in great detail, including the effects of inlet management, either by jetty stabilization or by dredging. He found that some of the general rules of inlet behavior predict very nicely the behavior of New Topsail Inlet, and the subsequent impact of potential property damage as well.

*Figure 7.11 is a photo-collage of long-term shape changes of New Topsail Inlet. Points labeled "0" and "1" are fixed so that southerly migration of the inlet to the left can be visualized. How the changing shape and orientation of the inlet channel changes the shape of the ebb tidal delta shoals, and the corresponding change in shoreline erosion also are shown. In addition, the channel itself can impact the shoreline position on the margin of the inlet and cause significant erosion.

*Figure 7.12 shows the history of the southerly migration of New Topsail Inlet. Between 1738, when the inlet opened, and the present, the inlet has migrated six miles! The dates on the figure refer to the year in which the inlet was at a given position.

*The main point of the figure is that, as the inlet migrates, the "drumstick" shape of the end of the island is translated in space. The "fat" end of the drumstick, the previously mentioned shoreline bulge, is moving with the inlet and thus is temporary at any one location. As the inlet migrates, the bulge moves causing shoreline retreat at its previous location. The Sea Vista Motel in Topsail Beach is located in just such an area.

*This natural process of inlet migration shouldn't cause any problem, except when development is unwisely sited within the active inlet zone, especially if located downdrift or "in the way" of the migration. In some states, such as Florida, few inlets remain in an unengineered state, without jetties, so inlet migration is now longer a problem. The migration problem, however, is replaced with a shoreline erosion problem caused by the sand trapping role of the jetties (Chapter 5).

*From Long Island to Florida to Texas, the coast abounds with communities sited next to inlets with histories of migration, and the cost of maintaining such inlets in stable (metastable) positions by dredging or through jetties is another major growing expense of the unreliable barrier island infrastructure. Those who are prudent in regard to their property investments will demand studies to define inlet hazard zones for land use planning. Better yet, prudent property owners will stay away from inlets. Figure 7.13 shows diagrammatically the differences in managing coastal zoning for migrating versus stable inlets.

*Migrating inlets can have a profound effect on property damage mitigation. The interior of Topsail Island is open and flat, a topography typical of younger barrier islands built in the track of a migrating inlet with insufficient time lapsed to form large dunes. Potential for new inlet formation exists at all narrow, low-elevation locations along this island, especially where finger canals are present. The frontal dune, where it exists, protects the interior "grasslands" and maritime forest.

*Topsail's opportunities for maintaining elevation naturally by overwash and dune formation have been altered through development because the frontal dune is kept fixed, not allowing sand overwash and dune migration to move sand in a landward direction. Thus, the island interior remains at low-elevation and narrow. This problem is common on most developed islands. The elevation of Topsail (and other islands) could be artificially maintained through a program of replenishment (or augmentation) of sand volume in the island's interior. Several methods by which this might be accomplished include sand fencing, bringing in new sand and building and stabilizing interior dunes with vegetation, and establishing maritime forests.

*A Dredged Modern Inlet, New River Inlet.--Some modern inlets are artificially stabilized or held in place through dredging. The inlet channel of New River Inlet, NC is artificially held in place by channel dredging which, in turn, determines the position of inlet-associated shoals, which provide protection for the town of North Topsail Shores on Topsail Island (fig. 7.14). As maintained, the inlet channel makes several sharp turns between ocean and lagoon. Left to its own devices, the channel would create a more direct, less winding link, and the inlet would migrate to the south as it has done historically. A storm is likely to realign the inlet through a narrow section of West Onslow Beach or at least to shift the inlet channel to the south. Once the inlet shifts, rapid erosion will occur on the northern end of Topsail, as it will no longer be in the "lee" of the inlet shoals.

*The dynamics of New River Inlet, at the northern end of the island, are illustrated in Figure 7.14, courtesy of Professor William Cleary. The figure shows photographs from 1938 and 1986, before and after the Army Corps of Engineers began maintaining the channel by dredging. White triangles on either side of the inlet in each photograph provide static reference points. Note the change in the size of the ebb tidal delta (the offshore sand shoals delineated by the breaking waves), the change in the width of the inlet channel, and the relative change in offset of the two island shorelines.

*The ramifications of maintaining the New River Inlet channel by dredging are: (1) the beach on Onslow Beach (part of the Marine Corps' Camp Lejune) to the north (right in the photos) is now severely eroding and steepening, making it difficult for beach-landing military vehicles and exercises; and (2) the northern end of Topsail Island is out of equilibrium as the inlet wants to migrate to the south, presenting a real danger from flooding and inlet channel switching that will occur during the next major storm. Development across the width of the island in this zone of potential inlet migration or reformation is at extreme risk.

*Figure 7.15 suggests possible mitigation for the very dangerous area adjacent to a migrating inlet. Obviously, the best way to reduce the potential for property damage is to remove the houses in this zone. Other than that, adding some interior island elevation, or well-vegetated sand dunes along the inlet margins may slightly reduce the potential of inlet channel migration during a storm.

*Relocated Modern Inlet, Sunset Beach, NC.--Sunset Beach is the southernmost developed barrier island in North Carolina, and actually trends almost east to west. The island is about 3 miles long and a little under a half mile wide. Sunset Beach has elevations between 10 and 15 feet, but only light vegetation in its central developed portions (i.e., few trees or large shrubs). What it does have along most of its length is a protective, wide, vegetated dune field. The central portions of Sunset Beach, between the sand spits at the east and west ends, has historically been accretional. In fact, almost everything (beach and shrub thicket) seaward of the development has built out in the last 30 to 40 years. This accretion zone is a buffer which was significantly reduced in size during Hurricane Hugo, but remained intact and protected the houses behind it. The dune field should be encouraged to continue growing and stabilizing for maximum protection.

*The presence of such a natural buffer allows the area west of the finger canals to be upgraded to high risk rather than extreme. Storm surge flooding from the tidal marsh and sound is still a significant threat. Everything east of the finger canals is in an extreme risk zone. Much of this land is actually artificial fill built on old marsh. Tubbs Inlet has a history of migrating to the west. In 1966 the inlet was relocated at public expense by the U.S. Army Corps of Engineers to the east, approximately to its 1930 location (fig. 7.16). Between 1930 and 1966 it had migrated all the way to the position of the last finger canal, a distance of approximately one-half mile. But the inlet will likely tend to migrate again. Unfortunately, when the inlet was artificially relocated, the "new" land (formed by pumping sand) east of the former inlet position was reopened or reprivatized, providing for a development rush into an extreme-hazard zone. Also, the wide dune field on the western end of the island narrows significantly to the east of the pier. Such relocation projects should be viewed by communities as an opportunity to build in mitigation to reduce risk potential. Instead of permitting new buildings on the newly-formed low-elevation sand flat, reconstruction of interior dunes and the establishment of vegetative cover could have been set as prerequisites for development. Better yet, no new development should have been allowed and the state or local governmental unit held title to land more suitable for public recreation, thus avoiding investment vulnerability and the associated extreme-to-high-risk.

Potential Inlet Locations and Newly Formed Inlets

*The lessons of Hurricane Hugo were clear--inlets can, and will, form across low-elevation, narrow portions of islands, during storms. Hurricane Hugo left Pawleys Island, SC with extensive property damage, overwash and a new inlet (fig. 3.11). Northeasters can form inlets, too. The 1962 Ash Wednesday storm formed new inlets across islands in New Jersey as well as at Buxton, NC. All these inlets were filled in artificially as have virtually all new inlets that have formed across developed barrier islands in the 1980s and 1990s.

*Examination of Pawleys Island revealed a spectrum of property damage that appears to have been a function of the presence of protective dunes, setback and vegetation. Relatively little damage occurred where houses were well elevated, well back from the beach, behind the frontal dune, and enveloped by dune and maritime forest. The most severe damage was found where the interior dunes and maritime forest had been removed for roads, houses, driveways and parking areas.
The low elevation, narrow spit on the southern end of Pawleys Island was, predictably, the site of an inlet formed by Hurricane Hugo (fig. 3.11). The new inlet cut off access to a few homes so it was filled in artificially, but plugging it up took two tries.

*Inlets on Padre Island, TX tend to be temporary. They occupy well-defined passes which become dry land again, shortly after the storm goes by. One of the passes near South Padre, TX is currently occupied by a condominium.

*New inlet formation was likely on Pawleys Island in such a low and narrow location and should have come as no surprise (Table 7.2). Historically, inlets had previously formed in the same locations and then migrated to the south. Regrettably, houses were rebuilt on precisely the spot where the inlet opened. Mother nature is giving us a hint here--avoid this entire spit!! A new inlet will open here again and the only viable mitigation strategy is to stay clear.

A fundamental rule is avoid both active and potential inlet zones.

Stop 13. Massive dune removal -- early development.
Stop 14. Dynamic inlet site -- Bogue Inlet.