Learn about coastal hazards

Chapter 6: Property Damage Mitigation Back from the Beach

*When a storm strikes a developed barrier island, the typical reaction is to cleanup the debris and rebuild. When Hurricane Frederic wiped out the Alabama-Mississippi coast in 1979 the community of Gulf Shores indicted they were coming back "bigger and better." The motto for the recovery from 1992's Hurricane Andrew in Florida was "We Will Rebuild!"

*Frederic cost $2.3 billion and 13 years later Andrew cost over ten times more at $25 billion. True, the come backs are bigger as relief money spurs redevelopment; cottages are replaced by duplexes, duplexes by condominiums, low-rise by medium-rise if not by high-rise buildings. But better? Not really, because rarely do the "come backs" result in planned communities based on island carrying capacity or planned island-wide mitigation of future property damage that will occur from future hazardous events.

*The lessons nature is trying to teach us are going unheeded. The assumption is made that you can't do much about hurricanes anyway! But if something isn't done to mitigate property damage, one or two more ten-fold increases in the cost of storm damage will break public treasuries, sink individual bank accounts, and erode the assets of insurance companies. Every barrier island property owner should be looking for new and better approaches to property damage mitigation. Every barrier-island community should be working to reduce the risk faced by their population.

How to Come Back "Better"

*The "traditional" property damage mitigation options (Chapter 5) such as rebuilding frontal dunes using sand fences, stabilizing sand with vegetation, replenishing the beach, relocating damaged or threatened houses, and even armoring the shoreline under exceptional circumstances, should continue to be applied (fig. 5.1). More rigorous design and construction requirements as well as zoning controls also are needed, and will continue to be developed. While all of these approaches are useful, they remain insufficient and, at times, insignificant because they do not take the entire island system into account.

*Initial preservation of natural environments, better recognition of coastal processes, conservation of sand and vegetation, recognition of the impact of historical storms, post-storm redefinition of coastal hazard areas, post-storm redesign of compatible development, augmentation and "repair" of island environments to enhance or restore protective capabilities of the natural setting, and public education need to be the basis for aggressive and effective mitigation programs. It cannot be overstated how important it is to increase the communication between property owners, managers, and coastal scientists.

*The more complete systems approach is to consider the larger area of related natural processes and materials including the entire island, the offshore shelf, inlets, and lagoon in a barrier island setting, or several city blocks inland for mainland ocean shoreline settings (e.g., the Grand Strand, SC). The following discussion outlines property damage mitigation options from this more "holistic" perspective, including: designation of inlet hazard zones (both potential and historic); building, augmenting, and repairing both frontal dunes and interior dunes; blocking shore-perpendicular and cross-island channeling features such as roads and finger canals, constrictions between buildings, and beach-access dune gaps; grass plantings and reforestation; change of road elevation and orientation by adding curves and rises; and development of long-term relocation plans for large buildings and entire communities (fig. 6.1). For example, Table 6.1 is similar to Table 5.1 but emphasizes mitigation of the entire coastal zone instead of just the beachfront.

Mitigation Based on Mapping

*Preliminary risk assessment through mapping identifies the distribution of low-risk areas, defined on the basis of high elevations and forest cover (fig. 4.1). These characteristics need to be conserved. Virtually all remaining risk zones are candidate areas for remedial enhancement of elevation and/or vegetative cover. The revised risk assessment map identifies specific areas of the island that might benefit from mitigation efforts aimed at particular processes (e.g., storm-surge flooding, existing or potential overwash passes, potential inlet zones for new inlet formation or migration of existing inlets, back-island erosion zones, interior flood zones, deteriorating dune fields).

*Evaluation of specific areas is usually tied to landforms and vegetation. Landforms (e.g., dunes) reflect the interaction of island processes with materials (e.g., wind and sand), and, in part, vegetation (e.g., dune grasses). Remember that landforms are dynamic (e.g., dunes migrate) and are important in the island's response to storms (e.g., protective role of dunes against wave erosion, flooding, and wind in the shadow of the dune) as well as the island's migration (e.g., dunes may move along or across the island platform).

*Vegetation almost always has a stabilization effect on the soil and topography from the trapping of sediment by grasses and mangroves to the anchoring of that sediment by dense mats of roots. Plant communities go through a succession when environments remain stable, from grasses to shrubs to trees that form the canopy of a climax forest that protects the diverse plants of the forest floor. Given the short existence of barrier islands and the fact that they are primarily sand, the soils which support the plant cover are very fragile in the sense of nutrient organic content and texture. Removal of vegetation usually is accompanied by the loss of this thin soil and its remobilization by wind and water. Reestablishment of any vegetative cover is difficult, and even the hardy native plants adjusted to salt spray and the harsh environment of barrier islands will not reestablish quickly when the soil is disturbed. The construction phase for buildings and services often marks the beginning of on-going problems for those same structures.

*Working mitigation plans can be expressed as a mitigation map for an island or community in the coastal zone (See Chapter 9). By way of specific examples the major environments of typical barrier island systems are reviewed below, and possible mitigation actions listed. Virtually every developed barrier island in America needs several or most of these types of mitigation applied if future property damage is to be controlled or reduced.

Island Interior Dunes

*The importance of frontal dunes was discussed in Chapter 5. Although frontal dunes were removed in the past, through mining or for a better ocean view, few would argue the valuable role played in property damage mitigation by healthy, vegetated frontal dunes. Dunes found on island interiors also serve an important function with respect to property damage mitigation. Dunes provide elevation and protection (buffer) from storm surge flooding and waves. Their height can act as a shield against damaging winds. Frontal dunes are protected in many states, but interior dunes often are not given the same consideration. As a result, precious sand volume is reduced and elevation lowered as has happened over and over again. Bogue Banks, NC (fig. 6.2) illustrates this problem. Near the western end of Bogue Banks are the highest sand dunes on the island and the largest in North or South Carolina south of Jockey's Ridge, NC. Large volumes of sand were removed from this location in order to provide a flat siting for residential development (fig. 3.7). The original dunes were very high and two rows were completely removed. The highest dunes, just to the west of the end of Ocean View Drive, are over 35 feet high. Here one can see the sharp bulldozed edge of the natural dune field and can get an idea of the magnitude of sand removal. In terms of storm damage mitigation, dune removal creates very serious hazards from floods, storm surge, and waves. Fortunately, onshore sand supply was great enough, and property construction here was set back far enough, so that sand fencing was effective in trapping sand and rebuilding dunes to afford some protection for property owners from the threat of hazardous overwash. However, the new dunes are nowhere near the volume of the original dunes.

*The lesson from Hugo in neighboring South Carolina was that the higher, wider, and more continuous the dune system, the greater the protection against damage. On Bogue Banks, continued sand fencing and even addition of sand from an off-island source are good options for frontal property. If a wide healthy beach exists, island-front dunes have a sand supply for healing and growth. Interior dunes on developed islands do not. The sand supply is covered by houses, lawns, and streets. Once an interior dune is destroyed, it can only be rebuilt or repaired artificially. The better choice is to conserve the dunes and design the architecture around and over the landforms. Damage potential will be less, the costs of dune reconstruction eliminated, and a safer, more aesthetically pleasing development will result. Table 6.2 provides a brief description of field trip stops shown by circled numbers in Figure 6.2.

Coastal Vegetation

*Natural coastal vegetation, where little disturbed by development, offers some of the best defense against property damage during storms. For very large storms, no amount of vegetation or dense forest cover may be sufficient. For many moderate-sized storms, however, dense forest, especially native-species of maritime forest, provides significant protection to buildings.

*Maritime Forest.--Maritime forests usually grow only on stabilized dune systems and generally on the back sides of islands. Although true maritime forests grow in areas exposed to the ocean, they flourish where island width, topography, and orientation provide sufficient protection from storm exposure (Bourdeau and Oosting, 1959). Such forests have evolved to survive under the harsh conditions found within the coastal zone, including salt spray, wind shear, nutrient-poor soils, and low water availability (Barbour et. al., 1985).

*The protective nature of maritime forest was well illustrated when Hurricane Hugo hit Pawleys Island, SC in 1989 (fig. 6.4). Overwash penetration and storm wave damage to property was noticeably greater where maritime forest has been removed for development. Neighboring houses suffered vastly different degrees of damage from Hurricane Hugo. Many houses located within the maritime forest were essentially untouched except for some cosmetic damage. Many houses built in cleared areas were destroyed. Dauphin Island, AL provides a similar example. Hurricane Frederic, 1979, did extensive damage to the unforested western segment of the island, while the forested eastern segment suffered less damage. That pattern of destruction on the island was repeated in later events.

*The densely forested, higher elevation areas are the most stable and lowest risk portions of barrier islands during a storm. Every measure should be taken to protect and preserve forest growth. Other aspects of island vegetation are also noteworthy and discussed below. Devegetation of the coastal zone only increases the susceptibility and likelihood of storm damage. Not only should as much forest as possible be retained, but also where appropriate, areas where trees have been removed should be reforested with native species. Once newly built dunes are stabilized with grassy vegetation, forest growth could be encouraged.

*The town of Pine Knoll Shores, Bogue Banks, NC is one of the most enlightened communities with respect to sound development practices for living with the shore. Town restrictions dictate that a permit is needed to cut down any tree over 2 inches in diameter. The community also controls density of development, attempting to preserve the ground water as well as minimizing destruction of vegetation. It helps that much of Pine Knoll Shores is on a naturally elevated part of the island. Their secret is: develop on the lower-risk portion of the island and protect the natural amenities that make it less vulnerable as well as a more pleasant place to live.

*Bogue Banks also provides common examples of the "bad" including extensive clearing of maritime forest (as well as leveling of interior dunes). In one example, near the western end of the island (fig. 6.2) about 40 acres of maritime forest were removed and sand dunes were leveled for the siting of a large motel complex (fig. 6.5). This occurred before laws were in place protecting frontal dunes. These buildings sit on top of what amounts to a sandy bluff, 12 feet or so high, eroding at a long-term average rate of 2 feet per year. In addition, several nearby roads run perpendicular to the shore and the dunes are notched allowing for increased overwash penetration and storm-surge ebb flow.

*There really was no need to clear so much forest for the development. The motel is left with a large, flat lawn instead of natural nearshore vegetation. The lawn will cost more to maintain and will offer no protection from storms. Recommendations for this area are to rebuild the frontal dunes, and the interior dunes as extensively as possible. The maritime forest should be re-established, although that is a much longer-term project. Ocean ends of roads should be closed (plug dune gaps), or curved and elevated where possible.

*Mangroves.--The dense mangrove forests of the south Florida mainland helped reduce the coastal impact of Hurricane Andrew in 1992. Mangroves serve as a primary sediment trapper and anchor for tropical/subtropical barrier islands and low-lying shores. When mangroves are removed for development, to provide boat access or simply provide an unobstructed view, shoreline erosion greatly accelerates (fig. 6.6).

*The protective effects of mangroves and coastal vegetation in general were noted on the coast of the Yucatán Peninsula, Mexico after Hurricane Gilbert in 1988. South of Cancún, the eastern (Caribbean) coast of the Yucatán is largely undeveloped except for the towns of Puerto Morelos and Playa del Carmen. Here is a striking example of the protective value of natural, undisturbed grasses and forest. In zones of undisturbed vegetation (fig. 3.18-A) the penetration of overwash sand was limited to a few tens of meters except along the one shore-perpendicular road. In the development in Puerto Morelos, however (fig. 3.18-B), where the natural vegetation and dunes were removed, overwash sand penetrated the width of the development, a distance of at least 200 meters.

*Lagoonside Marsh Growth.--Salt marsh grasses, typically Spartina, are efficient trappers of fine-grained sediments in the shallow lagoons and tidal mudflats behind barrier islands and along embayments. The extensive salt marshes behind America's barrier islands trap tons of sediment before it reaches the ocean, providing the substrate and nutrients that sustain the associated ocean fisheries for both shellfish and fin fish. Marsh grasses thrive on burial and trap muddy sediment by baffling wave and current energy. A marsh meadow essentially eliminates wave energy. When marshes die or their grasses are removed, rapid erosion begins immediately. Reestablishing marshes is a very effective way to combat erosion on the back sides of barrier islands, and other low-energy salt water shores.

*A salt marsh was cultivated to control erosion on the lagoon side of central portion of Bogue Banks, North Carolina (fig. 6.2) at the Pine Knoll Shores Country Club (fig. 6.7). A salt marsh was successfully cultivated here to stabilize a shoreline which was eroding at a rate of more than 20 feet per year (Stancyck, 1975). A narrow strip of salt marsh, only a few feet wide, was planted around 1973. Today the marsh is over 100 feet wide. The salt marsh acts as buffer to wave action and is a simple way to build up the lagoonside shore and reduce the effects of a major storm. Artificial plantings of salt marsh grass to establish protective marshes is a common mitigation practice for low-energy shorelines. In Chesapeake Bay show that success of marsh planting as a stabilizing barrier is partly dependant on fetch. Planting is most successful where less than one mile of open water is available for wave generation. Large fetch allows large waves during storms to prevent the establishment of marsh.

*Backbarrier marshes help to contain flood waters, dampen lagoonside wave energy, and add width and elevation to the island by trapping sediment. The back-island shores, however, may be bounded by tidal creeks and channels, or artificial channels such as parts of the Intracoastal Waterway. Where marsh is absent, the backsides of islands tend to be erosional. Bulkheading has been the response to lagoonside erosion. Yet, this ultimately decreases the island's width, and in a scenario of rising sea level could weaken an island's defenses (e.g., Topsail Island, NC).

*One of the main processes by which an island migrates in response to sea level rise is by sand overwashing the island during storms and being deposited in the lagoon. Bulkheading of the lagoon side of an island tends to prevent overwash sand from reaching the lagoon by interrupting the sediment transport, and keeping the lagoon shore from migrating toward the mainland. Bulkheading tends to help maintain depth on its lagoon side so that marsh grass does not reestablish growth. Bulkheading may also cause wave reflection and increased scouring; enough to erode or discourage marsh grass growth. As erosion continues on the front side, the island will tend to narrow.

*The island narrowing problem is real, though there is no way to predict the time frame over which it will occur. It could be centuries, or perhaps only decades, depending on the sea-level rise, storm climate, sediment supply, and other variables. Even some mitigation recommendations might exacerbate the problem of island narrowing. For example, methods to reduce overwash certainly prevent the island from natural migration (and elevation increase) in the long term. A method that is used in some places to combat narrowing is to replenish the lagoonside shore of the island; in effect, acting as artificial overwash. Planting marsh grass and conserving existing marsh are alternatives to bulkheading.

Orientation and Placement of Roads and Services

*After observing damage patterns from Hurricanes Gilbert (1988), Hugo (1989), and other storms, it is clear that road orientation, design, and placement usually are major factors contributing to increased storm damage. Some of the problems stemming from standard road development plans and practices can be remedied or at least ameliorated. Most mitigation measures concerning development are along the line of engineering and building codes. Buildings and services may be built stronger, but their placement is not consistent with the natural processes to which those structures are subjected. Such considerations should be addressed by the engineering community. For example, roads, streets, water lines, and other utilities are laid out in the standard inland grid pattern. Buildings block natural flow (e.g., overwash) while the ends of streets and gaps between rigid buildings funnel and concentrate flow, accentuating the erosive power of flood waters.

*During Hurricane Hugo, for example, water, sand, and debris were carried inland along shore-perpendicular roads in several South Carolina communities. On the northern end of Pawleys Island such roads acted as storm water conduits which led to a great deal of property damage. In addition, Pawleys' boat ramps provided ideal conduits for the return of storm waters (storm-surge ebb) back onto the island from the lagoon. Storm-surge ebb caused scour channels which undermined roadways and damaged adjacent houses and property. Such damage patterns were widespread after Hugo, and in Mexico after Hurricane Gilbert.

*If the direct line created by straight roads perpendicular to the shore could be interrupted, the amount of damage done by overwash, and storm-surge flood and ebb waters could be reduced. Exactly how to block roads in order to reduce the potential for overwash and/or storm-surge ebb may generate controversy. Obviously, if major gaps exist in the frontal dune ridges, the gaps could be plugged as described below. Simply building a small "bump" on the oceanward terminus of the shore-perpendicular roads could at least slow down the storm-surge ebb velocity enough to reduce the scouring potential of the flow, and reduce or delay the intrusion of storm waves into the community. Such a design is presented in the Texas General Land Office (1991) booklet (see page 18, Figure 30, of that report).

*Just adding a few simple curves in roads, instead of all access roads running perpendicular to shore, would greatly reduce the impact of overwash and storm-surge ebb. In Volusia County, Florida, east-west roads parallel Township and Range lines and are thus at some angle to the shoreline (fig. 6.8). This was probably a simple coincidence of "laying out the city" but will probably help lower property damage during a major event. The Lands End subdivision near the western end of Bogue Banks, NC has a very curved road layout (fig. 6.9). While it is likely this was done for aesthetic reasons, it certainly will help mitigate property damage in the next storm.

*Typically, more recent developments and planned island communities such as Kiawah and Hilton Head islands, SC avoided shore-perpendicular roads. Older communities must consider relocating roads, adding curves, or otherwise interrupting the grid pattern so as to eliminate the conduit effect. Placement of T-mounds (fig. 6.10) at alternate intersections is effective, and does not place any demand on private property (as road relocation might). Traffic flow is reduced in neighborhoods, and the T- mounds can be designed to function as mini-parks, playgrounds, or for aesthetic plantings. T-mounds can serve a local function of protective dune. Similar sediment mounds can be constructed around other likely flood conduits, e.g., at the ends of finger canals, or on the back side of the island in the vicinity of boat ramps.

*Road orientation and placement can have a major impact on interior areas flooded, overwashed, and subject to storm-surge ebb. It will not, however, reduce frontal wave impacts or have a significant mitigative effect against waves in the V-zone. Although services and utilities do not contribute to erosive processes like roads, their presence can create the mentality of a "line in the sand" which must be protected or held. Instead, their placement can be planned with hazards in mind. For example, buried lines may be less subject to wind damage, however, burial must be deeper than anticipated depth of scour and out of any erosion zones. Trunk lines should be in the interior of the island.

*Road Elevation.--Every community on Topsail Island, NC should prohibit the construction of any new structures seaward of the main road. A particular problem is the population density on the northern end of Topsail which is at a critical point. State Route 1568 is often overwashed along a wide front, cutting off the only evacuation route from the northern end of the island. Some spots on this solitary road are overwashed even in moderate storms and are protected only by a small, essentially inconsequential, sand ridge that is artificially maintained by bulldozing (fig. 6.11). Moreover, the newly built section of the road, where it has been relocated landward to the backside of the island, is in danger from flooding because of its low elevation.

*When roads are located, or rebuilt/relocated after storms, they should be elevated for several reasons. Most fundamental is that the road should be above the initial flood level of a storm in order to allow for evacuation. The added elevation may offer some protection against storm-surge currents if not flooding. However, road elevation design must give careful attention to natural processes so as not to interfere with natural drainage or sediment transport. This applies as well to overwash sediment which was described in Chapter 3 as nature's way of building up island elevation as well as moving an island landward during a rising sea level. If the sand is removed every time a road is buried by overwash, then island elevation cannot build up. This management strategy is used along NC Highway 12 on Hatteras Island and has resulted in Highway 12 being in a "furrow" along part of its length (fig. 6.12). Wouldn't it be better to let island elevation build up naturally, sacrifice the paved section of road buried by overwash, and replace those short sections of paved road with (very inexpensive) gravel road. Sure, it would mean having to drive at 35 mph instead of 55 (or faster), but what an incredibly inexpensive, environmentally sensitive option!

*These same concepts apply when designing bridge approaches and access causeways to islands. The causeway to Figure Eight Island, NC blocked the tidal flow in the salt marsh crossed by the causeway resulting in loss of marsh and impact on the protective environment of the backside of the island. Maintaining the natural flow and currents in the marsh wetlands should be part of any construction design.

The Complexity of Barrier Islands: Island Morphology

*Modern barrier islands rim most of the USA Atlantic Ocean and Gulf of Mexico coasts. These relatively young geologic features are only the most recent of several generations of barrier islands that have existed in the same or similar locations over recent geologic time. Sea level has risen over the continental shelf countless times, each time bringing with it another generation of barrier islands. Each series of islands were left stranded as the sea, inevitably, began to recede. Depending on the maximum level of the sea, some islands were at higher elevations and more inland positions than the present ones; and some were at lower elevation and more seaward positions. The coastal plains are filled with remnants of ancient barrier islands. More significant, many present-day barrier-island complexes include portions of these older barrier islands, and some mainland shorelines are formed along these older, stranded barrier islands (e.g., the Grand Strand, SC).

*Ancient barrier islands can play an important role in the relative risk of a modern island with respect to potential property damage, as is the case in Kitty Hawk, NC. Here, a remnant of the ancient barrier island remains to form the back side of the island complex. The higher elevation, forested portion of Kitty Hawk was left stranded by an ancient, higher sea level (fig. 4.14). The modern island (today's beach, dune, and overwash fan system) has been welded onto the ancient island, and the island complex illustrates how the island's geologic history controls landforms, processes, and materials, and forms a basis for a rational management program.

*Kitty Hawk is an old Outer Banks community, first settled in the mid-1800's. The town is bordered by Southern Shores to the north, and Kill Devil Hills to the south. Predominantly a tourist community, Kitty Hawk has approximately 1,700 full time residents, with the population swelling to well over 15,000 during the summer months. The island is over 2 1/2 miles (4 km) wide with significant areas of high-elevation dunes, mostly on the back half of the island along Albermarle Sound.

*The primary dune along the beach of Kitty Hawk ranges in height from 0 to 20 feet with approximately two-thirds of the beachfront possessing dunes below 10 feet in height. The original dune was constructed in the 1930's as part of the huge artificial dune line extending from the Virginia Border to Ocracoke, NC. Today, where a dune does exist, often in front of lots with no structures, gaps in excess of 10 feet are common and the dune is usually unvegetated and contains shell material, indicating that the sand was bulldozed. The three-mile stretch of Kitty Hawk ocean shoreline is experiencing average erosion rates of approximately five feet per year (NCDCM, 1992).

*The interior portions of Kitty Hawk are characterized by low, flat, partially-vegetated land, and this region is extremely vulnerable to overwash, storm-surge flooding, and wave action during storms. Significant vegetation exists in places, including live oak, pine, grass, and shrub thicket. The dominant feature of the island's interior is Kitty Hawk Woods, a combination of dense forest and marsh vegetation on Roanoke Sound. Elevation ranges up to well over 20 feet.

*Development along the beach of Kitty Hawk consists predominantly of small, 30+ year-old wooden homes, most elevated between 6-10 feet on wooden pilings. A large number of these homes are located shoreward of the primary dune field, within 100 feet of the high tide line. Figure 4.14 shows typical Kitty Hawk settings and development. Dozens of buildings have either been lost to erosion or moved back during the last two decades.

*Refer to Figure 2.2 and note that in a typical modern barrier island, the front (ocean) side has the highest elevation because frequent overwash events add sand and elevation to the island. The front side is also where the strongest winds blow, building loose sand up to form dunes. Usually, island elevation decreases from the ocean toward the lagoon, though dune field width and distribution may interrupt a simple profile.

*In contrast, the result of the two islands merging together is a high elevation back side of the island (the ancient island core) and a high elevation front side of the island (the modern overwash apron and dune field) with the lowest island elevations found in the island interior between the dune fields. In the case of Kitty Hawk, two other factors are important. First, the ancient island core is much higher in elevation than the modern, so the highest island elevations occur on the very back side of Kitty Hawk. Second, the frontal dune in Kitty Hawk (and along the entire Outer Banks of North Carolina) was artificially constructed during the 1930's. This dune has blocked the overwash process from occurring for some 60 years (Dolan, 19__) which seems like a good thing for protecting the island from flooding. However, the real end product is an island that has been cut off from its elevation-increasing process for decades, resulting in an island that is now lower than it would be otherwise. Today, the artificial dune line is breached in places and about to be in others, meaning smaller and smaller storms will be able to flood the island. Where will the flood waters go? Right into the "bowl" that is the lower elevation central portion of the island.

*The likelihood for Kitty Hawk (and other Outer Banks communities) is that the flood frequency will increase as the dune continues to be destroyed and the dune breach widens. Here is an example where some oceanfront homes may have to be sacrificed in order to have room to rebuild a dune to protect the island's much more extensive interior property. Of course, this solution perpetuates the problem of cutting off the overwash source of island sand. Artificially building up the island's interior elevation will be difficult, given the density of development, but such an option shouldn't be ruled out.

Finger Canals

*A common man-made island alteration that causes a variety of problems is the finger canal (fig. 6.13). "Finger canal" is the term applied to the ditches or channels dug from the lagoon or soundside of an island into the island proper for the purpose of maximizing the number of waterfront lots. Canals can be made by excavation along, or by a combination of excavation and infill of adjacent low-lying areas (usually marshes). The resulting substrate is not the best in which to anchor building supports.

*The major problems associated with finger canals are the (1) lowering of the groundwater table; (2) pollution of groundwater by seepage of salt or brackish canal water into the groundwater table; (3) pollution of canal water by septic seepage into the canal; (4) pollution of canal water by stagnation due to lack of tidal flushing or poor circulation with sound waters: (5) fish kills generated by higher canal-water temperatures; and (6) fish kills generated by nutrient overloading and deoxygenation of water.

*Bad odors, flotsam of dead fish and algal scum, and contamination of adjacent shellfishing grounds are symptomatic of polluted canal water. Thus, finger canals often become health hazards or simply places near which it is too unpleasant to live. Residents along some older Florida finger canals have built walls to separate their cottages from the canal! If the canal acts as a sediment trap, its function as an access navigational channel may be reduced over time.

*Finger canals have another destabilizing affect. Where canals cut deep into the island's interior almost to the ocean side, storm-surge ebb flow may be funneled through the canal and cut an inlet or adjacent finger canals may lead to lateral breaching creating small backside islands (e.g., Alabama, Hurricane Frederic, 1979; (fig. 1.3). Numerous finger-canaled barrier islands will be breached along these zones of weakness in future storms; creating greater risk during the storms and expensive post-storm island restoration. Pre-storm mitigation is recommended.

*As previously noted, building mounds at the landward end of finger canals or other potential points of breaching is a simple method of mitigation. Shortening existing canals by infilling their heads is a more costly approach, but may be warranted. In new developments, finger canals should be permanently banned. Sometimes there are trades, that is, old canals filled if new ones created. Check your state and local management office. When inlets do form due to finger canals, the post-storm reconstruction offers the opportunity to fill in both the inlet and the offending canal so as not to repeat the problem. Chapter 7 discusses the hazard set that is specific to inlets.

Swashbuckling

*Somewhat analogous to inlets cut through finger canals are the surge channels eroded across the islands interior topographic lows, either natural or man-made when development plats are excavated and leveled. These shallow waterways form during storms or from freshwater runoff and are known as breaches or swashes.

*The Grand Strand, SC area is a mainland coast that has a series of "swashes," instead of inlets at ends of barrier islands. Swashes are small, in some cases intermittent, streams draining fresh water from the mainland. They are also very low elevation areas and particularly prone to flooding, storm overwash and storm-surge ebb funneling. Development near swashes places property and infrastructure in danger. In addition, the main shore-parallel roads of the Grand Strand area pass over these swashes on small bridges.

*Swash locations are easy enough to spot. Even though they may be dry or have only intermittent freshwater flow, during storms they act like sluiceways. To reduce property damage, no more development should take place in these hazardous zones. Swashes and storm breaches should be treated more or less like inlets on barrier islands (see Chapter 7). The swashes will migrate or meander just as typical rivers and streams do. Property located behind the swash, even if well inland, is also in danger because of the low elevation of the swash area and the potential from swashes being exploited by storm surges.

*To completely fill the swashes is not reasonable because channels for storm water drainage must be provided. Replacement of some of the swashes with storm sewers is a possibility, but such systems sometimes cause flood waters to back up, raising flood level. Before modifying these natural drainage outlets, input from an urban storm water hydrologist and/or engineer should be sought to determine the impact on adjacent properties, including inland areas.

Hazards Under Construction

*Much of the damage to barrier island landforms and forest occurs during the construction phase of a building and can be prevented. Even for just a single-family house, land is leveled, dunes notched, and vegetation removed. These changes are usually meant to be temporary, but in reality, the damage is done, the risk greatly increased, and reconstruction is difficult. Buyers should work closely with architects and contractors to design and build in conformity with existing topography and natural vegetation. The class developments on barrier islands are the ones that have kept dunes and forest intact (e.g., Kiawah Island, SC; interior developments on Hilton Head Island, SC; Pine Knoll Shores, NC). The aesthetics are better, the resale value is higher, and better yet: the property is more likely to survive until its resale date!

*Offshore Rubble.--Landward transport of frontal houses and associated debris is part of the man-made construction hazard. Buildings are carried into island interiors, or across the island into the marsh; unless another house is encountered and smashed along the way. Less well known is oceanward transport of debris off of the island. Gayes (1991) in a side scan sonar survey taken immediately after Hurricane Hugo found a considerable amount of debris that was carried offshore by storm-surge ebb. In addition, he found that storm-surge ebb waters were funneled by development, carving channels into the nearshore environment. Small "deltas" of sediment and debris also were observed directly offshore of the swashes along the Myrtle Beach area. Rubble carried offshore into the ocean, inlet, and marsh during a hurricane is a real hazard to swimmers and boaters. A "Potential Debris Inventory" should be prepared immediately after major storms so that potential hazards can be identified and removed as part of the post-storm reconstruction.

*Manufactured Housing.--Manufactured housing presents a special case in terms of the construction-related hazard. Several coastal communities have very high densities of mobile homes. For this reason alone they would suffer extensive damage in a major storm. Mobile homes provide a different problem with respect to mitigating property damage, and their presence should raise a red flag for community planners and nearby property owners. Because of their vulnerability to wind damage, locating mobile homes in wooded areas and conserving protective vegetation is even more important. Their light weight makes mobile homes susceptible to overturning and floating, creating a hazard to adjacent units and other structures (e.g., missiling and ramrodding) (fig. 6.14). Mobile homes must be tied down as securely as possible. Again, trees and dense shrub cover between housing units may offer additional protection. In many instances mobile home parks are located on the lagoon side, on marsh fill (no longer legal), and at very low elevation, making them susceptible to flooding. Such parks should not be allowed intermixed with permanent dwellings, but similar facilities for day campers and RV's that can be evacuated might be permitted. The use of low elevation coastal areas for RV camps north of Myrtle Beach, SC is a good example. Again, avoid grid street patterns in mobile home parks.

Case Study: A Plan for Folly Beach, South Carolina

*Folly Island and the community of Folly Beach were introduced in Chapter 3 in the context of the impact of Hurricane Hugo. The island provides a case example of some specific hazards and ways to mitigate such storm damage. Although these approaches are suggested specifically for the community of Folly Beach, they can be utilized anywhere.
The general theme is that the community needs to develop a long-term migration plan; to roll with the shoreline migration, but in an organized fashion. Building elevation, particularly through building dunes and other sand additions and reestablishing a protective vegetation cover are ongoing objectives, but at the same time, small-scale projects and local actions can reduce potential losses immediately. Relocating buildings must also be incorporated into the intermediate time frame of the plan. Clearly, roads running perpendicular to the shoreline increase the potential for overwash, local storm surge flooding, and storm-surge ebb scour. The effects of these processes could be reduced easily and relatively inexpensively. The proposed method is to add T-mounds, artificial dunes, or other natural obstructions at critical points to block some of the paths for storm surge and overwash, as well as blocking paths that storm-surge waters would exploit in flowing back to the sea. Simply blocking the conduit effect of some of the roads running perpendicular to the shore would slow and dissipate the future return flow, or channel the water through undeveloped zones. The likelihood of increased flooding caused by ponding of flood waters is minor compared to the potential surge damage, and flooding will occur anyway unless the entire island is raised in elevation. Surge waters would be slowed enough to flow back to sea at a rate that would not cause scour, but not slowed enough to increase flooding significantly. Such a solution needs the design input of a hydraulic engineer.
*Selective addition of sand should include constructing a series of artificial sand dunes wherever possible on the island to add as much sand volume as practical, and to block up as many of the shore perpendicular (and in some cases, shore parallel) roads as possible. Figure 6.15 shows such a hypothetical dune, 30 feet wide at its base, 10 feet wide at the top, ten feet high, and 200 feet long. The volume of sand contained in the dune is 1500 cubic yards. A reasonable higher than average cost of emplaced sand in South Carolina is $5 per cubic yard. As such, this dune would cost about $7500. Again, sand placed on the island is "permanent". The same volume of sand placed on the beach for beach replenishment would be almost insignificant, as well as being removed quickly, probably by fair-weather waves, before it had a chance to afford any protection against storm waves. Such island interior dune construction projects are independent of shoreline mitigation, but should be given serious thought for inclusion in all future beach nourishment projects when large sand volumes are imported, and the equipment is in place for moving sand, thus reducing costs.

*Changing the orientation of some streets, so that they do not run directly perpendicular to shore, will likely decrease some of the effects of overwash and storm-surge ebb. Figure 6.16 shows three site-specific plans to reduce the impact of overwash and storm-surge ebb on Folly Beach. The circles represent structures (mostly single family homes). The shaded areas are dunes of the same volume shown in Figure 6.15 but in the shape and orientation as shown in Figure 6.10. The three examples on Figure 6.16 show approximately the same amount of dune building, but increasing amounts of street re-orientation. One important restriction placed on the proposed design is that no buildings were to be moved.

*Relocation is an option gaining popularity, but it is often difficult to get public support in situations where houses cannot simply be moved straight back on an owner's property. Folly Beach is so densely developed that in order for relocation to work, people would have to move their homes to other parts of the island, or off the island. It should be pointed out that demolition and rebuilding elsewhere is also considered a form of relocation. Economics, construction design, and lot size dictate the preferred method.

*Figure 6.16-A shows simple dune building and blocking of intersections with no buildings moved and access still easy for all residents. Figures 6.16-B and C show increasingly invasive forms of street blocking and re-orientation. Figure 6.16-B depicts minor road building, and moving of parts of roads, but basically it suggests simply blocking and curving road intersections to cut down the number of straight through passes for storm waters. The artificial dunes are placed as shown by the shaded areas. Points of placement should consider where some reduced ease of access would be considered beneficial (e.g., less through traffic), but all residents would be able to drive to their homes simply by re-routing around an additional block or so. Figure 6.16-C represents the most invasive (and thus most costly) plan where many of the streets are re-oriented necessitating building of streets on presently empty lots. The lots would need to be purchased by the town, donated, or traded for other land. Still no buildings would be moved. The T-shaped interior dunes (fig. 6.10) used to block intersections should be stabilizes either with vegetation,(native species) or with an erosion-resistant surface if designed for a secondary use (e.g., playground, park, overlook).

*Figures 6.17 and 6.18 show additional plans for blocking overwash and storm-surge ebb paths by artificial dune building. There would be little or no decrease in access, as the "buttress-shaped" artificial dunes block only a few intersections. The T-shape is a plan to get as much sand as possible perpendicular to the shoreline. Figure 6.17 shows 13 of the artificial dunes, a total of 19,500 cubic yards of sand. At a cost of about $7500 each, it represents approximately $100,000--less than the cost of one house! The same amount of beach replenishment sand would still be insignificant and temporary.

*Figure 6.18 shows an extremely invasive plan for blocking overwash and storm-surge ebb channels (that is, roads) by adding a tremendous amount of sand to the island. It is so invasive as to restrict access to some homes. The plan consists of completely blocking Arctic Avenue, the first shore-parallel road, as well as much of Ashley Avenue, the second shore-parallel road. Access to most of these homes would be from parking lots located in previously empty portions of the blocks. All homes to which access is available only from central parking lots are marked with a dot in the center of the open square representing the house. Areas with large "P's" are the proposed parking areas. The third shore-parallel road is left open as is the fifth. Access to homes in the third, fourth, fifth and sixth blocks is available from the third and fifth streets back, respectively. There would, however, need to be much re-designing of the driveways. Driveways that previously ran from the fourth street to homes in the third block would be re-routed to run from the third street to the fourth block, and so forth. A few homes on the backside of the island would need to be reached from central parking lots as on the front side.

*The plan shown on Figure 6.18 is unquestionably extreme, but money would be better spent, and serve the people better, if it were used to block roads and increase the sand volume of the island, rather than to replenish beaches, especially in the case of high density development of the central portion of Folly Beach. The artificial dunes shown on Figure 6.18 represent about 20,000 linear feet of the type dune shown on Figure 6.15. The total volume represented is about 150,000 cubic yards--a volume that would be a very small beach replenishment project. The portion of the community shown on Figures 6.16, 6.17, and 6.18 is between 3rd Avenue East and 7th Avenue east, across the width of the island. This is approximately one-half of the central part of the community. The total cost would be about $750,000; much less than a beach replenishment project and the sand is "permanently" emplaced. That is, the sand will still be there when the next major storm occurs, unlike sand placed on the beach which will start being removed immediately by fair-weather processes.

*Some obvious costs involved in the plan as presented need further study. For example, the dunes need to be vegetated in order to stabilize them. Vegetation plantings are often done on a volunteer basis, so the cost is variable. The reader is referred to some vegetation studies in the references, especially Broome et. al. (1982). In addition, the cost of re-orientation of driveways could be considerable if they are paved. Preferably, driveways are made out of gravel or left completely unimproved to maintain ground water recharge, decrease surface runoff, and reduce surge channeling.

*Questions may be raised about increased flooding on islands if some of the proposed storm-surge ebb reduction methods are employed. Based on observations made in undeveloped areas of the Yucatán Peninsula of Mexico after Hurricane Gilbert, it appears that flooding would not be a significant problem. Surges were comparable or higher in the Yucatán during Gilbert than on Folly Island during Hugo. Areas where several rows of well vegetated dunes existed were absolutely free of any evidence of storm scouring except where a beach access road existed prior to the storm. At that location, a 2 meter (7 feet) channel was scoured by storm-surge ebb flow. The dunes that had not been excavated slowed ebb flow enough to prohibit scouring, but did not cause any excess flooding. The point is that the surge water will return in time frames of hours if some road blocking method is employed versus fractions of hours (thus much higher flow velocities) when storm-surge ebb channels are exploited.

*It is recommended that less drastic methods than those described above be tested first. Perhaps something as simple as modified "speed bumps" on the shore-perpendicular roads could slow ebb flow enough to reduce scour. Similar features are illustrated for Texas by Texas General Land Office (1991, p. 18). Perhaps by adding some roughness to the road surface by paving with rounded pebbles would help, although increased turbulence might negate the benefits of slightly lowered flow velocity in terms of reducing erosive capability of the flow.

The Sand Commandments

*One of the great ironies of developed barrier islands is that everyone is living on a giant pile of sand, but the development has immobilized the pile! All of that sand, but communities are desperate for sand to nourish beaches and rebuild dunes---the landforms of sand actively or passively destroyed by the development. Sand is a precious commodity, in short supply on most islands and shorefront communities, and every grain should be safeguarded. Typical soft stabilization measures (as discussed in Chapter 5) include beach replenishment, dune building, beach bulldozing, and not much else. We propose additional measures as described below, especially the less traditional methods of adding sand and vegetation to the interior of the island.

*No matter what soft stabilization measures are utilized, the geological point of view takes into account the overall sand budget. The island, the beach, the dunes, the tidal deltas (old and new), and offshore features are all linked as one sand system. There are locations within this large sand system where sand is less mobile than others. For example, there is an advantage to adding sand to construct interior dunes over adding sand for traditional beach replenishment. Sand emplaced on the interior of the island will last much longer, at least in a several-decades time frame (i.e., until erosion catches up with it), whereas sand placed on the beach, while offering some protection, is easily removed by fair-weather processes as well as storm processes, and so it is temporary, that is, usually remaining less than five years.

*In light of the precious nature of sand and the concept of the overall sand system, there are some important "Sand Commandments" to keep in mind:

(1) Each island and oceanfront community should control its sand resources. No sand should be lost in offshore dumping or hopper dredging. All dredged sand should be dumped somewhere on the island. Not one grain of sand should leave the island!

(2) There should be no "Robbing Peter to pay Paul." Never should sand be taken from a low-risk part of an island to replenish a dangerous or higher-risk part of the island. By so doing, you only end up with two unsafe areas.

(3) Every island should own its own dredge so that when inlets are dredged for navigation, the sand can be put on the island's beaches.

(4) Every island and oceanfront community should have its own mainland sand source, located well inland away from the shoreline and the beach/dune systems of other communities.

(5) Every island community should have a "sand plan" as to where post-storm sand cleanup will be placed (e.g., to repair dune gaps, rebuild T-mounds, added to interior dunes, put back on the beach).

The sand commandments could be carried over into "Vegetation Commandments." Natural vegetation offers such good protection that not one blade of grass, not one shrub or tree should be needlessly cut, and so forth.

Abandonment
Relocation
Active (relocate before damaged)
Passive (rebuild destroyed structures elsewhere)
Long-Term Relocation Plans for Communities
Soft Stabilization
Adding Sand to Interior of Island
Rebuild interior dunes (including replacing roads)
Raise island elevation (build artificial dunes and raised terraces)
Infill ends of finger canals, or entire canal (potential new inlet)
Infill road cuts
Block interior cross-island roads (i.e., sand plugs, barriers, dead-end streets into forest)
Vegetation (native species planting)
Replace forest
Plant tree/shrub thicket windbreaks
Stabilize interior dunes
Stabilize interior overwash terraces (grasses and shrubs)
Plant marsh (Spartina and other natural marsh plants
Modification of Development and Infrastructure
Retrofit Houses
Elevate Houses
Reopen Ground-Level Floors of Elevated Houses (that were enclosed)
Curve and Elevate Roads
Partially Block, or Replace Roads with Interior Dunes
Zoning, Land Use Planning
Recognize Hazard Areas and Avoid (No Construction in):
Tidal inlets (past, present and future)
Swashes and breaches
Critical Environments (e.g., V-Zones, interior marshes, spits, shifting sand dunes, freshwater ponds)
Choose Elevated Building Sites
Lower Density Development
By Ordinance:
Protect interior dunes and other topographic highs against modification or removal
Protect vegetation cover against removal or heavy disturbance

*Things to keep in mind:
Each island or coastal community is different
Consider entire coastal zone not just oceanfront
Rising sea level must be considered
Table 6.2. Outline of field trip stops for Bogue Banks, NC. This trip is intended to illustrate property damage mitigation principles. The numbers refer to the circled numbers on Figure 6.2.

Stop 1. Triple ESS Pier. Beach Replenishment Project.
Stop 2. Old overwash pass -- "effect of notching dunes". Also part of beach replenishment project.
Stop 3. Old development, filled-in salt marsh.
Stop 4. Preservation of dunes and new development; plus vulnerable construction in dune gap.
Stop 5. John Yancey Hotel. Old and well set back development.
Stop 6. Finger canal.
Stop 7. Pine Knoll Shores Country Club. Artificial saltmarsh shoreline.
Stop 8. Controlling soundside erosion by planting marsh.
Stop 9. Beachfront condominium development.
Stop 10. Hurricane Hazel -- former inlet site.
Stop 11. Hurricane Hazel -- former inlet site.
Stop 12. Roads -- potential for overwash passes.
Stop 13. Massive dune removal -- early development.
Stop 14. Dynamic inlet site -- Bogue Inlet.