**Once the coastal zone, especiallybarrier islands, is recognized as  unique, the next rule in assessing coastal hazards to reduce property damage is identifying the natural storm forces or physical processes which result in environmental impact and potential property damage. Storm processes, acting singly or in various combinations, are the destructive forces, the hazards, of concern (fig. 3.1). These are: wind; waves; coastal and inlet currents; storm surge flooding; and storm-surge flood and ebb currents. Wind, waves, and rising water receive a lot of attention during hurricanes, and in fact account for most of the damage. Currents are responsible for moving vast amounts of sediment during storms. The onshore movement of water, the storm surge, causes flooding and may induce scouring currents around and behind structures. The rising water level allows the zone of wave attack to move inland, and sediment to washover onto the land. Storm-surge ebb, or the seaward return of storm surge, is a less familiar storm process which may erode new inlets and contribute to the overall erosional damage. Table 3.1 and Figure 3.2 summarize the various storm processes and their effects, and provides a basis for defining areas in which risks are likely (Rule #4, see Chapter 2).
Natural Processes: Energy in Motion
**Storm processes rarely act separately. That is, wind, waves, currents, are all active at the same time and combine to form secondary processes. For example, storm surge is formed by several processes acting together, any one of which may be dominant during any given storm or for a given period during a certain storm: wind pushes water toward shore, waves push water toward shore, low pressure allows doming of the sea surface, and the rotating winds of a hurricane actually cause the shallow water near shore to spiral higher. In the following section, we look at storm processes individually, to better understand their actions during storms.
**Wind.--The most common and, often, the most costly of storm hazards causing damage to buildings is direct wind impact on structures, including flying debris (known as missiling). In addition, strong winds can destroy vegetation by uprooting and knocking over trees, defoliating trees and other vegetation, blow downs of shrubs and grasses, and by damaging leaves directly either by blasting leaves with airborne sand and/or by carrying damaging salt spray inland (fig. 3.3). The same salt-spray pruning effect that produces the nearshore sloping profile of maritime vegetation will kill or damage inland vegetation that is not salt tolerant. Strong winds can also be responsible for transporting sediment onto and off of an island (NWS, 1993).
**Storm waves.--Property is damaged by direct wave attack on structures, or by pummeling structures with floating debris (a process called ramrodding, fig. 3.4). Probably the only type of buildings capable of surviving direct wave assault unscathed are concrete pillboxes. Even lighthouses have gone down under wave attack. Waves are also responsible for shoreline erosion (on both lagoon shores and ocean shores), as well as dune erosion, overwash, and destruction of vegetation.
**Storm surge.--The local rise in sea level caused directly by storm effects extends the zone of wave impact and causes flooding. The term is technically defined as "the superelevation of the still-water surface that results from the transport and circulation of water induced by wind stresses and pressure gradients in an atmospheric storm" (Simpson and Riehl, 1981). Pressure gradient refers to the lowered atmospheric pressure in storms which by itself can cause a rise in sea level. Storm surge impacts include flooding, floating structures off of their foundations, and floating debris inland, sometimes with ramrod force (fig. 3.5). The initial flow over and around obstructions (e.g., pilings) may cause scouring and sediment transport. The rising water also elevates waves and increases their landward incursion resulting in a wider zone of potential destructive impact. Waves combined with storm surge act to wash beach sand onto the island forming "overwash" deposits. Saltwater flooding kills or damages inland plants.
**Currents.--Storm-generated currents transport water, sediment, and storm debris both parallel and perpendicular to the coast. By far the greatest volume of water moved is in an alongshore direction, as storm waves approaching the shore at an angle set up a current in the general direction of wave travel. Because storm waves are so large, they begin to break much farther from shore than normal or fairweather waves. The net effect is a widening of the surf zone. The longshore current described above is a surf-zone phenomenon, so merely by enlarging the width of the surf zone, more water is moved parallel to shore. This current can move sediment (along with trees, sand fencing, dune-crossover stairways, decks, and other building debris) out of one local area and into another, resulting in a loss of sediment from one portion of the coast. This loss may be temporary or permanent, depending on many other factors. In some cases rip currents may be intensified during part of the storm making conditions even more dangerous for those foolhardy enough to try to surf or swim during a storm.
**Storms are also responsible for bottom currents which move at high angles, sometimes perpendicular, to the shoreline. These currents are set up because of the local rise in sea level caused by the storm surge. The storm surge sets up a sea surface which is actually tilted away from land. The water above normal sea level tends to "flow downhill" back out to sea (fig. 3.6). These types of currents may not be very strong, and they certainly move a much smaller volume of water than longshore currents, but they can be responsible for moving great quantities of sediment away from the beach and into deeper water. They are efficient at moving sediment because they are accompanied, and even partly created, by large storm waves. Storm waves, because of their great turbulence, are very efficient at resuspending sediment off the seabottom and carving up the beach and dunes. Once sediment is moved up off the seabottom and put into the water column, only the slightest current is needed to carry the sediment away. These seaward-flowing currents can actually carry sediment many miles offshore, often so far that the sand is lost from the beach system. This loss is true erosion of the shoreline; a permanent net loss of sediment from the beach and dunes.
**Currents and patterns of flow (channel positions) in inlets may be modified by the combined effects of storm surge, stage of the tidal cycle, increased rainfall runoff, and ebb scour. Changes in channel positions during storms may cause erosion of, or deposition on adjacent islands. Murrells Inlet, SC, for example, has historically migrated to the south with periodic updrift relocation during storms (FitzGerald et al., 1978). Others, such as Bogue Inlet, NC or Capers Inlet, SC, demonstrate relocation of the main inlet channel position within the tidal delta, not relocation of the entire inlet.
**Storm-Surge Ebb.--The "piled up" storm surge water flows back to sea, either by the force of gravity alone or when driven by offshore-blowing winds, generating an erosive ebb current. This type of current is different from the ones described above because it occurs while the storm is moving out of the area or diminishing. Storm-surge ebb can cause an existing inlet to change shape, can create a new inlet such as on Pawleys Island, SC during Hugo, can scour shallow cross-island channels, can transport storm debris (including houses) offshore, and can cause permanent removal of sand from the beach/dune system to the deeper offshore (fig. 3.6). After Hugo, the shoreface in front of Myrtle Beach, SC was covered with deep scour tracks, perpendicular to the shoreline. These were formed by the storm-surge ebb.
 **Human modification of the coast.--Construction in the coastal zone may enhance or otherwise alter the natural processes and their resulting impacts. Roads and beach-access paths perpendicular to the shore which penetrate the dune line may become overwash passes or focal points for storm-surge flood or ebb currents. Seawalls may redistribute wave energy or obstruct sediment movement. Jetties may block great volumes of sand from being transported along the coast, resulting in deposition of sand and beach widening on the updrift side and a long-term sand deficit and erosion on the downdrift side. Ground-level houses and closed-in ground floors of houses on stilts may obstruct the passage of overwash sand which is then lost to front-side erosion. Where vegetation cover has been removed, erosion by wind or water may occur.
Sediment Supply: No Deposit--No Return
**Sand is the barrier island's lifeline between sea and land. Wide sand beaches buffer storm waves. Sand dunes are stores that waves draw on in the big event, and as such are the last line of natural protection between the ocean and island property. Sand fill of former inlets makes up the barrier island's base, and the sand of overwash and dunes is the cap of the island standing above sea level. Put simply, under natural conditions a barrier island with a plentiful sand supply will be a healthy, robust island. But it will not be a stationary island if sea level is rising. Nature rearranges the sediment and landforms continuously in response to changes in wind, waves, currents, and changes in sea level whether short term (e.g., storm surge, tides) or long term (e.g., climate-controlled rise and fall of the sea). Use of the term "nourishment" to describe the artificial building or maintenance of a beach is an accurate reflection that beaches, dunes, in fact, the entire barrier island, needs a sand supply to survive. The same need exists for mud, fine-grained sediment, on the backsides of islands to build marshes and associated features.
**Community officials, planners, developers, and property owners should know:
o Where is our sediment coming from?
o What types of sediment are being supplied?
o What is the amount of the sediment supply?
o How does the supply vary?
o Where is the sediment going?
**If you don't know the answers, and proceed with construction, the chances are good that one or more processes will be modified, the sediment supply disrupted, and the property vulnerability increased. A century of beach erosion problems has taught us that shore hardening structures (e.g., groin fields, seawalls, breakwaters) cut off sand supply to down-drift beaches causing them to erode. The same principles apply across and along the entire barrier island.
**The questions noted above are not just for beaches on the island front, but apply to all island environments. Protective dune lines on natural barrier islands reform after hurricanes as the wind "banks a deposit" that waves will draw from on the next rainy day (i.e., northeaster or hurricane). The overwash carries sand to the interior and backside of the island, building its elevation or allowing it to migrate. When we densely develop islands, the natural, post-storm healing cannot take place. If we are to mitigate future property damage, then we must attempt to restore landforms as nature would restore them. Beach replenishment and rebuilding dune fields by planting dune grass or erecting sand fencing are obvious mitigation strategies, but are techniques applied primarily at the front side of the island.
In the past (and sometimes in the present), great quantities of sand were removed from various parts of barrier island systems. Dredging of navigation channels removed the "spoil," often good quality sand, to offshore "disposal" sites. Migrating sand dunes were considered a nuisance and were flattened and excess sand was removed from islands. Interior dunes (and maritime forests) were cleared, flattened, and removed for building sites (fig. 3.7). As part of the clean-up effort after storms, overwash sand was cleared from the streets and hauled off of the island. And tons of sand have been mined from barrier islands as a one-time construction commodity. Sometimes as on Edisto Island, SC, the island itself was mined to furnish sand for its beach replenishment.
**The rule to conserve sand (Rule #8, Chapter 2) must be applied island wide, front-middle-back, inlet-to-inlet, and not just at selected sites, although conservation measures may range from island-wide projects down to how each individual property is developed and maintained. If sand is required to be put back into the system, it should be borrowed from natural "sinks." A sediment sink is analogous to the accumulation at the end of a conveyor belt--a place where sediment transport ends.
**The sinks we would consider are mainly ancient types of sand deposits, not part of the active coastal system. Stranded barrier islands from ancient, higher stands of sea level might be one example. A river delta is another example of a sediment sink, but deltas are not present along many of the coasts where barrier islands form. Examples of exceptions include the Brazos River delta in Texas and the Santee River delta in South Carolina. Another sink would be sandy river channels from ancient river locations, which are numerous on most coastal plains and sometimes present in the older offshore sediments. Much of the sand used for replenishment projects in Myrtle Beach, SC comes from ancient river channels. Offshore sand shoals and tidal deltas are looked at longingly as sources of sand for today's island sand needs. However, these sinks are still part of active systems and their removal may have negative consequences, such as changing wave refraction patterns or tidal flow which may create erosion problems somewhere else. Also, the effect on plant and animal populations is often difficult to predict, but is almost always negative.
**Sediment supply of sand and mud is just as critical to the backsides of barrier islands as to beaches and interior dunes. Sand migration across the island and through the inlets provide mechanisms for landward migration of the entire island system. The resulting sand platforms (flood tidal deltas and tidal flats) provide the base for salt marsh growth. Salt marshes trap additional sediment and build up the backside of the island, and protect the backside from shoreline erosion.
Vegetation Cover: The Cap on the Deposit
**If there were no vegetation on barrier islands, the sedimentary landforms would never be stable. The scene would be like a desert dune field; remolding, shifting, constantly changing. Nature's addition of plant cover to the processes and material formula provides mechanisms to trap and hold sediment in place. The result is that vegetation plays a significant role in barrier island dynamics.
**Beach and dune grasses baffle wind currents so sand is deposited, burying the grass. Some of these species are adapted to burial--it stimulates their growth, and more sand is trapped in successively higher, wider, and laterally continuous dunes. In a similar fashion, marsh grasses trap sediment allowing the floor of the marsh to build upward. Roots of plants anchor the sediment. Plant succession from grasses to shrubs to trees creates a new environment and a protective cover. Fire, off-road vehicles, foot traffic, and, most often, construction, removes the protection allowing erosive processes to again dominate. Plantings of ornamental species ill-adapted to the barrier island setting are of little value in the role of protective plant cover. Often, dunes and their native vegetation cover are removed or flattened for planting of typical, inland grass lawns. These grasses, ill suited to the harsh coastal environment and the very sandy soil, require large amounts of water to survive. When they don't survive, sand is destabilized and a once flourishing and protective dune has been reduced to an active sand flat, offering absolutely no protection from storms.
**Again, the damage is not restricted to island fronts. Removal of marsh grass, channeling and causeway construction in marshes and sounds that result in plant kills, and mangrove removal (e.g., in Florida and the Caribbean) result in erosion on the backsides of islands. Opening clearings or breaks in the canopy of maritime forests results in loss of stabilizing undergrowth and loss of wind protection, increased likelihood of tree blow downs, and surface runoff or blowouts if on an old dune ridge.
**The rule is conserve plant cover. Communities, developers, and property owners should know:
o What is the natural distribution of native vegetation?
o Where are the areas of maritime and upland forest?
o How is the vegetation cover being, or likely to be, disturbed by development?
o What can be done to protect, restore, or expand natural vegetation?
The Physical Nature of Hurricanes:
All the Processes Rolled Into One
**Hurricanes are responsible for most of the storm-related coastal property damage in the United States, however, other types of storms, particularly northeasters along the east coast and southwesters on the Gulf coast, certainly are important. The actual processes that affect the coastal zone are similar in all storms, but they are most intense in hurricanes. During the relatively hurricane-free period from the 1960s until 1989's brush by Hugo, the majority of today's coastal residents and property owners had not experienced the full force of such storms. This led to an apathetic disregard of the hurricane menace, and increased development in high-hazard zones. In rapid succession, Hugo, Bob, Andrew, Iniki, and near misses by Emily and Gordon, changed all that. The odds are evening out; time is not on the side of coastal development.
**Each year on June 1st the official hurricane season begins. For the next five or six months conditions favorable to hurricane formation can develop over the tropical to subtropical waters of the Western Hemisphere. Early season tropical cyclones form mostly in the Gulf of Mexico or Caribbean Sea where the waters can heat up faster than the Atlantic Ocean. The monster hurricanes that strike the east and gulf coasts of the USA usually originate later in the season (August, September, and October) in the eastern North Atlantic, and grow on their long slow trek across the ocean.
**Once formed, the hurricane mass begins to track into higher latitudes and may continue to grow in size and strength. The velocity of this tracking movement can vary from nearly stationary to greater than 60 miles per hour. When a hurricane makes landfall, the destructive forces are at their maximum in the area to the right of the forward motion of the eye, but the entire landfall area will experience the severity of the storm. The right-of-the-eye effect means that significant destruction can be generated even by a storm that passes offshore, particularly for a hurricane that tracks north-to-south, along the shoreline. One should not feel any security, however, in the knowledge that you are to the left of the eye! Even where the tidal range is small, if the hurricane comes on a high tide, especially a spring tide (highest high tide), the effects of storm-surge flooding, waves, and overwash will be magnified.
Hurricane Probability and Rank
**The probability that a hurricane will make landfall at any given point along the coast in any one year is low, and the probability of a great hurricane makes such an event seem unlikely; but low probabilities give a false sense of security because the lesson of hurricane history tells us that in the lifetime of a structure such a storm is almost a certainty. Furthermore, the occurrence of a great hurricane one year does not reduce the likelihood that a similar storm will strike again the next year.
**In contrast, death tolls from modern hurricanes have been greatly reduced, thanks to the Weather Service warnings, radio and television communications, and evacuation plans. Nevertheless, we must not grow complacent; storm response can be improved. The hurricane watchers of the National Oceanic and Atmospheric Administration track hurricanes and provide advance warning for the evacuation of threatened coastal areas. Yet as little as 9 to 12 hours advance warning may be all that is possible, given the unpredictable turns a hurricane can take. Individuals need to be prepared, know their community's storm-response plan, and take appropriate action when the warning comes. Unsafe development and allowing population growth to exceed the capacity for safe evacuation must be prevented. A hurricane approaching the south Florida coast will trigger the evacuation of tens of thousands of residents and visitors from the Florida Keys into the Miami Metropolitan area. These people, plus the metropolitan area population, will then too need to be evacuated or sheltered. Add to this the large number of retired, elderly, and special needs people living in the area, and the emergency preparedness and response teams will certainly be taxed to the limit. The situation is similar for the New Jersey shore, parts of the Carolinas, New England's urban corridor, and the metropolitan New York City area which sits dead in the center of the New York Bight, a funnel-shaped stretch of shoreline perfect for augmenting storm surge to a frightening maximum!
**The National Weather Service has adopted the Saffir-Simpson Scale (Table 3.2) for communicating the strength of a hurricane to public safety officials of communities in the storm's potential path. The scale ranks a storm on three variables: wind velocity, storm surge, and barometric pressure. Although hurricane paths are still unpredictable, the scale communicates quickly the nature of the storm--what to expect in terms of wind, waves, and flooding. The Category 3 storm is used here to define the upper level conditions to guide property damage mitigation. Category 4 and 5 storms will cause massive property damage or destruction in spite of mitigation efforts.
**Do not be misled by such scales, however. A hurricane is a hurricane. The scale simply defines how bad is bad. When the word comes to evacuate, do it. Wind velocity may change, or the configuration of the coast may amplify storm-surge level, so the category rank can change. Don`t gamble with your life or the lives of others.
Wind: The Universal Agent of Destruction
**The strongest winds of a hurricane may exceed 200 mph, but the maximum winds of the largest storms to hit coastal areas are rarely recorded because wind-measuring instruments are destroyed or blown away! The areas impacted by the hurricane force winds are usually within 30-50 kilometers of the track of the eye. Frederic's winds reached 160 mph, at sea, and blew at 145 mph in the Dauphin Island, AL area, but Camille came ashore as one of the most intense hurricanes ever with devastating 190 mph winds. Andrew was a compact, but very intense, storm with peak winds of 180 mph over southern Florida.
**Considering that the diameter of a hurricane ranges from 60 to 1,000 miles, and that gale-force winds may extend over mostof this area, the total energy released over the thousands of square miles covered by the storm is almost beyond comprehension. No ship or seawall, cottage or condominium, or other static structure will be immune from the impact of such forces!
Waves: Interfacing with Air, Sea, and Land
**Some magnitude of hurricane, tropical storm, or northeaster impacts much of the USA coast almost every year. Major hurricanes that score "direct hits: are important as sediment movers, but almost any storm may have an impact and may bring torrential rain, causing river flooding as well as storm-surge flooding. Depending on the track of a given storm, waves can approach the coast from almost any direction. If the storm passes along the coast or at a low angle to the coast, the time-averaged effect will be that each point along the coast will be subjected to the same forces.
**The maximum height to which wind-generated waves will grow is controlled by three factors: velocity of the wind, the areal expanse over which the wind blows (called fetch), and the length of time the wind blows. The wave-height limit is essentially the extent to which energy from the wind can be transferred by friction, to the water surface. In the relatively simple case of a major winter storm, the long fetch (100s to over 1000 miles) and duration (up to several days), means that the maximum wave heights for that given wind speed will develop. This is called a fully developed sea.
**Estimating waves under hurricane conditions is much more difficult than for northeasters because hurricane winds vary with time, hurricane winds are circular, and because the hurricane quickly moves over waves generated at various angles to the path of the storm (Bretschneider, 1966). In addition, fetch lengths of constant wind speed and direction in hurricanes are so small that a fully developed sea is not achieved (CERC, 1984). The circulation of the wind and the forward motion of the storm influences the wave field development, increasing relative wind speeds in the right quadrants of the storm (relative to the track of the storm) and decreasing relative wind speeds in the left quadrants. The effect of forward motion of the storm on the wind field decreases with distance from the zone of highest winds. The maximum waves in hurricanes are generated to the right of the eye at exactly the radius of maximum winds.
Storm Surge: Landfall and Coming Ashore
**Hurricanes produce storm surges which result from the interaction of several forces. The water surface is actually set into circular motion by the fierce counterclockwise winds of the storm. This motion actually pushes water inward toward the eye of the storm, and generates a convergence of water mass in the surface layer. In addition, the low atmospheric pressure in the hurricane's eye means that air pressure is not pushing down on the ocean surface at the hurricane's center with the same weight or forces as on the storm's periphery. This intense low pressure causes a local rise of sea-level due solely to air pressure differences. A rule of thumb is that a 1 millibar change in air pressure translates into a change in sea level of about 1 centimeter. Not very much, but when you consider that a Category 5 hurricane can have a central pressure of less than 920 milibars compared to about 1013 milibars for normal atmospheric pressure, that means that almost one full meter of the storm surge can be due to atmospheric pressure differences as the eye crosses the shoreline!
**The combination of these effects creates a mound of water, the crest of which lies to the right of the hurricane center near the position of maximum winds. The counterclockwise wind circulation within the storm combined with the forward motion of the storm itself causes the greatest energy to be concentrated in the right front quadrant (for northern hemisphere hurricanes). Thus, surge heights and storm effects are usually greatest to the right of the eye.
**Storm characteristics are not the only controlling factors of storm surge elevation. The nature of the offshore continental shelf has an important influence. Storm surge is augmented by shelf width just as tidal amplitude is controlled in large measure by shelf width. On narrow shelves there simply is not enough "space" to pile up a large volume of the storm surge water, and the water ends up "leaking out" back to sea without piling up to appreciable heights. Narrow shelves such as those off Caribbean Islands, or off of Cape Hatteras, North Carolina or Miami Beach, Florida, have inherently lower maximum potential surge elevations. But when storm winds push surge water against a land mass, surge heights can grow to frightening proportions. On wide shelves, a broad expanse of shallow water is mobilized, and piled up as storm surge much as tides do. Storm surge piles up against the land forcing its absolute height to elevations well above the high tide mark resulting in flooding. Wide shelves such as off of St. Simons, Georgia, Galveston, Texas, and St. Petersburg, Florida. Storm surges reached 20 feet and possibly even 30 feet during Hurricane Camille on the Mississippi coast in 1969 where the shelf is very wide. The 1993 "Storm of the Century" blew for several days straight onshore in the panhandle area of Florida, creating storm surges of nearly 30 feet on the wide, very gently sloping shelf setting.
**Conversely, broad shelves typically have lower wave energy because the wide shelf allows more frictional damping of wave energy as the waves traverse the shelf. Research by Miles Hayes and his students at the University of South Carolina documented the relationship between shelf width and maximum tidal amplitude and average wave height for the southeastern USA (fig. 3.8). Nevertheless, all areas subject to storm surge, including those with wide shelves, can suffer severe storm wave damage.
FIRM's and SLOSH's: Defining the Storm Surge Risk
**Storm surge is commonly measured relative to the so called 100-year flood. The Federal Emergency Management Agency publishes maps showing various flood zones for the entire United States in order to identify flood hazard areas and to develop Flood Insurance Rate Maps (FIRM) for those areas. Coastal High Hazard Areas, according to the Flood Insurance Rate Maps are: V-Zones (floodplain subject to storm-driven waves), and A-Zones (100-year flood zone; a Æ1% chance of flood reaching or exceeding a predetermined area in any given year). Other zones shown on FIRM's are B-Zones (100- to 500-year flood zone) and C-Zones (>500-year flood zone). Criteria used to define V-Zones and A-Zones are shown in Figure 3.9, and are the preliminary basis for defining extreme to high-risk zones.
**A computer simulation model developed for the National Weather Service called SLOSH (for Sea, Lake and Overland Surges from Hurricanes) is used to predict the still-water superelevation (that is, the storm surge) of storm waters caused by the drop in barometric pressure, wind speed, forward speed of the storm, storm track, nearshore bathymetry, shoreline configuration, and nearshore topography. The SLOSH model is essential for developing hurricane evacuation plans in exposed coastal areas.
**SLOSH models give good examples of the differences in maximum potential storm surge on narrow shelves versus broad shelves. For example, on the northern shelf of Puerto Rico with a narrow shelf, less than a mile wide, the predicted maximum storm surge from a Category 4 hurricane is highly variable, ranging anywhere from 4 to 11 feet (1.2 to 3.4 meters). Hurricane Hugo was a Category 4 when it hit Puerto Rico in 1989 and caused surges of about 3.5 feet (about 1 meter) in San Juan. Surges were less than the predicted maximum because Hugo passed over just the northeastern corner of Puerto Rico; not a direct hit that would cause maximum surge levels (San Juan was to the left of the eye).
**In contrast, Hugo hit South Carolina head on and at high speed; both contributing to maximum storm surge. Predicted storm surge from a Category 4 hurricane in South Carolina is about 4-13 to 18 feet (5.5 meters). Measured surge elevations ranged from almost 20 feet (6 meters) in McClellandville, in the Cape Romaine area north of the eye, to 12 feet (3.6 meters) at Folly Island, south of the eye. The SLOSH models predict some of the highest potential storm surges, almost 36 feet (11 meters), for Wakulla County, in northwest Florida. Together, SLOSH and FIRM define minimum elevations to be above the level of wave and flood damage, but do not address other hazards (e.g., wind, erosion, inlet potential).
Storm-Surge Ebb: Water Returns to the Sea
**The counterclockwise circulation in tropical cyclones is responsible for abrupt changes in wind direction and intensity during storm passage. As a storm passes landward across a barrier coastline this circulation pattern produces abnormally high water level differences between ocean and lagoon. For a hurricane moving perpendicular to the Atlantic coast towards a barrier coastline (fig. 3.10), the initial winds of the storm are onshore to alongshore. Elevated water levels occur along the front of the barrier and on the mainland side of the lagoon. After passage of the center of the storm landward, an offshore wind will result south of the eye and an alongshore wind north of the eye. If the forward motion of the storm is rapid, reversal of wind direction is abrupt, giving rise to abnormally high storm-surge ebb against the backside of the barrier island at the same time sea level on the ocean side is low as winds push the water seaward. These conditions lead to flood flow across the island in the seaward direction, resulting in erosive scour and even the formation of new inlets, eroded through the island from the backside (fig. 3.11).
**Such a surge can contain a large volume of water if the lagoon is large, such as Pamlico Sound or Chesapeake Bay. In a coast-parallel hurricane, as the storm arrives at the coastline (fig. 3.12-A), onshore winds and storm surge are dominant. As the hurricane moves away from the area (fig. 3.12-B), offshore winds and storm-surge ebb are the dominant physical processes. This is exactly what occurred during Hurricane Emily as it passed offshore of Cape Hatteras on August 31, 1993. Emily was a weak Category 3 hurricane and stayed completely offshore. Winds blowing over the estuary (Pamlico Sound) caused maximum storm surge on the back side of Hatteras Island (Bush et al., 1995). Surge elevations were amplified by the concavity of the soundside shoreline of the Cape Hatteras cuspate foreland. Soundside water levels and wave heights were greater in magnitude than those at the open-ocean shoreline. The soundside of the island suffered more direct wave damage than the oceanside. Along this entire stretch storm-surge flooding from the sound carried mud, salt marsh grass, and other debris onto land. The rule is that backsides and interiors of islands need the same attention to planning and property damage mitigation as the front sides of islands. A mighty fortress (e.g., seawall) is worthless if the attack comes from the rear.
Hurricane Hugo: What We Learned
**Hurricane Hugo provided an excellent opportunity to document the effects of storm-related processes on developed shorelines. Field observations were made immediately after the storm, and aerial photographs and video were taken within 36 hours after landfall. These numerous subsequent studies show that the distribution of hurricane damage was often determined by the type of development (Gayes, 1991; Lennon, 1991; Thieler and Young, 1991). Damage, for example, was particularly severe where dunes were absent, where dunes were low and narrow, where elevations were low, where vegetation was absent, and along shore-perpendicular streets and beach access sites. The latter, together with gaps between large, solid, ground-level buildings, concentrated flow adding to the erosive power of currents. Houses that detached from foundations became battering rams, and debris missiled off of buildings by the wind became additional hazards.
**Some of the same lessons were learned from observations of the impact of Hurricane Gilbert (1988) on the Yucatán Peninsula of Mexico and of Hurricane Hugo in Puerto Rico (Thieler and Bush, 1991). Likewise, a look back at previous storms, and evaluation of storms impacting areas in a variety of geologic and climatic settings gives insight into shoreline response (See Chapter 9).
**Storm History.--Hurricane Hugo was detected on satellite imagery on September 9, 1989 when a cluster of thunderstorms moved off the coast of Africa (Golden, 1990; Brennan, 1991). Hugo moved westward at 20 mph across the tropical Atlantic Ocean, became a tropical storm on September 11 and a hurricane on the 13th (about 1100 nautical miles east of the Lesser Antilles Islands). Hugo passed over the Lesser Antilles reaping great havoc, and brushed the northeastern corner of Puerto Rico as a Category 4 hurricane. It weakened somewhat upon encountering the Puerto Rico landmass, but reorganized as it passed back over the Atlantic Ocean on its way north. Hugo increased in strength when over the Gulf Stream, and its forward speed accelerated.
**Hugo was a Category 4 storm (Table 3.2) when it made landfall in South Carolina just after midnight on September 22, 1989. Highest wind gusts occurred just before landfall, and were measured at 140 mph. The hurricane's central pressure of 934 millibars made Hugo the tenth most intense storm since 1900 when good barometric pressure records began.
**Final landfall was on the South Carolina coast near Charleston at the Isle of Palms, with the eye of the storm moving northwestward at nearly 30 mph. Moving inland and weakening, the center passed near Columbia, South Carolina. By 8:00am, 22 September, Hugo had weakened to a tropical storm and passed just west of Charlotte, North Carolina. The storm moved northwest across extreme western Virginia, West Virginia, eastern Ohio and near Erie, Pennsylvania, weakening to an extratropical storm, and tracked for two more days northeastward across eastern Canada and into the far north Atlantic Ocean.
**Impacts of Hurricane Hugo.--Total property damage in the coastal counties of South Carolina and southern North Carolina was estimated at near 7 billion dollars (U.S. Dept. of Commerce, 1990). Most of the coastal damage was caused by the intense winds, waves, and storm-surge flooding associated with Hugo. Once the storm passed inland, storm-surge ebb scour left a final imprint on the coast and caused additional damage. We all hear about loss of life, houses destroyed, infrastructure damaged, and peoples lives changed forever, but another impact of Hugo was the generation of tons and tons of debris--the wreckage of the storm. According to State officials, the debris generated by Hugo has shortened the lifespan of South Carolina's sanitary landfills by seven years. Seven years worth of garbage created in just a few hours!
**The South Carolina coast is located near the center of the Georgia Bight, the long concave coastal reach between Cape Hatteras, North Carolina and Cape Canaveral, Florida. The continental shelf here is about 60 miles wide. This setting creates a situation where water can be funneled somewhat, focusing and intensifying the storm surge. Records of high water stains and debris lines in conjunction with tidal records obtained from NOAA (NOAA/NOS, 1990) were used to ascertain the magnitude and time history of storm-surge and storm-surge ebb (reference USGS storm surge maps).
**The maximum storm-surge elevations (fig. 3.13) recorded at stations closest to the hurricane track exceeded historical highest water elevations (NOAA/NOS, 1990). Note also that the greatest storm-surge height was to the right of the eye. A major contributing factor to the severe storm surge and storm-surge ebb along the South Carolina shoreline was the coincidence of landfall with the occurrence of high tide, causing the storm surge to be superimposed on top of approximately 4 feet (1.2 meters) of astronomical tide. The maximum water elevation recorded at Charleston, South Carolina during Hurricane Hugo was 13 feet (3.9 meters). The station at Winyah Bay recorded a maximum elevation of 9.5 feet (2.9 meters). At Fort Pulaski, Georgia located near the South Carolina/Georgia border the maximum storm surge was 8 feet (2.4 meters). Personal inspection of high-water stains indicated a storm-surge level of 12 feet (3.6 meters) at Folly Island and Sullivans Island. Wave impact and flood damage was extensive from Kiawah Island, SC to Cape Fear, NC.
**Hundreds of houses were damaged or destroyed on the islands of the Charleston area. Houses on Pawleys Island were floated off their foundations and deposited in the marsh, intact, up to a mile away. Mobile homes were gathered like toothpicks in Garden City. Water ran through the first floor of houses elevated on stilts on Sullivans Island, Isle of Palms, and Folly Island. Storm surge flooding and waves wiped out 28 miles of dunes. Essentially all of the approximately 13 miles of shore-protective structures were overtopped or damaged (Theiler and Young, 1991).
**Hugo provided the hard lessons of experience. Frontal dunes and interior dunes were severely impacted by the Hurricane Hugo, however, they provided valuable protection against initial storm surge, albeit temporary, reduced penetration of overwash, and protected against the damaging effects of storm-surge ebb scour. Thieler and Young (1991) concluded that the minimum dune field that survived Hurricane Hugo and thus protected buildings was about 100 feet (30 meters) wide with crests of about 10 feet (3 meters) height. Most of the buildings damaged or destroyed by Hurricane Hugo were fronted by beaches less than 10 feet (3 meters) wide and dune fields less than 50 feet (15 meters) wide. Low dunes that were greater than 15 meters wide were overtopped quickly and not eroded severely. These latter dunes did not prevent flooding, but reduced the impact of storm surge and waves. All shore protection structures were overtopped, did not prevent flooding, and did not offer any wave-breaking protection to structures located behind them! Many of these stabilizing structures (seawalls, bulkheads, groins) were damaged or destroyed, adding to the total cost of storm damage. Chapter 5 discusses the storm's implications in more detail.
**Readings from six surviving tide gauge stations indicate that storm-surge ebb was complete in less than 12 hours. The storm surge and storm-surge ebb history during Hugo is shown on (fig. 3.14) for Charleston, South Carolina. In Charleston, ebb occurred in two separate stages. Water-level dropped 5 feet (1.5 meters) in less than one hour and then after a one-half foot (0.15 meters) resurgence fell another 6 feet (1.8 meters) in the next 5 hours. Storm-surge ebb in this case was completed in 9 hours. The wind field associated with Hurricane Hugo is shown diagrammatically in (fig. 3.15). Wind is one of the primary causes of storm surge, but the perpendicular track of Hugo, its high forward speed, and the concave shape of the South Carolina shoreline were important contributing factors.
**Storm-Surge Ebb Effects.--Storm-surge ebb is not a dominant process in all hurricanes, but its intense erosive scouring during Hugo was responsible for property damage to buildings, seawalls, roads, and water lines. Ebb scour channels eroded through washover deposits indicating that the process was separate from, and took place after, the erosion and deposition induced by storm surge and waves. Storm-surge ebb scour was observed in all of the developed areas affected by Hurricane Hugo.
**Storm-surge ebb scour was responsible for the creation of breaches in the narrow spit on Folly Island. One channel had dimensions of about 250 feet by over 30 feet (75 meters by 10 meters). The depth of scour channels were relatively shallow compared to scour pits. Steep-sided 6-12 feet (about 2-4 meters) wide scour pits with depths of up to (2 meters) and lengths of 15 to 25 feet (5 to 8 meters) cut through lawns and beach access paths. The absence or presence of vegetation controlled the size of scour pits. Once scouring was initiated, it grew to include areas where the soil and sand was not protected by shrubs, trees, or other vegetation. Again, specific development features influenced the occurrence of storm-surge ebb scour). Documented damage attributable to storm-surge ebb is so prevalent that the process must be seriously considered in planned property damage mitigation: Hurricane Gilbert on the northern coast of the Yucatán Peninsula of Mexico (Thieler et al., 1989; Hugo in South Carolina, Gayes, 1991; Lennon, 1991; Priddy, 1991; Thieler and Bush, 1991); the Chandeleur Islands of Louisiana after Hurricane Camille in 1969 (Wright et al., 1970); and along the Alabama barrier islands after Hurricane Frederic in 1979 (Penland et al., 1980).
**Another common affect of receding flow is the erosion of drainage channels across the beaches (fig. 3.16). Hugo's beach ebb scars were S-shaped in plan view, more strongly curved northwards, south of the hurricane eye. The northeast curvature at the seaward ends of these channels is consistent with the last direction of wind stress as the storm moved inland. Similar large linear scour channels and smaller rounded scour pits associated with Hurricane Gilbert (1988) were found landward of beaches with seawalls and revetments, suggesting a scouring effect behind the structures (fig. 3.17).
**A study of Hurricane Hugo's storm-surge ebb in South Carolina (Priddy, 1991) found that the occurrence of storm-surge ebb scour correlated with one or more development features: (1) shore-perpendicular roads and finger canals, (2) beach access sites between multistory buildings, and (3) orientation changes in seawalls and other types of shoreline armoring (Priddy, 1991). Development enhanced the ebb flow velocity and associated erosion.
**Morphological changes on land produced by the funneling of storm-surge ebb with development were extended into the nearshore. A nearshore sonar survey of the area from Myrtle Beach to Folly Island (Gayes, 1991) showed extensive shore-perpendicular scour channels on the inner shelf. These scour channels indicated strong offshore bottom currents caused by storm-surge ebb. Nearshore bars or shoals extending 50 to 75 meters further offshore in the gaps between multistory hotels than in the areas in front of large development, suggesting that sand was carried offshore by storm-surge ebb currents. Extensive debris was also identified offshore of most of the heavily developed areas of the South Carolina coast. Divers identified a wide variety of debris including sections of damaged seawalls, swimming pools, and mobile homes up to 100 meters (over 300 feet) offshore of the beach in water depths of 2 to 4 meters (about 7-15 feet) (Gayes, 1991). Such currents may move sediment far enough offshore to be considered permanently lost from the beach system.
**The occurrence of ebb flood erosion resulted in the opening of a new inlet on Pawleys Island as noted (fig. 3.11). Old eye witness accounts described the same process during earlier storms in North Carolina where portions of the barrier island coast consist of up to 40% old inlet fill, suggesting that new inlet formation is to be expected (Pilkey et al., 1980). The characterisitics of the backside and interior of the island, particularly in terms of low elevation and lack of protective vegetation, are determinants in where inlets are likely to form. The expanse of open water in the lagoon or sound, river, and creek mouths behind the islands, and the lack of protective salt marsh are clues to identify points of vulnerability. Finger canals and streets across low, narrow portions of an island will increase the likelihood of a new inlet formation. Where inlets migrate, the lengthening spit end of the updrift island ultimately will be cut off when the new inlet reforms in the earlier inlet position; nature's way of maintaining more efficient drainage from the backside of the island. Artificially closing these newly formed inlets maintains a level of inefficiency in the drainage, increasing the likelihood of another round of new inlet erosion in the next big storm, and another round of property loss. those who own the property on the Pawleys Island filled inlet can expect nature's instant replay in a future hurricane. America's barrier islands are peppered with relict and historic inlet positions. Some closed naturally and stabilized as island environments evolved; others remain as topographic lows, vulnerable to future episodes of ebb scour erosion.
**Such potential inlet positions may be stabilized by augmenting the topography (e.g., constructing dunes) and planting vegetation.
** Role of Vegetation.--Hurricane Hugo once again demonstrated that a heavy cover of vegetation reduces overwash penetration and storm-wave damage (Bush and Pilkey, 1994; Thieler and Bush, 1991). On Pawleys Island, SC neighboring houses suffered contrasting degrees of damage, the poorly vegetated properties being heavily damaged or destroyed. Less wind damage occurred to houses located behind or within the maritime forest on the same island. Similarly, during Hurricane Gilbert (1988) in Mexico the devegetated areas were heavily overwashed and buildings damaged (fig. 3.18). As noted, poorly vegetated areas are more susceptible to ebb scouring.
Extratropical Storms: Don't Write Them Off
**A variety of non-hurricane or extratropical storms affect all U.S. coasts. These winter storms are called nor'easters, sou'westers, and other names depending on the direction the wind blows and on local customs. The big storms are typically given unofficial names, usually related to the date of their arrival. Two of the biggest storms in the U.S. east coast's modern history were named "The Ash Wednesday Storm of 1962" and "The Halloween Storm of 1991." A large storm during December 1992 didn't fall on any day of particular note and was left with the designation of "The No-Name Storm." The northeaster/southwester of March 1993 was called "The Storm of the Century" and although it was not a record breaking coastal storm, the damage was widespread; storm surge and wave erosion in the Florida Panhandle, record breaking snow cover inland along the east coast, and cosmetic but costly wind damage to coastal buildings. Communities like Sunset Beach and Topsail Beach, NC were littered with the shingles from stripped roofs, and property owners scrambled to get tarpaulins over the bare roofs to prevent water damage from rainfall. The storm also tore fiberglass insulation out from underneath elevated homes and covered some sections of islands with a snow-looking blanket of fluff.
Anatomy of a Winter Storm
**Hurricanes get the headlines, but for sheer size, number, and duration, winter storms are the major threat to damage coastal property and to cause true shoreline erosion. From the Gulf of Alaska and along the Pacific's shores, through the Gulf of Mexico (southwesters) and all along the Atlantic seaboard (northeasters), winter storms cause widespread property loss almost every year. Much of what we know about these major storms comes from the studies of Robert Dolan and Robert Davis of the University of Virginia (e.g., Dolan and Davis, 1991; Davis and Dolan, 1993; and Watson, 1994).
**Extra-tropical cyclones, (referred to here northeasters) as distinguished from tropical cyclones (or hurricanes) form in mid-latitudes. The ideal breeding grounds for northeasters are coastal areas where there is a large temperature gradient in the winter time between air over the cold continental land and the relatively warmer ocean waters. The low-pressure systems (cold fronts) we see sweeping across the nation every day on the evening weather can intensify as the center moves over the Atlantic ocean.
**The key to a low-pressure center growing into a full-fledged northeaster is the front (that is, temperature gradient) between polar air from the north and tropical air from the south. As a northeaster tracks up the east coast or across the Gulf of Mexico, the storm intensifies by picking up energy from the relatively warmer waters. The damaging winds blow from the southwest in the Gulf of Mexico or the northeast in the Atlantic and are usually slowed somewhat by the forward motion of the storm center in the opposite direction. (Remember that winds are named for the direction from which they blow, so a northeast wind blows from the northeast, but a northeast moving storm moves toward the northeast. The presence of a strong stable high pressure center in eastern Canada will block a northeaster from moving very quickly up the east coast, providing the storm more time to gather strength from the ocean waters.
**Northeasters typically cover larger areas, have much lower wind speeds, and move slower than hurricanes; sometimes remaining off a coast for several days. Unlike fast moving hurricanes, the winds and waves of a northeaster may persist through several tidal cycles, amplifying the shoreline damage due to waves at any given location. Like California waves, generated far away in storms of the Gulf of Alaska, the waves of an east coast storm may travel far from the storm center and arrive at the beach on bright sunny days with only light local sea breezes in evidence (figs. 3.19 and 3.20). Generally, northeaster winds are not the major cause of property damage, and loss of life is usually low. A major exception to this was the great 1953 Dutch storm which struck Holland, breaking many dikes, leaving more than 1,500 people dead in its wake. More important, as a rule, is the direct impact of waves on buildings, extensive flooding, and shoreline erosion undercutting building foundations.
Ranking Northeasters
**An intensity scale for U.S. Atlantic coast northeasters, similar to the Saffir-Simpson scale for hurricanes, was developed by Dolan and Davis (1992). (Table 3.4). The Dolan-Davis classification is not based on wind velocity, but instead on the size of the waves and the duration of the storm, and is expressed in terms of intensity of property damage. This classification is intended for use along the U.S. Atlantic seaboard. Since 1960, eight Class V northeasters have occurred. On the average since the early 1980s, about two dozen northeasters have occurred each year along the U.S. Atlantic coast (Davis and Dolan, 1993).
What Happens in a "Storm of the Century?"
ASH WEDNESDAY 1962 (www.nyc.gov) |
**The most memorable storm of this century in the USA was the 1962 Ash Wednesday Storm, a class V northeaster. It struck during spring tides resulting in extreme storm surge. Because the storm persisted over five high tides, the damage grew more and more extensive. Beachfront communities from Fire Island, New York to Nags Head, North Carolina were devastated (fig. 1.4). Some damage occurred to all beach front communities between southern Massachussets and northern Florida. Damage was particularly severe to beachfront buildings. The loss of beaches was so severe that the '62 storm marked the entry of the U.S. Army Corps of Engineers into the arena of beach replenishment. Between 1962 and 1965 the Corps pumped up millions of cubic yards of sand to widen the beaches of New Jersey, many of which had actually substantially recovered through natural processes. A new inlet was opened along the Outer Banks of North Carolina (fig. 3.21).
**As the nation enters the 21st Century every new hurricane and winter storm will indeed be the "storm of the century" until the next storm breaks that record. If the impact of these yet unborn storms is to be blunted, the most thorough risk assessment and mitigation programs possible must be undertaken and emplaced.
**Table 3.1: Storm processes and their effects.
**Storm Wind:
direct wind attack on buildings
flying debris (missiling)
sand onto/off island (burial or erosion)
vegetation loss (blow down, salt spray kills, sand blasting of leaves)
**Storm Waves*:
direct wave attack on buildings
floating debris (ramrodding) from buildings and attachments
scouring around foundation footings
shoreline retreat on lagoon shore (erosion)
shoreline retreat on ocean shore (backbeach erosion)
overwash (burial and blockage)
dune loss
scarping of fastland (nondune)
vegetation loss (erosion, salt water kills)
local flooding
strong longshore currents, remove sand from area
**Storm Surge*:
flooding
floating debris (rafting)
lagoon shore retreat
ocean shore retreat
widening inlets
changing channel locations in inlets
new inlet formation
increases zone of wave influence (elevates waves, moves waves landward)
overwash (burial and blockage)
scouring of cross-island channels and undermining of structures
scouring around foundation footings
vegetation kills (salt water kills including saline groundwater contamination)
saltwater flooding impacts (sterile soil, contaminated groundwater)
drives offshore-directed currents, permanently removing sand from system
**Storm-Surge*:
widening inlets
changing channel locations in inlets
formation of new inlets
scouring of cross-island channels
scouring of offshore channels
scouring around foundation footings and other hard structures
emplacement of debris offshore (swimming and boating hazard)
sand removal/permanent sand loss
**High Rainfall:
water damage to buildings when coupled with high winds
enhanced flooding
enhanced erosion due to runoff
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