| CHAPTER
 **Mapping the level of relative property damage potential for barrier islands assists community officials in planning for and reducing impacts from natural disasters and allows individual property owners to choose sites and purchase property in a more knowledgeable way. Understanding that vulnerability is determined by the natural processes and the environmental settings suggests nature should be our guide in mitigation methods, rather than relying solely on engineering and social regulation (e.g., seawalls, building codes, zoning regulations). A useful approach is to classify risk into categories such as extreme, high, moderate, low, and even very low in some cases (Table 4.1). All barrier islands have a high element of risk, and any given island may not have areas that fall into all four categories. In fact, low-elevation barrier islands, such as Dewees Island, South Carolina, may fall entirely into the extreme-risk category. Table 4.2 summarizes field evidence useful to determine risk of property damage.
**Virtually all coastal areas are at extreme risk if struck by a category 5 hurricane (Saffir-Simpson Scale, Chapter 3)! Differentiation into risk zones is useless for such a storm. For a category 4 or lower storm, however, risk mapping can be meaningful. The risk mapping presented here is based on the risk afforded by a low category 3 hurricane, hitting directly at the site under question. A low category 3 hurricane will have winds of about 111 to 120 miles per hour. Typical accompanying storm surges will range from 5 to 12 feet. A category 4 storm as a basis for mapping would assume higher energy conditions, and require stricter limits on risk categories.
**Note that this hazard mapping involves only risk of property damage and is not concerned with risks to inhabitants. In general, of course, areas with high potential for property damage are also areas of high risk for human inhabitants, but a low-risk site also can be a death trap in a hurricane. Difficulty in evacuation is an example of a human risk which may be entirely independent of homesite safety. Being trapped or isolated by a great storm is extremely dangerous and may be independent of property vulnerability. For example, draw bridges may be stuck in the open position, evacuation routes blocked by flooding, downed trees, wrecks, or cars that run out of gas, with many miles to go to escape the flood zone (as on the West Florida mainland).
Storm Experience Sets New Directions
**Observations of several barrier island communities after Hurricanes Gilbert (1988) and Hugo (1989), as well as several smaller hurricanes and numerous winter storms, suggest that property damage potential can be lessened significantly by prudent site selection and proper location of structures on property. Storm impacts can also be greatly reduced by island-wide mitigation based on risk mapping. Barrier island risk assessment can be done at any scale: island-wide, individual communities on one island, neighborhoods within a community, a block within a neighborhood, and for individual structures and building sites. Naturally, property owners focus on site evaluation as do many mitigation plans. But working from an island-wide base map of risk potential down through the community level to neighborhoods and finally to individual sites, will provide a better basis for developing mitigation plans.
**Determining relative hazard ranking of large sections of barrier islands is relatively straightforward (Table 4.3). Important determinants of damage potential for a given structure include its elevation above sea level, elevation above ground level, exposure to wind and wave hazards (presence or absence of thick maritime forest or shrub cover), presence or absence of high, wide dune fields, and distance from the ocean or sound which determines the likelihood of impact by storm surge waters. Using these criteria it is possible to define areas of islands with similar risk. Response to previous storms in an area also gives critical insight into risk mapping.
Coastal Environments and Associated Hazards
**The concept of different parts of a barrier island or other coastal environment responding differently to the same storm is fundamental in developing any approach to hazard recognition and risk assessment (Tables 2.1, 3.1, 4.1, 4.2, and fig. 2.3). For example, during a storm, low-elevation environments, such as beaches and low-elevation dunes, will be subject to high wind, storm-surge flooding, and direct wave attack. During the same storm, higher-elevation dunes and maritime forests may only experience heavy rainfall, associated surface runoff of the rainwater, high wind, and possibly flooding in some maritime forest areas depending upon elevation (figs. 2.3 and 2.4).
**Elevation and exposure to wind (forest or dense shrub thicket cover) are the most important characteristics of the natural setting from the standpoint of potential for property damage. Stable higher elevation sites have lower potential for flooding and wave attack. Sites with lower exposure to wind because of the presence of dense maritime forest or the presence of nearby high, wide dune fields also are at significantly lower risk. High elevation may also mean greater exposure to wind forces. However, wind is much easier to engineer for in buildings than are waves or the forces of rising water. In large measure, then, the higher elevation and more densely forested portions of islands have lower potential for damage from storms and are rated as moderate to "low" risk sites. depending on secondary factors. These and similar criteria provide a basis for delineating portions of islands with similar risk or damage potential as "Low," "Moderate," "High" or "Extreme." In general, most barrier island environments fit neatly into these categories (fig. 4.1 and Table 4.4). Of course, some local variation in these categories should be expected. For example, "high" elevation at Nags Head, North Carolina may be less than the lower limits of "high" elevation on a Texas island where storm surge would be higher. In other words, the overall classification is qualitative, and absolute quantitative limits for determining parameters such as elevation and percent or density of forest cover must be determined for each region; again based on storm experience and the criteria established for developing FIRMs and applying predictive models such as SLOSH. The same is true for additional "qualifier" hazards (e.g., erosion rates, inlet migration rates, potential for inlet formation, type and quality of existing development, engineering structures, dune width and height). Absolute quantification may be debatable, however, when dealing with barrier islands the qualitative approach is a very good first approximation of risk level based on past experience. Application of this risk assessment/mitigation technique is relatively simple, quick, and inexpensive, providing both individuals and community officials with a useful evaluation tool.
**Location of a coastal environment relative to the open-ocean shoreline can influence hazard potential and complicate the ranking scheme. For example, a low-risk maritime forest can be exposed on the beach in a high-erosion situation (fig. 4.2), leading to its classification as high risk. The general assumption in applying each of these parameters is stability (e.g., the maritime forest isn't falling over an eroding scarp; the protective dunes are grass covered and not blowing out).
**Finally, climate must be taken into account. The nature of the vegetation cover will vary from tropical (rainforest, mangroves), to temperate (maritime and mainland forest as in this discussion), to arid (grass and shrub cover). In the latter case, the wind hazard is enhanced by lack of forest cover, but if dune formation has been active, high elevations may be present.
Hazard Zones
 **EXTREME HAZARD
Areas rated as extreme hazard areas are the lowest elevation portions of islands and those most exposed to wave-generated winds during storms (Table 4.1 and fig. 2.3). By definition they are within the 100-year flood level and exposed to potential wave-generating winds enough to have waves greater than three feet formed on top of the possible storm surge, that is, in V-zones as defined on FEMA Flood Insurance Rate Maps (FIRM's, discussed later in this chapter are available in any city hall and from FEMA; see Appendix A for addresses). Extreme-hazard zones typically have little vegetation except sparse growths of low beach or dune grasses. There is no maritime shrub thicket or forest, either never existing, or having been removed for development. These zones experience flooding from storm surge waters as defined by the SLOSH model and from heavy rains, are likely to be overwashed during storms, are susceptible to wave attack, usually affected by erosional shoreline retreat, and may experience storm-surge ebb scour. In addition to oceanfront areas within the FIRM "V-zones," island areas adjacent to inlets, lagoons, and estuaries as well as low, barren areas such as overwash terraces located away from the shoreline may fall into the extreme-hazard category.
**HIGH HAZARD
High hazard zones are typically low-elevation areas within the 100-year flood plain (FIRM A-zone) and typically have no or poor vegetative cover (Table 4.1 and fig. 2.3). Areas within maritime forest and/or shrub thicket may also be classified in this high-hazard zone because of susceptibility to flooding and even wave attack even though maritime forest is important for reducing exposure to high wind. The low results in vulnerability to flooding from storm surge and heavy rains. Potential for wind damage remains high. These zones are usually located more toward the island interiors, and are usually less susceptible to erosion damage. Such areas may be susceptible to soundside erosion as well as flooding from the backside of the island, and locally to new inlet formation. If forest is present, the likelihood of impacts from both wind and storm-surge ebb scour is reduced.
**MODERATE HAZARD
Moderate hazard zones generally include areas above the 100-year flood zone, although often without good maritime forest or dense shrub thicket cover (Table 4.1 and fig. 2.3). The high elevation means these areas are not generally subject to flooding and are unlikely to suffer direct wave attack. Flooding should not, however, be ruled out (i.e., these areas may be in FIRM B or C zones, that is, having potential to be flooded in a greater than 100-year flood) because unusually high water levels can be generated in a category 3 hurricane. In some cases an A-zone may be assigned to the moderate category if there is a good cover of vegetation, few secondary hazards, and additional natural protection like a fronting of a wide dune field. Having flood insurance is recommended for moderate-hazard zones. Wind damage is the most likely hazard; the lack of a protective forest means a high degree of exposure to wind as well as to missiling.
**LOW HAZARD
Low hazard zones are areas above the 100-year flood zone, and that are well forested (Table 4.1 and fig. 2.3). These vegetated areas are generally not subject to flooding or wind hazards. Typically these areas are inland, away from wave erosion and potential overwash or inlet effects. Sites for development within this zone are "least risky" in terms of potential for property damage. A caveat is that removing forest for development obviously reduces the amount of protection, and often leads to increased degradation of newly exposed portions of the forest from salt spray. Another caveat is that building in the forest could result in damage from falling tees and limbs during storms, but most maritime forests are relatively resistant to wind destruction. Furthermore, a few holes in the roof are better than a destroyed home. As with the previous category, the rare flood event can not be ruled out, especially if a long-term mitigation view is taken.
**No location on a barrier island or coastal area can be called truly safe for development. Moreover, the potential for property damage is often increased by human alterations to the coastal environment which reduce the natural protective capabilities that existed. (Table 4.5) lists typical coastal development and its impact on the environment. Each type of human adjustment alters the natural environment, usually exacerbating the damaging effects of natural processes. Mitigation plans must look beyond site-specific requirements or island-front approaches and consider all aspects of the island and island chain. Mitigation efforts in the interior of a barrier island may be as important as on the shoreline. In the final analysis, if low risk to life and limb and property is a high priority for a potential property owner, a carefully selected mainland site is a superior choice.
**Potential for property damage is high in almost any coastal setting, and will only increase as development density increases, or as single-family homes give way to multifamily dwellings and commercial establishments. Consider Gulf Shores, Alabama, which was heavily damaged by Hurricane Frederic (1979). Frederic eroded away protective frontal dunes and oceanfront buildings. The oceanfront single-story buildings, once protected by dunes, were replaced by larger multiple-storied buildings in the extreme and high-risk zones. If a new dune line had been allowed to form, its location would have been inland where the new buildings were located! The final result is greatly increased development density, population density, and the potential for greater financial loss in the next hurricane. Similar stories have been repeated elsewhere. In Garden City, South Carolina, small cottages destroyed by Hurricane Hugo have been replaced by much larger cottages, facing a duneless, storm-narrowed beach. This development pattern is taking place over the entirety of barrier islands, often proceeding into areas of higher and higher risk (e.g., inlet zones).
**The rule is that coastal risk assessment is not a one-time exercise. Risk mapping is an evaluation for the specific conditions at the time of mapping. Barrier islands change at a rate much faster than mainland environments, both in terms of natural alteration and economic development. Therefore, risk mapping must be repeated periodically, particularly after big storms, and the mitigation plan altered accordingly.
Coastal Hazard Mapping Technique
**Responsible officials, planners, conscientious developers, and individual buyers and property owners face a multitude of processes on barrier islands, all of which carry varying levels of risk to cause property damage. These processes, and their resulting landforms, become hazard assessment parameters (Tables 4.6 and 4.7 and continued). But which ones are most important? Which of these parameters are the best measure of the level of risk potential? And at what scale and level of detail should such risk analysis be carried out? To take into account all of the parameters in (Table 4.7 and continued) would be a daunting task, and many of the factors listed are difficult to quantify.
**Our objective is to reduce the number of critical parameters to the smallest number of broadly meaningful measures of risk potential; produce a preliminary risk map; and then refine the risk evaluation based on added levels of related factors that are of more local extent and impact. Given that the major processes are wind, waves, storm surge, and flooding, then two universal controls emerge as most important, namely elevation and protective vegetation cover (primarily forest). How the elevation is distributed in terms of landforms (e.g., dunes, overwash, interior beach/dune ridges, troughs) can be evaluated for local refinement, say an area within a barrier island. Parameters that are more site specific (e.g., erosion rates, potential overwash passes, inlet position) are incorporated at the neighborhood or individual property level for final risk evaluation.
**This simple-to-complex approach allows property-risk mapping to be carried out at various scales, beginning with the overview level of the entire island before trying to be site specific. As noted, the approach is qualitative which allows application with a minimum of background, is cost efficient, and can be communicated to the most important audience, the citizens of the island in question. At the same time, quantitative limits on many of the parameters can be assigned for individual islands or island chains, based on past storm experience, and ultimately used to define mitigation regulations. Some of these quantifiers are already in use in the form of FIRM's (the 100-year flood level plus a calculated wave height is the regulatory elevation) and storm-surge flood elevations based on the SLOSH model. Minimum dune heights and widths to sustain storms and protect property are known for specific storm conditions (e.g., Hurricane Hugo in South Carolina or Hurricane Frederic in Alabama). Specific erosion rates may be designated and utilized as in calculating setback requirements. Of course, these "quantifiers" must be measured and monitored frequently to be applied usefully, adding the requirements of expertise, time, and cost. Other parameters can be quantified simply as present or absent (e.g., overwash gaps, vegetation, inlets), and some factors are difficult to quantify (e.g., what is the most effective measure of forest cover?). Ultimately, the broad list of parameters (Table 4.7 and continued) must be addressed if the overall island mitigation activities are to be successful.
**The following sample mapping outline provides a model that can be adopted for a variable set of assumptions or objectives. The approach could be upgraded to a lower-level of acceptable loss, i.e., define the level of risk for a Category 2 hurricane or a 200-year flood level. Similarly, downgrading might allow a higher-level of acceptable loss and adopt a 50-year flood level or the conditions of a category 4 hurricane. Other factors on the list could be given more weight in the risk definition where deemed more important (e.g., overwash and inlet potential for narrow islands). The community of Nags Head, North Carolina is used to illustrate the map making and risk assessment procedure. (Table 4.3) Outlines the procedure.
Preliminary Analysis Map
**Keep in mind that the following mapping is based on island response to a low Category 3 hurricane (Table 3.3). The most important aspect of the evaluation process is to get a handle on which areas will be most exposed to high winds, which areas will be flooded and how deeply, and which areas will be susceptible to waves as well as rising water. Coastal elevations are critical in this regard and the initial and most important first approximation at hazard mapping can usually be done with the FEMA Flood Insurance Rate Maps (FIRM's), available at any town hall, at nearby county courthouses, or from FEMA (see Appendix A for addresses). These maps delineate V-Zones (100-year flood level plus waves of 3 feet height or greater), A-zones (100-year flood level), B-Zones (100-500-year flood level), and C-Zones (areas above the 500-year flood level). Probable extreme and high-hazard zones are derived from the FIRM's (fig. 4.3). Very simply, areas within V-zones will always be extreme hazard, and areas in A-zones will be high or moderate hazard depending on the amount of forest cover. Areas above these most floodable zones will be moderate to low risk, at this first level of rating, depending on the amount of forest cover. If hurricane inundation maps (SLOSH maps) are also available, potential overwash zones can be noted, adding detail for fine tuning the initial risk assessment (fig. 4.4). The U.S. Geological Survey has produced post-hurricane hydrologic survey maps for several hurricanes, and these maps are excellent sources for identifying future flood potential and expected storm-surge water elevations.
**In general, U.S. Geological Survey Topographic maps are of limited use in determining detailed land elevations on barrier islands because contour intervals are typically too large. The FIRM's must be field checked as there are errors on some of them, and the maps tend to be conservative in outlining the V- and A-zones. SLOSH maps of potential overwash can become quickly out-of-date if dunes have been removed in a recent storm or if new frontal dunes form either naturally or with the help of bulldozers and sand fencing. Likewise, removal of dunes for construction, whether legal or illegal, will increase risk from storm surge as well as wind.
**The second step in the preliminary risk assessment is to evaluate vegetation type, distribution, and density. As mentioned previously, dense maritime forest can be invaluable in reducing exposure to wind and wind-borne debris. Thus, any area with a "significant" cover of dense maritime forest must be considered lower risk than areas without the forest. Thick shrub thickets may also provide enough protection to make the difference between no damage, damaged, or destroyed buildings.
**Velocity (V-zones) are in trouble, period, and should not be developed. A-zones are still possible high hazard even with forest cover because of their susceptibility to flooding and possibly to wave action. High elevation areas with forest are lowest risk, but high elevation areas without forest are considered moderate risk because of wind exposure. Low elevation areas with forest are high risk, and low elevation areas without forest (unfortunately often the most heavily developed areas) are at extreme risk.
**Vegetation cover can be determined initially from aerial photographs or USGS ortho-quads, but must be field checked for accuracy and ongoing changes. The field effort can require from a few hours for islands entirely in A- or V-zones, to 2 or 3 days of mapping for an island with more complex topography.
Revised Risk Map
**The first draft of the property damage risk map is now complete. The map so produced is referred to as the "Preliminary Analysis" map (fig. 4.5). Based on the character of the island, the appropriate parameters (Table 4.5) can now be considered and alterations made on the preliminary map accordingly. One of the most important considerations is the retreat rate (erosion rate) of nearby shorelines. Various historical shoreline change maps often are available through state coastal management agencies or U.S. Army Corps of Engineers reports (see Appendix A for addresses). Clearly, if a moderate-risk zone is likely to be adjacent to the beach in a decade owing to high erosion rates, the hazard classification must be changed to high or extreme risk. Similarly, former inlet positions are mapped, past storm effects recorded, widths of dunes fields measured, and patterns of good or poor development identified, allowing map revision. These secondary factors are summarized on the risk revision map (fig. 4.6) which is the basis for deriving the final ranking of the risk zones.
**One of the more difficult parameters to evaluate is the quality of construction. A poorly constructed house or building truly is a "loose cannon" and will launch or float off of its piers or foundation into adjacent houses during a storm. This battering-ram effect and missiling of debris from adjacent houses is a genuine hazard that increases the vulnerability of houses that may be well built. Strong building codes do not necessarily reduce the risk. Building codes are only as good as their enforcement. Newer isn't necessarily better, as newer designs are often less wind resistant. The hurry to build new houses can lead to shoddy building techniques. One of the most telling incidents come from Hurricane Andrew (1992, FL). Houses built by Habitat for Humanity stood nearly unscathed while neighboring houses were destroyed or heavily damaged. Houses built with care, in part by amateurs, stood better than houses built by professionals. The quality of adjacent construction is a factor in risk analysis. One loose cannon may sink your ship.
**Another factor that's difficult to evaluate is the role of existing or planned engineering structures, either designated for shoreline stabilization or navigation. For existing structures, their performance and impact for past storms can be evaluated. New structures are unknowns, but impacts may be predictable.
**Other common questions that may need to be qualitatively addressed are: Is there a nearby jetty planned or in place that will increase the rate of erosion? Are the frontal dunes still present along the shoreline or have gaps been cut by storms or by communities making vehicle beach access points? Are the frontal or interior dunes about to be removed (within a decade or so) by natural erosion or human activities (development)? Is there a likelihood of inlet migration away from or toward a particular site? Is new inlet formation likely or was there an inlet here in a previous storm which was filled (and is likely to form again)? Are there cross island streets that may allow flooding or overwash into the interior? Do the map zones correspond well to known island response in previous large storms? Is there a wide beach storm buffer, natural or a replenished beach that is likely to quickly disappear? Are there mobile homes nearby which can be assumed to blow apart and furnish wind-blown "missiles?" Are there nearby buildings, old and pre-building code, that are also likely to be destroyed and contribute to missiling or ramrodding?
**Using Tables 4.2 - 4.6 as a guide, each process/feature which applies to the coastal region in question is investigated and evaluated. Historic storm impacts, for instance, may be recorded in popular magazines, newspapers, books, or garnered from "man on the street" interviews. Talking with long-time residents can be a wellspring of information, however, wave heights and wind speeds often seem to "grow" over the years with each retelling of the story! The so-called "grey" literature (e.g., agency reports, consultant reports, unpublished theses) and nonscience sources (e.g., tax maps, histories, newspaper accounts, community historical documents) can provide useful data or general insights.
**All of this information is now considered for incorporation in the final risk ranking (fig. 4.7), occasionally resulting in reclassification of zones to more hazardous categories than those of the preliminary analysis which are based only on elevation/vegetation considerations. Although the initial draft of the community risk map based on the FIRM's, SLOSH maps, elevation, and vegetation is a semi-quantitative approach, incorporation of the numerous other factors is largely subjective. The accuracy of this input and the revision depends on the quality of information (e.g., inlet behavior, response to previous storms) and the experience of the individuals doing the mapping. In actual practice, we have found that the changes from the preliminary analysis map are usually minor.
**The FIRM and SLOSH maps also have a certain qualitative aspect even though boundary lines are drawn at specific elevations. Many unknown factors control flood elevations and overwash penetration on barrier islands. Many, if not most, big storms have "unusual" or "unexpected" aspects to them, resulting in more intense flooding or wave penetration overall or at any given location than predicted. This imprecision is a result of both our lack of complete understanding of all the physical aspects of both barrier islands and storms, and of random turbulence and the general chaotic nature of storms. If we can't predict the weather with absolute precision, then we certainly are unable to predict it's effects any better.
**On the other hand, our experience is that the risk zones on barrier islands defined in Tables 4.1 , continued and 4.4 are quite obvious and their mapping fairly reproducible. Delineation of risk zones, while fairly simple to those familiar with island storm responses, may not be nearly as obvious to individual homeowners or even local elected officials and planners, yet with a little effort the process of mapping can be understood.
**Hazard mapping is ideally suited to the application of Geographic Information System (GIS) computer technology. Island physical and geomorphic (landform) descriptive criteria are entered into a computer data base, then, using GIS, any set of criteria can be combined to give the user the type of assessment desired. The preliminary analysis can be made by GIS summing of elevation and forest cover digitally entered as separate layers. The secondary factors for revising the preliminary map can be added each as a separate digital layer, or summed separately depending on the users needs.
**The GIS system has four advantages over more traditional mapping systems:
o it is digital, making data handling and transfer simple;
o maps can be produced depicting part or all of an island or community at any desired scale;
o new data can be entered into the data base as it is developed;
o updated maps can be easily produced.
**For certain criteria (e.g., dune gaps, cement block houses, houses on grade versus on pilings), a house by house assessment of ranking criteria can be made while driving along a street and entering observations into a laptop computer.
Variable Scales
**Based on the criteria just described, three hazard assessment scales are recommended:
ointer-island scale compares relative overall risks between entire islands;
ointra-island scale examines risks within portions of a single island;
oindividual building scale provides a site-specific ranking by building or small groups of buildings.
(1) Inter-island Scale.--The island scale can be thought of as a comparison of large-scale coastal segments, through which the overall risk factors for an entire island or shoreline reach are determined relative to other islands. The ranking is based on an overall geologic setting and coastal processes evaluation which considers property damage risk, but ignores political boundaries and hazards to life. Many of the processes which affect barrier islands are large in reach or area (e.g., wave refraction patterns, longshore drift, and currents which may affect long stretches of shoreline on front, back, or inlet sides of an island), and are influenced by the overall island geometry (e.g., tidal deltas, beach/dune ridges, dune fields). This big-picture scale addresses island-length and cross-island processes relative to sediment supply, long-term changes in island morphology, and potential changes in plant distribution.
**Figure 4.8 is an island-scale ranking of the North Carolina barrier islands. The ranking is on a 0-4 scale, 0 is poor (high risk) and 4 is good (least risk). Table 4.8 gives the basis for the ranking. This type of ranking is useful to pinpoint areas of concern, and areas in which more study or resources may be needed to develop a property damage mitigation plan. Maps of this scale are also useful as a first level of evaluation for individuals seeking to purchase coastal property.
(2) Intra-island Scale.--The second level of risk evaluation, the intra-island scale, ranks various hazard zones of one portion of an individual island relative to other portions of the same island. The risk map in Figure 4.7 is an example of the intra-island scale map for the northern portion of Nags Head, North Carolina. The intra-island, or community, scale is ideally suited to application of Geographic Information System (GIS) computer technology, and provides a basis for developing a community mitigation plan. At this scale one can evaluate features such as the continuity of a dune line or forest cover within a specific stand of forest. Service grids would be considered at this scale (e.g., Are streets running perpendicular to the shoreline? Are utility or water lines parallel to the shore likely to be cut by storm surge processes?). Locating in the middle of an area designated extreme or high risk is not wise economically even if the specific site can be mitigated against hazards.
(3) Individual Building Scale.--The third level of risk assessment, that of individual buildings or groups of buildings involves more detailed description and assessment of sites and local alterations of the natural island environment. At this scale, building codes and compliance as well as architectural design and construction can be considered, if desired. Figures 4.9 - 4.12 are examples of this most detailed scale of assessment. This site-specific scale of assessment also lends itself to application of Geographic Information Systems (GIS) technology, making on-site building-by-building assessments. Community planning maps (tax maps, zoning maps) can be used as possible base maps.
The Need for Updating
**Barrier islands are dynamic features and even in their native state, without human alteration, risk assessment of sites can change dramatically, either gradually over a decadal time frame, or rapidly in a single big storm. Shackleford Banks, North Carolina, on the Cape Lookout National Seashore, affords an example of such change. At the eastern end of this east-west oriented island, large dune fields once existed. They were even used as lookout points to spot whales by the island's inhabitants in the 1800's. Then in the late 19th century, a series of hurricanes destroyed houses and discouraged their agricultural pursuits. People disassembled their houses and moved them to Harkers Island (a lower-risk island in Pamlico Sound) and the mainland. The settlements on the island have long since disappeared, the last inhabitants moving off after the 1899 hurricane. The dunes at the eastern end now are completely gone, having been lost to storms and rapid shoreline erosion, mostly within the last three decades. An area that would have been rated as a moderate risk zone for development has changed naturally to high or extreme risk as the island character evolved from a dune dominated system to one impacted mainly by storm overwash. At about the same time, farther to the west, an active dune field developed and migrated over the maritime forest, significantly changing elevations and eliminating forest cover.
**Equally significant changes can be made by humans. On Ocean Isle, North Carolina, an interior dune ridge, the only one on the island, was flattened to make way for development. The lowered elevation put the entire development in the V-zone, leading to a change in risk classification from the preliminary to the final mapping from moderate-risk to extreme risk. The change of the island topography didn't happen 50 years ago or even 25, but in 1994 (fig. 4.13)! That risk increase will be reflected immediately in property owner's insurance costs because V-Zone properties pay the highest flood insurance rates. Remember: most developers seek to maximize their profit which may not be in the purchaser's best interest with respect to potential property damage.
**Another example of change, impacting on property damage risk, can be seen in Kitty Hawk, North Carolina. The dune along the shorefront of this community once extended along the entire community. It was constructed in the 1930's by the Civilian Conservation Corps to halt the shoreline erosion, and to provide a "protected" area along which to build a road. The artificial dune failed to change erosion rates, but it did afford some protection for buildings. After 50 years of dancing, however, the piper now demands payment. During the 1980's, the dune began to deteriorate due to storm penetration, and the 1991 Halloween northeaster finished the job by creating large gaps in the dune resulting in flooding of portions of the community. The dune cannot be rebuilt in place because the old dune location is now occupied by the beach, backed up against the frontal road. Between the time of dune construction and 1991, the community had only experienced major flooding once, in the great 1962 Ash Wednesday storm. Since the 1991 storm, the community has been flooded four times (fig. 4.14)!
**The effect of shoreline engineering on a whole-island system is starkly portrayed in Figure 4.15. It has taken several decades to be fully realized, but the impact of the Ocean City, Maryland jetties on the next island to the south, Assateague Island, Virginia can now be seen, over 50 years after the jetties were constructed. Similar stories abound along the coast. The Morris Island lighthouse, once on the backside of Morris Island, now stands some 2000 feet at sea; a sentinal that watched Morris Island rapidly migrate away after the Charleston jetties were built in 1898 (fig. 4.16).
**Risk mapping must be viewed as a dynamic process. Risk maps should be re-examined at least on a decadal time frame, and after each major storm. Even without storms, dune systems may change, shorelines can retreat, forests can be killed or removed. Major storms are bound to make changes on the island and of course they are the ultimate test of the validity of the risk maps. Meaningful mitigation must be ongoing, adaptable to natural and human-induced changes, and applied island wide, not just along the island front.
Table 4.1 and continued. Characteristics of the coastal property damage risk categories used in this book. Also included are skeletal descriptions of the type of damage that might be expected from a storm and recommendations for mitigation of property damage potential.
**RISK Categories of Evaluation
Primary Hazard Controls on Vulnerability (determine storm surge, flood, overwash, and wind hazards) Secondary Controls on Vulnerability (other modifying hazards)
**Vulnerability
Probable Damage/Destruction in Category 3 Hurricane of equivalent wind/wave/surge
Mitigation Recommendations
Elevation Vegetation Erosion
Rate Inlet
Potential Construction
Factor
**EXTREME Low; mostly in V-zone; some A-zone sites. Typically back beach, low frontal dune, active overwash front, similar on soundside and near inlets None or sparse beach/dune grasses only; no maritime forest or shrub thicket high to moderate Near migrating, historic, or potential inlet position Older buildings, built before building codes, not to code, or with code violations, closely spaced, or new building that were not built to code or violate code
MAXIMUM (likely to be impacted by 4 or more hazardous processes) Total destruction to very heavy damage by direct wave attack for all elements of buildings (roof, windows, walls, foundations, decks, porches, stairs, garages), as well as out buildings, utilities, services, and landscaping Relocate before storm or abandon site after storm. DO NOT REBUILD DESTROYED BUILDINGS IN PLACE. Protect marshes. Preserve, augment, and restore all natural environments.
**HIGH Low; in A-zone and lacks forest or shrub thicket cover. Typically overwash apron or fan extension but away from V-zone; inner dune trough or frontal blowout in dunes; perhaps flat interior grasslands
**Sparse to none, or greatly disturbed by development or past salt-water kills High to moderate As above As above HIGH (likely to be impacted by at least 3 hazards) Total destruction is not uncommon and heavy damage is likely; wind damage and flood damage most probable; all building elements at risk (roof, windows, walls, foundations, attachments) as well as outbuildings, services, and landscaping. Relocation is most prudent; elevate and vegetate; build protection in region around site (e.g., dune projects, forestation), plug dune gaps, change street layout; maintain existing projects (e.g., beach nourishment), but not shore hardening. Protect marshes.
**MODERATE Moderate to high, and not in V-zone; possibly A-zones if heavily forested. B-zone or out of flood zone (but downgraded by secondary hazards). In dunes or other elevated landforms such as built up terrace. Stable vegetation of maritime forest or shrub thicket (not seriously disturbed by development), interior and frontal dunes well vegetated, island backed by healthy marsh. A-zones even with good vegetative cover can still be at high risk.
**Low to accretionary Should be away from migrating inlet; downgrade if near historic or potential new inlet. Downgrade if as above. All buildings should meet or exceed building code standards. MODERATE (likely to be impacted by at least 2 hazards, and nuisance hazards such as overwash and dune migration are likely). Heavy to moderate damage should be expected. Wind damage very likely (e.g., windows, roof, walls, attachments). Flood damage less likely. Landscaping affected by overwash and blowing sand. Above ground utilities and services likely to be interrupted. Elevate and vegetate on site and surrounding region (e.g., vegetated dunes, rehabilitate forest, plug overwash gaps or work to maintain natural overwash sites; promote sand additions to island interior; modify street layout). Protect marshes.
LOW High, C-zone (or B-zone but upgraded by healthy maritime forest and no secondary problems). maritime forest cover is health and largely undisturbed. Surrounding environs also well vegetated. Zero to accreting. No inlet potential. All construction is at or above code and is well maintained. LOW (no more than one hazard likely). Expect damage, but not heavy. Wind damage is most likely (cosmetic to exterior roof, walls, windows, attachments). Potential serious damage from falling trees or blowing debris. Rain damage if glazing fails or roof/wall leaks develop.
Augment protective vegetation and dunes as above. Do not remove sand. Protect marshes.
VERY LOW High on mainland away from direct coastal winds. Mainland forest cover (buildings under canopy level).
**As above; open land space. VERY LOW (hazards unlikely). Possible cosmetic damage from wind or flying debris; potential for tree falls. Possible water leakage.
Replant and maintain forest cover.
Table 4.2. Field parameters that can be used to determine the category of risk of property damage in coastal storms. These are generalized and must be modified somewhat for individual islands.
Geo-indicator High Risk Moderate Risk Low Risk
Site Elevation < 3 m 3 m - 6 m > 6 m
Erosion/Accretion Rate severely to slowly eroding stable accreting
Beach Width, Slope, and Thickness narrow and flat, thin with mud, peat, or stumps exposed wide and flat, or narrow and steep wide with well-developed berm
Overwash overwash apron (frequent overwash) overwash fans (occasional overwash) no overwash
Site Position Relative to Inlet or River Mouth very near within sight distant
Dune Configuration no dunes
(see overwash) low, or discontinuous dunes high, continuous, unbreached ridge
Bluff (unconsolidated) Configuration bare face, recent or no talus ramp vegetated face and well-developed ramp low slope angle (large ramp), mature cover of vegetation
Coastal Shape concave or embayed straight convex
Vegetation on Site little vegetation or toppled vegetation well-established shrubs and grasses, none toppled mature vegetation, forested, no evidence of erosion
Drainage poor moderate good
Area Landward of Site lagoon, marsh, or swamp (e.g., mangrove) floodplain or low-elevation terrace upland
Table 4.3. The coastal risk assessment method.
Table 4.4. Typical risk categories of barrier island environments of southeast U.S. barrier islands. These categories may vary from island to island.
LOW HAZARD:
1. Mainland forest (high elevation)
2. Maritime forest (high elevation)
MODERATE HAZARD:
3. Maritime shrub thicket (high elevation)
4. Vegetated interior dunes (high elevation)
5. Active dune fields (high elevation)
6. Vegetated interior dunes (varying elevation)
7. Dune swales and blowouts
HIGH TO EXTREME HAZARD:
8. Overwash Apron (low elevation, no forest)
9. Washover fans (low elevation, no forest)
10. Frontal dunes
11. Ocean beaches
Table 4.5: The impact of development on the natural environment and on the risk of property damage (modified after Nordstrom, 1987).
Development Type
Building site modification:
-grading
-paving
-dune removal
-paths through dunes
-forest removal
-roads and other infrastructure
Construction of buildings:
-single family
-multi-family
-high, medium, low rise multi-family
Hard shore stabilization:
-seawalls
-groins
-jetties
-breakwaters
Soft shoreline stabilization:
-beach nourishment
-dune building
-sand fencing
-bulldozing
Inlet channel alteration:
-dredging
-relocation
-artificial closure
Direct Effects
-Changes landform configuration.
-Eliminates sources of sediment.
-Decreases sediment stability, changing rates of on/off-island sediment exchange.
-Alters wind patterns.
-Obstruction to sediment flow.
-Truncates beach or dune zone.
-Channels storm surge and ebb flow.
-Reflects waves.
-Armors shoreline.
-Alters sediment flows.
-Changes location of erosion/deposition and its severity.
-Reduces or prohibits on/off island sediment exchange.
-Changes sedimentation rates and severity of erosion.
-Interferes with on/off island sediment exchange.
-Alters natural current pattern.
-Changes location of erosion/deposition and its severity.
Indirect Effects
-Establishes property in hazardous areas.
-Increases exposure of property to wind and wave hazards.
-Focuses human use and human impacts.
-Leads to construction of support infrastructure and implementation of protection structures.
-Increases population density.
-Leads to further development, putting more property at risk.
-Leads to need for more structures.
-Destroys recreational beach.
-Masks nature of erosion problem.
-Leads to further development.
-Can encourage development in inlet hazard zones.
-Leads to calls for structural improvements.
Table 4.6: The secondary factors involved in property damage risk categorization (see Table 4.3).
Table 4.7 and continued: Factors that must be considered to determine the potential for property damage in the coastal environment. In a given situation, some of these factors may be of little importance but this list can be a useful checklist to be certain some parameter or other has not been dropped.
Table 4.8. Relative island scale risk ranking of North Carolina barrier islands. This inter-island level ranking includes the evacuation hazard (People Danger) and also a subjective ranking of future development trends (Crystal Ball).
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