A variety of laws, executive orders, and regulations clearly charge the NPS with preserving cultural resources and providing for their enjoyment "in such manner and by such means as will leave them unimpaired for the enjoyment of future generations." Parks offer special opportunities for people to experience their cultural inheritance by offering special protection for cultural resources.
The NPS Management Policies recognize five broad categories of cultural resources, with many resources often classified. into multiple categories.
Archeological resources are organized bodies of scientific evidence providing clues to the mystery of past events, primarily objects in context, ranging from household debris in a site from a past culture, to foundations of buildings, to pottery and tools, to paintings or writings.
Cultural landscapes are settings humans have created in the natural world showing fundamental ties between people and the land, ranging from formal gardens to cattle ranches, and from cemeteries or battlefields to village squares.
Structures are large, mechanical constructions that fundamentally change the nature of human capabilities, ranging from Anasazi cliff dwellings to statues, and from locomotives to temple mounds.
Museum objects are manifestations and records of behavior and ideas that span the breadth of human experience and depth of natural history, and may include archeological resources removed from the context where they were found.
Ethnographic resources are the foundation of traditional societies and the basis for cultural continuity, ranging from traditional arts and native languages, spiritual concepts and subsistence activities which are supported by special places in the natural world, structures with historic associations, and natural materials.
An important aspect of cultural resources is their non-renewability: If they lose significant material aspect, context, associations, and integrity, they are lost forever. The responsibility of the NPS is to minimize loss of pre-historic and historic material. Closely related but secondary responsibilities include maximizing the expression of historic character, integrating site development with natural processes, sustaining the lifeways of ethnic groups, increasing our knowledge of past human behavior, and supporting the interpretation of park resources.
Cultural resources of the NPS affected by overflights range from Anasazi cliff dwellings and museum objects, to the faces at Mount Rushmore, to Civil War battlefields and cemeteries, to religious ceremonies at Hawaiian temples, to the Statue of Liberty and the Jefferson National Expansion Memorial, to
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reenactment of important events in history or living examples of everyday life during an historical time period. They encompass museums, ships, and factories as well as paintings, clothing, dishes, books and fragile artifacts.
Possible adverse aircraft overflight impacts on cultural resources entrusted to the NPS include physical impacts from vibrations, loss of historical or cultural context or setting, and interference with visitors' park experience. The term "adverse effect" has special meaning when used in association with historical properties. The definition put forth in The National Historic Preservation Act of 1966 states:
"An undertaking is considered to have an adverse effect when the effect on a historic property may diminish the integrity of the property's location, design, setting, materials, workmanship, feeling or association."
While physical impacts can permanently harm objects, impacts to context or setting, such as when aircraft fly over an 1800's reenactment or an ancient religious ceremony, can significantly reduce the associations and integrity of the objects, and the enjoyment and understanding of the cultural heritage.
Section 1 of Public Law l00-91 requires the NPS to assess the effects of aircraft overflights on historical and cultural resources:
"The research at each such [park] unit shall provide information and an evaluation regarding each of the following...
"(3) Other injurious effects of overflights on the natural, historical, and cultural resources for which such units were established....”
At a large number of parks, cultural and historical resources are the focal point of the park. In many cases these resources were the primary reason for the park's creation, and they continue to be the reason for its existence. For example, cultural resource preservation is the primary mission at park units such as Chaco Culture National Historical Park, Canyon de Chelly National Monument, Colonial National Historical Park, Gettysburg National Military Park, Gila Cliff Dwellings National Monument, Mesa Verde National Park, Pu'uhonua o Honaunau National Historical Park, San Antonio Missions National Historical Park, Statue of Liberty National Monument, and numerous historical forts around the country. This section addresses whether overflights adversely affect cultural sites, structures, objects, as well as sacred sites and ceremonies.
4.1 Extent of Concern by Park Management and Visitors
Two recent surveys provide information on the extent and intensity of concern by park managers and by visitors.
4.1.1 Park Management Assessment
Park managers are responsible for safeguarding the resources in their parks. Cultural and historical resources are no exception. In order to learn more about these concerns, in the context of aircraft overflights, the Park Manager Survey (HBRS, Inc., 1994) asked questions about cultural and historical
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resource preservation. Survey questionnaires were sent to 98 park managers whose units had been previously identified as having some level of aircraft overflight problems. The responses provided by the managers are reported below. Although the results cannot be applied to the entire National Park System, they reflect the perspective from nearly one-third of the units in the System with 76 percent of the acreage and 53 percent of the visitation. And they reflect the subset of park units where aircraft overflights have generated some level of concern.
To determine the proportion of parks where cultural and historical resources were considered important, managers were asked how important it was for their park to provide an opportunity for visitors to appreciate the historical and cultural significance of the park. Their responses are shown in Figure 4.1. Out of 91 park units responding, 90 percent of the managers said that providing this opportunity was "moderately," to "extremely" important to them.
The managers were then asked the degree to which they felt aircraft activity interfered with their ability to provide this opportunity. Their ratings are shown in Figure 4.2. Opinions are clearly distributed across the entire range, from "not at all" to "extremely." However, over 50 percent of the managers rated the degree of interference in their parks as "moderate" to "extreme, " and over 10 percent rated interference in the "extreme" category alone.
The responses provided by the managers to these two questions make two important points. First, cultural and historical resources exist in, and are important to, the vast majority of the 91 park units who responded to the survey. This finding suggests that the potential for impact exists in many park units. Second, the extent to which this potential has been realized is sizeable. As was true in the case of managers’ evaluations of overflights in general, (as discussed in Chapter 2), the impacts in cultural and historic sites are judged to vary widely from park to park, with serious impacts occurring in some parks.
Conclusion 4.1
Park managers believe that providing an opportunity for visitors to appreciate the historical and cultural significance of the parks is an important goal, and that aircraft overflights, in certain circumstances, can significantly interfere with that opportunity. A systematic approach for addressing overflights must account for any special cultural or historical opportunities provided by specific parks.
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During the spring, summer and fall of 1992, the NPS conducted the Visitors Survey of park visitors at 39 park units selected to represent the National Park System (McDonald et al, 1994). Of these, eight were primarily cultural or historical parks. The parks were selected through a very careful, system-wide sampling of NPS units in order to achieve a statistically valid sample of the entire National Park System (excluding Alaska). In this survey, visitors were asked to complete a brief questionnaire as they exited the park.
As part of the survey, visitors were asked how much the sounds of aircraft interfered with their appreciation of the historical and/or cultural significance of the park. Figure 4.3 summarizes the responses from the eight cultural and historical parks sampled. A great majority of visitors felt that little interference had occurred. Similarly, system wide only about two percent of the visitors (about 7 million visitors) reported a moderate to extreme degree of interference.
These results present an expected difference from those of the managers' survey shown in Figure 4.2 (where 49 out of 91 managers reported moderate to extreme interference with their ability to provide cultural or historical opportunities). This difference is attributable to two primary factors:
As with other forms of impacts on visitors discussed in chapter 6, the impact on appreciation of the historical / cultural significance varies considerably park to park. Figure 4.4 shows for each of the Visitor Survey parks the percent of visitors reporting interference with appreciation of history / culture and the percent of visitors who reported hearing aircraft. Like the other impacts investigated, an understanding can be gained only by examining individual parks.
Conclusion 4.2
There is no wide-spread impact on the appreciation of historical and cultural resources by visitors from aircraft overflights. But under certain combinations of aircraft overflights and cultural / historical park opportunities, visitors’ experience may be impacted. The NPS believes this impact is occurring at a limited number of areas.
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4.2 Acoustic Impact on Cultural and Historical Resources
Because of the diversity in the cultural and historical resources of the National Park system, as well as in the types of aircraft overflights the parks experience, many types of impact can occur. For convenience, these impacts may be divided into two major categories (1) audible acoustic impact, and (2) noise-induced vibration. In addition, the NPS has some concern that the rotor wash from helicopters approaching too close to cliff dwellings could disturb materials in context (e.g., pollen, soils, etc.) Audible non-indigenous sound affects solemnity, natural quiet, and speech communication. Noise-induced vibration affects artifacts and structures. These impacts are addressed in subsequent subsections.
The sound from aircraft activity can impinge on the solemnity of sacred sites as well as interfere with Native American ceremonies. National parks provide opportunities for quiet generally unavailable in common non-park settings. Such quiet park surroundings provide unique opportunities for visitors to experience cultural and historic sites in an historically accurate audible environment - the environment that existed before the introduction of mechanized power.
An illustrative example of how overflights can impact site solemnity, speech communication, and historic structures is the situation at Taos Pueblo, one of the oldest living communities still existing in the United States, a candidate World Heritage Site. In a 1992 letter and position paper on the subject of military overflights of the Taos Pueblo, pueblo spokesperson gave this overview of their problem:
"The Pueblo of Taos has serious concerns with continual overflights of aircraft which intrude into the sensitive areas of our village and our sacred wilderness lands under trust protection with the United States Government.
Our Blue Lake Wilderness is a place of retreat and prayer to regain the Strength of Life for our People. Our sacred shrines lie throughout the region. Within our village is the place of ceremony where, within our Kivas and ancient homes, a season of quiescence is observed in reverence for the Earth. In recognition of these sacred ways and our ancient architecture, our village is denominated a National Historic Site and is under consideration as a World Heritage Site. Intrusions to our privacy cannot be tolerated, for they threaten the continuance of an ancient way of life.
In recent years, we have experienced an astonishing increase of both proposed and actual airflight activity over Taos Pueblo lands. The FAA and Town of Taos have proposed a commercial air corridor directly over our village and wilderness, originating approximately three miles from our borders. This project is currently in the EIS process. Private overflights are increasing. During our annual Blue Lake pilgrimage, our People were buzzed by low-flying Cessna aircraft, sixty to eighty feet high, attempting to film the sacred "Journey for Life." In 1991, the U.S. military proposed a low-level flight training corridor across our lands. This process has been temporarily halted from community outrage. Six months later, at least two out- of-state military bases adopted flight patterns across our wilderness, resulting in forty to seventy overflights a day. The flights alternate between low-level passes through our canyons, and B-1 high-altitude refueling flights, which echo off the mountain passes because of the size of these bombers."1
1. October 5, 1992 Letter and position paper from the Taos Pueblo to Senator Daniel Inouye (Hawaii) seeking relief from military overflights of the pueblo.
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In an earlier letter that year to the National Congress of American Indians on this subject, the Taos Pueblo more fully communicated the nature of their concern on this issue:
"Under the traditional ways of Taos Pueblo, air is part of the "sacred realm' to which we hold inseparable responsibilities from land and water protection. Our ceremonial ways protect all things of the Earth and all things overlying the earth. It is impossible for us to conceive of adequately carrying forth our traditional responsibilities when we can no longer control what happens within the sacred realm of the "upper domain." Among our people, it is understood that there is no separation between Earth and the realm overlying the Earth. It is an inseparable extension of life and the responsibilities toward life.
"These are concepts that are not understood by non-Indian people. Native American Indian people alone understand the unique responsibilities that Tribes hold toward the perpetuation of life, and how the successful outcome of our ceremonial responsibilities provide the link for human beings to maintain their connections to all life.
“The space overlying our sacred shrines and wilderness areas which are used for the perpetuation of tradition must be protected at all cost. Airspace to Native American Indian people is more than a resource for the generation of income. It is the sacred medium through which we make our connection the Spirit of Life, and through which all life is maintained.”2
National parks in Alaska, Hawaii, and the contiguous 48 states contain abundant resources traditionally defined and used as sacred and subsistence grounds by Native Americans and others associated with areas now under NPS stewardship. Some resources that Native Americans define as meaningful fall into the historic preservation category of "cultural resources," or rites, structures, and landscapes. Others may be termed "natural resources" for land management purposes, but are defined culturally by Native Americans and other traditional user groups as places of religious meanings. These include naturally configured shrines, power rocks and caves, ethnobotanical gathering areas, and traditional ceremonial hunting areas. Subsistence areas and wildlife, especially in Alaska, are among the resources invested with cultural significance for food-gathering purposes. Characteristically, Native American food-gathering also occurs within a religious context.
Native Americans note that undisturbed habitats, particular resources, and contexts are pivotal to the success of religious practices. Contemplative activities involving communication with holy beings require the intense concentration that quiet, restful surroundings engender. Unnatural disturbances during religious ceremonies portend harm to traditional practitioners of sacred acts and their intended beneficiaries. Mark Schoepfle (Schoepfle, 1989), for example, suggests that "...disruptions may cause important supernatural power to be misdirected, at considerable peril to the beneficiary of the ceremony or medicine man or shaman. In these cases sickness or harm may require further ceremonial or religious intervention, at considerable expense to the people involved." In this same vein, Thomas Greider's work (Greider, 1993) for the U.S. Air Force on the effects of low-level flyovers on Native American religious practices indicates there is a noticeable effect on the practice of Native American curing ceremonies from the flyovers.
2. June 10, 1992 Letter from the Taos Pueblo to the National Congress of American Indians (NCAI) explaining the overflight issue at the Taos Pueblo.
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Data on traditional ceremonials disrupted in parks by low flying craft have not been systematically collected, but informal comments suggest problems. At risk, for example, from disruptive overflights are religious activities and practitioners associated with the Timbisha Shoshone Tribe who live at Death Valley National Monument. Overflights at Grand Canyon can threaten religious activities of neighboring Hopi, Navajo, Hualapai, Havasupai, and Paiute. Observers have noted that on the flight corridors that run immediately south of the Kaibab Paiute Indian Reservation, adjoining Pipe Spring National Monument, caravans of B-52s can be observed traveling east to west close enough to the ground for their aircraft numbers to be read easily.
Conclusion 4.3
Just as the sound of overflights can impair opportunities for experiencing natural quiet, so too can these sound levels adversely affect not only the experience of visiting historic, cultural or sacred sites, but also the preservation of traditions that are an inherent part of a way of life. A process for identification and resolution of overflight impacts must recognize these significant adverse effects though they may be subtle to the uninformed.
4.3 Vibration Impact on Cultural and Historical Resources
The sound from aircraft activity can cause archeological resources, structures, and museum objects to vibrate. Depending on the character of the sound, the effects range from audible rattle, to items "walking" across surfaces, to fatigue cracking, and potentially to direct or indirect structural damage (Hanson et al, 1991). Considerable government sponsored research has been conducted on the effects of aircraft noise on structures. Most of this research, however, has been related to sonic booms. Research includes work sponsored by the U.S. Air Force (USAF) under its Noise and Sonic Boom Impact Technology (NSBIT) program, by the National Aeronautics and Space Administration (NASA), and by the FAA. Environmental impact assessments conducted by the USAF for proposed low-level military training routes, measurements of noise-induced vibration of building conducted by NASA near rocket launch sites, and measurements of airborne noise effects from blasting by the Bureau of Mines also provide valuable information. By comparison, only a limited number of vibration measurements have been conducted on archeological ruins exposed to fixed- and rotary-wing aircraft noise. This work has been conducted by the U.S. Geological Survey and the USAF Geophysics Laboratory.
Airborne sound is a form of energy which travels in waves through the air. When sound waves encounter a structure or solid object, part of the energy is transferred to the structure and part is reflected. The portion which is transferred causes the object to vibrate. The magnitude of the resultant vibration is dependent on the characteristics of both the sound source and object itself.
A very important characteristic of the structure is the way it responds to being shaken at different rates. Structures are very much like a series of springs and weights in the way in which they respond to acoustic pressure loadings. The type of construction (masonry, wood frame, etc. ) determines both the springiness and the weight. Together, the springs and weights create natural frequencies in the structure. If structures are shaken at their natural frequencies they will vibrate more than if they are shaken at other frequencies, even though the shaking force is the same.
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An example of the natural frequency phenomenon may be observed by attaching a small weight to a rubber band (a weight sufficient to stretch the rubber band about 2 inches). Holding the rubber band between the fingers, if the rubber band is shaken very slowly (up and down about once per second) the weight moves up and down about the same distance as the hand inducing the motion. When the speed of the shaking (the frequency) is slowly increased, the weight will begin to travel a greater distance than the hand. As the frequency is increased further, this phenomenon will reach a maximum. Shaking at even greater frequencies will reduce the motion of the weight, and as the speed is increased even further, the weight will hardly seem to move at all. The rate of shaking which produced the maximum movement of the weight is the natural frequency of the system.
The most important observation from the preceding experiment is that the weight moves a considerably greater distance than the source of the movement (the hand) when the system is shaken at its natural frequency. The spring and weight system actually magnifies the amplitude of the motion. Buildings and structures behave the same way. Thus, seemingly minor acoustic pressure loadings on a structure can have significant damage potential. When the sound wave contains considerable energy in the vicinity of the structure's natural frequency, the damage potential is maximized. Typical natural frequencies of structures are in the region of 8 to 12 complete cycles per second. The cause for concern is that sonic booms, helicopters and jets generate considerable energy at these frequencies.
4.3.2 Types of Aircraft Noise That Can Excite Structural Response
Damage potential from aircraft activity depends on the character of sound produced by the aircraft. For analytical purposes, aircraft sounds are divided into three categories: Sonic booms, subsonic fixed-wing noise, and helicopter noise.
SONIC BOOMS. These are caused when an aircraft flying faster than the speed of sound passes an observer or structure. The result is a very brief pressure pulse similar to that shown in Figure 4.5 (Sutherland et al. 1990). Because of its shape it is frequently called an "N-Wave." The amplitude of the wave can be very large, even when the aircraft is several miles away. During the wave, the pressure rapidly increases to a maximum (above atmospheric pressure), then decreases less rapidly to a minimum (below atmospheric pressure), and finally returns rapidly to atmospheric pressure. The upper peak of the "N" is the overpressure and the lower peak is the underpressure. These pressures are affected by the size, speed and altitude of the aircraft.
This wave usually sounds like two sharp booms in rapid succession (one from the steep initial part of the wave, and one from the steep return to atmospheric pressure). To a structure, however, the experience is much different. The structure generally responds to the lower frequency components of the pressure wave which are not audible to the human ear. The structure experiences a large push from the positive part of the wave, and then a pull from the negative part. Much like an ocean wave, the angle at which the wave encounters the structure determines whether particular surfaces experience the load of the wave head-on or as the wave ripples by. Either way, a unique bending load is placed on the structure. In general, the larger the surface, the greater the load because the pressure has a larger surface to act on. Sonic booms often produce sizable amounts of energy at the natural frequencies of structures.
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SUBSONIC, FIXED-WING FLYBYS. Heavy aircraft (bombers) at close range can produce substantial low frequency energy. Depending on factors such as airspeed, wing area, and distance, heavy low-flying aircraft can generate substantial turbulent vortices. The vortices from heavy aircraft are of concern at all major airports because of their ability to compromise the safety of smaller aircraft following in their wake. The FAA has strict air traffic control guidelines to ensure adequate distance separation to prevent incidents. The same turbulent vortices of concern to the FAA can also place large turbulent loads on structures.
SUBSONIC, ROTARY-WING FLYBYS. Helicopters produce a substantial amount of their energy at the natural frequency of structures. The size of the rotor (which provides lift as well as propulsion) produces significant acoustic energy, and the relatively slow speed of the rotor causes this energy to be concentrated at low frequencies. In general, the heavier the helicopter, the greater the radiated low frequency energy. The main rotor is a very directional sound source: it produces sound that has unique radiation patterns depending on where the observer is located in relation to the aircraft.
There are two important radiation phenomena of helicopters when it comes to structural response: blade vortex interaction (BVI), and "thickness" noise. Blade vortex interaction (sometimes referred to as "blade slap") occurs when the helicopter is in forward motion. A blade passing along the back side of the aircraft encounters (slaps) the turbulent wake created a fraction of a second earlier by another blade passing along the forward side of the aircraft. This sound radiates down and to the front of the aircraft, as shown in Figure 4.6. It is very directional, and not audible after an aircraft passes overhead.
Thickness noise from helicopters is not routinely experienced by observers on the ground. This is because the noise generating phenomenon radiates sound only in a narrow angle (+/- 10 degrees) above and below the plane of the main rotor, as shown in Figure 4.7. Hence, an observer must be at about the same elevation as the aircraft to observe thickness noise. Substantially more energy at the natural frequency of structures is radiated in thickness noise than in blade vortex interaction.
Conclusion 4.4
Aircraft overflights create sound levels of frequencies low enough to induce natural frequency vibrations in structures. Supersonic aircraft flight, overflights by very large aircraft, and helicopters can all produce levels that may cause structural vibrations. In the case of subsonic flight, the aircraft must fly relatively near the structure for vibrations to be great enough to result in risk of damage.
Damage potential from aircraft activity also depends on the structure itself. Structural vibrations, especially with repeated exposures, can eventually lead to structural damage of irreplaceable resources. These resources include historical and archeological structures such as sites on the National Register of Historic Places and National Landmarks, and under certain circumstances, archeological sites and artifacts and cultural resource objects inside structures. In looking at damage potential, there are short term and long term effects.
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SHORT TERM EFFECTS. A short term effect is one in which one or two noise events are sufficient to produce a permanent displacement in a structural element. A collapsed roof, or a broken window are dramatic examples of acoustic pressure loads that are capable of producing structural failure or a major compromise to structural integrity in only a few flexure cycles. Bric-a-brac or historical artifacts falling off shelves is another example. These effects are most often associated with large amplitude single events such as sonic booms. Damage risk criteria have been developed for different types of construction, and boom strength estimating procedures have been developed for different types of aircraft and supersonic maneuvers. Together, they provide a first level means for predicting damage potential.
LONG TERM EFFECTS. More insidious are the long term effects created by repeated exposures at lower acoustic levels. While the dramatic effects of sonic booms can result from only a few, large-amplitude pressure cycles, equal damage can be accomplished with greater numbers of lower amplitude pressure cycles (a single helicopter hovering for 30 seconds with a blade passage rate of 12 per second would produce 360 pressure cycles). In contrast to a major structural displacement, the smaller pressure cycles may initiate a slower process consisting of three stages: 1) fatigue cracking, 2) moisture damage, and 3) erosion damage. The lower amplitude acoustic pressure cycles can initiate fatigue cracking. Although these cracks are often no more than hairline in width, they begin when brittle materials, such as masonry, are momentarily stretched beyond their breaking strength. Careful observations have confirmed that repeated exposures to acoustic excitations produce ever-widening cracks. Most masonry construction is particularly vulnerable because of its brittle nature. Mortar joints, as well as plaster or mud veneers, are construction elements most susceptible to fatigue cracking.
Once fatigue cracking has begun, nature can complete the damage without further assistance (although repeated exposure to acoustic loads hastens the process). Mortar joints are important to masonry structures because they hold the various building blocks of the structure together. Veneers are also important because they act as moisture seals to protect the more vulnerable interior core wall constructions, particularly adobe, brick or rubble. Once a crack has been initiated (or accelerated) by acoustic excitation, moisture penetration can occur. The moisture then initiates a further disintegration process, either by eroding the structural integrity of a wall or roof directly, or by feeding the forces of freeze-thaw cycles. Freeze-thaw cycles are particularly damaging after moisture invasion has occurred because the freezing moisture expands and widens cracks even further. As cracks widen, further moisture is admitted and the process accelerates. Eventually, erosion occurs, and in time the structure is finally compromised.
This entire process may take several years to occur, but the origins are in the initial fatigue cracking. The length of time between the initial cracking and the final collapse of a structure can be years or decades depending on the natural forces at work. This slow process, along with myriad intervening factors, make it difficult to prove conclusively that any particular structure failed due to aircraft noise exposure alone, or even that the process was hastened by such exposure.
Damage risk criteria have been developed for heavy fixed-wing aircraft and also for helicopter flyovers. The major risk factors identified are low flying heavy helicopters and bomber aircraft. Lacking however, are risk assessments for potentially equally damaging exposures of aircraft flying at the same elevation as structures (such as cliff dwellings), an emerging area of interest to the air tour industry. Neither the effects nor the acoustic loads have been carefully documented for this activity.
Conclusion 4.5
Cause and effect are extremely difficult to determine for sound/vibration induced damage to structures. Accordingly,
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the NPS needs to develop a systematic method to inventory sensitive structures likely to be subject to potentially damaging sound exposures, and prepare methods for minimizing damage risk.
Mitigation is a three step process. It involves: 1) Assessing the acoustic pressure levels associated with potentially damaging aircraft activity, 2) identifying the types and structural conditions of vulnerable structures, and 3) predicting the damage risk for the identified combinations of acoustic loads and structures. Currently, no formal compendium is available enabling park managers to assess for themselves the potential impacts of different types of aircraft activity to be encountered in parks. Sound level prediction methods have been developed for subsonic aircraft (fixed- and rotary-wing), and for supersonic activity. Risk assessments have been published by various researchers for particular combinations of aircraft operations and structures. While much is known, some areas of risk assessment are incomplete. Most noteworthy for the absence of criteria is the damage risk posed by helicopters flying at the same elevation (or vertically within +10 degrees) of at-risk structures.
Much of the damage risk assessment literature focuses on probabilities of damage from single occurrences of acoustic events. Estimates of cumulative effect, the probabilities of damage from continued and repeated exposures, are also available. The best available cumulative effect information is crucial to NPS planning because long-term assessment strategies are needed to preserve valuable resources over very long periods of time. Perhaps the best mitigation method to prevent potential adverse effects to nationally recognized cultural resources is by establishing standoff distances.
Conclusion 4.6
Some damage risk criteria are available, but important areas of information regarding long term exposure and helicopter sound levels are not. If the NPS is to develop guidelines for minimizing risk to structures, such guidelines will require additional data, or they should be based on conservative (protective) assumptions about vibration-induced damage.
NPS managers believe overflights interfere with the historical or cultural significance of some national parks. Across the National Park System an estimated 4-5 million visitors feel their opportunities to experience the historical and cultural resources in parks is impacted. However this small percentage of park visitors increases significantly at parks where managers perceive overflight problems. Consequently, the NPS finds that aircraft overflights are impacting a limited number of cultural and historical parks in the National Park System. Studies show that at parks where aircraft overflight problems are perceived, visitors do indeed notice aircraft and react to them. And it is clear that although there is a need for a systematic approach to problem solving, it must be flexible enough to respond the unique park-by-park, location-by-location problems.
Resolving NPS concerns will require addressing how to prevent vibration related damage from occurring, how to prevent loss of historical or cultural context, how to ensure solemnity for sites and ceremonies, and how to provide interpretation for visitors without serious speech interference.
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