Mitigation Measures for Natural Hazards

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1 Mitigation Measures for Natural Hazards Mitigation measures are actions that eliminate or reduce risks from future disaster events, including: Physical construction projects such as replacing a highly vulnerable facility with a new facility or retrofitting an existing facility, and Evacuation planning to minimize casualties in future disaster events. Mitigation planning is complementary to emergency planning. Emergency planning includes measures to respond more effectively to future disasters. The mitigation measures listed below do not include emergency planning measures, but rather focus specifically on measures that eliminate or reduce damages and/or casualties in future disaster events. 1.0 Earthquake Mitigation Measures Earthquake mitigation measures are typically intended to reduce both casualties and damage in future earthquakes. Common earthquake mitigation measures include: Structural mitigation measures to improve the capacity of a building to resist seismic forces. Structural measures include improving the building elements that hold up a building and resist lateral forces from winds and earthquakes, including: foundations, columns, load-bearing walls, floor diaphragms, roof diaphragms and the connections between these structural elements. Nonstructural mitigation measures to restrain, brace, anchor or otherwise improve the seismic resistance of nonstructural building components such as parapets, chimneys, non-load bearing walls, fire sprinkler systems, HVAC systems, suspended ceilings and lights, windows, water heaters, furnaces, air conditioners and emergency generators. Nonstructural mitigation measures to restrain, brace or anchor building contents, especially tall and/or heavy items that pose life safety risks if they fall, such as bookcases, file cabinets, storage shelves, computers, monitors, televisions and others. Replacement of an existing building with substantial seismic deficiencies with a new current code building. Replacement is typically more expensive than retrofit, but may be appropriate if retrofit costs are high, especially if the existing building is in overall poor condition, needs non-seismic repairs, is near the end of its useful life, is functionally obsolescent, or has other deficiencies such as not being energy efficient. Replacement with a new building on a different site may be appropriate if an existing site is also subject to other 1

2 natural hazards, such as tsunamis or floods, or to significant anthropogenic hazards such as being near a major hazmat site. Design and construction of a new facility to higher than the minimum seismic standards required by building codes, especially for facilities that may serve as emergency shelters. Practicing drop, hold and cover drills, which may reduce injuries from falling objects, but which are not a substitute for other mitigation measures for buildings with substantial structural seismic deficiencies and that may be subject to collapse in major earthquakes. 2.0 Tsunami Mitigation Measures Tsunami mitigation measures are focused predominantly on life safety, with reducing damage being as secondary priority, if considered at all. The primary tsunami mitigation measures are: Developing and practicing evacuation plans. This is an essential, urgent priority for facilities within mapped tsunami inundation zones and strongly recommended for facilities near mapped tsunami inundation zones or simply near the coast at low elevations. An essential component of developing a tsunami evacuation plan is an evaluation of whether it is possible to reach designated evacuation assembly locations in the available time period between the end of earthquake ground shaking and the first arrival of tsunami waves. However, unless there is a high confidence that a safe haven location can be reached in the available time period, then an evacuation plan provides a minimal, if any, increase in life safety. Constructing new facilities well outside of tsunami hazard zones. For existing facilities, where reaching a safe haven location takes longer than the available time period, implementing one of the following mitigation measures is strongly recommended: Replacement of an existing facility with a new facility located well outside the tsunami inundation zone, at an elevation of at least 50 feet and an elevation of 100 feet or more, if possible. Construction of vertical evacuation structures such as platforms or engineered berms in close proximity to an at-risk facility. Designation of existing multi-story buildings as vertical evacuation sites. Existing multistory buildings are suitable for tsunami evacuation if and only if they have been rigorously evaluated by qualified, experienced engineers and found to have all of the following characteristics with a high confidence level: 2

3 o Have accessible floors or roofs at an elevation well above the anticipated tsunami inundation level. Ideally, at least 50 feet above sea level. o Have immediate access for the public on a 24/7/365 basis. o Have adequate structural capacity to withstand the very strong earthquake ground shaking expected from nearby tsunami-generating earthquakes such as large or very large magnitude earthquakes on the Cascadia Subduction Zone or large magnitude earthquakes on the Seattle Fault Zone or Tacoma Fault Zone. Buildings suitable for vertical evacuation must withstand the earthquake ground shaking without major structural damage. o Have adequate structural capacity to withstand the tsunami forces without significant structural damage. If an existing building doesn t meet all of the above criteria, one alternative is to retrofit the building to meet these criteria. It is also possible to construct new multistory facilities suitable for vertical evacuation. In this case, the same selection criteria apply as those listed above for existing buildings. For facilities near the fringe of expected tsunami inundation areas, construction of berms or other barriers to protect the facility may be possible. However, this is done infrequently because of cost considerations. Furthermore, such protective measures do not obviate the need for developing an evacuation plan and for immediate evacuation when a tsunami is possible. 3.0 Volcanic Hazards Mitigation Measures For K-12 facilities, the principal volcanic hazard is lahars (volcanic debris flows), although some campuses are within possible lateral blast zones. Volcanic hazards mitigation measures are focused predominantly on life safety, with reducing damage being rarely, if ever, considered. K-12 facilities are also exposed to volcanic ash falls. However, dealing with volcanic ash falls is more in the domain of emergency planning. The primary volcanic hazards mitigation measures are: Developing and practicing evacuation plans. This is an essential, high priority for facilities within mapped lahar zones and other volcanic hazard zones and strongly recommended for facilities near mapped hazard zones. An essential component of developing a lahar evacuation plan is an evaluation of whether it is possible to reach designated evacuation assembly locations in the available time period between awareness that a lahar has been initiated and the arrival at a given campus. However, unless there is a high confidence that a safe haven location can be reached in the available time period, then an evacuation plan provides a minimal, if any, increase in life safety. 3

4 For facilities with lateral blast zones, evacuation is impossible once a blast occurs. Therefore, evacuations must be pro-active, when USGS volcanic event warnings reach a pre-determined level. Constructing new facilities well outside of lahar hazard zones. For existing facilities, where reaching a safe haven location takes longer than the available time period, implementing one of the following mitigation measures is strongly recommended: Replacement of an existing facility with a new facility located well outside the lahar inundation zone, ideally at an elevation of at least 50 feet above the elevation of nearest boundary of a mapped lahar zone. Construction of a pedestrian bridge to expedite evacuation in locations where access to the nearest suitable evacuation location is precluded by a river without a bridge. For completeness, we note that it might be possible to build vertical evacuation structures for lahars, although cost-considerations for a structure robust enough to withstand lahar forces may well make this impractical and we are not aware of any such structures having been built. For K-12 facilities subject to volcanic ash falls, preparation of dealing with ash falls is recommended to be included in a district s emergency plan. 4.0 Flood Mitigation Measures In most situations, the primary objective of flood mitigation measures is to reduce the potential for future damages and other economic losses. Life safety risk is minimal for most flood situations, because there is typically more than adequate time to evacuate a campus before flooding occurs. However, for locations subject to flash flooding, evacuation planning is a high priority. Most of the mitigation measures outlined below apply to individual buildings, although flood barriers may also apply to an entire campus. Common flood mitigation measures include: Replacement of an existing building with a new building well outside of the flood hazard area. This is the only flood mitigation measure which is 100% effective in avoiding future damages. Replacement is typically considered for facilities with high levels of flood risk, most often with a history of repetitive losses. Replacement may also be appropriate when a building has substantial vulnerability to other hazards such as tsunamis or earthquake, or if the existing facility is in overall poor condition, needs non-hazard related repairs, is near the end of its useful life, is functionally obsolescent, and has other deficiencies such as not being energy efficient. 4

5 Elevation of an existing building is a very common mitigation measure for residential buildings and for small commercial buildings. Elevations have also been done for some school buildings. Whether or not elevation is possible, from both an engineering perspective and a cost perspective, depends on the size and on construction details, including the type of foundation. Relocation of an existing building to a new site is a much less common mitigation measure than elevation, but may be possible for some school buildings, depending on the same factors as for elevations, as well as on the transport routes, distance and rights of way. Flood barriers such as flood walls or berms may be constructed to protect an entire campus or single buildings. Floodproofing of an existing building by waterproofing existing exterior walls and adding removable flood gates for door openings is possible for buildings where the walls have adequate strength to resist hydrostatic forces from floods and anticipated flood depths are not more than several feet. In addition the above whole-campus or whole-building mitigation measures, there are several types of smaller-scale flood mitigation measures which eliminate or reduce flood damages to a limited extent, including: o Elevating at-grade outside utility infrastructure or key in-building infrastructure such as furnaces and electrical panels. o Moving expensive equipment or other expensive contents to an upper story. For campuses subject to flash flooding, developing and practicing an evacuation plan is a high priority mitigation measure. Flash flooding may occur in some narrow valleys in mountainous or hilly areas, in arid areas with limited vegetation but that are subject to occasional heavy rainfalls. Flash flooding is also possible for locations behind levees or downstream of dams or large water reservoirs. Constructing new facilities well outside of flood hazard areas. 5.0 Wildland/Urban Interface Fire Mitigation Measures Mitigation measures for wildland/urban fires typically focus primarily on evacuations for life safety. However, there are also physical mitigation measures to reduce the risk of damage to buildings. Common mitigation measures for wildland/urban interface fires include: Developing and practicing evacuation plans. The most urgent evacuations are for situations in which a wildland or wildland/urban interface fire poses an immediate threat. Evacuations may also be necessary when heavy smoke from a more distant fire poses health risks. 5

6 Mitigation measures to reduce the potential for damage include: Vegetation management and fuel reduction measures for high vegetative fuel load areas near developed areas. Such measures are almost always developed, implemented and maintained by fire agencies and/or other city or county departments, rather than by school districts. Maintaining defensible space around buildings to minimize the potential of a vegetation fire spreading to structures. Implementing fire-resistant construction details for buildings, such as nonflammable roof coverings, nonflammable exterior walls, screening ventilation openings to prevent embers from entering and many others. Siting new facilities in locations with lower risk of wildland/urban interface fires. Vegetation management, fuel reduction measures and maintaining defensible space apply to an entire campus. However, implementing fire-resistant construction details for buildings applies on a building-by-building basis. Fire-resistant construction details, which may go beyond minimum building code requirements, can be included in the design of new buildings and this is may be desirable for new construction in areas at high risk from wildland/urban interface fires. Fire safe measures can also be incorporated into existing buildings, especially when building elements are being repaired or upgraded. For example, it may be most costeffective to install a non-flammable roof covering when an existing flammable roof covering has reached the end of its useful life and needs to be replaced. Similarly, if windows are being replaced for energy efficiency, it may be desirable to install fireresistant windows 6.0 Landslide Mitigation Measures Possible mitigation measures for landslides include: Slope stabilization by dewatering and other methods, Protecting at-risk buildings by building berms or other structures to stop or deflect shallow landslides such as debris flows. For very high risk locations, where a building is located very close to a deeply incised stream with eroding banks or is located at the base of a very steep, very unstable slope, the only viable mitigation measure may be to abandon the existing building and locate a replacement outside of landslide hazard areas. Evacuation when landslide risk appears imminent such as evidence of downslope movements above a building or a rapidly eroding steep slope is approaching a building. Constructing new facilities outside of landslide hazard areas. 6