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Natural Hazards and Security

by the WBDG Secure/Safe Committee

Last updated: 09-26-2013

Overview

Buildings in any geographic location are subject to a wide variety of natural phenomena such as windstorms, floods, earthquakes, and other hazards. While the occurrence of these incidents cannot be precisely predicted, their impacts are well understood and can be managed effectively through a comprehensive program of hazard mitigation planning.

Only after the overall risk is fully understood should mitigation measures be identified, prioritized, and implemented. Basic principles underlying this process include:

Gulfport, MS after Hurricane Katrina

Gulfport, Mississippi after Hurricane Katrina

  • The impacts of natural hazards and the costs of the disasters they cause will be reduced whether mitigation measures are implemented during new construction (preventively) or as retrofits (correctively). Proactively integrating mitigation measures into new construction is typically more economically feasible than retrofitting existing structures.

  • Risk reduction techniques must address as many applicable hazards as possible. This approach, known as all-hazard mitigation, is the most Cost-Effective approach, maximizes the protective effect of complementary mitigation measures and optimizes all-hazard design techniques with other building technologies.

Recommendations

Design professionals agree that the most successful way to mitigate losses of life, property, and function is to design buildings that are disaster resistant. This approach should be incorporated into the project planning, design, and development at the earliest possible stage so that design and material decisions can be based on an integrated "whole building approach."

A variety of techniques are available to mitigate the effects of natural hazards on the built environment. Depending on the hazards identified, the location and construction type of a proposed building or facility, and the specific performance requirements for the building, the structure can be designed to resist hazard effects such as induced loads. Later in the building's life cycle, additional opportunities to further reduce the risk from natural hazards may exist when renovation projects and repairs of the existing structure is undertaken. When incorporating disaster reduction measures into building design, some or all of the issues outlined below should be considered in order to protect lives, properties, and operations from damages caused by natural hazards.

Earthquakes

FEMA News Photo: Earthquake Damage Paso Robles, California

FEMA News Photo: Earthquake Damage Paso Robles, California

Building design will often be influenced by the level of seismic resistance desired. This can range from prevention of nonstructural damage in frequent minor ground shaking to prevention of structural damage and minimization of nonstructural damage in occasional moderate ground shaking, and even avoidance of collapse or serious damage in rare major ground shaking. These performance objectives can be accomplished through a variety of measures such as structural components like shear walls, braced frames, moment resisting frames, and diaphragms, base isolation, energy dissipating devices such as visco-elastic dampers, elastomeric dampers, and hysteretic-loop dampers, and bracing of nonstructural components. Note that the primary focus of earthquake design is life safety, getting people out of the building safely, not the ability of a building to withstand the effects of an earthquake.

Hurricanes, Typhoons, and Tornadoes

FEMA News Photo: Tornado Damage Mena, Arkansas

FEMA News Photo: Tornado Damage Mena, Arkansas

The key strategy to protecting a building from high winds caused by tornados, hurricanes, and gust fronts is to maintain the integrity of the building envelope, including roofs and windows, and to design the structure to withstand the expected lateral and uplift forces. For example, roof trusses and gables must be braced; hurricane straps must be used to strengthen the connection between the roof and walls and walls and foundation; and doors and windows must be protected by covering and/or bracing. The load path and connectors are just as important as bracing. When planning renovation projects, designers should consider opportunities to upgrade the roof structure and covering, and enhance the protection of openings by considering the addition of impact-resistant windows, doors, louvers, etc. The Additional Resources section of this page includes several FEMA publications for designing community shelters, constructed to protect a large number of people from a natural hazard incident, and "residential safe rooms" for occupant refuge during windstorms.

Flooding

FEMA News Photo: Flooding in North Dakota

FEMA News Photo: Flooding in North Dakota

Flood mitigation is best achieved by hazard avoidance—that is, risk-informed site selection away from coastal, estuarine, and riverine floodplains. Still flooding and velocity flooding also present hazards.

  • Riverine hazards are associated with flooding from stream networks.
  • Coastal hazards are associated with flooding from oceans or lakes.
  • Still-water events are characterized by rising water with no horizontal movement.
  • Velocity events are characterized by fast moving waters at any depth.

Should buildings be sited in flood-prone locations, they should be elevated above expected flood levels to reduce the chances of flooding and to limit the potential damage to the building and its contents when it is flooded. Flood mitigation techniques include elevating the building so that the lowest floor is above the flood level; dry flood-proofing, or making the building watertight to prevent water entry; wet flood-proofing, or making uninhabited or non-critical parts of the building resistant to water damage; relocation of the building; and the incorporation of floodwalls into site design to keep water away from the building. Note that levees require a significant amount of care and are discouraged as a mitigation measure. Berms for frequent events are usually a better choice.

Rainfall and Wind-Driven Rain

One of the primary performance requirements for any building is that it should keep the interior space dry. All roofs and walls must therefore shed rainwater, and design requirements are the same everywhere in this respect. For example, roof drainage design must minimize the possibility of ponding water, and existing buildings with flat roofs must be inspected to determine compliance with this requirement. Recommendations for addressing rainfall and wind-driven rain can be found in the International Building Code (IBC) series.

Differential Settlement (Subsidence)

Ground subsidence can result from mining, sinkholes, underground fluid withdrawal, hydrocompaction, and organic soil drainage and oxidation. Subsidence mitigation can best be achieved through careful site selection, including geotechnical study of the site. In subsidence-prone areas, foundations must be appropriately constructed, basements and other below-ground projections must be minimized, and utility lines and connections must be stress-resistant. When retrofitting structures to be more subsidence-resistant, shear walls, geo-fabrics, and earth reinforcement techniques such as dynamic compaction can be used to increase resistance to subsidence damage and to stabilize collapsible soils.

Landslides and Mudslides

FEMA News Photo: Mudslide El Paso, Texas

FEMA News Photo: Mudslide El Paso, Texas

Gravity-driven movement of earth material can result from water saturation, slope modifications, and earthquakes. Techniques for reducing landslide and mudslide risks to structures include selecting non-hillside or stable slope sites; constructing channels, drainage systems, retention structures, and deflection walls; planting groundcover; and soil reinforcement using geo-synthetic materials, and avoiding cut and fill building sites.

 

 

Wild Land Fire / Urban Interface

FEMA News Photo: Wildfire Rancho Bernardo, California

FEMA News Photo: Wildfire Rancho Bernardo, California

As residential developments expand into wild land areas, people and property are increasingly at risk from wild land fire. Fire is a natural process in any wild land area and serves an important purpose; however, if ground cover is burned away, erosion, landslide, mudflow, and flood hazards can be exacerbated. A cleared safety zone of at least 30 feet (100 feet in pine forests) should be maintained between structures and combustible vegetation, and fire-resistant ground cover, shrubs, and trees should be used for landscaping (for example, hardwood trees are less flammable than pines, evergreens, eucalyptus or firs). Only fire-resistant or non-combustible materials should be used on roofs and exterior surfaces. Roofs and gutters should be regularly cleaned and chimneys should be equipped with spark arrestors. Vents, louvers, and other openings should be covered with wire mesh to prevent embers and flaming debris from entering. Overhangs, eaves, porches, and balconies can trap heat and burning embers and should also be avoided or minimized and protected with wire mesh. Windows allow radiated heat to pass through and ignite combustible materials inside, but dual- or triple-pane thermal glass, fire-resistant shutters or drapes, and noncombustible awnings can help reduce this risk.

Tsunami

A tsunami is a series of ocean waves generated by sudden displacements in the sea floor, landslides, or volcanic activity. In the deep ocean, the tsunami wave may only be a few inches high. The tsunami wave may come gently ashore or may increase in height to become a fast moving wall of turbulent water several meters high. Although a tsunami cannot be prevented, the impact of a tsunami can be mitigated through urban/land planning, siting away from shorelines, community preparedness, timely warnings, and effective response.

Areas of Refuge

An area of refuge is a location designed to protect facility occupants during an emergency, when evacuation may not be safe or possible due to area contamination, obstruction, or other hazard. Occupants can wait there until given further instructions or rescued by first responders. Areas of refuge can be safe havens, shelters, secure rooms, or protected spaces, each intended for protection from specified hazards and for different durations of occupancy. A safe haven is designed for temporary protection from various types of attacks in which building occupants may take refuge when forewarned of an imminent or on-going attack. A shelter is an expedient retreat that provides "sheltering in place" for a relatively short duration, of hours, not days or weeks, and does not normally involve air filtration. A secure room is constructed to meet specified hardening criteria based on the design basis threat. Protected areas are readily available locations that afford safety to people by their shielding characteristics, such as a firewall. Areas of refuge may be freestanding or integrated in a building.

A risk assessment to identify likely threats is a prerequisite practice for validating the need and design criteria for an area of refuge. Events that might warrant an area of refuge include, but are not limited to: natural events such as tornados; public disturbances such as riots or demonstrations, a terrorist event involving weapons of mass destruction, firearms, or other weapons, life-threatening accidents such as the release of chemical or biological contaminants, or situations in which there is a potential for loss of life or injury due to their immobility, such as a hospital intensive care unit. A collection of factors to be considered in the risk assessment process could include the type of hazard event, probability of event occurrence, severity of the event, probable consequences of a hazard, and a benefits/cost analysis of options.

Planning, rehearsal, and preparatory procedures (routine maintenance/testing of equipment, checking shelf life of stored provisions and materials) are paramount to a successful area of refuge. Well-written standing operating procedures (SOP) should outline who makes decisions, when and how to evacuate, responsibilities and operating procedures, and when and how to leave the area of refuge. Depending on the function, planning may include, but is not limited to, personnel accountability protocol, communications for notification and instructions (transmitters/receivers, megaphones, whistles), comfort provisions such as seating and bedding, life-support systems such as fire apparatus, dedicated ventilation/filtration, plumbing, emergency power/fuel supply, lighting (including emergency lighting), decontamination, and maintenance. In addition to storage provisions of food, water, and first-aid, other items/supplies may include: current emergency contact numbers, dust masks, flashlights, vests or other identification/apparel, wind-up or battery-operated radios, toiletries, stationary for emergency administration (assignments, name/billet labels, instructions, etc.), an audible sounding device that continuously charges or operates without a power source (e.g., canned air horn) to signal rescue workers if refuge egress is blocked, tools and maintenance manuals.

Key design considerations include reaction time (travel distance to refuge and activation time for emergency support systems), duration of occupancy, privacy, security (secure storage, doors, locks, windows/view ports), isolation areas for ill or contaminated occupants/equipment, and hardening criteria to mitigate specified threats. The refuge size is based on the maximum intended occupancy. The location should be readily accessible to all people who are to be protected. Travel time is especially important when refuge users have disabilities that impair their mobility and may need assistance from others to reach the refuge. Alternate accessible routes should be considered for use in the event that the primary route is blocked or inoperable. If the refuge is integrated into a building, a centrally located, interior room/space is preferable. If the refuge has windows, they should be capable of being sealed. Stand-alone refuges should be sited away from potential hazards, such as debris, excessive glazing, flooding or storm surge, power lines, or nearby structures that may be toppled or become airborne. In some situations, multiple small shelters may best suit the situation rather than one large shelter. The location should not be selected based on height above ground level if it would increase the travel time to the refuge. Signage and warning signals are critical for users to know when and where to seek refuge. Critical support systems located outside of the area of refuge area should be afforded the same protection criteria as the refuge.

During an emergency, preparatory actions may be required by designated team members outside the area of refuge, such as the turning off of fans, air- conditioning, or forced heat systems, the closing of windows or blinds, and the shutting of doors (securing doors in a lock-down situation). To close all doors quickly in a large building, there must be a notification system such as a public address system. Also, occupant furniture may have to be relocated to allow the maximum number of occupants in the room or building.

Related Issues

Hazard Mitigation and Sustainability

Unsustainable development is one of the major factors in the rising costs of natural disasters. Given that hazard mitigation is at the core of disaster resistance, then, many design strategies and technologies serve double duty, by not only preventing or reducing disaster losses but serving the broader goal of long-term community sustainability. For example, erosion control measures designed to mitigate flood, mudslide, rainstorm, and other damage to a building's foundation may also improve the quality of runoff water entering streams and lakes. Similarly, land use regulations prohibiting development in flood-prone areas may also help preserve the natural and beneficial functions of floodplains.

Regardless of whether hazard mitigation is undertaken before or after a disaster—on the basis of a risk assessment in the former case, and as a result of unforeseen damages or a renewed understanding of vulnerability in the latter—it is always an inherently proactive endeavor. In situations where mitigation efforts are integrated into a holistic post-disaster recovery strategy, the principles of sustainability should guide every aspect of the recovery effort. By carefully balancing the full spectrum of interests relating to the built environment, psychosocial recovery, economic redevelopment, and preservation or restoration of the natural environment, an impacted community can ensure that the net result is enhanced long-term disaster resilience.

Climate change

Ongoing changes in climate patterns around the world may alter the behavior of hydrometeorological phenomena within our lifetimes. The frequency and severity of floods, storms, droughts and other weather-related disasters is expected to increase, as is the risk from associated changes in the manifestation of other hazards such as wildland fires. High-performance buildings should be designed to be part of the solution rather than part of the problem wherever possible, incorporating strategies to both mitigate climate change itself (e.g., greenhouse gas emission reduction) as well as to adapt to changing environmental conditions by leveraging traditional hazard mitigation strategies (e.g., elevating structures in increasingly floodprone areas, creating clear zones around buildings in areas with increasing wildland fire risk, etc.).

Relevant Codes and Standards

Regulations, codes, standards, and best practices will guide the design of buildings to resist natural hazards. For new buildings, code requirements serve to define the minimum mitigation requirements, but compliance with regulations in building design is not sufficient to guarantee that a facility will perform adequately when impacted by the forces for which it was designed. Indeed, individual evaluation of the costs and benefits of specific hazard mitigation alternatives can lead to effective strategies that will exceed the minimum requirements. Additionally, special mitigation requirements may be imposed on projects in response to locale-specific hazards. When a change in use or occupancy occurs, the designer must determine whether this change triggers other mitigation requirements and must understand how to evaluate alternatives for meeting those requirements.

Finally, designers should augment the codes and standards to consider the importance of nonstructural elements, assets, and mission of the building, i.e., windows, hoods, parapets and balcony railings, and electrical and mechanical systems, because they may account for more than 70% of the value of a building.

Many states and municipalities have also adopted supplemental codes to meet local requirements for all-hazard protection. Examples of such codes include:

General All-Hazards

  • Public Law 106-390, Disaster Mitigation Act of 2000

Earthquake

Hurricane, Typhoon, and Tornado

Flood

Tsunami

  • Public Law 109-424, Tsunami Warning and Education Act (2006)

Rainfall and Wind-Driven Rain

Major Resources

Organizations and Associations

General All-Hazards

Disaster Management

Earthquake

Hurricane, Typhoon, and Tornado

Flood

Forest Fires

Publications

General All-Hazards

Earthquake

Hurricane, Typhoon, and Tornado

Flood

Landslide, Mudslide

Progressive Collapse

Tsunami

Wild Land Fire / Urban Interface

NOTE: To order FEMA publications that are not available online, request by title or document number from the FEMA Publications Warehouse at (800) 480-2520.

Tools

Training Courses