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Author: T.D. O’Rourke
Resilient physical and social systems must be robust, redundant, resourceful, and capable of rapid response.
The concept of critical infrastructure is evolving. In the 1980s, concerns about aging public works led the National Council on Public Works Improvement (1988) to focus on infrastructure in the public sector, such as highways, roads, bridges, airports, public transit, water supply facilities, wastewater treatment facilities, and solid-waste and hazardous-waste services. In the 1990s, as a result of increased international terrorism, infrastructure was redefined in terms of national security. After 9/11, the number of “critical” infrastructure sectors and key assets listed in the National Infrastructure Protection Plan was expanded to 17 (DHS, 2006). The list includes agriculture and food systems, the defense-industrial base, energy systems, public health and health care facilities, national monuments and icons, banking and finance systems, drinking water systems, chemical facilities, commercial facilities, dams, emergency services, nuclear power systems, information technology systems, telecommunications systems, postal and shipping services, transportation systems, and government facilities.
Adjusting the definition to reflect current concerns has provided for flexibility and adaptability but has also led to some ambiguities about which assets are critical and which criteria should be used to define them. In addition, the proliferation of critical-infrastructure sectors has added complexity to an already complex field. To develop basic principles that govern performance and clarify interactions, it is helpful to consolidate our thinking into unifying concepts and a smaller number of sectors based on common traits.
The concept of a “lifeline system” was developed to evaluate the performance of large, geographically distributed networks during earthquakes, hurricanes, and other hazardous natural events. Lifelines are grouped into six principal systems: electric power, gas and liquid fuels, telecommunications, transportation, waste disposal, and water supply. Taken individually, or in the aggregate, all of these systems are intimately linked with the economic well-being, security, and social fabric of the communities they serve. Thinking about critical infrastructure through the subset of lifelines helps clar-ify features that are common to essential support systems and provides insights into the engineering challenges to improving the performance of large networks.
Lifeline systems are interdependent, primarily by virtue of physical proximity and operational interaction. Consider Figure 1 [see PDF version for figures], for example, a photograph of the corner of Wall Street and Williams Street in New York City in 1917. The congestion shown in this photograph has not improved in the last 90 years, and similar locations can be found in a multitude of cities worldwide. Critical systems in crowded urban and suburban areas like these are subject to increased risk from proximity. Damage to one infrastructural component, such as a cast-iron water main, can rapidly cascade into damage to surrounding components, such as electric and telecommunications cables and gas mains, with system-wide consequences.
To complicate matters, much of this critical infrastructure is underground, which obscures the location and condition of components. The proximity of aging, weakened pipelines to other important facilities, such as high-pressure gas mains and electric power substations, is frequently not recognized, increasing the potential for unanticipated accidents for which no preparations have been made.
Lifeline systems all influence each other. Electric power networks, for example, provide energy for pumping stations, storage facilities, and equipment control for transmission and distribution systems for oil and natural gas. Oil provides fuel and lubricants for generators, and natural gas provides energy for generating stations, compressors, and storage, all of which are necessary for the operation of electric power networks. This reciprocity can be found among all lifeline systems.
The use of electric power at pipeline pumping stations is especially important. After Hurricane Katrina, the supply of crude oil and refined petroleum products was interrupted because of a loss of electric power at the pumping stations for three major transmission pipelines: the Colonial, Plantation, and Capline Pipelines. As a result, major lines of refined products were not available for delivery to southern and eastern states, and gasoline and diesel production in the Midwest was seriously affected by lack of supply. About 1.4 million barrels per day of the crude oil supply were lost, accounting for 90 percent of the production in the Gulf of Mexico. Nearly 160 million liters per day of gasoline production was lost, accounting for 10 percent of the U.S. supply. The three major pipelines were not fully restored until September 14, 2005, more than 17 days after Katrina made landfall in southern Louisiana.
Similar difficulties have been experienced at water-supply pumping stations. After the 1994 Northridge earthquake, electric power was lost for nearly 24 hours in the Van Norman complex, which receives and treats about 75 percent of the potable water for the city of Los Angeles. As a result, the largest water pumping station in the city system could not be operated.
A smaller station where pumps were activated by combustion engines made up for some of the loss. Note, however, that the amount of fuel that can be stored on site at pumping stations, even facilities equipped with combustion engines, is often restricted by environmental regulations. Thus, if fuel runs out, refueling depends on the transportation system, which is also likely to be damaged and difficult to negotiate after a disaster.
The World Trade Center Disaster
The World Trade Center (WTC) disaster has been studied in detail with respect to structural failure, building performance, and the impact of fire on building integrity. WTC also has lessons for lifeline performance and interdependencies. When the twin towers collapsed, water mains servicing the WTC complex were ruptured primarily by falling debris and impact. Records of water flow to the WTC area and nearby neighborhoods show that immediately after the buildings collapsed, water flow suddenly increased by 210 million liters per day, then rose gradually another 30 million liters per day (O’Rourke et al., 2003). The initial jump was caused by water pouring through broken water mains beneath and around the WTC complex. The additional flow represents, approximately, the amount of water drawn from fire hydrants to fight fires in adjacent buildings. Water pressures at hydrants around the WTC complex declined throughout the afternoon. Measurements at 6:00 p.m. showed pressure two to three blocks from the site at approximately one-third of normal. Of course, firefighting was impaired by the falling pressure.
The primary source of water at the WTC complex was fireboats on the Hudson River. Figure 2 is an aerial view of the WTC site, showing the deployment of four fireboats (Firefighter, McKean, Kane, and Smoke II). The tie-up locations and hose paths are shown for each boat. Although the combined pumping capacity of the fireboats was 180,000 liters per minute, only a small fraction of that, approximately 28,000 liters per minute, was conveyed to the WTC complex, partly because the water was relayed through relatively small hoses (90-mm and 125-mm-nominal-diameter) (O’Rourke et al., 2003). Nevertheless, water from the fireboats was about 150 percent of the water available from hydrants and was critical to containing and extinguishing fires on the site.
Water from the ruptured underground pipelines flowed into the underground sections of the WTC complex and flooded the Port Authority and Trans-Hudson (PATH) tunnels beneath the Hudson River. PATH trains had transported commuters from Exchange Place Station on the New Jersey side of the Hudson to the WTC Station in the WTC underground complex. Exchange Place Station, which is approximately 6 meters lower in elevation than the WTC Station, was also flooded.
Water flooded the cable vault of the Verizon building at 140 West Street, where 70,000 copper pairs and additional fiber optic-lines had been severed by falling debris. Nearly 41,600 cubic meters of water had to be pumped from the vault during recovery. The seventh and ninth floors of the telecommunications building also sustained water damage.
The capacity of the telecommunications office at 140 West Street had been one of the largest in the world. The building housed four digital switches, 500 optical-transport systems, 1,500 channel banks, 17,000 optical fiber lines, 4.4 million data circuits, and 90,000 message trunks. As a result of the damage and flooding, Verizon lost 200,000 voice lines, 100,000 private branch exchange lines, 4.4 million data circuits, and 11 cell sites. More than 14,000 business and 20,000 residential customers were affected.
The WTC disaster provides a graphic illustration of the interdependencies of critical infrastructure systems. The building collapses triggered water-main breaks that flooded rail tunnels, a commuter station, and the vault containing all of the cables for one of the largest telecommunication nodes in the world. These included the Security Industry Data Network and the Security Industry Automation Corporation circuits used to execute and confirm block trades on the stock exchange. Before trading resumed on the New York Stock Exchange on Monday, September 17, 2001, the telecommunications network had to be reconfigured. Hence, ruptured water mains were linked directly with the interruption of securities trading and the restoration of international financial stability.
Resilience is defined in Webster’s Unabridged Dictionary as “the ability to bounce or spring back into shape, position, etc., after being pressed or stretched.” Definitions vary slight, but they all link the concept of resilience to recovery after physical stress.
Since Hurricane Katrina, there has been a notable shift in emphasis from protecting critical infrastructure to ensuring that communities are resilient. When translating new ideas or concepts that connote a particular quality, such as resilience, into policy and implementation in the real world, we must remain mindful of the human dimensions of communities, which cannot be easily adapted or convolved into concepts based on the recovery of physical entities.
In addition, the concept of resilience, like the concept of critical infrastructure, is evolving. In its current form, the resilience of a community is an overarching attribute that reflects the degree of community preparedness and the ability to respond to and recover from a disaster. Because lifelines are intimately linked to the economic well-being, security, and social fabric of a community, the initial strength and rapid recovery of lifelines are closely related to community resilience.
Debate is likely to continue about the concept of resilience, and refinements and elaborations of the term are to be expected. Engineers and social scientists at the Multidisciplinary Center for Earthquake Engineering Research (MCEER) have proposed a framework for defining resilience (Bruneau and Reinhorn, 2007; Bruneau et al., 2003). According to Bruneau et al. (2003), resilience for both physical and social systems can be conceptualized as having four infrastructural qualities: