Ensuring the Reliability of Water Supply Systems During Natural Disasters
by Ronald T. Eguchi and Douglas G. Honegger
In an effort to reduce the vulnerability of u.s. water supply systems in future earthquakes, several stakeholders, supported by the Federal Emergency Management Agency and the American Society of Civil Engineers, have established Subcommittee F36.30 to create standard guidelines to assess the seismic fragility of water transmission systems.
Recent earthquakes in the United States have underscored the need to address the vulnerability of its lifelines, that is, those systems necessary to support post-earthquake response and recovery activities. For example, the latest estimates of lifeline damage and loss as a result of the 1994 Northridge Earthquake in California are in excess of $2 billion. While this amount may appear low relative to other types of losses, namely damage to buildings, it only reflects those costs associated with the repair of damaged systems.
Other costs that may more accurately reflect the impact of damaged or inoperable systems, such as business losses due to lifeline disruption, or fire damage resulting from loss of water supplies, may be several factors higher than these repair costs. Also, it must be recognized that the Northridge earthquake was a moderate-sized event and that the Los Angeles area is capable of generating earthquakes of much larger magnitude. Therefore, the relatively good performance of lifelines compared to buildings in the Northridge earthquake should not promote complacency in current or existing design measures for lifelines systems.
The engineering community has long recognized the importance of lifeline systems during disasters. After the 1971 San Fernando earthquake an event that caused catastrophic damage to virtually every type of lifeline many efforts were launched to better understand the cause of these failures and ways to mitigate future earthquake damage. Lifeline damage in the San Fernando earthquake was the key catalyst in prompting the American Society of Civil Engineers to establish the Technical Council on Lifeline Earthquake Engineering (TCLEE) in 1974.
The goal of TCLEE is to advance the state-of-the-art and practice of lifeline earthquake engineering in the United States. Since its formation, TCLEE has sponsored five national conferences, published numerous monographs and reports, and participated in over a dozen reconnaissance investigations of earthquakes worldwide. TCLEE continues to serve as an important forum for discussing all issues that affect the seismic design, construction, operation, and maintenance of lifeline systems. Table 1 shows other important milestones or events in the history of lifeline earthquake engineering in the United States.
The American Lifelines Alliance
Despite the efforts of TCLEE and other organizations with similar goals and objectives, we have yet to develop and implement consensus standards and guidelines that will dramatically improve the performance of existing utility and transportation systems in large earthquake events. This need for uniform guidance has been repeatedly supported in numerous workshops held over the last 10 years and was explicitly identified in the 1990 reauthorization of the National Earthquake Hazards Reduction Program. In 1998, the Federal Emergency Management Agency and the American Society of Civil Engineers (ASCE) entered into a cooperative agreement to establish the American Lifelines Alliance (ALA), a public-private partnership project whose goal is to reduce risks to lifeline systems from natural hazards. ALAs charter is to facilitate the creation, adoption and implementation of design and retrofit guidelines and other national consensus documents that, when implemented by lifeline owners and operators, will systematically improve the performance of utility and transportation systems to acceptable levels in natural hazard events, including earthquakes. Already, the ALA has produced several products that represent the beginning of this process of standardizing seismic vulnerability assessment and design. One of the first reports that has been prepared is Seismic Fragility Formulations for Water Systems.(1)
While seismic measures have generally been considered by most West Coast water utilities, they have yet to be implemented on a regular basis in other parts of the country. Furthermore, what measures are being implemented are usually done so on an uneven or inconsistent basis. One reason for inconsistent implementation of seismic mitigation measures is the lack of a consistent set of standards or guidelines that quantify the level of seismic vulnerability of water system components. Until a set of consensus-based guidelines is developed, it will be difficult for water utilities and others to gauge the relative effectiveness of different seismic design and retrofit strategies.
Key Role for ASTM
In response to an ALA request, ASTM is supporting the transformation of guidelines developed under ALA into ASTM standards. In late 2001, Subcommittee F36.30 on Seismic Fragility of Water Transmission Systems was formed to define standardized methods and procedures for assessing the seismic vulnerability of a broad range of water system components. This activity falls under ASTM Committee F36 on Technology and Underground Utilities. In total, over 20 experts in the water and earthquake areas comprise this subcommittee.
The starting point for the F36.30 subcommittee activities is an ALA report prepared in 2001 by a team of practicing engineers, water utility personnel, and academics under contract to G&E Engineering Systems, Inc. This report covers six different water system components: aqueducts, distribution pipelines, storage tanks, tunnels, canals, and valves and SCADA system components. In addition, the report provides special guidance on how to evaluate or assess seismic vulnerabilities based on different hazard effects, i.e., fault offsets, liquefaction and lateral spread, landslides, and strong ground shaking.
Sample Fragility Model
Figure 1 shows an example of the type of approach the ALA report uses to quantify the seismic vulnerability of a major water system component. In this case, the component is an anchored steel water tank. The schematic, often referred to as a fault tree, identifies several tank damage states and the failure mechanisms that can lead to these states. For example, Damage State 4 describes a condition where the tank is damaged, requires major repairs and is functionally out of service. The figure shows those failure mechanisms that are likely to lead to Damage State 4, i.e., damage to inlet pipes, tank wall uplifts leading to leakage, elephant foot buckling leading to tank leakage, and hoop overstress to the tank wall. Any of these failures will result in the tank being shut down for repairs. The benefit of using a fault tree approach is that it allows the engineer to break down the assessment of vulnerability into a series of steps or calculations that can be easily combined at the end to arrive at a failure rate or damage state probability.
Figure 2 shows a set of fragility curves for an anchored steel water tank, as presented by the ALA report. The ordinate is measured in terms of probability; the abscissa indicates severity of ground shaking in terms of peak ground acceleration. Four damage states are specified in the figure: minor, moderate, extensive, and complete. With access to such curves, an engineer is able to estimate the probability or likelihood of the tanks survival given different levels of ground motion. This information can be used in selecting the appropriate design specifications of a new tank or deciding whether an existing tank should be upgraded or not. When combined with economic information or data, these probabilities can also be translated into expected repair costs over the life of the tank.
Over the next year, Subcommittee F36.30 will review the ALA report, Seismic Fragility Formulations for Water Systems, as well as other relevant documents, and prepare a set of standard guidelines for assessing the seismic fragility of water transmission systems. Some of the key questions and issues that will be addressed by the subcommittee, particularly as they relate to the ALA report, are:
Were the reviews that were conducted in the development of this report comprehensive enough, i.e., are there reports, data or studies missing?
Are the models in the report technically defensible?
Are there technical areas where improvements are needed?
What information or data are needed in order to use or implement these models?
Are the models easily understood and applied by 1) water agency personnel, 2) consultants, and 3) regulatory officials?
Are the models transferable to 1) different users, and 2) different regions of the country?
Are the models flexible enough to incorporate new data or datasets?
When completed, these guidelines will allow engineers to use a common set of tools and databases to determine what the seismic design level should be for new and existing water facilities.
So what are the benefits in establishing standard guidelines for assessing the seismic fragility of large water supply systems? First, these guidelines will help to quantify the expected seismic performance of major water transmission system components. We know from past experience that failure of key components can shut down an entire water system. If we are to be successful in ensuring the continuity of service after a major earthquake event, we must identify critical components within the system, assess their likelihood of failure in a series of events, and either design or retrofit these facilities to conform to some acceptable level of performance. Second, knowing the expected performance for all critical water system components is essential to performing a systems analysis to assess the level of service that can be provided by the water system following an earthquake. Third, understanding the improvements in component performance that can be gained by implementing mitigation measures, is important to properly prioritize investments in water system upgrades that will help assure that adequate water supplies are available to fight post-earthquake fires and minimize health risks. Finally, establishing a set of standard guidelines to evaluate the seismic fragility of water systems will help pave the way for other lifeline systems. As stated at the beginning of this article, the end goal is to have practical and consensus-based standards for all lifeline systems. Only when we have such standards in place can we ensure the reliability of U.S. lifeline systems. //
(1) Available at the ALA Web site.
Copyright 2002, ASTM