Reliability Engineering and System Safety 42 (1993) 1-3 I

Guest Editorial

THE RELIABILITY OF WATER DISTRIBUTION SYSTEMS

The quality of the services provided by a nation's physical infrastructure is dependent on and can be directly related to the reliability of such infrastructure. The USA's highways and bridges, mass transit, aviation facilities, water transportation, wastewater treatment, water distribution systems, and a host of other public works and public facilities are deteriorat- ing, and their maintenance and rehabilitation have not been receiving the needed resources to do the job. ~ The failure of these public works facilities to perform their intended functions over their expected physical life has been the subject of studies by a myraid of national commissions and panels. In its final report to the President and to the Congress in 1988, the US National Council on Public Works Improvement 2 states: 'After two years of study, the Council has found convincing evidence that the quality of America's infrastructure is barely adequate to fulfill current requirements and insufficient to meet the demands of future economic growth and development.'

A similar concern was raised in a June 1987 US Congressional Office of Technology Assessment (OTA) staff paper. 3 The OTA paper recognizes that the nation's infrastructure is the physical framework that supports and sustains virtually all domestic economy activity; it is essential to maintaining international competitiveness as well. The paper further states that actual expenditures for public works are expected to meet only 35 to 60% of the estimated public works construction and repair needs. As a direct consequence of neglecting and mismanging the maintenance of the US infrastructure, it is estimated that over 100000 bridges do not meet current engineering safety standards and that thous- ands of miles of pipes that constitute water distribution systems are leaky. The US Environmental Protection Agency, for example, estimates at $76 billion the construction cost of wastewater plants needed between now and the year 2003.

A background paper published in January 1989 by

Reliability Engineering and System Safety 0951-8320/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England.

the US OTA 4 highlights the enormous magnitude and unimaginable dimensions of the problems the US faces in hazardous waste management: 'In our 1985 report "Superfund Strategy," we estimated the cost of future cleanups at about $300 billion by government and industry over about 50 y e a r s . . . A more realistic estimate is perhaps $500 billion in cleanup cos t s . . . However, until now government and in- dustry have probably spent between $5 and $10 billion on cleanups--only 1% to 2% of what they may ultimately spend.'

The importance of research and development (R & D) to restore the USA's physical infrastructure has been recognized recently by a wide spectrum of constituencies. R & D that can improve the infrastructure is just as important, but its urgency has not yet commanded national attention. A report published by the US National Research Council 5 states that 'current research and development on infrastructure is uneven across the various modes of infrastructure, with some commanding considerable resources while others are underfunded and facing significant challenges.

Water distribution systems epitomize the entire infrastructure reliability and maintenance problem. This special issue on the reliability of water distribution systems is an attempt to address some limited, albeit important, dimensions of the broader subject of the reliability of the entire physical infrastructure. Here we focus to water distribution systems from their multifarious technical dimensions, such as reliability analysis in optimal design, capacity analysis for deteriorating networks, and computer control.

Ostfeld and Shamir review the role of reliability in the optimal design of water distribution networks, offer new insights and suggest new concepts on the subject. They emphasize the importance of quantify- ing the reliability of water distribution systems at the design stage and argue that the less than desired attention paid on such quantification is due primarily to the difficulties associated with the task. Indeed, one can hardly allocate optimal resources at the design stage of public works if the tradeoffs between cost and reliability cannot be quantified and evaluated. How can a design engineer argue for a more costly option if he or she cannot establish an improvement in the

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reliability of alternative systems? Ostfeld and Shamir divide their paper into three major parts: (1) conceptual discussion of reliability definitions from different points of view (system versus consumers), (2) a literature survey of existing techniques to incorpor- ate reliability in optimal design of water distribution networks, and (3) a new concept for explicitly including reliability in optimal design of water distribution networks.

In his paper titled 'Practical Aspects of Providing Reliability in Water Distribution Systems,' Walski echoes the sentiments expressed by Ostfeld and Shamir on the importance of reliability and its quantification in the design and maintenance of water distribution systems. Walski reviews the state of the art of reliability in water distribution systems. He maintains that criteria for designing water distribution systems have been ambiguous, and he focuses in his paper on the practical aspects as opposed to the mathematical methods of reliability. In his second paper, entitled 'Water Distribution Valve Topology for Reliability Analysis,' Walski concentrates on the centrality of the valving system in the reliability water distribution systems and argues that it provides a better representation of reliability than the link node approach. This is because of the failure of the link-node representation to describe correctly what happens when a pipe breaks and the breakage must be isolated for maintenance or repair. To take advantage of the valve paradigm, Walski introduces the concept of segment (a collection of pipes) to describe the portion of a water distribution system that can be isolated by closing valves.

In their paper titled 'Capacity Reliability of Water Distribution Networks,' Li, Dolezal and Haimes recognize the important role of internal roughness of water mains in decreasing the carrying capacity of aging water distribution networks. They develop a probabilistic approach to treat the internal roughness coefficient as a random variable due to its large uncertainty. The concept of critical vectors, which they introduced in capacity networks, allows the ultimate calculation of capacity reliability of a distribution network. The developed methodology is applicable to general capacity networks. Furthermore, they propose an algorithm to generate a hydraulic reliability measure by combining the capacity reliability of a distribution system with its mechanical reliability. The authors maintain that the study of capacity reliability is still in its infancy with many theoretical and methodological aspects needing to be explored and solved. In particular, there is a need to research the capacity reliability of water distribution networks with more than one demand node.

Wu, Yoon and Quimpo address the deficiency of quantifying the reliability of water distribution systems

merely on the basis of the connectivity of the demand point to the water sources. In their paper entitled 'Capacity-weighted Water Distribution System Reli- ability,' the authors introduce a capacity-weighted reliability index that takes into account partial satisfaction of demand through a weighted index. The authors demonstrate through examples how a capacity-weighted index of reliability may be formu- lated and applied to small and moderately sized water distribution systems. They conclude that the inclusion of capacity makes the reliability measure more realistic and the allowance for partial satisfaction of demand makes the analysis more reasonable.

In their paper titled 'Reliability Analysis of Water Supply Systems Integrated with Treatment Plant Operation.' Fujiwara and Chen consider the following random variables in the assessment of the reliability of water distribution systems: (a) the random fluctuation of the quality of water at the source, (b) the limited water processing rate of the treatment plant, (c) the random pump failure and repair, and (d) the random demand variations. They employ dynamic program- ming to model and optimize the water processing rate of the treatment plant at each period so that the total expected water shortage over a finite time horizon is minimized. They demonstrate the usefulness of their approach through an example problem with three water quality patterns: dry, normal and wet seasons. The system that they consider is composed of a water source, a water treatment plant, storage, a pumping station and a lumped demand node in series.

Finally, in their paper 'Essential Considerations in the Computer Control of Water Distribution Syst- ems,' Coulbeck and Orr present a reliability perspective of the required systems and activities for the control of water distribution networks. The authors focus their effort on the control modules, the data reliability and the operational reliability of the individual systems and components of water distribu- tion systems. The authors also provide a detailed discussion of the on-line control and operational reliability of the system, where the consequential effects of hardware, software, and management are analyzed. The authors share their experience in the design and implementation of an on-line control system for a city distribution network in the UK.

The reliability of water distribution systems is a broad complex subject that deserves much wider attention than any seven papers can offer. Neverthe- less, we hope that exposing these complexities in this special issue of the journal of Reliability Engineering and System Safety constitutes an important step toward placing this subject higher on the agenda of reliability engineers, water resource managers and public officials. It is also hoped that this volume will encourage more professionals to engage in the study

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of the reliability of physical infrastructure and contribute to improving their design, maintenance, rehabilitation and ultimate management.

Finally, we would like to express our gratitude and appreciation to the following reviewers who have volunteered their time and effort in making invaluable recommendations to the authors of this special issue: Laurence Baxter, Okitsugu Fujiwara, Benjamin Hobbs, Larry Mays, Chun-Hou Orr, Rafael Quimpo, Uri Shamir, Thomas Walski and Ben Yen. We also would like to thank all the authors for their contributions as well as George Apostolakis, the Editor-in-Chief of this journal, for his encouragement and support.

This work was supported in part by a Grant from the National Science Foundation titled: Optimal Maintenance-Related Decisionmaking for Large-Scale Water Distribution Systems. The valuable encourage- ment, guidance and support provided by Dr. Jack Scaizi, Program Director, Structures and Building Systems, National Science Foundation, are greatly appreciated.

R E F E R E N C E S

1. Haimes, Y. Y., Options for national infrastructure renewal (editorial), IEEE Trans. on Systems, Man, and Cybernetics, 21(4) (1991)701-3.

2. National Council on Public Works Improvement. Fragile foundations: A report on America's public works. Government Printing Office, Washington, DC, 1988.

3. Office of Technology Assessment, US Congress, Construction and materials research and development for the nation's public works. Staff paper, June 1987.

4. Office of Technology Assessment, US Congress, Assessing contractor use in superfund. A background paper of OTA's assessment on superfund implementa- tion, Jan. 1989.

5. National Research Council, Infrastructure for the 21st Century: Framework for a Research Agenda, National Academy Press, Washington, DC, 1987, 188 pp.

Yacov Y. Haimes & Duan Li

Centre for Risk Management of Engineering Systems, University of Virginia, Thornton Hall, Charlottesville, VA 22903, USA