We all know the meanings of the R words listed in the title of this blog (at least we think we do). In everyday speech, however, we use them somewhat carelessly and sometimes interchangeably. I’m guilty of this as well.
However, when we work with various water systems, we need to use precise language because careless talk can lead to less-than-ideal solutions to our problems. It bothers me to hear these terms used imprecisely.
One of the papers that tried to best nail down these words for water resources systems was by Hashimoto, Stedinger, and Loucks (1982). “These measures describe how likely a system is to fail (reliability), how quickly it recovers from failure (resiliency), and how severe the consequences of failure may be (vulnerability).”
While the above paper is considered the definitive work in this area by some, other terms should be added to the list, and the three terms―reliability, resilience, and vulnerability―could use clarification.
Here are the Walski definitions of these terms, at least as they apply to water. In industry papers, you’ll find many definitions and many metrics used to calculate them.
Quantification of these reliability-related terms depends heavily on the property measured. For example, does a failure of a point in the distribution system occur when the pressure drops to zero, when the pressure drops to below 20 psi (or some comparable standard), or when the available fire flow drops below the needed fire flow? Metrics are important.
There is a great deal of literature in the general area of reliability, specifically in mechanical engineering, electrical engineering, and computer science. There have been several papers on the topic in water, but there doesn’t seem to be a consensus on the terminology. In the 1980s, an ASCE task committee on Risk and Reliability Analysis in Water Distribution Systems (ASCE, 1989) developed a book on reliability but didn’t focus on terminology.
Water journal papers on reliability tend to focus on system hydraulics. However, there are many other aspects impacting reliability. I wrote about these in a paper on the practical aspects of reliability (Walski, 1983). Some of the considerations include standby power, emergency interconnections, spare parts inventory, good communications with field crews, and adequate training for operation and maintenance personnel. These aren’t exciting topics for researchers. (Excerpts from that paper can also be found in the chapter on reliability in Mays (2000).)
Imprecise terminology is not usually a fatal problem, but discussions would be more productive if we agreed on things like whether excess capacity is the same thing as resilience.
There may have been some publication on this terminology that I missed and is widely used. I couldn’t find any. So, until we can come up with something better, I’d like to see people use the terminology I’ve described above. While I like to think my writing is perfect, I know it isn’t. If you want to comment on anything I’ve written above, send me an email at email@example.com.
ASCE, 1989, Reliability Analysis of Water Distribution Systems, Ed. Larry Mays, ASCE.
Hashimoto, T., Stedinger, J., and Loucks, P., 1982, “Reliability, resiliency, and vulnerability criteria for water resource system performance evaluation,” Water Resources Research, Vol. 18, No. 1. P. 114-20.
Mays, L. (Ed.), 2000, “Chapter 18. Reliability Analysis for Design,” Water Distribution Systems Handbook, McGraw Hill, New York.
Walski, T., 1993, “Practical Aspects of Providing Reliability in Water Distribution Systems,” Reliability Engineering and System Safety, Vol. 42, No. 1, p. 13.
If you want to look up past blogs, go to https://blog.virtuosity.com/tag/water-and-wastewater. And if you want to contact me (Tom), you can email firstname.lastname@example.org.
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