Condition Assessment Modeling for Drinking Water Distribution Systems: An Example of Shared Frailty Analysis

It has been estimated that replacement of drinking water infrastructure will need to increase from the current estimated replacement rate of 0.3 percent per year to 2.0 percent per year by 2040 in order to meet the needs of US drinking water utilities. This is approximately four times the current replacement rate. In addition to maintenance, structural and regulatory issues, there is concern over the ability of US drinking water distribution systems to maintain water quality to the consumer. Consequently, drinking water utilities are increasingly interested in the potential for the application of techniques such as Condition Assessment (CA) methodology for managing drinking water infrastructure. The USEPA has therefore initiated a project (carried out by the Eastern Research Group) with the goal of developing new CA techniques that can assist utilities in dealing with some of these issues. Several tools have been developed (which will be discussed) including a modeling approach that can be applied to a drinking water utilities break and leak database and that will help managers understand the factors affecting the "survival" of drinking water pipe sections. The model is based on the application of the Cox Proportional Hazard Model (CPHM) modified by a frailty function. Shared frailty represents unobserved external factors, known to be important, which vary randomly and which are more consistent among sections in the same pipe run than among sections in different pipe runs. Examples of factors which may be represented in this way are soil corrosivity, variations in external mechanical loading, pressure transients, vibration, nearby construction, etc. The CPHM has been applied to a pipe break data base collected by the Laramie Water Utility (located in Laramie, Wyoming (USA)). Preliminary results, for example, indicate that, on the average, metallic pipe has fewer beaks than Poly Vinyl Chloride (PVC) pipe but is more subject to undefined random factors. That is, metallic pipe has more "random frailty" then PVC pipe. This has implications for infrastructure management policies such as the application of advanced monitoring techniques and replacement strategies. In addition to the CPHM for individual pipe sections a general pipe break model has been developed.