Considering Consequence of Failure in Water Main Replacements

Posted by Doug Hatler on May 10, 2019 10:06:29 AM

When an old six-inch cast iron main breaks on Franklin Street on a cold February morning, the crews rush in to shut off the water and start assessing damage. A rupture like this is relatively standard, but it happens to be on a water main providing water to a large elementary school. Now the school is out of water, parents are notified and school is closed for the day.

Early morning news crews pick up the story and are on-site filming the repair crew. The city’s Water Deputy Director is asked to discuss the situation – the talking points are ready from another break that made the news some months earlier. 

“The city has aging infrastructure, but we are actively going through the system and replacing all the old cast iron mains. This is a 15-year program that will modernize the water system. Our replacement program had not reached Franklin Street yet, so this unfortunate break happened.”

This is an imaginary story, but it might ring true for many water agencies across the country. Is it unavoidable to experience breaks like these, or could there be improved ways to prioritize replacement plans? 

Some breaks in our aging water systems are unavoidable, but by using solid asset management practices and targeting the highest risk parts of the system, utilities can mitigate vulnerabilities.

In water system asset management, the asset risk is typically assessed by calculating asset criticality or business risk exposure (BRE). The main components of risk assessment are the likelihood of failure (LOF) and consequence of failure (COF). 


LOF is a value that can be derived purely mathematically. An objectively calculated LOF should be the foundation of the asset risk assessment. The LOF calculation should be as comprehensive as possible, resulting in LOF scores for every water main segment.

Pipe failures result in various consequences, including economic, social, and environmental impacts. COF calculations are a bit more complicated. COF can’t be derived with purely objective, mathematic rules. 

Best practice for COF determination is to consider the various factors of consequence in a “triple bottom line” analysis that considers financial, social and environmental impacts of main breaks. Measuring financial impacts, i.e. repair costs of a break, is possible and quite straightforward; calculating social impacts, on the other hand, requires a more subjective, opinion-based approach.

One way to aid in simplifying the complex challenge of defining COF is to build standard methodologies to assess the different aspects by defining typical COF categories:

Direct Costs:

  • Repair cost
  • Repaving and restoration cost

Indirect Costs:

  • Customers without water
  • Critical facilities
  • Property damage
  • Traffic disruption
  • Environmental fines 

It is possible to rank every pipe segment in these categories, but there are still challenges - how is this done systematically across the entire distribution system, and how do these categories compare against one another to determine the final COF impact?

In order to systematically assess the entire distribution system and calculate the COF impact, a scalable software approach is needed where asset properties such as diameter, material and various geospatial features that include proximity to critical facilities, important traffic patterns, and environmentally sensitive areas are assessed. 

We are still left with the challenge of comparing the COF categories against one another. One way to do this is to convert all categories to a financial impact value.  The Water Research Foundation report, Managing Infrastructure Risk: The Consequence of Failure for Buried Assets, explains how these different COF categories can be assessed monetarily.

Monetized COF provides a range of risk values, not just simple scoring from 1-5. Why is a range of values a benefit? Let’s illustrate this by considering a risk matrix with scores 1-5 and compare it to a risk matrix with LOF probabilities and monetized COF values.


The above graph illustrates the benefit of a monetized risk matrix. Using the score-based matrix, the highest COF is only 25 times greater than the lowest COF; however, in using the monetized matrix, the difference is 3200 times greater. Differentiating between higher risk assets and lower risk assets is beneficial when it comes to rehabilitation and replacement planning. 

When using a score-based matrix there is potential for overweighting parts of the system with a higher COF. If a replacement plan is built on these scores, perfectly good water mains could be prematurely replaced because of inaccurate ranking. 

For example, a six-inch cast iron main installed in 1932 in a typical neighborhood might receive a LOF score of 4, but a COF score of 1, resulting in total business risk of 4. A brand new 12-inch PVC main close to a school could have a LOF score of 1, but a COF score of 4, resulting in the same business risk in a score-based matrix. However, using a monetized matrix, the business risk would be $1,125 and $300 respectively, more accurately showing COF impacts.

Business risk based on objective, data-driven LOF and systematic, monetized COF sets the best foundation for balanced, proactive replacement plans. 

Breaks will happen in water systems even with perfect risk assessment and prioritization, but when a proactive program is implemented, the breaks will be fewer as high LOF mains are proactively replaced, and those that do break will have a lower COF, meaning there will not be news crews on-site on an early February morning.

Developing a comprehensive pipe prioritization program does not need to be complicated and unattainable. Fracta has developed a fast, affordable and accurate software solution for prioritizing water main replacement using objective LOF and systematic, monetized COF to calculate detailed business risk values for all segments in the distribution system.

Contact Fracta for more details

Topics: Capital Planning, Leak Detection, Condition Assessment

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