Risk-Based Approach Produces 10-Year Utility Master Plan

Dallas Water Utilities (DWU) provides wastewater services to 1.2 million customers throughout a 340-square mile service area within the City of Dallas as well as to 11 wholesale “customer cities” in the Dallas/Fort Worth Metroplex.

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by Bill McHie, Jane McLamarrah, and Chris Kaakaty

Dallas Water Utilities (DWU) provides wastewater services to 1.2 million customers throughout a 340-square mile service area within the City of Dallas as well as to 11 wholesale “customer cities” in the Dallas/Fort Worth Metroplex. Approximately every 10 years, DWU develops a wastewater master plan for the city to address capacity needs and regulatory compliance throughout its 4,000 miles of pipes and 14 collection system lift stations, which provide flow to two wastewater treatment plants.

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For nearly 9,400 modeled gravity pipeline segments, a capacity condition score and consequence of failure score were produced to determine DWU’s most critical capacity improvement projects.
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Previously, DWU’s planning efforts focused on meeting capacity needs largely associated with population growth; however, DWU, like many utilities throughout the United States, is facing an aging infrastructure and the threat of increased sanitary sewer overflows (SSOs).

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Twenty-two high capacity risk projects solve overflows and 13 medium capacity risk projects solve sewer surcharging within three feet of the ground.
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To be proactive in addressing these issues, DWU teamed with MWH, a global provider of environmental engineering, construction and strategic consulting services, to develop its most recent 10-year wastewater collection system master plan. Unlike previous DWU master plans, this plan centered on a risk-based approach to prioritize and schedule capacity improvement and asset renewal projects. Using this unique risk-based approach to capital improvements, the current master plan is designed to:

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High risk pipes, identified by functional condition and criticality, and high risk sewersheds, identified utilizing pre-defined and weighted decision criteria, such as infiltration/inflow factors, pipe condition and pipe criticality, were prioritized for assessment in the CIP.
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  • Achieve regulatory compliance by reducing SSOs;
  • Ensure collection system capacity and improve customer service; and
  • Meet the need for sustainable renewal of collection system assets.

Capacity Risk-Based Approach

The concept of developing inspection strategies and renewal projects according to risk is at the heart of best practice asset management programs being implemented by leading utilities throughout the world. For DWU’s master plan, the utility used a risk-based philosophy for both capacity improvement and asset renewal needs. Although risk-based approaches are becoming more common for asset management and renewal projects, it is unique to use the same approach for prioritizing and scheduling capacity improvement projects.

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Based on the renewal risk rating, DWU is able to identify and prioritize those individual pipe segments with the highest risk.
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“We [Dallas Water Utilities] needed a logical and defensible approach to prioritize and schedule capital improvement projects throughout our wastewater system,” said Chris Kaakaty, DWU Assistant Director of Wastewater Operations. “This approach allows us to be more proactive in managing our assets.”

The basis of a risk-based prioritization of asset renewal needs is to categorize the installed asset according to its condition and the potential consequence if it fails. This same risk-based prioritization methodology is used for capacity improvement projects; however, the asset condition score is replaced with a condition score that represents the probability of overflow rather than the probability of functional failure. The consequence of failure is the same for asset renewal (pipe failure) and capacity (sewer overflow); both result in a discharge of sewage into the environment, which can affect the local community. Factors considered in the consequence of failure analysis included economic, environmental and social/public health impacts.

For DWU’s master plan, once the consequence of failure score and the capacity condition score were defined for each of the nearly 9,400 modeled gravity pipeline segments, a capacity risk rating was assigned as the product of the two scores. The resulting capacity risk ratings were categorized as high (red), medium (yellow) and low (green) capacity risk as identified in Table 1.

Based on the capacity risk rating, DWU is able to identify its most pressing capacity improvement projects and prioritize the expenditure of capital funds. The high capacity risk projects resolve modeled overflows. The medium risk capacity projects resolve high surcharging. None of the low capacity risk projects were scheduled for improvement because of the low potential for overflows during the planning period.

Renewal Risk-Based Approach

Since DWU has limited field condition assessment data to characterize condition scores for all 100,000 gravity pipeline segments, this master plan assigned initial, “first-cut” renewal risk ratings based primarily on pipe age and historical operational knowledge.

Table 2 provides a breakdown of the total number of segments and the length of gravity pipes in each renewal risk category. The high renewal risk categories are shown in red, the medium renewal risk categories are shown in yellow and the low renewal risk categories are shown in green.

Based on this renewal risk rating, DWU is able to identify and prioritize those individual pipe segments with the highest risk. In addition to identifying individual pipe segments by risk rating, the planning team also prioritized the sewersheds for condition assessment by high, medium and low risk. This was accomplished by using pre-defined and weighted decision criteria, including infiltration/inflow factors, criticality of pipes and probability of failure. High risk pipes and high-risk sewersheds identified by this approach, were prioritized for assessment in the CIP.

As field condition data is acquired to refine the condition scores, the city will be able to fully integrate renewal risk-based prioritization and scheduling into the existing capacity risk-based CIP.

Benefits of Proactive Approach

Because of the increasing national awareness of the potential harmful impacts of SSOs, regulatory agencies have been implementing a “zero tolerance” policy where any SSO is a “violation” subject to enforcement. As a result, more utilities are looking for a proactive approach to providing adequate capacity and minimizing the potential for overflows.

“Knowing definitively which capacity-limited pipeline segments have the greatest potential for overflows allows us to target our capital funds to both save money and maintain our good reputation with regulatory agencies and our customers,” said Kaakaty.

DWU’s CIP was accepted by the city in late 2007, allowing for improvement work to begin in 2008 through 2018. Over the next 10 years, DWU will work to address the high risk capacity and asset renewal projects identified by this CIP and continue to prioritize projects as more condition assessment data becomes available WW

About the Authors

Bill McHie, P.E. (MWH) is a Vice President and Division Project Manager for MWH located in Baton Rouge, LA. He has 27 years of experience in the municipal water and wastewater sector, including planning, design and construction phases of large capital improvement programs. Jane McLamarrah, Ph.D., P.E. is a Vice President and Environmental Engineer for MWH in Dallas. In addition to being the technical manager for DWU’s master plan, Dr. McLamarrah is MWH’s Capacity Assurance, Management, Operations, and Maintenance (cMOM) National Practice Leader. Chris Kaakaty, P.E., is the Assistant Director of Wastewater Operations at DWU. He has worked for over 18 years at DWU providing engineering, regulatory, wastewater, laboratory and pretreatment expertise to various projects conducted by the city.

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