Pacing Reinvestment: Developing a Long-Term Collection System Rehabilitation Plan

Following an EPA 308 Letter of Violation in 2014 citing high rates of SSOs within the collection system, Winston-Salem/Forsyth County (WSFC) Utilities identified system condition as a key driver of gravity sewer failures. A rehabilitation plan was quickly developed and implemented to assess and rehabilitate poor condition assets within the system. Five years later the plan has underwent significant changes. By understanding the basis of this program and using the results to date to improve the initial plan, a sustainable long-term collection system has been developed to achieve WSFC Utilities level of service goals. Initial rehabilitation needs were projected using a Statistically Significant Sample Survey (4S) which sampled the variety of pipes in the system to determine which features correlated with the poorest condition. Based on these findings, rehabilitation yield rates were developed for the system. Areas with assets most likely to yield high volume of rehabilitation work were prioritized for assessment and rehabilitation measures were developed based on these asset-specific inspections. This allowed WSFC Utilities to prioritize the inspection of assets most likely to be in poor condition, maximize the number of asset failures identified, and efficiently address these failures to improve overall system condition as quickly as possible. Five years later findings indicated changes from those initial assumptions. Pipes were yielding nearly double the rehabilitation requirements. Additionally, the cost of rehabilitation had begun to escalate quickly. Initial plans were no longer appropriate and needed to be modified to be sustainable over the long-term. Figure 1 shows where data has and has not been collected to date. Using the much larger data set that now included five years' worth of condition assessment data in addition to initial 4S findings, a variety of statistical and machine learning approached were used to update rehabilitation yield rates. The types of rehabilitation required for poor condition assets could now be projected as well. These improvements, combined with updated unit prices, were used to develop a update cost of bringing the system up to target service levels. Updated failure rate projections of initial asset classes can be seen below in Figure 2. Understanding the high levels of investment required to fully improve the condition of the collection system to target levels of service, WSFC Utilities elected to focus their rehabilitation program on the worst performing asset classes. By targeting condition assessment and rehabilitation activities on these poorest condition assets, overall system can be improved most efficiently. Challenges exist in collection of data on these assets only. Throughout the collection system these in-program assets are intermixed with non-critical assets. To assess condition and rehabilitate these in-program assets only would be inefficient. Even for sub-basins with the highest concentration of in-program asset classes non-critical asset data is captured. When accounting for this factor the total cost of condition assessment and rehabilitation for all in-program asset classes is approximately $158M in present value, as illustrated with Figure 3. This cost must be spread over multiple years. Resource constraints, including funding and workforce limitations, prevent the rapid deployment of these rehabilitation activities. However, as the timeline for this investment is extended the system continues to deteriorate. A sustainable long-term rehabilitation plan must invest enough resources to offset normal condition deterioration and improve conditions to target levels of service. By modeling condition deterioration and existing rehabilitation needs, WSFC Utilities can test funding scenarios to develop a plan that balances long-term service needs with financial limitations, as illustrated in Figure 4.