Manhole Inflow - The unexpected reality of how much of our peak wet weather flow is related to sheet runoff

Manhole Inflow - The unexpected reality of how much of our peak wet weather flow is related to sheet runoff SUMMARY More than a decade of sewer rehabilitation work was conducted for the Kline's Island Sewer System of Lehigh County, including inspection of all manholes and significant manhole rehabilitation. Following this rehabilitation effort, hydrographs from a year-long I&I investigation program for the same system showed that many basins continued to be badly impacted by inflow sources. Investigations including smoke testing and basement inspections confirmed there were very few roof drain connections and no storm drain connections, suggesting that manhole inflow was the major contributing factor. Keystone Consulting Engineers (KCE), an engineering firm working with the County, conducted field trials using cameras mounted inside manholes to assess sheet runoff impacts on various frame, cover, and inflow dish configurations. These trials confirmed that inflow through manhole lids, lid-frame interfaces, dish to frame interfaces, and frame to chimney interfaces were much higher than previously realized, particularly during the type of downpour events that were leading to SSOs, basement backups, and/or pump station wet well alarms. Based on these findings, the various consulting engineers for this group of 15 signatories noted that their manhole inspections were focused on the manhole body — a source of infiltration - and not on the inflow potential of the lid/dish/frame/chimney components. Arcadis developed a revised manhole inspection rubric with added focus on inflow risks based on these realizations. Initial trials revealed a huge range of lid/dish/frame/chimney permutations, necessitating three months of field trials with 3 revisions to the inspection rubric. Once the logic was finalized, the inspection protocol was programmed into a Fulcrum-based cell phone/tablet manhole inspection application to ensure the required focus on inflow and provide a standardized inspection approach all 15 municipalities could use. The application was designed to automatically populate with rehabilitation recommendations based on observed defects in the given permutation to standardize recommendations across the different signatories. To date, nearly 1000 manhole inspections have been completed on manholes that had been previously inspected and, in many cases, rehabilitated. The new inspection protocol has found that approximately 2/3rd of the manholes had frames/covers/chimneys with sufficient inflow issues to warrant corrective action. These issues range from leaking grade rings to inflow prevention devices (IPDs) that are filled with holes. Issues and corrective actions vary between Signatories, but fall into the categories of frame and cover replacement/resetting, replacement/resealing of faulty inflow dishes, chimney sealing, cover gasket replacement, or cleaning and sealing via caulk or gasket. Relevance, Usefulness, and Takeaways This presentation will 1.Show the videos of the KCE field trials 2.Demonstrate the standardized manhole inspection rubric 3.Show photographic examples of the inflow defects found 4.Present the percentages of manhole with inflow-related defects encountered 5.Present the corrective actions taken for each combination of defects found. This presentation will provide operations and engineering staff with specific data and lessons learned directly applicable to every sanitary sewer system. The results will shed light on the significance of manhole inflow on peak wet weather flow under specific sewer system characteristics, and provide direction on future approaches to manhole rehabilitation areas of focus. The manhole inspection rubric will provide an example of how to leverage large amounts of data from multiple contributing signatories. Demonstrated Results The percentage of manholes found to have defects using the revised inspection protocol, the types of defects found, and the costs to repair each type of defect will be presented. These findings will have implications on the recommended approach to manhole rehabilitation, shedding light on the significance of manhole inflow on peak wet weather flow.