Moscow sewer study puts Basin 3 at top of I&I priority list
City of Moscow, Idaho · Meeting date: October 19, 2026 · Published: October 20, 2026
A four-month monitoring campaign found that a relatively small Moscow sewer basin generated consistently high rainfall-derived flows and experienced surcharge during a major March storm. Consultants recommend Basin 3 and Basin 5 as the first targets for further investigation and rehabilitation planning, with the much larger Basin 2 presenting a longer-term hydraulic concern.
Key points
- Basin 3 ranked highest on normalized I&I measures despite covering about 173 acres, with 30,657 feet of sewer main and 578 equivalent dwelling units.
- During the 2.46-inch March 11 storm, Basin 3 generated an estimated 7.78 million gallons of I&I and experienced surcharge associated with a downstream hydraulic bottleneck.
- Basin 2 produced the largest total I&I volumes in every evaluated event, including 9.85 million gallons in the March storm, and recorded prolonged, deep surcharge.
- J-U-B Engineers recommended further investigation and rehabilitation planning first in Basins 3 and 5; staff said the city will use the study as a heat map for future work.
Full report
Moscow has identified Basin 3 as its highest-priority area for sewer inflow and infiltration work after a citywide basin-level monitoring study found that the comparatively small service area consistently produced disproportionate wet-weather flows.
City Council received J-U-B Engineers’ 2026 flow-monitoring report on October 19. The four-month study monitored six gravity-sewer manholes between February 2 and June 1, collecting five-minute flow data through multiple rain events. The work was intended to identify where groundwater and stormwater are entering the sanitary collection system so the city can focus future capital spending and investigation.
Basin 3, monitored near Taco Time, covers approximately 173 acres, includes 30,657 feet of sewer main and serves 578 equivalent dwelling units. Yet it ranked highest among all six basins when I&I was normalized by sewer-main length, connections and basin area. It was also the second-largest contributor by total I&I volume despite being among the smaller monitored basins.
During the March 11 storm event, which brought 2.46 inches of rain, Basin 3 generated an estimated 7.78 million gallons of I&I. The basin recorded 254 gallons of I&I per foot of sewer main, 13,468 gallons per equivalent dwelling unit and 44,959 gallons per acre—each the highest result among the six basins for that event.
The study also found a surcharge at the Basin 3 monitoring location. The surcharge began at about 2.74 million gallons per day, increasing the flow depth from about 11 inches to 27 inches at the same flow rate. J-U-B said this indicated a downstream hydraulic bottleneck rather than a local pipe-capacity constraint.
Basin 5 was the second-highest basin in the normalized I&I analysis and recorded the highest peaking-factor ratios during every monitored rain event, indicating a pronounced and rapid inflow response. The report said such a response points to potential illicit connections, cracks or holes allowing surface water to enter the sewer system quickly.
Basin 2, monitored near Spence Hardware and covering roughly 1,979 acres, had the highest total I&I volume in each of the four evaluated storms. In the March 11 event it contributed an estimated 9.85 million gallons. The location experienced prolonged and deep surcharge, with depths reaching the manhole lid but not overflowing. J-U-B recorded a 4.4mgd peak flow at the maximum surcharge depth, compared with a theoretical full-pipe capacity of 3.45mgd.
The report recommends Basins 3 and 5 as the highest priorities for further investigation and rehabilitation planning, while describing Basin 2 as a suitable longer-term investigation target because of its size and hydraulic concern. Potential next steps identified include more focused flow monitoring, smoke and dye testing for illicit connections, CCTV inspection, manhole assessments, pipe rehabilitation and replacement.
Deputy City Supervisor Tyler Palmer told the council that the city would use the study as a “heat map” to direct its resources. He also said staff may return to the council with a proposal for a programme to encourage sewer-lateral replacement. The city does not own private laterals, but Palmer said connections and older clay lateral pipes can be sources of inflow and infiltration.
Palmer said reducing extraneous flow can avoid unnecessary collection-system costs, limit strain on lift stations and the water reclamation facility, and help the city meet the 85% pollutant-reduction standard in its discharge permit. During the March event, flows at the water reclamation facility rose from around 2mgd to 12mgd, according to the presentation.
This summary was generated by AI based on the source document and may contain errors.
Source: City Council Regular Meeting, City of Moscow