Two-Stage Mini-Basin Metering Quickly Isolates Sources of RDII in Bear Brook

The City of Saco is located on Maine’s southern coast, approximately 14 miles south of the City of Portland. Saco is an attractive destination for both residents and visitors due to Saco’s beautiful beaches, the Saco River, and the historic downtown and shopping district. Saco took an innovative approach to locating the sources of Rainfall Dependent Infiltration and Inflow (RDII) in its Bear Brook sewer shed.The Bear Brook pump station has experienced severe RDII flows and sewer overflows have occurred during heavy rains. Part of the sewershed parallels Bear Brook and it was assumed that most of the RDII was originating in that section of sewer. Instead of using the traditional smoke testing and CCTV in the 33,220 m (109,000 lf) sewer shed, the City elected to deploy a two-tier mini-basin metering strategy.Tier 1 metering was conducted in five sub basins at an average size of approximately 4,276 m (14,000 lf) of sewers. Data from 36 days of metering and three rain events were analyzed by ADS using its Sliicer.com web-based software to compare dry and wet weather performance and rank the five basins by the severity of RDII. Two of five basins produced low RDII and were eliminated from further examination leaving 13,138 m (43,103 lf) to study. Tier 2 metering was conducted in the three worst Tier 1 basins and involved relocating and installing additional meters to create 8 smaller micro-basins at an average size of 1,642 m (5,388 lf) of sewers. It was found that during a 54.1 mm (2.13inch) storm, 88% of the RDII was generated in three of the micro-basins. The three basins contained 6,827 m (22,397 lf) or 20% of the total system size.From the City’s perspective, there are three possible options for locating sources of RDII;The old way of doing SSES over the entire Bear Brook sewershed.Doing SSES on the worst of the five large basinsDoing SSES on the worst of the micro-basins.As a result of successfully identifying the sources of RDII with this micro basin technique, the town estimates it saved approximately $327,000 in unnecessary SSES (CCTV, smoke, dye test, manhole inspections). The metering cost for both phases of the work was $25,000 and the payback (over straight SSES) was greater than 15:1. Subsequent sewer rehabilitation work has reduced the peaking factor at the Bear Brook pump station from 6:1 to 2:1.