Lexington, KY - Continuous Flow and Groundwater Metering with Remote Sensing to Monitor Sanitary Sewer Surcharging

West Hickman 7 Sub-Sewer Shed Flow and Groundwater Monitoring, LFUCG, (2006–2007) - This study collected and examined 1 year of 5-minute continuous flow measurements in the sanitary sewers and nearby groundwater levels to determine if the groundwater was a controlling factor when the sewer surcharged. The study focused on the trunk line near Lexington Mall, which is an area with recurring sanitary sewer overflows (SSOs). The study concluded that groundwater was not the controlling factor for surcharging in the sanitary sewers, but it was limited downstream capacity. This was determined by examining the timing of the surcharge activity through the study area. Overflow volumes were calculated and extrapolated to estimate the size of a future storage basin. Also, larger pipe sizes were projected to permit capacity for the 2-yr 24-hour rain event.The study area encompassed approximately 187,000 LF of sanitary sewerline. This year-long program consisted of 17 flow monitor sites, 4 groundwater sites, and 1 rain gauge site. Remote sensing technology was employed at one of the sites.The significant conclusions that were drawn from this in-depth study are:Of the 30 rain events that were greater than 0.5 inches of rain, there were 21 (or 70 percent) rain events that caused an overflow to occur in study area.One rain event was a 100-year event and caused almost 4.3 MG of overflow volume in the three known overflow areas combined.There was not a clear seasonal trend in the diurnal patterns, where any one season was consistently higher or lower than the other seasons.Overall, the flow balance calculations throughout the system were excellent, with the difference between upstream and downstream flow meter sites consistently falling within the +/− 10 percent tolerance.A flow diversion was discovered under the Lexington. It does not have a known access point.The peak flow rates measured during heavy surcharging conditions indicate a maximum of about 3.2 million gallons per day (MGD) can flow out of this sub-sewer shed through an 18-inch pipe.The control point for surcharging in this system appears to originate downstream of the study area. An analysis of the timing of the surcharging activity suggests the surcharge originates below manhole WH7_23 and then progresses upstream all the way to the manholes behind Breckinridge Elementary School during severe rain events. A detailed hydraulic model will provide further details and potential confirmation of this conclusion.Typical peaking factors observed for the entire sub-sewer shed ranged from 2–4, but these values are somewhat skewed to the low side due to known overflow activity within the sub-sewer shed.Wet antecedent moisture conditions led to higher peak flow rates in the sanitary sewers. They were found to be 50 percent higher for a comparison between the May 25, 2006 and October 26, 2006 rain events.The average inflow and infiltration (I&I) coefficient for the sub-sewer shed was 5 percent and this value experienced the same skewed effect as the peaking factors.There was not a clear seasonal pattern in the dry weather level patterns found in the groundwater datasets.The levels in the sanitary sewer rise prior to the rise of the groundwater levels, and the levels in the sewer return to normal levels while the groundwater level remains above the sewer invert. The inflow component of the I&I flow is the dominant component when compared to the influence of the infiltration component.The groundwater levels typically recede within a few days after a rain event has ended.Overall, the vault approach to protect the flow meter equipment was very effective.Flow monitoring is an effective tool for evaluating the performance of sanitary sewers and locating problem areas. Flow meter data also permits detailed hydraulic models to be calibrated for the purpose of evaluating sewer system improvement scenarios.