IMPROVING CONFIDENCE LEVELS FOR MEASUREMENT OF INTERCEPTOR CAPACITY AND I/I

Community sewer utilities use flow monitoring and hydraulic analysis to assist their engineers in making design decisions for system improvements. This information is used primarily to identify pipe (or facility) capacity or remaining available capacity for growth. In the past, such studies relied heavily on an analyst's judgment and lacked objective statistical parameters about the level of confidence in the data and the projected flows. This paper presents a case study conducted for the Bowling Green Municipal Utilities in Bowling Green, Kentucky that illustrates the typical steps that should be accomplished for completion of a successful study. The study analyzed the results from 44 days of flow monitoring at four locations along the 46-cm (18-in) interceptor downstream from the Campbell Lane Pumping Station. The immediate purpose of the study was to identify the limiting capacity of that interceptor so new pumps could be selected to upgrade that pumping station. Although this was a relatively small study, several conditions were identified that represent typical “pitfalls” for analysts evaluating flow monitoring data. Additionally, the case study illustrates a practical application of the previously proposed “Standardized Method for I/I Analysis” (Kurz, et al, 2003) That method projects the level of I/I (rainfall dependent I/I) for a design storm. It also identifies the range of values within a 95% confidence interval so that the engineer can plan for different degrees of uncertainty in future designs. This procedure is not proprietary and will be described in sufficient detail for a municipal engineer to successfully adapt it to the needs of any community.The monitoring revealed that the limiting capacity is approximately 9,840 m3/day (2.6 mgd). This limit occurs in a reach which has an invert grade of 0.15%, which is the smallest slope that occurs along this interceptor. Slope alone indicates that this reach controls the capacity. However, the scattergraph analysis revealed additional capacity degradation under high flow conditions. This capacity is based on observed surcharge conditions and is less than what was projected theoretically. However, the pipe is deep and some surcharge to develop head pressure for peak flows was deemed acceptable.This paper will discuss the procedures used to evaluate the reliability and quality of the flow monitoring data. All measurements were depicted on scattergraphs by plotting velocity vs. depth.These data points were compared to “best-fit” curves representing depth and velocity from the Manning equation with various “hydraulic coefficient” values – expressed as the “square root of slope divided by the friction factor” in the Manning equation.