Flow knowledge - tools for sewer operation and targeting renewal

Flow monitoring in gravity sewer systems poses challenges of complexity and interpretation of data. The success of sewer renewal work needs objective measures to determine effectiveness of I/I removal, overflow abatement, and customer service. The value of sewer renewal has been underestimated in many cases because the initial loading of I/I was underestimated due to misinterpretation of data. This paper will explain how to avoid these errors, and how to develop baseline relationships between I/I and rainfall. Case histories for Nashville and Jackson demonstrate that improved interpretation enabled designers to identify instances when upsizing was not needed. Analysis of the data 'signature' showed that correction of downstream blockages would recover the required capacity. The US-EPA's national CMOM initiative will require flow monitoring for documenting municipal program performance. A common strategy for deploying temporary meters is to use two consecutive months between September and April. During a year of flow monitoring in a North Carolina community, only one of the seven possible combinations of two 'wet' months would have replicated the correct estimate of annual I/I with that strategy. Analysis of 12 individual months resulted in projections of annual I/I that varied from -49%, to +85% compared to the total I/I for that location in one calendar year. Permanent flow monitoring conducted for periods greater than a year will help increase the reliability of projected sewer system characteristics. However, annual variations must still be considered based on variations observed in system characteristics during a study of four permanent monitoring sites over a period of three years in Nashville. (author)