ACCURATE REPRESENTATION OF INFLOW AND INFILTRATION IN SEPARATE SEWER SYSTEM MODELS

Separate sewer system inflow and infiltration (I/I) impacts just about every utility throughout the United States. At present many utilities are currently under or are facing consent decrees which include provisions for addressing problems driven by I/I, such as basement backups, sanitary sewer overflows, treatment plant impacts, development capacity impacts, and pump station impacts. More often than not, solutions required to address these problems are costly, complex, and require an extensive knowledge of system hydraulics to properly design and implement. Computer models allow for testing numerous solution scenarios and comparing simulation results during the project planning phase. The models provide an excellent means of ensuring that problems related to I/I are addressed in the most cost-effective manner and will achieve targeted levels of control and system performance. Successful use of computer models requires accurate representation of sewer system assets and flow generation parameters for both dry and wet weather conditions. Model simulation of I/I includes flow components for dry weather groundwater infiltration, rainfall induced inflow and infiltration, seasonal variations, and variations due to antecedent storm conditions. This paper discusses methods to develop an accurate model representation of separate sewer systems under various antecedent moisture conditions. Results will be presented from a model developed for the Northern Kentucky Sanitation District No.1 that was calibrated to over 1 year of continuous time series data. The paper also discusses model calibration standards and selected parameters for predicting peak flows from future development.The District utilizes Wallingford's InfoWorks CS model software for sewer system analysis. Many of the I/I model procedures which will be discussed are also applicable to or can be used in other commonly available model software packages.