Methods for Estimating Inflow and Infiltration into Sanitary Sewers
Excessive inflow and infiltration during wet weather periods into capacity- constrained sewer systems cause sanitary sewer overflows (SSOs). The two major components of wet weather flow are inflow and infiltration, and are the main factors found in sanitary sewer evaluation studies (SSES) or inflow/ infiltration (I/I) studies. Control and reduction of inflow and infiltration directly relates to effective controls for SSOs.
The interaction and relative proportions of inflow and infiltration determine the extent, effectiveness and cost of control measures. Usually, control of direct inflow is the first source pursued, with the infiltration component either lumped into part of the inflow as an immediate response, or neglected because of the dominance of peak flow rates induced by inflow. The peak flow rate, as compared to sustained elevated flows from infiltration, is usually the sought-after result in SSES or I/I studies. Successful and accurate estimates of both rainfall-derived inflow and sustained flows from rainfall-derived infiltration are therefore the prime determinants of the effectiveness and cost of the controls.
The objective of this chapter is to summarize and provide some critique of common flow projection methodologies, particularly methods that predict rainfall--derived inflow and rainfall-induced infiltration or rainfall-derived inflow/ infiltration (RDII). A summary of the most common methodologies is included. Space limitation precludes site by site comparisons of each technique, even if project objectives (and funding) allowed such comparisons to be made.
The chapter will include discussion of the approach used in two communities, one of medium size (population 160,000) and one large (800,000). Both communities have examples of varied infrastructure age and conditions, and climatic differences that affect inflow or infiltration prediction. The two communities have large capital improvement programs that are significantly impacted by the quality and accuracy of flow estimates of design storm flows. Design flows are estimated for current flow conditions as well as for future basin conditions with growth projections and infrastructure improvements. Infrastructure improvements in general and specific to Salem, Oregon and Honolulu, Hawaii are to reduce inflow and infiltration, normal life cycle system maintenance, or to enlarge sewer system capacity.
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