Conclusion

In this study, three major rainfall derived infiltration and inflow (RDII) sources: roof downspout, sump pump, and leaky lateral were identified, and the physical process of each source was modeled using physics-based models. These three sources represent three different flow paths: a direct connection of runoff catchments, coarse porous media, and compacted soil, respectively. The typical flow response of each RDII source was expressed as impulse response functions (IRFs) that indicate the flow responses to a representative rainfall. The three IRFs displayed distinctly different flow patterns. Roof connection IRF directly reflected the input rainfall in terms of flow duration. The leaky lateral IRF showed a delayed and dampened flow hydrograph as percolation through porous media being the major hydrologic process. The sump pump IRF hydrograph fell between the roof IRF and the leaky lateral IRF. The sump pump flow path also involved a flow through the porous medium, but the process was “faster” than the leaky lateral flow path. It is due to the travel distance of surface water in the sump pump model was shorter than that of the leaky lateral model, and the medium usually has a larger hydraulic conductivity. The shapes of the three IRFs were easily distinguishable from one another, which in turn made them suitable for use as building blocks of an RDII estimation model.
The RDII estimation using the three IRFs was achieved by superposing the IRFs to best fit the monitored RDII hydrograph. To reproduce the total RDII, each IRF was multiplied by weighting factors that were calculated using a genetic algorithm (GA) technique. This method was applied to a study sewershed in a suburb of Chicago, IL, where sewer flow monitoring data is available. The IRF model performance was compared to a more widely used method, RTK, to evaluate the advantages and disadvantages of using the suggested approach (Camp Dresser & McKee Inc., 1985; Rossman, 2010).