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).