Identifying sources of rainfall derived infiltration and inflow using
impulse response functions
Abstract
Rainfall derived infiltration and inflow (RDII) are extraneous water in
a sanitary sewer system that are originated from rainfall in a surface
runoff form. Most RDII enters sanitary sewer systems through illegal
connections or mechanical faults, especially in aged sewer systems. In
this study, the physical process of three primary RDII sources: roof
downspout, sump pump, and leaky lateral, are investigated using
physics-based models. These three sources represent three different flow
paths: direct connection of impervious catchments, mixed flow through
coarse porous media followed by a direct connection, and percolated flow
through compacted soil, respectively. Due to the differences in medium
and the flow paths, flow responses of these three RDII sources differ in
time and magnitude, and they can be distinctly identified from each
other. The typical flow response of each RDII source is represented as
an Impulse Response Function (IRF) that is a flow response to a
pre-specified representative rainfall computed using physics-based
models. The total RDII flow hydrograph is presented as a combination of
these three IRFs, and the weighting factors of each IRF is calibrated
using a genetic algorithm (GA) technique in a test sewer catchment. The
results may shed light on identifying the contributions of different
RDII sources in a sewershed and help public water managers to understand
the local RDII issues better, which in turn facilitates more effective
management of a sewer system.