Introduction
Infiltration and inflow (I&I) is an urban water resources term that
describes the unwanted water existing in sewer systems that are not
originated from the typical sewer sources, e.g., domestic and industrial
discharge. Infiltration is water seeping into the sewer pipes,
preferably through broken pipe cracks and joints (Figure 1). The origin
of infiltration can be surface water percolated down to the sewer pipes
or groundwater with the water table above the pipe invert. Inflow is
surface water entering the sewer system through direct connections from
runoff catchments or cross-connections from storm sewer or combined
sewer. The term rainfall derived infiltration and inflow (RDII) is a
type of I&I that originates from rainfall, often in a surface runoff
form.
I&I is one of the major problems affecting sewer systems in terms of
flow overloading that causes sewer overflows, basement flooding, street
flooding, increase in pumping costs, water pollution, and decrease in
treatment efficiency in water treatment plants (Backmeyer, 1960; Field
& Struzeski, 1972; Gottstein, 1976l; Lai, 2008). Based on the
estimation by Petroff (1996), roughly 50% of the water entering
wastewater treatment plants in the U.S. is from I&I. Depending on the
age and the condition of the sewer system, the relative volume of I&I
to the dry weather flow (DWF) could be ranged from 0.4 to 9 (Bishop et
al., 1987; National Small Flows Clearinghouse, 1999; Ertl et al., 2002;
Weiss et al., 2002; Lucas, 2003; Pecher, 2003; Jardin, 2004; Kretschmer
et al., 2008; Bhaskar & Welty, 2012). For example, I&I for Baltimore
City was nine times greater than the DWF, and it was also larger than
the gauged streamflow from the urban watershed (Bhaskar & Welty, 2012).
This indicates that I&I volume can affect the capacity of a sanitary
sewer system significantly.
Various I&I estimation modeling methods have been developed since the
1980s to quantify the amount of I&I (De Bénédittis &
Bertrand-Krajewski, 2005). Bishop et al. (1987) developed a simple
synthetic hydrograph method for 300 study basins to estimate I&I and to
evaluate flow data. Gustafsson (2000) presented a leakage model that
takes account of the two-way interaction between pipes and the aquifer
using MOUSE (Lindberg et al., 1989) and MIKE-SHE (DHI Software,
2007a;b). Karpf and Krebs (2004) also used the same leakage approach.
The model was calibrated using a leakage factor that is a function of
groundwater infiltration rate, groundwater level, the water level in the
sewer pipe, and water level at the pipe surface to which the groundwater
is exposed. Schulz et al. (2005) used the same modeling approach to
estimate the potential benefits of sewer pipe rehabilitation with
different hypothetical infiltration rates. Qiao et al. (2007) presented
a groundwater infiltration model using a two-reservoir approach: one
reservoir for soil storage in an unsaturated zone and another for
groundwater storage in a saturated zone. The elevations of the reservoir
openings determine the trigger points that initiate infiltration into
sewer pipes.
One of the most common practices of estimating I&I contribution to
sewer flow is the RTK method that was developed by Camp Dresser and
McKee (CDM) Inc. et al. (1985). According to Lai (2008), ”the RTK method
is probably the most popular synthetic unit hydrograph (SUH) method” in
the stormwater management field. This method uses unit hydrographs to
estimate the response times associated with the effect of fast,
moderate, and slow I&I by a linear convolution. A user may calibrate
the model by comparing it to an observed I&I hydrograph. This SUH
method is the foundation of the EPA Sanitary Sewer Overflow Analysis and
Planning Toolbox, or SSOAP Toolbox (Vallabhaneni et al., 2008). EPA
SWMM5 (Rossman, 2010) also adopted the RTK method. Despite its
popularity, the model does not reflect the underlying physics of each
I&I response, and it may leave a user with a vast number of possible
solutions. Also, there is little guidance for calibrating these models
and for I&I modeling in general (Allitt, 2002).
InfoWorks CS (Innovyze, 2011) is another popular stormwater modeling
tool that has an option for I&I simulation. InfoWorks simulates I&I
using two components: rainfall-induced infiltration, and groundwater
infiltration. In the InfoWorks CS infiltration module, the percolation
flow from the surface depression storage is assigned to the soil storage
reservoir after a runoff occurs. When the soil reaches the percolation
threshold, a proportion of this percolation flow goes to the sewer
network, which represents RDII. The remainder of the percolation flow
goes down to the groundwater storage reservoir. When the groundwater
level reaches the sewer system invert level, groundwater infiltration
occurs. The method enables engineers to model groundwater infiltration
into a sewer system, but this approach lacks the representation of the
full physical process. For example, according to the model assumption,
groundwater infiltration occurs when the groundwater level is higher
than the pipe invert elevation, not the water level in the sewer pipe.
InfoWorks CS is popular because it provides an easy-to-use
representation of RDII, and it is useful for operational design.
However, the empirical approximations in this approach to model RDII and
infiltration limit the ability to use this model to provide an
understanding of the process behind I&I for a given system.
Both SWMM and InfoWorks take simple I&I estimation approaches that
represent I&I with unit hydrographs or constant rates. Simplified
modeling methods are labor- and cost-effective and easy to apply, but
such approaches do not provide an understanding of processes and need
much more calibration data for parameter estimation. Various I&I
prediction methods, including the above methods, are well documented by
Crawford et al. (1999), Wright et al. (2001), Vallabhaneni et al.
(2007), and Lai (2008).
Often, the complexity of the system and lack of data prevents
identifying the sources and origins of the RDII from happening. Though
for convenience, the I&I sources are often categorized as fast, medium,
and slow sources. The RTK method is a good example of categorizing I&I
sources into different response times, where three triangular
hydrographs represent short-term, intermediate-term, and long-term
responses (Rossman, 2010).
In the physical world, the fast I&I source indicates a direct
connection of impervious surface runoff catchments, e.g., roof
downspout, connected to a sewer pipe. The slow I&I is the infiltration
component of I&I that indicates flow through porous media. The
medium-speed I&I falls in between the fast and slow I&I in terms of
the time to peak. Walski et al. (2007) defined medium response as ”more
delayed and attenuated response to rainfall” or ”rapid infiltration.”
Hodgson and Schultz (1995) used the footer drain as an example of the
medium response. Nogaj and Hollenback (1981) pointed out that foundation
drains and storm sumps are not highly sensitive to changes in rainfall
intensity, which makes these inflow sources classified as medium-speed
I&I sources.
The fast and medium sources are examples of illegal connections to
sanitary sewer systems that lead surface water into sewer pipes. The
standard practice of treating the runoff from impervious areas is to
”drain to light” or drain to a gravity flow—a ditch, a storm sewer,
or an overland flow surface, ideally with permeable soil. In case the
storm sources are connected to sanitary sewer systems, the extra water
becomes RDII. Compared to the fast- and medium-speed I&I sources, slow
infiltration occurs when the sewer system fails to keep groundwater out
of the system.
The objective of this paper is to identify three representative RDII
sources and understand the hydrologic characteristics of the flow using
the impulse response functions (IRFs). The model is calibrated using a
genetic algorithm (GA) technique in a study area and eventually used to
verify the relative predominance of each RDII source in the test
community.