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 used
when I&I origin is surface runoff which is caused by rainfall.
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, water level in sewer
pipe, and the pipe surface to which the groundwater is exposed. Schulz
et al. (2005) used the same modeling approach to estimate 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 to an observed I&I hydrograph. This method is
embedded in EPA SWMM5 (Rossman, 2010) and EPA SSOAP toolbox
(Vallabhaneni et al., 2008). 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 a 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. This 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 the full physical process is not taken into
account. 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
widely used because it provides 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 understanding of 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 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).
In terms of modeling, often the sources and origins of the RDII are not
identified due to the complexity of the system or lack of data. Though
for a convenience the I&I sources are often categorized as fast,
medium, and slow sources. The RTK method is a good example of this
practice where three triangular hydrographs are used to represent
short-term, intermediate-term, and long-term responses, respectively
(Rossman, 2010).
In 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 response time. Walski et
al. (2007) defined medium response as “more delayed and attenuated
response to rainfall” and this is also referred to as “rapid
infiltration.” Hodgson and Schultz (1995) used footer drain as an
example of the medium response. Nogaj and Hollenback (1981) pointed out
that foundation drains and storm sumps are not expected to be highly
sensitive to changes in rainfall intensity, which makes these inflow
sources classified as medium 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 a permeable soil. In case the
storm sources are connected to sanitary sewer systems the extra water
becomes RDII. Contrast to the fast and medium sources, slow infiltration
occurs when the sewer system fails to keep groundwater out of the
system. Here, the term groundwater was used loosely to indicate water
exists in porous soil medium.
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
sewershed.