An Inflow and Infiltration Reduction Study was completed in Basement Flooding Study Area
9 in 2015/2016 by the City of Toronto and Civica Infrastructure.
The pilot project area is located in the Etobicoke York District of the City of Toronto and consists
of 557 properties, the majority of which are residential properties constructed in the
mid-to-late 1950s.
The objective of the project was to identify how extraneous rainwater and groundwater enters
the sanitary sewers, to quantify the problem, and, to then evaluate possible solutions that
could reduce basement flooding.
This extraneous flow is defined as "Inflow and Infiltration", or I&I, in municipal engineering.
One of the significant findings of this study was the impact of residential foundation drain
connections on the generation of I&I in a given catchment.
A desktop review of all available foundation drain records for Area 9 indicates that the
majority of residential foundation drains, also referred to as weeping tiles, are connected
to the sanitary sewer system.
This was standard practice in Etobicoke for homes constructed prior to 1962.
Of the total 557 properties in the study area, 451 residential foundation drains were connected
to the sanitary system.
One sub-area in the north-west section of the study area, defined as 9-San-1, has a
cluster of homes that does not match the same foundation connection practice of the rest
of the study area.
In sub-area 9-San-1, each home has a separate service lateral that directs water from foundation
drains to the municipal storm sewer.
An extensive field investigation, including monitoring of rain and sewer flow levels,
groundwater levels, smoke and dye testing, and visual sewer inspections during rainfall,
was conducted in order to validate the desktop results and to rule out other sources of I&I.
The flow monitoring program, which had eight (8) flow monitors distributed over the 45
hectare area, measured chronic levels of I&I in all but one (1) sub-area.
As expected, sub-area 9-San-1, where homes have their foundation drains connected to
the storm sewer, showed significantly less I&I in all rainfall events.
Smoke and dye testing of the entire study area, which included a detailed inventory
of each drainage feature, identified other, minor defects that could only account for
a fraction of the measured I/I.
These defects included residential downspouts, cleanout caps, basement walkout drains, and
window wells.
All traditional field inspection and record data indicated that the residential foundation
drains were the major factor in the chronic historical flooding experienced in this area
due to the amount of water they contribute to the sanitary sewer during wet weather.
To further validate these results, additional CCTV pipe inspections were commissioned to
visually capture the foundation drains in action during wet weather.
Inspections were completed on two (2) dates: October 28, 2015 and August 16, 2016.The October
28 event had 53.1 mm of rain over 17 hours while the August 16 event had 15.5 mm of rain
over 12 hours.
The Oct 28, 2015 CCTV pipe inspection completed in the sanitary sewer indicated that the sewer
pipes were in good condition under wet weather conditions, with no significant contributions
noted from pipe defects.
However, significant amounts of clear water were observed coming in from the sanitary
service laterals.
These service laterals corresponded to homes where records show foundation drains were
connected to the sanitary sewer system.
Using the results from the October 28 investigation to pinpoint problem areas, a second CCTV pipe
inspection was completed on August 16, 2016, this time with a focus on the sanitary service
laterals from the houses to the street.
The intent of these inspections was to distinguish between I&I contributions from possible defects
in the service lateral pipes and water coming from the foundation drains.
This was completed through the use of a CCTV camera with a lateral launch camera: a robotic
device that is able to launch a secondary camera from the main sanitary sewer into the
home's sewer lateral.
Assuming that the lateral is clear, this allows the camera to inspect up to the house's foundation,
and verify the source of water.
At a property in Area 9, where the lot surface drainage pattern is typical of most residential
settings in Toronto, dyed water was poured on the ground surface to replicate water flowing
from the neighbour's downspout.
The water was poured at a rate less than the approximate 3 L/s expected during a significant
rainfall event.
It is important to note that dye testing was completed during a sizeable rainfall event
to reflect the saturated soil conditions that would exist during a storm causing basement
flooding.
The time it took for the dyed water to percolate through the backfill material adjacent to
the foundation wall was the most startling finding.
It only took (10) minutes for the water to get from the ground surface, into the foundation
drain, through the sanitary lateral, and into the sanitary sewer.
The observed flow rate during this test is also similar to what was observed during the
October 28, 2015 event.
In total, four (4) homes were tested in this way, and they all responded within ten (10)
minutes.
Given the density of foundation drains connected in the study area, the fast response of these
foundation drains is considered the significant contributor to the basement flooding problems
in this area.
Field inspections identified poor lot grading at a number of properties in Area 9, which
can further amplify this issue.
Using the findings of the field inspection program, a sophisticated numerical model was
defined using the Infoworks ICM software to determine the possible benefits of disconnecting
the foundation drains.
In Area 9, it was determined that the reduction of I&I impacts resulting from the remediation
of typical engineered I&I defects – such as downspouts and manhole pickholes – will
only reduce peak flows by 5%.
This limited reduction will not enable the City to achieve the targeted level of protection
against sanitary sewer backup in the study area.
In addition to removing all I&I contributions from downspouts and manhole pickholes, an
80% reduction of I&I flow generated by foundation drains in the study area will also be necessary.
To achieve this requires disconnecting the foundation drains of YYY homes from the sanitary
sewer system – this constitutes 65% of the homes in Area 9.
It has been demonstrated in Area 9 that foundation drains respond rapidly to rainfall and are
the primary source of I&I in the study area’s sanitary sewer system.
This quick response contributes to high flows in the sanitary sewer, which leads to basement
flooding.
Reducing the I&I contribution from foundation drains will significantly reduce the risk
of basement flooding from sanitary sewer backups.
Furthermore, reducing the amount of rainwater currently draining to the sanitary sewer through
foundation drains will reduce environmental spills as well as operation and maintenance
costs at pumping stations and treatment plants.