Garrison Creek Relief Sewer
Garrison Creek Sewershed
Year of Construction:
Built between 1910-1912 as one of several improvements to the Garrison sewer system that were installed as components in the city's first interceptor network. While on some plans this tunnel is depicted with a brick arch, it appears to have been built from the beginning in concrete, with only the floor laid in brick. The arch is up to 11 feet tall in the largest section of the relief.
There are great, arched, empty spaces beneath the old City of Toronto, and the Garrison Creek Relief Sewer is one of them. Completed around 1912 along with the city's first interceptor network and the new Garrison Creek Sewer to the north, it served to expand the capacity of the existing Garrison sewer system by providing a second outlet for the watershed during periods of high flow. Beginning in an overflow chamber just above Dundas Street (pic), the relief sewer carries overflows from the main sewer 3 km to a separate outlet at the foot of Strachan Avenue. This outlet is now diverted into the final tank of the Western Beaches Storage Tunnel (WBST), but for ninety years overflows at Dundas Street went more or less unchecked (save possibly for a low capacity connection to the Exhibition Interceptor/Strachan Avenue Pumping Station) into the lake east of the Exhibition. With the installation and apparent operation of the WBST, the Garrison relief sewer can now be considered in a revisionist way to be a pseudo-extension of the storage tunnel.
Built right at the moment when underground engineering projects in Canada were moving from brick to concrete, the Relief Sewer is built of both materials: the enormous concrete arch shows off the structural stability, durability and lower cost of the new aggregate material, while the floor is still laid with paving bricks to resist erosion. The dimensions of the tunnel vary considerably along its length, the product one surmises of having been installed along a route that had largely been developed in the twenty-five years since the original Garrison sewer had drained a less constrained terrain. The Relief's builders would have had to deal not only with new property lines and the need to limit disruptions to above-ground neighbourhoods and street traffic, but with the existing combined sewer system. And indeed, as it runs along the western edge of Trinity-Bellwoods down towards Queen Street, the Relief conduit takes on a decidedly oblong form (pic) to accomodate a Garrison tributary sewer that parallels the Relief for a few hundred meters on its way to a junction with the old Garrison sewer somewhere beneath the park.
In addition to providing overflow capacity for the main Garrison sewer, more recently the Garrison relief sewer has also received connections to the modern storm sewer pipes that have allowed for partial sewer separation in some west end neighbourhoods. One of these smaller storm sewers carries rainwater into the relief sewer from Beaconsfield, while another comes down from College Street where it appears to collect precipitation from a reasonably large swath of the Dufferin Grove and Bickford neighbourhoods.
Exploring the College Street Extension
That latter connection proves incredibly interesting. From a junction just downstream of the overflow chamber that begins the Relief, we walk up a 2m diameter concrete pipe that runs beneath Ossington Avenue. When the Dundas or College streetcars are on a diversion, the run along this stretch of Ossington and can be heard in the tunnel. At College Street, the storm sewer splits in two at a T-shaped junction where the wood forms that were used to shape the curve of each pipe left perfect impressions that are still visible today (pic).
Go west from the junction, and this level of the storm sewer terminates in a large chamber where runoff plunges fifteen feet from a shallower sewer that is unreachable from within the chamber. That shallower sewer runs from the east, and probably accepts flow from a number of small streetside pipes that drain the catchbasins along north-south streets like Concord, Roxton and Shaw.
On the other hand, if you go east from the T-junction, you reach a fascinating chamber (pic) that actually surrounds the structure of the new Garrison Creek Sewer where it crosses College Street. A vaguely rounded, concrete berm roughly fifteen feet wide crosses the chamber perpendicular to the pipe from the T-junction. Ladders and a metal catwalk (pic) have been provided to allow access to both sides of the chamber, while small siphons have been installed to carry beneath the Garrison sewer what water and sewage occasionally enters the chamber from the east. For reasons that aren't completely clear, one of these siphons is surrounded on both sides by a low wall, while the other is wholely exposed (and shouldn't be stepped into). On the east side of the room, there is an access shaft that seems to lead to a manhole in the vicinity of Beatrice Avenue, and a connection to a small, brick pipe that is mostly dry and may be a remnant portion of the old Garrison Sewer or alternately an overflow connection from a combined sewer that runs beneath Bathurst Street.
Downstream in the Relief
Downstream in the Garrison relief sewer's main, arched conduit, several smaller pipes (including the afforementioned Brockton-Beaconsfield tributary combined sewer) cross the Relief in pipes suspended at head-level (pic). Further on, what appears to be a force main has been installed on one side of the tunnel (pic) — it may carry the solids pumped from the bottom of the Strachan Avenue tank of the Western Beaches Storage Tunnel up to the High-Level Interceptor where they can be carried east to the Ashbridges Treatment Plant.
Near the Princes' Gates, the force main crosses the tunnel, as does a concrete weir about twenty feet downstream of it (pic). There is a small diversion that leads to the Strachan pumping station, and this weir may also have served to prevent lake water (and now backlogged overflows at the WBST) from backwashing up the tunnel. Beyond this weir, a slightly newer and very filthy tunnel leads to the WBST's Strachan tank. We've seen this tunnel both flooded and empty, and as it has sat submerged for extended periods in the past (and may not have been built at a terribly useful grade to begin with), a significant amount of solids have settled to coat the floor and the side-mounted forcemain.
Pushing further, we eventually wade into a terrible blackwater beneath Remembrance Drive, just in advance of the tank. A metal gate (pic) blocks the tunnel and ensures that we don't do anything stupid like wade into the tank itself, and based on what we can see beyond this fence we accept its limitation gladly. Beyond the bars, a gyre turns slowly in the darkness, its currents driven by what must be a fairly small opening into the Strachan tank. Floating litter and various unmentionables allow us to see the slow dance of the greywater as it waits to fall through the control gate (just out of sight around the corner to the west) into the storage tank.
Michael Cook is available to speak to your organization about infrastructure history, lost creeks, current conditions, and opportunities for change in our management of and communication about urban watersheds, and to work with teams proposing or implementing such change. Get in touch.