Aging Stormwater & Sewer Infrastructure
The sewer system infrastructure, primarily the Highland Creek Sanitary Trunk Sewer at Lawrence & McCowan was largely constructed by the former, Municipality of Metropolitan Toronto (pre-1998) and Scarborough Public Works between 1954-1960. The infrastructure is aging and largely original despite having received critical emergency patch repairs and liners preventing imminent structural collapse in 2009, 2011 and 2021.[1]
The City of Toronto’s - 2023 Scarborough Centre (OurSC) Study – Phase 4 Final Report (relating to Scarborough Town Centre) identified significant storm and sewer challenges in the resident subdivision’s areas directly south of the study area (eg. Lawrence and McCowan).
o The main objectives of the future wastewater servicing strategy were to minimizing the flow through the residential neighborhood to the south.
o The modelling results indicate that the residential neighborhood to the south already experiences surcharging under existing conditions.
o The city minimizing future flows through it by redirecting the sewers to ensure that surge conditions did not worsen.[2]
According to data obtained from Toronto Water for Lawrence & McCowan:
o Over 69% of the storm sewer system was built in the 1950-1960s.[3]
o Just under 31% was constructed in and/or after the 1970s.[4]
o The storm/sanitary sewers on Lawrence, west of McCowan have not been upgraded since 1978 (+46 years) and have received no major updates.[5]
o A minor sewer rehabilitation program on the original infrastructure was launched between 2017-2023 which included cleaning the interior of the largely original sewers, liners to seal cracks and/or joints and repair catch basins or maintenance holes, etc.[6]
Stress cracking of the 900mm-1050mm Highland Creek Sanitary Trunk Sewer System serving Lawrence & McCowan was first discovered in Bendale Park along the walking/bicycle path directly over the 900mm sanitary trunk sewer in 2009.[7]
Stress cracking of the 900mm-1050mm Highland Creek Sanitary Trunk Sewer System serving Lawrence & McCowan was again discovered in 2011.[8]
In the 2011 report, two consulting engineering firms reviewed the inspection data and concluded that the trunk sewer under Lawrence Avenue (near McCowan) was structurally unsound and a number of sections of the 1050mm sewer had pieces of concrete missing along with substantial fracturing causing significant pipe deformation.[9]
At the time, inspection crews reported that the system had been compromised for decades and was at immediate risk of a complete failure.[10]
As part of the 2011 report, crews found the 900mm and 1050mm sewer segments… “were severely damaged and structurally deformed. In addition, the brick maintenance hole where the sewer transitions from 900mm to 1050mm has fractured and settled. There is an immediate risk of a complete failure of this infrastructure.” [11]
City of Toronto Water Staff also reported that a failure of the trunk sewer could result in “basement flooding, sewage spill into Highland Creek, legislative non-compliance, impact to public health and safety, and the collapse of a portion of Lawrence Avenue denying emergency services and the public access to a major east-west transportation corridor...”[12] [13]
In 2021, new road depressions were observed above the Highland Creek Sanitary Trunk Sewer (STS) in the southbound lanes of McCowan Road north of the Scarborough Health Network - General Hospital entrance (3050 Lawrence Avenue East).[14]
The report outlined that damage to the trunk sewers indicated “…a high probability of complete failure potentially resulting in sewage spilling into Highland Creek (at 43 million liters per day).”[15]
Based on the initial Infrastructure Ontario TOC study and reports, the current system at Lawrence & McCowan would not be able to handle a storm surge event. The report outlined the proposed sanitary servicing results are as follows:
o Under existing “Dry Weather Flow”, the sewer free-flow conditions were determined to be 72% of existing pipe capacity. However, under “Wet Weather Flows” (sewage flow + water runoff into the sanitary sewer systems during a storm event), the report outlined that surcharge condition (where the sewer entrance/exit are submerged and the pipe is flowing full and under pressure) would overwhelm the system at 235% of existing pipe capacity.[16]
o Proposed increases from the Lawrence East TOC under “Dry Weather Flow”, sewer free-flow conditions were determined to be 81% of existing pipe capacity. However, under “Wet Weather Flows” (sewage flows + water runoff into the sanitary sewer systems during a storm event), the report outlined that surcharge condition (where the sewer entrance/exit are submerged and the pipe is flowing full and under pressure) would see 244% of pipe capacity with the Lawrence East TOC development.[17]
Based on these results, further review is required during subsequent design submissions to evaluate the surcharge condition within the downstream pipe. [18]
The TOC admits that further study is required with regards to hydrants and other infrastructure …. “The fire flow requirement for the site has been determined to require 283 litres/s, which will necessitate additional fire hydrant coverage and verification of pressure within the system.”[19]
As of March 2024, as the study for Lawrence & McCowan (Basement Flooding Study Area 59) is still ongoing, a replacement or upgrade of the Highland Creek Sanitary Sewer System is still not identified in the 10-year Capital Operating Budget (2024-2032) for Toronto Water.[20]
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Sources:
[1] 1960 – Municipality of Metropolitan Toronto – Annual Report, Sewage https://maps.library.utoronto.ca/datapub/digital/metro_1960/metro_1960.pdf
[2] City of Toronto’s - 2023 Scarborough Centre (OurSC) Study – Transportation, Servicing, Energy And Heritage Assessment, p. 28
https://www.toronto.ca/wp-content/uploads/2022/03/9002-CityPlanning-OurSC-Phase-3-Report.pdf
[3] City of Toronto Public Consultation Unit. Study Area 59 - Toronto Water - Basement Flooding Protection Program, (Personal Communication, 25 Mar. 2024.)
[4] Ibid.
[5] Ibid.
[6] Ibid.
[7] 2009 - City of Toronto – Toronto Water, Procurement of Required Services for the Emergency Repair of Highland Creek 900mm Sanitary Trunk Sewer Located in Bendale Park https://www.toronto.ca/legdocs/mmis/2009/gm/bgrd/backgroundfile-24090.pdf
[8] 2011- City of Toronto – Toronto Water, Emergency Repair of Highland Creek 900mm -1050mm Sanitary Trunk Sewer https://www.toronto.ca/legdocs/mmis/2011/pw/bgrd/backgroundfile-40369.pdf
[9] Ibid., p.3
[10] Ibid., p.2
[11] Ibid., p.1
[12] Ibid.
[13] Ibid., p.2
[14] 2022 - City of Toronto Report for Action - Emergency Non-competitive Contract with AECOM
Canada Ltd., for Repairs to the East Don Sanitary Trunk and Highland Creek Sanitary Trunk Sewers https://www.toronto.ca/legdocs/mmis/2022/gl/bgrd/backgroundfile-222528.pdf
[15] Ibid., p.1
[16] Infrastructure Ontario - Functional Servicing Report - Lawrence East Station Transit Oriented Community (TOC) https://engageio.ca/sites/default/files/2024-02/functional_servicing_report.pdf
[17] Ibid.
[18] Ibid.
[19] Ibid.
[20] 2024 - Toronto Water Capital and Operating Budget Notes., p. 25 https://www.toronto.ca/legdocs/mmis/2024/bu/bgrd/backgroundfile-242420.pdf