Monitoring and Modeling of its Drainage System for the Town of Huntsville, Ontario

The Town of Huntsville owns more than 34 km of storm sewers spanning an area of over 361 hectares. The storm sewer network includes approximately 1,432 pipes, along with catch basins and stormwater management facilities used for stormwater collection, treatment, and flow control. Tulloch was retained by the Town to complete three major studies over several years involving inventory, condition assessment, and hydrologic/hydraulic analysis of the storm sewer system:

• Pipe CCTV Condition Survey (2017–2020)
• Storm Network Data Collection (2023–2024)
• Storm Sewer Network Modelling (2023–2024)

These investigations evaluated the condition, capacity, and functionality of the network. The work began in 2017 with CCTV inspections, which included surveying, flushing, and examining all Town-owned storm sewers. Data collected included geophysical locations of storm structures and pipe condition information based on Pipeline Assessment Certification Program standards and Ontario Provincial Standards Specifications.

The resulting condition data was compiled into a spreadsheet and GIS database, which supported the development of storm sewer infrastructure maps. These maps identified the location, size, grade, and depth of all Town-owned storm sewers. Because the Town’s storm system connects to the District of Muskoka’s storm drainage network, the modelling effort also incorporated data from the District to understand its influence on the Town’s system.

The next phase involved analyzing storm sewer capacity and identifying system deficiencies. After verifying all data, the network was modeled using PCSWMM, enabling assessment of system performance under various rainfall events. Additional inputs required for modelling included subbasin delineation, soil types, impervious areas, slopes, and curve numbers. The model assumed all pipes were in new condition without corrosion, collapse, or debris accumulation.

PCSWMM analysis evaluated the system based on the following criteria:

• Minimum cover depth of 1.5 m
• Minimum flow velocity of 0.6 m/s
• Surcharging during the 10-year storm event

Based on this assessment:

• 432 storm sewer pipes have less than 1.5 m of cover, representing 13,852 m or 51% of the Town’s network.
• During the 10-year storm event, 14 pipes have velocities of 0.6 m/s or less, increasing risk of sediment buildup and maintenance needs.
• Of these low-velocity pipes, two have slopes under 1%. Since velocity is influenced by slope, pipe size, and Manning’s n, resizing these pipes may be necessary to achieve minimum velocity.
• A total of 119 pipes surcharge during the 10-year storm, representing 3,764 m or 14% of the system.
• To eliminate surcharging, 133 pipes would need to be upsized, totaling 4,377 m or 16% of the network.

As a final exercise, the storm sewers that were modelled as surcharged were enlarged (upsized) to eliminate the surcharging within the storm sewer network. It was determined that a total of 133 storm sewers would have to be enlarged. This would a total of 4377 m of storm sewers or 16% of all the storm sewers.

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