Focussed Investigation

case study: flow control structures

In the City of Toronto, the combined sewer system within the downtown core is controlled through a series of flow control structures including weirs, gates and overflow pipes. Some structures include modern, remote-controlled gates while others include turn-of-the-century brick weirs.

Andrews.engineer was retained to carry out the inspection and performance evaluation of flow control structures as part of the City of Toronto’s Don and Waterfront Trunk Sewers and CSO Control Strategy EA project. Structures varied in complexity from large, multi-level, multi-gate control facilities to small, brick sewer overflow chambers. A total of 190 structures containing some 300 control elements were inspected, catalogued and assessed over a ten-week period.

A detailed hydraulic model of the combined sewer network had been created by the consulting team to support development of the CSO control strategy. The purpose of evaluating the flow control structures was to ensure that active structures were properly identified and that hydraulic characteristics were appropriately reflected in the model. As a result the critical first step in preparing the inspection plan was to establish key performance characteristics that best reflected system operation including:

• Weir length, height and cross-section
• Date type, size and current position
• Status and condition of interconnecting pipes and outfalls
• Evidence of operation; evidence of inappropriate or on-going overflows

All findings were thoroughly documented with measurements, photographs, specific comments and elevations. A complete database was generated and exported to the hydraulic modelling team.