Jimmy Jimmy (Judd) Slough Culvert Replacement

Background
Upper Jimmy Jimmy (Judd) Slough was once a branch of the Squamish River until construction of diking in the mid 1970s largely cut off flows to the area. When the dike was first built, a small culvert was installed through the dike to maintain flows to the slough. In the early 1980s, in response to flooding, the culvert was closed off but left buried within the dike. This cut off flow to the slough which has resulted in the accumulation of sediment and organic material within the slough and degradation of fish habitat.

The abandoned culvert within the dike poses a possible flooding threat as it forms a potential seepage path through the dike. As a result, the District’s Integrated Flood Hazard Management Plan (IFHMP) recommended to remove the abandoned culvert.

Removal of the abandoned culvert requires a large excavation across the dike which creates a significant opportunity to replace the abandoned culvert with a properly designed culvert and gate system to re-water Jimmy Jimmy (Judd) Slough and improve fish habitat.

Jimmy Jimmy (Judd) Slough also provides important stormwater storage for Brackendale during large storms. Re-watering the slough, therefore, requires careful consideration to ensure the project will not increase community flood risk. The District has engaged engineering consultants to complete engineering analysis and design for this project.

Project
The District plans to remove the existing culvert within the dike and replace it with a 2400mm x 900mm box culvert. The culvert will have a large chamber within the dike equipped with two automatically operated sluice gates that will be opened and closed to allow water into the slough according to the operating strategy (described below). Two gates are provided for redundancy purposes in case of gate failure or blockage. In addition, the gate chamber access and operators are provided on the dike top in order to ensure operability and access during high water events.

In order to reduce debris build-up within the culvert that could block the gates, a bar screen will be installed on the upstream side. The culvert will be equipped with instrumentation to measure rainfall, water levels within Jimmy Jimmy (Judd) Slough and the Squamish River as well as flow through the culvert to ensure the culvert is operating as intended.

A Feasibility Strategy was prepared to support the design and ensure the proposed project would not create adverse impacts such as erosion or increased flood risk. See links below for additional details.

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Culvert Replacement Project

• Operating Strategy

  Based on discussions with the Department of Fisheries and Oceans, Squamish Nation and our consulting engineer, and technical analysis, a preliminary operating strategy has been developed. The strategy was developed with the following considerations in mind:

  • Maintaining flood protection
  • Achieving flow velocities required for scouring sediments and exposing gravels to encourage fish spawning
  • Providing flow in the slough during fish migration periods
  • Balancing capital costs with operational costs

  A summary of the operating strategy is as follows:

  • The gates will be closed for the majority of the year. Gates are recommended to only be opened in July and August. This is a time when the river is high enough to achieve scouring velocities in the slough while minimizing operations during storm events.
  • The gates will be closed during significant rain events.
  • The gates will be closed if the slough level rises above a specified elevation that will be selected to avoid flooding of private property.
  • The total flow through the culvert will be limited by throttling the gates based on river level. The flow rate will be determined to achieve scour while minimizing erosion or increased water levels in the slough.

• Schedule

  The District will begin construction in late January/early February 2022 and work is scheduled to be complete by summer 2023. This timing will fit within the fisheries window and at a time when flood risk is considered low. 

• Supporting Documents

  1. Feasibility Memorandum
  2. Operating Strategy
  3. Preliminary Design

• Feedback

  The District of Squamish is seeking feedback on this project as we move into the detailed design stage of the project. Please contact David Roulston, Manager of Municipal Infrastructure, [email protected] with any questions or feedback.