The City of Chilliwack’s WWTP Engineering Services needed to rehabilitate the WWTP’s effluent outfall pipe and diffuser. In February 2021, a survey revealed 170 meters of pipe had been undermined by scour associated with Fraser River flows. In June 2021, Stantec was engaged for preliminary engineering and detailed design/construction services. By September 2021, Stantec completed geomorphic and constructability review and rehabilitation design. Stantec’s assessment indicated a side channel adjacent to the outfall could widen and deepen, conveying a larger portion of flow and increasing scour. The rehabilitation design was based on these findings, considering when and how to reach the site based on current capital and regulatory requirements.

In November 2023, a bathymetric survey revealed the diffuser portion of the pipe within the main channel of the Fraser River had severed at a coupling, posing a navigational hazard and requiring an immediate repair. This necessitated an amendment to the original design scope to include pipe replacement and armouring. Stantec quickly expanded the design and expedited permitting revisions to secure emergency authorization. Revised designs were issued for tender in December 2023. An emergency design-build contract for construction services was established, and construction began in January 2024. Construction involved close collaboration between the contractor, client, and Stantec to design an approach to retrieve the diffuser laying on the river floor, 170m from the closest land feature—a gravel bar. The project consisted of mid-construction design of a sanitary bypass, HDPE pipe ballasting, a riprap scour apron, and habitat enhancements and restoration, all while removing a navigational hazard and demobilizing from the project site prior to spring freshet. The new outfall was successfully installed and armoured by May 6, 2024, with demobilization completed before freshet on May 13, 2024.

The 1220mm outfall was originally installed in 1995 and comprised of carbon steel, with coal tar enameled coating, and wood lagging. Discharge from the WWTP is governed by the Municipal Wastewater Regulation, and the diffuser was made to meet requirements. Fabricating a new diffuser and completing the project before freshet was not feasible. The team resolved to find the diffuser, assess its condition, and re-connect to the new outfall pipe. HDPE was chosen as the replacement pipe material. Though lightweight and durable, the HDPE outfall’s connection to the existing diffuser posed challenges, including the design of pipe fittings and ballasting to counteract buoyancy, to be resolved during construction. Due to shallow depths at the tie-in, there was insufficient draft for a barge to operate; therefore, a temporary road made of riprap, road base, and alluvial soil was required to access the work area and shelter it from extreme river hydraulics. The diffuser was then staged to connect to the already floating HDPE outfall and filled with water to gently sink to bedding. A stage-storage relationship adapted from the CLEVER model was used to predict site specific water levels. Environmental offsetting, fish habitat enhancements, and restoration of temporary work areas was completed prior to freshet, avoiding the need to re-establish work area access to complete the permitting conditions.

The new outfall is SDR21 HDPE with concrete ballast set 5.5m on center. The outfall rests on imported bedding extending to the river floor. The pipe is armoured with two-1.2m lifts of class 500 riprap over top 100mm minus pipe bedding and ballasting based on the design scour elevation. The armoured section is 190m long, terminating at the diffuser which extends another 41m. Treated effluent discharges through 56 diffusers. Armouring extends 5m into the bar, keyed in to withstand freshet.