Metro Vancouver’s South Surrey Interceptor (SSI) sewer line crosses the navigation channel of the Fraser River immediately downstream of Alex Fraser Bridge. Over the past 35 years, a scour hole downstream of the SSI deepened and progressively migrated upstream toward the crossing. The scour threatened to undermine the sewer line in a short time frame, presenting a substantial environmental, cultural, social and economic risk.

The scour hole is driven by turbulence generated by flow contraction at the Alex Fraser Bridge; as such, conventional methods to design scour protection were inappropriate for the site. NHC developed a Computational Fluid Dynamics (CFD) numerical model of the river reach and employed a unique approach based on turbulent energy to design a riprap apron for the upstream face of the scour hole to protect the crossing.

Construction was complicated by the need to work in the deep-sea navigation channel. To ensure clear communication and reduce the risk of a collision, TyPlan Ltd. developed a Marine Construction Staging Plan (MCSP). The MCSP developed and implemented for the SSI project represented an unprecedented approach to managing communication during construction projects within the navigation channel on the lower Fraser River. NHC was responsible for project management and construction field review, ensuring liaison between TyPlan, Vancouver Pile Driving and Metro Vancouver.

Construction of the scour protection apron occurred without any incidents, with local marine users considering the project to be highly successful. In line with Metro Vancouver’s objective, the project was delivered prior to spring freshet 2019.