Planning and Building a Marine Outfall in a Sensitive Ecosystem: Lessons Learned

In 2004, our design team began a four-year project with the City of Bellingham, Washington (City) to design, permit, and construct a replacement for the City's Alternative Outfall. The Alternative Outfall serves as a second outfall for the Post Point Wastewater Treatment Plant (WWTP) when peak flows exceed the capacity of the 1520 mm (60-inch) Primary Outfall. The old 760 mm (30-inch) diameter Alternative Outfall was undersized, had numerous joint failures, and the end of the 150 meter (500 ft) long outfall had become aspirated with sediment due to extended periods of no flow. The design team developed several options to repair the pipeline, including sliplining, pipe ramming, and replacement with a larger pipeline. After careful evaluation, the City decided to replace the existing pipeline with a stronger, higher capacity 1400 mm (54-inch) diameter pipeline outfall protruding farther from the shoreline into Bellingham Bay. To minimize ecological impacts to a large and very healthy eelgrass (Zostera marina) bed, the outfall alignment was shifted to pass through a thinner section of the eelgrass, reducing overall impact. The project team also developed an extensive eelgrass mitigation plan for this alignment. The design team worked closely with the Washington Department of Fish and Wildlife and the Washington State Department of Natural Resources to implement several innovative procedures to aid in eelgrass mitigation, including an experimental clamshell dredge removal method and the first large-scale use on the western U.S. coast of the Transplanting Eelgrass Remotely with Frames (TERFs; developed by Short et al. [2002]) methodology. Divers were also deployed to remove and transplant eelgrass using more traditional methods. During construction, difficulties were encountered sinking the pipeline, sealing the 1400 mm (54-inch) rubber check valve, and stabilizing the marine backfill. This paper discusses some of the design considerations, construction challenges, the lessons learned in working in a sensitive marine environment, and the preliminary results of the experimental eelgrass transplanting methods.