In 2020 and 2021, Northwest Hydraulic Consultants Ltd. and Thurber Engineering Ltd. (NHC and Thurber) collaborated with the Líl̓wat Nation and other members of the Pemberton Valley Emergency Management Committee (PVEMC) to develop the Phase 1 design of the upgrade to the Poleyard Flood Defence System. Needed improvements to this flood defence system were identified by NHC in a 2020 Pemberton Valley Flood Mitigation Planning report for the PVEMC, which comprises the Líl̓wat Nation, Village of Pemberton, Squamish-Lillooet Regional District and Pemberton Valley Dyking District (PVDD). This report characterized multiple risks, including inundation, scour, erosion, deposition of sediments and debris, and impacts to sensitive habitats from contaminants transported by flood water. NHC’s hydraulic modelling also showed imminent potential for the Birkenhead River to start overtopping the Poleyard Dike at the 50-year flood level, thus requiring upgrades to the Poleyard Flood Defence System.

In 2022, the team completed the design and construction of approximately 750 m of upgrades, constructing a new dike along an existing 140 m road prism and upgrading 610 m of existing dike to meet the guidelines stipulated in the Dike Design and Construction Guide for registered dikes in BC. Material for this work was primarily sourced from an annual sediment removal program on the Lillooet River, which is a separate flood mitigation measure being undertaken by the PVDD that added considerable cost savings and enabled the Líl̓wat Nation to maximize the length of dike constructed during the project’s first phase.

NHC is presently working with the Líl̓wat Nation and the PVEMC to evaluate conceptual flood mitigation options for the lower Pemberton Valley region. Upgrading and extending the Poleyard Flood Defence System is a common element of the flood mitigation options being considered.

The Poleyard Dike upgrade was constructed with a crest elevation to the 200-year design flow with climate change effects and freeboard. The engineering team used a collaborative approach with Líl̓wat Nation and the PVDD in design and construction planning of the dike upgrade. A key design request of the Líl̓wat Nation is to maintain riparian habitat for fish species in the Birkenhead River. NHC accommodated this request by proposing an offset dike alignment to protect existing riparian habitat while incorporating the pre-existing dike prism into the design. By using the alignment offset, the team reduced riprap requirements and enabled construction of a longer portion of the flood defence system than originally proposed. NHC and Thurber also worked with Líl̓wat Nation and the PVDD to more affordably source dike fill that meets geotechnical requirements, since dike fill is typically a major project cost. As part of the Pemberton Valley’s flood mitigation program, the PVDD has been removing sediment annually from the Lillooet River. NHC and Thurber collaborated with all project partners to develop a composite dike fill design that uses aggregate diverted from this sediment removal program. Future phases of the Poleyard Flood Defense System upgrade and the broader Pemberton Valley flood mitigation program are expected to continue to benefit from this collaborative approach to dike design and construction between the engineering team, Líl̓wat Nation, PVDD and other PVEMC members.

NHC successfully assessed the flood risk associated with the potential overtopping of the Poleyard Dike and collaborated with Thurber to complete the detailed engineering design for upgrades to the Poleyard Flood Defence System.