Designing a Pump Station Replacement within Tsunami and Sea Level Influenced Zone

Purpose: The purpose of this presentation is to provide the attendees with planning and design criteria and solutions when dealing with the design of a sewer lift station within a tsunami and sea level rise impact zone. Background: The City of Bellingham is located approximately 100 miles north of the City of Seattle in Washington State, 20 miles from the Canadian border. The city's Roeder Lift Station is located near the industrial area along Bellingham Bay and serves much of the northern part of the city. The existing lift station was suspected to be operating under reduced reliability conditions during peak storm events. The city contracted with Carollo Engineers to complete an alternatives analysis and design to increase the existing lift station's pumping capacity from approximately 8 million gallons per day (mgd) to 18 mgd. The existing dry-pit/wet-pit station is located on a small city-owned parcel of land surrounded by Port of Bellingham (Port) and Burlington Northern Santa Fe (BNSF) railroad properties. The station's existing 18-inch diameter force main is routed through a city utility easement on Port and BNSF properties including crossing under a BNSF spur track to reach the discharge manhole location. Two lift station retrofit alternatives and one new submersible lift station alternative were evaluated, including installation of a parallel force main alternative. The selected alternative included a new submersible lift station 1,100 feet north of its current location. The lift station design included a combination of variable speed driven non-clog submersible pumps and screw centrifugal pumps in self-cleaning pre-rotation basins with a pumping range of 1 to 18 mgd. Other improvements included 2,500 lineal feet of parallel 14-inch and 28-inch diameter HDPE force mains with trenchless installation beneath the railroad, installation of a 36-inch diameter PVC gravity sewer main and manholes, and miscellaneous 24-inch diameter HDPE water main replacement. Design Challenges: During the planning and design of the sewer lift station replacement, several design challenges came to light. These included the limited property owned by the city and the lack of a viable location near the existing station, the presence of a creek just west of the proposed station location, and issues related to the stations location near Bellingham Bay including tsunami and seal level rise impacts. The first challenge encountered by the design team was the lack of city-owned property near the existing station. Two of the replacement alternatives included retrofitting the existing station infrastructure while a third alternative included constructing a new adjacent station. The lack of property available to the city required relocating the proposed lift station. Further validating the need to relocate the station was the presence of Squalicum Creek approximately 2,000 feet upstream of the proposed station location. The existing gravity sewer crosses the creek before entering the station and the top of the pipe is exposed in the creek bed. The city has future plans to improve the creek which would require lowering the sewer line. An analysis was completed to look at installation of a deeper gravity sewer or installation of an inverted siphon. The two challenges having the greatest impact on the design of the new sewer lift station were sea level rise and tsunami impacts. Based on FEMA mapping, the location of the new lift station is within the 100-year flood plain. The potential impact of flooding became painfully apparent during November 2021 when a major rain event caused significant flooding at the proposed lift station site. Adding to the challenge, the FEMA flood maps do not take into consideration future long-term change due to climate change and the rising sea level. Sea level rise predictions for our project area ranged from 1.1 to 11.7 feet above the base flood elevation. Through multiple modeling efforts the design team settled on a sea level rise impact of 1.5 meters and a 100-year storm event to set the initial water surface elevation used for design. The final challenge impacting the design of the replacement lift station was the fact that the new site was located within the Tsunami Design Zone. With the State's adoption of the International Building Code, we were required to use the Washington Tsunami Design Zone Map to determine whether Risk Category III or IV structures needed to be designed for tsunamis. The tsunami inundation elevation at the project site was identified as 21 feet based on mapping. Ultimately, the impacts related to tsunami inundation controlled the site design and the new station was elevated 9 feet above the existing grade. Status and Conclusions The project was bid for construction in April of 2023 with an engineer's estimate of $18 million including tax. The city received four bids ranging from a low bid of $17.5 million to $21 million. The project is currently under construction with an estimated completion date of Spring 2026. The design of the replacement sewer lift station was impacted by multiple surrounding water bodies which lead to many lessons learned throughout the design. Scour within Squalicum Creek caused exposure of the existing gravity sewer line. Future improvements to the creek impacted the depth of the future sewer upgrade requiring a deeper lift station wet well. This factor required construction of a new lift station and would not allow retrofit of the existing station. The location of the new lift station site and its proximity to Bellingham Bay caused the need to design for sea level and tsunami impact zones. Lessons that were learned during the design included understanding that FEMA flood plain mapping does not take sea level rise into account and that tsunami inundation can have a more significant impact on the final grade of the proposed site. Additionally, the design of the on-site structures themselves relative to the thickness and strength of the structure walls can be affected by the potential for tsunami impact loads. Finally, construction in a historically intertidal zone overlaid by fill required ground improvement design to stabilize the soils before construction could begin.