Arcata WWTF Phase 1 Project Sea Level Rise Planning

The City of Arcata is undertaking the largest capital improvement project the community has ever experienced. The existing land-based pond and wetland system has met the community’s economic needs for more than 30 years. Now the City needs to address new nutrient limits, deferred maintenance, flooding, and future sea level rise (SLR), all while maintaining the existing natural treatment system benefits. Benefits include sustainable wastewater treatment, marsh and wildlife sanctuary preservation, and community recreation amenities. Currently the best available science on SLR projections in California is provided in the SLR Policy Guidance (California Coastal Commission (CCC) 2018). This document present SLR projections for use by coastal communities, which were used during the initial stages of project planning and preliminary design. The 2018 guidance projected 2.7 to 3.7 feet of sea level rise for the Arcata area through the 30-year life of this project, and rising seas of up to 7.6 to 10.9 feet by the end of the century. To address anticipated impacts, the City has been actively engaged in SLR adaptation planning for both city-wide and for this facility. This presentation is a case study that outlines how the City’s team developed a plan to mitigate flooding impacts over the 30-year remaining life of the facility. Planning and design included identification of existing levee low points, that could be over topped in wind driven storm surge in the next 10 to 20 years. A hazard analysis was completed that identified needed improvements to the existing stormwater handling system to address potential flooding of the central portion of the plant. Other improvements included raising all new electrical and control facilities, including the new electrical service and distribution facility above anticipated flooding elevations.