Adapting wastewater treatment plants to sea level rise: A case study of Half Moon Bay, California

Abstract

Wastewater treatment plants around the world are becoming increasingly at risk of sea level rise impacts as these facilities are typically located in low-lying areas to utilize gravity flow for influent (incoming sewage) and effluent (discharged treated sewage). As these risks become realized, treatment plant managers and decision-makers must know when and how to adapt their facilities so that this critical service goes uninterrupted. This research highlights the Sewer Authority Mid-Coastside (SAM) wastewater treatment plant in Half Moon Bay, California, as a case study of assessing vulnerability and analyzing adaptation alternatives with an approach that may be replicated for adaptation studies of other treatment plants. The vulnerability assessment is based on a worst-case greenhouse gas emissions scenario and corresponding sea level rise projection data from the California Ocean Protection Council and U.S. Geological Survey. Adaptation strategies are analyzed based on their likelihood of success considering the site-specific risks posed to the SAM plant. At only 15-20 feet elevation NAVD88 and surrounded by creeks, the SAM plant is currently at risk of groundwater inundation and creek flooding during storm events and is expected to be subject to tidal flooding with 6.6 feet of sea level rise and an annual storm event as soon as the year 2080. Nature-based solutions for sea level rise adaptation are both strongly preferred by regulatory agencies and more suitable for the SAM plant due to the surrounding sensitive habitat, including the riparian corridors, dune scrub, and coastal marsh. A phased adaptation approach will be most successful for addressing short- and long-term risks of sea level rise, with an ultimate recommendation for managed retreat of the plant to avoid inevitable flooding impacts and accommodate future sewage treatment capacity needs.