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Creating a conceptual model to inform coastal adaptation and flood protection strategy around a marsh-based wastewater treatment plant

Arcata, California

Featured image for the project, Creating a conceptual model to inform coastal adaptation and flood protection strategy around a marsh-based wastewater treatment plant

(Figure 1) Map of Arcata Bay showing highlighted restoration areas (in yellow), protection zones (red), and proposed new salt marsh living shoreline protection (green) around Arcata Marsh & Wildlife Sanctuary Recreational Area. Map from City of Arcata, Environmental Services.


Arcata is located in northern California on the coast of rural Humboldt County along Arcata Bay estuary. Home to about 17,200 residents, including nearly 7,000 students at Humboldt State University (HSU), the coastal margins of the city were built on historically submerged area currently protected from inundation by dikes and levees. With rising sea levels, Arcata Bay’s shores are at risk to increasing impacts from flooding and higher than average tides.

The city government of Arcata recognizes the pending threats of sea level rise due to climate change. In particular, the city is considering adaptation strategies for the myriad public and private developments potentially affected. The city’s innovative marsh-based wastewater treatment plant is vulnerable due to its location (Figure 1) and is a critical priority for the city. Mitigating the impacts of increased flooding on homes and businesses in the South G Street corridor is also of great importance. Furthermore, preserving, and ultimately transitioning, ecological function and habitats in Arcata Marsh and Wildlife Sanctuary are primary adaptation objectives.

A Sea Level Rise Risk Assessment conducted in 2018 modeled sea level rise scenarios in the City’s Local Coastal Program (LCP) planning area. The modeling indicated the effects could be significant. While the South G Street area itself is an elevated structure created by historic fill, higher maximum tides will affect backwater flooding, groundwater elevation, and stormwater runoff rates. Even best-case modeled scenarios will require adaptation and accommodation.

Arcata residents have been engaged in the city’s adaptation planning efforts. Residents have participated in annual king tide monitoring and photo documentation events since 2017. The Friends of the Arcata Marsh, a local non-profit focused on education and outreach around the Arcata Marsh, provides tours, events, and education, some of which have been focused on sea level rise. The city has partnered with Humboldt State University on events. In addition, staff has held several stakeholder meetings, public forums, and study sessions with the Planning Commission and City Council on sea level rise. Arcatans are very engaged in the topic.

Living shorelines offer potential benefits for stabilizing sediment while maintaining shoreline migration capacity and preserving valuable habitat areas, and are one coastal protection strategy of high interest to the city. In the near term, building up hard structures like existing levees may provide sufficient protection around oxidation ponds. At some point, shifting toward nature-based living shorelines would provide co-benefits to preserve the salt marsh habitat and to dampen wave energy for flood protection. The City of Arcata is considering the merits of constructing living shoreline structures around the wastewater treatment plant oxidation ponds in tandem with building “eco-levees” (i.e. gradation between salt marsh/mudflats to upland), recognizing that this would be a temporary solution for shoreline erosion. Fully addressing the risks of rising ground and surface water from sea level rise will necessitate a measured retreat from former tideland areas in the long term.


The Project

Arcata is striving to develop a well-researched plan for successfully executing shoreline protection measures between now and a higher sea level future. For this reason, Arcata seeks assistance in creating a conceptual model to visualize the salt marsh habitat in transition and identify tipping points at which to change adaptation strategy. A partner scientist should be able to synthesize existing information and recommend options to increase resilience and to inform the city’s coastal protection efforts over the next 5-10 years (short term), 10-25 years (mid-term), and 25-50+ years (long term).

An expert is requested to create a conceptual model showing the cross section of the salt marsh habitat on different time scales as erosion occurs, as living shorelines are built out, and as the marsh retreats landward. This work would build on an existing body of work done related to sedimentation, flood vulnerability mapping, hydrology and sea level rise scenario modeling in Humboldt and Arcata Bay for a more complete picture of how Arcata’s shorelines are changing and where the city government should target construction of stabilizing structures first. The work completed by the partner scientist should result in an accessible, well-visualized conceptual model that can be used for community outreach and engagement with data-driven recommendations to the Arcata government project team, and a brief written report for use by the project team.

Completing this work would benefit the overall Arcata community by enabling the city government to move forward confidently in strategically protecting eroding shoreline areas for stabilization, increasing the resilience of wastewater treatment infrastructure, homes, businesses, recreational areas and coastal habitats. It would also allow the city to more easily articulate the planned coastal adaptation strategy to a public audience, to set up permitting and remove barriers to compete for future funding opportunities.



The project should start by May 2021, meeting monthly as a project team over the course of roughly 12 months. The team will work to refine the outcomes and deliverables with the partner scientist, and then hold check-ins with the partner scientist about once each month throughout the duration of the work. The City of Arcata is eager to move forward in identifying the most feasible options for shoreline protection, to support planning efforts, review of funding sources, and applications for additional grants if necessary. Arcata’s draft Sea Level Rise Policy, LCP Plan and Wastewater Treatment Plan are currently being developed, and ideally, this project work would coincide with the LCP plan’s adoption by the city in mid-2021. Recommendations resulting from this work also have potential to shape the direction of steps toward implementation. The current project work would help leverage future grant funding by laying the groundwork for appropriate placement of stabilizing structures while supporting plans to scale up a living shorelines matrix for Arcata.

About the Community

The scientific partner can expect to work in close communications with David Loya, Community Development Director with City of Arcata, in shaping the overall direction of the project. David’s role in the project is focused on policy and strategic planning. Susan Diehl McCarthy, Community Development Specialist with City of Arcata, is the secondary project lead keeping the team on task. Susan is involved with Arcata outreach and engagement efforts, including citizen science monitoring of king tides.

The team includes Emily Benvie with the City of Arcata Environmental Services Department, who is experienced with current living shorelines work, environmental permitting, and applying for grant funding to implement a living shorelines pilot. Mark Andre, former Environmental Services Director, will likely remain involved as a volunteer, as his knowledge and experience of living shorelines gained prior to his retirement in December 2020 will be invaluable. Additionally, the team includes Netra Khatri as a representative from the city’s Engineering Division.

Jeff Anderson with Northern Hydrology Associates brings technical expertise in engineering and will lend his support ad hoc. Jeff conducted sea level rise hydrodynamic modeling and inundation vulnerability mapping for the 2018 Risk Assessment work for Arcata, and continues to advise the project team as needed.

Project Team

Community Leads

David Loya, Community Development Director, City of Arcata

Susan Diehl McCarthy, Community Development Specialist, City of Arcata

Emily Benvie, Environmental Services Department, City of Arcata

Mark Andre, Former Environmental Services Director

Netra Khatri, Engineering Division, City of Arcata

Jess Clifton, Engineering Division, City of Arcata

Jeff Anderson, Northern Hydrology Associates


Community Scientists

Maddie McNerthney (Photo and bio coming soon!)

Dr. Patrick Biber (Photo and bio coming soon!)

Dr. Wei Wu (Photo and bio coming soon!)


Community Science Fellow

Morgan Corey is dedicated to the conservation of marine and coastal environments through applied science, management and policy. Currently a Fishery Management Specialist with NOAA and based in Washington, DC, Morgan has experience working toward restoration in the Chesapeake Bay watershed, researching in the Gulf of Mexico and around Lake Erie. She grew to embrace the power of community organizing for improved water infrastructure in Michigan and previously served as a Peace Corps Response volunteer in the Philippines, working with a local government to support the island’s solid waste management, habitat assessments and fish catch monitoring. She holds a M.S. in Coastal Science from the University of Southern Mississippi and a B.S. in Zoology (Ecology and Evolution) from the Ohio State University. Morgan is excited to learn with the Arcata, CA community how nature can make shorelines more resilient to sea level rise.