Humboldt Bay Region at Risk from Sea Level Rise

Red areas in graphic above show regions of diked shoreline that could be overtopped by two feet of sea level rise on left (11 miles total) and by three feet on the right (23 miles total). Graphic: Aldaron Laird.
Red areas in graphic above show regions of diked shoreline that could be overtopped by two feet of sea level rise on left (11 miles total) and by three feet on the right (23 miles total). Graphic: Aldaron Laird.

The local chapter of the Sierra Club hosted a well-attended panel discussion on Sea Level Rise on Humboldt Bay in early November. The panel consisted of Michael Richardson, Senior Planner for Humboldt County; Larry Oetker, Executive Director of Humboldt Bay Harbor, Recreation and Conservation District; Jennifer Kalt, Director of Humboldt Baykeeper; and Aldaron Laird, Co-Chair of the HSU Sea Level Rise Initiative.

Aldaron provided an overview presentation conveying that Humboldt Bay is on the threshold of change from existing vulnerabilities and future risks. Seventy-five percent of Humboldt Bay’s shoreline is an artificial structure. Humboldt Bay has lost approximately 9,000 to 10,000 acres of salt marsh—a consequence of the historic changes to the shoreline from building 41 miles of earthen dikes and 11 miles of the North Pacific Railroad. These dikes and railroad grade need to be maintained in perpetuity, or thousands of acres of former salt marsh would be inundated today.

Because much of the critical utility and transportation infrastructure (municipal water, sewer, natural gas, optical fiber, electrical  transmission towers and distribution poles, and Highway 101 and 255/Samoa Blvd.) that supports the Humboldt Bay region was built on these diked former salt marshes, they too are  vulnerable to being tidally inundated if the artificial shoreline is breached.

Graphic illustrates the area of Humboldt Bay that is vulnerable if shoreline barrier structures such as dikes are compromised by king tides (blue regions) versus potential tidal inundation area with two meters of sea level rise (red). Graphic: Aldaron Laird.
Graphic illustrates the area of Humboldt Bay that is vulnerable if shoreline barrier structures such as dikes are compromised by king tides (blue regions) versus potential tidal inundation area with two meters of sea level rise (red).
Graphic: Aldaron Laird.

Aldaron revealed that two or three feet of sea level rise will likely breach 23 diked hydrologic units on Humboldt Bay. King tides could cross that threshold before 2050 putting nearly 56 percent of the diked shoreline at risk of breaching.

Research by Cascadia Geosciences and Northern Hydrology Engineers found that Humboldt Bay has the highest rate of sea level rise (18” over 100 years) on the west coast of the U.S. due to tectonic subsidence.

We now have some of the best tools available to assess our vulnerability and risk to sea level rise, thanks to a hydrodynamic model and tidal inundation maps of Humboldt Bay and local sea level rise projections, created by Northern Hydrology Engineers, funded by the Coastal Conservancy. Vulnerability assessments have been completed for all of the Local Coastal Programs (LCP) on Humboldt Bay (Humboldt County, City of Eureka and City of Arcata). Adaptation planning is underway and focused assessments on the diked shoreline and Communities at Risk have been completed (reports are available at the LCP web sites for each of these agencies). Lastly, the need for collaboration between sea level rise planning agencies, utility and transportation asset owners, and shoreline property owners was identified, as we are all in this together and rising water knows no boundaries.

In the panel discussion, Larry Oetker shared the many planning efforts that the district is presently engaged in—its dredged sediment management plan in particular, which is looking into how dredged sediment can be utilized to build resiliency to sea level rise on Humboldt Bay.

Jennifer Kalt highlighted the need to address the numerous hazardous waste sites that are still untreated, which are at risk of being tidally inundated by sea level rise or breaching shorelines.

Michael Richardson informed us all that the County’s Public Works Department has secured a major grant from Caltrans to initiate Phase I of a Sea Level Rise Adaptation Plan for Humboldt Bay Transportation Infrastructure, which will also address how to integrate stakeholders in the planning process.

The County Planning Department has just completed a Sea Level Rise Policy Background Study for the Humboldt Bay Area Plan. The County is exploring many options to address the risks posed by sea level rise, such as creating a Sea Level Rise Zone for areas that are vulnerable to tidal inundation by three feet of sea level rise, real estate disclosures, and siting and development standards.

Humboldt State University’s Marine and Coastal Sciences Institute recently created a Sea Level Rise Initiative that Aldaron Co-Chairs with Professor Laurie Richmond. The Initiative is creating a sea level rise web site that will be online before the end of the year, which will serve as a one-stop depository for sea level rise documents and databases with an emphasis on Humboldt Bay.

The Humboldt Bay Area Plan Sea Level Rise Vulnerability Assessment, produced by Aldaron Laird Trinity Associates in January 2018, is available at http://bit.ly/2rLrAbz (pdf file will automatically download).