Relative sea level rise on Humboldt Bay highest in state


Climate change has been called a “long emergency,” with impacts ranging from the current extreme drought in California to globe-spanning disruptions of weather patterns and ecosystems predicted for the coming decades. Here in Humboldt County, one of the many predicted impacts is sea level rise, which experts say could threaten underground utilities and U.S. Highway 101. Here’s a look at what the county is doing to adapt.

Rising seas

As water warms, it expands. Combined with melting ice caps and glaciers, which sequester about 69 percent of the Earth’s fresh water, this phenomenon is expected to lead to rising sea levels. Estimates vary as to how much the oceans are could rise, but even a “modest” increase of a few feet could cause huge problems for coastal cities. 

In 2012, the National Research Council — the “operating arm” of the National Academy of Sciences — published findings that suggest California, Oregon and Washington should prepare for up to a foot and a half of sea level rise by 2050, and up to 4-and-a-half feet by 2100. 

While those are the “high-end” predictions, Arcata- based environmental planner Aldaron Laird of Trinity Associates said that “all of the observations of actual tide changes and sea level changes have been higher and greater than what the models have shown we should expect.”

Laird recently completed a “walkabout” inventory and mapping project of the entire 102-mile shoreline of Humboldt Bay as part of a project funded by the Coastal Conservancy. 

“The main thing we learned from going out and surveying and mapping the shoreline, which had never been done before, was that we really had no idea that we’ve been living on borrowed time this past 100 years,” Laird said. “People in the 1890s through 1910 diked off essentially 30 percent of the bay and ‘reclaimed’ it for agriculture, but the time has come due and the bay is ready to take all that land back.”

Laird said that even without sea level rise, “You can’t hold back the sea forever, and when you build 41 miles of dikes and you don’t maintain them and keep them in good working order, they fall apart.

“It’s like what we saw a couple of weeks ago at the wildlife refuge when a dike breached: all of a sudden 50 acres were flooded overnight,” he continued. “If the dikes on Humboldt Bay fail, the bay is going to expand by 50 percent. So we’re really faced with this historical legacy of failing to maintain this artificial shoreline that was built over a century ago. Then we went and built all of our critical utilities on those lands, and our critical highway structures, so we have to live with that.” 

Estimates based on the mapping project, published on the Humboldt Bay Harbor, Recreation and Conservation District’s website (, show 75 percent of the bay’s shoreline as “artificial,” with 53 percent made up of earthen dikes (about 21 miles) and 14 percent railroad grade (about 5 miles). About 36 percent (27.6 miles) of the shoreline is “fortified” with “revetment” (retaining walls), a process that Laird estimates costs between $900,000 and $2 million per mile.

Even with fortifications, however, “dikes will not be able to withstand projected sea level rise above 3 or 6 feet,” Laird wrote at

Laird said that sea level on Humboldt Bay has already risen by about 18 1/2 inches over the past century due to subsidence — “the gradual caving in or sinking of land” — and increasing tide elevation.

“Most of these old dikes, and even the railroad, are right at their limits as far as high tide,” he said. “One or two feet more of sea level rise, and all those things will be overtopped. The bay will likely reclaim a lot of those 9,000 acres (of diked salt marsh).”


Though Eureka and Arcata have been built up to the point where they’re not directly at risk, Laird said that U.S. Highway 101 and the utility lines that serve the cities are a different story.

“We’ve buried all of our gas lines, electrical lines and water transmission lines … in the farmlands behind those dikes. … All these underground utilities weren’t designed to be saturated by saltwater, and we won’t be able to get out there to maintain them if those areas become part of the bay again. Same thing with Highway 101. They built 101 over these low-lying areas, and over the last century a lot of that land has been compacted by as much as 3 feet. … So that’s where our vulnerability lies.”

The cost of addressing these issues is “a huge unknown” at this point, and Laird pointed out that “we’re not going to be the only coastal community that will need to relocate their gas lines or wastewater facilities, or elevate their highways. There are 76 counties and cities on the coast of California, and we’re just one of them.”

The NRC report notes that San Francisco International Airport could flood with as little as 16 inches of sea level rise, for instance, suggesting the scope of the problem and the massive costs that will result from any significant increase.

Adaptation planning 

An Adaptation Planning Working Group formed of staff members from more than a dozen local and federal agencies is currently examining options, with the stated goal to “support informed decision-making and encourage a unified, consistent regional adaptation strategy to address impacts associated with sea level rise in the Humboldt Bay region.” 

Hank Seemann, deputy director of Environmental Services for the Humboldt County Public Works Department, said the group will hold a public meeting the third week of November to release its maps and reports to the community.

“For sea level rise, we’re dealing with a scale that’s an order of magnitude bigger than we’re accustomed to dealing with in terms of the extent of the natural hazard and the scope and cost of potential adaptation measures,” Seemann said. “We’re entering a new age in terms of understanding and responding to sea level rise, and part of the planning group’s role has been to get all the jurisdictions together to discuss options. The infrastructure all connects, and things are connected hydraulically, so we need to look at this from a regional perspective. … We need a consistent, orchestrated approach so we can incorporate sea level rise into normal capital improvement planning, which is already beginning to take place.”

Seemann said the region is confronted with a “double- whammy.” 

“We’re facing sea level rise like everyone else, but also subsidence,” he said. “It’s still a work in progress to scientifically understand all the details, but I believe the relative sea level rise on Humboldt Bay is the highest in the state.” 

He said the county will be heavily dependent on state and federal funding for most of the necessary improvements, so “we’re going to have to be ready, have a plan and know what we need to do so that we can fall in line with what the state and federal government develop over the next few years.” He cited a recent federal call for infrastructure projects nationwide that identified more than $9 billion worth of proposals, but had only $600 million available to fund them. 

“That’s 15 times the available amount, so the need for improvements to current infrastructure already overwhelms the amount available. It’s going to be an incremental, long-term process to find funds for adaptation,” he said.

Read Original Article