Is Humboldt Bay vulnerable to sea level rise from climate change? Yes, absolutely. Within several decades, it is likely that impacts from sea level rise to diked former tidelands, such as flooding from emerging groundwater, backwater flooding during stormwater runoff, and tidal inundation from overtopping or breaching of dike shorelines, will become common. However, the agricultural lands and transportation and utility infrastructure located on the diked former tidelands around the bay are at risk from tidal inundation now.
Seventy-five percent of the 102-mile-long shoreline around Humboldt Bay is artificial. Dikes (barrier-like earthen structures) comprise approximately 41 miles (53 percent) of the artificial shoreline. Diking off salt marsh began in 1890 and converted thousands of acres of salt marsh to agriculture land. Since then, critical transportation and utility infrastructure has been built on these lands including Highway 101, Highway 255, municipal water lines, PG&E gas lines, sewer lines and lift stations, and optical fiber lines. If the diked shoreline is compromised (overtopped or breached) during a king tide or by storm-induced wind waves, such infrastructure could be inundated.
There are 23 diked hydrologic sub-units on Humboldt Bay—all of which are vulnerable and at-risk unless protective measures are employed to increase the resiliency of these structures to rising tides. Several of these diked sub-units have already been breached by king tides or storm surges in the last 15 years. Today, king tides occur on average four times a year. In the future, with 1.6 feet of sea level rise, tides could equal or exceed breaching elevation 125 times a year. With 3.3 feet of sea level rise, breaches could exceed 355 days a year.
There are also 62 tide gates designed to drain stormwater and tides from the diked former tidelands. With increased low tide elevations from sea level rise, the effectiveness of these gates would be reduced, impairing the ability of stormwater to drain.
Groundwater in diked former tidelands is generally within 3.3 feet of the surface. Eventually, regardless of the condition of dike structures, low-lying former tideland areas are therefore also vulnerable to flooding from emerging groundwater and saltwater intrusion in response to sea level rise. These modes of flooding will begin as nuisance flooding during winter and spring storms and king tides, then increase in frequency as the seas rise over time until the flooding becomes chronic. Ultimately, diked low-lying former tideland areas will become
continuously inundated.
On Humboldt Bay, transportation and utility infrastructure on diked former tidelands will have to either accommodate sea level rise or be relocated from this hazard area. We need to begin planning now for adaptation strategies to protect, accommodate, and relocate this critical transportation and utility infrastructure. These strategies will be expensive to implement, but are not optional. We likely need to employ a phased approach spanning many decades. It is critical that all Humboldt Bay stakeholders become involved and address sea level rise as a region and collaborate, plan, and build resiliency together.
