11/28/12
This year America watched sea levels rise and the ocean flood cities across the east coast in late October. Humboldt County faces a similar threat.
Mid-November to early December is when flood risk is the highest. A storm during the high tides of that time can overtake the dikes around Humboldt Bay, flood the area and cost the county millions of dollars to recover.
The California Coastal Conservancy proposed a $250,000 grant last month for a Humboldt County regional group to make preparations for future floods. The regional group includes the Humboldt Bay Harbor District, Eureka and Arcata. The money will be used to research flood risks and bolster the dike system around Humboldt Bay.
Dan Berman, director of conservation at the Humboldt Bay Harbor, Recreation and Conservation District, said the grant money is necessary to prepare for the future. The Coastline of Humboldt Bay is protected by dikes that are old, and have a history of failure.
“When high tide and storms hit us, we’re protected by earthen dikes that were built 107 years ago,” Berman said.
Aldaron Laird, founder of Trinity Associates, said the earthen dikes were not made with flood safety in mind.
“It’s not that they weren’t engineered well; they weren’t engineered at all,” Laird said. “They just used a dredge to excavate a ditch, then just piled it into a wall.”
The century-old dikes failed in winter 2003 when water breached the dike at Mad River Slough and damaged the nearby pastures with salt water.
The Federal Emergency Management Agency allocated money to fix the overtopped dike. Berman said it cost $500,000 per mile to reinforce the face of the dike with rock to prevent erosion.
The final cost totaled $11 million.
The dikes failed again in winter 2005 when a storm breached dikes along Humboldt Bay. Highway 101 and Arcata’s wastewater treatment ponds and agricultural fields flooded with water.
Laird said the damage from high tides and floods that followed warranted a declaration of a state of emergency by the governor.
In the grant proposal to Humboldt Bay the conservancy said 7,800 residents live in high-risk areas for floods, along with $1.4 billion in property value. Two-hundred and forty miles of roads, including 58 miles of highway, are at risk.
Berman said the flat land that Highway 101 runs alongside would be likely to flood in a big storm.
Rising sea levels put Humboldt Bay at a greater risk.
Jennifer Kalt, policy director of Humboldt Baykeeper, said sea levels are projected to increase 18 inches by 2050, and 55 inches by 2100. As the sea level rises, larger areas are vulnerable to floods during storms.
At the same time, the Humboldt Bay area is sinking. In a process called subsidence, plate tectonic movement has pushed the Humboldt Coastline downward. The rise in sea levels, combined with the lower coastline, creates a two-fold problem, and a greater risk for floods.
“In Crescent City, the coastline is actually uplifting at the rate sea levels are increasing,” Kalt said, “but we are not so lucky.”
Plans to lessen damage from future floods vary from person to person. To Kalt, this means all future development should retreat from the coastline and out of flood risk zones.
“Sea levels are only going to increase and erosion is going to continue to happen,” Kalt said. “development needs to steer further inland.”
But to Berman, the decision to improve existing dikes or retreat inland is not so simple. “Both are extremes, and there is a range of middle-ground options to consider,” Berman said.
“If a wastewater treatment plant is in a high-risk area, we can protect it for 50 years with better dikes as they gradually move operations inland,” Berman said.
Berman said the regional group developed by the Coastal Conservancy grant is an important step in the right direction. “The Harbor District is excited that the local community and the district are looking at the issue more seriously,” Berman said, “and thinking ahead instead of reacting.”
Kalt agrees that the regional group plays an important role.
“Dealing with the flood threat can’t be done parcel by parcel,” Kalt said. “It must be a bay-wide approach.”
