Cliff collapse has caused Metro Parks Tacoma to permanently close two miles of Five Mile Drive, a popular park road built on the cliffs of Tacoma’s Point Defiance 109 years ago.
City officials blame climate change for worsening erosion of the 150-foot sea cliffs that frame the Point Defiance peninsula as it juts out into Puget Sound.
“Increased rainfall, storm intensity at the top of the hill, and increased wave energy and erosion dynamics at the foot of the slope all combined to accelerate the rate of receding of the cliffs,” said Metro Parks Tacoma Deputy Planning Director Marty Stump.
In some places, the cliff edge is within 20 feet of a recreational trail built in 1913. The path will remain open to pedestrians or cyclists.
“I have fond childhood memories of this drive and will miss the occasional ride on my motorbike through it,” Tacoma resident Jeromy Crom said on Twitter. “But being able to walk the dog without worrying about an idiot driving while being distracted by a raccoon is a win.”
In January, geotechnical consulting firm Geoengineers told Tacoma officials the consequences of continued erosion of Point Defiance Park’s cliffs “could be severe.”
“As a result of wave action at the tip of the slope and erosion of exposed soil, the cliffs will continue to experience surface erosion that propagates further inland towards Five Mile Drive and Gig Harbor Viewpoint,” they reported.
Consultants said retaining walls or other engineering measures to save the cliffside road would be very expensive and impractical. Instead, they recommend relocating it in the woods 150 feet from the cliff.
Park officials concluded that it would require cutting too many trees from one of the country’s largest forested urban parks.
Metro Parks Tacoma is now considering converting an existing service road that runs through the middle of the forest-covered peninsula into a publicly accessible road, although the agency does not have the funds for such a project.
Scientists say decisions like this – to relocate or re-engineer threatened coastal infrastructure – will become more common around Puget Sound as the global climate warms and sea level rise accelerates.
“Where we have the things we value at the top of the cliff — roads, homes, buildings, infrastructure, utilities — basically your decision is: Pull the thing back out of the danger zone or try to keep it in place,” oceanographer Washington Sea Grant Ian Miller said.
Miller said defensive measures against rising sea levels were often very expensive and short-lived.
Sea levels in Puget Sound have risen about eight inches since 1900. In Tacoma, it is likely to rise 8 to 10 inches by 2050 and 22 to 28 inches by the end of the century, according to the University of Washington’s Climate Impact Group.
The storm is expected to grow stronger as the climate continues to warm.
“Rain of high intensity and short duration is an important driver of slope failure,” University of Washington climate scientist Guillaume Mauger said in an email.
The Point Defiance Park crew reported seeing immediate changes to the cliffs after heavy spring rains.
How much sea levels rise or storms in the coming years will depend on how much heat-trapping pollution humans continue to pump into the atmosphere.
The outer two miles of Five Mile Drive are close to motorized on Fridays.
Metro Parks Tacoma also announced that Owens Beach, another popular attraction in Point Defiance Park, is reopening after 16 months of renovation. Picnic areas, parking and other features have been moved inland in anticipation of rising Puget Sound eating away at the shoreline.