Coastline changes

HANALEI — A wall of tangled roots separates the park at Black Pot from the beach, and the erosion has drastically changed the beach from six months ago.

While geologists say shifting sands aren’t unusual, it’s a prediction of what could be coming in the next few decades and scientists say long-term studies of the area are necessary.

“Right now the beach appears to be recovering from an erosion event that started in late August/early September,” said Ruby Pap, Coastal Land Use extension agent with Hawaii Sea Grant.

She said it was probably due to elevated water levels, high tides and north swells.

Other areas of Hanalei experienced erosion as well, including Waikoko on the other side of the bay.

The County of Kauai received photos of the exposed roots and severe erosion in September, which prompted county staff to trim the ironwood trees fronting the shoreline and place boulders along the vehicular access, blocking cars from the area.

Three months later, the beach has changed so much that the boulders are no longer doing their job. The line between the beach and the grass has become a three or four-foot drop, and large pieces of driftwood are caught in the exposed roots.

The cause is still a mystery in many ways, but researchers say discharge from the Hanalei River and seasonally large waves most likely play their part.

“Both periodic large ocean waves and high discharge river flood events can erode both rivermouth sandbar deposits and coastal beach deposits and move the sand offshore, in this case into Hanalei Bay,” said Kauai geoscientist, Chuck Blay.

He continued: “A coincidental combination of both events could have caused the movement of the Hanalei Rivermouth sandbar bayward.”

And bayward it has been pushed — with all of the sand that used to be on the beach causing shallower nearshore waters that extend into the bay.

“The water is shallower because the sand that was formerly on the beach has eroded offshore,” Pap said.

It’s not the first time the park at Black Pot has bumped shoulders with the beach. According to the Kauai Shoreline Study Erosion Maps, the beach was eroded up to the lawn at the beach park in the 1960s. There are typical erosion episodes and then periods of recovery.

“This area of shoreline is very dynamic due to the influence from the Hanalei River on sand movement, in addition to waves and tides,” Pap said.

Blay said the Hanalei River floods frequently, especially during the winter months, and that such high river discharge floods move the rivermouth sandbar around.

“We also know that large, highly variable ocean waves, especially during winter months, can narrow a beach by grabbing the sand and taking it offshore,” Blay said.

The geoscientist has been studying sand movements around Kauai for years, and said the story of shifting sands plays out all around the island, with different results depending upon existing conditions.

“Just the right combination of river and wave process could have caused the recent, apparently abnormal erosion of Black Pot Beach,” he said.

And while these natural processes are playing out an age-old story, the section of coastline that houses Black Pot Beach has a historical erosion rate of about a foot per year, according to the shoreline study.

At the same time the shorelines are crumbling at Black Pot Beach, the Hawaii Climate Change Mitigation and Adaptation Commission released its first Hawaii Sea Level Rise Vulnerability and Adaptation Report. The document combines scientific analysis of seal level rise vulnerability with recommended best practices for the future.

Predictions within the document point to a sea level rise of more than three feet by the year 2010 and the more extreme predictions pointing to reaching that number by 2060.

That kind of sea level rise would chronically flood about 6,500 structures statewide and cost around $19 million just in losses from chronic flooding of land and structures in the sea level rise exposure area. Nearly 38 miles of coast roads are predicted to be be affected, as well as 550 cultural sites. More than 13 miles of beaches are predicted to be underwater as a result.

That highlights the importance of long-term studies at places like Black Pot Beach, scientists say.

“It’s important to monitor our beaches closely so we can better understand their patterns,” Pap said. “Sea level rise from global climate change is likely to increase erosion significantly and we may be seeing some of that impact already.”