Experts discuss possible causes of boulders crashing down on Oahu

HONOLULU (KHON2) — In the last two weeks, boulders have come crashing down from Oahu hillsides with two of them blasting their way into residents’ homes.

Nearly two weeks ago, a boulder crashed into a Palolo home, Wednesday morning another boulder into an Aiea home and on Sunday, rocks tumbled down onto Kamehameha Highway near Waimea Bay. Experts said there are many reasons why boulders can come tumbling down like construction, excavation and mother nature.

“Rainfall can weaken the soil or it can also cause a little bit of erosion around the base of boulders that are sitting on the hillside,” said Rex Baum, USGS Supervisory Research Geologist.

The National Weather Service said it’s not just severe wind and rain that can cause potential rockfalls, but also dry conditions.

“You can think of it as a dry sponge, when you have a dry sponge and you first get it wet, it takes a little while for that to kind of get it,” said Maureen Ballard, National Weather Service Lead Forecaster. “So what happens to the ground, you’ll have a dry period and the ground will be super hard and at that point you’ll just have rain roll off.”

The USGS published a landslide susceptibility report in 2018 highlighting areas of risk. Experts said boulders crashing down should be an indication of a bigger problem.

“The boulders may be a precursor to indication of landslide risk,” said Dennis Hwang, University of Hawaii Sea Grant Program.

Experts added rockfalls and landslides can be hard to predict so identifying risk factors ahead of time are ideal.

“If you see trees or poles that are leaning on the slope or occasional boulders or even smaller debris falling down the slope,” Hwang said.

The USGS is working with state and federal agencies to build a nationwide database on landslides.