Erosion likely result of supermoon tidal increases rather than storms

Stumps in the sand lapping in the shorebreak on the eastern side of Baldwin Beach Park in Paia are more likely the result of “unusual erosion” linked to “supermoon” tidal increases than the high surf generated by storm systems Iselle and Julio, said a coastal expert with the University of Hawaii Sea Grant program.

Tara Owens, coastal processes and hazards specialist who also serves as a technical liaison with the Planning Department, said she has been receiving reports of erosion on the north shore since late July, around the first summer occurrence of the supermoon.

“Tides are purely astronomical, caused in most part due to the gravitational attraction of the moon,” she explained Thursday.

Supermoons are given their name because they pass closest to the Earth in their orbit, causing a brighter and larger moon in the night sky and raising tides.

The summer has brought a number of supermoons. There was one July 12 and another Sunday. The third supermoon of the summer is set for Sept. 9. Sunday’s supermoon was the closest pass of the moon of the year, 221,765 miles from the Earth.

“Interestingly, it appears that unusual erosion along the north shore has been happening even before the storms in association with the occurrences of a supermoon,” Owens said.

In July, around the time of the supermoon, “higher-than-predicted tides” appeared, 3 to 4 inches higher than expected levels.

“And, sure enough, with the more recent supermoon, we had higher-than-predicted tides starting on Aug. 6 and getting up to about 5 inches above predicted,” Owens said.

Increases in sea level of this magnitude “are often associated with increased erosion events,” she said.

Owens said her “gut feeling” is that the current erosion is “more directly related to the high tides than the swell” from Iselle and Julio, which led the National Weather Service to post high-surf warnings for east-facing shores.

Still, the wave setup with the storm swell on top of the increased sea level “may have contributed to additional erosion because waves can reach further inland,” she said.

“We are very lucky that we didn’t have larger storm waves, because the combination of big storm waves with the high tides we were experiencing had the potential to be very damaging,” she said. “We dodged a bullet.”

The eastern side of Baldwin Beach normally would be experiencing seasonal erosion at this time of year, Owens explained. In the summer, erosion peaks along the eastern section of the beach, near Montana Beach, due to the stronger trade winds that pick up in April and May, she explained. The trades send sand to the Kahului side of the beach.

As the trades die down, the winter swells start to dominate and return sand to the eastern side. The sand moves back and forth on the beach depending on the seasonal conditions, she explained.

When the winter shift occurs, there only will be sand covering the stumps in the eastern area with the water receding, Owens said. There could be a 100-foot difference in area affected from winter to summer.

“That’s a natural process that happens every year,” she said.

The “longshore transport of sand” on the eastern side of the beach is particularly susceptible to disruptions due to man-made issues, a lime kiln revetment and rocks offshore, a natural situation.

“It’s very dramatic at that particular site,” Owens said.

The erosion at Baldwin Beach Park offers a preview of “what we expect to see in the future with increases in sea-level rise.”

“It is notable that even a few inches of extra water can result in extensive erosion and flooding,” Owens said.

What is happening at Baldwin Beach Park “is just going to be the more common scenario as we move into the future,” she said.

* Lee Imada can be reached at