Summer tides make shoreline erosion likely on South Shore

At Waikiki in summertime, big waves charge the air with negative ions and salt spray, long blue walls breaking into white lei of foam out to the horizon. Summer also brings the year’s highest tides, which can interrupt an ill-timed beach stroll, towel nap or picnic with a sudden soaking. Worse, those who walk out onto jetties or sea walls can get washed off and hurt on the rocks and reef below.

This summer will see 36 days with peak high tides above 2.2 feet in June, July and August, according to National Oceanic and Atmospheric Administration predictions. When combined with big waves, these high tides will make shoreline inundations and erosion likely, said Dolan Eversole, a geologist and coastal processes specialist for the Sea Grant program at the University of Hawaii at Manoa who serves as technical adviser to the Waikiki Beach Special Improvement District.

“These (will be) our seasonal high tides we get every summer, nothing really exceptional or unusual,” Eversole said.

However, he added, another factor might contribute to more severe beach inundation and erosion this summer, when “a regional anomaly (in the seas around Hawaii) will make the high tides 2-4 inches higher than what’s on the NOAA (charts).”

In any given year, he explained, the anomaly, produced by cyclical high water levels driven by winds back and forth across the Pacific, coupled with bulging ocean eddies around the Hawaiian Islands, varies from zero to 4 inches but in the “king tide” summer of 2017 reached 9-12 inches, Eversole said. Inundation at high tides forced beachboys’ concessions to close, and the makai facades of beachside restaurants were boarded up.

Even 2-4 inches can pose problems, said Charles “Chip” Fletcher, professor of earth sciences at UH Manoa’s School of Ocean and Earth Science and Technology and vice chairman of the Honolulu Climate Change Commission. “This water level Dolan identified is something to worry about: Erosion may be worse this summer than expected, particularly on southern shores,” Fletcher said.

“All this variability is superimposed on a long-term rise of sea water levels because of global warming,” he added.

High summer tides regularly cover beaches from Ala Moana through Waikiki to Diamond Head but don’t necessarily pose risks to anything more than personal property, said Paul Merino, lifeguard captain for South Shore operations for the city Emergency Services Department Ocean Safety Division. “While you can be up to your waist in water walking on that narrow sliver of Diamond Head Beach in summer, it’s a surge, (not) a large shorebreak that can injure people like at Sandy Beach.”

Combined with seasonal high surf, though, high tides can spell danger for those unfamiliar with the ocean who ignore lifeguards’ warnings and venture out on jetties and sea walls, Merino said.

“I’ve seen people get knocked off the wall (Kapahulu Groin), hit their heads, and we’ve had to spine ’em (put them on a backboard to help prevent further injury to the spinal cord) and get them to the hospital,” the captain said. “It’s due to the sheer volume of water coming over that wall.”

Beachboys prepare for seasonal high waves and tides as a matter of course, digging trenches in the sand if necessary to deflect wave energy and closing operations if conditions become too dangerous for water activities.

“I’ve been on the beach 70 years, and every summer it’s a normal occurrence,” said Ted Bush, 72, who runs Waikiki Beach Services, founded by his father, in front of the Royal Hawaiian Hotel. Because of sand erosion, he no longer runs canoes from the Sheraton Waikiki. “We still do from the Royal, we’ve got plenty of sand there, but high tide allows higher surf to come landward, right up against the hotel wall.”

This summer’s highest tide will be 2.6 feet on July 31, plus another 2-4 inches, Eversole said.

Lifeguards assess conditions and forecasts throughout the day, so Merino encouraged people to stop by lifeguard towers and seek advice when they arrive at the beach.

Also to be on alert for this summer: NOAA has announced a 70% chance our current mild El Nino conditions will continue through fall, producing warmer ocean temperatures and a 70% chance of an above-normal hurricane season for the islands.

A six-day wave inundation forecast for Waikiki is available at pacioos.hawaii.edu/shoreline/runup-waikiki.