Tuesday, August 30, 2016

Hurricane Madeline Threatens Hawaii's Big Island with Destructive Storm Surge and Surf

All eyes are on Hawaii as Hurricane Madeline approaches the Big Island. High surf and coastal flooding are concerns with any hurricanes that approach Hawaii, as storm surge can suddenly build along island chains, with water levels that are quite localized.

Hurricane Madeline approached Hawaii's Big Island as a category-3 hurricane on the afternoon of Tuesday, August 30, 2016, in this color-enhanced satellite image from NOAA.

Due to the deep bathymetry, or offshore water depth, storm surge levels tend to stay suppressed in Hawaii, while waves build to enormous heights. In historic literature, including scientific documentation, coastal flood observations are often referred to as “surf.”

The Central Pacific Hurricane Center forecasts Hurricane Madeline to approach the Big Island, either making landfall near the southern tip, or passing just south of the island.

In 1992, Hurricane Iniki generated the highest recorded storm surge levels in Hawaii, with water levels rising to 6 ft above normal astronomical tides on the South Kauai Coast. During that event, surf levels were recorded at 30 ft (U.S. Department of Commerce 1993).

On the Big Island, where Madeline’s impact will be felt the strongest, Hurricane Diana in 1972 generated surf levels also reached 30 ft along the Puna coast, while storm surge levels at Hilo ranged from 4 to 5 ft (Central Pacific Hurricane Center 1972). Although this may not sound impressive, modest storm surges combined with tremendous wave heights can still be destructive, because waves are riding on top of the storm surge, enabling them to push a destructive force of water inland, well beyond the storm surge water limit. Diana’s combined storm surge and high surf swept four homes from their foundations and eroded 200 feet of a private road (Central Pacific Hurricane Center 1972).

Hurricane Iniki made landfall in Kauai as a cat-4 hurricane in 1992. Although Iniki's storm surge was only measured at 6 ft, the storm generated 30-ft waves. Photo courtesy Nick Galante via Weather Underground.

More recently, Hurricane Iselle (2014) generated a 4-ft storm surge at Kapoho and Vacationland, near the eastern tip of the Big Island. Massive waves enabled water to push inland, causing flooding and  erosion.

Even though Hurricane Iselle (2014) weakened to a Tropical Storm before landfall, is still generated a 4-ft storm surge near the easternmost point not the Big Island.

Such problems are enhanced in Hawaii, where steep mountains meet the ocean and only one coastal road exists in many locations. This can be contrasted to the mainland U.S., where numerous alternative routes often exist.

Both Diana and Iselle managed to inflict coastal flood damage along the eastern shore of Hawaii although they were both downgraded to tropical storms before their closest approach. This should be of concern to residents of the Big Island, as Hurricane Madeline is forecast to make landfall or pass just south of the Big Island as a category-1 hurricane. Even if Madeline tracks just south of the island, the eastern shore will be to the right of the track, in the strongest quadrant, making this area quite vulnerable to wind and flood damage.

NOAA's WaveWatch III program predicts significant wave heights exceeding 16.4 ft (5 m) on the eastern shore of the Big Island, with the passage of Hurricane Madeline.

The biggest concern for coastal flood potential will be on the southeastern shores of the Big Island, near the villages Naalehu and Pahala, where hurricane-force winds may wrap around the “back side” of Madeline after its closest approach. Such winds come on strong from the opposite direction than they were previously blowing, quickly changing a low-water event into a sudden storm surge event, which can be surprisingly destructive.

NOAA's WaveWatch III program has forecast that significant wave heights, or the height of the highest one-third waves, could exceed 16.4 ft (5 m) on the eastern shore of the Big Island during the passage of Hurricane Madeline.

Under such circumstances, we should not be surprised to see storm surge levels exceed 4 ft, accompanied by massive waves that wash inland on top of the surge. Such conditions could be locally destructive.

Central Pacific Hurricane Center (1972), The central Pacific tropical cyclone season of 1972, National Weather Service- Centra Pacific Hurricane Center, Honolulu, Hawaii. [Available at: http://www.prh.noaa.gov/cphc/summaries/1972.php.]

U.S. Department of Commerce (1993), Natural disaster survey report, Hurricane Iniki, September 6-13, 1992. [Available at http://www.nws.noaa.gov/om/assessments/iniki/iniki1.pdf.]