Back-to-Back Hurricanes Approach Hawaii

Aloha everyone-

Today our focus shifts to the beautiful Hawaiian Islands. Rare back-to-back tropical cyclones will impact Hawaii over the next several days. Hurricane Iselle is forecast to make landfall on the Big Island as a hurricane this evening, while Hurricane Julio is forecast to track north of the islands as a tropical storm on Sunday.

 The Central Pacific Hurricane Center forecasts Hurricane Iselle to make landfall on Hawaii’s Big Island this evening. Source: http://www.prh.noaa.gov/cphc/tc_graphics/2014/graphics/EP092014W.gif

According to Jeff Masters’ Aug 6 blog on this topic (see http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2750), Hawaii has only been hit by two direct hurricanes since 1949. The most devastating was Hurricane Iniki in 1992, which made landfall on the island of Kauai as a category-4 hurricane.

I wanted to share a little about Hawaii’s storm surge history in this blog post. I also would like to provide a broader context of the processes that produce other coastal hazards, like tsunamis and high waves in the winter on the north shore of the islands.

Last year, I conducted a scientific literature search on coastal flooding in Hawaii, and found evidence of 13 tropical cyclones that have generated coastal floods in Hawaii since 1925. Hawaii stands out in the storm surge literature, because large swells and waves often reach these islands from both distant storms, and nearby tropical cyclones. However, for several reasons, Hawaii’s surge levels tend to be modest, but wave heights can be enormous. For this reason, high water events in Hawaii are most commonly listed as “surf,” as in, “surf levels were 15-20 feet,” or, “25-foot surf pounded the reef.”

The deep bathymetry, or offshore water depth, around the Hawaiian islands, helps moderate storm surge levels, but actually enhances wave heights. This is because displaced water and high waves approaching coastlines “feel” the coast and start breaking before they actually reach land. In a shallow offshore environment, like along the northern U.S. Gulf Coast, bathymetry is shallow, therefore, waves often break offshore and sometimes multiple times before reaching the coast. This process serves to dissipate wave energy before it reaches shore.

However, the deep bathymetry off Hawaii’s coast enables wave energy approaching the islands to remain mostly intact until waves are right along the shore. In this process, very little energy is dissipated until the waves break along the coastline. This, in part, explains why Hawaii observes monster waves along the north shore in the winter months. The energy in these waves comes from deep low pressure centers off the coast of Japan, Russia and the Aleutian Islands of Alaska, and travels thousands of miles without striking land. When the wave energy reaches Hawaii, we see enormous waves that draw world-class surfers to the North Shore.

Hawaii's North Shore has world-famous waves that attracts surfers from around the world. Surf is highest in the winter, as swells produced by strong extratropical storms in the North Pacific travel thousands of miles to the shores of Hawaii. Photo: http://www.thesurfbum.com/travels/bansai-pipeline-north-shore-of-oahu/

At the end of last summer, I was in Hawaii and I spoke to a lifeguard at Ke’e Beach (pronounced kay-ay) on Kauai’s north shore. The day we were talking the first big set of swells was coming down from the north and breaking hard on the reef. He told me that in the really big events, he has seen 50-foot waves crashing on the reef! I will tell you that it’s one thing to hear about big waves, but when you’re out there with a board, the power of the ocean is truly impressive. I was out one day with around 8 foot waves and the power is tremendous when they break on you!

Anyway….back to our hurricane history.

Due to deep bathymetry offshore, storm surge heights are modest in Hawaii, but waves can be massive. The highest recorded surge height in Hawaii in the SURGEDAT database is a 6-foot (1.83 m) surge near Poipu Beach, on Kauai’s south shore. However, wave heights on the coast were near 30 feet (9.14 m) during Iniki (U.S. Department of Commerce 1993).

Tsunamis have historically generated the highest total water level runups in Hawaii. By wave runup, I mean the elevation to which the maximum water, including waves, runs up on the land. On April 1, 1946, a powerful earthquake in Alaska’s Aleutian Islands produced a Pacific-wide tsunami that produced wave runup as high as 52.5 feet (16 m) (Lander and Lockridge 1989; Johnson and Satake 1997). The May, 23, 1960, Chilean earthquake, also generated a Pacific-wide tsunami that generated a 35-foot (10.67 m) wave at Hilo, on the Big Island’s north shore (Eaton et al. 1961).

People in Hilo, Hawaii, run to escape the 1946 tsunami. This event claimed 159 lives in Hawaii. Photo: http://www.prh.noaa.gov/itic_pr/The%20Great%20Waves/tsunami_great_waves_8.html

Wave runup from the 1946 tsunami in Hawaii reached as high as 52.5 ft (16 m). Note the man in the bottom-left corner of this photo. Photo: http://celebrating200years.noaa.gov/transformations/tsunami/image1.html.

While both hurricanes and tsunamis have caused considerable damage to Hawaii, the impact from tsunamis has been more severe. The 1946 Aleutian Island Earthquake produced a massive tsunami that killed 159 people (Lander and Lockridge 1989; Johnson and Satake 1997), making this the most deadly coastal flooding event in Hawaii’s history. This tsunami inflicted $300 million of damage in Hilo, adjusted to 2011 dollars, and destroyed many waterfront buildings (Western States Seismic Policy Council 2013). The tsunami from the 1960 Chilean earthquake killed inflicted $20 million in damage and killed 61 people in Hawaii (Eaton et al. 1961). By comparison, no deaths were officially reported from Iniki’s waves and storm surge in 1992, however, coastal flooding destroyed 63 homes on Kauai’s South Coast (Central Pacific Hurricane Center 1992).

Hurricane Iniki made a direct hit on Kauai in 1992. The storm inflicted catastrophic wind impacts, while surf as high as 30 ft (9.14 m) and storm surge as high as 6 ft (1.83) m, damaged 63 homes on Kauai's south shore, in places like Poipu Beach. Photo courtesy: Bruce Asato, found on Weather Underground post about Iniki. See: http://www.wunderground.com/blog/Civicane49/remembering-hurricane-iniki

While many people are focusing on how rare it is to have back-to-back tropical cyclones impact Hawaii, another rare aspect of these storms is that they are approaching Hawaii from the east. Historically, most hurricanes that have impacted Hawaii have approached from the south. Storm tracks play a huge role in storm surge development, especially in island locations, where one side of the island will observe onshore winds and another side will observe offshore winds.

A unique aspect about Hurricane Iselle, is that it is forecast to maintain hurricane intensity, until landfall on Hawaii’s big island. Because winds around hurricanes circulate counter-clockwise in the northern hemisphere, this means that strong onshore winds could impact the north shore of several islands. Substantial coastal flooding impacts are possible, therefore, in places like Hilo, as the harbor at this location faces northeast. It is possible that Hilo will observe higher water levels from Iselle than any previous cyclone. The highest surge level found in SURGEDAT for Hilo is a 4-5 ft (1.22-1.52m) storm surge from Hurricane Diana in 1972. Diana also produced 30-ft (9.14 m) surge along the Puna Coast of the Big Island (Central Pacific Hurricane Center 1972).

While Hawaii will certainly see wind and heavy rain impacts, some locations, especially along the east and north shores, such as the city of Hilo, are under threat for coastal flooding impacts. Also, keep in mind that on any side of any of the Hawaiian islands, dangerous coastal conditions may occur. Hawaii is famous for very strong currents, and persistent strong winds will displace water and create hazardous coastal conditions.

I am always available for follow-up questions and interviews related to storm surge. Feel free to send me an email at hal”at”srcc.lsu.edu.

On a personal note, I (finally) graduate with my Ph.D. tomorrow, so this is an exciting week for me. I have been working full time for LSU while working on a Ph.D., and will continue on at LSU, so don’t worry about the blog disappearing anytime soon.

Mahalo,

Hurricane Hal

REFERENCES

Central Pacific Hurricane Center, 1972: The Central Pacific Tropical Cyclone Season of 1972. National Weather Service – Central Pacific Hurricane Center, Honolulu, Hawaii. Available on the Web at: http://www.prh.noaa.gov/cphc/summaries/1972.php.

Central Pacific Hurricane Center, 1992: The 1992 Central Pacific Tropical Cyclone Season. National Weather Service- Central Pacific Hurricane Center, Honolulu, Hawaii. Available on the Web at http://www.prh.noaa.gov/cphc/summaries/1992.php.

Eaton, J.P., D.H. Richter, and W.U. Ault, 1961: The tsunami of May 23, 1960, on the Island of Hawaii. Bulletin of the Seismological Society of America, 61, 2, 135-157.

Lander, J.F., and P.A. Lockridge, 1989: United States Tsunamis (Including United States Possessions) 1690-1988, 265 pp., National Geophysical Data Center, Boulder, Colorado, 1989.

Johnson, J.M., and K. Satake, 1997 : Estimation of seismic moment and slip distribution of the April 1, 1946, Aleutian tsunami earthquake. Journal of Geophysical Research, 102, B6, 11,765-11,774.

U.S. Department of Commerce, 1993: Natural Disaster Survey Report, Hurricane Iniki, September 6-13, 1992. Report available online at: http://www.nws.noaa.gov/om/assessments/iniki/iniki1.pdf.

Western States Seismic Policy Council, 2013: 1946 Aleutians Tsunami. Available on the Web at: www.wsspc.org/resources-reports/tsunami-center/significant-tsunami-events/1946-aleutians-tsunami/.