Islands under Attack: Coastal Flood Threats in Bermuda and Hawaii

Hurricanes in both the Atlantic and Pacific Oceans threaten mid-oceanic islands today. In the Atlantic, category-3 Hurricane Gonzalo is bearing down on Bermuda, as it threatens to make a direct strike. In the Pacific, category-1 Ana intensified into a hurricane today, and is forecast to maintain hurricane strength, while the center of circulation tracks south of the Hawaiian Islands. This blog post provides an overview of storm surge history for Bermuda and Hawaii, while discussing storm surge/ surf height potential from these two systems.

Hurricane Gonzalo bears down on Bermuda today in this NOAA/AP image

Bermuda’s Storm Surge History

Hurricane Fabian in 2003 generated the largest storm surge in Bermuda’s hurricane history, according to the SURGEDAT database, hosted by Louisiana State University’s Southern Climate Impacts Planning Program (SCIPP). Although Fabian’s surge was estimated at only 10 ft (3.05 m), this surge was particularly destructive because 20 - 30 ft (6.1 – 9.1 m) waves were riding on top of this surge. These data were provided by Pasch et al. (2003), and quoted as follows…

“There were huge (estimated 20 to 30 ft high) battering waves on the south shore of the island, with the reported storm surge estimated near 10 ft. Significant structural damage was inflicted as a result of wave action and/or surge. Property damage estimate in Bermuda is estimated to be at least 300 million U.S. dollars” (pg. 2).

Hurricane Fabian made a direct strike on Bermuda in 2003.

Source: NOAA/ http://bernews.com/weather/2010/09/nasas-hurricane-fabian-imagery-sept-2010/

This coastal flooding event produced substantial damage to buildings and infrastructure, in part because of large, destructive waves.

Fabian's storm surge and destructive waves caused considerable damage on Bermuda's coast in 2003.

Source: AP News

Turpin (1982) provides coastal flooding information for other storm surge events in Bermuda’s history. He mentions that storm surge has not been a major problem for harbors in Bermuda, and provides a highest storm surge height of 7 ft (2.13 m) from Hurricane Arlene in 1963. However, this was written before Fabian’s destructive surge. This source also documents damaging surges in 1878, 1899 and 1917.

Quote: 

“Storm surge during tropical cyclone passage has not been a major problem for the Bermuda harbors. The highest surge height of recent record is about 7 ft, occurring with Arlene in August of 1963. Unofficial records report earlier cases of significant storm surge damage: in 1917 "unprecedentedly high" tides were reported; in 1899 the causeway was demolished; and in 1878 the sea made a clean breach of the dockyard breakwater.”

Bermuda’s Coastal Flooding Forecast for Hurricane Gonzalo

The latest Special Weather Statement issued by Bermuda’s Weather Service at 5:44PM local time in Bermuda forecast seas building to 30-40 feet. Earlier statements mentioned the likelihood of destructive storm surge, as well.

It should be noted that a forecast height for “building seas” refers to wave height, and not storm surge depth. Storm surge is essentially the height of sea level above predicted tide levels. We can think of storm surge as a “new sea level,” NOT including the height of large, destructive waves. We also sometimes hear the term storm tide, which refers to the combined height of storm surge and tidal oscillations. It is rare for mid-oceanic islands to observe storm surges higher than around 16.5 ft (5 m), however, waves will sometimes exceed 30 ft (9.1 m).

Although storm surges in Bermuda tend to be lower than other locations that observe tropical cyclones, large, destructive waves will ride on top of a storm surge, delivering a knock-out blow to coastal buildings. Such destruction may occur even if the buildings only observe a slight amount of storm surge flooding.

Gonzalo pushed high swells and waves into Bermuda before the strongest wind arrived. This webcam shot shows waves building along coastal Bermuda.

Source: www.portbermudawebcam.com; http://www.dailymail.co.uk/news/article-2795813/Bermuda-braces-Category-4-Hurricane-Gonzalo.html

Hawaii’s Storm Surge History

Earlier this Hurricane Season, I provided a blog about Hawaii’s storm surge history. That blog post can be found here: http://stormsurge2010.blogspot.com/2014/08/back-to-back-hurricanes-approach-hawaii.html.

In summary, hurricanes near the Hawaiian Islands tend to produce massive surf/ waves, but relatively low storm surges. However, similar to Bermuda, the enormous waves that strike Hawaii may greatly increase the destruction of even minor storm surge events.

According to SURGEDAT, the highest storm surge in Hawaii’s history was a 6-ft (1.83 m) surge that was observed near Kauai’s Poipu Beach during Hurrican Ikini in 1992 (U.S. Department of Commerce 1993). However, this source indicates that Iniki’s waves were 30 ft (9.14 m) high, which combined with the surge to create much damage on Kauai’s south shore.

Iniki devastated Kauai in 1992 with catastrophic winds and destructive waves. Even though the surge was only 6 ft (1.83 m), large waves on top of the surge caused much damage on Kauai's south shore.
Photo courtesy Bruce Asato on Weather Underground: 

http://www.wunderground.com/blog/Civicane49/remembering-hurricane-iniki

In early August, Hurricane Iselle weakened to a tropical storm, before making landfall on the Big Island of Hawaii. Although the winds weakened as the storm approached Hawaii, it still generated a destructive storm surge, particularly near Kapoho Vacationland.

These sources provide information/ videos about the storm surge damage:

Source 1: http://www.kitv.com/news/kapoho-residents-digging-out-of-iselle-aftermath/27451152

This source provided the highest storm tide level I could find, which was 5 ft above high tide level, near Kapoho Vacationland, on the Big Island of Hawaii. This would be considered a 5 ft storm tide, above the high tide reference line.

Source 2: http://www.youtube.com/watch?v=TFxpi5rjyIc

Starting at 1:40, this video shows storm surge debris that washed 150 years inland and 8 feet above sea level. Waves likely helped push debris higher than the 5 ft storm tide level.

Iselle's storm surge washed in at least 150 yards and pushed debris to 8 ft (2.44 m) above sea level on Hawaii's Big Island.
Source: http://www.youtube.com/watch?v=TFxpi5rjyIc.

Coastal Flooding Forecast for Hurricane Ana

According to the National Weather Service in Honolulu, surf along south-facing shores in the Hawaiian Islands will rapidly build on Saturday. For example, the forecast surf heights along Oahu’s south-facing shores, “will be 1 to 3 feet today, rising rapidly to 8 to 12 feet Saturday morning, continuing to rise to 12 to 20 feet by Saturday evening” (National Weather Service 2014).

A Hurricane Hal’s Storm Surge Blog fan from Oahu wrote to me today, indicating that such surf levels would be rare. It may be prudent for people with marine interests on south-facing shores to secure boats or other property that could be impacted by high waves.

A satellite image of Ana as it approached Hawaii. Ana developed into a hurricane today.
Source: NOAA/ Reuters. http://in.reuters.com/article/2014/10/17/usa-hawaii-ana-idINKCN0I60UR20141017.

I have not found storm surge forecasts for the Hawaiian Islands. However, the Tropical Storm Watch bulletin issued by the National Weather Service at 12:02 HST this afternoon forecasts surf heights. Apparently, Ana will produce 10-15 ft surf along south-facing shores late Saturday and Sunday, while an incoming NW swell produced 10-15 ft surf along north and west facing shores during the same period.

This means most of Hawaii’s coastline is going to observe high waves this weekend. It also means large swells from Ana will be moving NW and large swells coming down from higher latitudes will be traveling SE. I can't even imagine how tumultuous the seas are going to be near Hawaii this weekend, or how much more force will propel dangerous currents, in places like the Napali Coast.

Even if Ana stays well south of the Hawaiian Islands, high surf on south-facing coasts could certainly impact marinas, beaches, and possibly coastal infrastructure.

REFERENCES

National Weather Service, 2014: Surf forecast for O’ahu from the Weather Forecast Office at Honolulu, Hawaii. Available on the Web at: http://www.prh.noaa.gov/hnl/pages/SRF.php.

Pasch, R.J., E.S. Blake, and D.P. Brown, 2003: Tropical Cyclone Report, Hurricane Fabian, 27 August – 8 September 2003. National Hurricane Center, Miami, Florida. Available on the Web at: http://www.nhc.noaa.gov/data/tcr/AL102003_Fabian.pdf.

Turpin, R.J.B, Lieutenant Commander, Royal Navy Exchange, 1982: Analysis of the Tropical Cyclone Threat at Key West. Published as section 3 of 5 in Hurricane Havens Handbook for the North Atlantic Ocean, available on the Web at: http://www.nrlmry.navy.mil/~cannon/tr8203nc/keywest/text/frame.htm.

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.