Wai‘anae Ecological Characterization

Strong Winds and Coastal Storms

Besides warm temperatures, a prominent feature of the weather in Hawai‘i's subtropical climate is the occurrence of coastal storms that bring strong winds, heavy rains, and high waves. The trade winds are the primary source of wind in the Hawaiian Islands. These winds occur about 70 percent of the time and typically range from 10 to 25 miles per hour, occasionally reaching 40 to 50 miles per hour. The greatest impact of these northeasterly winds is felt along the east-facing, windward coasts of Hawai‘i.

Kona winds arrive from the southwest, the opposite direction of the trades. They are most common during the winter and spring. They may occur as light and variable winds or as strong winds associated with the passage of storm systems. When associated with storms, these winds can reach 50 miles per hour and last for days. Kona winds only occur about 30 percent of the time. However, it is these winds and their accompanying effects that typically have the greatest impact on Wai‘anae and other leeward coasts of the Hawaiian Islands.

Strong Winds

Hurricanes and tropical storms are a third source of winds in the Hawaiian Islands. When sustained winds associated with an organized weather system are greater than about 40 miles per hour but less than 70 miles per hour, the term "tropical storm" is used. When sustained winds reach or exceed approximately 75 miles per hour, a tropical storm becomes a hurricane. These rare but extreme events most commonly occur between the months of June through October. Typically they pass to the south and west of the Hawaiian Islands; occasionally, however, there is a direct hit.

Hurricane ‘lwa, which occurred in 1982, caused extensive damage, including inundation of the central sections of the coast southwest of the Wai‘anae Range as well as oceanfront areas on the south coast of O‘ahu from Sand Island to Diamond Head. Four hundred twenty-one (421) acres of land were flooded on O‘ahu by the combined effects of storm surge and high wave action. The height of the actual storm surge with Hurricane ‘Iwa is difficult to estimate. It probably reached to about three feet near Wai‘kīkī, five feet along the Wai‘anae coast (U.S. Naval Research Laboratory 2003).

Hurricane ‘Iniki, which occurred in 1992, is considered the strongest hurricane to hit the Hawaiian Islands this century. Based on estimated peak sustained winds of between 130 and 160 mph, ‘Iniki would be classified as a minimum Category Four storm on the Saffir-Simpson Scale (Wikipedia 2004). Despite the strength of the storm, ‘Iniki did not cause as much damage on O‘ahu as ‘Iwa did. Post storm estimates of wave heights range from a maximum of 16 feet (5 meters) on the Wai‘anae coast to 4 to 9 feet (1 to 3 meters) along the south coast of O‘ahu from Sand Island to Diamond Head (U.S. Naval Research Laboratory 2003).

The recent history of extreme storm events that have affected Wai‘anae include:

• Tropical Storm Dora - July 1993
• Hurricane ‘Iwa - November 1982
• Hurricane Dot - August 1959
• Hurricane Nina - December 1957
• Hurricane Hiki - August 1950

Recent work by the State of Hawai‘i Multihazard Science Advisory Committee, found that the "annual odds of occurrence" for a hurricane event on the island of O‘ahu is on the order of 1 in 50. With respect to damage to the island of O‘ahu during such an event, the Hawai‘i Hurricane Relief Fund estimates property losses would easily exceed $5 billion (City and County of Honolulu Oahu Civil Defense Agency 2003).

Coastal Storms

High waves and coastal erosion are associated with coastal storms. They can result in coastal flooding and shoreline retreat. In contrast to heavy rains and flooding, Kona and other more localized storms, high surf can also be caused by more distant hurricanes and tropical storms.

Typical Wave Swell Sources. Waves that reach the shores of the Hawaiian Islands originate from four primary sources: north Pacific swell, northeast trade wind swell, south Pacific swell, and Kona storm swell. North pacific swell delivers the highest waves annually (8 to 20+ feet in height). North Pacific swell occurs throughout the year, but is most common between October and May. It has the greatest impact on north-facing coasts.

The northeast trade winds produce swell that is characterized by waves of moderate height (4 to 12 feet). Even greater heights can be reached during intense trade wind events that tend to occur for 1 to 2 weeks each year. Trade-wind-generated swell is dominant between April and November and has its greatest impact on east-facing coasts. Having traveled great distances, south Pacific swell is characterized by waves with long periods and relatively low wave heights (1 to 4 feet). South Pacific swell can occur all year, but is most common between April and October. Its greatest impact is felt along south-facing shores. Kona storm swell is generated by central Pacific storms associated with fronts passing just north of the main Hawaiian Islands. Kona storm swell is characterized by waves of moderate height (10 to 15 feet) and short to moderate periods. These "steep" waves are most common between the months of October and May. Their greatest impact is felt on coasts, such as Wai‘anae, that face south and west.

High Waves. Another important source of high waves are hurricanes and tropical cyclones that pass in the vicinity of Hawai‘i. Waves from hurricanes and tropical storms can reach extreme heights (10-35 feet.) High waves from hurricanes generally occur between June and October. They most often hit the eastern shores as hurricanes approach the islands from the east, and south- and west-facing shorelines as the storm passes to the south and west.

High waves, in and of themselves, pose a significant threat to the lives of swimmers, boaters, and others who recreate along the shore. With respect to property damage, the greatest threat is due to coastal flooding and erosion that occurs when high waves combine with storm surge and high tides. This is what occurred along the Wai‘anae coast in 1992 during Hurricane ‘Iniki, when coastal roads and properties were overwashed by high waves.

Coastal Erosion and Beach Loss. The Hawaiian term for coastal erosion is "he kai a ai," literally, "sea that wears away the land." Along some segments of shoreline, coastal erosion and beach loss are a particular concern. Recent studies on O‘ahu, for example, have shown that nearly 24 percent, or nearly 17 miles of an original 72 miles of sandy shoreline, has been either significantly narrowed or lost since the 1940s (Fletcher and others 1997). With respect to Wai‘anae, approximately 1 mile of sandy shoreline in the Mā‘ili-Mākaha area has been lost or narrowed, and 25 percent of the shoreline has been classified as degraded (Coyne and others 1996, Fletcher and others 1997).

In addition to coastal storms, coastal erosion and beach loss in Hawai‘i has been attributed to reduced sediment supply, construction of shore protection structures, and sea level rise. Because the beach is such a vital economic, environmental, and cultural resource, its preservation is warranted. Maintaining a wide sandy beach enhances protection against the effects of high surf. Beaches are the basis for the visitor industry, which provides approximately three times more direct income to the State than all other industries combined (State of Hawaii Department of Land and Natural Resources 2000). The beach also provides habitat for marine and terrestrial organisms with beach-dependent life stages and is home to species of indigenous and endemic Hawaiian plants.

Storm Surge. Storm surge is the rise of water above sea level at the time of storm onset. Flooding from storm surge in the immediate coastal areas occurs primarily as a result of tropical storms and hurricanes. During these events, high winds and surf can push water several feet and even hundreds of yards inshore. Conditions can be exacerbated by large waves that form on top of rising water. The degree of damage caused by storm surge depends on the tidal cycle occurring at the time of the event.

During high tides, the increase in the water surface elevation caused by the storm surge on top of the high tide proceeds further inland, causing more extensive damage. The area of impact of storm surge flooding is confined to regions along the immediate coastline and typically extends to a few hundred feet inland. Storm surge floods and erodes coastal areas, salinizes (contaminating with salt) land and groundwater, causes agricultural losses, results in loss of life, and damages structures and public infrastructure. About 90 percent of the deaths experienced in the past near the coast resulting from hurricanes are caused not by wind, but by storm surge.

High Surf. The O‘ahu Civil Defense Agency classifies high surf as a condition of very dangerous and damaging waves ranging in height from 10 feet to 30 feet or more. High surf is associated with large ocean swells typically generated from storms passing to the north of O‘ahu. Aside from the direct impacts of the large waves, high surf also leads to other threats: rip current and coastal damage. The volume of water contained in a large wave creates a very strong rip current when it recedes back into the ocean. Coastal damage is increased as well, due to the effect of temporary rise in sea level along the coast. The large volume of water associated with high surf can stack up along the coastline and cause prolonged exposure to extreme erosive wave forces.

References Cited

City and County of Honolulu Oahu Civil Defense Agency. 2003. Multi-Hazard Pre-Disaster Mitigation Plan for the City and County of Honolulu. http://www.mothernature-hawaii.com/county_honolulu/planning.htm

Coyne, M.A., R. Mullane, C.H. Fletcher, and B.M. Richmond. 1996. "Losing Oahu: Erosion on the Hawaiian coast." Geotimes 41(12): 23-26.

Fletcher, C.H., R.A. Mullane, and B.M. Richmond. 1997. "Beach loss along armored shorelines on Oahu, Hawaiian Islands." Journal of Coastal Research 13(1): 209-215.

State of Hawaii Department of Land and Natural Resources (DLNR). 2000. Coastal Erosion Management Plan (COEMAP). Land Division, Coastal Lands Program. http://www.hawaii.gov/dlnr/occl/files/coemap.pdf

U.S. Naval Research Laboratory. 2003. Pearl Harbor and Honolulu Harbor Hurricane Haven Study. In Typhoon Havens Handbook for the Western Pacific and Indian Oceans. Report NRL/PU/7543-96-0025. October 1996, modified August 2003. On-line address: https://www.cnmoc.navy.mil/nmosw/thh_nc/hawaii/text/frame.htm

Wikipedia. 2004. Hurricane Iniki. http://en.wikipedia.org/wiki/Hurricane_Iniki

Related References

Businger, S. 1998. Hurricanes in Hawaii. University of Hawaii. School of Ocean and Earth Science and Technology. http://www.soest.hawaii.edu/MET/Faculty/businger/poster/hurricane/

City and County of Honolulu Oahu Civil Defense Agency. 2004. High Surf Hazards. http://www.co.honolulu.hi.us/ocda/highsurf.htm

Coyne, M.A., R. Mullane, C.H. Fletcher, and B.M. Richmond. 1999. "Mapping coastal erosion hazard areas in Hawaii: Observations and errors." Journal of Coastal Research (Special Issue 28): 171-184.

Fletcher, C.H., E.E. Grossman, B.M. Richmond, and A.E. Gibbs. 2002. Atlas of Natural Hazards in the Hawaiian Coastal Zone. United States Geological Survey (USGS), University of Hawaii, State of Hawaii Office of Planning, and National Oceanic and Atmospheric Administration. http://pubs.usgs.gov/imap/i2761/

National Oceanic and Atmospheric Administration (NOAA). 2004a. Hurricane Preparedness: Storm Surge. National Hurricane Center. http://www.nhc.noaa.gov/HAW2/english/storm_surge.shtml

Pukui, M.K. 1983. ‘Olelo No‘eau: Hawaiian Proverbs and Poetical Sayings. Bishop Museum Press. Honolulu, HI.

State of Hawaii Department of Land and Natural Resources (DLNR). 2002. Coastal Erosion and Beach Loss in Hawaii. http://www.hawaii.gov/dlnr/exhibits/clp/CoastalErosion1.html

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