Wai‘anae Ecological Characterization

Heavy Rains and Flooding

The many Hawaiian words and terms referring to heavy rains and flooding reflect the frequency with which these events occur in the islands. Hawaiian terms for heavy rains include ua (drenching rain), ua hoeele (continuous rain), ua ho‘okina (excessive rain), and pīpīnoke (to rain continuously; Ulukau Electronic Hawaiian Library 2004). Hawaiian terms for flooding are wai hālana (flood water) and wai pi‘i (inland flooding).

Extreme coastal storm events often bring heavy rains as well as strong winds. With the steep slopes and small drainage basins characteristic of tropical island settings, heavy rainfall can lead to flash floods and landslides or debris flows. The most frequent and severe flooding occurs where steep sloping hillsides abruptly meet flat or low-lying coastal plains. Because the predominant northeasterly trade winds do not usually bring flood-producing rainfall, heavy rain is typically associated with non-trade wind conditions.

Hurricanes and tropical storms hitting or passing by the Hawaiian Islands can generate heavy rainfall. Such events tend to occur between the months of June and October.

Kona storms are a significant cause of heavy rains and flooding (see the Winds and Storms section). They can bring intense local showers affecting a small area for several hours or can blanket an entire island with rain for several days. Kona storms occur most frequently in January, but are common throughout the months of October through May. A review of 2,317 recorded annual peak discharges with known dates shows that 89 percent of the peaks occurred during this period. Eighty-four percent of the floods in the islands occurred during this period (City and County of Honolulu Oahu Civil Defense Agency 2003).

Other causes of flooding include frontal storms and storms caused by upper-level lows. Frontal storms are associated with the moving boundary of two air masses of different temperatures. This type of storm usually occurs during the period from December through March when cold air masses force warm air masses to rise, causing heavy rains. Upper-level lows form at elevations above 15,000 feet and can occur any time of the year. Extreme weather from the upper level low-pressure system can have the longest duration of any major storm type, because the system is large and moves or weakens slowly.

The Wai‘anae area's topography is dominated by the Wai‘anae mountain range, the southwestern side of which is characterized by steep slopes, short broad valleys, and small watersheds. The streams associated with this type of watershed generally are very narrow (constrained) and do not have floodplains. The lack of flood water storage within the channel causes the streams to be susceptible to flash floods. With high-rainfall-intensity storm events covering an entire watershed, it is not uncommon for this to result in a rapid rise of water levels in streams and channels.

When compared to other natural hazards affecting Wai‘anae, the rare and extreme storm events of the types noted above pose the greatest risk to life and property. During the first 15 days of November 1996 in Wai‘anae, for example, a record-breaking rainfall of 21 inches fell in an area where the average annual rainfall is only 2 inches. In Nānākuli during October of 1991, 15 inches of rain fell in 48 hours. In both of these instances, flooding and landslides caused extensive damage.

Rare and Extreme Storm Events

Following is a summary of rare and extreme storm events (excluding hurricanes) that affected the Wai‘anae area of the island of O‘ahu for the period between 1920 and 1996. The figure details the recorded flooding events for the entire Island of O‘ahu.

Wai‘anae Extreme Rainfall and Flooding Events

Storm Event	Comments and Impacts
1996 November	Wai‘anae received 21 inches of rain in a 15-day period, resulting in flash floods and mudslides.
1991 October	Nānākuli received 15 inches of rain in 48 hours, resulting in flash flooding.
1991 September	Wai‘anae received heavy rains resulting in flooding.
1985 January	Wai‘anae and Nānākuli received heavy rains and flooding.
1978 February	Wai‘anae area received heavy rains and flooding.
1965 November	O‘ahu encountered strong winds, heavy rains. Mākaha Stream had debris flows.
1964 December	Statewide strong winds, heavy rains, high seas. Mākaha Stream flooded. Four lives lost and $0.5 million in damage.
1962 March	Heavy rains caused flooding on Mākaha Stream. Almost $0.5 million in damage.
1954 November	Heavy rains on O‘ahu caused flooding on Mākaha Stream. Almost $1 million in damage.
1927 December	Statewide heavy rains and flash flooding (including Wai‘anae and Wailuku). Five lives lost.

Source: City and County of Honolulu Oahu Civil Defense Agency (2003).

Landslides and Debris Flow

A landslide happens when gravity forces land downward. Often, precipitation, runoff, or ground saturation are contributing factors to slope movement. Debris flows are a particular type of landslide. They are characterized by a combination of loose soils, rocks, or other materials and water that flows rapidly downslope, commonly in association with periods of intense rainfall. Once formed, they can continue to grow, with water and materials being added as they flow down hills and through channels. Upon reaching flatter ground, they can travel great distances, eventually slowing down as they spread out over a large area. Debris flows are dangerous because they occur suddenly and move rapidly. Because they can carry large items such as boulders, trees, and cars, debris flows can cause extensive damage. The most hazardous areas are canyon bottoms, stream channels, areas near the outlets of canyons, and slopes excavated for buildings and roads.

Landslides and debris flows are not uncommon in the history of O‘ahu. More than 1,779 have been recognized in aerial photographs of the Honolulu District flown during a period of approximately 50 years (Peterson and others 1993). Particularly large numbers of landslides and debris flows have accompanied major rainstorms, such as the New Year's Eve storm of 1987-1988. They generally occur during or immediately after severe rainfall of more than three inches in a peak six-hour period (City and County of Honolulu Oahu Civil Defense Agency 2003). Most of these debris flows caused relatively little direct property damage because they occurred in undeveloped or relatively inaccessible upland areas; however, some of the areas affected by past debris flows have since been developed. If development continues in these upland areas, the impacts from debris flows in future storms could become more frequent and costly.

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

Peterson, D.M., S.D. Ellen, and D.L. Knifong. 1993. Distribution of Past Debris Flows and Other Rapid Slope Movements from Natural Hillslopes in the Honolulu District of Oahu, Hawaii. Open-File Report 93-514. U.S. Geological Survey.

Ulukau Electronic Hawaiian Library. 2004. Hawaiian Dictionaries: Search for a Hawaiian Word or Definition. http://wehewehe.org/cgi-bin/hdict

Related References

Bakutis, B. 1996. Downpour Staggers Oahu. Honolulu Advertiser. November 15, 1996. Honolulu, HI.

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/

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

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