Climate and Water Resource Case Study

Sea Level, Storms, and Oahu

            Some evidence of how sea level rise, sometimes in conjunction with high tide periods, can impact Oahu may have been provided between November, 2003 to March, 2004.  During this interval, termed the “ho′oilo” or wet season in Hawaiian, the Hawaiian Islands were repeatedly hit with heavy storm activity that was severe enough to spawn very rare occurrences of waterspouts and tornados.  Over this interval, the island of Oahu and the remaining Hawaiian Islands received extensive intense rainfall (Table 3). 

Table 3. Oahu rainfall station totals (in inches) from November 2003 through March 2004 for the Wilson Tunnel, Honolulu International Airport, and Aloha Tower (see Figure 38 for rainfall gauge locations). The percentage of the monthly rainfall total compared to the normal monthly rainfall is given in parentheses.  Source: National Weather Service, Honolulu Forecast office.

¹ Aloha Tower is close to Mapunapuna; ² by the end of March 2004, Honolulu International Airport had received within an inch of its yearly average of total rainfall.

            Some examples of rainfall totals received were: (1) November 29th-30th, 2003, Oahu received up to 29.1 centimeters (11.47 inches) in 24 hours (Wilson Tunnel, Oahu), (2) December 7th and 8th, parts of Oahu received over 28 centimeters (11 inches) of rain, (3) on February 27th, Oahu received almost 20 centimeters (8 inches) in a 24 hr period.  These storms resulted in extensive flooding, especially in certain Honolulu industrial areas (e.g. Mapunapuna).  The flooding was a function of a variety of factors such as saturated soil, clogged drainage, and high tide.  Because the storm events lasted many times longer than a day, the interval of their precipitation and resultant runoff occurred over enough time to encompass the daily high tide.  The high tide (over 0.6 meters or 2 feet at times; Table 4) imposed on the long-term sea level rise trend (Figure 37) resulted in the flooding of the island drainage system to the ocean – i.e., runoff water can not drain to the ocean because the elevated sea level floods the drainage system.  This backup due to elevated sea level increased the opportunity for flooding, especially given that the ground was already saturated and therefore unable to absorb more rainwater.

Figure 38. Rainfall gauge locations on Oahu.

Table 4. Maximum high tide on major storm days for Honolulu (www.almanac.com).

            Although the storms of late 2003 – early 2004 did not cause any significant damage to Waikiki, they did cause considerable damage elsewhere in Honolulu. Mapunapuna, an industrial section of Honolulu, was flooded several times during this period of time.  Parts of Mapunapuna were under as much as several feet of water and mud resulting in significant damage to many businesses.  During high tide, even without heavy rainfall, the ocean can back up onto streets in Mapunapuna through the drainage system.  Sinking soil in the area has also put some areas below sea level further exacerbating the situation.  Damage alone from the December 7^th to 8^th storm induced by flooding in Mapunapuna was estimated at $20 million.

           Sea level change due to global warming has already been attributed to relocation for some Pacific Islanders. In 2002, rising sea levels in Vanuatu have forced the relocation of an entire village. This has been described as the first case in the world of the formal displacement of an entire human population due to global warming. More than 100 residents of Tegua Island had to abandon their settlement for higher ground after major flooding made their village uninhabitable. This could be just the beginning of a trend in the region. There are now some 2000 people on the Carteret Islands off Papua New Guinea planning to move to Bougainville Island because of similar flooding problems.

Read more about the situation in Vanuatu here.