Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentHigh surf events can lead to strong rip currents and drownings and to coastal run-up, inundation,coastal erosion, and property damage. Run-up refers to the vertical height of the saltwater onwhat is normally dry land and inundation refers to the inland distance of the saltwater. High surf(that triggers advisories) on Guam is defined as 9 feet or greater on north, west, and southexposures and 12 feet or greater on eastern exposures. Fifteen (15) feet on any exposure triggersa high-surf warning. Advisory events can last from 2 to 10 days (average duration is 4 days) andwarning events can last from 1 to 3 days. Worst conditions usually occur during new or fullmoons. The number of days of high surf are not as critical as the number of high-surf events forproperty damage, because it only takes one short period of flooding to cause property damage.Duration is important for impacts on coastal erosion. Table 5-11 shows the estimated monthlyfrequency of high-surf events requiring advisories or warnings.Table 5-11Estimated Monthly Frequency of High Surf Event Advisories/WarningsJan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual3/1 3/1 3/1 1/0 0/0 0/0 1/0 2/0 3/1 3/1 2/1 3/1 24/7Super Typhoon Paka, which passed directly over Guam, produced waves ranging from 6 to 30feet along the northern coast, the entire western coast, and the entire southern coast of Guam.Super Typhoon Pongsona produced waves that were estimated to have been 25 to 30 feet on thehigh sea cliffs of northeast Guam.Several occurrences are documented in which the rough seas associated with typhoons that haveaffected the island have damaged sewage outfall structures, which channel sewer water (withsolids removed) to the open ocean. These events have caused treated sewage to drain into the seaat locations much closer to the coastline than the locations of the outfall structures. Damage tooutfall pipes occurred during Typhoon Pamela in 1976, Typhoon Dale in 1996, and SuperTyphoon Paka in 1997.Ocean swells caused by monsoon surges can generate high seas and surf as high as 18 to 30 feet.These waves affect the western coast of Guam and have been known to cause coastal erosion andprevent ships from entering or leaving Apra Harbor for long periods. A large wave eventbetween August 11 and 13, 1974, caused by a strong monsoon surge resulted in the sinking of a700-foot, 40,000-ton passenger liner at the mouth of Apra Harbor. This 3-day high-surf hazardresulted in more than $4 million in damage.The high surf that struck Guam during Typhoon Andy in 1982 and Typhoon Dale in 1996 wasbelieved to have been produced by a combination of monsoon-surge-generated ocean swells andswells generated before the typhoons arrived in Guam. The high surf from both of these stormscaused large amounts of coastal erosion.Probability of Future EventsAs discussed above, high waves on Guam are produced by multiple climatic events. High-waveevents with a recurrence interval of less than 20 to 25 years that affect the reefs and open bays onthe western side of the island are generally produced by monsoon surges. High waves in thissame area with a return period that is greater than 25 years are generally generated by typhoonswith a wind intensity of 125 mph or greater. Table 5-12 illustrates the probability, as calculatedby WERI (1999), for waves to affect Guam.5-33