Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmentof a tropical cyclone when it makes landfall, and the amount of time a tropical cyclone will taketo make landfall, but many exceptions and errors can occur in forecasting for a tropical cyclone.The disastrous effect of tropical cyclones on islands in the western North Pacific Ocean can besubclassified into several hazards causing widespread damage. Each of the following hazardsthat may be associated with a tropical cyclone is addressed separately and can be found in thefollowing sections.• Coastal Erosion (Section 5.3.1)• Flooding (Section 5.3.5)• High Surf (Section 5.3.7)• Salt Spray (Section 5.3.10)• Severe Wind (Section 5.3.11)• Slope Failure (Section 5.3.12)LocationAll of Guam is susceptible to a tropical cyclone. Most tropical cyclones that pass near Guam aremoving in a westward direction. Sixty percent have approached Guam from the east through thesoutheast, 19 percent have approached from the southeast through the south, and 7 percent haveapproached from the northeast through the east.Previous OccurrencesGuam is located in an area of the western North Pacific Ocean known as “Typhoon Alley.”Thirty-three percent of the world’s cyclones develop in the immediate area around Guam. Guamhas been affected by approximately 111 tropical cyclones from 1900 to 2010. Although recordsprior to 1946 are likely incomplete, approximately 85 of these tropical cyclones, at least 61 ofwhich were typhoons or super typhoons, have made landfall onto Guam and have resulted insevere winds, heavy rainfall, or flooding. The 26 reported tropical cyclones that did not makelandfall on Guam caused high surf on Guam that generally resulted in large rescue operations,injuries, and/or fatalities. Presidential Disaster Declarations have been made for six tropicalcyclones: Typhoon Russ, Super Typhoon Yuri, Super Typhoon Paka, Typhoon Chata’an, SuperTyphoon Pongsona, and Typhoon Tingting. Historical records from 1900 to 2010 haveaccounted for 86 fatalities and 461 injuries from tropical cyclone–related and monsoon-relatedhazards.Probability of Future EventsHistorical data show 12 typhoons passing over Guam between 1923 and 2002 in which the eyeof the storm passed over the island. This methodology assumes that the most devastatingtyphoons to have struck Guam occur when the eye of a typhoon passes over the island. However,several of the typhoons with very high recorded wind speeds on Guam did not have their eyepass over the island. For instance, the eye of Typhoon Dale (1996) did not pass over Guam, but itproduced 98 mph sustained winds. Typhoon Kim in 1977, whose eye passed over Guam, hadsustained winds recorded at 89 mph.In WERI (1999), a risk assessment was conducted for the probability and magnitude of tropicalcyclones to occur on Guam primarily using the HURISK (Hurricane Risk) Model. The HURISK5-48