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Guam Hazard Mitigation Plan - Western States Seismic Policy Council

Guam Hazard Mitigation Plan - Western States Seismic Policy Council

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SECTIONFIVERisk AssessmentThe one recorded event of flooding from stormwater runoff on <strong>Guam</strong> occurred in August 1984;the flooding occurred after a rainfall of 15.18 inches was recorded. The Governor of <strong>Guam</strong>issued an emergency declaration that noted that flooding in the Fineli Beach Area of Agatoccurred as a result of stormwater runoff from higher areas.Many urban storm drains are overwhelmed if rainfall exceeds a rate of 1 inch per hour. Suchrainfall rates can occur on <strong>Guam</strong> during strong monsoon surges and tropical cyclones. Rainfallrates during typhoons and super typhoons often exceed 4 to 5 inches of rainfall per hour at theirpeak of rainfall intensity. Poor storm drain conveyance in conjunction with storm surges canexacerbate coastal flooding. No street flooding data are readily available for <strong>Guam</strong>. In built-upareas generally subjected to coastal flooding, street flooding and coastal flooding are generallyinseparable events.Flash Flooding: Flash floods in <strong>Guam</strong> are poorly documented; no comprehensive record ofthese events in <strong>Guam</strong> is available. However, a few occurrences of flash flooding have beennoted. In 1999, the Red Cross responded to a flash flood, providing assistance to 14 families. Themost significant flash flooding event occurred in June 2004, during Typhoon Tinting. TyphoonTinting was still a tropical storm when it passed over <strong>Guam</strong>, but it brought record-breakingrainfall to <strong>Guam</strong>. In 24 hours 21.85 inches of rain fell on <strong>Guam</strong>; this rainfall shattered both therecord for highest single-day rainfall on <strong>Guam</strong> and the record for the highest monthly rainfall forJune. Although over 20 inches of rain fell in 24 hours over much of <strong>Guam</strong> during this event,rainfall was somewhat less over the northeastern portion of <strong>Guam</strong> where 11.33 inches fell atAndersen Air Force Base in 30 hours. These extreme levels of rainfall caused island-wide flashfloods and mudslides, road closures, and inundations.Although the flash flooding event in 2004 caused much damage for <strong>Guam</strong>, this type of event isnot common. WERI has reported that flash floods generally occur in the mountainous areas of<strong>Guam</strong> and do not result in significant damage.The distribution of rainfall on <strong>Guam</strong> is mixed; the events that produce more than 10 inches ofrain per day come from near-direct or direct passages of tropical cyclones. According to a 2004WERI report, the maximum rainfall rate observed in the eye-wall cloud of typhoons affecting<strong>Guam</strong> was measured in Typhoon Pongsona (2002) at about 0.12 inches/minute over 60 minutesor about 7.22 inches per hour. However, rainfall rates could be somewhat higher during strongertyphoons or during comparable typhoons passing over the more mountainous terrain of central<strong>Guam</strong>. This likelihood is reflected in the increasing rates with respect to return periods (Tables5-8 and 5-9). For events producing less than 10 inches per day, the source may be peripheralthunderstorms of more distant tropical cyclones, island-induced thunderstorms, or thunderstormsadvected into <strong>Guam</strong>. These events are most commonly associated with upper cold low-pressuresystems embedded in the Tropical Upper Tropospheric Trough (a common feature of thesummer troposphere in the western North Pacific Ocean). These events are more short-lived thantyphoon events, but can have higher, short-term intensities.Table 5-8Rainfall for Typhoon Eye-Wall (inches)Return Period 1-minute 5-minute 10-minute 15-minute2-year 0.09 0.45 0.90 1.355-year 0.10 0.50 1.50 1.5010-year 0.11 0.55 1.10 1.655-27

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