Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentThe one recorded event of flooding from stormwater runoff on Guam occurred in August 1984;the flooding occurred after a rainfall of 15.18 inches was recorded. The Governor of Guamissued 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 Guam 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 Guam. In built-upareas generally subjected to coastal flooding, street flooding and coastal flooding are generallyinseparable events.Flash Flooding: Flash floods in Guam are poorly documented; no comprehensive record ofthese events in Guam 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 Guam, but it brought record-breakingrainfall to Guam. In 24 hours 21.85 inches of rain fell on Guam; this rainfall shattered both therecord for highest single-day rainfall on Guam and the record for the highest monthly rainfall forJune. Although over 20 inches of rain fell in 24 hours over much of Guam during this event,rainfall was somewhat less over the northeastern portion of Guam 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 Guam, this type of event isnot common. WERI has reported that flash floods generally occur in the mountainous areas ofGuam and do not result in significant damage.The distribution of rainfall on Guam 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 affectingGuam 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 centralGuam. 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 Guam. 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