Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentFlash Flooding: WERI has reported that flash floods occur in the mountainous areas of Guam,but no comprehensive data are available on the locations of past occurrences of flash flooding onGuam.Previous OccurrencesCoastal Flooding: Although previous occurrences of this hazard are not well documented,coastal flooding has been a significant and recurring hazard on Guam. The combination of heavyrains, storm surges, the presence of developed urban areas at low elevations along the coast, reefstructure, and small bay entrances has produced frequent coastal flooding during tropicalcyclones.Table 5-7 shows the major typhoons that passed over Guam from 1950 to 2010, with theirmodeled wave heights. The coastal inundation from Super Typhoon Yuri in November 1991,Typhoon Omar in August 1992, Typhoon Gay in November 1992, and Super Typhoon Pongsonain December 2002 has been well documented. Typhoon Omar and Typhoon Gay caused coastalinundation of 9 and 11 feet, respectively, above mean high tide in areas that were (and still are)developed at low elevations along the western coast. Super Typhoon Yuri caused near-historicinundation on Guam’s eastern coast from Pago Bay at the northern end to the Saluglula Poolsand the Tipoco Cemetery in Inarajan at the southern end. Super Typhoon Pongsona producedmaximum coastal inundation of approximately 18 feet on coastal areas of eastern Guam. Thecoastal inundation associated with Typhoons Tingting and Chaba was estimated at 6 feet.Coastal flooding has generally occurred in southern and eastern Guam along bays that have smallrestricted entrances that focus and contain the high water generated by the storm surge, windgeneratedwaves, and large upland discharges of heavy rains that result during tropical cyclones.Inarajan Bay, Talofofo Bay, Tlig Bay, and Pago Bay experience frequent coastal flooding alongtheir adjacent uplands when tropical cyclones pass to the south of Guam. Seawater in Pago Bayis sometime driven over an elevated ridge of land to form an inland lake. During Super TyphoonYuri, several houses floated off their foundations in this temporary coastal flood lake.Table 5-7 Major Typhoons with Modeled Wave Heights, 1950–2010Date Name Modeled Significant Wave Height (feet)08/11/1951 Tropical Cyclone Marge 13.711/11/1962 Super Typhoon Karen 19.909/05/1964 Typhoon Sally 811/23/1968 Tropical Storm Ora 6.205/21/1976 Typhoon Pamela 22.611/08/1977 Typhoon Kim 12.508/17/1979 Tropical Depression Judy 310/03/1983 Tropical Storm Mac 12.611/12/1984 Typhoon Bill 17.411/27/1991 Super Typhoon Yuri 3008/28/1992 Typhoon Omar 17.510/21/1992 Typhoon Brian 10.511/23/1992 Typhoon Gay 14.55-25