Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentTropical cyclones create swells that emanate from the region just outside the cyclone’s center oreye. The swells associated with a cyclone generally arrive at a location up to several days aheadof the actual storm. The ocean swell and the high waves at a location increase in size as the stormgets closer. Larger tropical cyclones produce larger swells due to the larger fetch length andwidth. For storms that pass over or near to an island, the specific locations where devastatingwaves occur depend on the direction the tropical cyclone is traveling and the track that the stormtakes.Although the hazard of high surf is often associated with tropical cyclones, high surf is notalways associated with tropical cyclones. Generally four sources other than nearby tropicalcyclones can lead to high surf:• Swells or a combination of swells and wind waves from easterly trade winds. Thesegenerally occur in winter and spring.• Swells or a combination of swells and wind waves from westerly monsoon winds. Thesegenerally occur in summer and fall.• Swells from distant winter storms near Japan.• Swells from slow-moving typhoons that are less than 300 nautical miles away.Strong monsoon surges can last from a few days to a more than 2 weeks. The persistentsouthwestern winds of a monsoon surge can produce a long fetch, generating large ocean swellsand high surf when they reach land. Swells caused by monsoon surges can combine with swellsgenerated by tropical cyclones; the result can be amplified swell sizes and higher surf. Generally,this event occurs when a large swell from a monsoon surge travels through a tropical cyclone.The western North Pacific Ocean is susceptible to large ocean swells that have been generatedfrom distant tropical cyclones that will not come near the area. Most commonly these swellsoccur with large, intense, slow-moving tropical cyclones that take a track south of Japan. Thesestorms generate large swells that can arrive as a surprise because no nearby storm is associatedwith the high surf.LocationTropical cyclones that pass north of Guam generally produce high waves on the northwest coastlines. Tropical cyclones that pass to the southwest or the west of the island can produce high surfand rough seas on the southern and western coasts. A tropical cyclone approaching from thesoutheast produces hazardous waves on the east and southeast sides of Guam. Westward-movingstorms produce the highest surf on the northeast side of the island if they pass over or to thesouth of the island. Rapidly moving tropical cyclones that pass north of Guam generally do notproduce damaging swells on the western side of Guam.Previous OccurrencesLarge ocean swells from passing and distant cyclones, monsoon surges, and trade winds haveresulted in hazardous high surf on all coasts of the island. According to the NCDC’s Storm EventDatabase, between 1993 and 2010 high surf, rip currents, and rough seas resulted in 34 deathsand 41 injuries on Guam. Five of these fatalities occurred on June 29, 2004 when three kayakswere overturned by high surf related to Typhoon Tingting, which had passed the Mariana Islandsthe day before (440 miles north-northwest of Guam).5-32