Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentTable 5-7 Major Typhoons with Modeled Wave Heights, 1950–2010Date Name Modeled Significant Wave Height (feet)12/16/1997 Super Typhoon Paka 22.212/8/2002 Super Typhoon Pongsona 25–30Sources: WERI 1999, NWS-WFO 2008, NCDC 2010.Riverine Flooding and Stormwater Runoff: The flood events on Guam reported in theNational Climatic Data Center’s (NCDC’s) Storm Event Database are attributed to localizedheavy rainfall events from monsoon surges, tropical cyclones, or a combination of the two. Inaddition to coastal flooding (addressed in the preceding section), flooding on the island alsooccurs in riverine areas and urban areas. Generally, flood events in Guam are irregularlydocumented. Some data are available for riverine floods but only one case of flooding due tostormwater runoff has been found. Damage from flooding is not considered to be a widespreadproblem on Guam.Typhoon Chata’an was recorded to have delivered rainfall totals that exceeded 21 inches over themountainous areas of southern and central Guam. Flooding and siltation occurred in FenaReservoir during this event; as a result, there was a lack of potable water for several days. Thisstorm also caused flooding in southern Guam that resulted in numerous landslides and severeerosion along watercourses. All the stream-gauging stations operated by the U.S. GeologicalSurvey (USGS) were damaged or destroyed during these floods. Peak flows in many rivers insouthern Guam reached record levels during this storm. Four of the nine gauging sites had waterelevation recurrence intervals that were determined to be greater than the 100-year flood level. Ariver gauge on the Ylig River recorded a peak height of 23.45 feet, which was 4.82 feet higherthan the previous maximum level recorded in 1953. The Fena Reservoir level was 5.36 feetabove the spillway crest and 0.86 feet higher than the previous maximum recorded level in 1953.The northern part of Guam received less than 10 inches of rain, and the central part of the islandreceived 10 to13 inches. Recorded rainfall totals were determined to be less than what wasactually received because severe winds caused water to be sucked out of gauges or the gaugesdid not catch the rain well when the rainfall was occurring along a horizontal plane due to severewinds.Few direct historical accounts of flooding on Guam exist for events before 2002, so the mostuseful historical information about floods on Guam can be obtained by examining historicalrainfall events. Large rainfall events on Guam are generally attributed to tropical cyclones thatmove slowly across the island. In 1976, Typhoon Pamela dropped over 27 inches of rain in 24hours as it moved across the island at 7 mph. Over half of the heaviest rain events on Guamoccur from weak tropical cyclones or monsoon surges. The highest recorded hourly rain rate wasfrom Super Typhoon Pongsona, which delivered 7.67 inches. It is believed that many of theintense typhoons and super typhoons that have crossed Guam, such as Typhoon Pamela andSuper Typhoon Karen, have delivered large quantities of rain that were under-recorded due torain gauge destruction, malfunction, or power outage. Most recently, Typhoon Tingtingdelivered a recorded 16 inches in a 24-hour period on June 27, 2004. Typhoon Chaba producedheavy rains with a peak 24-hour total of 9.05 inches.5-26