Flooding Associated with Typhoon Chata'an, July 5, 2002, Guam

Abstract

Introduction On July 5, 2002, starting at about 8 a.m., the southern half of the eye of Typhoon Chata'an passed directly over the northern part of the island of Guam. Data collected on Guam indicate that the typhoon had sustained winds of 85 to 90 miles per hour (mi/hr) with gusts of up to 115 mi/hr (Charles Guard, National Weather Service, written commun., 2003). Storm rainfall totals exceeded 21 inches (in.) over the mountainous areas in south-central Guam. During the peak of the storm, rain fell at rates of up to 6.48 inches per hour (in/hr). Because of the damage caused by Typhoon Chata'an, the President signed a major disaster declaration on July 6, 2002. Damages associated with Typhoon Chata'an, while considered moderate relative to other storms that have affected Guam, amounted to several tens of millions of dollars. In excess of 1,000 single-family and multi-family homes were either extensively damaged or destroyed. Electrical power was out for several days over most of the island and no potable water was available through public distribution systems (Federal Emergency Management Agency, 2002). The extreme rainfall led to flooding in southern Guam and caused numerous landslides and severe erosion along water courses. The most significant evidence of these effects could be found in the Fena Valley Reservoir, where elevated sediment concentrations made the water unsuitable for use as a domestic water supply for several days. During normal operation, Fena Valley Reservoir supplies most of the drinking water for the military and some of the general public in southern Guam. All of the stream-gaging stations operated by the U.S. Geological Survey (USGS) on Guam were damaged to some extent during the flood and three of the stations were totally destroyed. Peak flows in many rivers in southern Guam reached record levels during Typhoon Chata'an. New record peak stages and/or flows of record occurred at 14 of 15 sites where the USGS has collected data. In some areas, the magnitude of flood peaks exceeded previous records significantly. Peak flows had recurrence intervals of 80 years or more at 9 of the 13 sites where sufficient data were available to make the computations. Four of the 9 sites had recurrence intervals that were determined to be greater than 100 years. In this fact sheet, storm rainfall totals and maximum rainfall totals for durations of 1-, 3-, 6-, and 12-hours are summarized for 12 rain gages on Guam. Peak stages and/or flows were computed at 15 USGS streamflow-gaging stations and recurrence intervals for the peaks determined. Rainfall and streamflow-gaging stations operated by the USGS on Guam are supported by funding provided by numerous agencies including the U.S. Navy, the U.S. Army Corps of Engineers (USACE), and the University of Guam through the Water and Environmental Research Institute (WERI). The USGS Office of Surface Water, as part of a national program to document the effects of extreme floods in the United States, provided funding to support the preparation of this fact sheet.