Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk Assessmentof rainfall, inadequate drainage design, storm surges, and/or debris blocking the storm drainconveyances. As the amount of paved surface increases in Guam due to the growth of urbanareas, the amount of stormwater runoff can increase. This increase, when combined withinadequate stormwater runoff conveyances, can lead increased flooding.Seismic forces and heavy rainfall generally propagate slope failure on Guam. Seismic forces tendto destabilize slopes and heavy rainfall can saturate the destabilized slopes and dislodge looserocks. (A detailed discussion of slope failure is provided in Section 5.3.12 [Slope Failure].)These events can result in rockslides, mudflows, and debris flows. These hazards can furtherexacerbate floods or result in the changing of floodplains.Flash Flooding: A flash flood, also a fresh water source, is the fastest-moving type of flood; thishazard can fill a normally calm area with a rushing current in a relatively small amount of time.Flash floods in Guam are caused by heavy rain that is often, but not always, associated with atropical cyclone. Flash floods occur when water falls too quickly on saturated soil or dry soil thathas poor absorption ability. This water cannot be absorbed into the soil and therefore flowselsewhere.The main defining characteristic of a flash flood is the timescale in which it develops; a flashflood generally develops in less than 6 hours. Flash flood waters also move at very fast speedsand have the power to move boulders, tear out trees, and destroy both buildings andtransportation infrastructure. During a flash flood, walls of water can reach heights of 10 to20 feet. This combination of power and suddenness makes flash floods particularly dangerous.Heavy persistent rainfall on Guam is more often associated with slow-moving tropical cyclonesthan with fast-moving storms of comparable intensity. Many of the rain events on Guam occurfrom storms that are of less-than-typhoon intensity. The heaviest rains in typhoons occur in aconcentrated area near the eye. Therefore, a typhoon needs to pass relatively close to an area forthe area to receive the heaviest rains of the typhoon. Weaker storms are less organized than atyphoon, but heavy rains can extend farther from the center and can therefore have a broadersweep of heavy squalls than a typhoon. Monsoon surges can combine or continually feedmoisture into a tropical cyclone.Rainfall is generally higher in mountainous areas than over flat terrain. For this reason, flashfloods are generally constrained to the southern half of the island of Guam. Flash floods canoccur when heavy rain (e.g., 2 inches per hour) falls for 1 hour or less heavy rain (e.g., 1 inch perhour) falls for more 2 or more hours in mountainous areas. Even rainfall of 1 inch per hour for1 hour or heavier rates of rainfall for shorter periods can cause drainage systems to overflow andforce manhole covers to pop up, a hazard to both vehicles and pedestrians.LocationCoastal Flooding: As shown on Figure D-17, the entire coast of Guam is susceptible to coastalflooding, and Apra Harbor is particularly susceptible.Riverine Flooding and Stormwater Runoff: As shown on Figure D-17, riverine floodinggenerally occurs in the southern portion of Guam, near the villages of Agat, Santa Rita, andTalofofo. Although stormwater runoff is recognized as a hazard, the areas subject to this type ofsubhazard have not been well documented to date and are not identified on FIRMs because theseevents are typically relatively small in size and extent.5-24