Guam Hazard Mitigation Plan - Western States Seismic Policy Council

SECTIONFIVERisk AssessmentTable 5-8Rainfall for Typhoon Eye-Wall (inches)Return Period 1-minute 5-minute 10-minute 15-minute15-year 0.12 0.60 1.20 1.8025-year 0.13 0.65 1.30 1.9550-year 0.14 0.70 1.40 2.10100-year 0.16 0.80 1.60 2.40Rainfall amounts in inches for Typhoon Eye-Wall occurrences for the designated time periods and the designatedreturn periodsTable 5-9Rainfall for Non-Tropical Cyclone Thunderstorms (inches)*Return Period 1-minute 5-minute 10-minute 15-minute2-year 0.10 0.50 0.89 1.255-year 0.12 0.60 1.07 1.5010-year 0.14 0.70 1.24 1.7415-year 0.15 0.75 1.33 1.8625-year 0.16 0.80 1.42 1.9950-year 0.18 0.90 1.60 2.24100-year 0.20 1.00 1.78 2.49Rainfall amounts in inches for Non-Tropical Cyclone Thunderstorms for the designated time periods and thedesignated return periods.*Table 5-9 slightly modified based on a personal communication with Dr. Bill Merkel and Dr. Merkel’s draft Rainfall-Frequency tables for Guam and the CNMI.Probability of Future EventsCoastal Flooding: Coastal flooding on Guam is mainly caused by storm surges associated withtropical storms. On average, three tropical storms and one typhoon pass within 180 nauticalmiles of Guam each year.Riverine Flooding and Stormwater Runoff: On Guam, riverine flooding is typically caused byprolonged periods of rainfall from slow-moving tropical cyclones or monsoon surges during thewet season (June – December). As noted above, on average, three tropical storms and onetyphoon pass within 180 nautical miles of Guam each year.No methodology has been employed to quantitatively determine the frequency of flooding onGuam due to stormwater runoff. To a certain extent, the probability for flooding due tostormwater runoff can be based on the designed conveyance capacity of a stormwater facility andthe ability of the system to prevent the settling of sediments at drains; however, no detailedanalysis of this nature has been conducted for Guam.Flash Flooding: High levels of rainfall are common on Guam, but flash floods require highlevels of rainfall in very short periods. Flash flooding is most likely to occur during the wetseason (June – December), when westerly moving storm systems bring heavy showers or steadyand sometimes torrential rain.5-28