Guam Hazard Mitigation Plan - Western States Seismic Policy Council

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SECTIONFIVERisk AssessmentPrevious OccurrencesThe most destructive winds on Guam have occurred during typhoons and super typhoons. DuringGuam’s most intensive storms, wind-measuring devices tend to fail. However, the strongest windgust experienced in recent history on Guam is estimated to be about 200 mph during SuperTyphoon Karen. Table 5-14 shows the highest recorded or estimated wind speeds on the island.Table 5-14 High Wind Speeds Recorded or Estimated during Typhoons, 1950–2010Typhoon Name Date Recorded or Estimated Wind SpeedsSuper Typhoon Lola 11/16/1957 97 mph gustsSuper Typhoon Karen 11/11/1962 estimated 200 mphTyphoon Olive 4/29/1963 100 mph gustsTyphoon Wendy 7/11/1963 57.6 mph sustainedTropical Storm Ora 11/23/1968 89 mph gustsTyphoon Pamela 5/21/1976 159 mph gustsTyphoon Kim 11/8/1977 89 mph gustsTyphoon Betty 10/31/1980 91 mph gustsTyphoon Bill 11/12/1984 97.9 mph gustsTyphoon Roy 1/12/1988 113 mph gustsTyphoon Russ 12/20/1990 128 mph gustsSuper Typhoon Yuri 11/27/1991 115 mph gustsTyphoon Omar 8/28/1992 121 mph sustained, 150 mph gustsTyphoon Brian 10/21/1992 75 mph sustained, 92 mph gustsTyphoon Hunt 11/18/1992 75 mph sustainedTyphoon Gay 11/23/1992 98 mph sustained, 121 mph gustsTyphoon Dale 11/8/1996 104 mph sustainedSuper Typhoon Paka 12/16/1997 est. 145 mph sustainedTyphoon Chata’an 7/5/2002 85-90 mph sustained, 105 mph gustsTyphoon Halong 7/10/2002 >100 mphSuper Typhoon Pongsona 12/8/2002 115 mph sustained, 150 mph gustTyphoon Tingting 6/27/2004 51 mph sustained, 66 mph gustTyphoon Chaba 8/21/2004 58 mph sustained, 79 mph gustTyphoon Nabi 08/31/2005 43 mph sustained, 63 mph gustSources: WERI 1999; HS/OCD 2003; NCDC 2011; NWS-WFO 2011.Probability of Future EventsModeling of the recurrence intervals for typhoon-induced sustained wind speeds shows that75 mph wind speeds occur every 4.1 years, 115 mph wind speeds occur every 16.5 years,150 mph wind speeds occur every 64 years, and 173 mph wind speeds occur every 175 years.5.3.12 Slope FailureSlope failure can lead to a variety of subhazards. For the island of Guam the concerns are withlandslides, mudslides, and post-fire debris flows.5-40
SECTIONFIVERisk AssessmentNatureLandslide: Landslides are the dislodging and falling of a mass of soil or rocks along a slopedsurface; the dislodged mass itself is also referred to as a landslide. Landslides can be earthquakeinducedor non-earthquake-induced. Earthquake-induced landslides occur as a result of groundshaking. The most common earthquake-induced landslides include shallow rock falls, disruptedrock slides, and disrupted slides of earth and debris. Non-earthquake-induced landslides mayinvolve a wide range of combinations of natural rock, soil, or artificial fill. The susceptibility ofhillside and mountainous areas to non-earthquake-induced landslides depends on variations ingeology, topography, vegetation, and weather. These landslides may also occur due toindiscriminate development on sloping ground or the creation of cut-and-fill slopes in areas ofunstable or inadequately stable geologic conditions. Non-earthquake-induced landslides oftenoccur as a result of intense or prolonged precipitation that can saturate slopes and cause failures.Mudslide: Mudslides are another type of soil failure; mudslides are defined as flows of rivers ofliquid mud down a hillside. They occur in relatively steep areas of clay when soils becomesaturated and moderate to heavy rain is occurring. If no brush, tree, or ground cover is present tohold the soil, mud will form and flow down the slope.Post-Fire Debris Flow: Post-fire debris flows are defined as fast-moving, highly destructiveflows of rain, water, rock, and soil within a burned area and downstream of that area. They aremost common in the 2 years after a fire and are usually triggered by heavy rainfall.The threats of erosion, flooding, and debris flows are significantly increased by the followingprocesses:• Reduced infiltration and increased runoff: A fire’s consumption of vegetative cover increasesthe exposure of the soil surface to raindrop impact. Soil-heating destroys the organic matterthat binds the soil together. Extreme heating may also cause the development of waterrepellant,or “hydrophobic,” soil conditions, which further reduce infiltration.• Changes in hill slope conditions: Fires remove obstructions to overland flow, such as trees,downed timber, and plants. The removal of these obstructions can increase flow velocity andtherefore erosive power. Increased sediment movement also fills depressions, reducingstorage capacity and further contributing to increased velocity and volume of flow. Thesefactors combine to allow more of the watershed to contribute flow to the flood at the sametime, and this combination of factors increases the volume of the flood.• Changes in channel conditions: Increased overland flow and sediment transport result inincreased velocity and volume of flow in defined channels. These conditions increasechannel erosion and peak discharges.The occurrence of erosion, floods, and debris flows in burned areas also depends on precipitationintensity—storms with high intensity are more likely to initiate the processes described aboveand result in flood events. Also, easily eroded types of soils facilitate changes in hill slopeconditions and increase the volume of runoff.In extreme situations, the conditions described above combine to form a post-fire debris flow.These flows are often the most destructive events resulting from heavy rainfall in fire-affectedareas. They occur with little warning, carry vast quantities of rock and other material, and strikeobjects with extreme force. Because of their viscosity and density, debris flows can move orcarry away objects as large as vehicles and bridges, and they can travel great distances down5-41
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ReferencesTABLE OF CONTENTS7.2.4 HM
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List of Tables, Figures, and Append
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List of Tables, Figures, and Append
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Acronyms and Abbreviations°F degre
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Acronyms and AbbreviationsNCTSNFIPN
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AcknowledgementsRay Calvo, Guam Env
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1. Section 1 ONE PrerequisitesSECTI
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2. Section 2 TW O Backgrou ndSECTIO
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SECTIONTWOBackgroundanalyses should
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3. Section 3 THR EE Planning Pro ce
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SECTIONTHREEPlanning Process Docume
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4. Section 4 F OUR Island D escript
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SECTIONFOURIsland DescriptionThe El
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7. Section 7 SEVEN Plan Mainten an
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SECTIONSEVENPlan Maintenance Proces
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SECTIONEIGHTReferences8. Section 8
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SECTIONEIGHTReferencesGovernment of
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SECTIONEIGHTReferencesGuam Departme
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SECTIONEIGHTReferencesLautner, Beth
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SECTIONEIGHTReferencesPacific ENSO
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SECTIONEIGHTReferencesUSGS. 2004.
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Appendix AAdoption Resolution
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Appendix BFEMA Crosswalk
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STANDARD STATE HAZARD MITIGATION PL
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Appendix CDefinitions
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Appendix CDefinitions100-hundred ye
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Appendix CDefinitionsConstruction o
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Appendix CDefinitionsconcept of dis
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Appendix CDefinitionsthat significa
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Appendix CDefinitionsInfestations m
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Appendix CDefinitionsPrivate activi
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Appendix CDefinitionsSpecial events
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Appendix CDefinitionsflooding, all
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Appendix DFigures
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Appendix EEssential Facilities, Maj
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Appendix FVulnerability and Potenti
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Appendix GPlan Maintenance Document
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GOVERNMENT OF GUAM CERIFICATION:App
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