Guam Hazard Mitigation Plan - Western States Seismic Policy Council

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