Glendale (PDF) - Hazard Mitigation Web Portal - State of California

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Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSECTION 9:Section 9 – LandslidesLANDSLIDESWhy are Landslides a Threat to the City of Glendale?Landslides are a serious geologic hazard in almost every state in America. Nationally, landslidescause 25 to 50 deaths each year. The best estimate of direct and indirect costs of landslidedamage in the United States ranges between $1 and $2 billion annually. In California, landslidesare a significant problem, in part because of the region’s seismic activity, and in part because theregion is underlain by weak soils and rocks, especially when saturated. Some landslides resultin private property damage, whereas other landslides impact transportation corridors, fuel andenergy conduits, and communication facilities. They can also pose a serious threat to humanlife.The San Gabriel Mountains are located in the central part of the Transverse Ranges, wherethey rise abruptly to heights of more than 7,000 feet above the valley floor (several peaks aremore than 9,000 feet high and Mount Baldy is the highest at 10,064 feet). Bounded by the SanAndreas fault system on the north and the Sierra Madre fault zone on the south, the mountainsare essentially a large block of the Earth’s crust that has been squeezed up and thrust over thevalley floor by north-south compression along the Big Bend portion of the San Andreas tectonicplate boundary. Tectonic forces that initiated the rise of the mountains are thought to havestarted about 3.5 million years ago, at a time when scientists now believe there was a change inthe relative motion of the Pacific and North American tectonic plates from strike-slip (slippinghorizontally past one another) to transpressive (oblique movement that is a combination ofstrike-slip and compression). Uplift of the mountains accelerated in mid-Pleistocene time, about500,000 years ago, and continues today (Wright, 1991). The current rate of uplift, in thecontext of geologic time, is one of the fastest in the world.The steep southern flank of the San Gabriel Mountains is deeply incised by gorges and canyonsthat drain south into the La Cañada Valley, where they have been channelized, conveying theirflows south to Verdugo Wash. The three canyons that are located mostly within city ofGlendale limits include Ward, Dunsmore, and Cooks. Several other streams draining the SanGabriel Mountains that are also channelized through the La Crescenta area and into thenorthern portion of Glendale; these include the Eagle Canyon, Pickens, Hills and WineryCanyon channels (see Plate H-1 in Appendix H). Nearly all the tributaries flowing northerlyand easterly out of the Verdugo Mountains and westerly out of the San Rafael Hills also emptyinto Verdugo Wash. South of the mountains, Verdugo Wash turns to the west-southwest andjoins the Los Angeles River near the junction of Highway 134 with the 5 Freeway (Interstate 5).Drainage from the southwestern slope of the Verdugo Mountains flows directly across thealluvial fan and into the Los Angeles River. As discussed further in Section 8, Verdugo Washhas been confined to a man-made channel through most of Glendale to reduce the potential forit to flood the city.Nearly half of the land in Glendale consists of steep hillslopes and rugged mountains. Theseareas have for the most part been preserved in their near natural state, while most of thedevelopment in the city occurs in the flat to gently sloping alluvial surfaces at the base of themountains. However, some development (primarily residential) is present in and adjacent tosteep hillsides. These areas include the canyons within the Verdugo Mountains and the SanRafael Hills, and the alluvial fans situated at the front of the San Gabriel and VerdugoMountains. Such areas are locally vulnerable to slope instability, particularly in winters ofheavy rainfall and in winters following wildfires.What is a Landslide?2006 PAGE 9 - 1
Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 9 – LandslidesLandslides are downslope movements of relatively large landmasses, either as nearly intactbedrock blocks, or as jumbled mixes of bedrock blocks, fragments, debris, and soil. Landslidesare a type of “mass wasting” which denotes any down slope movement of soil and rock under thedirect influence of gravity. The term “landslide” encompasses events such as rock falls, topples,slides, spreads, and flows, and in general, landslides can be broken down into two categories: 1)rapidly moving (generally known as debris flows), and 2) slow moving. Rapidly movinglandslides or debris flows pose the greatest risk to human life, and people living in or travelingthrough areas prone to rapidly moving landslides are at increased risk of serious injury. Slowmoving landslides can cause significant property damage, but are less likely to result in serioushuman injuries.Geologists also describe the type of movement of a landslide as either 1) translational (wheremovement occurs along a flat surface), 2) rotational (where sliding material moves along acurved surface) or 3) wedge (where movement occurs on a wedge-shaped block formed byintersecting planes of weakness, such as fractures, faults and bedding). The size of a landslideusually depends on the geology and the initial cause of the landslide. Landslides vary greatly intheir volume of rock and soil, the length, width, and depth of the area affected, frequency ofoccurrence, and speed of movement. Slow-moving landslides can occur on relatively gentleslopes and can cause significant property damage, but are far less likely to result in seriousinjuries than rapidly moving landslides. Rotational and translational slides are generally slowmoving and can be deep; whereas wedge failures generally occur suddenly and are shallow.Slumps are small rotational slides that are generally shallow.Landslides can be initiated by rainfall, earthquakes, volcanic activity, changes in ground water,disturbance and change of a slope by man-made construction activities, or any combination ofthese factors. Landslides can also occur underwater, causing tidal waves and damage to coastalareas. The potential for slope failure is dependent on many factors, including slope height, slopesteepness, shear strength and orientation of the underlying geologic unit, as well as moisturecontent. For example, water can increase the plasticity of weak clays lining joints or shears,forming planes of weakness along which a landmass can fail.For engineering of earth materials, these factors are combined in calculations to determine if aslope meets a minimum safety standard. The generally accepted standard is a factor of safety of1.5 or greater (where 1.0 is equilibrium, and less than 1.0 is failure). Natural slopes, gradedslopes, or graded/natural slope combinations must meet these minimum engineering standardswhere they impact planned homes, subdivisions, or other types of developments. Slopesadjacent to areas where the risk of economic losses from landsliding is small, such as parks andmountain roadways, are often allowed a lesser factor of safety. From an engineeringperspective, landslides are generally unstable (may be subject to reactivation), and may becompressible, especially around the margins, which are typically highly disturbed and broken.The headscarp area above the landslide mass is also unstable, since it is typically oversteepened,cracked, and subject to additional failures. Although existing landslides are not widespread inthe Glendale area, it is probable that many of the steeper hillsides do not meet the minimumfactor of safety, and slope stabilization may be needed if development reaches these areas.Failure of a slope occurs when the force that is pulling the slope downward (gravity) exceedsthe strength of the earth materials that compose the slope. They can move slowly, (millimetersper year) or can move quickly and disastrously, as is the case with debris-flows. Debris-flowscan travel down a hillside of speeds up to 200 miles per hour (more commonly, 30 – 50 miles perhour), depending on the slope angle, water content, and type of earth and debris in the flow.These flows are initiated by heavy, usually sustained, periods of rainfall, but sometimes canhappen as a result of short bursts of concentrated rainfall in susceptible areas. Burned areascharred by wildfires are particularly susceptible to debris flows, given certain soil characteristicsand slope conditions.2006 PAGE 9 - 2
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City CouncilDave Weaver, MayorRafi
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Table 4-2: STAPLEE Ranking of Actio
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NOTES:This map is intended for gene
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City of GlendaleUpdateHazard Mitiga
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Similar to Safety ElementIncludesEa
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HospitalsPartnershipsWithin City an
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Impact on CityMost significant area
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