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

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Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 9 – LandslidesWhat is a Debris Flow?This type of failure is the most dangerous and destructive of all types of slope failure. A debrisflow (also called mudflow, mudslide, and debris avalanche) is a rapidly moving slurry of water,mud, rock, vegetation and debris. Larger debris flows are capable of moving trees, largeboulders, and even cars. This type of failure is especially dangerous as it can move at greatspeeds, is capable of crushing buildings, and can strike with very little warning. As with soilslips, the development of debris flows is strongly tied to exceptional storm periods of prolongedrainfall. Failure occurs during an intense rainfall event, following saturation of the soil byprevious rains.A debris flow most commonly originates as soil slip in the rounded, soil-filled “hollow” at thehead of a drainage swale or ravine. The rigid soil mass is deformed into a viscous fluid thatmoves down the drainage, incorporating into the flow additional soil and vegetation scouredfrom the channel. Debris flows also occur on canyon walls, often in soil-filled swales that do nothave topographic expression. The velocity of the flow depends on the viscosity, slope gradient,height of the slope, roughness and gradient of the channel, and the baffling effects of vegetation.Even relatively small amounts of debris can cause damage from inundation and/or impact(Ellen and Fleming, 1987; Reneau and Dietrich, 1987). Recognition of this hazard led FEMA tomodify its National Flood Insurance Program to include inundation by “mudslides.”Watersheds that have been recently burned typically yield greater amounts of soil and debristhan those that have not burned. Erosion rates during the first year after a fire are estimated tobe 15 to 35 times greater than normal, and peak discharge rates range from 2 to 35 timeshigher. These rates drop abruptly in the second year, and return to normal after about 5 years(Tan, 1998). In addition, debris flows in burned areas are unusual in that they can occur inresponse to small storms and do not require a long period of antecedent rainfall. These kinds offlows are common in small gullies and ravines during the first rains after a burn, and canbecome catastrophic when a severe burn is followed by an intense storm season (Wells, 1987).The United States Geological Survey (USGS), as part of its National Landslide HazardsProgram, is currently developing tools and methodologies to identify and quantify slopestability hazards posed by burned watersheds. Such tools will help communities withemergency planning and in dealing with post-fire rehabilitation (USGS, 2001).2006 PAGE 9 - 3
Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 9 – LandslidesHistoric Southern California Landslides:Historically, there have been many landslides in the southern California area. Landslidesrecorded in the 20 th century alone caused losses of more than $5 billion (in 2000 dollars). Manyof these landslides have occurred after particularly wet winters, others in response to strongground shaking during an earthquake. Some of the most dramatic of these cases are brieflydescribed below.1956 Portuguese Bend Landslide, Palos Verdes:Cost: $14.6 million (2000 dollars), on California Highway 14 in the Palos Verdes Hills. ThePortuguese Bend landslide is a reactivated ancient slide that began its modern movement inAugust 1956, when displacement was noticed at its northeast margin. Movement graduallyextended downslope so that the entire eastern edge of the slide mass was moving within 6weeks. By the summer of 1957, the entire slide mass was sliding towards the sea.1969 Glendora, Los Angeles County:Cost: $26.9 million (2000 dollars), in Los Angeles County. The winter of 1969 was one of thewettest on record in the southern California area. The rain caused many of the canyons at thebase of the San Gabriel Mountains to overflow, causing debris flows that damaged 175 housesin the Glendora area alone.1977-1980 Monterey Park and Repetto Hills, Los Angeles County:Cost, $14.6 million (2000 dollars); 100 houses damaged due to debris flows.1978 Bluebird Canyon, Orange County:Cost: $52.7 million (2000 dollars); 60 houses destroyed or damaged. On October 2, 1978, aportion of the Bluebird Canyon slope gave way, in great part due to the unusually heavy rains inMarch that may have contributed to initiation of the landslide. Although the 1978 slide area wasapproximately 3.5 acres, it is suspected to be a portion of a larger, ancient landslide.1979 Big Rock, California, Los Angeles County:Cost: approximately $1.08 billion (2000 dollars; rockslide causing damage to CaliforniaHighway 1.1980 Southern California slides:Cost: $1.1 billion in damage (2000 dollars). Heavy winter rainfall in 1979-90 caused damage insix southern California counties. A sequence of five days of continuous rain that started onFebruary 8 dropped more than 7 inches of water by February 14. Slope failures began todevelop by February 15 and then very high-intensity rainfall occurred on February 16. Asmuch as 8 inches of rain fell in a 6-hour period in many locations. Records and personalobservations in the field on February 16 and 17 showed that the mountains and slopes literallyfell apart on those last two days.1983 San Clemente, Orange County:Cost: $65 million (2000 dollars), California Highway 1. Litigation associated with this landslideultimately cost approximately $43.7 million (2000 dollars).1983 Big Rock Mesa, Malibu, Los Angeles County:Cost: $706 million (2000 dollars) in legal claims; 13 houses condemned and 300 morethreatened due to rockslide triggered by intense rainfall.1978-1979, 1980 San Diego County:Experienced major damage from storms in 1978, 1979, and 1979-80, as did neighboring areas ofLos Angeles and Orange County, California. One hundred and twenty landslides were reportedto have occurred in San Diego County during these 2 years. Rainfall for the rainy seasons of 78-79 and 79-80 was 14.82 and 15.61 inches (37.6 and 39.6 cm) respectively, compared to a 125-2006 PAGE 9 - 4
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City CouncilDave Weaver, MayorRafi
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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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