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

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Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 9 – LandslidesTable 9-1: General Slope Instability Potential within the City of GlendaleAreaSan GabrielMountainsSan RafaelHills (north ofHighway 134)Geologic ConditionsSteep to extremely steep rockslopes, most in excess of 40 degrees;Highly fractured, sheared, faulted,and crushed crystalline bedrock;Soils and loose debris in tributarydrainages;Stream terrace deposits along majordrainage channels;No known landslides in Glendale.Moderate to very steep rock slopes,most in excess of 26 degrees, manyin excess of 40 degrees;Highly fractured and weatheredcrystalline rock;Soils and loose debris in tributarydrainages and swales;Several small existing landslides.Types of PotentialSlope InstabilityMost Probable:Rockfalls, soil slips on steep slopes, soilslumps on the edges of active streamchannels, small to large debris flows.Less Probable:Large, deep-seated landslides.Most Probable:Soil slips and slumps on moderate to steepslopes and in drainage swales, small debrisflows, small slides or rockfalls, surficialsoil failures on steep man-made slopes.Less Probable:Large, deep-seated landslides.VerdugoMountainsMajor DrainageChannels –Verdugo WashValley PlainModerately steep to extremely steeprock slopes, most between 26 and 40degrees, with some slopes steeperthen 40 degrees;Highly fractured, sheared, faulted,and crushed crystalline bedrock;Soils and loose debris in tributarydrainages;A few remnant stream terracedeposits along major drainagechannels;Several small existing landslides;Rockfalls common according to R.T.Frankian & Associates (1968).Gentle to moderate sloping channelwalls with steeper channel banks(26-40 degrees) in a few isolatedareas;Poorly bedded Holocene alluviumconsisting of silt, sand and gravel,with coarse sand, gravel andboulders near the mountain front;No mapped landslides.Very gentle slopes, typically about10 degrees or less;Poorly bedded Holocene andPleistocene alluvium consisting ofsilt, sand and gravel;No mapped landslides.Most Probable:Soil slips and slumps on moderate to steepslopes and in drainage swales, small debrisflows, small slides or rockfalls, surficialsoil failures on steep man-made slopes.Less Probable:Large, deep-seated landslides.Slope instability generally not an issue.Slope instability generally not an issue.2006 PAGE 9 - 11
Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 9 – LandslidesLifelines and Critical Facilities:Lifelines and critical facilities should remain accessible, if possible, during a natural hazardevent. The impact of closed transportation arteries may be increased if the closed road orbridge is critical for hospitals and other emergency facilities. Therefore, inspection and repair ofcritical transportation facilities and routes is essential and should receive high priority. Losses ofpower and phone service are also potential consequences of landslide events. Due to heavyrains, soil erosion in hillside areas can be accelerated, resulting in loss of soil support beneathhigh voltage transmission towers in hillsides and remote areas. Flood events can also causelandslides, which can have serious impacts on gas lines that are located in vulnerable soils.Landslide Mitigation Activities:Landslide mitigation activities include current mitigation programs and activities that are beingimplemented by local or city organizations.All proposed development projects require a site-specific geotechnical evaluation of any slopesthat may impact the future use of the property. This includes existing slopes that are to remain,and any proposed graded slopes. The investigation typically includes borings to collectgeologic data and soil samples, laboratory testing to determine soil strength parameters, andengineering calculations. Numerous soil-engineering methods are available for stabilizingslopes that pose a threat to development. These methods include designed buttresses (replacingthe weak portion of the slope with engineered fill); reducing the height of the slope; designingthe slope at a flatter gradient; and adding reinforcements such as soil cement or layers ofgeogrid (a tough polymeric net-like material that is placed between the horizontal layers of fill).Most slope stabilization methods include a subdrain system to remove excessive ground waterfrom the slope area. If it is not feasible to mitigate the slope stability hazard, building setbacksare typically imposed.For debris flows, assessment of this hazard for individual sites should focus on structureslocated or planned in vulnerable positions. This generally includes canyon areas; at the toes ofsteep, natural slopes; and at the mouth of small to large drainage channels. Mitigation of soilslips, earthflows, and debris flows is usually directed at containment (debris basins), or diversion(impact walls, deflection walls, diversion channels, and debris fences). A system of baffles maybe added upstream to slow the velocity of a potential debris flow. Other methods includeremoval of the source material, placing subdrains in the source area to prevent pore waterpressure buildup, or avoidance by restricting building to areas outside of the potential debrisflow path.There are numerous methods for mitigating rock falls. Choosing the best method depends onthe geological conditions (i.e., slope height, steepness, fracture spacing, bedding orientation),safety, type and cost of construction repair, and aesthetics. A commonly used method is toregrade the slope. This ranges from locally trimming hazardous overhangs, to completelyreconfiguring the slope to a more stable condition, possibly with the addition of benches tocatch small rocks. Another group of methods focuses on holding the fractured rock in place bydraping the slope with wire mesh, or by installing tensioned rock bolts, tie-back walls, or evenretaining walls. Shotcrete is often used on the slope face to prevent raveling in highly fracturedrock, but its primary purpose is to offer surface protection only. A third type of mitigationincludes catchment devices at the toe of the slope, such as ditches, walls, or combinations ofboth. Designing the width of the catchment structure requires analysis of how the rock will fall.For instance, the slope gradient and roughness of the slope determines if rocks will fall, bounce,or roll to the bottom. Rock slope stabilization may also include the addition of drains in orderto reduce water pressure within the slope (Wyllie and Norrish, 1996).There are a number of options for management of potential slope instability in developedhillsides.2006 PAGE 9 - 12
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City CouncilDave Weaver, MayorRafi
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Natural Hazards Mitigation PlanCity
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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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