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

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Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 9 – Landslidesincreases the amount of water flowing into landslide-prone slopes can increase landslidehazards. Broken or leaking water or sewer lines can be especially problematic, as can waterretention facilities that direct water onto slopes. However, even lawn irrigation in landslidepronelocations can result in damaging landslides. Ineffective storm water management andexcess runoff can also cause erosion and increase the risk of landslide hazards. Drainage can beaffected naturally by the geology and topography of an area. Development that results in anincrease in impervious surface impairs the ability of the land to absorb water and may redirectwater to other areas. Channels, streams, ponding, and erosion on slopes all indicate potentialslope problems.Road and driveway drains, gutters, downspouts, and other constructed drainage facilities canconcentrate and accelerate runoff flow. Ground saturation and concentrated velocity flow aremajor causes of slope problems and may trigger landslides.Changes in Vegetation:Removing vegetation from very steep slopes can increase landslide hazards. Areas thatexperience wildfire and land clearing for development may have long periods of increasedlandslide hazard. Also, certain types of non-native ground covers require extensive irrigation toremain green. As a result, clearing and replacement of native ground covers with non-nativecovers can lead to an increase in slope failures.Landslide Hazard AssessmentHazard Identification:Identifying hazardous locations is an essential step towards implementing more informedmitigation activities. Evidence of past slope failures are found throughout the mountain andfoothill regions of the City of Glendale. The crystalline rock of the San Gabriel Mountains,weakened by fracturing, shearing, and crushing along numerous fault zones, particularly nearthe range front, combined with the moderate to extremely steep slopes that have resulted fromrapid uplift of the mountains, are important elements that create the setting for the developmentof slope failures. Similar conditions are present in the Verdugo Mountains and the San RafaelHills, where rocks are highly weathered and slope gradients of 30 degrees or steeper arecommon.Significantly, however, areas of gross instability such as large deep-seated landslides have notbeen mapped in the Glendale area, primarily because the highly fractured crystalline rocks thatunderlie the San Gabriel and Verdugo Mountains and the San Rafael Hills rarely fail as largecohesive units. Three small landslides (Qls) of probable Holocene age have been mapped on theslopes of the Verdugo Mountains and the San Rafael Hills. Because the bedrock in these areas ishighly fractured and weathered, the slides consist of small blocks and rock fragments ratherthan large cohesive masses. These landslides are shown on Plates H-2 and H-11. Numerousother smaller landslides have also occurred in the area, but their size is too small to show on themaps that accompany this report. Large prehistoric landslides have been mapped in the SanGabriel Mountains just to the east of the city, but not in the Glendale area. The distribution ofexisting landslides in the Glendale area and vicinity was compiled from various publications,including Morton and Streitz, (1969), Crook et al. (1987), and Dibblee, (1989a, 1989b, 1991a,1991b, 2002).Areas of surficial instability are common along the steep slopes and canyons of the San GabrielMountains, Verdugo Mountains and San Rafael Hills. Unfortunately detailed maps showingprevious sites of surficial slope failures, such as small landslides, slumps, soil slips, and rockfallshave not been compiled or published for the Glendale area. However, an unpublishedengineering geology report records several talus rockfalls on steep slopes and roadcuts in the2006 PAGE 9 - 7
Natural Hazards Mitigation PlanCity of Glendale, CaliforniaSection 9 – LandslidesVerdugo Mountains (R. T. Frankian & Associates, 1968). The common occurrence of rockfallscan also be inferred by the abundant talus at the base of steep slopes and in canyons of the SanGabriel Mountains.The Southern California Area Mapping Project (SCAMP), a cooperative effort between the USGeological Survey (USGS) and the California Geological Survey (CGS), has produced a series ofDebris-Flow Occurrence Maps, at a scale of 1:100,000, that predict in a general way areas thatwill be prone to debris flows in normally vegetated hillsides (SCAMP, 2001). The maps arebased on their studies of recent El Nino events, specifically relating the relationships betweenrainfall thresholds, terrain, and past debris flow events. Their studies indicate that in uplandareas underlain by sedimentary rock and fractured crystalline rock (such as that found in themountains of Glendale), essentially all past debris flows have occurred on slopes with gradientsof 26 degrees or steeper. The mapped debris flow susceptibility areas in the San GabrielMountains include most slopes steeper than 26 degrees, but do not include the heads of thelarge alluvial fans at the base of the mountains because the flood control dams and debris basinsthat have been built in these areas are thought to be adequate to contain flows from unburnedareas.However, flows can overwhelm flood control structures during periods of extreme rainfall on arecently burned hillside. For instance, during winters of exceptional rainfall (such as 1934,1969, 1978, and 1980), debris flows caused widespread property damage and loss of life incommunities in and near the base of the San Gabriel Mountains, with areas below burnedwatersheds receiving the bulk of the damage. For example, in November 1933, there was a largefire in the Montrose-La Crescenta area that burned more than 5,000 acres. Then, on January 1,1934, intense rainfall fell on the same area that had burned. La Crescenta and Glendale receivedthe brunt of the damage. Several people died, swept away by debris-ladden flows thatovertopped the canyons in the area. Streets were clogged with debris, and several bridges werewashed out (see Plate H-9). In 1978, several canyons within burned watersheds near theGlendale area overtopped their debris basins (Davis, 1980). These canyons include ZachauCanyon located north of Sunland, Shields Canyon north of La Crescenta, and Rubio Canyonnorth of Altadena. In 1980, the Rubio basin again overflowed, partially inundating one homeand threatening several others (Davis, 1980). Therefore, if the right conditions are met, such ashigh rainfall within burned watersheds, the possibility that debris flows will overtop basins inthe Glendale area cannot be precluded.A recent detailed study of burned watersheds (including in the San Gabriel Mountains duringand after the 1997-1998 winter rains) indicate that less than half of the drainage basinsproduced debris flows, although the debris flows that did occur were most frequently inresponse to the initial heavy rainfall. In addition to rainfall and slope steepness, the studyhighlights the many other factors that contribute to the formation of post-fire debris flows,including the underlying rock type, the shape of the drainage basin, and the presence or absenceof water-repellent soils. The goal of these studies is a better understanding of the processes andconditions that generate this hazard, an understanding that is needed in order for communitiesto make appropriate decisions on public safety and slope mitigation (Cannon, 2001).Vulnerability and Risk:Vulnerability assessment for landslides will assist in predicting how different types of propertyand population groups will be affected by a hazard. Data that includes specific landslide-proneand debris flow locations in the city can be used to assess the population and total value ofproperty at risk from future landslide occurrences.The potential for slope failure is dependent on many factors and their interrelationships. Someof the most important factors include slope height, slope steepness, sheer strength, andorientation of weak layers in the underlying geologic unity, as well as pore water pressures.2006 PAGE 9 - 8
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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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HospitalsPartnershipsWithin City an
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Impact on CityMost significant area
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