Fountaingrove Environmental Impact Report - City of Santa Rosa ...

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3.6 GEOLOGY, SOILS, AND SEISMICITY addition of material (surcharge) to the slope, application of foundation loads, or seismic loading. Slope shear strength can be reduced through erosion or removal of supporting material at the slope toe, increased pore water pressure within the slope, and weathering/ decomposition of supporting soils. Zones of low shear strength within slopes are generally associated with the presence of certain clays, bedding, or fracture surfaces. Strong earthquakes often cause landslides, particularly in areas already susceptible to landslides due to other factors, including the presence of existing landslide deposits. Landslides are typically a major effect of ground shaking during earthquakes with magnitudes of 5 and greater, especially where earth materials are water-saturated. Failure of steep slopes, collapse of natural-stream banks, and reactivation of existing landslides may occur widely during a major earthquake. There are numerous landslides depicted on published geologic and slope stability maps in the Project vicinity. The recent trenches excavated by Giblin (2007) encountered evidence of landslides and slope instability of varying sizes and ages within the Project site. These landslides and soil instability were grouped into three categories, based mainly on the landslide age (Giblin, 2007): • An older bedrock displacement affecting much of the Project site. This displacement is likely to have occurred between 40,000 to 80,000 years before present, during seismic events. Because the displacement occurred tens of thousand of years ago and is relatively small, the potential for renewed movements is judged to be very low. • A 20- to 25-foot-thick landslide colluvial soil deposit in the swale near the north end of the Project site. This deposit, consisted of debris flow and pond sediment, and the underlying bedrock appear to be part of an ancient landslide movement. The potential for renewed movements is judged to be moderate. • Landslide deposits on the west-facing slopes adjacent to and east of Thomas Lake Harris Drive. These landslides are old, but they show evidence of recent movements (small open tension cracks). The potential for movements is judged to be high. Areas where heavy soil creep was suspected to be occurring were also observed during previous subsurface explorations, including the two drainage swales on the property. Maps and cross sectional drawings that show the landslide and soil instability areas are presented in Appendix L (Giblin, 2007). Dam Failure and Inundation The California Division of Safety of Dams has stated that it has no concerns regarding the seismic safety of the Fountaingrove dams using current seismic criteria (Giblin, 2006a). Earthquake-induced waves in lakes and bays are known as seiches. The size and behavior of a seiche would depend on a number of factors, including the magnitude of the earthquake, the water depth, and the configuration of the water body and shoreline. Strong seismic shaking from an earthquake on the Healdsburg-Rodgers Creek fault may produce a seiche in Fountaingrove Lake. If a seiche were to occur, a number of factors suggest that the amount of water overtopping the dams would be limited: 3-86 BAO\082970001
Reference: Giblin Associates, Investigation of Fault Surface Rupture Hazard Report, Plate 3, November 2007. ES032007014BAO_Fountaingrove_GeologyMap_082708_lho FIGURE 3.6-4 LOCAL GEOLOGY MAP FOUNTAINGROVE LODGE PROJECT SANTA ROSA, CALIFORNIA
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Aegis Senior Living Continuing Care
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CONTENTS Response to Comment Letter
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ACRONYMS AND ABBREVIATIONS HFC hydr
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1.0 INTRODUCTION were accepted. A p
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2.0 List of Commenters and Response
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2.0 LIST OF COMMENTERS AND RESPONSE
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28) Comment noted. 2.0 LIST OF COMM
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Page 161 and 162: 2.0 LIST OF COMMENTERS AND RESPONSE
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Page 189: Executive Summary Project The Aegis
Page 193: Mendocino £¤ 101 0 ± 5 10 Miles
Page 197: 2.0 PROJECT DESCRIPTION • Swimmin
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Page 202 and 203: 3.6 GEOLOGY, SOILS, AND SEISMICITY
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Page 221 and 222: 4.0 MITIGATION MONITORING AND REPOR
Page 235: 5.0 References California Board of
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Fountaingrove Environmental Impact Report - City of Santa Rosa ...

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