3.6 - Geology, Soils, and Seismicity

City of Calistoga – Enchanted Resorts Project Draft EIR Geology, Soils, and Seismicity
Michael Brandman Associates 3.6-1 H:\Client (PN-JN)\3808\38080001\3 - Draft EIR\38080001_Sec03-06 Geology, Soils, and Seismicity.doc
3.6 - Geology, Soils, and Seismicity
3.6.1 - Introduction This section describes the existing geology, soils, and seismicity setting and potential effects from project implementation on the site and its surrounding area. Descriptions and analysis in this section are based on geotechnical reports and addenda prepared by RGH Consultants, which are included in this EIR as Appendix G.
3.6.2 - Environmental Setting Regional Geology
Napa County is located within the California Coast Range geomorphic province. This province is a geologically complex and seismically active region characterized by sub-parallel, northwest-trending faults, mountain ranges, and valleys. The oldest bedrock units are the Jurassic-Cretaceous Franciscan assemblage, and the Jurassic-Cretaceous Great Valley sequences sediments originally deposited in a marine environment. Subsequently, younger rocks such as the Tertiary-age Sonoma Volcanics group; the Plio-Pleistocene-age Clearlake Volcanics; and sedimentary rocks such as the Guinda, Domingine, Petaluma, Wilson Grove, Cache, Huichica and Glen Ellen formations were deposited throughout the province. Extensive folding and thrust faulting during late Cretaceous through early Tertiary geologic time created complex geologic conditions that underlie the highly varied topography of today. In valleys, the bedrock is covered by alluvial soils. The project site is located on the eastern flank of the southern Mayacamas Mountains overlooking the northern Napa Valley.
Seismicity
The term seismicity describes the effects of seismic waves that are radiated from an earthquake as it ruptures. While most of the energy released during an earthquake results in the permanent displacement of the ground, as much as 10 percent of the energy may dissipate immediately in the form of seismic waves. The probability of one or more earthquakes of magnitude 6.7 (Richter scale) or higher occurring in the project area has been evaluated by the United States Geological Survey (USGS). Based on the results of the USGS evaluation, there is a 62-percent likelihood that such an earthquake event will occur in the Bay Area between 2003 and 2032. The faults with the greater probability of movement with a magnitude of 6.7 or higher earthquake are the Hayward Fault at 27 percent, the San Andreas Fault at 21 percent, and the Calaveras Fault at 11 percent. A discussion of faulting and seismic hazards is provided below.
Faulting Faults form in rocks when stresses overcome the internal strength of the rock, resulting in a fracture. Large faults develop in response to large, regional stresses operating over a long time, such as those stresses caused by the relative displacement between tectonic plates. According to the elastic rebound theory, these stresses cause strain to build up in the earth’s crust until enough strain has built up to exceed the strength along a fault and cause a brittle failure. The slip between the two stuck plates or
City of Calistoga – Enchanted Resorts Project Geology, Soils, and Seismicity Draft EIR
3.6-2 Michael Brandman Associates H:\Client (PN-JN)\3808\38080001\3 - Draft EIR\38080001_Sec03-06 Geology, Soils, and Seismicity.doc
coherent blocks generates an earthquake. Following an earthquake, strain will build once again until the occurrence of another earthquake. The magnitude of slip is related to the maximum allowable strain that can be built up along a particular fault segment. The greatest buildup in strain that is due to the largest relative motion between tectonic plates or fault blocks over the longest period of time will generally produce the largest earthquakes. The distribution of these earthquakes is a study of much interest for both hazard prediction and the study of active deformation of the earth’s crust. Deformation is a complex process, and strain caused by tectonic forces is not only accommodated through faulting but also by folding, uplift, and subsidence, which can be gradual or in direct response to earthquakes.
Faults are mapped to determine earthquake hazards, since they occur where earthquakes tend to recur. A historic plane of weakness is more likely to fail under stress and strain than a previously unbroken block of crust. Faults are, therefore, a prime indicator of past seismic activity, and faults with recent activity are presumed to be the best candidates for future earthquakes. However, since slip is not always accommodated by faults that intersect the surface along traces, and since the orientation of stresses and strain in the crust can shift, predicting the location of future earthquakes is complicated. Earthquakes sometimes occur in areas with previously undetected faults or along faults previously thought inactive.
The Mayacama, Healdsburg-Rogers Creek, West Napa, Hunting Creek, and San Andreas are the five faults closest to Calistoga. These faults and their characteristics are summarized in Table 3.6-1.
Table 3.6-1: Fault Summary
Project Site (miles)
Healdsburg-Rogers Creek Strike-Slip Southwest 9 7.00
West Napa Normal-Oblique Southeast 16 6.50
Hunting Creek Strike-Slip Northeast 17 6.75
San Andreas Strike-Slip Southwest 30 7.75
Source: RGH Geotechnical and Environmental Consultants, 2002; California Department of Transportation, 1996.
Seismic Hazards Seismicity describes the effects of seismic waves that are radiated from an earthquake as it ruptures. While most of the energy released during an earthquake results in the permanent displacement of the ground, as much as 10 percent of the energy may dissipate immediately in the form of seismic waves.
Seismic hazards pose a substantial danger to property and human safety and are present because of the risk of naturally occurring geologic events and processes impacting human development.
Therefore, the hazard is influenced as much by the conditions of human development as by the frequency and distribution of major geologic events. Seismic hazards present in California include ground rupture along faults, strong seismic shaking, liquefaction, ground failure, landsliding, and slope failure.
Fault Rupture Fault rupture is a seismic hazard that affects structures sited above an active fault. The hazard from fault rupture is the movement of the ground surface along a fault during an earthquake. Typically, this movement takes place during the short time of an earthquake, but it also can occur slowly over many years in a process known as creep. Most structures and underground utilities cannot accommodate the surface displacements of several inches to several feet commonly associated with fault rupture or creep.
Ground Shaking The severity of ground shaking depends on several variables such as earthquake magnitude, epicenter distance, local geology, thickness, seismic wave-propagation properties of unconsolidated materials, groundwater conditions, and topographic setting. Ground shaking hazards are most pronounced in areas near faults or with unconsolidated alluvium.
Based on observations of damage from recent earthquakes in California (e.g., San Fernando 1971, Whittier-Narrows 1987, Landers 1992, Northridge 1994), ground shaking is responsible for 70 to 100 percent of all earthquake damage. The most common type of damage from ground shaking is structural damage to buildings, which can range from cosmetic stucco cracks to total collapse. The overall level of structural damage from a nearby large earthquake would likely be moderate to heavy, depending on the characteristics of the earthquake, the type of ground, and the condition of the building. Besides damage to buildings, strong ground shaking can cause severe damage from falling objects or broken utility lines. Fire and explosions are also hazards associated with strong ground shaking.
Ground Failure Ground failure includes liquefaction and the liquefaction-induced phenomena of lateral spreading, and lurching.
Liquefaction is a process by which sediments below the water table temporarily lose strength during an earthquake and behave as a viscous liquid rather than a solid. Liquefaction is restricted to certain geologic and hydrologic environments, primarily recently deposited sand and silt in areas with high groundwater levels. The process of liquefaction involves seismic waves passing through saturated granular layers, distorting the granular structure, and causing the particles to collapse. This causes the granular layer to behave temporarily as a viscous liquid, resulting in liquefaction.
Liquefaction can cause the soil beneath a structure to lose strength, which may result in the loss of foundation-bearing capacity. This loss of strength commonly causes the structure to settle or tip.
Loss of bearing strength can also cause light buildings with basements, buried tanks, and foundation piles to rise buoyantly through the liquefied soil.
Lateral spreading is lateral ground movement, with some vertical component, caused by liquefaction. In effect, the soil rides on top of the liquefied layer. Lateral spreading can occur on relatively flat sites with slopes less than 2 percent, under certain circumstances, and can cause ground cracking and settlement.
Lurching is the movement of the ground surface toward an open face when the soil liquefies. An open face could be a graded slope, stream bank, canal face, gully, or other similar feature.
Landslides and Slope Failure Landslides and other forms of slope failure form in response to the long-term geologic cycle of uplift, mass wasting, and disturbance of slopes. Mass wasting refers to a variety of erosional processes from gradual downhill soil creep to mudslides, debris flows, landslides and rock fall—processes that are commonly triggered by intense precipitation, which varies according to climactic shifts. Often, various forms of mass wasting are grouped together as landslides, which are generally used to describe the downhill movement of rock and soil.
Geologists classify landslides into several different types that reflect differences in the type of material and type of movement. The four most common types of landslides are translational, rotational, earth flow, and rock fall. Debris flows are another common type of landslide similar to earth flows, except that the soil and rock particles are coarser. Mudslide is a term that appears in non- technical literature to describe a variety of shallow, rapidly moving earth flows.
Surface Subsurface Profile
RGH Consultants evaluated the surface and subsurface characteristics of the project site. The findings are summarized on the following pages. Exhibit 3.6-1 depicts the geologic characteristics of the project site.
Surface The project site extends primarily over moderate to steeply sloping woodland on a northwest-to- southeast-trending ridge. Vegetation at the site consists mostly of fir trees with various other trees including sycamores, oaks and manzanita. Two existing structures are located adjacent to Foothill Boulevard (State Route 29/State Route 128). A partially paved and rocked access road begins at the Foothill Boulevard entrance north of the site and winds to the top of the property at the southern property boundary. Several other rough-graded routes and ungraded paths access most other areas of the remainder of the property.
In general, the ground surface at the time of site reconnaissance was moderately hard. However, soils in the area that appear hard and strong when dry will typically lose strength rapidly and settle under the loads of fills, foundations and slabs as their moisture content increases and approaches saturation.
Michael Brandman Associates
38080001 • 12/2011 | 3.6-1_proj_site_geological_characteristics.cdr
CITY OF CALISTOGA • ENCHANTED RESORTS PROJECT ENVIRONMENTAL IMPACT REPORT
Exhibit 3.6-1 Project Site Geological CharacteristicsN
O R
T H
This typically occurs because the surface soils are weak and porous. On sloping terrain (5:1 or steeper), the weak surface materials (topsoil, residual soil, colluvium) undergo a gradual downhill movement known as creep. Soil creep is inherent to hillsides in the area and its force is directly proportional to slope inclination, the soils plasticity, water content and expansion potential.
Drainage off the property is by sheet flow to four major drainage ravines that drain toward the north to the Napa River. Some of the flows are collected by the inboard roadway ditches and outlet through surface ditches to the previously mentioned ravines.
Subsurface RGH Consultants’ borings, pits, and laboratory tests indicate that the portion of the site is blanketed, in general, by topsoils and colluvial soils that range in depth from 1 to 10 feet deep. Detailed mapping of these soils was not practical, but generally, the shallow soils are located along the tops of the ridges in the upper elevations and the thicker colluvial soils in the lower elevations in areas of accumulation such as swales and ravines. The surface soils are underlain mostly by volcanic bedrock consisting of rhyolite, tuff and tuff breccia, and localized alluvial deposits overlying the volcanic bedrock. On hillsides 5:1 or steeper, the topsoil, colluvium, and residual soils typically creep. Fill soils are located at the site associated with existing access routes where cut and fill operations were performed to construct roads. These fills are not known to be engineered.
Borings located at the lowest elevations near the northern entrance encountered alluvial fan or valley alluvial deposits of silts, sands, and clays to the maximum depth explored (50 feet). Elsewhere on the site, the Sonoma Volcanics group bedrock extends from beneath the surface materials (topsoils, colluvium, residual soils, alluvial soils,) to the maximum depths explored (40 feet). The bedrock is a complex association of volcanic flow rock (rhyolite), ash fall deposits (tuff), and volcanic rubble deposits (breccia). All the bedrock materials exhibit varying degrees of weathering from slightly to highly weathered and varying degrees of fracturing from widely to closely fractured. The strength and hardness of the bedrock are characterized as complexly varied and friable to very strong and firm to very hard.
Soils
The USGS geologic maps reviewed indicate the project site is underlain by bedrock of the Sonoma Volcanics group. In the site vicinity, the Pliocene age Sonoma Volcanic group are represented to comprise rhyolitic lava flows, locally containing intercalculated rhyolitic tuff. Scattered exposures of Holocene age non-marine origin sedimentary deposits overlie the volcanics.
Mapping by the United States Soil Conservation Service has classified soil over the lower portion of this project site as belonging to the Forward series. In the upper elevations of the property, the soils belong to the Boomer-Forward-Felta complex, a mixture of three series.
The Forward series soils consist of clay to gravelly clay with low to moderate plasticity and low expansion potential. Runoff over these soils is very rapid and the hazard of erosion is high to very high depending on slope. The risk of corrosion is given as moderate for uncoated steel and high concrete. The Boomer-Forward-Felta complex is about 45 percent Boomer Series (clay to clayey silt), 35 percent Forward Series (clay to gravelly clay) and 15 percent Felta Series (gravelly clay and sandy clays). Runoff over these soils is rapid. The hazard of erosion is slight on the Boomer soils and severe on the Forward and Felta soils. The expansion potential ranges from low for the Forward Soils up to the moderate for the Boomer soils. The risk of corrosion of the Boomer and Felta soils is given as moderate for uncoated steel and generally moderate for concrete with the Forward soils identified as high.
Groundwater
Free groundwater was not observed in the test pits at the time of excavation. Groundwater seeps were noted in some of the borings at depths ranging from about 5 to 23 feet below the ground surface in alluvial deposits or near the interface of the colluvium and the bedrock. On hillsides, rainwater typically percolates through the porous topsoil and migrates downslope in the form of seepage at the interface of the topsoil and bedrock, and through fractures in the bedrock. Fluctuations in the seepage typically occur because of variations in rainfall and other factors such as periodic irrigation.
3.6.3 - Regulatory Framework State Alquist-Priolo Earthquake Fault Zoning Act In response to the severe fault rupture damage of structures by the 1971 San Fernando earthquake, the State of California enacted the Alquist-Priolo Earthquake Fault Zoning Act in 1972. This act required the State Geologist to delineate Earthquake Fault Zones along known active faults that have a relatively high potential for ground rupture. Faults that are zoned under the Alquist-Priolo Act must meet the strict definition of being “sufficiently active” and “well-defined” for inclusion as an Earthquake Fault Zones. The Earthquake Fault Zones are revised periodically, and they extend 200 to 500 feet on either side of identified fault traces. No structures for human occupancy may be built across an identified active fault trace. An area of 50 feet on either side of an active fault trace is assumed to be underlain by the fault, unless proven otherwise. Proposed construction in an Earthquake Fault Zone is permitted only following the completion of a fault location report prepared by a California Registered Geologist.
California Building Standards Code The California Building Standards Code establishes building requirements for construction and renovation. The most recent version of the California Building Standards Code was adopted in 2010 by the California Building Standards Commission and took effect January 1, 2011, and it is based on the National Fire Protection Association, International Association of Plumbing and Mechanical Officials, and the International Code Council’s Building and Fire Codes. Included in the California
Building Standards Code are the Electrical Code, Mechanical Code, Plumbing Code, Energy Code, and Fire Code.
Local City of Calistoga General Plan The General Plan establishes the following goals, objectives, and policies related to geology, soils, and seismicity that are applicable to the proposed project:
• Goal SAF-1: Reduce risk to the community from earthquakes and other geologic hazards. • Objective SAF-1.1: Enforce measures related to site preparation and building construction that
protect life and property from seismic hazards. • Policy P1: All construction in Calistoga shall conform with the Uniform Building Code, which
specifies requirements for seismic design, foundations, and drainage. • Objective SAF-1.2: Regulate new land development to prevent the creation of new geologic
hazards. • Policy P1: Development in or adjacent to hillside areas shall minimize geologic hazards by
undertaking site-specific geotechnical investigation. • Policy P2: In areas with significant identified geological hazards, development shall be sited
and designed to minimize exposure to damage resulting from geological hazards and to minimize the aggravation of off-site geological hazards.
• Policy P3: As part of site planning review, a geologic/seismic report that includes analysis of soils foundation, grading, erosion, and sediment control shall be required under any of the following circumstances:
- When warranted by the results of a geologic/seismic evaluation. - For new residential developments, roads or highways proposed to be located on parcels
which contain identifiable landsliding or slumps. - For all proposed structures and facilities open to the public and serving 100 persons or
more. - For projects proposed in hazardous geologic areas.
• Policy P4: Where alterations such as grading and tree removal are made to hillside sites, rendering slopes unstable, planting of vegetation shall be required to protect structures at lower elevations.
• Policy P5: The use of drought-tolerant plants for landscaping in the hills shall be required as a means to eliminate the need for supplemental watering, which can promote earth movement.
3.6.4 - Methodology RGH Consultants prepared a Geotechnical Investigation in January 2001 that evaluated the proposed Diamond Hill Estates Subdivision. Eight subsequent addenda were prepared between 2002 and 2010 to address design changes to the Diamond Hill Estates Subdivision and revisions to the California
Building Standards Code. The Geotechnical Investigation and the addenda are contained in Appendix G. Below is a summary of the activities associated with the preparation of the geotechnical reports.
RGH Consultants reviewed previous geotechnical investigations for the project site as well others in the vicinity. On June 30, 2000, RGH Consultants performed a geotechnical reconnaissance of the site. The subsurface conditions were explored by excavating a total of 40 test pits to depths ranging from about 3.5 to 20 feet on July 19 and 20 and August 9 and 10, 2000. The test pits were excavated with a track-mounted backhoe.
In addition, to the above-described test pits, the subsurface conditions were explored by drilling a total of 14 test borings (B-1 through B-14) to depths ranging from about 9.0 to 49.5 feet on July 19, 26, and 27; October 9; and November 2, 2000 at the approximate locations shown on the Exploration Plan included in the Geotechnical Investigation (Appendix G). Borings B-1 through B-4 were drilled with a rig mounted on a balloon-tired, all-terrain vehicle equipped with 8-inch-diameter, hollow-stem augers. Borings B-5 through B-13 were drilled with a truck-mounted rig equipped with 6-inch- diameter, hollow-stem augers. Boring B-14 was drilled with a track-mounted rig equipped with 6- inch-diameter, solid augers. The field engineer (B-1 through B-4 and B-14) or geologist (B-5 through B-13 and all test pits) located and logged the borings and pits, and obtained samples of the materials encountered for visual examination, classification, and laboratory testing.
Relatively undisturbed samples were obtained from the borings at selected intervals by driving a 2.43- inch-inside-diameter, split-spoon sampler, containing 6-inch-long brass liners, using a 140-pound hammer dropping approximately 30 inches. The sampler was driven 12 to 18 inches. The blows required to drive each 6-inch increment were recorded and the blows required to drive the last 12 inches, or portion thereof, were converted to equivalent Standard Penetration blow counts for correlation with empirical data. Disturbed “grab” samples were also obtained at selected depths from the test pits and placed in plastic bag or buckets.
The logs of the test pits and borings showing the materials encountered, depth of free water, sample depths, and converted blow counts are presented on Plates 3 through 56. The soils were described in accordance with the Unified Soil Classification System, outlined on Plate 57. Bedrock is described in accordance with Engineering Geology Rock Terms shown on Plate 58.
The samples obtained from the borings and pits were transported to RGH Consultants’ office, and re- examined by the project engineer to verify soil classifications, evaluate characteristics, and assign tests pertinent to the analysis. Selected samples were laboratory tested to determine their water content, dry density, classification (Atterberg limits, percent of silt and clay), consolidated undrained triaxial compressive strength, consolidation, expansion potential (UBC Expansion Index) and R- value. The test results are presented on the test boring and test pit logs, with the results of the strength data presented in accordance with the Key to Test Data on Plate 57. Results of the Atterberg
limits, particle size analysis, triaxial compression (two plates), consolidation, and R-value (two plates) tests are presented on Plates 59 through 65, respectively.
Additional fieldwork was conducted in associated with the eight subsequent addenda prepared between 2002 and 2010. Similar procedures were used as described previously, including the drilling of additional borings, digging of new test pits, and laboratory testing of soils. Refer to Appendix G for complete descriptions of activities associated with each addendum.
3.6.5 - Thresholds of Significance According to Appendix G, Environmental Checklist, of the CEQA Guidelines, geology, soils, and seismicity impacts resulting from the implementation of the proposed project would be considered significant if the project would:
a.) Expose people or structures to potential substantial adverse effects, including the risk of loss, injury or death involving:
i. Rupture of a known earthquake fault, as delineated on the most recent Alquist-Priolo Earthquake Fault Zoning Map issued by the State Geologist for the area or based on other substantial evidence of a known fault? Refer to Division of Mines and Geology Special Publication 42.
ii. Strong seismic ground shaking?
iii. Seismic-related ground failure, including liquefaction?
iv. Landslides?
b.) Result in substantial soil erosion or the loss of topsoil?
c.) Be located on a geologic unit or soil that is unstable, or that would become unstable as a result of the project and potentially result in on- or off-site landslide, lateral spreading, subsidence, liquefaction or collapse?
d.) Be located on expansive soil, as defined in Table 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property?
e.) Have soils incapable of adequately supporting the use of septic tanks or alternative wastewater disposal systems where sewers are not available for the disposal of wastewater? (Refer to Section 7, Effects Found Not To Be Significant.)
3.6.6 - Project Impacts and Mitigation Measures This section discusses potential impacts associated with the development of the project and provides mitigation measures where appropriate.
Seismic Hazards
Impact GEO-1: The proposed project may expose persons or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving seismic hazards such as fault rupture, strong ground shaking, seismic-related ground failure, or landslides.
Impact Analysis This impact assesses the potential for exposure to seismic hazards, including fault rupture, strong ground shaking, seismic-related ground failure, and landslides. Each issue is discussed separately.
Fault Rupture The project site is not located within a currently designated Alquist-Priolo Earthquake Fault Zone. RGH Consultants did not observe any land forms within the project site that indicate the presence of active faults. This condition precludes the possibility of fault rupture occurring within the project site.
Strong Ground Shaking RGH Consultants concluded that the project site is located in a seismically active region and, therefore, may be exposed to strong ground shaking during a seismic event. Accordingly, RGH Consultants recommended that the proposed project adhere to building code standards for earthquake resistant construction. This recommendation is reflected in Mitigation Measure GEO-1. With the implementation of this mitigation measure, impacts would be reduced to a level of less than significant.
Seismic-Related Ground Failure RGH Consultants found that the project site may be susceptible to seismic slope failure during an earthquake. As such, RGH Consultants recommended that the proposed project’s foundations be adequately supported into the underlying bedrock. A design-level geotechnical study would provide specific guidance regarding foundation design. Mitigation Measure GEO-1 requires that a design- level geotechnical study be prepared and, therefore, reflects this recommendation. With the implementation of this mitigation measure, impacts would be reduced to a level of less than significant.
Landslides RGH Consultants indicated that the project site contains two areas where landslides have occurred or may have potentially occurred. These areas are located along the access road and in the area where the custom residential lots are proposed; refer to Exhibit 3.6-1.
As part of the previous construction of the access road, the slope was stabilized and reinforced. Furthermore, no structures are proposed on this slope; therefore, it can be concluded that the proposed project would not significantly increase exposure to landslides in this area.
Regarding the potential for landslides in the custom residential lot area, this is confined to Lot 12 (refer to Exhibit 2-4). RGH Consultants provided recommendations for soil engineering and foundations to abate this condition. A design-level geotechnical study would provide specific guidance regarding soil engineering and foundation design. Mitigation Measure GEO-1 requires that a design-level geotechnical study be prepared and, therefore, reflects this recommendation. With the implementation of this mitigation measure, impacts would be reduced to a level of less than significant.
Level of Significance Before Mitigation Potentially significant impact.
Mitigation Measures MM GEO-1 Prior to issuance of building permits for the proposed project, the project applicant
shall submit grading and building plans to the City of Calistoga for review and approval that reflect the applicable recommendations from the previously prepared design-level geotechnical report and addenda. The applicant shall have the option of commissioning a new design-level geotechnical report in lieu of relying on the previous reports. The proposed project’s plans incorporate all applicable seismic design standards of the latest adopted edition of the California Building Standards Code or local amendments. The project applicant shall adhere to these approved plans in constructing the project.
Level of Significance After Mitigation Less than significant impact.
Erosion
Impact GEO-2: Development of the proposed project may have the potential to create substantial erosion or loss of topsoil.
Impact Analysis Construction activities associated with the proposed project would involve vegetation removal, grading, and excavation activities that could expose barren soils to sources of wind or water, resulting in the potential for erosion and sedimentation on and off the project site. National Pollutant Discharge Elimination System (NPDES) stormwater permitting programs regulate stormwater quality, which includes erosion and sedimentation, from construction sites. Under the NPDES permitting program, the preparation and implementation of a Stormwater Pollution Prevention Plan (SWPPP) is required for construction activities that would disturb an area of 1 acre or more. The SWPPP must identify potential sources of erosion or sedimentation that may be reasonably expected to affect the quality of stormwater discharges as well as identify and implement Best Management Practices (BMPs) that ensure the reduction of these pollutants during stormwater discharges. Typical BMPs intended to control erosion include sand bags, detention basins, silt fencing, storm drain inlet
protection, street sweeping, and monitoring of water bodies. These requirements have been incorporated into the proposed project as Mitigation Measure HYD-1.
Additionally, the applicant is required to obtain approval of a Timber Harvest Plan/Timber Harvest Permit from the California Board of Forestry and Fire Protection. As part of the requirements of this approval, the applicant will be required to identify measures to prevent erosion during timber harvesting activities. These requirements have been incorporated into the proposed project as Mitigation Measure AFR-2a.
The implementation of these mitigation measures would reduce potential erosion impacts to a level of less than significant.
Mitigation Measures Implement Mitigation Measures AFR-2a and HYD-1.
Unstable Soils or Geologic Units
Impact GEO-3: The proposed project may expose persons or structures to hazards associated with unstable geologic units or soils.
Impact Analysis As discussed in Impact GEO-1, the project site contains unstable geologic units and soils that may potentially expose persons or structures to hazards if left unabated. RGH Consultants prepared a design-level geotechnical study and addenda that provided various recommendations for abating these conditions, which are reflected in Mitigation Measure GEO-1. With the implementation of Mitigation Measure GEO-1, the proposed project could safely be developed and would not expose persons or structures to hazards associated with unstable geologic units or soils. Impacts would be less than significant.
Mitigation Measures Implement Mitigation Measure GEO-1.
Expansive Soils
Impact GEO-4: Development of the proposed project may expose persons or structures to hazards associated with expansive soils.
Impact Analysis RGH Consultants indicated that the proposed project contains soils belonging to the Forward series and the Boomer-Forward-Felta complex. The Forward series is located in the lower elevations of the projects site, while the Boomer-Forward-Felta complex is located in the upper elevations.
The Forward series has low expansion potential and, therefore, would not be classified as an expansive soil.
The Boomer-Forward-Felta complex is about 45 percent Boomer Series (clay to clayey silt), 35 percent Forward Series (clay to gravelly clay) and 15 percent Felta Series (gravelly clay and sandy clays). The expansion potential ranges from low for the Forward soils up to moderate for the Boomer soils. The proposed residential and resort structures are proposed in the upper elevations of the project site, where soils would have a moderate expansion potential.
RGH Consultants prepared a design-level geotechnical study and addenda that provided various recommendations for grading and soil engineering activities that would abate an expansive soil conditions that may be present where structures are proposed. Mitigation Measure GEO-1 requires that the project plan adhere to these recommendations and the applicable California Building Standards Code requirements. With the implementation of this mitigation measure, impacts would be reduced to a level of less than significant.
Mitigation Measures Implement Mitigation Measure GEO-1.
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3.6 - Geology, Soils, and Seismicity

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