Section 4.6 Geology and Soils Introduction

Transcription

1 4.6 invisible_toc_marker County of Kern Section 4.6 Geology and Soils Introduction As described in Chapter 2 of this Supplemental EIR (SEIR), an EIR was previously certified for the Alta Oak Creek Mojave (AOCM) Project. Two addenda to that EIR were subsequently approved for a land division action and the Alta Infill project. Collectively, these actions have been addressed in the AOCM EIR and are referred to as the AOCM Project. This SEIR has been prepared to address the current project, known as the Alta Infill II Wind Energy Project (project). This chapter serves to update the analysis contained within the AOCM EIR with regard to geology and soils associated with the current project. Applicable information from the AOCM EIR is incorporated into this chapter by reference, in accordance with the provisions of Section of the California Environmental Quality Act s (CEQA) Guidelines. The full text of the AOCM EIR is presented in Appendix B of this SEIR Environmental Setting The environmental setting for the project with regard to geology and soils is very similar to the AOCM Project, with several updates as listed below: In contrast to the AOCM Project, the project encompasses fewer soil types and geologic formations, and portions are situated along the foothills (rather than some ridgelines) of the Tehachapi Mountains. The Garlock Fault traverses both the AOCM Project and the Alta Infill II Wind Energy project. Geologic Setting Regional and local geology as described in Section of the AOCM Final EIR also accurately describes the geology of portions of the project area. The project area and vicinity is geologically young and seismically active. The geology of the project area is primarily underlain by pre- Tertiary crystalline rocks mainly composed of granitic-type rocks of the Sierra Nevada batholith of Mesozoic age and roof pendants of older metasedimentary rocks. These crystalline rocks are overlain by Tertiary volcanic and sedimentary rocks, and/or by Quaternary alluvium and colluvium as slope wash and talus mantling the slopes. The valleys in the area are also underlain by Quaternary to Holocene age alluvial and lacustrial deposits (Leighton, 2008). Soil types mapped in the project vicinity include those typical of the eastern side of the Tehachapi Mountains and the western end of the Mojave Desert, as described in the AOCM EIR. Soils within the project area are coarse-grained to moderately fine-grained sand, gravel and cobbles, which are moderately to excessively drained, and generally have low shrink-swell potential (SEI, 2008 and NRCS, 2011). Surficial geologic units within one mile of the project area are shown on Figure Seismic Setting As with the AOCM Project, the project area is seismically active. Several principally active faults occur in the vicinity of the project area including the Garlock Fault (which extends through the northern portion of the project site), the White Wolf Fault (24 miles west of the project site), the San Andreas Fault (23 miles southwest of the project site), the Pleito Thrust Fault (31 miles west Alta Infill II Wind Energy Project August 2011

2 County of Kern of the project site), and the San Gabriel Fault (36 miles southwest of the project site) (CGS, 2011a). These faults are described in the AOCM EIR and their locations relative to the project area are illustrated in Figure Numerous historic earthquakes have affected the Mojave Desert region where the project is located. Major seismic events proximate to the project area are presented in Table of the AOCM EIR. The majority of recent seismic activity in the vicinity of the project site has occurred along the White Wolf Fault and the San Andreas Fault. Historic earthquakes along the Garlock Fault, which underlies the northern portion of the project area, have not occurred within the last 200 years (SEI, 2008). Geologic and Seismic Hazards Slope Stability The project area is located at the foothills of the Tehachapi Mountains and the desert floor of the Antelope Valley. The southern portion of the project area on the valley floor is gently sloped and apparently stable (SEI, 2008). The foothills are relatively steeper and in these areas seismicallyinduced landslides may occur when ground motion from earthquakes causes unstable or sloping and loosely aggregated soils and rocks to move down-slope under the force of gravity. The project area is not within a California Seismic Hazard Zone for landslides (CGS, 2011b) and was not mapped by Kern County as being susceptible to landslides. The potential for seismicallyinduced slope failure or rock falls in the project area is considered low. Soil Hazards Erosion. As with the AOCM Project, soils at the project site are generally composed of sand, gravel, and cobbles with little to no fine-grained soils. Due to these soil characteristics, the potential for wind erosion to affect the structural integrity of project features would be low. Expansion ( shrink-swell ). Expansive soils may cause damage to aboveground infrastructure as a result of density changes that shift overlying materials. Fine-grain clay sediments are most likely to exhibit shrink-swell patterns in response to changing moisture levels. Due to the characteristics of the soil in the project area, as described above, there is a low probability of expansion. Settlement. In the event of a major earthquake, minor settlement (less than one inch) could occur in the project area (SEI, 2008). The potential for seismically induced settlement at the project site is considered low. Faults and Seismicity As with the AOCM Project, the controlling fault affecting the project area is the Garlock Fault. The Garlock Fault is a near vertical shear zone with a slip rate of 6 millimeters (mm) per year with a maximum credible earthquake event of 7.3. A geotechnical evaluation performed for the adjacent AOCM Project (SEI, 2008) determined that the Garlock Fault is capable of generating peak bedrock acceleration (PBA) of 0.55 based on the Maximum Credible Earthquake event of 7.3. It is expected that the PBA would be similar at the project area; however, given the proximity to the Garlock Fault Earthquake Fault Zone, a site-specific geotechnical evaluation would be required to more accurately assess the PBA for the project area. August Alta Infill II Wind Energy Project

3 County of Kern Alta Infill II Wind Energy Project August 2011

4 Lo s A n g e l e s A E(2 1/2 )R S A County of Kern K e r n White Wolf Fault Garlock Fault Project Site Pielto Fault L o s A n g e l e s San Andreas Fault S B e r n V e n t u r a San Gabriel Fault I Miles Alta-Oak Creek Mojave Project Alta Infill II Wind Energy Project Faults Proposed Project Counties Figure Major Faults December 2008 August 2011

5 County of Kern Strong Ground Shaking As with the AOCM Project, the project area has experienced seismic activity with strong ground shaking in the past. The Garlock Fault has an estimated slip rate of 6 millimeters per year and a maximum earthquake magnitude of 7.1. An 8.0 magnitude earthquake within 50 miles would result in strong ground shaking at the project site (SEI, 2008). As such, the project site could experience strong ground shaking caused by moderate to strong earthquakes during the lifetime of the project. Ground Rupture Ground rupture is when an earthquake produces fault displacement at the ground surface. The Garlock Fault, which extends through the northern portions of the project area as well as the AOCM Project, is designated as an Alquist-Priolo Earthquake Fault Zone (EFZ). Construction of occupied structures within an EFZ is regulated under the Alquist-Priolo Earthquake Fault Zoning Act and must conform to strict building codes. Ground rupture could occur within 500 feet of an active trace of the Garlock Fault (SEI, 2008); therefore, the potential for ground rupture in the project area is high. Liquefaction Liquefaction potential is based on the density and textural characteristics of the alluvial sediments, the intensity and duration of groundshaking, and the depth to groundwater. The lithology within the project area predominately comprises dense decomposed granitic material and rock. Depth to groundwater is between 40 and 50 feet or deeper below ground surface (SEI, 2008). The project area has not been identified by Kern County as an area that is subject to liquefaction hazards. The potential for liquefaction to be a hazard at the project area is negligible Regulatory Setting The Federal, State and County regulatory setting for geology and soils is the same as that described in Section of the AOCM EIR., with the following exceptions: Portions of the project area fall within the boundaries of the Mojave and Soledad Mountain Elephant Butte Specific Plans and a portion of the site is within an area designated as a part of the Cache Creek Interim Rural Community Plan. The Cameron Canyon Specific Plan would not apply to the project. These plans are described as follows: Mojave Specific Plan Chapter 9 Seismic and Safety Element Goal Protect structures from potential damage caused by earthquakes. Objective Objective 9.1: Minimize the potential damage to structures and loss of life that could result from earthquakes. August Alta Infill II Wind Energy Project

6 County of Kern Policies Policy 9.1.1(A-1,A-3,M-2) Safety measures required by the Uniform Building Code and the Kern County Seismic Safety Element during construction of new buildings are hereby incorporated by reference. Policy (H-1, M-2) Encourage residential property owners to implement seismic safety improvements in older buildings, such as anchoring buildings to foundations, bolting water heaters to walls, and performing other preventative measures. Policy (M-2) Participate in State-sponsored earthquake preparedness programs. Goal Promote awareness of potential flood and geologic hazards. Objective Objective 9.2: Prevent loss of life, reduce personal injuries and property damage, and minimize economic loss resulting from flood hazards. Policy Policy (C-1, C-4, M-1, M-3) Encourage clustering of residential development within areas identified on the Physical Constraints Map to mitigate physical hazards. Soledad Mountain Elephant Butte Specific Plan This Specific Plan does not contain any recommendations, analysis, or implementation measures that are directly applicable to the geology and soils analysis of the project. Cache Creek Interim Rural Community Plan A portion of the project site is designated 4.2 (Interim Rural Community Plan) by the Kern County General Plan (KCGP). Map Code 4.2 is used to identify settlements in the County that have individual character which, in past plans, have been broadly merged with the surrounding countryside. These settlements are recognized as unique communities. Appendix B of the KCGP contains details and maps of interim rural community plans the provisions that shall apply to the area until a Specific Plan or precise development plan is adopted within the rural community area. The interim land use guidelines are general in nature, however, and are not specific to geology and soils Impacts and This section describes the potential geology and soils impacts associated with development of the project. As described in Section 3.9 (Decommissioning and Repowering) of the SEIR, activities that are reasonably expected to occur throughout the life of the proposed project, including construction and installation of wind turbines, operation and maintenance, and decommissioning, may extend over a period of 30 years. The potential environmental effects are evaluated based on the significance criteria identified below (Thresholds of Significance). Alta Infill II Wind Energy Project August 2011

7 County of Kern The Methodology and Thresholds of Significance described in Section of the AOCM EIR are incorporated by reference. Impact 4.6 1: Expose people or structures to substantial adverse effects involving the rupture of a known earthquake fault The project would result in installation of the same types of structures in the same general location and with the same general project design as for the AOCM Project. Both projects are traversed by the active Garlock Fault and portions of each are within the Alquist-Priolo Earthquake Fault Zone (EFZ). Impacts related to potential exposure of people or structures to adverse affects attributable to ground rupture of the Garlock Fault within the project area are the same as those described in the AOCM EIR. As with the AOCM Project, structures on the project site may be subject to moderate to severe ground shaking. Damage to project facilities and injury of workers could occur from direct surface rupture along the Garlock Fault. This structural damage and personal injury would be significant and mitigation would be required , 4.6-2, and from the AOCM EIR require that a full geotechnical study be conducted to evaluate soil conditions and geologic hazards and that project facilities (including transmission lines) be sited and designed based on the findings of this study. Implementation of these mitigation measures would also be required for the current project and would reduce the impact below the level of significance. Accordingly, the project would not result in a seismic hazard related to ground rupture beyond that described in the AOCM EIR. Implement MMs through as described in Section 4.6 of the AOCM EIR. No additional Impact 4.6 2: Expose people or structures to substantial adverse effects involving strong seismic ground shaking As with the AOCM Project, the Garlock Fault, which is considered an active fault, crosses the project site and other active and potentially active faults are located within 50 miles. Impacts related to potential exposure of people or structures to adverse affects attributable to strong ground shaking are the same as those described in the AOCM EIR. As with the AOCM Project, the project would likely be subjected to at least a moderate or larger earthquake occurring close enough to produce strong ground shaking at the project location during the life of the project. This may result in structural damage or personal injury, which would be significant and require mitigation through (described above) from the AOCM EIR require that a geotechnical study is conducted and project facilities sited and designed accordingly. In addition, Mitigation Measure from the AOCM EIR specifically requires that project facilities be designed to withstand substantial ground shaking in accordance with applicable California Building Code (CBC) seismic design standards and Kern County building codes. Implementation of these mitigation measures would also be required for the project and would reduce the impact below the level of significance. Accordingly, the project would not result in a seismic hazard related to strong ground shaking beyond that described in the AOCM EIR. August Alta Infill II Wind Energy Project

8 County of Kern Implement MMs through as described in Section 4.6 of the AOCM EIR. No additional Impact 4.6 3: Expose people or structures to substantial adverse effects involving seismic related ground failure, including liquefaction As described in Section 4.6.2, ground rupture and ground failure could occur within 500 feet of an active trace of the Garlock Fault. Seismic events also have the potential to result in liquefaction, although the potential for liquefaction at the project area is low. The project would result in installation of the same types of structures in the same general location (i.e., near the Garlock Fault) and with the same general project design as for the AOCM Project. Given the similar geologic setting, impacts related to potential exposure of people or structures to adverse affects attributable to seismic-related ground failure are the same as those described in the AOCM EIR. As with the AOCM project, seismic-related ground failure could potentially result in structural damage or personal injury, which would require mitigation and (described above) from the AOCM EIR require that a geotechnical study is conducted and project facilities sited and designed accordingly. Implementation of these mitigation measures would also be required for the project and would reduce the impact below the level of significance. Accordingly, the project would not result in a seismic hazard related to seismic-related ground failure beyond that described in the AOCM EIR. Implement MMs through as described in Section 4.6 of the AOCM EIR. No additional Impact 4.6 4: Expose people or structures to substantial adverse effects involving landslides As with the AOCM Project, portions of the project area located in the foothills of the Tehachapi Mountains may experience seismically induced landslides if/when ground motion causes unstable or steeply sloping and loosely aggregated soils and/or rocks to mobilize under the force of gravity. During construction, destabilization of natural or constructed slopes could occur as a result of excavation and/or grading operations required for installation of project infrastructure. As described in Section 4.6.2, the project site is not located within a State California Seismic Hazard Zone for landslides and the potential for landslides to occur in the project area is considered low. The project would result in installation of the same types of structures in the same general location and with the same general project design as for the AOCM Project. Therefore, impacts related to potential exposure of people or structures to adverse affects attributable to landslides are the same as those described in the AOCM EIR. As with the AOCM project, landslides could potentially result in structural damage or personal injury, which would require mitigation. Alta Infill II Wind Energy Project August 2011

9 County of Kern 4.6-5, 4.6-6, and from the AOCM EIR require that constructed slopes be designed for safety based on the results of the geotechnical analysis and that project structures are not sited in landslide, mudflow or other unstable area. Implementation of these mitigation measures would also be required for the project and would reduce the impact below the level of significance. Accordingly, the project would not result in a seismic hazard related to landslides beyond that described in the AOCM EIR. Implement MMs through as described in Section 4.6 of the AOCM EIR. No additional Impact 4.6 5: Result in substantial soil erosion or loss of topsoil The project would result in installation of the same types of structures in the same general location and with the same general project design as for the AOCM Project. Both projects would require ground disturbance for construction, including excavation and grading. Impacts related to soil erosion of loss of topsoil within the project area are the same as those described in the AOCM EIR. As with the AOCM Project, construction activities could loosen soil or remove stabilizing vegetation and expose areas of loose soil. Due to soil characteristics and topography in the project area, the potential for substantial wind and stormwater erosion and subsequent loss of topsoil is low. However, these exposed areas of loose soil, if not properly stabilized during construction, could be subject to increased soil loss and erosion by wind and stormwater runoff. This would constitute a potentially significant impact and mitigation is required to ensure soil stabilization and to mitigate this impact through from the AOCM EIR require various best management practices to prevent wind and water erosion during construction, including: limiting grading to the minimum area necessary for construction and operation of the project, retaining a California registered professional engineer to review the final grading earthwork and foundation plans prior to construction, watering disturbed areas frequently to control dust, salvaging topsoil and using existing roads. Implementation of these mitigation measures would also be required for the project and would reduce the impact below the level of significance. Accordingly, the project would not result in soil erosion or loss of topsoil beyond that described in the AOCM EIR. Implement MMs through as described in Section 4.6 of the AOCM EIR. No additional August Alta Infill II Wind Energy Project

10 County of Kern Impact 4.6 6: 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 The project would result in installation of the same types of structures in the same general location and with the same general project design as for the AOCM Project. Given the similar geologic setting, impacts related to potential geologic hazards attributable to locating project structures on unstable geologic units are the same as those described in the AOCM EIR. As with the AOCM Project, the potential for landslide, lateral spreading, subsidence, liquefaction, or collapse in the project area is low. Although it is unlikely that project infrastructure would be sited on any unstable geologic unit or soil given the low probability of their occurrence in the project area, seismic events could potentially result in damage to project facilities which may subsequently result in on- or off-site landslide, liquefaction, or collapse and from the AOCM EIR require that a full geotechnical study be conducted to evaluate soil conditions and geologic hazards and that project facilities be sited and designed based on the findings of this study. Implementation of these mitigation measures for the project would prevent locating project infrastructure on unstable or potentially unstable geologic units or soils, thereby reducing this impact below the level of significance. Accordingly, development of the project would not result in landslide, lateral spreading, subsidence, liquefaction, or collapse beyond that described in the AOCM EIR. Implement MMs through as described in Section 4.6 of the AOCM EIR. No additional Impact 4.6 7: Be located on expansive soil as defined in Section of the California Building Code creating substantial risks to life or property The project would result in installation of the same types of structures in the same general location and with the same general project design as for the AOCM Project. Given the similar geologic setting, risks to life or property attributable to locating project structures on expansive soil are the same as those described in the AOCM EIR. As with the AOCM Project, soils within the project site exhibit low probability for shrink-swell patterns or expansive characteristics. Therefore, the risk to life or property from ground failure attributable to development of the project is also low and from the AOCM EIR require that a full geotechnical study be conducted to evaluate soil conditions and that project facilities be sited and designed based on the findings of this study. Implementation of these mitigation measures for the project would prevent locating project infrastructure on expansive soils, thereby reducing this impact below the level of significance. Accordingly, development of the project would not result in associated risks to life or property beyond that described in the AOCM EIR. Alta Infill II Wind Energy Project August 2011

11 County of Kern Implement MMs through as described in Section 4.6 of the AOCM EIR. No additional Impact 4.6 8: Have soils that are incapable of adequately supporting the use of septic tanks or alternative wastewater systems As with the AOCM Project, the project includes warehouse facilities to support O&M activities for the project. These proposed warehouses would require running water and bathroom facilities. If not designed correctly and sited on suitable soils, septic systems could result in health impacts, adversely affect natural habitat, and pollute groundwater. As with the AOCM Project, this impact is therefore considered to be potentially significant and mitigation is required. Mitigation Measure from the AOCM EIR requires that the project septic system and leach field be constructed to comply with applicable requirements of the Kern County Environmental Health Department, including siting these facilities away from surface waters in the project area to avoid potential runoff effects to these features. Implementation of this mitigation measure would also be required for the project and would reduce the impact below the level of significance. Accordingly, the project would not result in impacts related to siting septic facilities on inadequate soils beyond that described in the AOCM EIR. Implement MM as described in Section 4.6 of the AOCM EIR. No additional mitigation measures are required. Cumulative Setting Impacts and Cumulative impacts are two or more individual impacts that, when considered together, are considerable or that compound or increase other environmental impacts. The geographic scope for considering cumulative impacts to Geology and Soils includes the extent of the project area and the AOCM Project area, because impacts to geology and soils are generally site-specific, as described in the AOCM EIR. Impact 4.6 9: Contribute to Cumulative Geologic Impacts With regard to the project s potential to expose people or structures to hazards associated with the rupture of a known earthquake fault or from strong seismic groundshaking, damage to wind turbines and associated project facilities could occur from direct rupture along the Garlock Fault, where it crosses through the project site. Portions of SCE s Tehachapi Renewable Transmission Project (TRTP) will be constructed directly adjacent to and on portions of the project site. In the event of a strong earthquake along the Garlock Fault, structural damage to wind turbines, overhead transmission lines, and other associated facilities from both projects could injure workers at the project site. However, both projects would be required to site project facilities outside of fault August Alta Infill II Wind Energy Project

12 County of Kern traces and to construct project facilities in conformance with relevant building codes, which would minimize placement of structures in active faults zones. Additionally, operation of the TRTP would not require onsite staff except for periodic inspection and maintenance activities, further reducing the potential for people to be present in the event of an earthquake. Therefore, impacts of the project would not combine with impacts from past, present, or reasonably foreseeable projects to result in a cumulatively considerable impact, as was disclosed in the AOCM EIR. With regard to the project s potential to expose people or structures to hazards associated with seismic-related ground failure, including liquefaction, it is considered possible that ground rupture and/or failure could occur within 500 feet of an active trace of the Garlock Fault, and that such an event could result in damage to project infrastructure; liquefaction is not likely to occur at the project site. Such an impact would be site-specific, and would be reduced to less-than-significant levels with the implementation of and Therefore, impacts of the project would not be expected to combine with impacts from past, present, or reasonably foreseeable projects to result in a cumulatively considerable impact, as was disclosed in the AOCM EIR. With regard to the project s potential to expose people or structures to hazards associated with landslides, destabilization of slopes in the north portion of the project site in the foothills of the Tehachapi Mountains could occur during construction of project infrastructure. However, implementation of through would reduce the potential for structures to be subject to landslides or slope instability. Additionally, none of the cumulative projects identified in Section 3.11 are located on the portions of the project site that consist of steep slopes. Therefore, this impact of the project would not have the potential to combine with similar impacts of past, present, or reasonably foreseeable projects to result in a cumulatively considerable impact, as was disclosed in the AOCM EIR. Regarding the project s potential to result in substantial soil erosion or loss of topsoil, the characteristics of soil at the project site indicate that the potential for substantial erosion or loss of topsoil would be low. Implementation of through would reduce this impact of the project to less-than-significant levels. SCE s TRTP would be constructed directly adjacent to and on portions of the project site. Soil erosion from the TRTP would have the potential to combine with erosion from the project to result in a cumulative impact. However, both projects would be required to implement a SWPPP, which would include site-specific Best Management Practices (BMPs) for erosion and sediment control for each project, which would reduce this cumulative impact to a less-than-significant level, as was disclosed in the AOCM EIR. With regard to the project s potential to place infrastructure on soil that is unstable or expansive, geotechnical assessments at the project site would be conducted prior to construction to assure that soils are suitable for the placement of project infrastructure. Therefore, this impact would not have the potential to combine with similar impacts of past, present, or reasonably foreseeable projects to result in a cumulatively considerable impact, as was disclosed in the AOCM EIR. The project site is not located in a populated area and, in compliance with WE Combining District requirements, project facilities would be set back from any existing residences, as would other wind energy and transmission projects in the cumulative scenario. All projects in the cumulative scenario are subject to the requirements of the laws and regulations described in Section 4.6.3, which would mitigate project impacts and reduce hazards to property and individuals, as related to geology and soils. In summary, the project would not contribute to new or substantially more adverse cumulative impacts to geology and soils than those disclosed in the AOCM EIR. Alta Infill II Wind Energy Project August 2011

13 County of Kern Implement MMs through as described in Section 4.6 of the AOCM EIR. No additional Cumulative impacts would continue to be less than significant, as discussed in the AOCM EIR. August Alta Infill II Wind Energy Project