4.6 GEOLOGY AND SOILS

Transcription

1 4.6 GEOLOGY AND OIL ummary able summarizes the identified environmental impacts, proposed mitigation measures, and residual impacts of the proposed project with regard to geology and soils. Additional detail is provided in ection (Impact Analysis). able Impact and Mitigation ummary: Geology and oils Impact Mitigation Measures Residual Impact Impact GEO-1 eismically induced ground shaking could destroy or damage structures and infrastructure, resulting in loss of property or risk to human safety. However, the project s mandatory compliance with applicable California Building Code requirements renders impacts Class III, less than significant. No mitigation is required. Impact GEO-2 Due to the depth of groundwater on the project site, the potential for seismic related ground failure from liquefaction of underlying soils is moderate. Impacts would be Class III, less than significant. Impact GEO-3 Construction of the proposed project could result in soil erosion or loss of topsoil. However, compliance with existing regulations would reduce impacts to a Class III, less than significant, level. Impact GEO-4 he project site is not located on a geologic unit or soil that is unstable, and would not result in landslides, lateral spreading, subsidence, collapse, or soil expansion. Impacts would be Class III, less than significant etting No mitigation is required. No mitigation is required. No mitigation is required. Impacts would be less than significant without mitigation. Impacts would be less than significant without mitigation. Impacts would be less than significant without mitigation. Impacts would be less than significant without mitigation. a. Regional etting. he Carmel Valley is geologically complex and seismically active. he predominant structural feature in the California Coast Ranges is the an Andreas Fault, which is the structural boundary of the Pacific and North American tectonic plates. Uplift along faults is the primary force that created the mountains and valleys of the outhern Coast Ranges, including the anta Lucia and ierra de alinas Mountains. Erosion and deposition of soil from the uplifted mountains formed broad alluvial fans of well-drained, nutrient rich soil, including the soils found in Carmel Valley. his region has three active faults with evidence of historic or recent movement. he an Andreas Fault runs through the southeastern portion of Monterey County for approximately 30 miles and poses the greatest seismic hazard to the County. he two other active faults affecting 4.6-1

2 Monterey County include the Palo Colorado-an Gregorio fault zone and the Monterey Bay fault zone. he Palo Colorado-an Gregorio fault zone connects the Palo Colorado Fault near Point ur, south of Monterey, with the an Gregorio fault near Point Año Nuevo in anta Cruz County. he Monterey Bay fault lies seaward of the City of easide extending northwesterly to the Pacific Ocean. b. Project ite etting. he site is located on the floor of the Carmel Valley approximately 1.5 miles east of the mouth of the Carmel River and the Pacific Ocean. It is situated atop an older river terrace about 1,050 feet north of the active Carmel River channel. he project site is located within the Unites tates Geological urvey (UG) Monterey 7 ½- minute topographic quadrangle at approximate latitude N and longitude W. his section covers the geology of the project site, its topographic relief, seismic hazards, landslide hazards, and soil characteristics. opography. he site is primarily flat and lies at an elevation of approximately 30 feet. here are no substantial slopes on or adjacent to the site. he site is drained by sheet-flow towards the southwest corner of the site. Val Verde Drive is present along the eastern boundary. Geology. he project site lies 35 miles west of the an Andreas Fault with basement rocks of alinian Block metamorphic and granitic rocks of Paleozoic and Cretaceous age. he alinian basement rocks have been detached and moved northward along the an Andreas Fault system to their present position. he basement rocks are overlain by younger late Cretaceous to ertiary, and Quaternary age marine and terrestrial sedimentary rocks with volcanic rock locally present. According to the Geologic Hazards tudy (Earth ystems Pacific, 2010; refer to Appendix F), the site is underlain by older flood-plain deposits of Holocene age with younger flood-plain deposits of Holocene age on the southwest site corner. hese deposits are described as unconsolidated, relatively fine-grained, heterogeneous deposits of sand and silt with common thin layers of clay. eismic Hazards. he UG defines active faults as those that have had surface displacement within Holocene time, or approximately within the last 11,000 years. Evidence of surface displacement can be recognized by the existence of cliffs in alluvium, terraces, offset stream courses, fault troughs and saddles, the alignment of depressions, sag ponds, and the existence of steep mountain fronts. Potentially active faults are those that have had surface displacement during Quaternary time, or within the last 1.6 million years. Inactive faults have not had surface displacement within the last 1.6 million years. he site is located within the seismically active central California Coast Ranges geomorphic province, but is not located in an Alquist-Priolo Earthquake Fault Zone. he major active faults capable of producing large magnitude events and that have a high seismic activity rate recognized in the region are the an Andreas, Hayward, and an Gregorio faults. Other active faults in the region include the Zayante-Vergeles, Monterey Bay-ularcitos, and Calaveras faults. here are no faults mapped on or adjacent to the site (Earth ystems Pacific, 2010). Figure shows the faults in the area in comparison to the project location. Potential seismic hazards resulting from a nearby moderate to major earthquake can generally be classified as primary and secondary. he primary effect is fault ground rupture, also called 4.6-2

3 Carmel Rio Road Project EIR 87 d ar yw Ha 85 Za y 82 te 152 -V er ge l a n An dr ea s es er a l av Ca an s Mo e nt y re Ba y an Gre gon io in es a on u la a n _ ^ 0 im eo os Quaternary Faults 12 Miles rc it n Project Location 6 da _ ^ nc Ri Ch up 68 ± Imagery provided by Google and its licensors 2016; Additional data provided by UG, Regional Faults Figure 4.6-1

4 surface rupture. Common secondary seismic hazards include ground shaking, liquefaction, and subsidence. Each of these potential hazards is discussed below. urface Rupture. urface rupture is an actual cracking or breaking of the ground along a fault during an earthquake. ince the site is not located in a tate Earthquake Fault Zone and no faults considered to be active lie on or adjacent to the site, the potential for surface ground rupture is considered to be low. Ground haking. Fault displacement can generate seismic ground shaking, which is the greatest cause of widespread damage in an earthquake. Whereas surface rupture affects a narrow area above an active fault, ground shaking covers a wide area and is greatly influenced by the distance of the site to the seismic source, soil conditions, and depth to groundwater. he site is in a region of generally high seismicity and has the potential to experience strong ground shaking from earthquakes on regional and/or local causative faults. Liquefaction. oil liquefaction occurs when ground shaking from an earthquake causes a sediment layer saturated with groundwater to lose strength and take on the characteristics of a fluid, thus becoming similar to quicksand. he site lies within an area deemed to have a moderate susceptibility for liquefaction (, 2012). According to the Geotechnical Engineering Report and Liquefaction tudy prepared for the project (Buena Geotechnical ervices, 2007; refer to Appendix F), soils below a depth of 23 feet at the site are likely to liquefy in the event of a nearby large magnitude earthquake. eismically-induced ettlement. eismically-induced settlement of sufficient magnitude to cause structural damage is normally associated with poorly consolidated, predominantly sandy soils. he project site has loose soil conditions and slightly clayey silty find sands (Buena Geotechnical ervices, 2007). Landslides. Landslides and other forms of mass wasting, including mud flows, debris flows, soil slips, and rock falls, occur as soil or rock moves down slope under the influence of gravity. Intense rainfall or seismic shaking could trigger landslides. he site is essentially flat and there are no substantial slopes on or adjacent to the site. In the site vicinity, large landslide complexes are present approximately 2,000 feet south of the site on the valley wall. Based on their distance from the site, these landslides do not appear to be capable of impacting the site. According to the Geologic Hazards tudy, there was no evidence of landsliding on or adjacent to the site in the 1950 aerial photographs reviewed and no evidence of landslides was found during site reconnaissance (Earth ystems Pacific, 2010). oil Characteristics. As mapped by the U.. Department of Agriculture (UDA), Natural Resource Conservation ervice (NRC), the soil type of the project site is Pico Fine andy Loam. Pico soils are found on floodplains and alluvial fans and have slow to medium runoff (UDA, 2003). Evaluation of the subsurface indicates that soils are generally dark brown slightly clayey silty fine sands. Reported soil types at the site consisted of sands with some local gravel zones. Groundwater was encountered at an approximate depth of 23 feet (Buena Geotechnical ervices, 2007). oil erosion is the removal of soil by water and wind. he rate of erosion is estimated from four soil properties: texture, organic matter content, soil structure, and permeability. Other factors 4.6-4

5 that influence erosion potential include the amount of rainfall and wind, the length and steepness of the slope, and the amount and type of vegetative cover. he erosional hazard for soils at this site is moderately low (UDA, 2016). c. Regulatory etting. Federal. National Pollutant Discharge Elimination ystem. tormwater-related erosion is one major source of soil-related impacts. tormwater discharges from construction activities (such as clearing, grading, excavating, and stockpiling) that disturb one or more acres, or smaller sites that are part of a larger common plan of development or sale, are regulated under the National Pollutant Discharge Elimination ystem (NPDE) stormwater program. Prior to discharging stormwater, construction operators must obtain coverage under an NPDE permit. In California, the General Permit for Discharges of tormwater Associated with Construction Activity are regulated by the tate Water Resources Control Board and administered through the local Regional Water Quality Control Board. he Construction General Permit requires the development and implementation of a torm Water Pollution Prevention Plan (WPPP). he WPPP should contain a site map(s) which shows the construction site perimeter, existing and proposed buildings, lots, roadways, storm water collection and discharge points, general topography both before and after construction, and drainage patterns across the project site. he WPPP must list Best Management Practices (BMPs) the discharger will use to protect storm water runoff and the placement of those BMPs. Additionally, the WPPP must contain a visual monitoring program; a chemical monitoring program for "non-visible" pollutants to be implemented if there is a failure of BMPs; and a sediment monitoring plan if the site discharges directly to a water body listed on the 303(d) list for sediment. ection A of the Construction General Permit describes the elements that must be contained in a WPPP. tate. Alquist-Priolo Earthquake Fault Zoning Act. he Alquist-Priolo Earthquake Fault Zoning Act was signed into California law on December 22, 1972 to mitigate the hazard of surface faulting to structures for human occupancy. he Alquist-Priolo Act provides for special seismic design considerations if developments are planned in areas adjacent to active or potentially active faults. he project site is not located in an Alquist-Priolo Earthquake Fault Zone. eismic Hazards Mapping Act. he eismic Hazards Mapping Act (HMA) of 1990 (Public Resources Code, Chapter 7.8, ection ) directs the Department of Conservation, California Geological urvey to identify and map areas prone to earthquake hazards of liquefaction, earthquake-induced landslides and amplified ground shaking. he purpose of the HMA is to reduce the threat to public safety and to minimize the loss of life and property by identifying and mitigating these seismic hazards. he HMA was passed by the legislature following the 1989 Loma Prieta earthquake. he eismic Hazards Mapping Act addresses geoseismic hazards, other than surface faulting, and applies to public buildings and most private buildings intended for human occupancy

6 California Building Code (CBC). he 2013 CBC incorporates by reference and amends requirements in the 2012 International Building Code pertaining to geologic hazards, including seismically resistant construction and foundation and soil investigations prior to construction. he CBC also establishes grading requirements that apply to excavation and fill activities, and requires the implementation of erosion control measures. he County is responsible for enforcing the 2013 CBC. Local. Local regulations include Monterey County Code Chapters (Grading) and (Erosion Control). he project would also be required to comply with the requirements of the Monterey County 2010 General Plan. pecifically, the Monterey Conservation and Open pace Element and afety Element provide criteria for evaluation of geologic hazards and geotechnical requirements related to new development. Consistency with specific geotechnical policies that apply to the project is evaluated in ection 4.10, Land Use and Planning. Monterey County Code, Chapter 16.08, Grading. Chapter of the Monterey County Code regulates grading activities. he purpose of these regulations is to safeguard health, safety, and public welfare, to minimize erosion, protect fish and wildlife, and to otherwise protect the natural environment. A grading permit is required for all activities that would exceed 100 cubic yards of grading. Where grading operations obstruct and/or otherwise impair the flow or runoff of a drainage course, appropriate drainage facilities are required to be implemented to convey flows past the point of obstruction ( ). Chapter also contains measures to protect water quality from grading related activities and associated erosion. hese requirements are codified in of the Monterey County Code, which requires that all areas disturbed in connection with grading related activities shall be consistently maintained to control erosion. he project would be required to comply with these requirements. Monterey County Code, Chapter 16.12, Erosion Control. Chapter of the Monterey County Code requires that development activities control runoff to prevent erosion. he purpose of these regulations is to eliminate and prevent conditions of accelerated erosion that have led to, or could lead to, degradation of water quality, loss of fish habitat, damage to property, loss of topsoil or vegetation cover, disruption of water supply, increased danger from flooding. An erosion control plan is required to be submitted to the prior to any land disturbing activities ( ). his plan is required to indicate methods to control erosion. Runoff control must be implemented to control runoff from a 10-year storm event ( ). All runoff must be detained or dispersed so that the runoff rate does not exceed the pre-development level. Any concentrated runoff which cannot be effectively detained or dispersed without causing erosion is to be carried in non-erodible channels or conduits to the nearest drainage course designated for such purpose or to on-site percolation devices with appropriate energy dissipaters to prevent erosion at the point of discharge. Runoff from disturbed areas must be detained or filtered by berms, vegetated filter strips, catch basins, or other means as necessary to prevent the escape of sediment from the disturbed area. he project would be required to comply with these requirements

7 4.6.3 Impact Analysis a. Methodology and ignificance hresholds. his evaluation is based in part on a Geotechnical Engineering Report and Liquefaction tudy (Buena Geotechnical ervices, June 2007) and Geological Hazards tudy (Earth ystems Pacific, January 2010) prepared for the proposed project. Both of these reports are attached to this EIR as Appendix F. Both reports were peer reviewed by a Rincon Consultants, Inc. certified Engineering Geologist. he analysis also included a review of existing information and other available regional sources, including data from the California Department of Conservation and NRC. Based on the environmental checklist included in Appendix G of the tate CEQA Guidelines, impacts would be considered potentially significant if the proposed project would: 1. 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 tate Geologist for the area or based on other substantial evidence of a known fault; ii. trong seismic shaking iii. eismic-related ground failure, including liquefaction, iv. Landslides; 2. Result in substantial soil erosion or the loss of topsoil; 3. 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; 4. Be located on expansive soil, as defined in able 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property; and/or 5. Have soils incapable of adequately supporting the use of septic tanks or alternative waste water disposal systems where sewers are not available for the disposal of waste water. here are no faults mapped on or adjacent to the site and the site is not located in an Alquist- Priolo Earthquake Fault Zone. he potential for surface ground rupture is therefore considered low. he project site is relatively flat and would not be subject to the risk of landslides. he proposed project also would not involve installation and use of septic tanks or alternative waste water disposal systems. Wastewater from the project site would be collected and conveyed through a conventional gravity system to an existing Carmel Area Wastewater District sanitary sewer main located at the southwest corner of the property. herefore, impacts related to hresholds 1.i, 1.iv and 5 are not discussed further in this section, but details are provided in ection 4.15, Effects Found Not to Be ignificant. b. Project Impacts and Mitigation Measures. hreshold 1.ii: Impact GEO-1 Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving: strong seismic shaking. eismically induced ground shaking could destroy or damage structures and infrastructure, resulting in loss of 4.6-7

8 property or risk to human safety. However, the project s mandatory compliance with applicable California Building Code requirements renders impacts Class III, less than significant. he project site is located within the seismically active central California Coast Ranges, but is not located in an Alquist-Priolo Earthquake Fault Zone. he major active faults capable of producing large magnitude events and that have a high seismic activity rate recognized in the region are the an Andreas, Hayward and an Gregorio faults. Other active faults in the site region include the Zayante-Vergeles, Monterey Bay- ularcitos, and Calaveras faults. Based on the Geologic Hazards tudy (Earth ystems Pacific, 2010) and Figure 4.6-1, there are no faults mapped on or adjacent to the site. However, the project would potentially experience strong ground shaking from earthquakes on any active or potentially active faults in the area, as would other properties in the Carmel Valley. he greatest site peak ground acceleration would result from an earthquake on the Monterey Bay ularcitos or an Gregorio (south) faults (Earth ystems Pacific, 2010). Despite the potential for ground shaking, the project would be required meet the current CBC seismic resistance standards, which ensure that new structures are engineered to withstand the expected ground acceleration at a given location. he also has policies and standards in place that regulate construction in areas subject to fault rupture and ground shaking. For example, General Plan Policy -1.5 requires that structures in areas that are at high risk from fault rupture not be permitted unless measures recommended by a registered engineering geologist are implemented to reduce the hazard to an acceptable level (County of Monterey, 2010). In addition, in accordance with Policy -1.8, new development may be approved only if it can be demonstrated that the site is physically suitable and the development would neither create nor significantly contribute to geologic instability or geologic hazards (, 2010). Compliance with all applicable provisions of federal, state, and local construction and design standards, and implementation of the recommendations of the Geotechnical Engineering Report and Liquefaction tudy prepared for the project (Buena Geotechnical ervices, 2007) would ensure that potential impacts would be less than significant. Mitigation Measures. No mitigation is required. ignificance After Mitigation. Impacts would be less than significant without mitigation. hreshold 1.iii: hreshold 3: Impact GEO-2 Expose people or structures to potential substantial adverse effects, including the risk of loss, injury, or death involving seismic-related ground failure, including liquefaction. 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. Due to the depth of groundwater on the project site, the potential for seismic related ground failure from 4.6-8

9 liquefaction of underlying soils is moderate. Impacts would be Class III, less than significant. he site lies within an area deemed to have a moderate susceptibility for liquefaction (County of Monterey, 2012). Based on the project Geotechnical Engineering Report (Buena Geotechnical ervices, 2007), soils below a depth of 23 feet at the site are likely to liquefy in the event of a nearby large magnitude earthquake. eismically-induced settlement of sufficient magnitude to cause structural damage is normally associated with poorly consolidated, predominantly sandy soils or variable consolidation characteristics within the building areas. he Geotechnical Engineering Report (Buena Geotechnical ervices, 2007) noted loose soil conditions and slightly clayey silty fine sands at the site. It is possible that these soils are susceptible to seismically induced settlement. Implementation of the recommendations of the Geotechnical Engineering Report and Liquefaction tudy prepared for the project (Buena Geotechnical ervices, 2007) would ensure that potential impacts would be less than significant. Mitigation Measures. No mitigation is required. ignificance After Mitigation. Impacts would be less than significant without mitigation. hreshold 2: Impact GEO-3 Result in substantial soil erosion or the loss of topsoil. Construction of the proposed project could result in soil erosion or loss of topsoil. However, compliance with existing regulations would reduce impacts to a Class III, less than significant, level. According to the NRC soils mapping for the project site, the project site is underlain by one soil type, Pico Fine andy Loam, which has a moderately low erosion potential. Grading associated with construction would temporarily expose bare soils, which could be removed from the site and transported through wind shearing or stormwater runoff. As discussed in ection 4.8, Hydrology and Water Quality, implementation of a NPDE-compliant tormwater Pollution Prevention Plan (WPPP) and additional requirements detailed in Chapter of the Monterey County Code of Ordinances and other applicable standards would be incorporated into the design of the project and would reduce potential impacts related to soil erosion to a less than significant level. Mitigation Measures. No mitigation is required. ignificance After Mitigation. Impacts would be less than significant. hreshold 3: 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

10 hreshold 4: Impact GEO-4 Be located on expansive soil, as defined in able 18-1-B of the Uniform Building Code (1994), creating substantial risks to life or property. he project site is not located on a geologic unit or soil that is unstable, and would not result in landslides, lateral spreading, subsidence, collapse, or soil expansion. Impacts would be Class III, less than significant. opography at the project site is relatively flat and the proposed project would not affect this topography such that an increased likelihood of landslides would result. he potential for lateral spreading at the site is low due to the flat topography. No documented areas of subsidence have been identified on or near the project site. Additionally, the Geotechnical Engineering Report and Liquefaction tudy states that on-site soils have a very low expansion potential (Buena Geotechnical ervices, 2007). Impacts related to landslides, lateral spreading, and expansion would therefore be less than significant. Impacts associated with unstable soils would be reduced through implementation of recommendations made in the Geotechnical Engineering Report and Liquefaction tudy. Mitigation Measures. No mitigation is required. ignificance After Mitigation. Impacts would be less than significant. c. Cumulative Impacts. he geographic scope for considering cumulative impacts to geology and soils is the project site along with the immediately adjacent areas. he geographic scope would also include off-site lands where earth movements at the project site could affect the local watershed. his scope is appropriate because geologic materials and soils occur at specific locales and are generally unaffected by activities not acting on them directly or immediately adjacent to them. In addition any geologic impacts of the project would be sitespecific. Past, present, and reasonably foreseeable future projects in Carmel Valley, as shown in able 3-1 in ection 3.0, Environmental etting, would add an estimated 524 dwelling units to the Valley. uch development would expose new residents and property to seismic and other geologic hazards. However, these seismic and soil issues are specific to each project and therefore, for purposes of this cumulative analysis, the geographic context is more narrow as well. It is expected that because of the site-specific nature of these issues, each development would be required to address said issues on a case-by-case basis through preparation of required soils and geotechnical engineering studies and adherence to the recommendations therein, in addition to adherence to existing local and state laws and regulations including the applicable CBC standards and requirements. hus, the combination of the project with other cumulative developments would not have a significant cumulative impact. Furthermore, with adherence to the applicable laws and regulations, the project s contribution to any cumulative geology and soils impacts would be less than significant