Section 5.7 Geology and Seismic Hazards SECTION 5.7 GEOLOGY AND SEISMIC HAZARDS This section describes the City of Buena Park’s existing geologic, seismic, and soil conditions, and the existing Federal, State, and local regulations with which development must comply. Geologic and seismic impacts that could result from implementation of the proposed General Plan Update are identified, and where appropriate, mitigation measures are recommended to avoid or lessen impacts. 5.7.1 EXISTING SETTING This section identifies existing earth resources, and seismic and geologic hazards within the City. Earth resources in this context include geologic and soil conditions. GEOLOGY Regional Conditions The City of Buena Park is located in the southeastern section of the coastal plain of Los Angeles and Orange counties. Specifically, Buena Park is located within the central lowland coastal plain of Orange County, which stretches northeasterly from the vicinity of Irvine, beyond Santa Ana and Garden Grove, and into Los Angeles County. The central lowland comprises the Downey and Tustin Plains, an approximately 300 square mile area. The coastal plain of Los Angeles and Orange counties is bounded by the Santa Ana Mountains, and the areas of Elysian, Repetto, and Puente Hills to the northeast; the Santa Ana Mountains to the southeast; the San Joaquin Hills to the south; and the Pacific Ocean on the west. The primary rivers traversing this coastal plain include the Los Angeles, San Gabriel, Rio Hondo, and Santa Ana rivers. The Rio Hondo River flows in a southwest direction across the coastal plain and merges with the Los Angeles River. The San Gabriel River flows south on the eastern portion of the coastal plain generally parallel to the Los Angeles River. The Santa Ana River stretches approximately 75 miles and has a total drainage of approximately 3,200 square miles. The Santa Ana River originates in the San Bernardino Mountains and traverses San Bernardino, Riverside, and Orange counties. The coastal plain of Los Angeles and Orange counties was formed from recent (Holocene) alluvial deposits. The alluvial fans of the Los Angeles, San Gabriel, Rio Hondo, and Santa Ana rivers resulted from the formation of a gently sloping plain through stream deposition. The portion of the coastal plain within Orange County is underlain by deep structural depression containing primarily sedimentary rocks. The subsurface of the County varies in thickness and lithology due to the rapid rate of deposition of rock units, folding, and faulting. The sedimentary deposits of the coastal plain are a hybrid of marine and continental sediment. A significant amount of the sedimentary deposits have been removed over time due to erosion, resulting in numerous discontinuities in the land surface. Draft EIR Page 5.7-1 Buena Park General Plan Update September 2010 Geology and Seismic Hazards Local Conditions The City of Buena Park consists of urbanized land on generally flat topography with a slope of less than 20 feet per mile. Buena Park consists of two different geomorphic areas: the Coyotes Hills and the Downey Plain. The Coyotes Hills form the northeast portion of the City and have an elevation ranging from approximately 400 to 600 feet. The Coyotes Hills were formed through earth movement and local faulting. The Downey Plain comprises the majority of the City’s land area and is characterized by nearly level topography. This plain was formed through a series of stream deposits from the Los Angeles, San Gabriel, Rio Hondo, and Santa Ana rivers and consists of weathered unconsolidated and semi-consolidated alluvial soils. The City is underlain by the Talbert Aquifer, and sedimentary deposits from recent time (15,000 years ago) and the Pleistocene period (one million years ago). The composition of these deposits is described below. RECENT TIME (15,000 YEARS AGO) SURFICIAL DEPOSITS Young Alluvial Channel Deposits (Qya) Locally young alluvial channel deposits include elements of late Holocene (approximately 11,000 years ago) alluvial. Young alluvium of Recent Time is found on or near the surface of the City and much of Orange County at a depth of approximately 175 feet. The top layer of young alluvium consists of recent stream channel deposits, including sand, silt, clay, and gravel, unconsolidated and semi-consolidated alluvial fan, and flood stream sediments. Alluvial deposits are typically poorly sorted and permeable. Lenses of fine sand and gravel along the coast and in scattered sections of the main portion of the groundwater basin contain perched and semi- perched water. Perched groundwater is a zone of saturation in a formation that is discontinuous from the water table (main body of groundwater) and the unsaturated zones surrounding this formation. Pleistocene Period (1 million years ago) Deposits Lower layers of soil composition in Buena Park consist of Pleistocene Period Stream Terrace and Older Alluvium, La Habra Formation, Lakewood Formation, Coyote Hills Formation, and San Pedro Formation. Stream Terrace and Older Alluvium The Stream Terrace and Older Alluvium are separated from Recent Time soils by unconformities in sedimentation. Older Alluvium and terrace deposits are composed of reddish brown, semi- consolidated silt, sand, gravel, and rubble. Older Alluvium deposits are generally above the water table. These deposits are generally permeable enough to transmit precipitation to underlying sediments. Page 5.7-2 Draft EIR September 2010 Buena Park General Plan Update Geology and Seismic Hazards La Habra Formation1 The La Habra Formation is found in the Coyote Hills and along the southern flank of the Puente Hills. It is a non-marine deposit and appears to have been an old flood plain. This formation consists of siltstone, thick-bedded friable sandstone, pebbly sandstone, and pebble-cobble conglomerate; locally abundant clasts of platy white siltstone. Lakewood Formation The Lakewood Formation ranges in thickness from 10 to 500 feet or more in the northern portion of Orange County. This formation consists of interbedded clay, silt, sand, and gravel that transmit water slowly. The Lakewood Formation merges with the La Habra Formation near the City of La Habra. Coyote Hills Formation The Coyote Hills Formation lies below the Lakewood and La Habra Formations. The Coyote Hills Formation has a maximum thickness of approximately 500 feet. This formation is composed of nonmarine sandstone and mudstone. The lower sandstone lies atop the San Pedro Formation and is approximately 250 feet thick. The upper mudstone has a maximum thickness of approximately 500 feet. According to the Department of Water Resources, sandstone deposits are permeable and mudstone deposits appear to be non-permeable. San Pedro Formation The San Pedro Formation contains deposits of marine sands, gravel, silts, and clays. Marine mollusks are locally abundant within this deposit. The San Pedro Formation ranges in thickness of approximately 275 feet in the Santa Ana Gap to approximately 800 feet in the northern portion of Orange County. There is a coarse sequence of sand and gravel interbedded with silt and clay lenses near the base of the San Pedro Formation. This unit is designated as the principal aquifers used for domestic water in the Orange County and Los Angeles areas. The Upper San Pedro Formation comprises the lower aquifer system below the San Pedro Formation and the Main aquifer. Local nonconformities and changes in the lithology of deposits within the Orange County Coastal Plain reflect varying degrees of tectonic movement around the end of the Pliocene Time. These deposits are composed of pebble conglomerates, conglomeratic sandstones, siltstones, and in some places abundant marine mollusks. The maximum thickness of the Upper San Pedro Formation ranges between approximately 900 feet at the eastern end of the East Coyotes Hills and approximately 1,400 feet north of Yorba Linda East Coyotes. SOILS The City of Buena Park is urbanized and primarily built-out. Surface soils in the City may no longer reflect the natural soil associations and characteristics identified below, since topsoil in the City has been predominately developed. Fill material of unknown origin and varying composition currently cover most of the City’s developed area. 1 Explanation for the Geologic Map of the Long Beach 30’ X 60’ Quadrangle, California, California Department of Conservation Geological Survey, 2003, http://www.conservation.ca.gov/cgs/rghm/rgm/ Pages/preliminary_geologic_maps.aspx, Accessed March 9, 2009. Draft EIR Page 5.7-3 Buena Park General Plan Update September 2010 Geology and Seismic Hazards Soil Associations A soil association is an overarching classification of similar soil types occurring on similar material or on a combination of rocks and soil types that have similar profiles, arrangements, sequence of layers, or other characteristics. The following soil associations underlie Buena Park:2 . Chino-Omi Association. The Chino-Omi association is characterized by nearly level, somewhat poorly drained, calcareous silt loams to clays on alluvial fans, flood plains, and basins. Hueneme-Bolsa Association. The Hueneme-Bolsa association is characterized by nearly level, poorly drained and somewhat poorly drained, calcareous fine sandy loams, silt loams, and silty clay loams on alluvial fans and flood plains. Metz-San Emigdio Association. The Metz-San Emigdio association is characterized by nearly level, somewhat excessively drained and well drained, calcareous loamy sands and fine sandy loams on alluvial