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HomeMy WebLinkAbout07_Sec4.2_FloodInundationHazard 4.2 Flood and Inundation Hazards 4.2FLOOD AND INUNDATION HAZARDS This section describes existing flood and inundation hazards, including creek overflow, dam and levee failure, and seismically-induced flooding, within the MEIR Study Area and describes whether implementation of the proposed project would cause a substantial change in the characteristics of local flood and inundation hazards. Surface water drainage, water quality, and other water resource issues, including flooding as a result of storm drain system deficiencies, are described in Section 4.13 (Utilities and Service Systems) of this MEIR. Preparation of this section used data from various sources. These sources include the City of South San Francisco General Plan prepared by Dyett & Bhatia and adopted in October 1999, and the East of 101 Area Plan prepared by Brady and Associates and adopted in July 1994. Full bibliographic entries for all reference materials are provided in Section 4.2.4 (References). One comment letter related to flood and inundation hazards was received in response to the December 9, 2005, Revised Notice of Preparation (NOP) circulated for the project. The County of San Mateo indicated that the MEIR Study Area is outside of the Colma Creek Flood Control Zone. In addition, no comments were received at the public scoping meeting held January 17, 2006. The NOP and comment letters are included in Appendix A of this MEIR. 4.2.1Existing Conditions MEIR Study Area The MEIR Study Area is located along the eastern shoreline of the City overlying artificial fill and Bay mud. The MEIR Study Area is largely paved and occupied primarily by buildings and parking lots. At the center of the MEIR Study Area is San Bruno Hill, which features the Wind Harp Sculpture Park. San Francisco Bay forms the eastern boundary of the MEIR Study Area, while the rest of the area is surrounded by mixed industry, warehouse, retail, office, and hotel uses in the East of 101 Area. The East of 101 Area generally slopes downward to the east, towards San Francisco Bay. The MEIR Study Area, itself, comprises a hilly region to the south, formed by southeast-trending Coyote Point Fault Zone, and low-lying areas to the northeast. Elevations range from 182 feet above mean sea level (MSL) at the top of San Bruno Hill to approximately 0 feet MSL at the low-lying areas in the northeast portion of the MEIR Study Area (USGS 1956). The City has a Mediterranean type of climate, characterized by dry, relatively cool summers and wet, mild winters. Average annual precipitation in the City is between 18 and 22 inches per years, increasing to 26 inches in the upper watersheds west of the City. Approximately 90 percent of the precipitation is received between November and April (D&B 1997). Runoff throughout the City is collected in the City?s storm drainage system, which discharges to Colma Creek or San Francisco Bay, and is described further in Section 4.13 (Utilities and Service Systems). 4.2-1 Genentech Corporate Facilities Master EIR Chapter 4 Environmental Analysis Floodplain Risk The City of South San Francisco is highly urbanized with relatively high runoff generation rates (D&B 1997). These conditions increase the potential for flood conditions in periods of heavy rainfall. Periodic flooding occurs along most of Colma Creek, but the principal flooding problems in the City exist near its eastern edge, just southwest of the MEIR Study Area, where flows of Colma Creek are limited by passage under the Caltrain railroad tracks and US 101. According to the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) for the City, no portion of the MEIR Study Area is located in the 100-year or 500-year floodplains, which are the result of flooding in Colma Creek. Portions of the City along US 101 are in the 500-year floodplain, but the 500-year floodplain does not extend much further east of Harbor Way. The location of the MEIR Study Area in relation to the FEMA-designated floodplain is shown in Figure 4.2-1. However, according to the City?s General Plan, the eastern shoreline of the MEIR Study Area is subject to 100-year flood conditions as a result of coastal flooding and wave action. The San Mateo County Flood Control District (SMCFCD) is responsible for maintenance of Colma Creek. The SMCFCD has developed a Colma Creek Flood Control Improvement Project, which is expected to reduce flood hazards on all reaches of Colma Creek from a 10- to 20-year storm and even to a 50-year storm recurrence (D&B 1997). The area constrained by flood hazards will be greatly reduced and may be eliminated after completion of the project. After its completion, FEMA will remap the 100- year floodplain. Other Inundation Hazards Dam Failure Dam failure often results from neglect, poor design, or structural damage caused by a major event such as an earthquake or flood. When a dam fails, the quantity of water held back by the dam (i.e., the contents of the reservoir) is suddenly released downstream, causing damage in its inundation zone. Although there are no dams within the City of South San Francisco, several dams are in the nearby cities of San Francisco, Burlingame, and San Mateo. Inundation zones resulting from failure of these dams would not reach the City of South San Francisco (ABAG 1995). Levee Failure As with dams, levee failure can occur in the event of a major earthquake or flood. The largest levees in the Northern California region are in the Sacramento-San Joaquin River Delta, on the American River, and on the Sacramento River. However, inundation zones resulting from failure of these levees would not reach the City of South San Francisco. Smaller levees are present throughout the Bay Area along the San Francisco Bay shoreline and in local ponds and creeks. These levees include those in the San Francisquito Creek flood control system, the Foster City and Redwood Shores levee trails, and the Cargill salt ponds. 4.2-2 Genentech Corporate Facilities Master EIR 4.2 Flood and Inundation Hazards Seiche Reservoirs, lakes, ponds, swimming pools, and other enclosed bodies of water are subject to potentially damaging oscillations (sloshing) called seiches. This seismic hazard is dependent upon specific earthquake parameters (e.g., frequency of the seismic waves, distance and direction from the epicenter), as well as site-specific design of the enclosed bodies of water, and thus difficult to predict. Genentech?s 1.5 million gallon storage reservoir on the top of San Bruno Hill poses the greatest risk of seiche hazards in the MEIR Study Area. Tsunami Earthquakes can also cause tsunami (i.e., tidal waves) in San Francisco Bay. As specified in the City?s General Plan: Existing Conditions and Planning Issues, two portions of the City are subject to inundation by tsunami; the northwest portion of the MEIR Study Area (Lower Campus) extending north past Oyster Point Marina Park, and the area of land south of the MEIR Study Area bordered by Littlefield Avenue and Swift Avenue (see Figure 4.2-1). Wave run-up is estimated at approximately 4.3 feet above mean sea level (MSL) for tsunami with a 100-year recurrence and 6.0 feet MSL for a 500-year tsunami (D&B 1997). Mudflow Mudflows (i.e., debris flows, mudslides) are rivers of rock, earth, and other debris saturated with water. They develop when water rapidly accumulates in the ground, such as during heavy rainfall, changing the earth into a flowing river of mud or "slurry." A slurry can flow rapidly down slopes or through channels, and can strike with little or no warning at avalanche speeds. A slurry can travel several miles from its source, growing in size as it picks up trees, cars, and other materials along the way. Mudflow hazards are primarily concentrated in the Hillside Zones of the City, where slopes are steep and are covered with exposed soil. Hillside Zones are at the southern flank of San Bruno Mountain and near Skyline Boulevard, but not in or near the MEIR Study Area (D&B 1997). 4.2.2Regulatory Framework Flood hazards are regulated under a variety of local, state and federal laws. The following summarizes the applicable flood damage protection regulations that apply to the City of South San Francisco. Federal National Flood Insurance Program FEMA is responsible for determining flood elevations and floodplain boundaries based on US Army Corps of Engineers studies. FEMA is also responsible for distributing the FIRMs, which are used in the National Flood Insurance Program (NFIP). San Mateo County participates in the NFIP. FIRMs identify the locations of special flood hazard areas, including the 100-year and 500-year floodplain. Portions of the MEIR Study Area that would contain structures are within the 500-year floodplain. Federal 4.2-5 Genentech Corporate Facilities Master EIR Chapter 4 Environmental Analysis regulations governing development in a Zone A (100-year) floodplain are set forth in Title 44, Part 60 of the Code of Federal Regulations (CFR), which enables FEMA to require municipalities that participate in the NFIP to adopt certain flood hazard reduction standards for construction and development within floodplains. Mudflows are also covered under the NFIP which San Mateo County participates in. State There are no state flood control regulations applicable to the proposed project. Local San Mateo County Flood Control District The San Mateo County Flood Control District (SMCFCD) is a countywide special district that was created by state legislation in order to provide a mechanism to finance flood control projects. The legislation requires that a flood control zone be formed over an entire watershed and a proposed funding source be determined before a flood control project is undertaken. Recent changes in the California Constitution require an election if a flood control zone is to be financed with property assessments or taxes. There are currently three active flood control zones?Colma Creek, San Bruno Creek, and San Francisquito Creek?none of which directly affect the MEIR Study Area. City of South San Francisco General Plan The City?s General Plan contains policies designed to protect people and development from damage associated with flooding. Policies applicable to the 2006 FMPU are as follows. Health and Safety Element: Flooding 8.2-G-1 Minimize the risk to life and property from flooding in South San Francisco. 8.2-I-2 Use the City?s development review process to ensure that proposed development subject to the 100-year flood provides adequate protection from flood hazards, in areas identified in Figure 8-3. City of South San Francisco Municipal Code The purpose of Chapter 15.56 (Flood Damage Prevention) of the South San Francisco Municipal Plan (SSFMC) is to promote the public health, safety and general welfare, and to minimize public and private losses due to flood conditions. To accomplish this purpose, this chapter includes methods and provisions to: Restrict or prohibit uses which are dangerous to health, safety, and property due to water or erosion hazards, or which result in damaging increases in erosion or flood heights or velocities. Require that uses vulnerable to floods, including facilities which serve such uses, be protected against flood damage at the time of initial construction. Control the alteration of natural floodplain, stream channels, and natural protective barriers, which help accommodate or channel flood waters. 4.2-6 Genentech Corporate Facilities Master EIR 4.2 Flood and Inundation Hazards Control filling, grading, dredging, and other development which may increase flood damage. Prevent or regulate the construction of flood barriers which will unnaturally divert flood waters or which may increase flood hazards in other areas. The provisions contained in SSFMC Chapter 15.56 are applicable only to development in special flood hazard areas as identified by FEMA. According to the FEMA FIRM, no portion of the MEIR Study Area is located in the 100-year or 500-year floodplain (Figure 4.2-1). However, according to the City?s General Plan, the eastern shoreline of the Study Area is subject to 100-year flood conditions, as a result of coastal flooding and wave action. East of 101 Area Plan The MEIR Study Area is within the East of 101 Area, which is bounded by the San Francisco Bay on the east, US 101 and rail lines on the west, the City of Brisbane on the north, and San Francisco International Airport on the south. The East of 101 Area Plan, prepared in July 1994, is a policy document that guides land use, circulation, noise control, geotechnical safety, public facilities provision, design, recreation and natural resource enhancement, and financing in the East of 101 Area. The Public Facilities Element of the East of 101 Area Plan includes the following policy applicable to the 2006 FMPU: Policy PF-8 Specific development proposals in the East of 101 Area shall be evaluated individually to determine drainage and flood protection requirements. 4.2.3Project Impacts and Mitigation Analytic Method The analysis in this section focuses on the effects of construction and operation of the proposed project on existing flood and inundation hazards. The increase in flood risks associated with implementation of the 2006 FMPU is also analyzed. The information in this section is based upon reviews of previously prepared reports documenting flood and inundation hazards in the City, including the City of South San Francisco General Plan: Existing Conditions and Planning Issues report. In determining the level of significance, the analysis assumes that construction and operation of the 2006 FMPU would comply with relevant federal laws and regulations, City General Plan policies, and ordinances. Thresholds of Significance The following thresholds of significance are based on Appendix G of the 2006 CEQA Guidelines. For purposes of this MEIR, implementation of the proposed project could result in potentially significant impacts on flood hazards and inundation if the proposed project would result in any of the following:: Place housing within a 100-year flood hazard area as mapped on a federal Flood Hazard Boundary or Flood Insurance Rate Map or other flood hazard delineation map. Place within a 100-year flood hazard area structures which would impede or redirect flood flows. 4.2-7 Genentech Corporate Facilities Master EIR Chapter 4 Environmental Analysis Expose people or structures to a significant risk of loss, injury or death involving flooding, including flooding as a result of the failure of a levee or dam. Expose people or structures to inundation by seiche, tsunami, or mudflow. Impacts and Mitigation Measures Threshold Place housing within a 100-year flood hazard area as mapped on a federal Flood Hazard Boundary or Flood Insurance Rate Map or other flood hazard delineation map Impact 4.2-1 Implementation of the proposed project would not place housing in a no impact designated flood hazard area and would result in . The 2006 FMPU would not construct housing in the MEIR Study Area. Furthermore, the MEIR Study Area is not within the 100-year flood hazard area. However, as discussed in 4.2.1 (Existing Conditions), the shoreline of the MEIR Study Area is considered a 100-year flood hazard area by the City?s General Plan. Coastal flooding and wave action during a 100-year storm would inundate the narrow strip of shoreline bordering the MEIR Study Area to the east. Since, housing would not be placed within the 100- no impact year flood hazard area under the 2006 FMPU, there would be . Threshold Place within a 100-year flood hazard area structures which would impede or redirect flood flows Impact 4.2-2 Implementation of the proposed project could place structures in a 100- year flood hazard area, but not in a manner that would substantially less than impede or redirect flood flows. This impact is considered significant . As described in Existing Conditions, the MEIR Study Area is not within a FEMA-designated floodplain. However, the shoreline of the MEIR Study Area is considered a 100-year flood hazard area by the City?s General Plan. Coastal flooding and wave action during a 100-year storm would inundate the narrow strip of shoreline bordering the MEIR Study Area to the east. A wider strip of land just inland and along the shoreline is designated by the Bay Conservation and Development Commission as the Bay Trail, a public open space area. For this reason, it is unlikely that development under the 2006 FMPU would place structures directly on the shoreline. However, potential opportunity sites identified in the 2006 FMPU are near the shoreline. Unlike flood flows along a drainage channel, flood flows at the shoreline would not travel a substantial distance on land. Coastal flood waters would run up onto land and recede back to San Francisco Bay. The flow of coastal flood waters that encounter a building constructed under the 2006 FMPU would not be substantially obstructed or redirected because the path of flow, i.e., the width of the shoreline, is short. In addition, structures that substantially impede flood flows, such as dams and levees, would not be constructed under the 2006 FMPU. Therefore, the impact of the 2006 FMPU on less than significant. flood flows is 4.2-8 Genentech Corporate Facilities Master EIR 4.2 Flood and Inundation Hazards Threshold Expose people or structures to a significant risk of loss, injury or death involving flooding, including flooding as a result of the failure of a levee or dam Impact 4.2-3 Implementation of the proposed project could place people or structures in an area susceptible to flooding, but would not expose people or structures less to significant risk of loss, injury, or death. This impact is considered than significant . As analyzed in Impact 4.2-2, above, while it is unlikely that development under the 2006 FMPU would place structures on the shoreline where there is a flood hazard area, potential opportunity sites identified in the 2006 FMPU are near the shoreline. Thus, structures near the shoreline could potentially be exposed to coastal flooding. Buildings constructed in flood hazard areas are required to comply with the construction standards contained in Chapter 15.56 of the SSFMC. Section 15.56.140 identifies standards specific to construction in coastal high hazard areas. Developments shall be elevated above the flood level, anchored, and constructed of materials resistant to flood damage. Thus, implementation of the 2006 FMPU would not expose people or structures to substantial risks involving flooding. As described in Existing Conditions, the MEIR Study Area is not prone to flooding in the event of dam or levee failure. Failure of a small-scale levee in the vicinity of the City would not release a volume of water such that the MEIR Study Area would become flooded. Thus, the 2006 FMPU would not expose people or structures to flood risks associated with dam or levee failure and this impact would be less than significant. considered For an analysis of potential flood hazards associated with storm drain system deficiencies, see Section 4.13 (Utilities and Service Systems). Threshold Expose people or structures to inundation by seiche, tsunami, or mudflow Impact 4.2-4 Implementation of the proposed project could expose people and structures to inundation by tsunami, resulting in a potentially significant impact. Compliance with flood damage prevention provisions of the City?s less than Municipal Code would reduce tsunami inundation impacts to significant . The 1.5 million gallon storage reservoir in the MEIR Study Area could experience seiching during an earthquake. Because the reservoir is situated on top of San Bruno Hill near the center of the Genentech Campus, water from the reservoir would flow downhill, potentially reaching structures and people in various parts of the MEIR Study Area surrounding San Bruno Hill. However, because the reservoir holds a relatively small volume of water, water released during seiching would be largely absorbed in the vegetated hillsides. Because the hillsides are not very steep, the flow of water would not be rapid. Also, water would drain away from the hill instead of ponding and resulting in high water levels. Thus, seiche less than significant inundation impacts are considered to be in the MEIR Study Area. 4.2-9 Genentech Corporate Facilities Master EIR Chapter 4 Environmental Analysis As shown in Figure 4.2-1, the northwestern portion of the MEIR Study Area in the Lower Campus could potentially be inundated by a tsunami. A few of the potential opportunity sites identified in the 2006 FMPU are in the tsunami inundation zone. As such, the proposed project could expose people and structures to inundation by tsunami. However, as described in Impact 4.2-3, buildings constructed in flood hazard areas are required to comply with the construction standards contained in Chapter 15.56 of the SSFMC. Specifically, Section 15.56.140 identifies standards specific to construction in coastal high hazard areas, which include areas subject to inundation by tsunami. Developments shall be elevated above the flood level, anchored, and constructed of materials resistant to flood damage. In addition, both Genentech and the City have in place emergency services to respond to natural disasters. Genentech?s Environmental, Health, and Safety (EHS) team and designated production and research personnel have training in emergency preparedness and response, and serve as first responder for the entire campus (D&B 2005). Genentech currently has an Alternate Command Post (ACP) located in the Upper Campus. The ACP is equipped with all necessary emergency gear and equipment to allow a focus of emergency activities in the event of a natural disaster. The City?s emergency services are established in SSFMC Chapter 2.72, which also provides for the preparation and implementation of plans for protection of people and property in the event of an emergency. Under the General Plan, the City?s Emergency Response Plan serves as the guide for emergency management. Thus, compliance with the SSFMC requirements would reduce potential impacts resulting from tsunami inundation to a less-than- significant level under the 2006 FMPU. The potential for inundation by mudflow is considered low because the MEIR Study Area does not contain steep slopes of exposed soil. Hillsides in the MEIR Study Area, such as that of San Bruno Hill, are covered by development and/or landscaping. Rainfall onto these areas would encounter vegetation or impervious surfaces, and would not pose a risk of causing saturated soil to loosen and flow downhill. Thus, there would be no mudflow inundation impact on the MEIR Study Area. 4.2.4References Association of Bay Area Governments (ABAG). 1995. Dam Failure Inundation Areas in the San Francisco Bay Region, http://www.abag.ca.gov/bayarea/eqmaps/damfailure/damfail.html, accessed 11 January, 2006. Brady and Associates for the City of South San Francisco. 1994. East of 101 Area Plan, July. Dyett & Bhatia (D&B) for the City of South San Francisco. 2003. City of South San Francisco General Plan, October 1999, as amended December 2003. ???. 1997. City of South San Francisco General Plan: Existing Conditions and Planning Issues, September. ???. 2005. Draft Genentech Central Campus Ten-Year Master Plan, November. USGS. 1956. San Francisco South Quadrangle, California, 7.5 Minute Series (Topographic), photo revised 1980. 4.2-10 Genentech Corporate Facilities Master EIR