Community Safety Element – January 2020 VIII CS - 70
COMMUNITY SAFETY ELEMENT
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Community Safety Element – January 2020 VIII CS - 71
Table of Contents
I. Purpose and Relation to Other Elements 72
II. Background Information 72
Seismic Hazards 72
Alquist-Priolo Earthquake Fault Zones 72
Surface Rupture 72
Ground Shaking 73
Ground Failure 73
Tsunami 73
Seiche 73
Dam Failure 74
Slope Instability 74
Land Subsidence 75
Liquefaction 75
Flooding 75
Urban and Wildland Fires 76
Emergency Operation Plan 77
Climate Change 78
Evacuation Routes and Peak Load Water Supply Requirements 78 III. Goals, Objectives and Policies 79
Community Safety Element Diagram 81
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Community Safety Element – January 2020 VIII CS - 72
COMMUNITY SAFETY ELEMENT
I. Purpose and Relation to Other Elements
The Safety Element is intended to describe natural and man-made disasters which may pose a hazard to the residents of Atherton. It sets forth policies for responding to threats to public safety. It includes identification of unreasonable risks, and policies for the protection of the community from such risks. The goal of the safety element is to reduce the potential short and long-term risk of death, injuries, property damage, and economic and social dislocation resulting from fires, floods, droughts, earthquakes, landslides, climate change, and other hazards.
The Safety Element is closely related to the Circulation, Land Use, and Open Space and Conservation Elements as development plans must adequately account for public safety considerations, and open space for public health often incorporates area of increased hazard (for example, increase hazard associated with dam safety within the Bear Gulch Reservoir open space).
II. Background Information
Seismic Hazards
The primary seismic threat to the Town of Atherton is represented by the San Andreas fault and its attendant rift valley which lies approximately five miles to the west of the Town. This fault has a long history of earthquake activity. While there are no known active or potentially active faults within the Town of Atherton, it is subject to periodic, very strong earthquakes which originate either on the San Andreas or from the Hayward and Calaveras faults in the East Bay. Most geologists agree that an earthquake of comparable magnitude to that which occurred in 1906 may well be experienced by the current generation of Bay Area residents.
Alquist-Priolo Earthquake Fault Zones
Alquist-Priolo Earthquake Fault Zones are regulatory zones, delineated by the State Geologist, within which site-specific geologic studies are required to identify and avoid fault rupture hazards prior to subdivision of land and/or construction of most structures for human occupancy. There are no AlquistPriolo Earthquake Fault Zones within the Atherton Town limits. The closest such zone, the San Andreas Fault Zone, is located in Woodside, approximately one-half mile southwest of I-280. Other such zones are located in the East Bay and include the Hayward Fault Zone and the Calaveras Fault Zone.
Seismic hazards associated with earthquakes include the following:
Surface Rupture
Seismically induced surface rupture refers to a break in the ground’s surface and associated deformation resulting from the movement of a fault. Surface rupture is usually limited to a narrow zone along the fault.
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Community Safety Element – January 2020 VIII CS - 73
Since there are no know active or potentially active faults within the Town of Atherton, it is unlikely that significant seismically induced surface rupturing will occur within Town.
Ground Shaking
Seismically induced ground shaking poses a serious potential hazard to Atherton. In the future, the major source of earthquake damage is likely to come from the San Andreas Fault system, including the Hayward Fault and the Calaveras Fault branches in the East Bay area. The principal effect of such an earthquake in most of the Town will be a sudden, unexpected initiation of a strong shaking motion of the ground, which could last approximately one minute or more. This ground shaking can be expected to be hazardous to people during the earthquake.
Ground Failure
Seismically induced ground failure refers to mudslides, landslides, liquefaction or soil compaction caused by a seismic event. The California Department of Conservation has mapped areas where previous occurrence of landslide movement, or local topographic, geological, geotechnical and subsurface water conditions indicate a potential for permanent ground displacements such that mitigation would be required. Mitigation in this context means those measures that are consistent with established practice and that will reduce seismic risk to acceptable levels.
Tsunami
A tsunami refers to a series of waves generated in a body of water by a rapid disturbance that vertically displaces the water. These changes can be caused by an underwater fault rupture that generates an earthquake, a volcanic eruption, or underwater landslides typically triggered by earthquakes. The California Emergency Management Agency has prepared a series of maps plotting the potential inundation line for a tsunami runup along the San Francisco Bay shoreline. The inundation line represents the maximum considered tsunami runup from a number of extreme, yet realistic, tsunami sources. In the Atherton vicinity, the potential inundation line follows the southwesterly shoreline of the Westpoint Slough and the Ravenswood Slough located in the salt evaporators within the margins of San Francisco Bay. A runup of approximately 4 feet at Ravenswood Point (East Palo Alto) could occur, as estimated by the US Geological Survey. The inundation line runs approximately ½ to 1-mile northeast of the dike protecting the east Menlo Park and Redwood City industrial area. As the inundation line is located approximately 1 to 1 ½ miles northeast of the Atherton City Limit along Bay Road, there appears to be little chance that a tsunami would affect land within the Town. Further since the inundation line is located approximately ½ to 1-mile northeast of US 101 there appears to be little chance that a tsunami would affect that major evacuation route.
Seiche
Seismic seiches (sloshing) are standing waves set up on rivers, reservoirs, ponds, and lakes when seismic waves from an earthquake pass through the area. A seiche can overflow or even erode an embankment, potentially releasing significant volumes of water that could flood and damage developed areas downstream. Bear Gulch Reservoir is the only body of water within Atherton large enough to be subject to a seiche. A potentially damaging seiche at this location could adversely impact properties and development downstream.
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Community Safety Element – January 2020 VIII CS - 74
Dam Failure
The Bear Gulch Reservoir Dam is the only dam in Atherton and is large enough to endanger lives and property in the event of a failure. A seismic event could cause the dam to fail and endanger an estimated population of approximately 1,000 people, according to the Atherton Emergency Operations Plan. The flood plain that would result from catastrophic failure of this dam has been mapped by California Water Service Company (Cal Water), the dam owner; the map is on file with the Office of Emergency Services.
The California Division of Safety of Dams (DSOD), (a division of the Department of Water Resources [DWR]) routinely and periodically inspects the dam for performance and problem identification. Should a problem be identified that could lead to failure potential the dam owner (or as directed by the DSOD) takes mitigating actions such as reducing the water level to avoid catastrophic loss of water or other actions specified in its Emergency Action Plan.
The Atherton neighborhood most seriously threatened by dam failure and wildfire hazard is the Walsh Road neighborhood. This neighborhood has only one primary evacuation route; Walsh Road, which is a narrow, two lane residential street that intersects with Alameda de las Pulgas. Two other evacuation routes have been identified:
• The main secondary automobile evacuation route is the road at the end of Reservoir Road through the Cal Water property adjacent to Bear Gulch Reservoir connecting to Moore Road. Cal Water must open the gate at the entrance of the road for this to be passable. Cal Water maintains a 24/7 presence at the Bear Gulch site. Calling 1-855-CAL-WATER, then selecting “1” for emergency will provide access to a Cal Water employee who will notify an on-site employee to unlock the gate. • There is a pedestrian only exit using the horse tunnel from Valley Court under highway 280.
In the event of a fire or flood, a warning siren has been installed at the Cal Water facility on Reservoir Road. The siren can be activated by the Fire or Police Department to advise residents that an evacuation should take place.
An all-volunteer group of concerned Atherton residents formed the Atherton Disaster and Preparedness Team (A.D.A.P.T.) to collaborate with town officials, Menlo Park Fire, Atherton Police and other professional emergency responders and the California State “Get Ready” and FEMA’s/US Citizens’ Corps programs to help organize, train, educate, communicate with and aid fellow Athertonians in preparing for major emergencies and natural disasters. A.D.A.P.T is sponsored by the Atherton Police Department and is linked to the Menlo Park Fire Protection District’s Community Crisis Management (CCM)/Community Emergency Response Team (CERT) Program.
Slope Instability
Landslides include all movements of soil, rock, or debris as a result of falling, sliding, or flowing. Most landslides are a combination of two or more types of motion and/or material. Landslides are categorized according to the types of motion and material involved. They can be directly caused by earthquakes or be completely independent of them.
• Falls describe the sudden movement of material from vertical or near-vertical slopes and are generally labeled by the type of material displaced (e.g. soilfall, rockfall).
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• Slides refer to movements in which the material moves more or less as a unit along recognizable shear surface. If the shear surface is concave, the slide movement will be rotational and is denoted by the term "slump." If the shear surface is planar, transnational movement occurs and the term "slide" is used alone. Both slides and slumps are further classified according to the type of material involved (e.g., earth slump, rockslide, debris slide where "debris" refers to combinations of soil, weathered bedrock and/or organic material).
• Flows describe the movement of material in which a myriad of small-scale movements rather than massive sliding is the dominant mechanism of transport. This category is further broken down by the type of material involved and the rate at which it moves (e.g., debris flow, mudflow). The modifier "avalanche" is used to describe exceptionally fast flows.
Much of the land surface in Atherton is relatively flat and not subject to slope instability. Land west of Alameda de las Pulgas however is steeper and therefore subject to slope instability. A map prepared by San Mateo County which shows the general location of existing landslides, characterizes the area west of Alameda de las Pulgas as having “few landslides”. Another map produced by the Association of Bay Area Governments (ABAG) identifies “earthquake induced landslide study zones” and “rainfall induce study zones” each contain a few acres on the south side of Walsh Road and near Bear Gulch Reservoir.
Land Subsidence
Land subsidence is defined as the lowering of the land surface. Many different factors can cause the land surface to subside. Subsidence can occur rapidly due to a sinkhole or underground mine collapse, or during a major earthquake. It may happen slowly in the case of groundwater withdrawal or natural gas extraction. In Atherton the subsurface composition is such that sinkholes have rarely occurred. There are there no known mines or natural gas fields. There has been groundwater withdrawal, however the withdrawal has not resulted in significant land subsidence. A program to monitor measurements of landsurface elevations and future subsidence is on-going and described in the Open Space and Conservation Element. While there could be seismically induced land subsidence in Town during a major earthquake, such an effect has not been known to have occurred in the past.
Liquefaction
Loose sand and silt that is saturated with water can behave like a liquid when shaken by an earthquake.
This phenomenon is called liquefaction. During an earthquake the soil can lose its ability to support structures, flow down even very gentle slopes, and erupt to the ground surface to form sand boils. Many of these phenomena are accompanied by settlement of the ground surface, usually in uneven patterns that damage buildings, roads and pipelines.
A map produced by ABAG identifies liquefaction susceptibility hazards in Atherton. In general, the area of Town northeast of a line formed by Euclid Avenue/Monte Vista Avenue/Camino por los Arboles is characterized as having a “moderate susceptibility”. The narrow band of land adjacent to the Atherton Channel is characterized has having a “very high susceptibility”. The balance of the Town is characterized as having a “very low to low susceptibility”.
Flooding
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Community Safety Element – January 2020 VIII CS - 76
Flooding has not presented a significant, extensive hazard in Atherton in the past. There have been numerous recurring localized areas of flooding. During the 2001 Town-wide Drainage Study, 97 localized flooding complaints were identified and evaluated. These events were classified by type of problem such as building floods, saturated or clogged drywell, channel or ditch overflow, driveway and intersection floods, and storm system overflow or clog. Many of these problem areas were addressed with improvement projects implemented since 2001 or with maintenance activities.
In 2015 the Town-wide Drainage Study was updated. During that process, 17 localized flooding complaints were identified; three of which coincided with flooding complaints from the 2001 Drainage Study. The report includes general and specific recommendations for mitigating these hazards.
There are no Federal Emergency Management Agency (FEMA) identified flood prone or hazard areas in Atherton. The Town has chosen not to participate in the National Flood Insurance Program.
There are areas within the Town, due to their proximity to the Atherton Channel or in portions of lowerlying Lindenwood, which require raised finished floor elevations (typically by approximately 1 foot) during new construction. Finished floor elevations in these areas are recommended by the project engineer based on studies required by the Town during the grading and drainage plan review process.
Flooding resulting from failure of the Bear Gulch Reservoir dam is a hazard that is addressed under the topic of Dam Failure in this Element.
Atherton has entered into a partnership with the jurisdictions of Redwood City, Menlo Park, and San Mateo County to complete the planning for the proposed Bayfront Canal/Atherton Channel Flood Protection and Restoration Project. The Atherton Channel and Bayfront Canal watersheds have experienced decades of repetitive flooding in the lower reaches of the channels in Redwood City. This project is further discussed in the Conservation Element.
The proposed Atherton Water Capture Project, a runoff diversion, storage and filtration system is discussed in the Open Space and Conservation Element.
The use of green infrastructure techniques and systems can help to reduce the impacts of localized flooding associated with stormwater runoff, the Atherton Channel, and Bayfront Canal while also assisting the Town in satisfying the provisions of the MRP.1
Urban and Wildland Fires
Fire protection for Atherton is provided by the Menlo Park Fire District; a special district that serves the cities of Menlo Park, Atherton, East Palo Alto and portions of San Mateo County. Backup assistance for the Fire District is available through mutual aid agreements. All fire agencies in San Mateo County have signed the California Master Mutual Aid Agreement and participate in mutual aid operations as required.
The Menlo Park Fire District also has specific Mutual Aid agreements with the cities of Palo Alto and Redwood City.
1 Source: CD+A
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Community Safety Element – January 2020 VIII CS - 77
The Fire District actively works to prevent structural and wildfires through its regulations, education and training programs; some of which include residential and commercial fire sprinkler requirements, plan review of new construction, periodic inspection of commercial buildings, weed abatement, defensible spaces, home ignition zones, disaster and emergency preparedness.
Wildland fire is a hazard that exists throughout the Town of Atherton. The California Department of Forestry and Fire Protection (CAL FIRE) has mapped fire threatened communities in the State. The entire Town of Atherton has been identified as a “Community At Risk”.
Wildfire hazard in the Walsh Road neighborhood is a topic that is addressed in the Dam Failure section of this Element.
Emergency Operations Plan
The Town of Atherton Police Department and the Menlo Park Fire Protection District have jointly prepared the Town of Atherton Emergency Operations Plan (EOP) which describes how the jurisdictions will manage and coordinate resources and personnel responding to emergency situations. The Atherton EOP, along with a companion document; the Atherton Hazard Mitigation Strategies (HMS) is the Town’s Local Hazard Mitigation Plan specified in the federal Disaster Mitigation Act of 2000 (P.L. 106-390). The HMS contains policies designed to mitigate hazards identified in the EOP.
The Atherton Town website currently has a section entitled “Evacuation Plan & Emergency Siren” that provides information about the Walsh Road emergency siren and evacuation routes in the event of a fire or flood. The same website section provides general information about the Atherton Disaster and Preparedness Team (A.D.A.P.T) a Police Department sponsored emergency preparedness and action program as well as other preparedness resources for the community.
The Town of Atherton EOP is designed to be consistent with Homeland Security Presidential Directive (HSPD)-5, National Incident Management System (NIMS) and the California Standardized Emergency Management System (SEMS) requirements. The plan:
• Conforms to the National Incident Management System (NIMS) and the Standardized Emergency Management System (SEMS)
• Provides Emergency Operations Center (EOC) responders with procedures, documentation, and user-friendly checklists to effectively manage emergencies • Provides detailed information of supplemental requirements such as Public Information, Damage Assessment, and Recovery Operations.
The Town of Atherton Emergency Operations Plan is a document that is continually evolving. The EOP provides a comprehensive emergency response document that includes detailed information covering Emergency Operations Center procedures, documentation and reference and support information.
Pursuant to California Government Code Section 65302.6, the Atherton Emergency Operations Plan together with the Atherton Hazard Mitigation Strategies are hereby adopted by reference and included in this Community Safety Element. Further, any future amendments to the EOP and HMS are adopted by reference and included in this Element.
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The Town also participates with a coalition of San Mateo County cities and special districts in updating and adopting the San Mateo County Hazard Mitigation Plan.2 This plan includes an assessment of the risk and vulnerability to the impacts of natural hazards, develops a mitigation strategy consistent with a set of uniform goals and objectives, and creates a plan for implementing, evaluating and revising the strategy.
Climate Change
Atherton’s proposals and policies related to climate change are contained in its adopted3 Climate Action Plan. The Town’s Climate Action Plan serves as a guiding document to identify methods that the Town and community can implement to significantly reduce greenhouse gas (GHG) emissions. The Plan provides a comprehensive roadmap of programs that can be implemented to reduce emissions and increase sustainability. Transportation aspects of the Climate Action Plan are addressed in the Circulation Element.
Energy, water and solid waste programs and policies are addressed in the Open Space and Conservation Element.
Atherton has adopted a target of reducing GHG emissions to 15 percent below 2005 levels by 2020.
Evacuation Routes and Peak Load Water Supply Requirements Pursuant to the State Planning Guidelines, evacuation routes have been designated in the Policies below.
State Planning Guidelines require the Safety Element to include a statement specifying the peak load water supply requirements of the Town. Peak load water supply requirements currently average just under five million gallons per day during the months of August and September.
Emergency evacuation routes are shown on the Community Safety Diagram, Figure CS-1. These routes are generally located along minor arterials that provide access from all Atherton neighborhoods to El Camino Real, and, ultimately U. S. 101 and I-280. The streets included are El Camino Real, Middlefield Road, Marsh Road, Alameda de las Pulgas, Atherton Avenue/Fair Oaks Lane, Valparaiso Avenue, Stockbridge Avenue, Glenwood Avenue, Encinal Avenue, Watkins Avenue and Ringwood Avenue. These routes would be used for general emergencies (e.g. earthquake, wildfire) and dam failure at Bear Gulch Reservoir. 2 Adopted October 19, 2016
3 Adopted October 19, 2016
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Community Safety Element – January 2020 VIII CS - 79
III. Goals, Objectives, Policies and Actions
Goal CS-1: The Town recognizes the potential danger to public safety that may result from natural or man-made causes and seeks to minimize the public risks in such hazards.
Goal CS-2: Reduce the risk of injury, structure and property damage from exposure to seismic activity.
Policy CS-2.1: Support the Goals, Objectives and Policies contained in adopted Atherton local hazard mitigation plans and Emergency Operations Plans.
Policy CS-2.2: Public education, research and information dissemination on seismic hazards and emergency response shall be encouraged.
Policy CS-2.3: The Town shall seek to improve interjurisdictional cooperation with other agencies for geotechnical safety in land use planning, hazard prevention and emergency response.
Goal CS-3: Reduce hazards related to natural flooding and potential inundation from failure of the Bear Gulch Reservoir Dam.
Goal CS-4: Support any Town Green Infrastructure programs that address stormwater infrastructure that may use natural processes to manage water and create healthier urban environments.
Goal CS-5: Prevent and reduce risks to property and protect residents from urban and wildland fire hazards.
Goal CS-6: Support the Town’s ability to respond effectively to natural and human-caused emergencies.
Policy CS-6.1: Support the preparation, implementation and regular update of local preparedness and evacuation plans, training and education; and multijurisdictional cooperation and communication for emergency situations.
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Policy CS-6.2: Continue to participate in regional emergency planning efforts.
Policy CS-6.3: The emergency evacuation routes established in this General Plan Element are El Camino Real, Middlefield Road, Marsh Road, Alameda de las Pulgas, Atherton Avenue/Fair Oaks Lane, Stockbridge Avenue, Valparaiso Avenue, Glenwood Avenue, Encinal Avenue, Watkins Avenue and Ringwood Avenue.
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BEAR GULCH
RESERVIOR
BAY RD
SELBY LN
MARSH RD
POLHEMUS AVE
PARK LN
JAMES AVE
SELBY LN
VALPARAISO AVE
FAIR OAKS LN
WATKINS AVE
STOCKBRIDGE AVE STOCKBRIDGE AVE
ATHERTON AVE ATHERTON AVE
AUST NI AVE
ALA
MEDA DE LAS PULG
AS
STATE H GI HWAY
82 HWY
WALSH RD
MIDDLEFIELD
RD MIDDLEFIELD RD
ALMENDRAL AVE
OAK GROVE AVE
Community Safety Diagram
Figure SC-1
0 Miles
0.5 1 /
Legend
Town Boundary
Earthquake Induced Landslide Zones
Emergency Evacuation Routes
Liquefaction Zones
Bear Gulch Dam Failure Inundation Area
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