Issues/ Opportunities/ Constraints
Significant potential hazards to public health and safety exist in Mono County. These hazards include: avalanches; floods; fires; geologic hazards such as landslides and mudflows; seismic hazards; and volcanic eruptions. The following section briefly discusses the constraints to development posed by each of these hazards.
Mono County covers an area that is relatively young by geologic standards. It is located at a stress point where the earth's crustal plates are exerting opposite pressures against each other. This combination creates both "tectonic" earthquakes (e.g., land mass movement) and volcanic activity that can trigger earth shaking (e.g., magma chamber movement and lava dyke formations).
Earthquakes are usually caused by sudden movement along geologic faults. The California Department of Conservation, Division of Mines and Geology (DMG), has evaluated potentially and recently active faults throughout Mono County including most of the community areas. Based upon these DMG studies, fault hazard zones (Alquist-Priolo Special Studies Zones) have been designated for the county (see the MEA or General Plan Map).
The primary seismic hazard in the county is strong to severe ground shaking generated by movement along active faults. The entire county, except for a small portion of the Sierra crest, is in an area where intense ground shaking is possible. This area has been designated as a Seismic Zone D, the zone of greatest hazard defined in the California Building Code. Probabilistic Seismic Hazard Assessment (PSHA) maps prepared by the California Geological Survey (CGS) and the USGS show that the areas with the greatest earthquake shaking hazard in Mono County include the Long Valley Caldera, the western portion of the Mono Basin extending north along the Eastern Sierra escarpment, the western edge of the White Mountains, the southeast corner of the county around Oasis, and the northern tip of the county around Topaz.
The Long Valley-Mammoth Lakes region has experienced numerous earthquakes caused by the movement of magma below the earth's surface. The oval-shaped Long Valley Caldera spans an area approximately 10 by 20 miles, and is among the largest volcanoes in the continental United States. Scientists suspect that the earthquakes are caused by shifts of buried stone slabs that are made unstable as magma moves within the volcano.
Ground failure induced by ground shaking includes liquefaction, lateral spreading, lurching, and differential settlement, all of which usually occur in soft, fine-grained, water-saturated sediments, typically found in valleys. Areas at high risk are mapped in the MEA. During the 1980 Mammoth Lakes earthquake sequence, ground failure was prevalent at Little Antelope Valley, along margins of the Owens River in upper Long Valley, along the northwest margins of Lake Crowley, and along Hot Creek Meadow.
All of Mono County is situated within Seismic Zone D, and consequently new construction in the county must comply with stringent engineering and construction requirements. Existing buildings that may be subject to seismic hazards must comply with the requirements of the unreinforced masonry building law (Government Code § 8875).
Other Geologic Hazards
Rockfall, Mudflow and Landslide Hazards
Rockfalls and landslides are particularly common along the very steep slopes of the eastern scarp of the Sierra Nevada, where talus slopes provide evidence of abundant past rockfalls. During the winter and spring months, rockfalls can be lubricated with snow and ice and can become extremely fast moving and destructive. Landslides in areas of hilly and mountainous terrain can be triggered by ground shaking, heavy rains or human activities such as road cuts, grading, construction removal of vegetation, and changes in drainage.
The state Department of Conservation, Division of Mines and Geology has yet to prepare maps of earthquake-induced landslide hazards for Mono County as required by the Seismic Hazards Mapping Act. Maps of rockfall hazard areas are based upon slope conditions and local and historical knowledge. Community areas in the county affected by rockfall hazards include Lundy Canyon and the June Lake Loop (primarily the Down Canyon area). The remaining rockfall risk areas are outside community areas.
Mud and debris flows involve very rapid downslope movement of saturated soil, sub-soil, and weathered bedrock. Large mud and debris flows, such as the one that occurred in 1989 in the Tri-Valley area, can be destructive, particularly at the mouths of canyons. Previous evidence of extensive mud and debris flows are evident in the large alluvial fans in the Tri-Valley area.
Subsidence in Mono County has been caused primarily by the tectonic movement of the earth and the movement of magma beneath the Long Valley Caldera. During the May 1980 sequence of earthquakes near Mammoth Lakes, the ground surface dropped about four inches at several locations near the Hilton Creek fault, and up to 12 inches of vertical offset occurred along the Mammoth Yosemite Airport fault zone. Magma movement in the Long Valley Caldera has caused bulging of the resurgent dome in the Casa Diablo area by about two and a half feet since 1980.
No subsidence has been observed in the county due to fluid withdrawals, or hydrocompaction of water impoundment. All major groundwater basins (see the MEA), however, have been identified by the Division of Mines and Geology as areas where subsidence could occur as a result of excessive groundwater pumping.
Evidence of volcanic activity in Mono County extends from Black Point north of Mono Lake to the deposits of Bishop Tuff in southern Mono County. The source of volcanic risk in Mono County is the Inyo-Mono crater chain and the Long Valley Caldera. Vents in the Inyo-Mono crater chain have erupted about every 500 years over the last 2,000 to 3,000 years, with the most recent eruption occurring approximately 500 years ago. Eruptions in the Long Valley Caldera have occurred approximately every 2,000 years over the last 7,000 years. The volcanic hazards mapped in the MEA estimate the extent of explosive blasts, hot flowing material, and ash flow.
The Federal Emergency Management Agency (FEMA) has prepared Flood Insurance Rate Maps illustrating 100-year flood hazard areas for several streams. Floods in these areas have a 1% probability of occurring in any given year. Such flooding could result in the loss of life and property, health and safety hazards, disruption of commerce and governmental services, and extraordinary public expenditures for flood protection and relief. Flood losses may be increased by the cumulative effect of obstructions in flood hazard areas that increase flood heights and velocities, and when inadequately anchored, can damage downstream uses.
Flooding is a potential risk to private properties situated in the vicinity of several waterways within the county. The community areas most likely to be impacted by a 100-year flood include properties along the East and West Walker River, Reversed Creek, and Spring Canyon Creek including portions of the Antelope Valley, Bridgeport Valley, the June Lake Loop, and the Tri-Valley area.
Some FEMA maps lack information regarding the base flood elevation, and are therefore of limited use for local development review and site-specific planning purposes. Some maps lack information concerning local alluvial fan and mudflow hazards. There is a significant need to update the flood hazard maps where these deficiencies exist. The California Department of Water Resources publishes flood-awareness area maps that, while non-regulatory, can provide additional flooding potential information, particularly for areas that remain unmapped by FEMA.
The Mono County Multi-Jurisdictional Local Hazard Mitigation Plan (LHMP) indicates that 18 dams are located in Mono County. The Lower and Upper Twin Lakes, Lundy Lake, Long Valley/Crowley Lake, Rush Creek meadows, and Saddlebag dams are identified as presenting some threat to downstream developed areas if dam failure were to occur.
The MEA illustrates the areas subject to flood hazards and dam failure inundation, as well as the area that would be inundated if the dam at Crowley Lake were raised an additional 20 feet to provide an increased storage area.
Seiches are earthquake-generated waves within enclosed or restricted bodies of water such as lakes and reservoirs. Similar to the sloshing of water in a bowl or a bucket when it is shaken or jarred, seiches can overtop dams and pose a hazard to people and property within their reach. There is no available evidence that seiches have occurred in Mono County lakes and reservoirs.
The combination of highly flammable fuel, long dry summers and steep slopes creates a significant natural hazard of wildland fire potential in most of Mono County. Wildland fires can result in death, injury, economic loss, and significant public investment in firefighting efforts. Woodlands and other natural vegetation can be destroyed resulting in a loss of timber, wildlife habitat, scenic quality and recreational resources. Soil erosion, sedimentation of fisheries and reservoirs, and downstream flooding can also result.
Fire hazard severity has been mapped by Cal Fire for most of the privately owned land in Mono County. All areas except the Bridgeport Valley and Antelope Valley have been rated as having a very high fire hazard. The Bridgeport Valley has a moderate fire hazard rating, and the Antelope Valley has not been rated. With the exception of the Antelope Valley, all privately owned lands in Mono County are within the State Responsibility Area (SRA).
The Mono County Community Wildfire Protection Plan (CWPP) and the Cal Fire San Bernardino/Inyo/Mono Unit Fire Plan are incorporated by reference into this Safety Element. The CWPP provides community-level data concerning fire hazards in the county, including community fuel reduction treatment areas and fuel breaks and other wildfire mitigation recommendations, particularly in Wildland-Urban Interface areas.
Much of the privately owned land in the county is located outside of fire protection districts, and therefore lacks formal emergency fire protection service. It is difficult for existing fire districts to receive additional property tax revenues for annexation of these unserved areas, or for new fire districts to be formed. Consequently, future development in these areas without adequate fire protection will be limited.
The State of California recently updated wildland protection regulations for future development in the SRA; Mono County has adopted and periodically updates a local ordinance that has the same practical effect as the Cal Fire regulations (Mono County Land Use Element Ch. 22, Fire Safe Regulations).
These fire safe regulations address requirements for adequate clearance of flammable vegetation around individual structures and clusters of structures and construction methods to prevent the spread of fire from the wildland to structures, and from structures to wildlands. Minimum water capacities for fire protection purposes are established in the regulations to ensure the availability of water for fire suppression purposes. Adequate road widths and load capacities are required to ensure ready movement of fire engines, and other heavy firefighting equipment to developed areas of the county; the Mono County Department of Public Works also has established similar road improvement standards for new development.
The 11 fire protection districts in the county provide fire-prevention services through such activities as education and development review. The districts also provide varying levels of fire suppression and emergency medical response services to community areas. The Community Services Section of the MEA provides a summary description of fire district service levels and capabilities, including the general capabilities and availability of local community water service in the county.
Although avalanches in Mono County occur primarily on national forests in the Sierra Nevada backcountry, some avalanche hazards present a significant risk to community areas. Both property damage and loss of life have resulted from avalanches in Mono County. Community areas influenced by avalanche hazards include Swauger Creek, Twin Lakes, Virginia Lake, Lundy Lake, June Lake, Long Valley/McGee Creek, and Wheeler Crest. In addition, roadway sections threatened by potential avalanches include portions of Lower Rock Creek Road; US 395 at Long Valley, Wilson Butte, and just north of Lee Vining; S.R. 158 entering the June Lake Loop; and several County roads entering eastern-slope community areas.
Avalanche Studies and Maps
In accordance with State law, avalanche hazard maps have been developed to illustrate areas of known avalanche occurrences. These maps were prepared by five Board-appointed avalanche advisory committees consisting of local residents and landowners. All pertinent information concerning the work of the five appointed committees and the avalanche policy formulation process – including committee recommendations and position papers – is on file in the county Planning Division. Other County avalanche hazard studies prepared by avalanche consultants and that project potential avalanche run-out areas, and an archive of photographs documenting evidence of avalanche damage and occurrences are also on file in the Planning Division.
Avalanche Monitoring and Evacuation
A backcountry avalanche monitoring program is operated by the Eastern Sierra Avalanche Center. This monitoring program issues avalanche hazard warnings during periods of high avalanche danger in the backcountry. The county Sheriff's Department keeps in contact with avalanche experts and should a hazardous situation develop, advises those within the hazard-prone area of the critical nature of the hazard.
The Mono County Local Hazard Mitigation Plan indicates that major routes (State and County), immediate access routes to community areas, and internal community street systems could be subject to closure by avalanches, landslides, snow and fog whiteouts, and flooding. In addition, imminent hazards such as high avalanche hazard conditions could prohibit travel even along open access routes. The developed areas of Wheeler Crest, Lundy Lake, Virginia Lakes, and Twin Lakes all have only one access.
The Mono County Local Hazard Mitigation Plan, sets forth site-specific evacuation plans as well as general evacuation procedures for various emergency situations. Several community area plans also call for development of additional emergency access routes into the community areas.