City Of Portola - Local Hazard Safety Summaries
The Division of Safety of Dams’ engineers inspect over 1,200 dams annually to assure they are performing and being maintained in a safe manner. In addition, the Division periodically reviews dam stability and stability of major appurtenances in light of emerging design approaches (earthquake hazards, hydrologic estimates) and construction requirements. Even with seismic activity, dam failures are rarely catastrophic events. Dam failures typically begin to indicate signs of wear or weakness through seepage noted in the downstream face or near the toe.
The City of Portola lies approximately 8 miles downstream of Lake Davis, which is the reservoir created by the Grizzly Valley Dam. The Grizzly Valley Dam was completed in 1915, is within the jurisdiction of the State of California, and is owned and operated by a federal agency. Government Code, Section 8589.5, requires dam owners to prepare and submit copies of inundation maps in the event of a dam failure. Inundation maps are available to appropriate public safety agencies and cities that can be affected. All Dam owners must also have an emergency action plan if a potentially unsafe condition develops.
California Division of Safety of Dams
Floods are the most prevalent hazard in the United States and are considered events where people and property can be most adversely affected. Due to their frequency and proximity to occupied structures and residences, floods present the highest death rate of any natural hazard. Because Portola straddles the Middle Fork of the Feather River, and is confined in a river canyon, the City of Portola is at significant risk for flood damage, especially in low lying areas along the river.
The National Oceanic and Atmospheric Association (NOAA) stage gage on the Middle Fork of the Feather River near Portola, shows that the “action stage” of 7 feet has been exceeded regularly, and that the “flood stage” of 8.5 feet was most recently reached on 3/17/2011 with a stage of 9.03 feet.
FEMA Get Ready – Floods
Cal EMA Planning and Preparedness – Floods
Wildfire is an uncontrolled fire that spreads through forests, over mountains, and across deserts. Unfortunately, land use, forest management concerns, weather patterns, and climate change have altered natural fire regimes. In addition, historical practices of fire suppression are now viewed to have resulted in creating forests with large fuel loads that are primed to burn. The City of Portola is surrounded by woodlands with moderate to very high fuel loads. Physical properties of the surrounding terrain such as slope, aspect, and topographic features also impact fire behavior.
Portola has an active history of fire ignitions within the Wildland-Urban Interface (WUI), a five-mile buffer around the city. In 1988 a lightning caused fire burned 783 acres directly south of the high school, well within the city limits. In 2008 the “Cold” fire burned approximately ten square miles about fifteen miles to the west of Portola. Figure xxx, presents a graphic of fire history and ignition points for areas in and around the City of Portola. More recently, the Chips Fire ignited approximately 45 miles northwest of Portola and burned more than 75,000 acres of forested land.
Ignition risks fall into two general categories, lightning and human caused. Human caused ignitions that have occurred within the WUI come from a variety of sources: equipment use, debris burning, playing with fire, arson, and campfires. Lightning strikes, however, present the greatest ignition frequency.
FEMA Get Ready – WildfiresCal EMA Planning and Preparedness – Fires
Winter Storms and Extreme Cold
Winter storms are relatively common in the Sierra Nevada Mountains and in the Portola area. Snow accumulations of 20 to 30 inches per event can occur. The maximum daily snowfall recorded since 1/1/1915 resulted in an accumulation of approximately 40 inches (Portola Western Regional Climate Center). Severe winter storms are classified as a blizzard if wind speeds exceed 38 mph and temperatures drop to less than 21°F. Extreme cold in the Sierras is not common, and atmospheric conditions can occur which push temperatures below 0oF. Cold air outbreaks, extreme cold, have been characterized as two or more days more in which the daily mean temperature is more than 2 standard deviations below daily mean temperature for December, January, and February. Based on data obtained from WRCC, this would indicate two consecutive days with temperatures below 5 to 10 oF. WRCC data shows extreme low temperatures reaching -30oF, but the lowest average low is about 10 oF. Winter storms and extreme cold can knock out power, immobilize travel and hamper service crews and repair vehicles, isolate residents in remote areas, and impede access to critical facilities and goods and services. Older structures, not consistent with current code requirements, can become distressed or damaged due to large snow loads.
An earthquake is a sudden motion or trembling caused by a release of stress accumulated within or along the edge of the earth‘s tectonic plates. Earthquakes usually occur without warning and, after just a few seconds, can cause massive damage and extensive casualties. The severity of ground motion generally increases with the amount of energy released and decreases with distance from the fault or hypocenter of the earthquake.
Coincidental events manifested by earthquake activity include:
Surface rupture occurs when the ground surface ‘breaks’ due to fault movement during an earthquake. The Alquist-Priolo Earthquake Fault Zoning Act (APEFZA) was passed in 1972 to attempt to mitigate the hazard of surface rupture to structures for human occupancy. The State has not designated any special study zones for Portola or the immediate surrounding areas indicating that Portola is not in a fault zone.
Ground shaking is a general term that refers to all aspects of motion of the earth‘s surface resulting from an earthquake and is usually considered the most important of all seismic hazards because all the other hazards are triggered by ground shaking. The amount of ground shaking experienced varies with each earthquake and each fault system. Structural design, construction materials and supporting soils/bedrock also significantly influence how the structure responds to the event and how much damage occurs. Portola is considered in an area of moderate ground shaking potential.
Liquefaction and Lateral Spreading
Some of the most infamous events associated with earthquake damage deal with liquefaction, the point during a seismic event when soils lose their strength and begin to act as fluids, and spread laterally or liquefy. The greatest potential for lateral spreading would follow the river corridor where significant changes in grade/elevation occur. Liquefaction can cause severe damage to structures, bridges, roadways, and buried utilities as well. Liquefaction typically manifests in somewhat isolated zones and areas. An area can be the same seismically and geologically, yet liquefaction may only impact a small portion of the area or an isolated structure.
FEMA Get Ready – Earthquakes
Cal EMA Planning and Preparedness – Earthquakes and Tsunamis
USGS Earthquakes and Hazards
Severe weather encompasses: thunderstorms, lightning, hail, microbursts (wet and dry), tornadoes, and windstorms. Severe weather is most commonly associated with the term thunderstorm. Thunderstorms develop when warm, moist, air rises and condenses as it cools at higher altitudes. As the moisture condenses it causes the surrounding air to continue to warm, intensifying the instability in the air mass. We’ve likely all experienced a severe weather event at some point in our lives; and probably followed it up with the thought, “Where did that come from?”
The following paragraphs present brief descriptions of what some of these hazards are and how they develop.