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© Hamilton County Florida Emergency Management

2016

NATURAL HAZARDS Severe weather events are a likely occurrence in Hamilton County.  The semi-tropical like climate offers sporadic tropical storms, tornadoes and flooding events.  While the county rarely deals with snow, sleet or ice, winter storms and freezes can significantly impact the agricultural industry. Many of the identified hazards are related (e.g., flooding can occur and tornadoes can develop during tropical storms) in the sense that other hazards may result from a disaster event, such as sinkholes stemming from flooding; in such instances, these hazards are not listed separately but concurrently. Hazards that most frequently affect Hamilton County: A. Hurricanes and Tropical Storms B. Thunderstorms and Hail C. Tornadoes D. Flooding E. Sinkholes F.  Wildland Fires Hurricanes and Tropical Storms  Hurricanes and Tropical Storms are a fact of life in Florida and the effect from these storms are felt equally in all of Hamilton County and the City of Jasper, Town of Jennings and the Town of White Springs. All hurricanes start off as tropical storms and the only difference between the two is wind speed. Hurricanes are tropical cyclones with winds that exceed 74 mph while tropical storms have wind speeds less than that. They are essentially heat pumping mechanisms that transfer the sun’s heat energy from the tropical to the temperate and polar regions. This helps to maintain the global heat budget and sustain life. Hurricanes are formed from thunderstorms that form over tropical oceans with warm surface temperatures. The ambient heat in the sea’s surface and moisture in the rising air columns set up a low pressure center and convective conditions that allow formation of self-sustaining circular wind patterns. Under the right conditions, these winds may continue to intensify until they reach hurricane strength. This heat and moisture from the warm ocean water is the energy source of a hurricane. Hurricanes weaken rapidly when deprived of their energy source by traveling over land or entering cooler waters. Hurricanes and Tropical Storms are characterized by torrential rainfalls, tidal flooding and damaging winds can spawn severe thunderstorms, lightening and tornadoes. Thunderstorms Thunderstorms are a storm of heavy rain accompanied by lightning, thunder, wind and sometimes hail. They occur when moist air near the ground becomes heated, especially in the summer here in Hamilton County. The air then rises forming clouds that produce precipitation. Ordinarily, thunderstorms cause little damage unless it grows in strength and reaches the severe level. The National Weather Service defines this is as having large hail of at least 1 inch (2.5 cm), surface wind speeds of 58 miles per hour (93 km/h) or greater. Hazardous conditions associated with thunderstorms include tornadoes, lightening, hailstorms, flooding, downburst and microburst winds. Strong “downbursts” (winds) are concentrated, straight-line winds created by falling rain and sinking air that exceed 125 mph. A separate wind phenomenon is the microburst, which are narrowly- concentrated downdrafts that can exceed speeds of 150 mph. Lightning occurs in all thunderstorms and can strike anywhere, both air and ground. Every thunderstorm produces lightning, which kills more people each year than tornadoes or hurricanes. Heavy rain from thunderstorms can cause flash flooding, and high winds can damage homes and blow down trees and utility poles, causing widespread power outages. Tornadoes NOAA identifies damaging winds as occurring in two categories: rotational (tornadic) or straight-line (downburst). Both forms of wind are bi-products of a thunderstorm and can cause a similar degree of damage. Tornado wind damage is often highly concentrated over narrow width in relation to their damage length. Downburst wind damage is normally spread out over a broader width but shorter damage length, in either case, both can cause substantial damage to structures. Tornadoes occur usually during daytime, from mid-afternoon till about early evening. Their movement is usually from the southwest to the northeast. Sometimes they move in any direction, and in the general path of the thunderstorm. The spinning winds cover an area of about 300 — 400 yards, and can travel on a path for about 5 miles (some tornadoes travel for over 80 miles), at a speed of about 5 — 60mph. Sometimes tornadoes develop in a very short time frame, leaving very little lead time for warning and preparation. Some examples of the damage tornadoes can cause include leveling homes, flipping mobile homes upside down, toppling large trees, picking up cars and dropping them miles away from their original location, and pushing metal shards into tree trunks. The worst damage a tornado can cause, however, is to the lives of the people involved, since injuries and fatalities are common when a tornado hits. Tornadoes materialize at the trailing edge of large frontal cyclones that result from the clash of high pressure and low pressure weather systems moving at continental scales across North America.  Because of climatic differences, southern states like Florida experience their most violent tornadoes in winter.  However a tornado event is not limited to winter.  They can also be generated during the summer in association with afternoon thunderstorms.  Most tornadoes are of short duration and do not touch down as the Category 4 or 5 that make national headlines. When a tornado threatens, only a short amount of time is available for life or death decisions.  The National Weather Service (NWS) issues two types of alerts: A Tornado Watch means that conditions are favorable for tornadoes to develop, and A Tornado Warning means that a tornado has actually been sighted. Flooding A flood is an overflow of water onto normally dry land. The inundation of a normally dry area caused by rising water in an existing waterway, such as a river, stream, or drainage ditch.  Ponding of water at or near the point where the rain fell.  Flooding is a longer term event than flash flooding: it may last days or weeks.  A flash flood is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours.  Flash floods are usually characterized by raging torrents after heavy rains that rip through river beds or urban streets.  They can occur within minutes or a few hours of excessive rainfall. Historically, floods have been a factor in over 80 percent of all Presidential declared disasters. Sinkholes For the residents of Florida, the sudden appearance of sinkholes is not an unusual occurrence.  Sinkholes occur because the entire State of Florida is underlain by limestone, a type of rock that is slowly dissolved by weak natural acids found in rain and in the pore spaces in soil. As rainfall is filtered through the ground to the limestone, it erodes and dissolves the soluble rock and creating cavities in the subsurface. The abrupt formation of sinkholes may follow extreme rain producing events such as tropical storms or hurricanes.  This is because the weight of a large amount of rain water at the earth’s surface may bring about the collapse of an underground cavity if its limestone “ceiling” has become thin. This tendency for sinkholes to form following events that produce large amounts of rainfall is made worse in times of drought.  During periods of drought, underground cavities that might normally be filled with water may be only partially filled.  These cavities are less likely to bear the weight of flood waters without collapsing. Ground water pumping in specific areas when water levels are already low and are forced lower can trigger a more sudden collapse of overburdened sediments and create sinkholes that might not have otherwise happened. Increases in ground water pumping, loading at land surface, retention pond building, and altering a landscape where you’re changing the overburdened thickness are all activities that can induce sinkholes. Wildfires As people search for a place to live they often desire two conflicting conditions: country living with urban services.  This desire for urban/rural living has been given the name “wildland/urban interface.”  As described by the Florida Forest Services, the wildland/urban interface “refers to that geographical area where two diverse systems - wildland and urban – meet and affect each other and give rise to conflicts between societal values and expectations concerning the management of natural resources.”  The major problem resulting from the wildland/urban interface is wildfire. A wildfire is any fire occurring in wildlands (i.e., grasslands, forest, brush land, etc.) Prescription burning is the process of igniting fires under selected conditions, in accordance with strict parameters. Wildfires have burned across the woodlands of Florida for centuries and are part of the natural management of much of Florida’s ecosystems.  Forest fires from natural causes such as lightning account for only a very small percentage of Florida’s wildfires, whereas, man is by far the leading cause of wildfires. Forest land is continuously susceptible to destruction by wildfires. There are four types of forest fires: •  Surface - A surface fire is the most common type and burns along the floor of the forest, moving slowly while killing or damaging trees. •  Ground - Ground fires (muck fires), which are usually started by carelessness, burn on or below the forest floor.  These fires are hard to detect, and even harder to extinguish. •  Crown - Crown fires are spread rapidly by the wind and move fastest of all types of fires by jumping along the tops of trees. •  Wildland-Urban Interface – WUI Fire is a wildfire in a geographical area where structures and other human development meet or intermingle with wild lands or vegetative fuels.