Natural hazards
From HFA-PEDIA
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About natural hazards
Natural hazards are natural processes or phenomena occurring in the biosphere that may constitute a damaging event. Natural hazards can be classified by origin namely: geological, hydrometeorological or biological. Hazardous events can vary in magnitude or intensity, frequency, duration, area of extent, speed of onset, spatial dispersion and temporal spacing.
Types of natural hazards
Hurricanes
A "hurricane" is a revolving tropical storm accompanied by torrential rain and wind speeds exceeding 119 kilometres (74 miles) per hour.
In the Americas and the Caribbean, these storms are called "hurricanes". In the western Pacific, East Asia and Australia they are "typhoons". And in the Indian Ocean they are "cyclones".
Storm strength varies from Category 1 to 5, the lowest referring to storms with winds of 119-153 kph (74-95 mph) and Category 5 to winds exceeding 249 kph (155 mph), according to the Saffir-Simpson Hurricane Scale.
The impact depends not just on wind speed, but also on where a storm strikes, how much flooding it causes, and the quality of buildings and infrastructure in the affected area.
The Atlantic hurricane season runs from June to November, posing a threat to coastal communities when tropical storms and hurricanes generate storm surges and trigger landslides and flash floods.
The Eastern Pacific hurricane season begins in mid-May and also ends on November 30.
The eastern Pacific has experienced suppressed hurricane activity since 1995, while the Atlantic Basin has experienced greatly enhanced activity.
Source: Alertnet - http://alertnet.org/db/crisisprofiles/CA_HUR.htm
Tropical cyclones
Tropical cyclones are powerful hydro-meteorological hazards. On average, over 78 million people are affected globally by between 50 to 60 events each year. Tropical cyclones are unevenly spread around the globe as their development depends on specific climatic and oceanic conditions. A tropical cyclone has multiple impacts on the affected areas, including:
- - Extremely powerful winds.
- - Torrential rains leading to floods and/or landslides.
- - High waves and damaging storm surge, leading to extensive coastal flooding.
The complexity of the multiple forms of impact triggered by tropical cyclones would call for integrated modelling of wind, rain, storm surge and landslides.
(Source: 2009 Global Assessment Report on Disaster Risk Reduction; UNISDR)
Floods
Floods are one of the most frequent natural hazards and occur in almost every country. A flood is generally defined as an excess of the amount of discharged water compared to the drainage capacity. At present there is no systematic global detection of flood events as there is for cyclones and earthquakes.
Floods are triggered by various phenomena and there are different types of floods. For example one often differentiates among flash floods, river floods, and urban floods, all of which are caused by a combination of heavy precipitation and poor drainage. The severity of these flood types depends on rainfall intensity, spatial distribution of rainfall, topography and surface conditions.
All coastal areas are vulnerable to flood events, which could be devastating when heavy rainfall occurs at the same time as high tide or storm surge. Many climate change models predict more frequent extreme precipitation events, which in combination with global sea level rise makes the situation even more critical in the future. Consequently, the risk associated with flooding is expected to increase significantly in coastal regions with high population density in the future. Because of the predictability of the flooding events, however, the main consequences will usually be damage to constructed facilities and discomfort of the exposed population, rather than loss of life.
(Source: 2009 Global Assessment Report on Disaster Risk Reduction; UNISDR)
Drought
Drought is a phenomenon that affects more people globally than any other natural hazard. Unlike aridity, which refers to a semi-permanent condition of low precipitation (desert regions), drought results from the accumulated effect of deficient precipitation over a prolonged period of time. Here “deficient” refers to values being less than the expected, or long-term average value at a particular location. Ultimately, drought refers to a condition of an insufficient supply of water necessary to meet demand, both being highly location-specific. For example, a few months of deficient rainfall can adversely affect rain-fed agricultural systems while several months to a year (or more) of drought may be necessary to impact a water supply system with substantial storage capacity. Given the varying impacts of drought several drought indicators are in use around the world.
Drought is often described as falling into three main categories: meteorological, agricultural, and hydrologic. Meteorological drought refers to a prolonged period of deficient precipitation that may last from a season to several years. Agricultural drought occurs when soil moisture is depleted to the point where it begins to adversely affect crops, pasture, or rangeland. A reduction in soil moisture is in part related to precipitation but also depends on other meteorological conditions such as temperature and wind as well as non meteorological factors such as soil type and terrain. Hydrologic drought refers to a condition of persistent, below-average surface water levels in rivers, streams, lakes and reservoirs or subsurface water such as an unusually low water table. These conditions are again partially related to precipitation variability but also to non-meteorological factors. Given the importance of non-meteorological factors, there is often a delay between the onset of meteorological drought and agricultural or hydrologic drought. The “best” indicator for drought is the one that most closely corresponds to the specific drought-sensitive application being considered.
Among natural hazards, drought risk is especially difficult to quantify. First, unlike earthquakes, floods or tsunamis that occur along generally well-defined fault lines, river valleys or coastlines, drought can occur anywhere (with the exception of deserts where it doesn’t have meaning). Defining what constitutes a drought across the wide range of regional climates around the globe is challenging in its own right, identifying what drought characteristic (its intensity, duration, spatial extent) is most relevant to a specific drought sensitive sector (agriculture, water management, etc.) poses another layer of complexity. Drought does not destroy infrastructure or directly lead to human mortality. Famines may be triggered by drought but increased human mortality during famine is ultimately linked to a broader set of issues surrounding food security.
(Source: 2009 Global Assessment Report on Disaster Risk Reduction; UNISDR)
Landslides
The term landslide (is used here to refer to) slides with rapid mass movement, like rockslides, debris flows, induced by both rainfall and earthquake; which pose a threat to human life. Slow moving slides have significant economic consequences for constructions and infrastructure, but rarely cause any fatalities. Rapid mass movement also includes snow avalanche as well as rock avalanches and submarine slides.
Landslides represent a major threat to human life, property and constructed facilities, infrastructure and natural environment in most mountainous and hilly regions of the world. Statistics from the Centre for Research on the Epidemiology of Disasters (CRED) show that, on average, landslides are responsible for a small fraction of all fatalities from natural hazards worldwide. The socio-economic impact of landslides is, however, greatly underestimated because landslides are usually not separated from other natural hazard triggers, such as extreme precipitation, earthquakes or floods in natural catastrophe databases. This underestimation contributes to reducing the awareness and concern of both authorities and general public about landslide risk.
Only the population exposure to precipitation-induced landslides was considered in the event-per-event analysis. The fatalities caused by earthquake-induced landslides are attributed to “earthquakes” in the EM-DAT database, and including them again under “landslides” would lead to an overestimation of risk.
(Source: 2009 Global Assessment Report on Disaster Risk Reduction; UNISDR)
Earthquakes
The term earthquake is used to describe any seismic event that generates seismic waves. They are caused mostly by rupture of geological faults and sometimes also by volcanic activity, landslides, etc, and are concentrated within specific areas around the world, mainly in geologically active areas such as the Pacific coast on North and South America, Indonesia, Japan, Himalayas, etc.
(Source: 2009 Global Assessment Report on Disaster Risk Reduction; UNISDR)
Tsunamis
Tsunamis are waves set in motion by large and sudden forced displacements of the sea water, having characteristics intermediate between tides and swell waves. Although tsunamis are infrequent (ca. 5-10 events reported globally per year), they do represent a serious threat to the coastal population in many areas, as demonstrated by the devastating effects of the 2004 Indian Ocean tsunami. Tsunamis are often generated by submarine earthquakes. However, submarine landslides are becoming increasingly recognized as important triggers as well. Other sources of tsunamis include collapsing/exploding volcanoes, and asteroid impacts. Tsunamis generated by large earthquakes in subduction zones (area where one continental plate moves beneath another) along the major plate boundaries contribute most to the global tsunami hazard.
When the tsunami is generated, it propagates in the open sea with speeds of several hundred kilometres per hour, and may hence reach coastlines distant from the earthquake within a relatively short time. The wave slows down when it reaches the shoreline, and its height increases. Because of its relatively large wave-length, the tsunami may travel far inland compared to wind waves and swells, and because of its relatively short period, it inundates much faster than tidal waves and storm surges. When the tsunami inundates land, flow velocities become large, enabling the tsunami to carry very large objects, erode the landscape, and destroy buildings. The tsunami becomes lethal both due to possibilities of being impacted by debris and flotsam, as well as drowning. Generally, tsunamis may cause damage to most coastal structures; however, buildings of poor quality are particularly vulnerable. The tsunami is most destructive close to the shoreline where the flow velocity and wave load are largest.
The results of the 2009 Global Assessment Report on Disaster Risk Reduction represent a first-pass assessment of the tsunami hazard and population exposure based on today’s knowledge. The study considered the tsunamis caused by earthquakes only, as these events will often contribute more to the risk than the smaller events.
Tsunami hazard is a combination of anticipated wave-height, travel time and exposed population within a region.
(Source: 2009 Global Assessment Report on Disaster Risk Reduction; UNISDR)
Biomass fires
According to a recent inventory (Tansey et al. 2008), wild land fires and other biomass fires annually burn a total land area of between 3.5 and 4.5 million km2, equivalent to the surface area of India and Pakistan together, or more than half of Australia. This makes it one of the most spatially prevalent hazards after drought.
Emissions from biomass burning inject pollutants into the atmosphere, as well as greenhouse gases (GHG). The Intergovernmental Panel on Climate Change (IPCC) attributes 17.3% of total anthropogenic emissions to biomass burning, making it the second largest source of GHG from human activities after the burning of fossil fuel. However, this figure may in reality be even higher, as it is based on pre-2000 data. Biomass fire is the only hazard that has both an impact on, and is exacerbated by, climate change. Most fires have human causes.
(Source: 2009 Global Assessment Report on Disaster Risk Reduction; UNISDR)
Tropical storms
Source: Reuters; 20-08-2008
"Hurricane", "cyclone" and "typhoon" are different terms for the same thing: a revolving tropical storm accompanied by torrential rain and wind speeds exceeding 119 kilometres per hour (74 miles per hour). The storms can be hundreds of kilometers wide and they bring destructive winds, torrential rain, storm surge and sometimes tornadoes. The worst recent storms include Cyclone Nargis, which hit Myanmar in 2008 leaving nearly 140,000 people dead or missing, and Hurricane Katrina, which destroyed New Orleans in 2005, killing more than 1,800 people and displacing 2.16 million.
In the Americas and Caribbean the tropical storms are called "hurricanes". In the western Pacific, East Asia and Australia they are "typhoons". And in the Indian Ocean they are "cyclones".
The storm strength varies from Category 1 to 5, the lowest referring to storms with winds of 119-153 kph (74-95 mph) and Category 5 to winds exceeding 249 kph (155 mph), according to the Saffir-Simpson Hurricane Scale. The impact of the storm, however, depends not just on wind speed, but also on where they strike, how much flooding they cause and the quality of the buildings and infrastructure.
Weather-watchers use satellite images to track tropical storms as they develop, and try to predict when and where one will hit land and at what speed; but storms are unpredictable and can suddenly weaken or swerve off course.
Wind speeds
- Less than 63 kph (39 mph) - tropical depression
- More than 63 kph - tropical storm and given a name
- More than 119 kph - designated either a hurricane, typhoon, severe tropical cyclone, severe cyclonic storm or tropical cyclone depending where it is in the world
Tornadoes
Some tropical storms bring with them tornadoes - destructive rotating columns of air which form from a storm cloud and reach towards the ground. These twisting spirals of wind can lift houses off foundations. The tornado ranges from just a few meters across to over a kilometer, and can last up to an hour or more. The strongest have winds of over 420 kph (260 mph). Scientists do not know exactly how they are formed, making them hard to predict. The average warning time is 12 minutes.
When do Tropical Storms occur?
The cyclone season in the Pacific/South East Asia region runs from May to November. The Americas/Caribbean hurricane season runs from Jun 1 to Nov 30, peaking in August and September. And the cyclone and typhoon season in East Asia, South Pacific and Australia normally runs from November to April.
In Northern India tropical cyclones usually occur from April to June, and September to November. The East Coast of Africa normally experiences tropical cyclones from November to April.
Geological hazards
Natural earth processes or phenomena that may cause the loss of life or injury, property damage, social and economic disruption or environmental degradation. Geological hazard includes internal earth processes or tectonic origin, such as earthquakes, geological fault activity, tsunamis, volcanic activity and emissions as well as external processes such as mass movements: landslides, rockslides, rock falls or avalanches, surfaces collapses, expansive soils and debris or mud flows. Geological hazards can be single, sequential or combined in their origin and effects.
Hydrometeorological hazards
Natural processes or phenomena of atmospheric, hydrological or oceanographic nature, which may cause the loss of life or injury, property damage, social and economic disruption or environmental degradation.
Hydrometeorological hazards include: floods, debris and mud floods; tropical cyclones, storm surges, thunder/hailstorms, rain and wind storms, blizzards and other severe storms; drought, desertification, wildland fires, temperature extremes, sand or dust storms; permafrost and snow or ice avalanches. Hydrometeorological hazards can be single, sequential or combined in their origin and effects.
Biological hazards
Processes of organic origin or those conveyed by biological vectors, including exposure to pathogenic micro-organisms, toxins and bioactive substances, which may cause the loss of life or injury, property damage, social and economic disruption or environmental degradation.
Examples of biological hazards: outbreaks of epidemic diseases, plant or animal contagion, insect plagues and extensive infestations.
Related pages
Related Internet links
Reuters - AlertNet
