دکتر اینترنتی
مطالب متنوع پزشکی و آموزشیدکتر اینترنتی
مطالب متنوع پزشکی و آموزشیWhat is a Landslide
Severe damage to road caused
by landslide
A landslide is the movement of rock, debris or earth down a slope. They result from the failure of the materials which make up the hill slope and are driven by the force of gravity. Landslides are known also as landslips, slumps or slope failure.
Some of the most common types of landslide in Australia are earth slides, rock falls and debris flows. The movement of landslide material can vary from abrupt collapses to slow gradual slides and at rates which range from almost undetectable to extremely rapid. Sudden and rapid events are the most dangerous because of a lack of warning and the speed at which material can travel down the slope as well as the force of its resulting impact. Extremely slow landslides might move only millimetres or centimetres a year and can be active over many years. Although this type of landslide is not a threat to people they can cause considerable damage to property.
Landslides can be triggered by natural causes or by human activity. They range from a single boulder in a rock fall or topple to tens of millions of cubic metres of material in a debris flow.
They can also vary in their extent, with some occurring very locally and impacting a very small area or hill slope while others affect much larger regional areas. The distance travelled by landslide material can also differ significantly with slides travelling from a few centimetres to many kilometres depending on the volume of material, water content and gradient of the slope.
Landslides in Australia have caused fatalities, environmental degradation and millions of dollars damage to buildings, roads, railways, pipelines, communication networks and agricultural land.
Since 1842, there have been 100 recorded landslide events which have resulted in the death of 105 people and injury to 129 (National Landslide Database, 2007). Although many of these landslides have resulted from natural phenomenon, almost half of those causing death and injury can be attributed to human activity. The basic types of landslide movement are:
Fall
This is generally characterised by a rapid to extremely rapid rate of movement with the descent of material characterised by a freefall period. Falls are commonly triggered by earthquakes or erosion processes.
Topple
This is characterised by the tilting of rock without collapse, or by the forward rotation of rocks about a pivot point. Topples have a rapid rate of movement and failure is generally influenced by the fracture pattern in rock. Material descends by abrupt falling, sliding, bouncing and rolling.
Flow
This is the most destructive and turbulent form of landslide. Flows have a high water content which causes the slope material to lose cohesion, turning it into a slurry. They are channelled by the landscape and move rapidly.
Slide
This is one of the most common forms of failure and can be subdivided into translational and rotational slides. Rotational slides are sometimes called slumps because they move with rotation. Translational slides have a planar, or two dimensional surface of rupture. Slides are most common when the toe of the slope is undercut. They have a moderate rate of movement and the coherence of material is retained, moving largely intact or in broken pieces.
Spread
This phenomenon is characterised by the gradual lateral displacement of large volumes of distributed material over very gentle or flat terrain. Failure is caused by liquefaction which is the process when saturated loose sediment with little or no cohesion such as sands or silts are transformed into a liquid-like state. This process is triggered by rapid ground motion most commonly during earthquakes
What is a Cyclone
Cyclone seen from Space
A tropical cyclone is a low-pressure system which develops in the tropics and is sufficiently intense to produce sustained gale force winds of at least 63km/h. If the sustained wind reaches hurricane force of at least 118km/h the system is defined as a severe tropical cyclone. In other parts of the world they are called hurricanes or typhoons.
Tropical cyclones can cause significant phenomena which can adversely, and sometimes favourably impact on communities and the environment. The most common features are destructive winds and heavy rainfall that can lead to flooding. Storm surge, or coastal inundation by seawater, is a lesser known phenomenon but can be the most dangerous element of a cyclone. Though rare in Australia, tornadoes have been reported during cyclone events.
Severe wind
In tropical cyclones, wind gusts in excess of 90km/h can be expected around their centre, or eye, while in the most severe events, gusts can exceed 360km/h. Although the strongest winds are near the eye, damaging winds can extend hundreds of kilometres from the centre. The eye can have quite calm winds and cloud-free skies, but this lull is temporary and is followed by destructive winds from another direction. This is because, from above, the winds spiral around the eye in a clockwise direction (in the Southern Hemisphere). The effect of this on the ground is that winds on opposite sides of the eye blow in different directions.
Wind damage is mostly caused by the maximum gusts in the cyclone. For this reason, the well-known tropical cyclone severity categories used by the Bureau of Meteorology to communicate warnings are based on maximum gust strengths.
Heavy rainfall
Because tropical cyclones form over warm tropical oceans, they generally hold enormous amounts of moisture and can produce heavy rainfall over extensive areas. Rain can create severe impacts by causing floods and landslides and through the direct damage of materials by contact, such as being driven by wind into buildings. Direct damage is generally the result of wind damage to walls, windows or roofs, which allows water to penetrate buildings.
Rainfall can be associated with the tropical cyclone when it impacts on the coast or further inland as it weakens to become a tropical depression. Heavy rain from tropical cyclones or tropical depressions can often reach Australia's more southerly latitudes where the rainfall is a major source of water for the country's inland river and ground water systems. Flooding can wreak havoc over vast areas, inundating land, isolating communities and destroying infrastructure.
Storm surge
Potentially the most dangerous hazard associated with tropical cyclones which make landfall is storm surge. Storm surge has been responsible for more deaths than any other feature of tropical cyclones. Storm surge is a raised dome of water about 60 to 80 kilometres across and typically about two to five metres higher than the normal tide level. It is caused by a combination of strong winds driving water onshore and the lower atmospheric pressure in a tropical cyclone. In the southern hemisphere the onshore winds occur to the left of the tropical cyclone's path. In Australia, this is the east side on the north west and north coasts and the south side on the east coast.
The largest surge usually extends between 30 and 60 kilometres from the crossing point of the tropical cyclone centre, or eye. Its influence also depends on the local topography of the seafloor and the angle at which the cyclone crosses the coast. If the surge occurs at the same time as a high astronomical tide the area inundated can be extensive, particularly along low-lying coastlines.
What is a Flood?
A simple definition of flooding is water where it is not wanted. Another, more comprehensive definition of a flood is:
A general and temporary condition of partial or complete inundation of normally dry land areas from overflow of inland or tidal waters from the unusual and rapid accumulation or runoff of surface waters from any source.
In November 2011, the Australian Government introduced a standard definition of flood for certain insurance policies. The announcement was part of the Government’s response to the recommendations in the Natural Disaster Insurance Review report.
The standard definition will apply when an insurer offers flood cover for a home building, home contents, small business or strata title insurance policy. For this purpose a flood is defined as:
The covering of normally dry land by water that has escaped or been released from the normal confines of: any lake, or any river, creek or other natural watercourse, whether or not altered or modified; or any reservoir, canal, or dam.
Floods can have both positive and negative impacts. They can bring welcome relief for people and ecosystems suffering from prolonged drought, but also are estimated to be the most costly natural disaster in Australia.
Every year in Australia, floods cause millions of dollars damage to buildings and critical infrastructure, such as roads and railways as well as to agricultural land and crops. They also disrupt business and can affect the health of communities. Between 1967 and 2005, the average direct annual cost of flooding has been estimated at A$377 million (Bureau of Infrastructure, Transport and Regional Economics analysis of the Emergency Management Australia database).
The losses due to flooding vary widely from year to year and are dependent on a number of factors such as the severity of a flood and its location. The most costly year for floods was 1974, when events resulted in a total cost of A$2.9 billion (Bureau of Transport and Regional Economics, 2001).
What is a Tsunami?
Tsunami (pron: 'soo-nar-me') is a Japanese word; 'tsu' meaning harbour and 'nami' meaning wave. The phenomenon is usually associated with earthquakes, landslides or volcanic eruptions in, or adjacent to oceans, and results in sudden movement of the water column. Until recently tsunami were called tidal waves, even though the event has nothing to do with tides.
A tsunami is different from a wind generated surface wave on the ocean. The passage of a tsunami involves the movement of water from the surface to the seafloor which means its speed is controlled by water depth. Consequently, as the wave approaches land and reaches increasingly shallow water it slows. However, the water column still in deeper water is moving slightly faster and catches up, resulting in the wave bunching up and becoming much higher. A tsunami is often a series of waves and the first may not necessarily be the largest.
When a tsunami travels over a long and gradual slope, it allows time for the tsunami to grow in wave height. This is called shoaling and typically occurs in shallow water less than 100m. Successive peaks can be anywhere from five to 90 minutes apart. In the open ocean, even the largest tsunami are relatively small with wave heights of less than one metre. The shoaling effect can increase this wave height to a degree such that the tsunami could potentially reach an onshore height of up to 30 metres above sea level. However, depending on the nature of the tsunami and the nearshore surroundings, the tsunami may create only barely noticeable ripples.