Page 39
CYCLONE:
We have already discussed the type of cyclones and their formation in detail in the Value adds of Climatology. Here we will discuss the disaster-related aspect of it.
A disaster is only caused by a Tropical cyclone, which is known by different names in different continents like Typhoon in China, Hurricane in North America, Willy Willey in Australia etc.
Tropical cyclones can produce heavy rainfall and sustained winds that can exceed 155 miles per hour (249 km/hr). The official seasons during which cyclones are predicted to become destructive hurricanes and typhoons are different in different areas, but the fact is that, at any time of year, a cyclone is capable of becoming a dangerous hazard.
Cyclones in India:
The Indian subcontinent is one of the worst affected regions in the world. The subcontinent with a long coastline of 8041 kilometers is exposed to nearly 10 per cent of the world’s tropical cyclones. Of these, the majorities of them have their initial genesis over the Bay of Bengal and strike the East coast of India.
On an average, five to six tropical cyclones form every year, of which two or three could be severe. More cyclones occur in the Bay of Bengal than the Arabian Sea and the ratio is approximately 4:1. Cyclones occur frequently on both the coasts (the West coast - Arabian Sea; and the East coast - Bay of Bengal). An analysis of the frequency of cyclones on the East and West coasts of India between 1891 and 1990 shows that nearly 262 cyclones occurred (92 of these severe) in a 50 km wide strip above the East coast. Less severe cyclonic activity has been noticed on the West coast, where 33 cyclones occurred the same period, out of which 19 of
Disasters & Disaster Management-IASbaba
Page 40
were severe. Tropical cyclones occur in the months of May-June and October-November. Cyclones of severe intensity and frequency in the North Indian Ocean are bi-modal in character, with their primary peak in November and secondary peak in May. The disaster potential is particularly high during landfall in the North Indian Ocean (Bay of Bengal and the Arabian Sea) due to the accompanying destructive wind, storm surges and torrential rainfall. Of these, storm surges cause the most damage as sea water inundates low lying areas of coastal regions and causes heavy floods, erodes beaches and embankments, destroys vegetation and reduces soil fertility. Cyclones vary in diameter from 50 to 320 km but their effects dominate thousands of square kilometers of ocean surface and the lower atmosphere. The perimeter may measure 1,000 km but the powerhouse is located within the 100-km radius. Nearer the Eye, winds may hit at a speed of 320 km. Thus, tropical cyclones, characterized by destructive winds, torrential rainfall and storm surges disrupt normal life with the accompanying phenomena of floods due to the exceptional level of rainfall and storm surge inundation into inland areas.
Cyclones are characterized by their devastating potential to damage structures, viz. houses; lifeline infrastructure-power and communication towers; hospitals; food storage facilities; roads, bridges and culverts; crops etc. The most fatalities come from storm surges and the torrential rain flooding the lowland areas of coastal territories.
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Cyclones are classified into five different levels on the basis of wind speed. They are further divided into the following categories according to their capacity to cause damage:- Cyclone Category Wind Speed in Km/h Damage Capacity 01 120-150 Minimal 02 150-180 Moderate 03 180-210 Extensive 04 210-250 Extreme 05 250 and above Catastrophic
Storm surges (tidal waves) are defined as the rise in sea level above the normally predicted astronomical tide. Major factors include:
A fall in the atmospheric pressure over the sea surface
Effect of the wind
Influence of the sea bed
A funneling effect
The angle and speed at which the storm approaches the coast
The tides
Related Hazards
Storm surge (rise in water level)
High winds and heavy rainfall
Flooding and landslides
Storm tide (the combination of storm surge and high tide)
Disasters & Disaster Management-IASbaba
Page 42
Tornadoes
Precautions
Have an evacuation plan and disaster supply kit ready, including a radio and batteries.
Install hurricane storm shutters or precut lumber for windows.
Remove branches and small trees that may fall on your house.
Clear clogged rain gutters and drains.
Stock extra food, water, and batteries, in case of power failures.
Fill large containers with water for cleaning and toilet flushing.
Purchase a generator for emergency power supply needs.
What to Do if a Tropical Cyclone Threatens
Secure loose objects that may blow away, such as outdoor furniture.
Turn off water, electricity, and propane tanks, and unplug all appliances.
Listen to local radio stations for official announcements and instructions.
Stay indoors and away from windows and exterior walls and doors.
Evacuate to sturdy buildings or public shelters, if advised or ordered to do so.
Leave areas that may flood.
Disasters & Disaster Management-IASbaba
Page 43
LANDSLIDE
Development of Slope Instability
1. Gravity is the agent or driving force producing slope instability. It acts at a constant rate at all times on slopes. When slope instability is produced by gravity, landslides are the resultant phenomena.
2. The strength of the material making up the slope resists the action of gravity- prevents landslides from occurring.
3. The balance between gravity acting on the slope and resistance by the slope material usually favors the resisting slope material. When this balance shifts to being equal or slightly favoring gravity, a landslide is imminent.
4. While gravity acts at a constant rate, changes happen- which increase the shear stress of gravity as it acts on the slope; reduce the resisting strength of the slope material, or a combination of both. This alters the balance between driving and resisting forces. If the change is sufficiently great, it may trigger a landslide.
Slope instability requires triggering mechanisms. These include:
Intense rainfall which temporarily alters the strength/stress relationship in the soil or rock
Earthquake ground motion which alters the stress on the slope
Mechanical reshaping of the slope by factors such as river erosion at the base of slopes, human excavation such as quarries, and human loading of slopes by placing materials such as mine spoils.
Disasters & Disaster Management-IASbaba
Page 44
The landslide process
Mass movement is one of the erosional processes shaping hill slopes. Landslides are the principal form of mass movement. Mass movement involves the down slope movement of slope materials under the influence of gravity. Landslide is a general term equivalent to slope movements.
Types of landslide movement:
1. Falls are masses dislodged from very steep slopes or escarpments which then free-fall, bounce, or roll downslope. Falls usually move very to extremely rapidly.
2. Topples are a forward rotation around a pivot point low or below one or more masses.
3. Lateral spreads are the result of movement involving lateral extension accommodated by shear or tensile fractures. This type of movement is earthquake-induced.
4. Slides displace masses along one or more discrete planes. Slides may either be rotational or translational in their movement.
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Rotational movement is where the plane is curved. The mass rotates backwards around a common point with an axis parallel to the slope.
Translational movement is where the plane is more or less planar or gently undulating. The mass moves roughly parallel to the ground surface.
5. Flows are masses moving as a deforming, viscous unit without a discrete failure plane.
6. More than one form of movement may be represented in some landslides. Movement in this case is often described as complex.
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Parts of Landslide:
Crown - unfiled area of slope above the landslides. May contain ground cracks called crown cracks.
Main scarp - a steep surface on the margin of the slide caused by the movement of displaced material away from the neighboring, unfiled slope. By projecting this scarp under the displaced material, it would become the surface of rupture.
Minor scarp - a steep surface on the displaced material produced by differential movement within the displaced material.
Toe of surface of rupture - the intersection (sometimes buried) between the lower part of the surface of rupture and the original ground surface. It marks the point separating the zones of accumulation and depletion.
Head - the upper part of the displaced material along the contact between the material and the main scarp.
Top - the highest point of contact between the displaced material and the main scarp.
Main body - the part of the displaced material that overlies the surface of rupture between the main scarp and the toe of the surface or rupture.
Flank - the side of the landslide.
Foot - that portion of the displaced material that lies downslope from the toe of the surface of rupture.
Toe - the margin of displaced material most distant from the main scarp.
Tip - the point on the toe most distant from the top of the slide.
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NOTE: Not all parts of a landslide may be present due to past movement deformation or the nature of the slope movement.
Elements defining the significance of landslide activity:
1. The toll in terms of human life lost to landslides represents a special element to significance. It is a cost which should be represented in addition to monetary costs to define significance. Injuries due to landslides should be represented along with fatalities.
2. Direct costs are those which involve actual physical damage and (or) are related to restoration costs to structures and land impacted by a landslide.
Physical damage would include the loss of agricultural crops as well as the value of destroyed structures.
Restoration costs are limited to the reestablishment of structures, other improvements, and the land to the same conditions and degree of usefulness as prior to the landslide.
3. Indirect costs are any costs associated with the landslide which are not direct.
Relocation of structures or roads would be an indirect cost as would measures taken to prevent or mitigate additional damage.
A major indirect cost is the effect on economic production. Lost wages due to the inability to reach work due to a landslide-closed road, tax losses due to decreased property value, secondary physical effects such as reduced crop production of affected slopes or silting of irrigation systems are all indirect costs to economic production.
Disasters & Disaster Management-IASbaba
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LANDSLIDE CAUSES:
1. Geological causes
a) Weak or sensitive materials
b) Weathered materials
c) Sheared, jointed, or fissured materials
d) Adversely oriented discontinuity (bedding, schistosity, fault, unconformity, contact, and so forth)
e) Contrast in permeability and/or stiffness of materials
2. Morphological causes
a) Tectonic or volcanic uplift
b) Glacial rebound
c) Fluvial, wave, or glacial erosion of slope toe or lateral margins
d) Subterranean erosion (solution, piping)
e) Deposition loading slope or its crest
f) Vegetation removal (by fire, drought)
g) Thawing h. Freeze-and-thaw weathering
h) Shrink-and-swell weathering
3. Human causes
a) Excavation of slope or its toe
b) Loading of slope or its crest
c) Drawdown (of reservoirs)
d) Deforestation
Disasters & Disaster Management-IASbaba
Page 49
e) Irrigation
f) Mining
g) Artificial vibration
h) Water leakage from utilities
Three basic strategies exist for designing slopes in soil and rock to limit the hazard posed to the new development—
Both the nature of the development (road, structure, and dam) and site conditions will dictate the suite of measures capable of producing stable cut and fill slopes-
1. Avoid the problem by changing the location or the design aspect which creates the increased hazard, remove the unstable materials, or bridge over the unstable area.
2. Reduce the driving forces- Changing the grade, draining surface or subsurface water, and reducing weight are typical actions.
3. Increase resisting forces- Draining subsurface water, installing buttress or counter-weights, and installing strengthening elements are common measures to accomplish this result.
Selection of an appropriate remedial measure depends on:
Engineering feasibility,
Economic feasibility,
Legal/regulatory conformity,
Social acceptability, and
Environmental acceptability
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Page 50
Landslide mitigation measures generally fall into four major categories:
1) Avoidance,
2) Water control,
3) Excavation and
4) Retaining structures
We cannot stop a disaster but can minimize its impact by preparing ourselves better for landslides. The Government of India has made plans to identify the areas where landslides occur repeatedly. This is achieved through Landslide Hazard Zonation (LHZ) maps which shows or demarcates areas by different colors. NDMA has published a guideline on Landslides and Snow Avalanches as given on its website.
DOs and DON’TS Do's
Prepare tour to hilly region according to information given by weather department or news channel.
Move away from landslide path or downstream valleys quickly without wasting time.
Keep drains clean,
Inspect drains for - litter, leaves, plastic bags, rubble etc.
Keep the weep holes open.
Grow more trees that can hold the soil through roots,
Identify areas of rock fall and subsidence of buildings, cracks that indicate landslides and move to safer areas. Even muddy river waters indicate landslides upstream.
Disasters & Disaster Management-IASbaba
Page 51
Notice such signals and contact the nearest Tehsil or District Head Quarters.
Ensure that toe of slope is not cut, remains protected, don't uproot trees unless re-vegetation is planned.
Listen for unusual sounds such as trees cracking or boulders knocking together.
Stay alert, awake and active (3A's) during the impact or probability of impact.
Locate and go to shelters,
Try to stay with your family and companions.
Check for injured and trapped persons.
Mark path of tracking so that you can't be lost in middle of the forest.
Know how to give signs or how to communicate during emergency time to flying helicopters and rescue team.
Don'ts
Try to avoid construction and staying in vulnerable areas.
Do not panic and loose energy by crying.
Do not touch or walk over loose material and electrical wiring or pole.
Do not built houses near steep slopes and near drainage path.
Do not drink contaminated water directly from rivers, springs, wells but rain water if collected directly without is fine.
Do not move an injured person without rendering first aid unless the casualty is in immediate danger.
CYCLONE:
We have already discussed the type of cyclones and their formation in detail in the Value adds of Climatology. Here we will discuss the disaster-related aspect of it.
A disaster is only caused by a Tropical cyclone, which is known by different names in different continents like Typhoon in China, Hurricane in North America, Willy Willey in Australia etc.
Tropical cyclones can produce heavy rainfall and sustained winds that can exceed 155 miles per hour (249 km/hr). The official seasons during which cyclones are predicted to become destructive hurricanes and typhoons are different in different areas, but the fact is that, at any time of year, a cyclone is capable of becoming a dangerous hazard.
Cyclones in India:
The Indian subcontinent is one of the worst affected regions in the world. The subcontinent with a long coastline of 8041 kilometers is exposed to nearly 10 per cent of the world’s tropical cyclones. Of these, the majorities of them have their initial genesis over the Bay of Bengal and strike the East coast of India.
On an average, five to six tropical cyclones form every year, of which two or three could be severe. More cyclones occur in the Bay of Bengal than the Arabian Sea and the ratio is approximately 4:1. Cyclones occur frequently on both the coasts (the West coast - Arabian Sea; and the East coast - Bay of Bengal). An analysis of the frequency of cyclones on the East and West coasts of India between 1891 and 1990 shows that nearly 262 cyclones occurred (92 of these severe) in a 50 km wide strip above the East coast. Less severe cyclonic activity has been noticed on the West coast, where 33 cyclones occurred the same period, out of which 19 of
Disasters & Disaster Management-IASbaba
Page 40
were severe. Tropical cyclones occur in the months of May-June and October-November. Cyclones of severe intensity and frequency in the North Indian Ocean are bi-modal in character, with their primary peak in November and secondary peak in May. The disaster potential is particularly high during landfall in the North Indian Ocean (Bay of Bengal and the Arabian Sea) due to the accompanying destructive wind, storm surges and torrential rainfall. Of these, storm surges cause the most damage as sea water inundates low lying areas of coastal regions and causes heavy floods, erodes beaches and embankments, destroys vegetation and reduces soil fertility. Cyclones vary in diameter from 50 to 320 km but their effects dominate thousands of square kilometers of ocean surface and the lower atmosphere. The perimeter may measure 1,000 km but the powerhouse is located within the 100-km radius. Nearer the Eye, winds may hit at a speed of 320 km. Thus, tropical cyclones, characterized by destructive winds, torrential rainfall and storm surges disrupt normal life with the accompanying phenomena of floods due to the exceptional level of rainfall and storm surge inundation into inland areas.
Cyclones are characterized by their devastating potential to damage structures, viz. houses; lifeline infrastructure-power and communication towers; hospitals; food storage facilities; roads, bridges and culverts; crops etc. The most fatalities come from storm surges and the torrential rain flooding the lowland areas of coastal territories.
Disasters & Disaster Management-IASbaba
Page 41
Cyclones are classified into five different levels on the basis of wind speed. They are further divided into the following categories according to their capacity to cause damage:- Cyclone Category Wind Speed in Km/h Damage Capacity 01 120-150 Minimal 02 150-180 Moderate 03 180-210 Extensive 04 210-250 Extreme 05 250 and above Catastrophic
Storm surges (tidal waves) are defined as the rise in sea level above the normally predicted astronomical tide. Major factors include:
A fall in the atmospheric pressure over the sea surface
Effect of the wind
Influence of the sea bed
A funneling effect
The angle and speed at which the storm approaches the coast
The tides
Related Hazards
Storm surge (rise in water level)
High winds and heavy rainfall
Flooding and landslides
Storm tide (the combination of storm surge and high tide)
Disasters & Disaster Management-IASbaba
Page 42
Tornadoes
Precautions
Have an evacuation plan and disaster supply kit ready, including a radio and batteries.
Install hurricane storm shutters or precut lumber for windows.
Remove branches and small trees that may fall on your house.
Clear clogged rain gutters and drains.
Stock extra food, water, and batteries, in case of power failures.
Fill large containers with water for cleaning and toilet flushing.
Purchase a generator for emergency power supply needs.
What to Do if a Tropical Cyclone Threatens
Secure loose objects that may blow away, such as outdoor furniture.
Turn off water, electricity, and propane tanks, and unplug all appliances.
Listen to local radio stations for official announcements and instructions.
Stay indoors and away from windows and exterior walls and doors.
Evacuate to sturdy buildings or public shelters, if advised or ordered to do so.
Leave areas that may flood.
Disasters & Disaster Management-IASbaba
Page 43
LANDSLIDE
Development of Slope Instability
1. Gravity is the agent or driving force producing slope instability. It acts at a constant rate at all times on slopes. When slope instability is produced by gravity, landslides are the resultant phenomena.
2. The strength of the material making up the slope resists the action of gravity- prevents landslides from occurring.
3. The balance between gravity acting on the slope and resistance by the slope material usually favors the resisting slope material. When this balance shifts to being equal or slightly favoring gravity, a landslide is imminent.
4. While gravity acts at a constant rate, changes happen- which increase the shear stress of gravity as it acts on the slope; reduce the resisting strength of the slope material, or a combination of both. This alters the balance between driving and resisting forces. If the change is sufficiently great, it may trigger a landslide.
Slope instability requires triggering mechanisms. These include:
Intense rainfall which temporarily alters the strength/stress relationship in the soil or rock
Earthquake ground motion which alters the stress on the slope
Mechanical reshaping of the slope by factors such as river erosion at the base of slopes, human excavation such as quarries, and human loading of slopes by placing materials such as mine spoils.
Disasters & Disaster Management-IASbaba
Page 44
The landslide process
Mass movement is one of the erosional processes shaping hill slopes. Landslides are the principal form of mass movement. Mass movement involves the down slope movement of slope materials under the influence of gravity. Landslide is a general term equivalent to slope movements.
Types of landslide movement:
1. Falls are masses dislodged from very steep slopes or escarpments which then free-fall, bounce, or roll downslope. Falls usually move very to extremely rapidly.
2. Topples are a forward rotation around a pivot point low or below one or more masses.
3. Lateral spreads are the result of movement involving lateral extension accommodated by shear or tensile fractures. This type of movement is earthquake-induced.
4. Slides displace masses along one or more discrete planes. Slides may either be rotational or translational in their movement.
Disasters & Disaster Management-IASbaba
Page 45
Rotational movement is where the plane is curved. The mass rotates backwards around a common point with an axis parallel to the slope.
Translational movement is where the plane is more or less planar or gently undulating. The mass moves roughly parallel to the ground surface.
5. Flows are masses moving as a deforming, viscous unit without a discrete failure plane.
6. More than one form of movement may be represented in some landslides. Movement in this case is often described as complex.
Disasters & Disaster Management-IASbaba
Page 46
Parts of Landslide:
Crown - unfiled area of slope above the landslides. May contain ground cracks called crown cracks.
Main scarp - a steep surface on the margin of the slide caused by the movement of displaced material away from the neighboring, unfiled slope. By projecting this scarp under the displaced material, it would become the surface of rupture.
Minor scarp - a steep surface on the displaced material produced by differential movement within the displaced material.
Toe of surface of rupture - the intersection (sometimes buried) between the lower part of the surface of rupture and the original ground surface. It marks the point separating the zones of accumulation and depletion.
Head - the upper part of the displaced material along the contact between the material and the main scarp.
Top - the highest point of contact between the displaced material and the main scarp.
Main body - the part of the displaced material that overlies the surface of rupture between the main scarp and the toe of the surface or rupture.
Flank - the side of the landslide.
Foot - that portion of the displaced material that lies downslope from the toe of the surface of rupture.
Toe - the margin of displaced material most distant from the main scarp.
Tip - the point on the toe most distant from the top of the slide.
Disasters & Disaster Management-IASbaba
Page 47
NOTE: Not all parts of a landslide may be present due to past movement deformation or the nature of the slope movement.
Elements defining the significance of landslide activity:
1. The toll in terms of human life lost to landslides represents a special element to significance. It is a cost which should be represented in addition to monetary costs to define significance. Injuries due to landslides should be represented along with fatalities.
2. Direct costs are those which involve actual physical damage and (or) are related to restoration costs to structures and land impacted by a landslide.
Physical damage would include the loss of agricultural crops as well as the value of destroyed structures.
Restoration costs are limited to the reestablishment of structures, other improvements, and the land to the same conditions and degree of usefulness as prior to the landslide.
3. Indirect costs are any costs associated with the landslide which are not direct.
Relocation of structures or roads would be an indirect cost as would measures taken to prevent or mitigate additional damage.
A major indirect cost is the effect on economic production. Lost wages due to the inability to reach work due to a landslide-closed road, tax losses due to decreased property value, secondary physical effects such as reduced crop production of affected slopes or silting of irrigation systems are all indirect costs to economic production.
Disasters & Disaster Management-IASbaba
Page 48
LANDSLIDE CAUSES:
1. Geological causes
a) Weak or sensitive materials
b) Weathered materials
c) Sheared, jointed, or fissured materials
d) Adversely oriented discontinuity (bedding, schistosity, fault, unconformity, contact, and so forth)
e) Contrast in permeability and/or stiffness of materials
2. Morphological causes
a) Tectonic or volcanic uplift
b) Glacial rebound
c) Fluvial, wave, or glacial erosion of slope toe or lateral margins
d) Subterranean erosion (solution, piping)
e) Deposition loading slope or its crest
f) Vegetation removal (by fire, drought)
g) Thawing h. Freeze-and-thaw weathering
h) Shrink-and-swell weathering
3. Human causes
a) Excavation of slope or its toe
b) Loading of slope or its crest
c) Drawdown (of reservoirs)
d) Deforestation
Disasters & Disaster Management-IASbaba
Page 49
e) Irrigation
f) Mining
g) Artificial vibration
h) Water leakage from utilities
Three basic strategies exist for designing slopes in soil and rock to limit the hazard posed to the new development—
Both the nature of the development (road, structure, and dam) and site conditions will dictate the suite of measures capable of producing stable cut and fill slopes-
1. Avoid the problem by changing the location or the design aspect which creates the increased hazard, remove the unstable materials, or bridge over the unstable area.
2. Reduce the driving forces- Changing the grade, draining surface or subsurface water, and reducing weight are typical actions.
3. Increase resisting forces- Draining subsurface water, installing buttress or counter-weights, and installing strengthening elements are common measures to accomplish this result.
Selection of an appropriate remedial measure depends on:
Engineering feasibility,
Economic feasibility,
Legal/regulatory conformity,
Social acceptability, and
Environmental acceptability
Disasters & Disaster Management-IASbaba
Page 50
Landslide mitigation measures generally fall into four major categories:
1) Avoidance,
2) Water control,
3) Excavation and
4) Retaining structures
We cannot stop a disaster but can minimize its impact by preparing ourselves better for landslides. The Government of India has made plans to identify the areas where landslides occur repeatedly. This is achieved through Landslide Hazard Zonation (LHZ) maps which shows or demarcates areas by different colors. NDMA has published a guideline on Landslides and Snow Avalanches as given on its website.
DOs and DON’TS Do's
Prepare tour to hilly region according to information given by weather department or news channel.
Move away from landslide path or downstream valleys quickly without wasting time.
Keep drains clean,
Inspect drains for - litter, leaves, plastic bags, rubble etc.
Keep the weep holes open.
Grow more trees that can hold the soil through roots,
Identify areas of rock fall and subsidence of buildings, cracks that indicate landslides and move to safer areas. Even muddy river waters indicate landslides upstream.
Disasters & Disaster Management-IASbaba
Page 51
Notice such signals and contact the nearest Tehsil or District Head Quarters.
Ensure that toe of slope is not cut, remains protected, don't uproot trees unless re-vegetation is planned.
Listen for unusual sounds such as trees cracking or boulders knocking together.
Stay alert, awake and active (3A's) during the impact or probability of impact.
Locate and go to shelters,
Try to stay with your family and companions.
Check for injured and trapped persons.
Mark path of tracking so that you can't be lost in middle of the forest.
Know how to give signs or how to communicate during emergency time to flying helicopters and rescue team.
Don'ts
Try to avoid construction and staying in vulnerable areas.
Do not panic and loose energy by crying.
Do not touch or walk over loose material and electrical wiring or pole.
Do not built houses near steep slopes and near drainage path.
Do not drink contaminated water directly from rivers, springs, wells but rain water if collected directly without is fine.
Do not move an injured person without rendering first aid unless the casualty is in immediate danger.
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