when do tropical storms occur in the northern hemisphere?
july to october
causes of hazardous mass movements
shear stress overcoming shear strength, pore pressure or increased rainfall or sudden thaw creating snow melt result of hazardous activity eg earthquakes they destabilise slopes volcanoes produce snow melt and engender lahard intense storms can also bring about slope instability human activities can also contribute although their influence is more limited deforestation undercutting and overloading can all affect slope stability as off piste skiing
when do slab avalanches occur and why?
spring partial melt occurs at depth leading to instability at a well defined failure surface
explain the threat of storm surges from tropical storms?
it can lead to extensive and very sudden costal inundation which will swamp many costal defences EG New Orleans where the levees and costal protection was completely overwhelmed by the storm surge resulting from Hurricane Katrina
What conditions are needed for tropical storms to form?
Sea temperatures must be 27 degrees, at a depth of 60m Far enough from the equator that the Corilosis force creates sufficent rotation in the rising air mass
describe the hazardous effects of a tornado?
destruction of property along a narrow belt extending for a few kilometres.
‘killer tornadoes’ (less than 2% - deaths of 689 in 1925) but loss of life is generally low, say, in comparison with tropical storms.
extreme high winds up to 500km/h (category 5 on Fujita scale) drawn into an ascending vortex and heavy downpours of rain with the possibility of massive hailstorms.
Movable bodies such as cars and lorries can be uplifted and dumped, and the extreme low pressure can cause some buildings to ‘explode’.
There is often widespread crop destruction.
what latitudes do tropical storms occur at
explain the hazardous impacts of tornados?
destruction of property as movable bodies are uplifted and dumped, extreme low pressure can cause some buildings to explode due to pressure difference flooding due to heavy downpours of rain and hailstorms extreme high winds drawn into an ascending vortex causing crop destruction killer tornados (less than 2%) may creates swathed hundreds of metres wide and up to 150km long loss of life is generally low
describe and explain the development of tropical storms
Intense evaporation from warm seas with warm air rising and condensing to form towering clouds (cumulonimbus) and heavy rainfall (125mm an hour).
The uplift of air is accentuated by the release of latent heat as a result of condensation.
The rising air also creates intense low pressure with warm air being sucked in which then also rises and so the hurricane develops.
The up flow of warm and humid air continues.
Air that surrounds the low-pressure zone at the centre flows in a spiral at very high speeds, anti-clockwise in the northern hemisphere.
The Coriolis force of the earth is needed to develop the spinning motion.
Air is ejected at the top of the eye.
why are there continental deserts?
more heat as land heats quicker than sea, is transparent and large of deep less rain as rain comes from the sea eg afghanistan is thousands of miles from sea
explain how a tornado develops
Tornadoes are spawned by violent uplift associated with violent thunderstorms, called supercells.
At Tornado Alley in the mid-west USA warm moist air masses from the Gulf of Mexico moves north and meets cooler, drier air from the northern Plains and the Rockies.
When these winds collide, a strong front develops which causes a big horizontal cylindrical vortex to form.
he warm air slides beneath the cold air, at different altitudes in a wind-shearing effect, the thunderheads and start to rotate rapidly (reasons not fully understood, thought to be air inversion)
Beneath the clouds violent up draughts caused by convection cause the horizontal cylindrical spiral of air to tilt into a vertical funnel and create intense low pressure
If it continues to grow, it will touch the ground and become a tornado.
when do powder avalanches occur?
Any time of the year following a heavy snowfall over a partially frozen snow surface
what factors cause a landslide
climate eg permeable rock overlying impermeable faults or joints allowing water buildings increase weight deforestation increases infiliytation removal of the foot slope heavy rainfall overwhelming through flow
describe the hazards associated with tropical storms
intense high rainfall up to 125mm recorded usually lasts for 6 hours from the storm centre - results in: extreme flood events, landslides, breaching of levees and slope settlements destroyed - eg hong kong winds of over 250km/hr denote a category five storm on the Safir-Simpson index - results in: structural damage, contribution to the development of storm surges by whipping up waves and driving them on shore storm surges develop from the low pressure, causing a rise in sea levels; wave generation from high winds being driven on shore very hazardous when combined with high tide - results in: costal inundation, especially when low lying (Bangladesh) or on a river estuary (New Orleans) combined with high rain this has a multiplied effect
areas most at risk of tropical storms
caribbean bangladesh SE america
fact about tornadoes in the USA
80% occur here tornado alley especially Kansas Oklahoma Missouri
what is the Coriolis force
force causing the rotation of the Earth
describe the nature of avalanches
All avalanches occur on slopes with a gradient of between 26-60 degrees (specifically, human triggered avalanches occur at 38 degrees)
Avalanches tend to have a speed of 60km/h, but this speed can reach 290km/hr, as was seen in the Galtur Avalanche 1999
Avalanches can be 50m high
In the Northern hemisphere, they tend to occur on north facing slopes as the lack of sun inhibits the slope stabilization
They are classified by the European avalanche size table with 1 being unlikely to cause injury, and 4 being an avalanche with the ability to destroy buildings
Single point vs slab avalanches (travel at 60–80 miles/hour and account for most fatalities)
The movement of the snow pack caries: either in an open valley, or powder (airborne), or flows along the ground.
It has three main parts: the starting zone which is the most volatile area of the slope, where unstable snow can fracture from the surrounding snow cover and begin to slide. The avalanche track is the path that the avalanche follows as it goes downhill, and the runout zone is where the snow and debris come to a stop.
appearance of tornadoes
small funnel shaped violently rotating vortex e tending down from cumulonimbus clouds vortex in contact with the ground
definition of a tropical storm
a generic term that includes hurricanes cyclones and typhoons low pressure weather system up to 600km/hr in diameter with wind speeds of up to 300km/hr and bringing up to 30-50cm of rainfall
describe the nature of tornados
intense rotating (anti-clockwise) masses of air funnel shaped swirling cloud
full of debris and dust
Around the funnel of rising air are very strong winds often over 300 kph
intense precipitation with large hail stones
local meteorological hazards (usually less than a km wide) 100-600m the track can extend for 2-4 km
last a few minutes
Distribution: famously occur at Tornado Valley in Mid-West USA as warm moist air from North Mexico collides with cold dry air from the Rockies.
explain the threat of intense torrential rainfall from tropical storms?
They destabilise slopes bringing about slopes landslides and mudflows particularly hazardous in LIC where shanty towns are located on unstable slopes
conditions for tornado formation
warm air high humidity near ground level fast moving winds about 70 knots about 3k. above ground level travelling in different directions
describe tornado alley
Mid west USA Oklahoma Kansas Missouri warm moist air from the north from Canada or the rocky mountains
why are he hazardous impacts of tropical storms worse than tornadoes?
power of tropical stones is immense, impact leads to thousands dead and widespread destruction of property death toll of tornadoes rarely reaches hundreds tropical storms affect costal areas that are often densely populated (receiving high winds storm surfers and floods) - lower course of river but tornadoes produce heavy hail storms cyclones have a diameter of 500-800kn whilst tornadoes rarely exceed 10km in path and may only periodically touch the surface BUT wind speed is higher for tornadoes as can reach 500km/hr, double that of tornadoes pressure at the heart of tornadoes is 20% below normal ambient pressure
how fast do slab avalanches travel?
60-80 miles an hour
how can high rainfall be protected against?
leads to flooding and landslides flood control measures can be put in place along rivers deforestation if slopes floodplain management can be developed to protect critical assets; building on slopes can be controlled or prevented Protective river embankments, levees and dukes should be regularly inspected for breaches and opportunities taken to plant mangroves to reduce braking wave energy Vegetation cover can be improved This helps to reduce the impact of erosion and landslides and facilities the absorption of rainfall to reduce flooding
list of human activities on slopes
deforestation building on or on top of cutting base of slopes mining or quarrying filling large resovirs weapons testing shear stress strength balance affected
What is a tornado?
A violent destructive weather system with powerful rotating winds up to 300km/h Intense low pressure systems Their development depends on instability in the atmosphere convergence and strong updrafts in the air
describe the distribution of tornados?
80% major tornadoes occur in the USA generally at their most hazardous tornadoes occur widely throughout the world from australia to birmingham
what latitudes do tropical storms occur at
definition of a tornado
S violent destructive weather system Powerful rotating winds up to 300km/he Tornadoes are intense low pressure systems and their development depends on instability in the atmosphere convergence and strong updrafts in the air
when do tropical storms occur in the southern hemisphere?
January to April
how does a tropical storm develop
over warm ocean area higher than 26 degrees more than 50m deep 500km both and south of equator 5 degrees in late summer when sea temperatures are at their maximum so moisture and energy enters the system via evaporation where the trade winds converge (NE and SW -NH) developing low pressure the undistrubed upper atmosphere allows the development of massive uplift of cold air is forced to rise rapidly forming cumulonimbus clouds no wind shearing the coriolis forces helps generate strong spinning winds, around the low pressure at the centre of the storm (eye)
describe the method of draining slopes to reduce mass movement hazards
Drainage pipes - hard engineering Reduce build up of water decreasing water pressure in the soil preventing oversaturation Effective, low cost covering a large volume of ground HOWEVER requires regular maintenance or it’ll become ineffective
explain how cold ocean currents cause deserts
less evaporation any air blown on shore is drier and cooler off coast of peru or chile cold peruvian current
what are the most damaging type of tropical storms?
Category 5 on the Saffir Simpson Scale Wind speeds in excess of 250km per hour Storm surge of more than 5.5m This can lead to extensive damage to property and cause deaths and injuries from flying objects
causes of instability in slope?
inherent: weaknesses in the rock structure, composition of soil caused by heavy rain or snow melt caused by tectonic or seismic activity human triggers: changes to slope angle and form decrease in toe support increased load on the head interference with drainage
explain rain shadow effect?
leeward sides of mountain are driver this is because the prevailing winds from the sea move towards the mountain, clouds pass over mountain and the flat land adjacent had descending dry air eg south america
methods of limiting the hazardous effects of avalanches
reinforcing buildings snow fences tree planting artificially triggering avalanches preventing access to dangerous slopes
ineffective response of a tornado
1996 Bangladesh tornado government underestimated disaster Red Cross not permitted with valuable medical supplies Despite widespread injury 1,000 injured in a 200 bed hospital in Tangail
how can high winds be protected against?
High winds Cyclone or hurricane shelters can be constructed Marked evacuation routes established Along with earnings The removal securing of vulnerable structures
definition of a tsunami
a series of water waves caused by the displacement of a large volume of a body of water, generally an ocean (can be a large lake)
how can storm surges be protected against?
Sea walls can also be constructed but these are not much defence against category 5 hurricanes, warnings given to costa moorings the evacuation of coastal areas
what is an avalanche
a type of mass movement involving the movement of snow and ice downslope under the influence of gravity
reducing the hazard of mass movements
Afforestation Prediction Draining slopes Regarding slopes
how large can the central eye be in a tropical storm
low pressure systems up to 600kn in diameter with wind speeds of up to 300km/hr typically 160km/h rostov around a calmer core eye bringing up to 30-50cm of rainfall
reducing vulnerability of structures and infrastructure against tropical storms
new buildings should be designed to be wind and water resistant - building codes communication and utility lines located away from coastal areas and installed underground improvement of building sites; raising ground levels to protect against flood and storm surges improved vegetation cover; helps to reduce the impact of soil erosion and landslides river embankments levees regularly inspected for breaches
concrete breakwaters and floodgates protecting ports and costal areas
Case study of mass movement prediction
MUDSLIDES OF SIERRA LEONE AUGUST 2017
Sierra Leone Meterological Office did not issue warnings due to a lack of funding/equipment : GNI of country is $1560
Sierra Leone's Office of National security advised evacuation when satelitte images of the hillside adjacent to the area of the inital slide showed another threat
how did the the human geography of the area affect the mudslide in Sierra Leone
Deforestation on slopes reduced soil stability and increased soil erosion
Unregulated construction; moratorium ignored and the drainage system regularly blocked
example of a mudflow caused by heavy rain
unusually wet rainy season - 104mm of rainfall since July 1st - triple the average, leaving the ground saturated, making it heavy, and eventually resulting in a mudflow that killed 1,000 people.
three days of unusually heavy rainfall prior to the event
example of land use planning to reduce the hazardous impact of mass movements
High risk areas that are therefore without human activity, completely mitigate the hazard posed by mudflows.
resettlement programs: the Indian Institute for Human Settlements seeks to reduce the hazard of mass movements in India via resettlement.
what is the difficulty with predicting mass movements
the triggers themselves are difficult to predict
e.g. earthquakes and possibly volcanoes
explain methods of reducing the gradient of a slope in order to prevent mass movement
This could be done by grading, which involves shaping of the rock slope into a more stable configuration, in flattening or leaving ‘benches’ in the slope face.
On the other hand, the slope can be excavated or filled to produce a gentler slope.
However, this is ineffective in the case of slopes that are not easy to reach, as the mechanical means of achieving slope grading or excavation, requires machinery
explain how afforestation reduces the hazardous impacts of mudflows
Vegetation reduces the risk of mudflows by removing water through transpiration, the mechanical reinforcement of roots and enhancing soil cohesion increasing shear strength
e.g. in Malaysia deforestation plays a major role in mass movement cause so in Pahang, the plant ubi kayu, a species of fast growing tree was planted - after 6 months 75% had sprouted and taken root. Landslides have not occured on these bioengineered plots since vs not managed slopes that have seen undercutting of slope face.
This measure is effective and low cost, only $3000 and can be locally constructed, minimal maintenance, attractive as soft engineering
can have adverse effects such as reducing bio-diversity and taking up space (which in urban areas, with high population density, is a pertinent issue).
explain how mass movement triggers be predicted
Hydrological triggers: prolonged and heavy rainfall are the easiest to forecast
but it is difficult to know which slopes are likely to be affected or exactly when it will occur (however some areas are more suceptible e.g. clay areas, steep slopes)
Monitoring is also possible with instruments used to measure ground deformation, the expansion of fissures and groundwater pressure
the United States Geological Survey has published a series of warning signs that can be observed shortly before a mass movement event:
leaning telegraph poles, new cracks or buldges in the ground (volcano) and springs in the ground that had not previously been wet
explain how perception of risk is flawed for mass movements
Italy is more concerned with technological risk e.g. car accidents than antural risks
Of the natural risks they are most concerned about earthquakes then floods and volcanic eruptions
POR is also deeply influenced by recent landslide events
e.g. the people of Calabria percieve landslide risk as very high, but the hazard to life and property as moderate as they have never had a fatal mass movement event
how can response be hampered in mudflows
e.g. Sierra Leone (2017)
e.g. Mount Ranier, Washington sttae
remote areas make resuce/evacuation difficult
visitors are unfamiliar to the area/respond slowly
soft mud that is difficult to move
e.g. Nevado Del Ruiz (1985) up to 4.5 m deep
several highways connecting Amero were destroyed by lahars so it took 12 hours for the first survivors to be rescued
sustainable managment/prediction of lahar mass movements
mount ranier stratovolcano in washington state
1995 US geological survey
automated system detects lahars using a network of small sensors, embedded underground to measure ground vibrations
computer base stations analyze signals from field stations
upon detection computer sends signal to 24 hour emergency monitoring and notification centres; people are notified via channels of communication e.g. tv, sirens, radio
in remote areas people are encouraged to be aware of natural signs:
roaring sounds similar to jet
10,000 people have between 40 mins and 3 hours to get to saftey
why does sea water have to be 50 degrees for a tropical storm to occur
waters of this temperature cause the overlying atmosphere to be unstable enough to sustain convection and thunderstorms
how was Galtur, a powder avalanche, caused?
4m of snowfall resulting from a combination of 3 atlantic weather systems - ended up being 170,000 tons
freeze thaw conditions created a weak layer on top of the eisting snow pack
more snow deposited on top
describe the Galtur avalanche
290km per hour
why was the impact so high for the Galtur avalanche
31 deaths mostly tourists, overturned cars, powder snow killed people by asphyxiation
galtur was marked as green in the hazard map - avalanche safe, with no building reinforcements required
1 in 150 year event wouldnt even reach the village
example of avalanche forecasting
hazard mapping in all populated reigons of alps
red: highly vulnerable no development permitted
yellow: moderate developed should be reinforced
green: avalanche safe, no reinforcement required
reason why impact was so high - Galtur was green with a 1 in 150 chance of an avalanche
no monitoring in place (took 60 seconds to hit anyway)
difference in size: tropical storm vs tornado
Tropical storms have a diameter of some 500–800 km
whereas the destructive path of tornadoes rarely exceed 10 km and may only periodically touch surface
tornados leave a footprint of some 50 metres wide and a track of 1–2 km
wheras ropical storms may also travel for hundreds of km
tropical storms can lasting days, destroying islands and coastal areas – large death toll, widespread property damage.
By contrast, tornados last for a few minutes to a couple of hours, causing limited damage – injury or very rare deaths ‘killed tornados’
describe the nature of mudflows
Mudslides involve the downslope movement of debris with a significant liquid content.
They contain at least 50% clay or slit and 30% water
Thus, they can travel on lower slope angles than other mass movements 25-45 degrees e.g. debris flows or rock falls
It has two main parts: a main scarp – the area of incidence and a toe – the last affected area
They have been known to be as deep as 30m
They can flow as fast as tens of m per second, carrying debris and smothering villages. For example, the Sierra Leone mudslides of 2017, the saturated mud flow carried mud, large boulders, tree trunks, and other material.
when are avalanches most common
24 hours after a snow storm dumps 30cm of fresh snow
what is wind shear
wind moving fast at different altitudes
describe tornados having greater winds than tropical storms
the wind speed of tornadoes can reach 500 km/h about double that of the most powerful cyclones and the pressure at the heart of a tornado can be 20% below normal ambient pressure.
example of landslides following a tropical storm
Hurricane Mitch 1988
rain fillled the creater of the dormant Casita volcano in Nicaragua
caused a decrease in shear strength
lead to four villages inundated, everything buried in 6m of mud
area of 16 by 8 km
explain the global distribution of tropical storms
Tropical storms develop in a narrow band between 5° and 15° N and S of the equator except in the South Atlantic and eastern Pacific oceans.
They do not originate on the equator because of a lack of Coriolis force.
They develop in areas where sea temperatures exceed 26 °C and are of sufficient depth to fuel tropical storms which develop from atmospheric disturbance at the meeting of warm and cooler air masses.
All tropical storms develop fully where there is no wind shearing.
They do not develop on the western coasts of Africa and South America because of the cold offshore currents.
Tropical storms track westwards driven by the westerly flowing air streams from the convergence of the NE and SE trades and they veer poleward before dying out.
contrast the prediction of tornados and of tropical storms
Tropical storms are much easier to predict than tornados.
The strength of wind in a tornado means that conducting investigations into them highly problematic.
Scientists can predict tornados using ground and air based radar systems, analysing storm clouds; however, these systems only give warnings a few minutes in advance.
By contrast tropical storms can always be seen approaching as they are large and slow moving, meaning evacuation orders can be enabled 3-5 days in advance.
For example, in 1998 100,000 people were evacuated from Honduras in advance of Hurricane Mitch. However, Cyclone Nargis was predicted but the Burmese government lacked sufficient infrastructure to do anything about it.
Also, the precise track of hurricanes is not always possible to predict, nor is the unexpected strengthening and weakening of the storm as It passes over sea areas of different temperatures.
why is prediction of tropical storms problematic
difficult in terms of the trajectory (within 80km)
not possible to predict the intensity at all;
strength and weakening as storms pass over hot/cold water
this is attributed to the complexity of tropical systems and an incomplete understanding of factors that affect their development
prediction: general likelihood of TROPICAL STORMS
Atlantic/Caribbean satellite imagery and computer modelling
predict in April how many hurricanes are expected to make landfall
(based on past occurrences and factors e.g. climate – as a %).
prediction: specific forecasting of a TROPICAL STORM
achieved via effective, continuous monitoring: information avalible every 6 hours TCWC
Once a hurricane has been formed, its path can be predicted 3-5 days in advance, giving people time to evacuate or hurricane-proof their property.
This enables small-scale preparedness to take place, the city itself can’t be completely prepared for landfall.
For example, hurricane Mitch was predicted 5 days prior, allowing thousands of citizens to be evacuate onto the bay islands.
The Indian Metrological agency predicted Cyclone Nargis, but Burmese government did nothing
preperation: general of TROPICAL STORMS
Local government building flood barriers in the case of storm surges, but these are often very expensive and not always effective.
For example, the Fox Point Hurricane Barrier is a 3,000-foot long tidal flood barrier spanning the Providence River in Rhode Island.
Alternatively, mangroves can be planted to reduce vulnerability to storm surges by slowing the flow of water.
Additionally, communication and utility lines can be located away from areas of water, or located underground.
Extreme winds mitigated in an elevated structure on an open foundation, reduces the risk of damage from flooding and storm-driven water
In New Orleans area V must have flood insurance to financially protect people from the hazardous impacts of tropical storms
preperation: specific event of TROPICAL STORMS
evacuation/fleeing – often late notice as governments are reluctant to enable the release of funds/creation of panic in order to coordinate an evacuation, if the storm will disperse
why is response so important to reduce the hazardous impacts of the tropical storms
the intensity of tropical storms can never be predicted, so government predictions, advise and preperation methods are often insufficent - response is the best way of dealing with specific hazardous impacts on an appropriate scale
but, it is necessary for a stable political climate and appropriate aid distribution methods - something that even HIC countries struggle with: after Hurricane Katrina saw US gov. criticised for inadequate planning
hazard perception: TROPICAL STORMS
According to the American Meteorological Society (2014), the majority of people misunderstood that the greatest threat was wind (the damage of this was also underestimate), rather than water.
immediate response for TROPICAL STORMS
digging through the rubble to rescue people; sending out search and rescue teams
immediately after Hurricane Mitch 1998 armed guards were sent to coordiante rescues and guard businesses that could be looted, 200 people arrested in Honduras
short term response for TROPICAL STORMS
after hurricane mitch US offered $70 million, Roman Catholic church helped to distribute aid across the country.
HOWEVER short term response can be hampered by political conditions:
Cyclone Nargis in Burma, 2008, the isolationist government only permitted certain flights and denied visa requests
Aid was only distributed 2 weeks after the event, making food rations unsuitable
long term response of TROPICAL STORMS
reform of policy towards building design, ensuring homes can withstand storm events;
after hurricane mitch 1998
homes were rebuilt and crops replanted over the years
the reconstruction efforts acted to lower unemployment rates
prediction: general likelihood of tornados
. FEMA issue hazard maps yearly in accordance with recent events, weather conditions etc.
This is a choropleth map which parallels the Fujita scale (categorising tornados based on damage).
Iowa, Kansas and Missouri.
Citizens of these areas are advised to build portions of the buildings designed for shelter.
prediction: specific forecasting of TORNADOS
the strength of wind in a tornado means that conducting investigations into them highly problematic; tornado formation not properly understood - forecasting gives few minutes of warnings
Scientists can monitor tornados using air based radar systems (5 satellites orbiting 22,000m above earth taking snapshots of cloud formation), analysing storm clouds
Also short term, storm spotters report to the national weather service, trained locally on the demands of the area.
general preperation: TORNADOS
government websites with advice such as FEMA
Schools/hospitals/malls identify small windowless rooms below ground
People in mobile homes or offices advised to go to a secure pre-identified location
If evacuation from a building is impossible, then the advice is to take cover under a sturdy table and cover themselves
The US has warning sirens/tornado drills
specific event preperation: TORNADOS
evacuation/fleeing; putting relevant action plans into action
Apps such as the American Red Cross notify mobile warnings, allowing people to indicate they are safe through social media.
hazard perception: TORNADOS
Simple and inexpensive enhancements to building codes could lead to 30% less damage in Oklahoma
opposed by conservative local government with false hazard perception, sceptical about climate change etc.
short term response: TORNADOS
During the 2013 Mississippi Tornado the response was largely internal.
Schools were reopened 4 days later, buses borrowed from neighbouring school districts. 13,000 power outages quickly fixed
The red cross and salvation army provided shelter.
Following the 1996 Bangladesh tornado, the government massively underestimated the disaster, meaning supplies were not distributed
A Government 200-bed hospital in Tangail was overwhelmed with more than 1,000 injured; whilst the Red Cross was not permitted into the country with valuable medical supplies.
long term response: TORNADOS
improved hazard perception
updated hazard mapping
reform to building codes
preperation: specific hard engineering for MUDSLIDES
reducing pressure on the slope,
controlling slope face processes, e.g. using sheet piles, draining the slope,
reducing slope foot processes, e.g. gabions,
changing the angle of the slope so it is nearer the stability angle
In Hong Kong, a system of ‘mulching’ has been employed, where a protective cover is provided, making it possible for vegetation to grow on the slope, binding the slope surface and securing it in place.
Also, reinforced fibre soil systems mixing polyester fibre in sandy soils are capable of resisting tension.
describe the nature of tropical storms
Size: diameter of some 500–800 km; travel for hundreds of km
Rotation: anticlockwise in accordance with the Coriolis force
wind speeds: highest (category 5 on the saffir-simpson index – above or at 157mph, however some reach speeds of up to 200mph). Hurricane Irma 2017.
They move from east to west to impact largely on eastern coastal areas
distribution: they start within 8° north and south of the Equator (outside the immediate equatorial area) where sea surface temperatures reach 27°C
gain or lose strength in accordance with sea temperature.
Areas most at risk are the Caribbean, South East USA and the low lying coasts of Bangladesh. In the northern hemisphere, they occur July – October when sea temperatures are at their highest.
rainfall amounts: on average 406mm but can be up to 1,540mm (hurricane harvey 2017)
lasts for 6 hours from the storm centre
describe the hazardous impacts of tropical storms
Vast quantities of rain (up to 125 mm in one hour have been recorded) which usually last for six hours each side of the storm centre.
The resultant hazards are extreme flood events and land/mud slides so that the impact can be massive where river levees are breached and slope settlements are destroyed (plenty of examples; Hong Kong, Rio).
Winds of over 250 km/hr denote a category five storm on the Safir-Simpson index.
The impact is structural damage and a contribution to the development of storm surges by whipping up waves and driving them on shore.
Storm surges develop minimally from the low pressure causing a rise in sea level and mainly by the wave generation from high winds being driven on shore. They will be especially hazardous when combined with a period of high tide.
The impact is coastal inundation of low lying coastal areas (Bangladesh) and river estuaries (New Orleans). Combined with river floods from high rainfall has a multiplied effect.
prediction: general likelihood of MUDSLIDES
hazrd mapping based on onger term on geology and hydrology of an area, but shorter term will depend on recent weather and changes to environment, such as cut and fill new road through mountains, excavations at foot of slopes for new buildings, etc.
FEMA produce a hazard map for California, which encompasses the volume of the possible mudsides and the relative hazard of a mudslide – mountainous areas are red.
With an 80-100% chance of mudslides within the year.
problems with specific warnings of mudslides
Even if there is sufficient monitoring, a warning is not always issued.
For example, prior to the Sierra Leone mudslides of August 2017, the Meteorological office did not issue a warning due to lack of funding and infrastructure.
However, at a later stage after the first mudslide had occurred, an evacuation was enabled.
general preperation for MUDSLIDES
Sign up for your community’s warning system, such as the Emergency Alert System
. Know your community’s landslide evacuation plans; Practice driving evacuation routes and identify shelter locations.
Gather supplies in case you have to leave immediately or if services are cut off. Keep in mind each person’s specific needs, including medication.
Plant ground cover and build walls to direct the flow around buildings.
Why is response the most effective way of dealing with mudslides
Only once an event has happened the specific needs of the affected area can be addressed
planning and preparation prior to an event are inevitably much more generalized and can’t necessarily anticipate the scale or intensity of the hazard
short term response to MUDSLIDE
Red Cross and government donated 15 vehicles to isolated areas.
UN sent forensic officers and specialized rescue units, dogs. WTP provided rations for 7,500 people.
Workers in regent constructed 4x 10,0000L water harvesting systems as there was a fear cholera would break out. The UK provided storage tanks, purification tablets and hygiene advice.
long term response to MUDSLIDE
government under international and domestic pressure to reform building policy to protect the poor who are inevitably hurt by their ability to only afford housing on unstable and marginal land
The IFRC approved 4.6 million Swiss Franks to be distributed through Sierra Leone agencies over 10 months following the disaster – helping to rebuild and provide temporary accommodation.
explain why mudslides occur
Tend to occur due to heavy rainfall, oversaturating the slope, increasing the weight of the soil.
Shear stress becomes greater than shear strength and a mudslide occurs.
For example the 2017 Sierra Leone mudslides were triggered by an unusually rainy season (triple the regions seasonal average for July).
This high level of rain was not appropriately handled in the infrastructure of the area, with blocked drains preventing infiltration.
This was exacerbated by the deforestation on the slope in order to meet the growing demand for homes (increased population pressure). Deforestation reduces the anchoring effect of vegetation, increasing susceptibility to soil erosion.
These buildings additionally increased the weight on the slope, increasing shear stress and so the vulnerability to a trigger.
Geology (rock types and structures) e.g. permeable strata overlying impermeable, faults or joints allowing ingress of water.
Removal of the foot slope, i.e. allow a stable slope to develop; undermining the cliff
describe the hazardous effects of avalanches
Wet snow avalanches are the most powerful and generate the most hazards; They occur in spring with partial snow melt.
Powder avalanches can cause asphyxiation as occurred following the Galtur avalanches (31 deaths). Powerful shockwaves precede it. The cold weather associated with heavy snowfall can create additional suffering - suffer hypothermia and frost-bite before help arrives. If triggered by earthquakes they are the most hazardous – as a secondary impact.
Animals caught off-guard may be swept to their deaths; hoofed mammals such as mountain sheep are particularly vulnerable.
Blockages of roads, mountain passes and streams and damage to electric and communication lines. Blockage of streams and later release of the impounded water create flash floods with disastrous effects. Most hazardous in the location of settlements.
explain the formation of slab avalanches
Snow avalanches occur mostly on slopes between 25 and 40 degrees
Slab avalanches tend to occur in spring when partial melt occurs at depth, due to a sudden rise in temperature.
This causes water to percolate through the snowpack, leading to instability (reduction in shear stress) at a well-defined failure surface.
A plane of weakness emerges in the upper layers of snow. Because of this weakness, human activity can then trigger an avalanche.
For example, the vibrations from the movement of a snowmobile or earth tremors due to movement in the earth’s crust. Snowmobiling is the leading cause of avalanche fatalities in Idaho.
Mass overcomes friction and a resultant avalanche occurs.
explain the formation of powder avalanches
Powder avalanches can occur at any time following a heavy snowfall over a partially frozen snow surface (however, tend to occur in winter seasons – Northern Hemisphere November-Februrary, Southern Hemisphere June-August).
Avalanches are most common after a storm that deposits 30cm of fresh snow. the Galtur avalanche 1999 was triggered by 4m of snow from 3 different weather systems.
This most often occurs on a smooth surface e.g. after rain – without the cohesion with the snow layer underneath, it cant settle. Freeze thaw conditions created a weak layer on top of the existing snow pack that prompted destabilisation
This then reaches a critical point where mass is overcome by friction and an avalanches results.