NATURAL HAZARDS

What are natural hazards?

Natural hazards are extreme natural events that can cause

• loss of life,

• extreme damage to property and

• disrupt human activities.

Some natural hazards, such as flooding, can

• happen anywhere in the world (flooding)

• happen in specific areas (tropical storms)

• need climatic or tectonic conditions (tropical storms, volcanic eruptions)

Tropical storms• A tropical cyclone is a very powerful

low-pressure weather system which

results in strong winds (over 120

km/h) and heavy rainfall (up to 250

mm in one day).

• In the US and the Caribbean they are

known as hurricanes, in South Asia -

cyclones, in East Asia- typhoons and

in Australia they are known as willy-

willies.

Hurricanes need a lot of heat to form and a sea surface temperature of at least 26°C, which is why they usually occur over tropical seas.

Warm ocean heats the air above

Rising warm air evaporates and starts to spin

The air then cools and condenses to form towering clouds

Intense low pressure sucks in air, causing very strong winds

The eye of the storm is formed because this is the only part of the hurricane where cold

air is descending.

ANATOMY OF A HURRICANE

EYECONVECTION CURRENTS

COOL DENSE AIR

HURRICANE WINDS AND RAIN WARM MOIST AIR

What are the hazards associated with tropical cyclones?

• Storm surges

• Strong winds

• Torrential rain and flooding

Prediction and preparationRich countries :

• have resources and technology to predict and monitor storms, ex. using satellites and specially equipped aircraft.

• train the emergency services appropriately and to educate people about necessary precautions.

Prediction and preparationPoor countries :

• rely on aid (sometimes reluctantly) from MEDCs

• intense winds of tropical storms can destroy whole communities, buildings and communication networks

• take longer to recover

Hurricane Katrina• Hurricane Katrina was the costliest storm in U.S. history, and its effects are still felt today in

New Orleans and coastal Louisiana.

• Katrina first formed as a tropical depression in Caribbean waters near the Bahamas on August 23, 2005.

• It reached hurricane status two days later, when it passed over south eastern Miami as a Category 1 storm.

• The tempest blew through Miami at 80 miles per hour, where it uprooted trees and killed two people.

• Katrina then weakened to a tropical storm, since hurricanes require warm ocean water to sustain speed and strength and begin to weaken over land.

• However, the storm then crossed back into the Gulf of Mexico, where it quickly regained strength and hurricane status.

• On August 27, the storm grew to a Category 3 hurricane. At its largest, Katrina was so wide its diameter stretched across the Gulf of Mexico.

• Before the storm hit land, a mandatory evacuation was issued for the city of New Orleans, which had a population of more than 480,000 at the time.

• Tens of thousands of residents fled. But many stayed, particularly among the city's poorest residents and those who were elderly or lacked access to transportation.

• Many sheltered in their homes or made their way to the Superdome, the city's large sports arena, where conditions would soon deteriorate into hardship and chaos.

• Officials initially believed New Orleans was spared.

• But later that morning, a leveebroke in New Orleans, and a surge of floodwater began pouring into the low-lying city.

• An estimated 80 percent of the city was soon underwater.

• The city of New Orleans was at a disadvantage even before Hurricane Katrina hit. Some of the city is below sea level so is particularly prone to flooding. Low-income communities tend to be in the lowest-lying areas.

• Katrina was the most destructive storm to strike the United States and the costliest storm in U.S. history, causing $108 billion in damage. An estimated 1,833 people died in the hurricane and the flooding.

• The rescue and recovery efforts following Katrina became highly politicized, with federal, state and local officials pointing fingers at one another.

• an aging and neglected federal levee system and a slow state and local response following the disaster

FloodingThere few places on Earth where flooding is not a concern. Any area where rain falls is vulnerable to floods, though rain is not the only cause.

The definition of a flood is land covered by water that is not usually covered by water.

Causes of floodingNATURAL CAUSES

• long periods of rain

• snowmelt

• impermeable rock (doesn't allow water through)

• steep slopes

• overflowing rivers

Causes of flooding

CHANGING LANDSCAPE

• urbanisation (surfaces of towns and cities are impermeable ex. tarmac)

• deforestation

• broken dams

• increase in population density (leads to building on inappropriate land)

Flooding in BangladeshBangladesh is a low lying country that is incredibly vulnerable to flooding.

It has 3 major rivers flowing through it (the Ganges, Brahmaputra and Meghna) and is vulnerable to coastal flooding as it is so low lying and flat.

Flooding in BangladeshBangladesh is a country that has both river and coastal flooding.

• River floods result from heavy rainfall an melting of snow in the Himalayas. Farmers rely on the floods to supply fine silt (fertile sediment deposited from rivers

• Coastal floods are created by cyclones which push water in the Bay of Bengal towards the land, causing a storm surge. This floods low-lying coastal areas.

Wildfires

How do wildfires start?•A fire needs three things: fuel,

oxygen and heat. Firefighters often talk about the fire triangle when they're trying to put out a blaze.

•On a hot summer's day when droughtconditions peak, something as small as a spark from a train wheel striking the track can ignite a raging wildfire.

•Sometimes, fires occur naturally, ignited by heat from the sun or a lightning strike.

Common causes for wildfires include:

•Arson

•Campfires

•Discarding lit cigarettes

• Improperly burning debris

•Playing with matches or fireworks

•Prescribed fires

Why can they spread so quickly?

•Once it's started a wildfire can be wind driven, slope driven or fuel driven. Low rainfall causing a drought, searing hot temperatures and wind all make the perfect recipe for a wildfire.

Benefits of wildfires• tree cones need to be heated before they open and release their seeds

• some seeds require fire before they germinate

• wildfires also help keep ecosystems healthy. They can kill insects and diseases that harm trees.

• fires can make way for new grasses that provide food and habitat for animals and birds

• they add nutrients to the soil, and open up space to let sunlight through to the ground. That sunlight can nourish smaller plants and give larger trees room to grow and flourish.

Avalanches

Many people think of a snow capped mountain as a peaceful scene, but things can get ugly pretty quick if an

avalanche comes!

• Avalanches are most common during the winter, December to April in the Northern Hemisphere, but they do occur year-round.

• To get an avalanche, you need a surface bed of snow, a weaker layer that can collapse, and an overlaying snow slab. The highest risk period is during and immediately after a snow storm.

What causes avalanches?Humans trigger 90 percent of avalanche disasters, with as many as 40 deaths in North America each year. Most are climbers, skiers, and snowmobilers.

Avalanches are usually triggered by :

▪ Heavy snowfall

▪ Rise in temperatures

▪ People skiing off-piste

▪ Strong winds

▪ Earthquakes

▪ Layers of snow which are not compacted

▪ Glacier movement

• If you get trapped in an avalanche, it is basically like getting buried alive.

• Since the snow is so heavy, avalanche victims are rarely able to dig themselves out. Once the avalanche stops, the snow settles like concrete and makes it nearly impossible to move.

• Victims of avalanches are also at risk for frostbite, a potentially deadly disease that occurs when the human body is exposed to extremely cold temperatures for an extended period of time.

• Victims are much more likely to survive an avalanche if they are rescued within 15 minutes, but after 45 minutes the survival rate drops drastically.

• Besides being dangerous to humans, avalanches can also cause flash floods and changes in weather patterns.

Predicting avalanches

Concrete wedges to

protect pylons

Buildings with reinforced roofs and

walls

Forests to trap

snow

Ramps and mounds

to slow down

avalanches

Explosives used

to start small

avalanches

Fences to stop

avalanches

Galtur avalanche

Galtür is a popular ski resort in Austria. Avalanches happen most years, but don't usually reach the village of Galtür. On 23 February 1999, an avalanche raced down the mountain.

Mudslides and landslides

• A landslide includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows.

• The biggest reason for a landslide is always that there is a slope and material goes down the slope because of gravity.

What triggers landslides?• erosion by rivers, glaciers, or ocean waves makes slopes

too steep

• saturation by snowmelt or heavy rains

• earthquakes create stresses that make weak slopes fail

• volcanic eruptions produce loose ash deposits, heavy rain, and debris flows

• vibrations from machinery, traffic, blasting and even thunder may trigger failure of weak slopes

• weight from much rain or snow, or from buildings may stress weak slopes

• groundwater pressure making the slope unstable

• removal of deep-rooted plants that bind the soil to bedrock

Vargas tragedyVargas tragedy is one of the worst disaster that struck the Venezuelan country's Vargas state on 15 December 1999. It was a result of heavy downpour of rain, flash floods and mudslides