NATURALNATURAL HAZARDS AND PEOPLEHAZARDS AND PEOPLE

GCSE REVISION

Elements to be covered:-

What are natural hazards?

How can natural hazards be classified?

Severity, frequency and duration of natural hazards.

Why do rivers flood?

Case studies- Bangladesh (LEDC)

The Rhine (EU)

Why live in a hazardous area?

What are natural hazards? a natural event which is perceived by people as a threat to life and property. It may be generated from within the Earth (geological hazards), or happen within the atmosphere (weather and climate hazards).

Weather and climateWeather and climate GeologicalGeological

FloodsFloods

Tropical stormsTropical storms

DroughtDrought

SnowSnow

FogFog

TornadoesTornadoes

EarthquakesEarthquakes

VolcanoesVolcanoes

TsunamisTsunamis

LandslidesLandslides

Severity, frequency and duration of Severity, frequency and duration of natural hazardsnatural hazards

The severity of a natural hazard can be assessed by

• the numbers injured

• the cost of the damage

• the type of location affected (large city or remote countryside).

So, an unexpected hazard that kills a lot of people and causes much damage in a large city is very severe.

Severity Duration

The length of time a natural hazards lasts for varies,

Short term- lasting for less than a day (earthquake)

Medium term- lasting for more than a day, less than a month (river floods).

Long term- lasting for more than a month (many volcanic eruptions).

Frequency

Some natural hazards occur frequently (flooding in Bangladesh). Others are isolated occurrences (Italian mudslide disaster in 1998).

What is flooding?River flooding is when a river overflows its banks.

Key termsWater moving downwards into the soil - Water moving over the land - Water moving through the soil – Water moving vertically down into rock - Water moving slowly through ground rock –

When no more water can infiltrate in to the soil the soil is said to be saturated. This is when overland flow is likely to be greatest.

Since water flowing over the land reaches the river very quickly, the river is likely to flood.

infiltrationoverland flowthroughflow percolationgroundwater flow

Factors that effect flooding

Flood (storm) hydrographThe amount of water in a river channel at a given time is called the discharge. Discharge is measured in cumecs (cubic metres of water per second). Following an increase in rainfall there will also be an increase in the level and the discharge of the river. The relationship between precipitation and the level of a river is illustrated by the flood (storm) hydrograph.

Peak discharge

Normal (base) flow

Peak rainfall

Lag time

Falling limb

Rising limb

When a storm begins, discharge does not increase immediately as only a little of the rain will fall directly into the channel. The first water to reach the river will come from surface runoff, and this will later be supplemented by water from throughflow.

Factors affecting the shape of the flood hydrograph

• It is possible that two drainage basins, located side by side, can receive approximately the same amount of rainfall and yet have very different hydrograph shapes.

• The river in basin A is likely to flood regularly

• The river in basin B will probably never flood.

• The reasons for this difference may be due to one factor or a combination of factors.

FactorFactor Drainage Drainage Basin ABasin A

Drainage Drainage Basin BBasin B

ReliefRelief Steep slopesSteep slopes Gentle slopesGentle slopes

Rock TypeRock Type ImpermeableImpermeable

e.g. granitee.g. granite

PermeablePermeable

e.g. chalke.g. chalk

SoilSoil Very thin less Very thin less infiltrationinfiltration

Deeper soil Deeper soil

Natural Natural vegetationvegetation

Thin grass Thin grass and moorlandand moorland

Forest Forest

Land useLand use UrbanisationUrbanisation Rural areaRural area

Drainage Drainage densitydensity

Higher densityHigher density Lower densityLower density

FLOODING CASE STUDIES

You MUST remember:

Causes of the flood

Effects of the flood

Flood prevention

LEDC Case Study : Bangladesh 1998 (July to September)

CAUSES• Melting of snow from the Himalayas added to volume of water in the summer months.

• 80% floodplain and delta

• Meeting of two huge rivers (Brahmaputra and Ganges) increases the flood risk.

• Deforestation in the Himalayas to make room for new housing meant fewer trees to absorb and transpire water, therefore more surface run off.

• Particularly heavy monsoon rain causing summer flooding.

• Rivers carried silt further downstream causing riverbeds to rise. The river channels could hold less water. Bed of Brahmaputra rising 5cm/year.

EFFECTS

66% of the land area was underwater

30 Million were made homeless

Over 1000 died from drowning, snake bites and diarrhoea Dhaka, the capital, was

2m under water and almost completely cut off

Water and electricity in Dhaka was cut off and the sewerage system collapsed

The entire annual stock of rice was destroyed130,000 cattle were killed

400 factories had to close and more than a thousand schools were destroyed

The disaster would have been much worse but for the flood embankments already there!!

PREVENTION Levees were built so that the rivers could hold more water.

Improved flood services, including a flood warning system, rescue services and flood shelters have been improved.

Re-afforestation has been undertaken in the Himalayas. In Nepal fruit trees were planted so that the trees prevented run off into the rivers.

FAP In 1990 a long-term solution, the Flood Action Plan was started. This is a joint venture between government and international aid agencies, funded by the World Bank, to tackle the flooding problem in Bangladesh. PROBLEMS COST Bangladesh are one of the poorest countries in the world and flood protection measures are too costly.

CORRUPTION Many Bangladeshis believe that officials have helped themselves to government money that should have been used for flood prevention.

EU Case Study : Rhine 1995 (January/February)

The Rhine is Europe largest river- 1320 km long.

Source- Alps.

Mouth- North Sea.

The drainage basin includes 9 countries and 40 million people.

Two areas that we have studies:

- Cologne (urban) - Well (rural)

Causes of the flooding

Natural

• Warm weather caused snow in the Alps to melt.

• Particularly heavy rain - 40% increase in rain since 1900.

• January had 3.5 times the average rainfall.

• Saturated soil after heavy rain increased overland flow.

• The mouth of the river is very flat and low lying.

Human

• Urbanisation – more impermeable, man made surfaces.

• Deforestation – for building and farming.

• Channel change – to reduce travel time for barges the Rhine has been straightened and is now 50 km shorter. This along with,

• Embankments have increasing the flow. Places further downstream now receive more flooding.

Effects of the flooding

Rural : Well

Evacuation of livestock cost money and infection was spread.

Agriculture was destroyed.

50 000 were evacuated.

Many greenhouses were saved but the huge quantity of flowers were lost.

Urban : Cologne

The military tried to stop the flooding by using sand bags. This gave residents time to evacuate.

750 000 were evacuated.

Industry was forced to shut, which led to economic concerns.

Roads and communications were unuasable.

Prevention

Short term

Home owners took belongings upstairs.

Carpets were raised.

Sandbags were put along the river in Cologne to give people extra time to evacuate.

People and animals were evacuated to prevent loss of life.

Long term

Improve flood warning system.

Ground floor of buildings raised.

Tiled flooring instead of carpets.

Dykes built parallel to river.

Metal walls that could be erected in the event of a flood (less unsightly than permanent ones).

Upstream

Afforestation

Retention basins where large areas of land can be flooded.

Why live in a hazardous area?

It is estimated that:-

o 360 million people live on the slopes of volcanoes.

o 450 million people live in areas at risk from flooding.

o Over a third of the fastest growing cities in the world are within areas at risk from earthquakes or volcanoes.

The reasons

- No choice People may be poor- fishing and farming only way of life

- Fertile land

- Economic activities The hazardous area may be a focus of tourism

- Risk not great enough Hazards that are less severe and in frequent may not deter people from living in these areas.

- Forecasting is improving.