Physical GEOG: Chapter 8 - Floods and Droughts


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Physical GEOG: Chapter 8 - Floods and Droughts

  1. 1. Causes of floods Description Example - Places which experience the - The Yangtze River and the tropical monsoon climate have Yellow River in China freq- seasons of heavy rain. uently overflow their banks Excessive rainfall because of the deposition of - When excess rainwater cannot be sediments washed into the held by the river, the river over- river, reducing the depth and flows their banks and floods the ability of the river to contain surrounding areas. water. - Can cause floods in coastal areas. - Bangladesh is a low-lying - Strong winds raise the waves in country with mostly sits on the ocean to higher levels which low-lying floodplains of Brah- Storm surges mahputra and Ganges Rivers. crash onto the coast, flooding the land. Due to its low elevation, it is frequently flooded by storm - Esp during tropical storms. surges from the Bay of Bengal. - Places which experience the cool - In 1986, the St John River temperate climate have winters in Canada flooded the followed by spring. surrounding areas as it could Melting snow - In spring, the melting snow flows not hold the excess water from into rivers increasing the volume of melting snow. Several homes water in the rivers, the river over- were submerged and families flows their banks and floods the were evacuated. surrounding areas. - El Niño: rise in air pressure over - In 1997-98, El Niño brought Indian Ocean where INA, AUS are heavy and prolonged rains to causes a fall in pressure over PER. Peru for several months, Piura Tradewinds move eastwards from River in Peru overflowed its SW of Pacific to SE of Pacific. banks to flood surrounding This pushes warm surface waters areas. eastwards along with winds. It brings droughts to SW Pacific and heavy rains to SE Pacific, causing floods in SE Pacific.Global atmospheric processes (El Niño and La Niña) - La Niña: fall in air pressure over Indian Ocean where INA, AUS are causes a rise in pressure over PER. Tradewinds move westwards from SW of Pacific to SE of Pacific. This pushes warm surface waters westwards along with winds. It brings heavy rains to SW Pacific, causing floods in SE Pacific and droughts to SE Pacific.
  2. 2. - Earthquakes results in landslides - In 2004, the Indian Ocean where soil is loosened and it flows Tsunami triggered by a strong down. When soil is deposited into undersea earthquake, destroy- rivers, it reduces the depth and ed Banda Aceh, Indonesia, a ability of the river to contain water coastal town.Movements of the Earth’s surface and the river overflows its banks to flood the surrounding areas. - Earthquakes can also trigger tsunamis that destroy coastal sett- lements. - Without vegetation, no roots can - Forests in mountainous hold soil together. Soil erosion regions of China are cleared increases. for housing and firewood. This has causes more floods in - Increases surface runoff as the China. ground is bare. - Forests are also cleared in - With more surface runoff, it Bangladesh causing more Clearing of forests increases the volume of water sediments to be deposited in flowing into the rivers. the Ganges and Brahmaputra Rivers leading to more floods. - Soil and mud are deposited in the river, reducing the depth and the ability of the river to contain water and the river overflows its banks to flood the surrounding areas. - More land is cleared for urban - Areas around River Thames, development as population London experience regular increases hence there are more flooding as the natural impermeable surfaces (eg. vegetation has been cleared for Urban development concrete pathways, tarred roads) concrete pathways hence more hence greater surface runoff that surface runoff, causing River leads to increased river volume Thames to overflow its banks which soon overflows causing a and the surrounding areas. flood. - Increase in global average temperatures cause the melting of Impacts of enhanced ice covers that rises the sea levels. ----------- greenhouse effect The excess water then floods low- lying areas in the world.
  3. 3. Causes of droughts Description Example - Places which experience the - Countries such as India and tropical monsoon climate have Bangladesh experience droughts seasons of very little rain. when the Southwest Monsoon isDelayed or insufficient rainfall delayed in June or the Northeast - When monsoonal winds are Monsoon is delayed in delayed, the dry season is December. prolonged and droughts occur.Global atmospheric processes - REFER TO EARLIER NOTES. - REFER TO EARLIER NOTES. - Reduced vegetation reduces - Deforestation in Amazon TRF rates of transpiration and the led to worst droughts in Amazon amount of water vapour drops. Basin in 40 years. - Less clouds are formed when - Water levels are so low that Reduced forest cover less water vapour condenses and exposed river banks dried up and less rain falls. are eroded by wind. - Direct heating onto the soil dries it up even faster and the soil becomes dry. - Can cause droughts in places - In Sahel, Africa, high with drier climates. temperatures evaporate water quickly, drying up lakes and Impact of enhanced - Also caused by increase in rivers. Due to the scarcity of greenhouse effect emissions of greenhouse gases water, little rain falls over a long that increase the global average period of time causing droughts. temperatures. - Overpopulated areas have high - Since 2001, Kothariya, India demand of water hence the use of have been experiencing a drought water for agriculture, homes and due to the overuse of water and Overuse of water industries exceed what the water the scarcity of water which they sources can provide. get from 5 water holes dug into the floors of ponds.
  4. 4. Impacts of floods Description Example - Floodplains are attractive places - In 2003, weeks of heavy for cultivation of crops due to the downpour led to rising flood- deposition of fertile alluvium waters in north-central Huai hence these places are densely River Basin in China which populated. claimed 298 lives and forced Loss of lives (-) many to evacuate. - When floods occur in these places, impact of it is amplified. Also, floods occur when people least expect them hence they drown in a flood. - When floodwaters submerge - Hurricane Katrina hit Gulf towns and villages, homes and Coast of USA causing infrastructure are destroyed. massive flooding esp to New Homes are destroyed. Roads are Orleans with US$44 billion of Damage of property inaccessible. Power supply and damage. and infrastructure (-) telephone lines are disrupted. - Such damages require high costs and long times to fix. - Homeless flood victims are housed in overcrowded makeshift shelters lacking in sanitation and Spread of diseases (-) --------- clean drinking water. Outbreak of diseases (eg. malaria and cholera) are common. - Floods destroy trees and other - On 17 July 2006, a tsunami plant life, as well as natural that struck Java caused habitats of animals. severed damage to coastal ecology along the southern Damage to environment (-) part. - Nature reserve near Pangandaran Beach was badly affected. - Regular flooding of the rivers - People live around the Nile provide floodplains with fertile Delta in Egypt as the riverFertile soil for agriculture (+) alluvium, “fertilising” the soil. provides fertile alluvium for cultivators to grow their crops.
  5. 5. Impacts of droughts Description Example - Lack of water in rivers and - In mid 1980s, more than 3 mil lakes causes people and animals people died from starvation and Shortage of food and water to die from dehydration and dehydration in the drought- crops are destroyed, leading to stricken Sahel region of Africa. starvation and famine. - Desertification occurs when - Sahara Desert has expanded deserts spread into areas with southwards since 1968. Damage to the environment little or no rain leaving dry and barren soil that is unable to - According to UN, deserts are support veg growth. spreading at 80000km2/year. - Heat dries the veg which catches fire easily and causes haze which spreads by wind. Forest fires and haze -------- - Cause breathing difficulties and eye irritations in people living in cities surrounding it.Adaptation to floods Description + - Examples - Walls of sand, - Increase - Easily over- - In China, dykes stone and concrete capacity of river, topped by a have been built in are built along river reducing chances higher flood the lower course banks which are of flood hence it will pro- of the Yellow prone to flooding tect the area River. Always Building artificial overflowed due levees or dykes - Constant dep- to constant osition of sedi- deposition of ments reduce the sediments in the depth of the river, soil. reducing the vol- ume of the river - Walls of dams - Controls amt of - Very expensive - Series of dams have gates to hold water flowing to construct and in Tennessee back or release downstream into must be well- Valley in USA Construction water. Forms an art- the river, prevent- constructed to spent $281 mil of dams ificial reservoir ing a flood down- prevent collapse behind the dam stream -Hydroelectricity can be generated
  6. 6. - Maps drawn up by - Reduce areas - Difficult to - In Singapore, local governments to prone to flood- carry out in ur- Ministry for En- show areas prone to ing as govern- ban areas facing vironment and floods ment would raise land constraints Water Resources the ground levels as there is a dem- (MEWR) requi-Building control - Contain valuable or by having a and for land res ground levels info for developers drainage system to be raised, dra- to construct their inage canals to buildings at safe be improved places - Conservation of - Most environ- - Requires strict - Royal Forest vegetation cover mentally friend- supervision to Department reduces surface ly way of reduc- patrol watersheds (RFD) in runoff and reduces ing floods as Thailand soil erosion, no there are no - Wardens may developed a Watershed deposition and no changes to the be corrupt and watershed prog- management reduction in river environment. accept bribes ramme to prevent depth from companies floods from who want to cut occurring. - Replanting of trees the trees down in watersheds is done to manage it - Residents living in - Provides finan- - People in LDCs - United King- flood-prone areas cial resources for cannot afford to dom has a can buy flood the rebuilding of pay the annual national flood insurance that gives houses after the premiums and insuranceFlood insurance them financial res- flood may not have it. programme to ources to rebuild provide financial after the flood help to people who lost their homes. - With modern tech- - When detected - Usually - In USA, when nology, scientists early, evacuation response is not floods are ex- can monitor weather can be carried fast enough to pected, govern- patterns and issue out to reduce loss evacuate all the ment advises warnings prior to the of lives people to safer people to leave Monitoring flood grounds hence their homes to and education some still die safer grounds - Countries have evacuation plans that are informed via media to the public so that they can evacuate
  7. 7. - Plans made by - Not reliant on - Not immediate - During Hurri- authorities to country’s efforts and takes time for cane Katrina, evacuate flood alone, other aid to arrive, by USA opened victims and provide countries and or- then some more Louis Armstrong humanitarian aid to ganisations can would have died New Orleans survivors also help to Airport to receive Post-flood reduce more loss humanitarian and management of lives rescue operations strategies - Aid is sent by countries such as China and organ- isations such as UNMethod against Description + - Examples droughts - Agenda 21 rep- - Chances of - Some crops - In Israel, apple ort teaches count- desertification cannot be substi- cacti, which bearsManagement of ries how to man- reduced tuted as they are a fruit 11 out of 12watershed and age watersheds culture in the months, is grown agricultural and the need to country as they require less practices change agricultural water practices - Proper irrigation - Less water is - Irrigation must - In Turkey, farms techniques through wasted hence be carefully done use spray Using proper the use of man- demand is less to prevent over- or irrigation where irrigation made channels under- that will precision techniques reduce water loss - More water is both waste water sprinklers drip conserved for small amounts of times of drought water onto crops - Spraying of - When successful, - Cost of cloud - In Malaysia and silver iodide or it can bring rain seeding is very Thailand, cloud dry ice using to drought-stricken high, LDCs cannot seeding was used planes or machines areas to reduce afford to pay large in 2005 to relieve on ground into the impact of drought sums to overcome the one of theCloud seeding sky to act as drought worst droughts that condensation was affecting rice nuclei for water - Silver iodide can harvests droplets then cause temporary clouds to form and incapitation in rain to fall humans
  8. 8. - Aid from - Not reliant on - Not immediate - US Agency for countries and country’s efforts and takes time for International international aid alone, other aid to arrive, by Development organisations is countries and or- then some more provides donations sent to drought- ganisations can would have died of money, food stricken areas to also help to reduce and water to help them more loss of lives Ethiopia in the 2006 droughtPost-droughtmanagement - Food for the measures Hungry Relief brought in water via tanker trucks to provide water to school cisterns so that they can benefit from the emergency water provision during the same drought.