1
Q
definition of floods
A
Occur when land that is usually dry is submerged by large amounts of water.
Considered as NATURAL HAZARD as it can endanger lives and destroy property.
*EXAMPLE: In 1999, a flood hit Venezuela in South America. About 650 people died and 150 000 people became homeless and 7 000 people went missing.
2
Q
types of floods
A
- coastal floods
- river floods
- flash floods
3
Q
what are coastal floods
A
-Occur in areas along the coast e.g. low-lying coastal areas, such as river mouths
-Caused by intense storms such as tropical storms and tropical cyclones (a.k.a. hurricanes and typhoons).
described as to have tall, large waves
4
Q
2
what are river floods?
A
- Occur when water in the river rises and overflows its banks.
- Occur on the flood plain which is the relatively flat land stretching on either side of the river.
5
Q
How are flood plains formed?
A
When there is heavy rain, water overflows the banks and floods the surrounding areas. Sediments are deposited on the banks. repeated flooding cause deposits to accumulate forming flat land also known as flood plains
6
Q
what are flashfloods?
A
- A sudden, localised flood of great volume
and short duration which usually affects a small area. - Caused by unusual heavy rainfall.
- Occurs when the ground becomes saturated with water that has fallen too quickly to be absorbed.
- Can be caused by coastal and river floods.
- Recognised by its quick rise in water level.
7
Q
5
causes of flooding (natural causes)
A
- Excessive rainfall (river/ flash)
- Storm surge (coastal)
- Snow melt (river)
- El Nino / La Nina (coastal)
- Movement of the Earth’s
surface (tsunamis) -> (river/ coastal)
8
Q
causes of flooding (human causes)
A
- Deforestation (river)
- Urban development
- Dam failure
- Climate change (coastal)
9
Q
Natural causes of floods -> excessive rainfall
A
** Excess rainwater is unable to seep into the ground - increase in surface runoff when ground is saturated.
- Especially in urban areas as ground is covered with concrete - little infiltration – high surface runoff.
- Surface runoff occurs quickly within a short span of time.
- Water flows swiftly into drains; if the urban drainage system cannot cope with sudden increase – it overflows.
- Need a good system of artificial drainage to prevent floods e.g. drains/canals.
** Rivers overflow their banks when river channels can no longer contain the excess water.
* E.g. Yangtze River and the Yellow River in China frequently overflow their banks after heavy rains because the rain washes large amounts of sediments into the rivers. These sediments are then deposited on the river beds, causing the rivers to become shallower. With time, flooding occurs.
10
Q
Natural causes of flood -> snow melt
ST John’s
A
- Places that experience cool temperature climate.
- Melting snow in spring releases large amounts of waters into rivers - sometimes cause rivers to overflow their banks.
- E.g. In 1986, the St John River in Canada overflowed its bank as it could not hold the excess water from melting snow in its channel.
Several homes became submerged in floodwaters and families had to be evacuated.
11
Q
natural causes of flood -> storm surge
bangladesh
A
- Cause floods in coastal areas.
- Occurs when strong winds raise the waves in the ocean to exceptionally high levels.
- Gigantic waves crash onto the coast and flood the land.
- E.g. More than 80% of Bangladesh sits on low-lying floodplains of the Brahmaputra and Ganges Rivers. About 35% of the country is less than 6m above sea level. As such it is frequently subjected to regular flooding caused by storm surges. When strong winds blow towards Bangladesh from the Bay of
Bengal, flooding occurs.
*Storm surge brought about by gales (strong winds) and tropical storms; resulting in increase of sea level and flooding of low-lying coastal areas subsequently.
12
Q
natural causes of flood -> el nino
alternate patterns
A
Alternate patterns of heating & cooling between eastern & western South Pacific Ocean:
-El Niño is an abnormal warming of ocean at the southeastern part of the Pacific Ocean. It occurs every 3 to 7 years and lasts for several weeks.
-Surface temperature of the ocean off the coast of South America heats up causing heavy rains and floods at southeastern part of the
Pacific Ocean.
e.g. Peru while droughts occur in the southeastern part of the Pacific ocean
e.g. Australia / Indonesia.
13
Q
natural causes of flood -> movement of the earth’s surface
indian ocean tsunami
A
- During an earthquake, soil may loosen along the hill or mountain slopes and cause a landslide. Loosen soil when deposited into a river may cause water in the river to overflow.
- Earthquakes that occur under the ocean displaces the ocean floor which generates tsunamis which may bring floods to coastal areas.
E.g. The 2004 Indian Ocean Tsunami, which is triggered by a strong earthquake in the Indian Ocean in Sumatra. Banda Aceh, Indonesia was badly affected as the raging waters of the tsunami engulfed most of the infrastructure of this coastal town (Dec 2004).
14
Q
natural causes of flood -> la nina
A
-La Niña follows after El Niño and it occurs every 3 to 5 years.
- Southwestern part of the Pacific Ocean
experiences abnormal warming thus bring heavy rains and floods to Australia & Indonesia.
- Drought like conditions are experienced in Peru in South America (Southwestern part of the Pacific Ocean).
15
Q
what are the human causes of floods
A
- deforestation
- urban development
- dam failure
- climate change
16
Q
human causes of floods- deforestation
A
- Reasons include creating land for agriculture, urban development, wood for fuel, etc.
- Little / no interception of rain - soil and mud are washed into the river beds causing the rivers to become shallower.
z Little infiltration of rain water into soil - more surface runoff enters river, increase in volume of water flowing into rivers
17
Q
human causes of floods- urban development
A
- In urban areas, concrete pavements and tarred roads prevent rainwater from seeping into the ground.
- Clearing of vegetation to construct settlement – less vegetation to intercept the rain resulting in an increase in amount of surface runoff.
E.g. In London, areas around the River Thames experience regular flooding as he natural vegetation around the river has been replaced with concrete pathways and buildings. This has led to increase surface runoff flowing into
the River Thames, causing it to overflow.
18
Q
human causes of floods- dam failure
A
- A dam is a barrier built across a river to retain and store water with the purpose of generating hydropower and regulating the amount of water flowing in a river.
- When continuous heavy rain occurs, reservoir overflows or when sediments pile up behind dam to destabilise it.
- Catastrophic flooding will occur.
E.g. Banqiao Dam failure in August 1975 in China, caused an estimated 26 000 deaths when villages downstream were flooded. An additional 204 000 people died from famine and diseases caused by the floods.
19
Q
human causes of floods- climate change
A
- Long term change – global warming
(enhanced greenhouse effect) - Temperature increased by 0.5 degrees celcius per decade
- Polar ice caps melt – sea level rises
z Flooding of low-lying cities, countries, islands e.g. London, Bangladesh, Maldives, etc.
20
Q
what are the 3 types of impacts of floods?
A
- social
- economical
- environmental
21
Q
what are the social impacts of floods?
A
- injuries and loss of lives
- loss of property*
- spread of diseases
22
Q
social impacts- injuries and loss of lives
A
- People may be injured by objects and debris washed by fast-moving floodwaters.
- Floodwaters may cause buildings to collapse and injure or kill people.
- People caught in the flood for a long period of time, they can suffer from hypothermia with symptoms such as constant shivering, confusion and loss of co-ordination.
- Floodplains around coastal areas and rivers provide fertile alluvium, making them very attractive places for cultivation of crops– densely populated. Impact is higher when flooding occurs.
- E.g. In 2003, heavy downpour led to rising floodwaters in the north- central Huai River Basin, located in the provinces, Anhui, Jiangsu and Henan in China. The flood claimed 298 lives and forced people to evacuate from the affected areas.
23
Q
social impact- loss of property*
A
- Floodwaters damage property and infrastructure, disrupts power supply and phone lines. Damage is costly and takes a long
time to repair.
E.g. In 2005, Hurricane Katrina hit the Gulf Coast of the USA, leading to massive flooding of several states, including New Orleans, in terms of property and infrastructure, cost around US$44 billion.
24
Q
social impact- spread of diseases
A
- Large numbers of homeless flood victims are often housed in overcrowded makeshift shelters that lack sanitation and clean drinking water– outbreak of waterborne diseases e.g. dysentery cholera.
- Contamination of water supply – freshwater polluted by floodwaters which may contain lead, oil and bacteria e.g. E. coli.
25
Economic impacts of floods
- Accumulation of debt – money needed for repair works, provide food and basic needs for homeless victims.
- Slowdown of economy – loss of crops, disruption of businesses thus resulting in loss of income.
- May need to get aid from international agencies e.g. World Bank, UN (United Nations).
26
environmental impacts on floods
- Destroy trees and other plant life as well as the natural habitats of animals.
E.g. Tsunami that struck Java on 17 July 2006 flooded the coastal regions along the southern part of the island, causing severe damage to the coastal ecology. The nature reserve near Pangandaran Beach was badly affected.
- Soil erosion – sediments washed away by floodwaters especially deforested land.
- Waterlogging – when soil becomes saturated with water, anaerobic condition may discourage plant growth.
27
positive impacts on agriculture
Fertile soil for agriculture:
- Regular flooding of rivers - deposition of fertile soil alluvium along the river banks. Soil rich in alluvium or river-borne silt - suitable for farming.
E.g. Nile Delta in Egypt where crops are cultivated has fertile
alluvium deposited by the River Nile.
28
FLOOD MANAGEMENT STRATEGIES
1. regulation-zoning
2. Elevate Properties - Minimum Platform Level (MPL)
3. Investment in Infrastructure – Levees and Floodwalls
4. Investment in Infrastructure – Channel Improvement
5. Disaster Preparedness – Flood Insurance
6. Disaster Preparedness – Monitoring and Education
7. Disaster Preparedness – Post Flood Management
29
1. regulation-zoning (not SG)
- Regulation refers to a rule of law which is enforced by an authority and which states how something should be done.
- Zoning is a way in which the government plans the physical development of the land and its uses. Zoning laws often specify the areas where residential, industrial, commercial or recreational activities may take place.
- Local governments drawn up plans showing areas prone to flooding – provide information for developers ensuring that floods would not affect the buildings they construct.
29
30
Areas that are prone to floods
- low elevation
- near coastal areas
-flood history
-low ground
31
Flood-prone areas are often zoned as low-value developments such as parks and fields for recreational activities. Why?
low value development doesn't cost much to develop and has activities that bring less income -> suffer less economic losses in the even of floods
E.g. In Eagan City, Minnesota, USA, areas near the Minnesota River which are prone to flooding are used for low-value developments such as the Minnesota Valley National Wildlife Refuge and parks. Beyond the zone of low-value developments is the zone of industrial areas. Residential areas are located further away so that houses may be safe from floods.
32
Elevate Properties - Minimum Platform Level (MPL) (SG)
-Ensure platform levels of the development site are not lower than the road level / ground level.
E.g. In 2011, national water agency PUB in Singapore unveiled tighter regulations to boost flood-protection measures. In areas with no flood history, the MPL is 30cm above the
adjacent road or ground level. However is areas that have experienced floods the MPL is
60cm above the highest recorded flood level. The MPL is revised when there are changes to
the highest recorded flood level. MPLs are imposed on new and existing developments.
33
Benefits and challenges of zoning
Benefits:
-Zoning creates spaces of green areas such as parks and fields which help to reduce the impact of flood damage.
Challenges:
-Zoning may limit the development of available land because only certain land uses are permitted.
-Zoning is difficult to carry out because of complete land uses and because people may not abide by the recommended land use
more trees-> more interception-> more infiltration-> less surface runoff-> less floods
34
Benefits and challenges of the Minimum Platform Level
Benefits:
-Development is protected against floods up to the height of the MPL
Challenges:
-It is challenging to implement MPL because
of the increased cost of development.
35
Investment in Infrastructure – Levees and Floodwalls (NOT SG)
- Both structures are higher than the floodplains and rivers so that they can help to keep out floodwater.
- Levees are raised river banks (refer to slide number 12) which can be natural or man-made.
E.g. Sacramento, USA, the Sacramento River flows south and drains into the San Francisco Bay. Starting in the 1870s, several kilometres of levees have been built along the river to protect the surrounding land from floods.
- Floodwalls are man-made structures and made of concrete.
E.g. River Wye flows through the city of Hereford, Herefordshire, UK, and places the city at constant risk of flooding during heavy rains. A floodwall was built in 2008 as part of the flood defence scheme.
36
benefits and challenges of Investment in Infrastructure – Levees and Floodwalls
Benefits:
-Cheaper to build levees and floodwalls to mitigate and prevent floods compared to raising the height of the surrounding land.
Challenges:
-Regular Maintenance is required to ensure that levees and floodwalls are strong enough to hold back floodwater.
- Levees and floodwalls are designed to hold back floodwater up to a certain level. They
are unable to prevent floods beyond that level.
37
Investment in Infrastructure – Channel Improvement (SG)
- Channel improvement refers to changes to the river channel or canal to increase its capacity to hold water or to enable water to flow quickly.
- Widening and deepening of the river channel or canal – increasing its ability to hold water. Also increases the amount of surface runoff from the surrounding areas into the river or canal because more surface runoff can enter without causing it to flood.
- River bed and banks may also be smoothened by replacing the soil with cement
and granite – less friction between water and river bed and banks. Thus, speed of the river will increase and water to flow away from area quickly.
E.g. Deepening and widening of Rochor Canal to improve its drainage capacity which began in 2011
38
Disaster Preparedness – Flood Insurance (NOT SG)
- In areas prone to flooding, residents can have their property insured against damages caused by floods.
- Provides people with financial resources to rebuild their property if a flood strikes.
- E.g. In the UK, the government plays an active role in providing flood insurance for its people. There is a national flood insurance programme to help reduce the financial burden of people who have lost their homes
due to a flood.
39
benefits and challenges of Disaster Preparedness – Flood Insurance
Benefits:
-Victims can rebuild their property with payouts
-Flood Insurance is priced according to the amount of risk in a geographic area
-> Its fair
-> Areas near the river/ coast are more likely to flood. Thus they get more insurance.
Challenges:
-There are many exclusions and limitations on what is covered by such a policy
e.g. cars are excluded and it does not cover the cost of temporary accommodation.
40
Disaster Preparedness – Monitoring and Education (SG)
z Help of modern technology – monitor weather patterns and issue warnings when a flood is about to occur.
E.g. In Singapore, water level sensors are placed in canals and drains to gather data about the changing weather levels at regular intervals. The water levels sensors are equipped with cameras to take photographs of the situation in the canals.
- Educate citizens what to do in the event of an emergency. Evacuation plans are created and put in place. Announcements about these plans are broadcast over television and radio to ensure that information reaches the masses.
E.g. In the USA, when a flood is expected, the government will advise its people to evacuate
their homes and seek shelter on higher grounds. The people are also advised on the safest route to the nearest flood shelter.
41
Benefits and challenges of Disaster Preparedness – Monitoring and Education
Benefits:
-A forecasting and warning system provides useful information so that there is time for people to take action to minimise flood
damage.
-People are educated on what to do when a flood warning is received. (less panic)
-Evacuation drills will enable cities to improve flood evacuation plans
Challenges:
-Cities may not have the expertise and technology to collect accurate data and make accurate forecasts.
-Limited participation will reduce an effectiveness of an evacuation drill.
42
Disaster Preparedness – Post Flood Management (NOT SG)
- When severe flood occurs and lives are threatened, local authorities may decide to evacuate flood victims.
E.g. When Hurricane Katrina struck on August 2005 in the USA, victims were evacuated by
boats and sometimes by helicopters to temporary shelters. The Louis Armstrong New
Orleans International Airport, which did not succumb to the flood, was reopened to
humanitarian and rescue operations, where food and water were provided to victims several days during the hurricane.
- Foreign aid may also be offered – donations of food, water, blankets and other necessities for victims, as well as first aid and medical teams to treat casualties.
43
Benefits and challenges of Disaster Preparedness – Post Flood Management
Benefits:
-A well executed and planned evacuation and emergency aid can save lives.
Challenges:
-People may choose to remain behind for fear of losing their property and valuables.
-Aid can increase the dependency of LEDCs on donor countries. Sometimes aid is not a gift, but a loan, and poor countries may repay.
-Aid may not reach the people who need it most. Corruption may lead to local politicians using aid for their own means or for political gain.
44
What is a hydrological cycle?
Is a sequence of processes that occur to
ensure that water is naturally replenished on the Earth.
45
Key terms in the hydrological cycle
-evaporation
-transpiration
-condensation
-precipitation
-infiltration
-percolation
-groundwater
-surface runoff
46
evaporation
During the day, the sun heats up the surfaces of the Earth and changes some of the water from the oceans, lakes, rivers and ground into water vapour.
47
transpiration
Plants give out water vapour through their leaves.
48
condensation
Warm air cools as it rises and this causes water vapour in the saturated air to be changed into water droplets.
49
precipitation + process
Rain, snow, sleet, freezing rain, or
hail that falls on the ground.
1. Warm moist air rises or is lifted.
2. Rising air will cool.
3. Cooled air becomes saturated at dewpoint temperature.
4. Further cooling of air results in condensation of water vapour into tiny water droplets, forming clouds.
5. Tiny water droplets coalesce (combine/ fuse / join together) to form bigger droplets.
6. When big water droplets in clouds are too heavy, they fall as precipitation.
50
Infiltration
Part of the water that reaches the
ground soaks into the soil subsurface.
51
Percolation
Some of the water moves further down into the unsaturated underlying soil and rock
* Water infiltrates into the ground and percolates deeper underground
52
Groundwater
water that seeps into the ground through pores in the soil due to gravity
53
surface runoff
when precipitation exceeds the infiltration rate, the water that is not absorbed fast enough, flows over the land
54
Benefits and challenges of channel improvement
Benefits: Channel improvement makes it for moderate value and high value developments to take place on the land adjacent to river channels and canals.
Challenges:
-Channel improvement is difficult to carry out when the land next to the river channels and canals are already used for other purposes
-Channel improvement may create problems for areas downstream of the improved areas
Geography Sec 2
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