Secondary 2 : Geography : Chapter 5 : Floods

1. Sec 2 Geography
Chapter 5
Floods
How Can Cities Prepare For Floods?
Slides prepared are to be used alongside textbook

2. Page 152
Turn to:
Page 152 of Sec 2 Geography
textbook

3. Floods
 Floods are a natural occurrence which can bring positive impacts.
 For example, floods are welcomed by farmers in Asia as floods support the cultivation of
rice crops.
 Flooding deposits fine silt (alluvium) onto the floodplain, making it very fertile and
excellent for agriculture.
 People living on or near floodplains may rely upon regular flooding to help support their
farming and therefore provide food.
 This process is known as irrigation.

4. Floods
 However, floods become a hazard when they damage property and cause loss of
lives
 The impact of floods on cities is of particular concern because there is a high
concentration of people, buildings, businesses and infrastructure in cities.
 There can be severe damages caused by floods.
 The last decade saw an increase in the occurrence of floods and many cities will
continue to face floods.
 Thus, it is important that cities prepare themselves for floods.

5. Investigating Floods
How do cities prepare for
floods?
- What are floods?
- Which cities are prone to
floods?
- Why are these cities more
prone to floods than
others?
- How should cities prepare
for floods?
- How do floods affect people
living in cities?

6. Investigating Floods
What are floods?
• Definition
• Types
• Causes
Which cities are prone
to floods?
Why are these cities
more prone to floods
than others?
• Knowing the general trend
of cities prone to floods
• Reasons for higher
occurrence
How do floods affect
people living in cities?
• Impact
How should cities
prepare for floods?
• Strategies used in cities
• How these strategies are
effective
• Describe benefits and
challenges of strategies

7. What Are Floods?
- Types of Floods
- Causes of Floods

8. Types of Floods
Coastal Floods River Floods Flash Floods
- Occur along the coast
- Low-lying coastal land are more prone
to coastal floods
- Caused by Intense Storms such as
tropical storms and tropical cyclones
- Tropical cyclones are also known as
typhoons and hurricanes
- Strong winds during these storms
generate large waves known as storm
surge
- Storm surges can bring about higher
water level and cause coastal floods
- Tsunamis can also cause coastal floods
- Occur when water in the river rises
and overflow its banks
- Occur on the floodplain
- Raised banks in the river are also
known as levees
- Levees prevent water from
overflowing
- Causes by snowmelt, heavy rainfall
and dam failure
- Takes place suddenly and lasts for a
short period of time
- Caused by heavy rainfall
- Coastal and river floods may also
cause flash floods
- Can be recognised by quick rise in
water level

9. Identifying the types of floods
 Flash Flood
 Coastal Flood
 River Flood

10. Answers : Identifying the types of floods
 Flash Flood
 Coastal Flood
 River Flood

11. Coastal Floods
3 Ways Huge Waves cause Coastal Flooding
1. Direct Inundation
- Sea height exceeds the elevation of the land, with no defences
2. Overtopping of a barrier
- Height of water exceeds height of defence
3. Breaching of a Barrier
- Defence is broken down due to strong waves
Intensity – Severe
Duration – Quite long, about 1 week on
average
Frequency – When there storm surges,
monsoon season etc
Caused by :
- Intense Storms which causes storm surges
- Sea Level Rise
- Tsunami
Coastal flooding occurs when normally dry, low-
lying land is flooded by sea water. The extent
of coastal flooding is a function of the elevation
inland floodwaters penetrate which is controlled
by the topography of the coastal land exposed
to flooding.

12. River Floods
Intensity – Subjective
Duration – Longer than flash floods, more
than a day
Frequency – Not Common in Singapore
Caused by :
- Increase in water level
- Snowmelt
- Heavy Rainfall
- Dam failure
River floods occur when the water in the
river rises and overflows its banks. River
floods occur on the floodplain which
consists of the river and the low-lying
land on either side of it.

13. Flash Floods
Intensity – Sudden
Duration – Short
Frequency – Subjective
Caused by :
- Heavy rainfall
A flash flood takes place suddenly and lasts
for a short period of time. It can be caused by
heavy rainfall. Both coastal and river floods
may also cause flash floods. You can
recognise a flash flood by its quick rise in
water level.

14. Causes of Floods
Rainfall
- Rainfall Intensity
- Rainfall Duration
- Rainfall Frequency
Snowmelt
- Surface runoff
resulting from
melting snow
- Can be caused
by volcanic
eruption
Storm Surge
- Sudden rise
of sea level
Failure of Man-
made structures
- Dams, levees,
floodwalls

15. Causes of Floods : Rainfall
• Rate at which rain falls
• Measured with millimetres
per hour
Rainfall
Intensity
• Duration of rain event
• Measured with hours
Rainfall
Duration
• Probability of occurrence of
a certain amount of rain
falling within a certain
period of time
Rainfall
Frequency
Rainfall refers to droplets of water
that fall to the ground.
Condensation of atmospheric
water vapour leads to the
formation of water droplets,
which form clouds. If the water
droplets become too heavy, they
fall as rain. Rainfall intensity,
duration and frequency are
characteristics of rainfall.

16. Causes of Floods : Snowmelt
Snowmelt is surface runoff
that results from melting
snow.
Snowmelt is common in
temperate countries during
the spring season when
temperatures increase.
Volcanic Eruption can also
cause snowmelt.
During winter,
ice and snow
form on the
ground and on
the mountains
When temperature
rises, the ice and snow
melts
Snowmelt flows
over land into
rivers
Snowmelt increases the amount of water in
the river and causes the river to overflow and
flood the surrounding areas

17. Causes of Floods : Storm Surge
A storm surge is a sudden rise of the seal level in which water is piled up against a
coastline beyond normal high tide conditions.
During a storm, strong winds push water towards the coast and create huge waves, giving
rise to a storm surge.

18. Causes of Floods : Failure of Man-made Structures
Failure of man-made structures, such
as dams and levees can result in
floods.
A dam is a barrier built across a river
to retain and store water. Besides
generating hydropower, a dam also
helps to regulate the amount of water
flowing in a river.
The failure of levees and floodwalls
also has disastrous impact on cities.

23. Assignment 1
Through today’s lesson, contemplate what you have
learnt and think about :
What can be potential strategies taken by the
authorities to cope with flooding and its impacts?
Write a 100 essay on the question WITHOUT looking
at the next section of the chapter.
Be prepared to present and share your ideas with the
class.
Remember to include elaboration and examples!

24. Lesson 2
- Which cities are prone to floods?
- Why are these cities more prone to
floods than others?
- How do floods affect people living in
cities?
- How should cities prepare for floods?

25. Which Cities Are Prone To Floods?

26. Which Cities Are Prone To Flood?
- Calgary, Canada (snowmelt)
- New Orleans, USA (hurricane)
- Rio Largo, Brazil
- Newport, UK (storm)
- Dhaka, Bangladesh
- Tacloban, Philippines (storm)
Mark these place on the map!

27. Why are these cities more prone to floods than others?
1. Height of Land above Sea Level
The height of land above sea level is an important factor affecting the likelihood that
a place would flood. Sea level is generally used to refer to the mean sea level. The
mean sea level is the average height of the sea surface at a given site over a long
period of time. It is the midpoint between the mean low tide and the mean high tide.
Cities that are located on low-lying lands, are more prone to floods.

28. The Netherlands
The relief of
Netherlands. Notice
that the land closer
to the sea is lower
than mean sea level
and is protected by
man-made
structures.

29. Why are these cities more prone to floods than others?
2. Proximity to Water Bodies
Water bodies are significant accumulations of water such as oceans, seas, lakes, river and reservoirs.
Cities which are located nearer to water bodies, such as coastal cities or cities with a river running
through them, are more prone to floods.
Some cities are located near rivers, thus they are more prone to river floods compared to other cities
which are further away.

30. Why are these cities more prone to floods than others?
3. Permeability of surface cover
Permeability refers to the ease with which water can infiltrate or seep into the ground. Vegetated
surfaces are more permeable and allow water to infiltrate the ground more easily. On the other hand,
concrete surface are impermeable and water is unable to infiltrate into the ground. This leads to high
surface runoff, thus increasing the likelihood of floods.
As cities expand, natural vegetation may be removed and replaced with concrete. This leads to an
increase in surface runoff in a rain event because concrete surfaces are impermeable. Thus, cities with a
higher proportion of impermeable surfaces might be more prone to floods.

31. Why are these cities more prone to floods than others?
4. Channel Capacity
Channel capacity refers to the volume of water that can be held in the channel of a river or channel
without overflowing. Floods may occur when there is too much rain and the capacity of the channel is
exceeded.
A city’s ability to drain water away quickly is very important because it has a high proportion of built-up
areas. The concrete surfaces in the built-up areas are impermeable, resulting in large amounts of
surface runoff during a rain event. In cities, a network of canals is built to remove surface runoff from
the area. The canals and rivers would have to be able to cope with the high surface runoff or a flood
would occur.
Large Surface Runoff Canals drain water away Canals are unable to cope, flood will occur

32. How do Floods affect people living in cities?
Impacts
1. Social Impacts : effect on the well-being of the people
2. Economic Impacts : effect on goods and services being produced, sold
and bought

33. Social Impact
Social impacts refer to the effect of an event on the well-being of the
people.
The social impacts of floods include
1.Injuries, spread of diseases and loss of lives
2.Homelessness
3.Disruption to clean water supply

34. Injuries, spread of diseases and loss of lives
People may be injured by objects and debris washed along by fast-moving floodwaters.
Floodwaters may cause buildings to collapse and injure or kill people.
When people are 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
coordination.
After a flood, diseases may spread and affect a larger population. Water supplies may be
contaminated by chemical spills from factories and sewage, people might drink
contaminated water and fall ill.
People may be stressed because they lost their homes. They may also suffer from
emotional distress from the loss of loved ones due to floods.

35. Homelessness
Floodwaters can cause damage to property and even sweep houses away. During
floods, people may have to evacuate to emergency shelters. Some people have
to stay in these emergency shelters for long periods of time after the flood while
trying to rebuild their houses.
Flooding in cities tend to affect many people due to the high population
densities in these areas.
Emergency shelters and trailer parks were set up for flood victims after the
Hurricane Katrina in 2005.

36. Disruption to Clean Water Supply
The supply of clean water may be disrupted due to damaged water pipes. Sewage
pipes may also burst, causing sewage to flow and contaminate water supplies.
During floods, pump houses, storage tanks and water pipes can be damaged, and
as a result, water supply will be disrupted in many areas.
People would have to walk long distances to get access to clean water.

37. Economic Impacts
Economic impacts refer to the effect of an event on goods and services being
produced, sold and bought.
During a flood, economic activities may stop due to the damage to machinery,
equipment and transport infrastructure, and disruption to energy supply.
After a flood, huge amount of money is required to repair the damage caused by the
floods

38. Read About
Assessment of Economic Loss Caused by Flood Damaged Transportation Network M. J. B. Alam, Alamgir
M. Hoque and Md. Mazharul Hoque Department of Civil Engineering Bangladesh University of
Engineering and Technology, Dhaka-1000, Bangladesh
Abstract : Flood causes direct as well as indirect loss to the economy of a country. The direct losses include
damage to the infrastructure, agricultural and industrial products, human being and livestock. The indirect losses
include the damage to the economic activities that cannot remain operative due to lack of communication
facilities during and immediately after flood. The indirect losses, particularly losses due to damage of
transportation facilities may be substantial and warrant special attention. This paper presents an estimate of the
indirect loss caused by flood damaged road network. The paper reveals that such indirect loss is much higher than
the direct loss. It suggests that some economically important transportation corridors should be constructed in
such a way that they remain workable even during flood.
Web : http://salekseraj.com/Page151-Jubair-2.pdf
Skill ! It isn’t just about reading content in the
textbook! You should also read research
papers to supplement your knowledge so that
you have more information to write about in
your essay!

39. Damage to machinery and equipment
Floods can damage machinery and equipment such as drilling machines, cranes and computers. The
machinery and equipment can be submerged in water and cause damage to the motors and other
electrical components.
As such, factories are unable to operate at full capacity and suffer economic losses. Companies may
lose profit and have to spend more money to repair or buy new equipment.
People may also lose their jobs.

40. Damage to transport infrastructure
Transport infrastructure refers to facilities such as roads and railway tracks. During a
flood, transport infrastructure can become impassable because they may be
submerged or damaged. The force of the floodwater may be so powerful that it
weakens the transport infrastructure. Cities will spend a considerable length of time
and high costs to repair the damaged infrastructure.
Flood destroys roads and bridges
Slows down rescue work

41. Disruption to energy supply
Floods can disrupt the supply of energy by affecting its distribution.
The disruption of electricity supply can affect the capacity of factories and companies to produce
goods and services. Power lines may be damaged and power outages may occur during floods.
These power failure can last for weeks before they are restored. Factories and companies are unable to
operate due to the lack of electricity.
This may result in the loss of employment and income for individuals and companies.

42. How should cities prepare for floods?
Mitigating measures used in Singapore and other cities
- Cities use a range of mitigation measures to reduce the impact of floods. Some of these measures can also help prevent floods
from occurring. To be successful in reducing flood risk, cities need to adopt an integrated flood management approach that
involves a combination of mitigating measures.
1. Regulation
- Zoning
- Elevated Properties
2. Investment in Infrastructure
- Levees and Floodwalls
- Channel Improvement
3. Disaster preparedness
- Forecasting and warning system
- Evacuation Drills

43. Regulation
Regulation refers to a rule or a law which is enforced by an authority and which states how
something should be done. Example of flood regulations include
1. Zoning
2. Elevated Properties

44. Zoning
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.
Flood-prone areas are often zoned as low-value developments, such as parks. With the use of zoning,
cities are likely to suffer fewer economic losses in the event of floods.

45. Zoning, in Singapore
Zoning is rather common in Singapore too.
For example, the Ulu Pandan River was flooded after a heavy rainfall. It was unable to cope with the high
demands of surface runoff and a flood occurred.
Look at how the government uses zoning.
Park
Connector
and Green
Spaces are
low-value
develop-
ments
Man-
made
structures
are built
to protect
the houses

46. Example of Zoning
Zoning creates green areas such as
parks and fields which help to reduce
the impact of flood damage
Zoning may limit the development of
available land because only certain
land use are permitted
Zoning is difficult to carry out because
of competing land uses and because
people may not abide by the
recommended land use.

47. Elevated properties
Another measure used to reduce the impact of floods is to elevate or raise properties to a
certain height.
In Singapore, the minimum ground level for a development is known as the minimum platform
level.
In areas with no flood history, the MPL is 300 millimetres above the adjacent road or ground
level.
In areas with flood history, the MPL is 600 millimetres above the highest recorded flood level.
The MPL is revised when there are changed to the highest recorded flood level.

48. Elevated properties
Benefits and
Challenges
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.

49. Investment in Infrastructure
Cities also invest in infrastructure such as levees, floodwalls, and channels to reduce the
impacts of floods. Beside building new infrastructure, there is also a need to maintain
existing infrastructure to make sure that they are in working condition.
1. Levees and Floodwalls
2. Channel Improvement

50. Levees and Floodwalls
Levees and Floodwalls are barriers against floods and protect development from floodwater.
Levees
- Raised river banks which can be
natural or man-made
Floodwalls
- Man-made structures made of
concrete
- Built higher than the
floodplains and rivers so
that they can help keep out
floodwater

51. Levees and Floodwalls
Benefits and
Challenges
Benefits
Cheaper to build
levees and
floodwalls to
mitigate and
prevent floods as
compared to raisin
the height of the
surrounding land
Challenges
Regular
maintenance is
required to ensure
that levees and
floodwalls are
strong enough to
hold back
floodwater
Challenges
Levees and
floodwalls are
designed to hold
back floodwater up
to a certain level.
They are unable to
prevent floods
beyond that level

52. Channel Improvement
Channel improvement refers to changes made to the river channel
or canal to increase its capacity to hold back water or to enable the
water to flow quickly.
River channels and canals can be widened and deepened so that
they can carry more storm water away.
They can also be straightened so that water can be carried away
quickly.
This will help to prevent water in the river channel and canals from
overflowing.
However, there is a limit to the
changes that can be made to river
channels and canals. Channel
improvement is difficult to carry out
when the land next to the river
channels and canals is already used
for other purposes.
A city should also improve its network
of river channels and canals if
channel improvement is to be
effective in mitigating floods.

53. Channel Improvement
Benefits and
Challenges
Benefits
Channel
Improvement
makes it possible
for moderate-value
and high-value
developments to
take place on land
adjacent to river
channels and
canals.
Challenges
Channel
improvement may
create problems
for areas upstream
or downstream of
the improved area.

54. Disaster Preparedness
Cities can take further steps to reduce the harm caused by floods.
Being warned of possible floods is important in reducing flood impact.
People can also be trained to be prepared and to know what to do when a flood occurs.
1. Forecasting and warning system
2. Evacuation Drills

55. Forecasting and Warning System
To reduce flood risks, cities may put in place a forecasting and warning system.
For forecasting to be accurate, there needs to be monitoring.
Monitoring is the systematic process of collecting and analysing information to identify changes.
Based on the information collected, a forecast is made.
A forecast is a statement about the way things might happen in the future.
If the situation is serious, a warning will need to be issued.
Monitoring Forecasting Warning

56. Forecasting and Warning System
In the case of flood mitigation, cities monitor water levels in river channels, canals, tide levels in seas
and surrounding weather conditions.
In Singapore, water level sensors are placed in canals and drains to gather data about the changing
water level at regular intervals.
The water level sensors can also be equipped with cameras to take photos of the situation in the
canals.
Monitoring of the tides and weather conditions is also carried out daily with the aid of equipment,
such as weather instruments.
Based on the information collected, flood forecasting can be made to identify the likely flood areas
affected by a flood event. Following this, a decision would be made if flood warnings should be
issued to the public.

57. Forecasting and Warning System
Monitoring Forecasting
• Water levels in canals, drains and river channels
• Tide levels in the seas
• Weather conditions
• Flood events
• Weather forecast
Benefits and
Challenges
Benefits
A forecasting and
warning system
provides useful
information so that
there is time for
people to take action
to minimise flood
damage.
Challenges
Cities may not have
the expertise and
technology to collect
accurate data and
make accurate flood
forecasts.

58. Read More!
Web :
http://www.epw.in/system/files/pdf/1954_6/43-
44/preventing_floods_by_channel_improvement.pdf

59. Evacuation Drills
Cities have large populations.
During emergencies, cities need to have a plan to evacuate a large number of people in the shortest
possible time.
Evacuation drills are a form of preparedness measure where people practise the steps to leave a
place in a safe and orderly way during an emergency.
Such evacuation drills create awareness among the people and reduce panic during a flood event.
Evacuation drills involve people moving to safe locations, following instructions given by trained
personnel and practising first aid.

60. Evacuation Drills
Benefits and
Challenges
Benefits
Evacuation drills
will enable cities to
improve their flood
evacuation plans
Challenges
Limited
participation will
reduce the
effectiveness of an
evacuation drill
Benefits
People are
educated on what
to do when a flood
warning is received

62. Timed Question (10 minutes)
Sample Question :
Explain the benefits and challenges of any two
measures a city could take to mitigate floods. (4m)

63. Suggested Answers
Suggested points:
• Zoning creates space for green areas such parks and fields which help reduce the impact of flood damage. However, zoning may
limit the development of available land because only certain uses are permitted. It is also difficult to carry out because of competing
land uses and people may not abide by the recommended land use. [2]
• Elevating properties means that a development is protected against floods up to the height of the MPL. However, it is challenging to
implement MPL because of the increased cost of development. The appearance of or access to building may be badly affected. [2]
• Levees and floodwalls are cheaper options to mitigate and prevent floods as compared to raising the height of the surrounding land.
However, levees and floodwalls need regular maintenance to ensure that they are strong enough to hold back floodwater. In addition,
they are designed to hold back floodwater only up to a certain level. Beyond that level, they are unable to prevent floods. [2]
• Channel improvement makes it possible for moderate- to high-value developments to take place on the land adjacent to channels
and canals. However, channel improvement may create problems for areas upstream or downstream of the improved area. [2]
• Forecasting and warning systems provide useful information so that there is time for people to take action to minimise flood
damage. Cities may not have the expertise and technology to collect accurate data and are unable to make accurate flood forecasts.
[2]
Any two points (2 marks each)
Remember to Elaborate and use Evidences!

64. The End