S2 GE Slides - Floods (Part 1)

Chapter 5
A natural occurrence

Floods can
bring
positive impacts…

a hazard…
But can they
also be

1. What are floods? What causes floods?
2. Which cities are prone to floods?
Why are these cities more prone?
3. How do floods affect city dwellers?
4. How should cities prepare for floods?
We want to learn about…

1. What are floods?
Definition:
Floods are large amounts of
water that overflows onto
land that is usually dry.

Geographical concept:
Scale
SCALE of floods
depends on the
extent of the
area that is
flooded
Brisbane flood: LARGE - scale flood

Geographical concept:
Scale
SCALE of floods
depends on the
extent of the
area that is
flooded
Singapore flood: SMALL - scale flood

Small-Scale flood Large-Scale flood
Only affects local area Flood affects wider area
SG orchard road flood
(2009)
Brisbane flood (2011)
Smaller extent of
damage (ie. lesser
casualties/losses)
Damages more property &
affects more
peoples’ lives

1. Types of floods
Flash floods
Coastal floods
(1)
River floods
(2)
(3)

Occur in areas along the coast (eg. river mouths)
Caused by intense storms
(eg. tropical storms,
typhoons or hurricanes)
Coastal floods
(1)

How storms cause
coastal floods:
Watch
–
h(ps://www.youtube.com/watch?v=Kd1tY1ekYSI

Coastal floods
(1)

Occur when water in the river
rises and overflows its banks.
A cross-section of a river channel
River floods
(2)

Occur when water in the river
rises and overflows its banks.
raised banks (aka levees)
River floods
(2)

Occur on the floodplain (river
+ low-lying land on both sides)
Floodplain
River floods
(2)

May be caused by:
Snowmelt
Failure of man-made
structures
(eg. Dam failure)
Heavy rainfall
River floods
(2)

Floodwater reached the roof level of houses
Flood due to overflow of Brisbane River
(in Brisbane, Queensland, Australia, in 2011)
River floods
(2)

Flash floods
Takes place suddenly
Lasts for a short period of time
Recognised by its quick rise in water level
(3)
h(ps://www.youtube.com/watch?v=uGTNf0i3uVI

Flash floods
River floods
(3)
May be caused by:
Heavy rainfall
Storm
surge
(eg.
Coastal
floods)

Q3a) What are the arrows pointing to?RECAP!
Natural
levees

Q3b) What are the arrows pointing to?RECAP!
Floodplain

S2 GE Slides - Floods (Part 1)

Failure of man-
made structures
Heavy
rainfall
Storm
surge
1. What causes floods? Snowmelt
(a)
(B)
(C)
(D)

Definition:
Rainfall refers to
droplets of water
that fall to the ground
1. What causes floods?
Heavy rainfall(a)

How rain is formed:
Heavy rainfall(a)

Rainfall characteristics:
Heavy rainfall(a)
Rainfall
dura3on

(hours)

How
long
the
rain
event
lasts
for.

Heavy rainfall(a)
Rainfall
intensity

(mm/h)

•  Rate
at
which
rain
falls
to
the
ground.

•  Tropical
storms
usually
have
a
high

rainfall
intensity.

Heavy rainfall(a)
Diﬀerent
rainfall
intensi3es

Heavy rainfall(a)
Rainfall
frequency

•  The
probability
that
a
certain

amount
of
rain
falling
within
a

certain
period
of
Mme
will
occur.

•  Eg.
1-‐in-‐50-‐year
storm
in

England
in
June
2012.

Heavy rainfall(a)
During a tropical
cyclone, rainfall is of
high intensity and
has a long duration.

Definition:
Snowmelt is surface runoff that
results from melting snow
Snowmelt
(B)

How snowmelt leads
to flooding:
Snowmelt
(B)
h(ps://www.floodsmart.gov/floodsmart/pages/flooding_flood_risks/flood_scenarios.jsp

Definition:
Storm surge is a sudden rise of
the sea level in which water is
piled up against a coastline
beyond normal high-tide
conditions.
Storm surge
(C)

How it looks like:
Storm surge
(C)

How it is formed:
During a storm,
strong winds push water
towards the coast.
This creates huge waves,
giving rise to a storm surge.
Storm surge
(C)
h(p://www.theguardian.com/world/video/2013/nov/18/typhoon-‐haiyan-‐storm-‐surge-‐house-‐video

Dams

A dam is a barrier built across a river
to retain and store water.
Failure of
man-made
structures
(D)

Dam failure can be disastrous for
people living downstream.
Failure of
man-made
structures
(D)
Dam
failure

Levees

Failure of
man-made
structures
(D)
Levees are raised river banks
that act as flood barriers
h(ps://www.floodsmart.gov/floodsmart/pages/flooding_flood_risks/levee_simulator.jsp

Failure of
man-made
structures
(D)
Can be
man-made or
natural.
Levees

Levees

Failure of
man-made
structures
(D)
Levees are designed to protect us
and our communities from flooding.
But overtime, they can
decay and fail.

Floodwalls are man-made structures
made of concrete.
They act as flood barriers.
Failure of
man-made
structures
(D)
Floodwalls

Austria’s
ﬂoodwalls:
h(ps://www.youtube.com/watch?v=hzQyTOaBHzI

Failure of
man-made
structures
(D)
Floodwalls

Failure of
man-made
structures
(D)
Levees
&

Floodwalls

Failure

1. What are floods? What causes floods?
Why are these cities more prone?
We want to learn about…

Floods can occur almost anywhere!

Case
studies
on
TB
page
164-‐165

Newport,
UK

Located
hear
coasts


coastal
flooding

due
to
storm
surges

New
Orleans,
USA

Located
in
the

path
of
hurricanes


floods
due
to

heavy
rainfalls

from
tropical

storm

Calgary,
Canada

Located
at
the

foothills
of

mountains


floods
due

to
snowmelt


Floods can occur almost anywhere!
Case
studies
on
TB
page
164-‐165

Tacloban,
Philippines

Located
along

the
coast


coastal
floods
&

storm
surges

Rio
Largo,
Brazil

Located
near
dams


floods
caused
by

heavy
rains
&

dam
failure

Dhaka,
Bangladesh

Located
at
low-‐lying

areas

floods
due

to
heavy
monsoon

rains

2. Why are these cities more prone to floods?
Height of land
above sea level(i)
Proximity to
water bodies(iI)
Permeability of
surface cover(iII)
Channel
Capacity(iv)

Height of land above sea level
(i)
Mean sea level is the average height of the
sea surface over a long period of time.

Height of land above sea level
(i)
Cities that are
located on
low-lying
lands
are more prone
to floods
Low-lying areas à prone to flooding

Proximity to water bodies
(iI)
Water bodies are significant accumulations
of water:
•  Oceans
•  Seas
•  Lakes
•  Rivers
•  Reservoirs

Proximity to water bodies
(iI)
Cities that are
located near water
bodies are more
prone to floods
Near to oceans / seas /
lakes / river mouths /
reservoirs à more prone
to flooding

Permeability of surface cover
(iII)
Permeability
refers to how
easy water can
infiltrate / seep
into the ground.

(iII)
Concrete
surfaces
Water cannot
infiltrate
the ground
High surface runoff
More prone to floods

(iII)
Vegetated
surfaces
Water can infiltrate
the ground easily
Less surface
runoff
Less prone
to floods

Channel Capacity
(iv)
Channel capacity refers to the volume of water
that can be held in the channel of a river / canal
without overflowing.
low

capacity

to
hold

water

high

capacity

to
hold

water

Channel Capacity
(iv)
Amount of rain exceed
the capacity of channel à Floods

Channel Capacity
(iv)
Cities have high proportion
of built-up areas and
concrete surfaces
Floods occur!
Canals may not
have the capacity to
carry this runoff away
fast enough
large amounts of
surface runoff
during a rain event
Decrease likelihood
of floods
Thus, a good network of
canals and rivers is
needed to drain
water away quickly

Textbook page 171 (Q1)
Explain how the factors shown in the sketch
can affect the area’s flood risk.


The
concrete
pavements
can
result

in
increased
overland
ﬂow
because

of
low
permeability.

The
narrow
canals
suggest
low
channel
capacity.

As
such,
the
canal
is
likely
to
be
unable
to
handle

large
amounts
of
surface
runoﬀ
during
a
rain
event.


The
area’s
proximity
to
the

river
makes
it
more
prone
to

ﬂoods
than
other
areas.

S2 GE Slides - Floods (Part 1)

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S2 GE Slides - Floods (Part 1)