Causes of floods
Make a copy of the column headings below.
Causes of floods
Rainfall
Snow and ice melt
Tidal influences
Other
(natural causes)
Put the causes into the correct column(s). Some can go in more than one column.
Affects mainly mountainous and high latitude areas.
Affects the lower course of a river.
Affects mainly the upper course of a river.
Occurs infrequently. Is due to large earthquakes or tsunamis.
Could also be caused by a landslide.
Long periods of heavy rain. High intensity rainfall exceeds the
infiltration capacity of the soil.
High antecedent rainfall can leave the soil saturated.
In some areas, 75% of the year’s river flow can occur when
temperatures suddenly rise.
Can affect all parts of a river.
Presence of downstream ice can act as a dam and force upstream
water to rise.
Occurs more frequently after large areas have been covered
recently by impermeable materials.
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Causes of floods
Flood-intensifying conditions
Write brief notes explaining how each of the following factors may increase the size of a
flood.
Urbanisation and urban growth
Bridges, dams and other
obstructions
Changes in vegetation cover
River engineering works
e.g. levees
Floodplain developments
Small drainage basins
Steep slopes
Global warming
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17509
Causes of floods
Teaching notes
Students could be asked to research a case study for each cause, presenting their
research to the class. Some possible hyperlinked examples are given below.
Causes of floods: answers
Rainfall
Other
(natural causes)
Snow and ice melt
Tidal influences
Long periods of
heavy rain, e.g.
Carlisle 2005.
Affects mainly
mountainous and
high latitude areas.
Affects the lower
course of a river,
e.g. Boscastle 2004.
High intensity rain
exceeds the
infiltration capacity
of the soil, e.g.
North-east England
and Scotland 2011.
Affects mainly the
upper course of a
river.
High antecedent
rainfall can leave
the soil saturated,
e.g. Boscastle 2004.
Can affect all parts
of a river.
Occurs more
frequently after
large areas have
been covered
recently by
impermeable
materials.
In some areas, 75%
of the year’s river
flow can occur when
temperatures
suddenly rise.
Occurs infrequently.
Is due to large
earthquakes or
tsunamis, e.g. Japan
2011.
Could also be caused
by a landslide, e.g.
Hunza Valley 2010.
Presence of
downstream ice can
act as a dam and
force upstream
water to rise, e.g.
Red River flood
2011.
© www.teachitgeography.co.uk 2012
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Causes of floods
Flood-intensifying conditions: answers
Urbanisation and urban growth
Bridges, dams and other
obstructions
Changes in vegetation cover
The increase in impermeable surfaces and increase
in drainage density (gutters and sewers, etc.)
leads to an increase in flood frequency and in the
degree of flooding.
These may act as temporary obstructions causing
the water to ‘pond up’. This may increase
pressure on the bridge and eventually cause the
collapse of the bridge. Downstream ice can also
act as a temporary dam causing the upstream
water to rise.
The removal of vegetation leads to less
interception of water, more overland flow and an
increased risk of flooding. In contrast, the
planting of vegetation can decrease the probability
of flooding and/or the size of peak discharge.
River engineering works
e.g. levees
Levees protect floodplains from a certain size of
flood (the ‘50 year flood’, for example) but they
do not offer protection from larger floods. They
cause local people to become complacent as to
the flood risk.
Floodplain developments
Any development on the floodplains (factories,
roads, housing and car parks, etc.) increases the
amount of impermeable surface. This increases
both the amount of surface runoff and reduces the
lag time, thereby increasing the risk of flooding.
It may cause other locations to flood, and
floodwaters to be deeper.
Small drainage basins
Small drainage basins respond quickly to high
rainfall events causing ‘flash floods’ and allowing
less time for taking action / response.
Steep slopes
Water flow is faster on steep slopes. The time lag
between maximum rainfall and the flood’s peak
discharge is reduced.
Global warming
Some places may become wetter and so flooding
may be more common. Other places however,
may become drier and so flooding may possibly be
reduced. Some places may become drier in
summer (drought conditions) but wetter in winter,
increasing the risk of flooding.
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17509