Floods, Flooding and Management
AS Geography
Plenary
• Why did the Mississippi floods of 1993 occur?• How do waterfall’s form?• What do we mean by competence and capacity?
Learning Objectives
• For students to understand the principle of flood frequency and how to calculate this
• Students to appreciate and demonstrate that there are both human and physical causes of flooding
• That there are various strategies that can help alleviate the effects of flooding
The Story so far...
• Students have already provided an in-depth analysis of the Mississippi floods of 1993.
• Have some knowledge of why Bangladesh floods• We understand the principles of flooding• So why does it happen and what can we do?
Why do Rivers Flood?
• Rivers flood naturally• People live near rivers from both a historical as well
as a cultural perspective• Floods are a natural hazard• Many rivers flood consistently• Humans alter natural landscapes flood risk can
increase.
Flood Frequency
• How to calculate flood frequency:• Size of largest flood for each year is ranked with 1
being the largest for all available records for any given year
T = n+1
m
T = Recurrence intervals
n = The number of years of observation
m = The rank Order
What does this tell us?
• It indicates the number of years within which a flood of this size can be expected
• What problems can you see in this equation?• It’s based on historical data• It doesn’t mean floods can’t occur less or more frequently
• The EA recommend that that flood defences should be built to withstand a 1 in 50 year flood
• The EA however build flood defences for a 1 in a 100yr flood
Activity
• Complete the flood frequency activity handed out
PHYSICAL Causes of Flooding
Physical Causes
• Flooding occurs when a river’s discharge exceeds the capacity of it’s channel to carry the discharge
• Excessive levels of precipitation over a prolonged period of time. Leads to soil saturation when water table reaches ground surface there is an increase in overland flow.
• Intensive Precipitation over a short period of time especially on hard sun baked ground.
Physical Causes
• Snowmelt especially when subsoil is still frozen as there is limited infiltration capacity.
• Climatic Hazards – Cyclones in Bangladesh, Hurricanes in Gulf of Mexico. Deep low pressure systems in mid latitude climates
• Relief – The slope of the river Basin• Vegetation – Can help increase infiltration and
thus reduce surface run off
Physical Causes
• Geology – Rock type both pervious and porous• Soil Type – Sand to clay type soils and changes in
infiltration• Drainage Density – Amount of surface streams
within the basin
Example of Floods
• Physical causes of floods have caused floods such as:
• Boscastle August 2004• Bangladesh 1998
What factors Increased the risks of floods in Boscastle?Watch the video and you decide.
Boscastle Floods
HUMANCauses of Flooding
Human Causes
• Urbanisation – Reduces infiltration and increases runoff
• Deforestation – Reduces interception• River Management – Dams can cause issues
along with other techniques• Climate Change – Heavier storms, more frequent,
melting of ice caps
Activity
• Complete the table activity on your sheets
Homework
• Complete the exam paper for Monday.
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