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.