Flooding and hydrological cycle L9-10

Surplus in the Hydrological Cycle - Flooding

Learning Objective

  • To understand how surpluses within the hydrological system can lead to flooding, with significant impacts for people.

Starter – Key Words Quiz 4

  • Write the definitions for:

    1. Flash flooding: Rapid surface runoff occurs when dry soil surfaces become waterlogged very quickly.

    2. Monsoon rains: Rains that accompany the monsoon wind blowing from the southwest, bringing rain across South and Southeast Asia between May and September.

    3. Jet stream: A band of fast-moving air located between 9 and 16 kilometers above the Earth, which determines the direction of depressions and their speed of movement.

    4. Flood-return period: A statistical estimate of how often a flood of a certain magnitude is likely to occur, based on past flood levels (e.g., a 1-in-50-year or a 1-in-100-year flood).

    5. Hard engineering: Human-made, artificial structures designed to protect the land from erosion.

Topic 5: The Water Cycle and Water Insecurity

  • Lesson 9-10 Enquiry question: What factors influence the hydrological system over short- and long-term timescales?

  • Key Words Task for lesson 11: Learn the definitions for:

    • The Walker Cell

    • La Niña

    • El Niño

    • El Niño Southern Oscillation (ENSO)

    • ITCZ

Mapping Flood Events

Large Flood Events (2001 to 2016)

  • Area Affected:

    • Data sources indicate various global regions suffered from significant flooding events, highlighting a distribution of risk (refer to map drawn on pages using data from G.R. Brakenridge's work).

Describe the distribution of flood events.
Suggest reasons for the distribution of these events.

Meteorological Causes of Flooding

Normal Floods

  • Caused by a prolonged build-up of rainfall over a period of time.

  • Typically involves several mid-latitude depressions passing over the same location, incrementally increasing discharge until the point of bankfull is reached.

Flash Flooding

  • Occurs without warning due to intense heavy rainfall, usually from intense tropical storms or convectional rainfall events.

Monsoon Floods

  • Definition: A monsoon is a seasonal change in the direction of prevailing winds causing wet and dry seasons in subtropical regions.

  • In India, the summer monsoon results in very heavy rainfall. Warm, moist air from the Indian ocean travels towards the north as the ITCZ moves northward.

The ITCZ (Intertropical Convergence Zone)

  • Explanation of how the ITCZ works: The ITCZ represents the area where the trade winds of the Northern and Southern Hemispheres meet, influencing rainfall patterns and the distribution of tropical weather systems.

  • Winds from the northern and southern hemispheres converge - causing increased precipitation and low pressure.

  • Converging winds and solar heating force the warm humid over the tropical ocean to rise through convection.

  • As the warm air rises it cools and water vapour condenses, into clouds and rain.

  • The hotter the air the more rain it can pull.

  • When conditions are right, tropical storms form.

  • The zone position fluctuates depending on the season

Flooding in the UK

Causes

  • Many flood events in the UK are produced by mid-latitude depressions. Each depression typically features two bands of rainfall:

    • Heavy rain associated with the cold front

    • Showers and rain related to the warm front

  • Initial ground absorption can occur, but it decreases as rainfall persists, leading to increased river flow and surface runoff.

Impact of Climate Change on Jet Stream

  • The warming Arctic and loss of sea ice have enabled the jet stream to meander more than its previous steady west-east flow, causing extreme weather patterns and trapping high and low-pressure systems, which can contribute to flooding events.

Changing Land Use

Causes of Flooding

  • An environmentalist, George Monbiot, describes the landscapes as 'sheep-wrecked uplands.'

  • Human Management: Mismanagement through hard engineering schemes like raised riverbanks can often exacerbate flooding risks.

Human Actions That Increase Flood Risk

  1. Population Pressure

  2. Deforestation (removal of vegetation)

  3. Increased surface run-off and shorter lag times

  4. Land Use Change

  5. Urbanization

  6. Agriculture

  7. Reduced interception and evapotranspiration

  8. Flood plain use

  9. Hard engineering approaches

Research Exercise

  • Conduct a mind map illustrating how human actions can increase the risk of flooding. Include diagrams and examples for each factor.

Case Study: Storm Desmond

Flood Event (5th-6th December 2015)

  • Analysis of River Cocker and River Greta flood data, discussing variability in flood impacts across regions.

Cockermouth Flood Defences

  • Analyze the types and locations of flood defenses in the town, specifically noting River Derwent, River Cocker, and Bitter Beck.

Effectiveness of Cockermouth's Flood Defenses

  • Map study to compare the extent of flooding and the effectiveness of implemented flood defenses.

Mitigating Flood Risk

Solutions

  • The Environment Agency critiques hard engineering as an expensive and often ineffective solution to extreme flooding events.

  • Increasing popularity of soft-engineering solutions such as:

    • Reafforestation of upland areas

    • Restoration of straightened river channels

    • Wetland restoration

    • Denial of planning permissions near rivers

    • Rewilding initiatives, while not directly flood-focused, have gained attention and may share relevance with other geographic aspects.

Benefits of Flooding

  • Discussion prompting: “Flooding isn’t always a bad thing.”

  • Flooding contributes to natural equilibrium and biodiversity—illustrated by regions such as the Okavango Delta in Botswana.

Exam Question

  • Assess the extent to which human actions can exacerbate flood risk, supported with examples.

    • Knowledge (AO1): Water surplus and its role in flood risks.

    • Application (AO2): Physical factors affecting flood risks including geography, vegetation, and geology.

    • Use of case studies like the flooding of Keswick, Cumbria due to Storm Desmond to exemplify points.

    • Suggest soft-engineering solutions as future flood risk management strategies.

Plenary Activity

  • Group activity to create a mark scheme for assessing the influence of physical and human factors in increasing flood risk.

Summary

  • In December 2015, Storm Desmond resulted in severe flooding across Cumbria due to record-breaking rainfall influenced by jet streams and low-pressure systems.

  • Physical factors include meteorological events, whereas human actions relate to mismanagement and land use changes.

  • Emphasis on the growing importance of soft-engineering solutions in mitigating flood risks and understanding longer-term implications of flooding.