Rivers and flooding flashcards

Rivers shape the land through erosion, transportation and deposition. Students need to understand how a river changes from its source to its mouth, including the formation of landforms such as waterfalls, meanders, ox-bow lakes and flood plains. The drainage basin and its key features, including the water cycle processes that affect river discharge, are essential knowledge. Students must be able to explain the causes of flooding, both physical and human, and evaluate hard and soft engineering strategies used to manage flood risk. Understanding how to interpret hydrographs, including lag time and peak discharge, is also required. Case studies of real flood events help students apply their knowledge to specific places and assess the social, economic and environmental impacts of flooding on communities.

1. Card 1

   What is a drainage basin?

   A drainage basin is the area of land drained by a river and its tributaries. It is bounded by a ridge of high land called a watershed.

2. Card 2

   What are the three main processes by which a river erodes its channel?

   Hydraulic action (force of water), abrasion (rocks scraping the bed), attrition (rocks hitting each other) and solution (dissolving minerals) are the key erosion processes.

3. Card 3

   How does a waterfall form?

   Water flows over hard rock above softer rock. The softer rock erodes faster, forming a plunge pool through hydraulic action and abrasion. The hard rock is undercut and eventually collapses, causing the waterfall to retreat upstream.

4. Card 4

   What is a meander and how does it develop?

   A meander is a bend in a river. Fast water on the outer bank causes erosion, forming a river cliff. Slow water on the inner bank deposits sediment, forming a slip-off slope.

5. Card 5

   How does an ox-bow lake form?

   As a meander tightens, the neck of land narrows. During a flood the river cuts through, and deposition seals off the old bend, leaving an isolated ox-bow lake.

6. Card 6

   What is a flood plain and how is it formed?

   A flood plain is the flat land either side of a river in its lower course. It is built up by alluvium deposited when the river floods and by lateral erosion widening the valley.

7. Card 7

   What does a storm hydrograph show?

   A storm hydrograph shows how river discharge changes after a rainfall event. Key features include peak discharge, lag time, rising limb and falling limb.

8. Card 8

   What is lag time on a hydrograph?

   Lag time is the delay between peak rainfall and peak discharge. A short lag time means the river responds quickly to rain, increasing flood risk.

9. Card 9

   Give two physical causes of flooding.

   Prolonged rainfall saturates the ground so water cannot infiltrate, and impermeable rock prevents water soaking in, both increasing surface run-off and raising discharge rapidly.

10. Card 10

    Give two human causes of flooding.

    Urbanisation increases impermeable surfaces such as roads and rooftops, and deforestation removes trees that intercept rainfall and take up water, both increasing run-off.

11. Card 11

    What is the difference between hard and soft engineering flood management?

    Hard engineering uses artificial structures such as dams and embankments to control water. Soft engineering works with natural processes, for example flood plain zoning and afforestation.

12. Card 12

    What are the advantages and disadvantages of building a dam for flood management?

    A dam controls discharge and can generate electricity, but it is very expensive, displaces communities and traps sediment, starving the river downstream of material.

• Many students confuse the inner and outer banks of a meander, incorrectly stating that erosion happens on the inner bank rather than the outer bank where velocity is greatest.
• Students often describe lag time as the time between the start of rainfall and peak discharge, when it is actually measured from the peak of the rainfall to the peak of the river discharge.
• A common error is to argue that hard engineering is always better than soft engineering, without recognising that soft engineering is often more sustainable, cheaper in the long term and less damaging to the environment.

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