Changing Channel Characteristics

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Changing Channel Characteristics Geography lesson. Using AQA A specification.

Changing Channel Characteristics Geography lesson. Using AQA A specification.

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  • 1. Velocity
  • 2. Lesson Aims
    • To understand the main factors which influence a river’s velocity.
        • Channel shape in cross-section
        • Roughness of the channel’s bed and banks
        • Channel slope
        • How will you do this?
        • You will achieve this by calculating the hydraulic radii of rivers and considering what happens as we move down a river from its source to its mouth.
  • 3. Channel Shape
    • This is best described by its hydraulic radius; that is, the ratio between the cross sectional area and the length of the wetted perimeter.
      • Hydraulic Radius =
    Cross-sectional area Wetted Perimeter
  • 4. Cross-sectional area
    • This is measured by multiplying the width and the mean depth of the channel.
  • 5. Wetted perimeter
    • The portion of the perimeter of a stream channel cross section that is in contact with the water.
  • 6. Which river is the most efficient? These two channels have the same cross sectional area but are different shapes and as such different hydraulic radii.
  • 7. Answer
    • Stream A – has a larger hydraulic radius with less water in contact with the bed or banks, so it has less frictional drag and more of the water can move at a higher velocity.
  • 8. The reality
    • Streams are often asymmetrical and as such we get varying velocity from one river bank to the other.
  • 9. The reality
    • A river’s velocity will change throughout the year. Consider the two diagrams below.
    Bankfull stage
  • 10. Complete the activities
    • Summarise the text from the paragraph at the bottom of page 10 into 4 bullet points.
    • From page 11 of the book.
  • 11. Demonstrate your understanding
    • Your understanding of this part of the work will be tested in Stansfeld when you investigate the following hypothesis.
      • AIM – A river’s channel characteristics change significantly as it moves downstream.
      • Objectives
      • 1) The further you move from the source of the river the greater the discharge and hydraulic radius.
      • 2) The further you move from the source of the river the smaller and smoother the bedload will become.
  • 12. Review your thinking
    • Explain to your table why river velocities increase as you move downstream.