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Changing Channel Characteristics
 

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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    Changing Channel Characteristics Changing Channel Characteristics Presentation Transcript

    • Velocity
    • 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.
    • 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
    • Cross-sectional area
      • This is measured by multiplying the width and the mean depth of the channel.
    • Wetted perimeter
      • The portion of the perimeter of a stream channel cross section that is in contact with the water.
    • 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.
    • 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.
    • The reality
      • Streams are often asymmetrical and as such we get varying velocity from one river bank to the other.
    • The reality
      • A river’s velocity will change throughout the year. Consider the two diagrams below.
      Bankfull stage
    • 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.
    • 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.
    • Review your thinking
      • Explain to your table why river velocities increase as you move downstream.