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# Changing Channel Characteristics

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Changing Channel Characteristics Geography lesson. Using AQA A specification.

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### Changing Channel Characteristics

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