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# Meander Formation

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This powerpoint looks at the formation of meanders. It leads students through their formation and then ends with an exam question.

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### Meander Formation

1. 1. Meander Formation
2. 4. Learning Outcomes <ul><li>You will be able to explain the sequence involved in the formation of a meander. </li></ul><ul><li>You will be able to explain the sequence using the key terminology. </li></ul><ul><li>KEY WORDS – helicoidal flow </li></ul><ul><li>sinousity </li></ul><ul><li>thalweg </li></ul><ul><li>centripetal force </li></ul>
3. 6. Meander Formation <ul><li>Riffles and Pools </li></ul><ul><li>Riffle – a shallow part of the channel </li></ul><ul><li>Pool – a deep part of the channel </li></ul><ul><li>On average pools occur 5-6 times the width of the river bed </li></ul>
4. 7. Meander Formation <ul><li>The riffle reduces the hydraulic radius of that section of the channel as the water flows inefficiently over it. </li></ul>
5. 9. Asymettrical shape
6. 10. The meandering Thalweg <ul><li>Thalweg – the line of fastest flow in a river </li></ul>
7. 11. Centripetal Force <ul><li>At times of high flow the water is thrown around the riffles and is thrown to the edge by centripetal force eroding and undercutting it. (MAINLY ABRASION AND HYDRAULIC ACTION) </li></ul>
8. 12. Deposition <ul><li>Slower flow on the inside of the bend leads to depostion there. </li></ul><ul><li>It is slower because the reduction in the rivers efficiency being reduced due to the frictional contact with the river bed. </li></ul>
9. 13. Helicoidal Flow <ul><li>The movement of water in a meander has been found to move from the outside to the inside. </li></ul><ul><li>This helicoidal flow allows material eroded from the outer bank to be moved to the point bar on the inside of the next meander downstream. </li></ul>
10. 14. Helicoidal Flow <ul><li>Corkscrew motion </li></ul><ul><li>In bends, water forced to outside of bend hits bank with force </li></ul><ul><li>Water piles-up on outside of bend </li></ul><ul><li>Forced downward along river cliff, back towards point bar </li></ul><ul><li>Transfers and reverses motion around next alternating bend </li></ul>
11. 16. Exiting the meander <ul><li>As the water exits the meander the channel becomes shallower. This reduces the hydraulic radius thus increasing deposition. </li></ul><ul><li>As such new riffles are created. </li></ul>
12. 18. Sinuosity <ul><li>Actual channel length </li></ul><ul><li>Straight Line Distance </li></ul><ul><li>A sinuosity of 1.5 is the dividing line between a straight and meandering channel. </li></ul>= sinousity
13. 19. Point Bar River Cliff Thalweg
14. 20. Thalweg Deposition Point Bar Helicoidal Flow River Cliff Abrasion Hydraulic Action Riffle Pool Sinuosity Meander Centripetal Force
15. 21. <ul><li>Explain the formation of a meander </li></ul><ul><li>(8 marks) </li></ul>
16. 24. Meander Formation <ul><li>Explain the formation of a meander using </li></ul><ul><li>helicoidal flow </li></ul><ul><li>sinousity </li></ul><ul><li>thalweg </li></ul><ul><li>centripetal force </li></ul>