Pools and Riffles <ul><li>Pools are areas of deep water and greater erosion. (Energy build up due to less friction) </li></ul><ul><li>Riffles are areas of shallow water caused by deposition of coarse sediment </li></ul>
Sinuosity <ul><li>Is the curving nature of a river’s course. </li></ul>
Thalweg <ul><li>The position of the rivers fastest flowing current. </li></ul><ul><li>(Least friction, greater energy, greatest erosion) </li></ul>
1. Origins of meanders in times of flood and related to the occurrence of sand bars, pools and riffles
2. Usual spacing of pool to pool or riffle to riffle is regular at 5 times the bed width.
3. This is thought to be because of secondary flows within the main flow. E.g. Helicoidal flow – corkscrew movement that erodes and deposits material. 4. Secondary flows increase the sinuosity of the meander.
5. These produce a regular meander wave length of 10 times the bed width. 6. A river is said to be meandering in the sinuosity index is above 1.5.
7. Meanders have an asymmetrical cross section with erosion greater on the outside bend where discharge and velocity are greatest and friction is at a minimum.
8. Material is deposited on the inside of a meander bend as a point bar. 9. At point bars particles are usually graded in size (sorted) with the largest particles found on the upstream side.
10. As erosion occurs on the outside of the meander bend, the whole meander migrates downstream. 11. Material forming the point bar becomes a contributory factor in floodplain formation.
12. As sinuosity increases, during flood events the flow may cut of the meander bend at the neck, thus shortening its course. 13. Temporary straightening of the channel means the main flow is in mid channel, meaning deposition now occurs at the river banks and the old curve is therefore abandoned to leave an ox-bow lake.