The Long Profile Characteristics. Why does a river meander in its middle and lower course?

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1 QU: How are Meanders formed? AIM: To describe and explain meander formation and identify the difference between GCSE and A/S knowledge and expectations. ST: Get the key rivers terms from the pictures A*/B Ext: Some have links to others, can you explain the links? 1

2 The Long Profile Characteristics. Why does a river meander in its middle and lower course? Transition of potential (gravitational) to Kinetic energy. Due to decreasing gradient. The river is closer to its graded profile so no longer works at vertical erosion but any surplus energy is put into lateral erosion. Hence the meandering and widening of the valley. 2

3 Erosion occurs on the outside of a meander whereas deposition happens on the inside of the meander. A* Velocity is greatest on the outside of the meander. Therefore there is more energy that can erode the banks. On the inside of the bend velocity is slower so the river has less energy. As a result it cannot carry sediment so deposits it on the inside of the bend. A/S Over to you. GCSE Structured, short answers. Describe and explain the formation of a meander. (7) 3

4 Today 4

5 In groups you are going to be allocated a stage of meander formation. You will have 5 mins to prepare a short presentation on what happens in your stage. You will present it with the aid of drawings on upturned tables. Success criteria: You use appropriate terminology. Diagrams Everyone chips in You deal with my killer questions. 5

6 Group 1 Straight river What is helicoidal flow? How is it created and how does it start the meander process? Group 2 Meandering starts What is the Thalweg? Why is it significant in further developing the meander? Group 3 Why are meanders asymmetric in cross section? Why is this a self accenuating process? What features are created across a meander? Group 4 The meander in cross section Bedload across a meander. Bedload across a meander is often sorted. What does this mean and how does it happen? Group 5 What are the last stages of a meander? What is an Ox bow lake? The meander matures 6

7 It all starts with our friends, the pool and riffle sequence... Riffles are shallow and due to increased friction create turbulance and helicoidal flow. As Helicoidal flow leaves the riffle section and enters a pool it has more energy to erode and will favour one side of the other of the channel. Once favouring one side of the river it will deepen this side and remain shallow on the other. This creates different velocities either side of the river. Erosion will happen one side (deeper more efficient) deposition the other (shallow less efficient). The meander process has begun. 7

8 lateral erosion lateral erosion deposition deposition deposition lateral erosion The thalweg denotes where greatest velocity is in a river. On a meander this is always on the outside of the bend where the water is deeper and more efficent as a result. This encourages erosion on the outside of the bend. The meander deepens and moves laterally as a result. 8

9 Well so what if the channel becomes asymmetric? Fastest flow. Why? Why does the inside of the meander have a smaller particles than the outside of bend. Erosion A B Deposition A B The deeper part will have a larger hydraulic radius therefore more energy to erode and increase velocity. This will lead to greater lateral erosion on this side which will lead to a positive feedback loop where the outside of the meander ( faster and more efficient ) grows larger. On the inside, the water has a greater hydraulic radius as more water is in touch with the bank / bed leading to reduction in velocity and ( as Hjulstrom says ) deposition will occur. This is a self accentuating process. (Process continues once started and is irreversable) The meander moves laterally across the floodplain from this process. 9

10 Due to having a higher settling velocity larger bedload settles on the bed on the outside of the meander. It is able to resist greater energy of the river whereas smaller bedload cannot resist and gets transported to the inside of the next meander downstream where in the lower velocities it is deposited. smallest material larger material Often a river beach shows evidence of sorting where the coarsest material is deposited first with the finest particles last due to settling velocities being reached in order of size. Therefore bedload is 'Sorted by size' 10

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13 Tasks: ( using these slides and man climbing a tree book pp ) 1) Describe how a meander first starts to form. Make reference to pools and riffles, helicoidal flow, the meandering thalweg, hydraulic radius changes and include at least a 2 step ( sequential ) diagram like fig. 5. 2) Draw a cross section of an asymmetric meander bend. Add annotations including slip off slope, river cliff. Where is hydraulic radius greatest and why? Based on this, explain where erosion and deposition are occurring. Ox Bow lakes: 3) What is an oxbow lake? 4) Draw a set of sequential diagrams to explain how a meander forms an ox bow lake 1) Describe this landform (4) 2) Explain how this landform is created ( 7 ) 13

14 Meander animation Overdo the answer to these. Rip it to shreds!! 14

15 What shape is the river cross section at a b, c d, e f? More importantly can you say why? 15

16 Attachments Meander animation