River Channels Velocity And Discharge


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to support AS geography, edexcel B, unit 1

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River Channels Velocity And Discharge

  1. 1. River Channels Geography Department Boston Spa School
  2. 2. Energy Variation <ul><li>A still body of water at any point above sea level has a certain amount of stored energy. </li></ul><ul><li>This = potential energy available to do work in the river channel. </li></ul><ul><li>Its quantity is in proportion the mass of the water body & the height it has to fall. </li></ul><ul><li>The kinetic energy of a river is caused by its movement & is derived from potential energy. </li></ul><ul><li>Between the two is some loss required to overcome the friction with the bed. </li></ul><ul><li>If the channel is steep, the change from potential to kinetic energy is rapid & velocity of the river is high & conversely low gradient, low velocity. </li></ul><ul><li>The amount of work done by the river is also affected by the volume of water. The greater the discharge, the larger the total energy. </li></ul><ul><li>So large rivers have more energy than small ones . </li></ul>
  3. 3. Velocity & Discharge <ul><li>Velocity varies vertically & laterally across the river </li></ul><ul><li>Discharge is dependant upon the velocity of water & size of channel at a given point </li></ul><ul><li>Q= A x V </li></ul><ul><li>e.g. if A = 11.32m 2 & V=0.153m/s </li></ul><ul><li>Q=1.73m 3 /s </li></ul><ul><li>Discharge varies at time of flood & low flow as velocity & cross sectional area varies </li></ul>
  4. 4. Friction of water with the bed <ul><li>The effect of the bed is determined by how much of the water comes into contact with it –wetted perimeter & cross sectional area of the channel. </li></ul><ul><li>Relationship between the wetted perimeter & the cross sectional area = hydraulic radius </li></ul><ul><li>R= A/Pw </li></ul><ul><li>If the value of the hydraulic radius is high, a large area of water in the cross sectional area is affected by each metre of bed & therefore the frictional effect of the bed is limited. If the value is low, the effect is high (e.g. in a very shallow river) </li></ul><ul><li>Friction of the bed of the river itself depends upon how ‘rough’ the bed is – smooth, silty bed has low friction compared to gravel & boulders. </li></ul>
  5. 5. Manning’s Equation <ul><li>Q= Ax R 2/3 x S 1/2 </li></ul><ul><li>N </li></ul><ul><li>Formula devised in 1889 </li></ul><ul><li>Investigate the total effect of internal friction, bed roughness, channel slope, size & shape & discharge on the velocity of a river. </li></ul><ul><li>S = slope, n= Manning’s coefficient of bed roughness, R = hydraulic radius, A = cross sectional area & Q=discharge </li></ul><ul><li>If bed roughness increases (n is large), velocity & therefore discharge are reduced. </li></ul><ul><li>Useful in estimating the discharge in flood conditions </li></ul>
  6. 6. Task <ul><li>Complete the three remaining entries in the data recording sheet </li></ul><ul><li>Why is the calculation of the hydraulic radius an important part of stream study? </li></ul><ul><li>Why is mean river velocity different from mean surface velocity? </li></ul><ul><li>Look at the labels on the sketch & site comments. How might they vary downstream? </li></ul>
  7. 7. Task……. <ul><li>Study Figure 2(b) The diagram shows how other channel characteristics change downstream. </li></ul><ul><li>5. Describe the changes shown in discharge and velocity downstream. </li></ul><ul><li>6. Give reasons for the changes in discharge and velocity </li></ul>