River Channels: Velocity and Discharge

River Channels Geography Department Boston Spa School Energy Variation • A still body of water at any point above sea level has a certain amount of stored energy. • This = potential energy available to do work in the river channel. • Its quantity is in proportion the mass of the water body & the height it has to fall. • The kinetic energy of a river is caused by its movement & is derived from potential energy. • Between the two is some loss required to overcome the friction with the bed. • 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. • 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. • So large rivers have more energy than small ones. Velocity & Discharge • Velocity varies vertically & laterally across the river • Discharge is dependant upon the velocity of water & size of channel at a given point Q= A x V e.g. if A = 11.32m2 & V=0.153m/s Q=1.73m3/s • Discharge varies at time of flood & low flow as velocity & cross sectional area varies Friction of water with the bed • 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. • Relationship between the wetted perimeter & the cross sectional area = hydraulic radius R= A/Pw • 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) • 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. Manning’s Equation Q= Ax R2/3 x S1/2 N • Formula devised in 1889 • Investigate the total effect of internal friction, bed roughness, channel slope, size & shape & discharge on the velocity of a river. • If bed roughness increases (n is large), velocity & therefore discharge are reduced. • Useful in estimating the discharge in flood conditions S = slope, n= Manning’s coefficient of bed roughness, R = hydraulic radius, A = cross sectional area & Q=discharge Task 1. Complete the three remaining entries in the data recording sheet 2. Why is the calculation of the hydraulic radius an important part of stream study? 3. Why is mean river velocity different from mean surface velocity? 4. Look at the labels on the sketch & site comments. How might they vary downstream? Task……. Study Figure 2(b) The diagram shows how other channel characteristics change downstream. 5. Describe the changes shown in discharge and velocity downstream. 6. Give reasons for the changes in discharge and velocity