1. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge 2.2What are the major fluvial processes? Part A

2. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge What happens to the sand grains inside the tube when water flows through it? They become wet, and then start moving down the tube. How does water flow in a channel? Do the different sizes of grains of sand move at the same time? No, they do not. The finest grains move first; the largest ones last. The faster the flow of water, the larger the grains it can move. How can a river move the sand grains in its channel? It needs energy. When a river has more energy, it can move more and heavier grains.

3. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge What is the discharge of a river? Discharge (Q) =  V Area of stream cross-section (A) = W  D (in the unit of m 2 ) A Stream width (W) Average depth (D) Velocity (V) Velocity (V): Measured by an instrument (in the unit of m/sec) The ( larger / smaller ) the discharge, the more the energy a river has. The amount of energy of a river is indicated by the discharge of a river.

4. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge Amount of channel water Speed of water flow Energy of a river What are the factors affecting the energy of a river?

5. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge Water table The water flowing in a river channel is called channel flow. 1Amount of channel water Overland flow Throughflow Base flow Channel flow increases with increasing overland flow, throughflow and base flow. Channel flow

6. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge Water table Overland flow Throughflow Base flow Vegetation cover Gradient Drainage density (more / less) rainfall and snowmelt; The amount of channel water increases when there is: (more / less) evaporation; a (larger / smaller) catchment area; (more / less) vegetation cover; a (faster / slower) infiltration rate; a (steeper / gentler) slope gradient; a (lower / higher) drainage density. Rainfall Evaporation Infiltration 1Amount of channel water

7. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge Water table 2Speed of water flow The speed of water flow of a river depends on the gradient and the friction of the channel. Channel friction Channel gradient

8. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge AChannel gradient Channel A Channel B Water in Channel ( A / B ) flows faster. The steeper the gradient, the faster the flow, and the (higher / lower) the energy of a river. 2Speed of water flow

9. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge BChannel friction Friction ( hinders / helps ) water from flowing downstream. Energy is needed to overcome friction. The reduced amount of energy causes water to flow ( faster / slower ). 2Speed of water flow This in turn reduces the energy of a river.

10. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge Wetted perimeter: The perimeter of the banks and bed which are in contact with the water. Channel friction Wetted perimeter 2Speed of water flow BChannel friction

11. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge Channel A has a shorter wetted perimeter, so, it has ( more / less ) friction. Channel AChannel B Which channel has a faster river flow? So, water in Channel A flows faster. Wetted perimeter = ( 9 + 6 + 6 ) m = 21 m Wetted perimeter = ( 18 + 3 + 3) m = 24 m 2Speed of water flow BChannel friction

12. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge Wetted perimeter = 24 m Hydraulic radius of Channel A 9 m  6 m = = 2.57 m 21 m Hydraulic radius of Channel B 18 m  3 m = = 2.25 m 24 m Hydraulic radius Cross-sectional area = Wetted perimeter The larger the hydraulic radius, the greater the efficiency of the channel. Channel AChannel B Wetted perimeter = 21 m 2Speed of water flow BChannel friction Therefore, water in Channel A flows faster.

13. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge Same cross-sectional area Channel AChannel B Same cross-sectional area Which channel has a slower river flow? Channel ( A / B ) has a slower river flow, because the channel is __________. Its wetted perimeter is ( longer / shorter ). Therefore, its friction is higher. rougher 2Speed of water flow BChannel friction

14. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge What are the factors affecting the energy of a river? Let’s review what we have just learned! Amount of channel water Speed of water flow Energy of a river Channel gradient Channel friction

15. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge P.27

16. © Oxford University Press 2009 Managing river and coastal environments: A continuing challenge After learning the factors affecting the energy of a river, we will learn how a river shapes the land in Part B.