© oxford university press 2009 managing river and coastal environments: a continuing challenge...
TRANSCRIPT
© Oxford University Press 2009
Managing river and coastal environments: A continuing challenge
2.22.2 What are the majorWhat are the major fluvial processes? fluvial processes?
Part APart A
© 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?
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?
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?
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.
© 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 m2)
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.
© 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?
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Managing river and coastal environments: A continuing challenge
Water table
The water flowing in a river channel is called channel flow.
1 Amount of channel water
Overland flow
Throughflow
Base flow
Channel flow increases with increasing overland flow, throughflow and base flow.
Channel flow
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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
1 Amount of channel water
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Managing river and coastal environments: A continuing challenge
Water table
2 Speed 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
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Managing river and coastal environments: A continuing challenge
A Channel 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.
2 Speed of water flow
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Managing river and coastal environments: A continuing challenge
B Channel 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 ).
2 Speed of water flow
This in turn reduces the energy of a river.
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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
2 Speed of water flowB Channel friction
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Managing river and coastal environments: A continuing challenge
Channel A has a shorter wetted perimeter, so, it has ( more / less ) friction.
Channel A Channel B
Which channel has a faster river flow?
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
2 Speed of water flowB Channel friction
© 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 A Channel B
Wetted perimeter= 21 m
2 Speed of water flowB Channel friction
Therefore, water in Channel A flows faster.
© Oxford University Press 2009
Managing river and coastal environments: A continuing challenge
Same cross-sectional area
Channel A Channel B
Same cross-sectional area
Which channel has a slower river flow?
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
2 Speed of water flowB Channel friction
© 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
© Oxford University Press 2009
Managing river and coastal environments: A continuing challenge
• P.27
© 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.