3717 - River Processes Presentation

7/29/2019 3717 - River Processes Presentation

1/24

Paul Warburton. Head of Geography. Manchester High School for Girls.

This presentation should help teachers and students involved with AS

and A2 Level Geography.

Important terms are highlighted, particularly the first time they are

used. Students will need to be able to define these terms and usethem in examination answers.

Use the left and right arrow keys unless otherwise instructed.

Also includes variations along a long profile and

Hjulstrm Curves

ZigZag Education 2010

Photocopiable/digital resources may only be copied by the purchasing

institution on a single site and for their own use


2/24

This is the wearing down of the landscape by a number offluvial processes. These can lead to the development of

erosion landforms. Click on each box below to find out

more about each process.

ABRASION

HYDRAULIC ACTION

ATTRITION

CORROSION

When you are ready to move on use the right arrow or click here.1


3/24

This takes place in a number of different ways click on the bars below

to find out more about each process.

SALTATION

SUSPENSION

BED or TRACTION LOAD

When you are ready to move on use the right arrow or click here.

2


4/24

There are two important concepts that relate to

transportation in rivers two terms which you willneed to learn.

COMPETENCE This refers to the largest particle

that can be carried in a river at a given velocity.

CAPACITY This refers to the total amount of load

that can be carried in a river at a given velocity.

When will capacity and competence be at their

maximum?

We will finally look at Deposition next slide

3


5/24

Deposition is relatively straightforward. A river deposits material as the velocity or

discharge slows. Material becomes too heavy to be carried and is dropped.

On this meander the water is flowing slower on the

inside and deposits have accumulated.

In this estuary water is slowing as it meets the sea

and there is plenty of visible deposition. An estuary

is tidal so at low tide the banks of silt and sand are

exposed.

Here braiding has occurred. This is common

towards the end of a river or where it flows across

flatter land and slows. Deposition creates small

islands called eyots. The course is split into many

separate channels.4 Source: US Geological Survey


6/24

Hopefully you will have realised by

now that rivers carry out much oftheir work during bankfull

conditions or when a river floods.

Then discharge is high, the river is

carrying a greater load and

erosion can be carried out more

effectively.

5

The rate of formation of erosional landforms increases under

these conditions. Some depositional landforms like levees are

formed when rivers flood. Some of the characteristics of aflood plain are again associated with flooding.

Use your AS textbook to understand this more fully.


7/24

The Hjulstrm curves are based

on laboratory experiments and

they show the relationship betweenvelocity and particle size and how

they interact to affect erosion,

transportation and deposition in a

river. A number of basic ideas are

easy to grasp.

As the river slows particles aremore likely to be deposited.

As velocities increase particles

start to be transported.

At higher velocities particles are

carried at a velocity that enable

them to erode.

Very fine particles of clay and silt need higher velocities than might be expected to disturb

these sizes as they are more cohesive (stick together). They are also so small that they need

higher velocities to be effective in erosion.

The next slide will ask you some questions to see how well you have understood the graph6

Cmsper

second


8/24

QUESTIONS

Think about each question then press the right arrow

key for the answer and again for the next question.

1. At what velocity would a 0.1mm sand particle start

to carry out erosion?

About 25cm/s

2. At what velocity would a 0.01mm silt particle start to

be transported?

About 0.1cm/s

3. Why do gravel particles need a higher velocity to

carry out erosion than sand particles?

Gravel particles are heavier than sand, more energy

(velocity) is needed to lift and carry these particles

so that they can carry out erosion.

4. Does the graph reflect competence or capacity?

Competence because this is about the size of particle that can be carried at a given velocity, i.e.

the relationship between the two. Capacity is the amount that can be carried at a given velocity

this is not in the graph there is no measure here of quantity or volume of load.

We will finally see how these processes vary along the course of a river.7

Cmsper

second


9/24

THE LONG PROFILE OF A RIVER (Cross profiles follow)

The long profile of a river is the course from the source to the mouth. In

reality it is clearly not as smooth as shown here, but all rivers are wearing

down the landscape and moving towards this theoretically smooth orgraded profile. Each stage is marked by dominant processes and typical

landforms these are outlined in a summary on the next slide. Of course

the processes and even landforms can be found in different stages but are

either at a different scale or are less common.

8


10/24

UPPER MIDDLE LOWER

PROCESSES PROCESSES PROCESSES

Dominated by vertical

erosion

Lateral erosion becomes

more significant

Deposition on inside of

bends

Deposition becomes

dominant as river

slows particularly

near the mouth

LANDFORMS LANDFORMS LANDFORMS Irregular course small

waterfalls and rapids

V shaped valleys as

river erodes vertically

Interlocking spurs

Valley sides more

gentle

Rivers develop more

pronounced meanders

Flood plain develops

Pronounced

meanders

Levees

Braiding may occur

where river slows and

is carrying a high load

Estuary at mouth

Deltas

The next slide takes you to the end

9


11/24

CHANNEL CROSS PROFILES

This refers to the cross sectional shape of a rivers

channel. The cross sectional area is obtained by

multiplying channel width by the average depth.

UPPER STAGE

In the upper stage, downstream from the source of a

river, it tends to be relatively shallow and irregular. As

this photo shows, the channel contains many

boulders and pebbles it is a rough channel.

The channel roughness leads to friction and lowervelocities. The discharge is low except in times of

flood.

Erosion is slow and is largely in a vertical direction.

A V-shaped valley is slowly developing.

10

Click in this box for details of Mannings Roughness Coefficient


12/24

Width/depth ratio and hydraulic radius

One way of describing channel cross section is to consider the width/depth ratio.

This involves measuring the width of the channel and dividing this by the

average depth. Some simple calculations will reveal that the ratio decreases as

the channel becomes more efficient in its shape.

A further way of measuring channel efficiency

is to calculate the hydraulic radius. This

involves dividing a channels cross sectional

area (in m2) by the wetted perimeter (in m).

The wetted perimeter (red line) is the length of

the bed and banks in contact with water. Thegreater the hydraulic radius, the greater the

efficiency of the channel and the less likely

the river is to flood. The highest values occur

when channels are deep, narrow, and semi-

circular in shape.

11


13/24

MIDDLE STAGE

With distance downstream the

channel becomes deeper and the

depth increases relative to the

width. This is a more efficient

channel as a smaller proportion of

the water is near the bed and

banks. So even though the gradient

may be less, discharge and velocity

increase as there is less friction to

slow the water.

You may also be able to see thatthe channel is less rough this also

reduces friction and encourages a

higher velocity and discharge.

It is useful at this point to introduce two measures of channel efficiency.

12

Erosion is now greater in a lateral direction. A

river will have a more meandering course and a

flood plain will develop.


14/24

LOWER STAGE

Towards the end of a river the

channel is smooth and often deep

relative to the width and continues

to be efficient.

Towards the sea though (or a lake)this can change. As a river slows

and loses its energy, deposition

becomes a more dominant

process. The channel can become

shallower and islands of deposits

develop. If a river flows into thesea the mouth will be tidal and a

wide estuary may develop. At low

tide extensive mud and sand flats

will be seen.This is the mouth of the river Mawddach near

Barmouth in North Wales.

13

Suggest reasons why the discharge of a

river would be higher in the lower stage.


15/24

UPPER MIDDLE LOWER

Irregular channel, rough

many boulders,

pebbles

Shallow relative to width

Inefficient channel

High width/depth ratio

Low hydraulic radius

Smoother channel

Deeper relative to its

width

More efficient

Lower width/depth ratio

Higher hydraulic radius

Smooth channel

Deep relative to width

Efficient channelalthough reduces towards mouth

Low width/depth ratio

Higher hydraulic radiusalthough lower towards mouth

A SUMMARY OF CHANNEL PROPERTIES AT STAGES

IN THE LONG PROFILE OF A RIVER

14 The next slide takes you to the end of this presentation.


16/24

End the presentation

(move the cursor and click on the square symbol in

the bottom left of the screen then End Show)


17/24

Corrosion is most effective on rocks that contain carbonates such as

limestone. Minerals in the rock are chemically broken down in water

that is slightly acidic and carried away in solution.

The formula is CaCO3 + H2CO3 Ca(HCO3)2Limestone Acidic water Calcium hydrogen carbonate (soluble)

Gordale

gorge in

the

Yorkshire

Dales

Where the watertable reaches

the surface

streams can

flow in

limestone areas even though

the rock is

permeable.

CORROSION

Click here to return to the menu


18/24

ATTRITION

This occurs when particles rub and grind against each other in moving water.

Particles gradually become smoother, smaller and more rounded.

What would most particles be like in the upper stage of a river?



19/24

HYDRAULIC ACTION

Turbulent water has a hammering and disturbing effect on the bed and banks. Air

can be trapped, compressed and suddenly released this also helps to break up

loosely consolidated material.

NB the combination of hydraulic action and abrasion below a waterfall is called

CAVITATION this leads to the development of a plunge pool.



20/24

ABRASION

The river cliff here has been created by a combination of hydraulic action and

abrasion. Abrasion is the sandpaper/abrasive effect of material carried in the water rubbing against and impacting on the banks.

Under what circumstances do you think this process would be most effective

/ at its maximum?



21/24

SALTATION

Saltation occurs where particles are too heavy to be carried in suspension

(entrained) but are light enough to be lifted by the flowing water. Particles

therefore bounce along the channel. Such particles disturb others when

they fall to the bed and set off a general movement of material downstream.

This process largely occurs with smaller particles.

What would enable larger particles to be moved in this way?



22/24

SUSPENSION

Suspension occurs when particles are small enough to be lifted andentrained (held in) the moving water. Suspended material often gives a river

a colouration usually brown but other colours depending on the colour of

the sediment. The Yellow River in China (above) is stained here with loess

(wind blown soil) carried in suspension.



23/24

BED or TRACTION LOAD

This is material often rocks

and boulders that is too large

to be lifted from the bed.

Instead it is rolled and dragged

along by friction with, and by

the force of, the moving water.

When or under what conditionswould these larger boulders be

moved?



24/24

MANNINGS ROUGHNESS COEFFICIENT

Manning was an engineer who developed a formula for calculating channel

roughness.

Where Q = discharge, A = cross sectional area, R = hydraulic radius, and S =

slope.

The formula gives a useful approximate value, the higher it is the rougher thebed and banks of the channel.

A(R x S)Q =

n

Click here to return to the slide you left

Documents

3717 - River Processes Presentation