cambridge geography as - hydrology and fluvial geomorphology: 1.3 river channel processes and...

River Channel Processes and Landforms

AS GEO 1.3

HYDROLOGY AND FLUVIAL GEOMORPHOLOGY

TOPICS

River ProcessesVelocityFlowHjulstrom Curve

RIVERDepends on:

Discharge - amount of water Velocity - rate of water movement

RIVER PROCESSESRiver carries three types of

work1. Erosion2. Transportation3. Deposition

EROSION

Occurs when rivers erode or wear away the land surface over which they are flowing

The rock particles

which are worn away

from the land surface are

called sediments

TRANSPORTATION Is a process where

the sediment produced by erosion

is carried away downstream by the

river

DEPOSITIONHappens when the sediment

may settle either on the river bed.

Where the water flows more slowly as in the flood plain,

or eventually on the sea bed.

HYDRAULIC ACTIONA process by which the force

of the flowing water may remove particles from the

banks or bed

CAVITATIONA form of hydraulic action caused by bubbles of air

collapsing.The resultant shock waves hit and slowly weaken the banks.This is the slowest and least effective erosion in process.

ATTRITIONA process by which as rocks are carried along by a river, they knock against each other so pieces break off and the rock fragments are

reduced in size and become rounded.

CORRASIONA process by which the rock particles which are then carried by the river may be used as

tools to help break more rock fragments from the river bed and banks.

Example: Circular holes called pot holes may be cut in a rocky river bed.

CORRASION

POTHOLES

SOLUTIONA process by which rivers can dissolve

some rocks such as limestone. Example

At Mulu in Sarawak, rivers have dissolved the limestone and created huge caverns (large caves) through

which they flow underground.

Rivers flow in channels and the sides of the channel are called banks, with the floor of the channel known as the river bed. Rivers can erode river channels in four main

ways such as:HYDRAULIC ACTIONA process by which the force of the flowing water may remove particles from the banks or bed

CORRASIONA process by which the rock particles which are then carried by the river may be used as tools to help break more rock fragments from the river bed and banksATTRITION

A process by which as rocks are carried along by a river, they knock against each other so pieces break off and the rock fragments are reduced in size and become rounded

SOLUTIONA process by which rivers can dissolve some rocks such as limestone

CASH

TRANSPORTATIONIs a process where the sediment produced by erosion is carried away downstream by the river

• 3 main processes:• Bedload• Suspended Load•Dissolved/Solution Load

BEDLOADLarger particles which cannot be picked up by current may be moved along the bed of the river in two ways:

- Traction- Saltation

TRACTIONWhen the large particles roll or

slide along the river bed. Large rocks are only moved after heavy

rain when the river has a large volume of water and is fast

flowing.

SALTATIONWhen particles are temporarily

lifted up by the current and bounced along the bed in a

hopping motion.

SUSPENDED LOAD0Then small particles such as sand and clay are carried along without touching the river bed. These small

particles are just floating, and lightest particles are near to the

surface of the water.

SOLUTIONWhen rainwater can slowly

dissolve limestone rock. They cannot be seen by the naked

eye.

RIVER TRANSPORTSUSPENSIONwhen small particles such as sand and clay are carried along without touching the river bed, small particles are just floating, and lightest particles are near to the surface of the water

SOLUTIONwhen rainwater can slowly dissolve limestone rock.

SALTATIONwhen particles are lifted up by the current and bounced along the bed in a hopping motion

TRACTIONwhen the largest particles roll or slide along the river bed, moved after heavy rain when the river has a large volume of water and is fast flowing

HOW DOES RIVER TRANSPORT HAPPEN?

RIVER DEPOSITIONThe speed of flow of a river is reduced the river

may no longer have enough energy to transport its load of sediment.

The larger particles will sink and settle first while the finer particles will be carried further before

settling, or they may be carried all the way to the sea. This sinking and settling of the river’s

sediment is called river deposition.Deposition may occur on the river bed, or on the inside curve of a river bend, or on the river banksThe sediment which is deposited in the sea at the

river mouth may build up new land known as delta.

RIVER DEPOSITIONDecrease in velocity less energy and no longer

had competence and capacity to carry all its load. Therefore, largest/heaviest particles, materials

begins to be deposited.

Occurs when:Low discharge following a period of low

precipitationLess velocity when river enter sea or lake

Shallower water occurs on inside of a meanderThe load suddenly increase (debris from landslide)River overflow its bank so velocity outside channel

is reduced (resulting in floodplain)

VELOCITY

VELOCITYVelocity is the speed of a river (m/s). Can influence the turbulence:

High Velocity:The amount of energy still available after friction will be greater and so turbulence increases.

The faster the flow of river the larger the quantity and size of particles (load) which can be transported.

Low Velocity:Less energy to overcome the friction.Turbulence decreases and may not be visible to human eye.Sediment will remains undisturbed.Reduction in turbulence may lead to deposition of sediment.

Velocity of a river is influenced by three factors:

(1) Channel shape in cross-section.

(2) Roughness of the channel’s bed and banks.

(3) Channel slope.

1. CHANNEL SHAPE IN CROSS SECTION

Simply describe by the term ‘Hydraulic Radius’ (cross section

area/wetted perimeter).Wetted perimeter - shape of the

channel or its cross section affects the extent to which water is in

contact with its channel. The greater the wetted perimeter, the greater the friction between the water and the banks and the

bed of the channel, and the slower the flow of river.

River volume: 6 sq m (2m x 3m) Wetted perimeter: 7 metres (2m + 3m +

2m). The 7 metres will be represent the

friction slowing the river down.

Volume: 24 sq metres Wetted perimeter: 14 metres.Shape of the river a major influence. A river with the same volume of water as Ex 2 but with a different shape will have a different friction value.

Volume: 24 sq metresWetted perimeter is 26 metres almost double

that of Example 2 which means that the river will

be slower as a larger part of the river energy

is used to overcome friction. The gradient of the river channel is only one factor to influence the speed of the river.

Example

Stream A: larger hydraulic radius

Small amount of water in contact with the wetted perimeter.

Creates less friction reduce energy loss allows greater velocity

Stream B: smaller hydraulic radius

Large amount of water in contact with the wetted perimeter.

Creates greater friction more energy loss reduce velocity

2. ROUGHNESS OF THE CHANNEL’S BED AND BANKS

Material such as rocks in the channel can influence the speed. Whether rocks on the river bed are smooth or rough or uneven. Rocks that protrude out from the bank can slow the pace of the water as friction slows it down as it passes the obstacles.

In figure A, the channel is smooth while that in figure B is rough or uneven with boulders on the river bed as well as rocks that protrude out from the bank.

A river that flows through such a river has to overcome such obstacles and therefore there will be more friction and the velocity of the river is reduced.

Figure A

Figure B

Velocity of a mountain stream is less than that of a lowland. Mountain stream is likely

to pick up loose material and carry it downstreamExample:

Mountainous / Upper course of a riverDespite high velocity in waterfalls, the large number of angular rocks, coarse-grained banks and protrusions increase

frictions and reduce overall velocityLower course of a river:

As there is little resistance from the smooth bed and banks, there is little

friction and river flows faster

3. CHANNEL SLOPEA river flowing down a steep slope or gradient has higher velocity than one which flows down

a gentler gradient.

For example, the speed of  flow in a river that plunges down a steep slope in the form of a

waterfall is much higher than the speed of flow in a river that winds down a gentler slope.

CHANNEL SLOPEChanges in gradient are related

to changes in discharge. Discharge is higher in the lower

course.Since gradient decreases as

discharge increases, river can transport the same quantity and

size of sediment load in the gentler lower course as it can in

the steeper upper course.

FLOW

PATTERNS OF FLOWRiver water has a certain amount of available

energy.Greatest when there is a large amt of water and

when there is steep gradient.Most of the river’s energy used up in overcoming

friction with the bed and banks.Friction high in the upper reaches of a river where

large boulders may protrude into large river’s flow.

There are three patterns of flow:

1. Laminar flow2. Turbulent flow3. Helicoidal flow

LAMINAR FLOWHorizontal movement of water. Travel over the sediment in the river bed without disturbing it. Rare in reality but common in the lower reaches.Condition:

SmoothStraight channelShallow waterNon-uniform velocity

TURBULENT FLOWSeries of erratic (inconsistent) eddies.

Both vertical & horizontal in downstream direction. Depends on the

amount of energy available after friction has been overcome.

Conditions:Complex channel shape eg. Winding

channels, riffles and poolsCavitation as eddies trap air in pores, cracks crevices which is then release

under great pressure.

T U R B U L E N T F L O W

HELICOIDAL FLOWUsually occur in meanders.A corkscrew movement in a

meander.It is responsible for moving

material from the outside of one meander bend and depositing on

the inside of the next bend.

HELICOIDAL FLOW

HJULSTROM CURVE

HJULSTROM CURVEA graph used by

hydrologists to determine whether a river will erode,

transport or deposit sediment.

The graph takes sediment size and channel velocity into

account.The curve shows several key ideas about the relationships

between erosion, transportation and

deposition.

HJULSTROM CURVE

Shows that particles of a size around 1mm require the least energy to erode, as they are sands that do not coagulate.

Particles smaller than these fine sands are often clays require a higher velocity to

produce the energy required to split the small clay particles which have

coagulated.

Larger particles pebbles are eroded at

higher velocities Very large objects boulders require the

highest velocities to erode.

When the velocity drops below this velocity called the line of critical velocity, particles will be deposited or transported, instead of being eroded, depending on the river's energy.

THE LANGUAGE OF HJULSTROM CURVE

Critical erosion velocity: The lowest velocity at which grains of a certain size can be moved.

Critical deposition velocity: The velocity at which particles of particular sizes are laid down

Entrainment: Materials being picked up by riverFlocculate: Materials stick together in the river

Clay particles: Tiny particles between 0.001 and 0.01mm in size

Sand particles: Sediments between 0.1 and 2mm in size

Cobbles: Sediments between 20 and 300mm in size

HJULSTROM CURVEKey:

Silt/sand are picked up (entrained) at the lowest velocities.Clays are difficult to pick up as pebbles – although they are small particles, they are very cohesive and the claybed is very smooth.Large boulders are dropped easily.Clay particles can be transported in suspension at very low velocities.

VOCABULARY CHECKHydraulic actionCavitationAttritionCorassionSolutionBedloadSuspended loadSolutionTractionSaltationHydraulic radius

Wetted perimeterLaminarTurbulentHelicoidalHjulstrom CurveCritical erosion curveCritical deposition curveEntrainmentFlocculateClay particlesSand particlesCobbles

HJULSTROM CURVE1. Name the type of sediment that requires

the lowest velocity to be eroded. [1]2. Name the type of sediment that is likely

to be transported at all velocities. [1]3. Describe and explain the relationship

between water velocity and the erosion of clayand sand particles. [4]4. Explain the variation in water velocity

that is required to transport and to depositsediments of different particle diameter.

[4]

ANSWER1. Sand2. Clay3. Clay - requires higher energy to be eroded - tend to stick together - are difficult to pick up as pebbles - although they are small particles, they are very

cohesive Sand - requires lower energy - sand particles are unconsolidated (loose)4. Boulders - require large velocities to be transported Small particles - Clay & silt – can be held in

suspension area at low velocity.Energy velocity to transport is always lower than energyto erode.