RIVER CHANNEL

PROCESSES

Topics:

River ProcessesVelocityFlowHjulstrom Curve

RiverDepends on:

Discharge: Amount of water

Velocity: Rate of water movement

River Processes:River carries three types of work:

1. 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

DEPOSITION

happens 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 ACTION

A process by which the force of the flowing water may remove

particles from the banks or bed

CAVITATION A 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

ATTRITION

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

CORRASION

A 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

SOLUTION

A 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

River Erosion 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 ACTION

A process by which the force of the flowing water may remove particles from the banks or bed

CORRASION

A 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

ATTRITION:

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

SOLUTION

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

CASH

TRANSPORTATION

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

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

Bedload Larger particles which cannot be

picked up by current may be moved along the bed of the river in two ways:

Traction Saltation

TRACTION

when 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

SALTATION

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

SUSPENDED LOAD when 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

SOLUTION when rainwater can slowly dissolve

limestone rock.

they cannot be seen by the naked eye

River TransportSUSPENSION

when 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

SOLUTION

when rainwater can slowly dissolve limestone rock.

SALTATION

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

TRACTION

when 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 banks

The sediment which is deposited in the sea at the river mouth may build up new land known as delta

River deposition decrease 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:1. Low discharge following a period of low

precipitation2. Less velocity when river enter sea or lake.3. Shallower water occurs on inside of a meander.4. The load suddenly increase (debris from

landslide)5. River overflow its bank so velocity outside

channel is reduced. (resulting in floodplain)

VELOCITY

Velocity: Velocity: speed of a river (m/s)

Can influenced turbulence:

High Velocity: the amt 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 influence by 3 factors:

(i) Channel shape in cross-section.

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

(iii) Channel slope.

i. Channel shape in cross-section.

Simply describe by the term ‘Hydraulic radius’

i.e: 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 (2mx3m) 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 Example 2 but with a different shape will have a different friction value.

Volume: 24 sq metres

Wetted 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 amt of water in contact with the wetted perimeter

- creates less friction reduce energy loss allows greater velocity

Stream B: smaller hydraulic radius

- large amt of water in contact with the wetted perimeter

- creates greater friction more energy loss reduce velocity

(ii) 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

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

Velocity of a mountain stream is less than that of a lowland

Mountain stream is likely to pick up loose material and carry it downstream

Example: Mountainous / Upper course of a river:

Despite high velocity in waterfalls, the large number of angular rocks, coarse-grained banks and protrusions increase frictions and reduce overall velocity

Lower course of a river: As there is little resistance from the smooth bed and

banks, there is little friction and river flows faster

(iii) Channel Slope A 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.

(iii) Channel slope.

Gradient = steepness As more tributaries and water from the

surface, throughflow join the main river

the discharge, channel cross-section and hydraulic radius increases.

less energy will be loss through friction erosive power will decrease river flows over a gradually decrease

gradient

Channel Slope

Changes 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.

How does velocity change with distance?

FLOW

Patterns of flow- River 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 flow

2. Turbulent flow

3. Helicoidal flow

Laminar Flow Horizontal movement of water

Travel over the sediment in the river bed without disturbing it

Rare in reality but common in the lower reaches

Condition: Smooth Straight channel Shallow water Non-uniform velocity

Turbulent Flow Series of erratic (inconsistent) eddies

Both vertical & horizontal in downstream direction

Depends on the amt of energy available after friction has been overcome

Conditions: Complex channel shape eg. Winding channels, riffles

and pools

Cavitation as eddies trap air in pores, cracks crevices which is then release under great pressure

Turbulent Flow

Helicoidal Flow

Usually 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 curve a 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 river

Flocculate: 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 Curve Key:

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 Check! Hydraulic action Cavitation Attrition Corassion Solution Bedload Suspended load Solution Traction Saltation Hydraulic radius

Wetted perimeter Laminar Turbulent Helicoidal Hjulstrom Curve Critical erosion curve Critical deposition

curve Entrainment Flocculate Clay particles Sand particles Cobbles

Hjustrom curve.

1. 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]

Answers1. 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 velocityEnergy velocity to transport is always lower than energyto erode