river channel processes landforms 1234591289846057 3

7/27/2019 River Channel Processes Landforms 1234591289846057 3

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RIVER PROCESSES:

Three river processes:

1. Transportation2. Deposition

3. Erosion


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RIVER TRANSPORTATION

The load is transported by 4 ways:(i) Saltation: when pebbles, sand and gravel (bedload) are lifted up

by current and bounced along the bed in a hopping motion.

(ii) Traction: when largest boulders and cobbles (bedload) roll or

slide along the bed.

traction saltation


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(iii) Suspension: very fine particle such as clay and silt (suspended

load) are dislodged and carried by turbulence in a fast flowingriver.

(iv) Solution: water flowing within a river channel contains acids (e.g.

carbonic acid from precipitation) dissolve the load such as

limestone in running water and removed in solution.

solution suspension


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River deposition

Deposition: when velocity begins to fall, it has less energy and no

longer had competence and capacity to carry all its load so largest

particles, materials begins to be deposited.

When occur?

1. Low discharge during period of low precipitation

2. Less velocity when river enter sea or lake.

3. Shallow 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)


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River erosion.Erosion: wearing away of river bed and bank.

There are four main process of erosion:

(i) Corrasion: occurs when the river picks up materials and rubs italong its bed and banks, wearing them away by abrasion,effective during flood. Major method by which river erodes bothvertically and horizontally.

Landforms: potholes. (turbulent eddies in the current can swirl pebblesaround to form potholes that are hollows in river bed and pebbles arelikely to become trapped)

potholes


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(ii) Attrition: As bedload moved downstream, boulders collide with

other material and the impact may break the rock into smaller

pieces. In time angular rocks become increasingly rounded.

(iii) Hydraulic action: The sheer force of the water as the turbulentcurrent hits banks (outside of meander) pushes water into cracks.The air in cracks compressed, pressure increased and in timebank will collapse.

Cavitation: is a form of hydraulic action caused by bubbles of aircollapsing.

(iv) Solution/corrosion: This process in independent of river dischargeand velocity. It is related to chemical composition of water e.g.

concentration of carbonic acid and humid acid.


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Hjulstrom curve


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Hjulstromcurve The Hjulstrm curve is a graph used by hydrologists to determine whether

a riverwill erode, transport or deposit sediment. The graph takes sedimentsize and channel velocity into account.

The curve shows several key ideas about the relationships betweenerosion, transportation and deposition.

The Hjulstrm Curve shows that particles of a size around 1mm require theleast energy to erode, as they are sands that do not coagulate. Particles

smaller than these fine sands are often clays which require a higher velocityto produce the energy required to split the small clay particles which havecoagulated.

Larger particles such as pebbles are eroded at higher velocities and verylarge objects such as boulders require the highest velocities to erode. Whenthe 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.
http://en.wikipedia.org/wiki/Graphhttp://en.wikipedia.org/wiki/Hydrologyhttp://en.wikipedia.org/wiki/Riverhttp://en.wikipedia.org/wiki/Erosionhttp://en.wikipedia.org/wiki/Erosionhttp://en.wikipedia.org/wiki/Riverhttp://en.wikipedia.org/wiki/Hydrologyhttp://en.wikipedia.org/wiki/Graph


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Velocity and Discharge:

Discharge: is the amount of water originatingas precipitation which reaches the channel

by surface runoff, throughflow and baseflow.

Q = A X V

(Q: discharge, A: cross-sectional area and

V: velocity)


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Velocity:

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

2. Velocity of a river is influence by 3 factors:

(i) Channel shape in cross-section.(ii) Roughness of the channels bed and

banks.

(iii) Channel slope.


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Patterns of flowAs water flows downhills under gravity, velocity decreases.

This is not only due to friction found along river bed andbanks, but also internal friction of water and air resistance

on the surface. There are two patterns of flow:

1.Laminar flow : horizontal movement of water

(rarely found), common in lava flow.

2. Turbulent: a series of erratic eddies, both vertical and

horizontal, in a downstream direction.


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3. Helicoidal flow: 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.


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Channel types

(a) Straight channel

(b) Braided channel(c) Meander channel


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Straight channels


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Braided channel

Braided reach of Lillooet River,

southwestern British Columbia.

What? A braided stream has islands/eyots of deposited material within the channel.

Description:Overall channel is straight with eyots and smaller channels.

It rapidly and frequently change position.

When?

It occurs when the load contain high proportion of coarser sands and gravel.

Area:

Semi-arid environment.


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MeanderWHAT? Bends in course of a river channel.HOW OCCUR?

1. Begin when a river approaches its middle course & gradient channelis less steep.

2. It results from helicoidal flow with faster current spirals downstream in

corkscrew fashion. Movement result in erosion on outside bend of meander to

form river cliff and deposit on inside bend called slip off slope.

CHARACTERISTICS:

1. River cliff on outside of bend and gentle

sloping slip- off slope called point bar on

inside of bend of meander.

Read textbook (Geography An integrated approach, Waugh, D., p:79)

write briefly formation of Meander and ox-bow lake.


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Riffle and pools

Riffles: deposition of a coarse material that create

areas of shallow water.

Pools: areas of deeper water between riffles.

Pools and riffles developed in section along river channel

which create different gradient of channel.

Coarse pebbles create steeper gradient than eroded pools.


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Fluvial landforms

Effects of fluvial erosion:1. (a) V-shaped valleys

A river erode vertically by traction or saltation which resulted in a steep-

sided valley called a V-shaped valley.

Steepness of valley sides depend of factors such as:

(i) Climate: valley are steeper where there is sufficient rainfall. (for

mass movement and allow river to transport bedload and erode

vertically)

(ii) Rock structure: resistant, permeable rocks such as limestoneproduce vertical sides.

(iii) Vegetation: it helps to bind soil together and keep the hillslope

more stable.


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(b) Interlocking spurs

It forms because the river is forced to follow a winding course around

the protrusions of the surrounding highland, resulting in spurs interlock.

Interlocking spur.
http://en.wikipedia.org/wiki/Image:Interlocking_spurs%2C_Ashes_Hollow.jpg


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2. Water fall:

A waterfall form when a river, after flowing over relatively hard rock

meets a band of less resistant rock flow over the edge of a plateau.

Over a period of years, the edges of this shelf will gradually break awayand the waterfall will steadily retreat upstream, creating a gorge of

recession

Havasu fall, Arizona.
http://en.wikipedia.org/wiki/Image:Havasu_Falls_1a_md.jpghttp://en.wikipedia.org/wiki/Image:Waterfall_formation23.png


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(3) Rapids:

Rapids develop where the gradient of the river bed increases without a

sudden break of slope (as in a waterfall) or where stream flows over a

series of gently dipping bands of harder rock. Rapid increase the

turbulence of a river and hence its erosive power.


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Effect of fluvial deposition.

Deposition of sediment takes place where there is a decrease in energy

oran increase in capacitywhich makes the river less competent to

transport its load.

It can occur anywhere from upper course, where boulders may be left,

to the mouth where fine clays may be deposited.

FLUVIAL LANDFORMS:

Floodplains:

A floodplain is a mostly flat area of land bordering a river subjected to

periodic flooding. It is made of silts and sands which have beendeposited over many years by the river.


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(2) Levees

When river overflows its banks, the increase in friction produced by the

contact with the floodplain causes material to be deposited. The

coarsest material is dropped first to form a small, natural embankment

(levee) alongside the channel. During subsequent periods of low

discharge, further deposition will occur within main channel causing

bed of the river to rise and the risk of flooding to occur.

Floodplains and levees.


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(3) Braiding channels:

For short periods of year, some rivers carry a very high load in relation

to their velocity e.g. during snow melt periods in Alpine or Arctic areas.

When a rivers level falls rapidly, competence and capacity are

reduced, and channel become chocked with material, causing the river

to braid ( divide into a series of diverging and converging segments)

Braided channels.
http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dbraided%2Bchannels%26ei%3DUTF-8%26fr%3Dyfp-t-460%26x%3Dwrt&w=142&h=107&imgurl=www.state.sc.us%2Fforest%2Fchannel.gif&rurl=http%3A%2F%2Fwww.state.sc.us%2Fforest%2Fbraid.htm&size=4.2kB&name=channel.gif&p=braided+channels&type=gif&no=20&tt=111&oid=041a880a842ae728&ei=UTF-8http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Dbraided%2Bchannels%26ei%3DUTF-8%26fr%3Dyfp-t-460%26x%3Dwrt&w=400&h=250&imgurl=z.about.com%2Fd%2Fgeology%2F1%2F0%2F7%2FO%2Fbraidedriver.jpg&rurl=http%3A%2F%2Fgeology.about.com%2Flibrary%2Fbl%2Fimages%2Fblbraidedstream.htm&size=54.8kB&name=braidedriver.jpg&p=braided+channels&type=jpeg&no=18&tt=111&oid=efe7dd2938878916&ei=UTF-8


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(4) Delta:

It is composed of fine sediment which is deposited when a river losses

energy and competence as it flows into an area of slow moving water

such as a lake or sea. The shape resembled that of delta, the fourth

letter of the Greek alphabet ( )

Delta provide worlds fertile land, while shallow and frequently changing

river channels hinder navigation.


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There are three types:

(a) Arcuate (fan-shaped delta) : having rounded, convex outer margin e.g.Nile.

(b) Cuspate (tooths delta) : where material brought down by a river is spread

out evenly on either side of its channel. E.g. Tiber(c ) Birds foot: where the river has many distributaries bounded by sediment

and which extent out to sea like the claws of a birds foot.e.g. theMississippi.

Arcuate delta e.g. River Nile. Birds foot e.g.Mississippi river.

Cuspate delta e.g. Tiber.
http://images.search.yahoo.com/search/images/view?back=http%3A%2F%2Fimages.search.yahoo.com%2Fsearch%2Fimages%3Fp%3Driver%2Bdelta%26ei%3DUTF-8%26fr%3Dyfp-t-460%26x%3Dwrt&w=1500&h=1001&imgurl=www.stchas.edu%2Ffaculty%2Fewilson%2FPhotos%2FMidwest%2FMississippi%2520River%2520Delta%2520001.jpg&rurl=http%3A%2F%2Fwww.stchas.edu%2Ffaculty%2Fewilson%2FPhotos%2FMidwest%3FC%3DS%253BO%3DA&size=596.3kB&name=Mississippi+River+Delta+001.jpg&p=river+delta&type=jpeg&no=8&tt=38,957&oid=5b39bf9359973556&ei=UTF-8


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FLOOD

Causes

Impact

Management Case studies


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FloodingCAUSES (Human and physical factors)

(I) PHYSICAL FACTORS:

When does flooding occur? Water overflows river banks onto surrounding area.

Occur when water available is more than infiltration capacity.

When does water overflow?

1. Intense precipitation

2. Prolong rainfall in saturated soil.

i.e. clay prone to overlandflow (smaller pores)

Soil already saturated thus reduce infitration capacity. (prone to flood)

3. Sudden increase in temperature (rapid snow melt)

(II) HUMAN FACTORS:

1. Dam burst2. Land use (drainage system, digging ditch, ploughing up and down slope)

3. Urbanisation (land made impermeable in road building)

4. Deforestation.


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Impact of flooding (case study: uk and brazil)Reference: AS Geography Pallister J. & Bowen, Ann, p: 34-38)

Flood management (Colorado & Bangladesh)

Ref: AS Geography, Pallister J. & Bowen, Ann, p:39-43)

IMPACTS AND FLOOD MANAGEMENT

Read from textbook below, draw a mind map/concept map

river channel processes landforms 1234591289846057 3

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