Fluvial process and landforms:

A river channel is part of a drainage basin the hydrological system.

A river channel is an open system which interacts with other open system, particularly the hill slope system.

Water in a river has potential energy created because it is above base level (sea level).

Potential energy of a river get change into kinetic energy as it flows down a hill slope.

In this conversion process about 95% of the energy is loss to overcome friction; external friction between the water and banks, bed and air and internal between eddies of water within the river.

The remaining energy is available to transport and erode debris.

Three main river processes:1. Erosion2. Transportation3. Deposition

Erosion: wearing away of the banks and beds of the river.

Eroded material is transported as load before deposition can occur.

Ability of a river to erode depend on it’s energy. Energy of a river depends on its velocity and how well it can overcome friction.

A river’s velocity is influenced:1. Channel shape in cross-section2. Roughness of the channel’s bed and banks3. Channel slope

1. Channel shape in cross-section Q: Can you find the Hydraulic radius of Channel A, B and C.

Formula: HYDRAULIC RADIUS = Area of cross section

Wetted perimeter.

1

*Wetted perimeter : the total length of the bed and bank sides in contact with the water in the channel.

Hydraulic radius;Channel A: ______ Channel B:_____ Channel C: ________

Qn: Which river channel has the highest radius?_______________________________________________________

Hydraulic radius measures the efficiency of a river channel. The river with the highest Hydraulic radius is more efficient thus has more energy for transport and erosion.

During flood the hydraulic radius increases (peak at bankfull discharge) thus efficiency increases but as water overflow the efficiency decreases.

Find the Hydraulic radius of the river channel below at time of normal discharge and bankfull discharge

2

2. Roughness of the channel’s bed and banks

Smooth river channel Irregular/ Rough river channel

• Flowing water in a river channel encounters various forms of resistance, which reduces its energy.

• Which river has high velocity. Why?River in fig. A has high velocity since less energy is used to overcome obstacles and there will be less friction

If water flows in a smooth, straight channel at slow velocity its flow is described as laminar.

As river flow is usually in a rough, irregular channel, laminar seldom exists in reality; typical river flow is turbulent with chaotic water movement involving eddies.

Types of flowIn a straight river channel river flows at a slow velocity by laminar flow.

(i) Laminar flow: a horizontal movement of water rarely experienced by a river. Such flow if existed will travel over sediment on the river bed without disturbing it.

(ii) Turbulence flow, consists of a series of erratic eddies, both vertical and horizontal in a downstream direction.

(iii) Helicoidal: a corkscrew movement usually in a meander where material is eroded from outside of meander bend, transported and then deposited on inside of next bend.

3

Helicoidal

3. Channel slope .River velocity depends on gradient of slope but also influence by channel shape and roughness.

Steep gradient needed in upper course since river need to overcome greater loss of energy due to friction.

In lower course river losses less energy from friction since channel is smooth thus greater efficiency. It has larger cross section with greater discharge. That is why in lower course with gentle gradient, it could maintain the same or high velocity than upstream river.

River long profile.

4

*Smooth river profile (graded) : river is curve, steep in upper and gentle in lower course due to active erosion and deposition which removed irregularity.

River is said to be in equilibrium state when it reach a graded profile.

Graded profile is when slope is adjusted to provide just velocity needed for transportation of river load. Slope has to constantly adjusted through erosion and deposition to change discharge and load of a river.

When is a river most active to erode and transport its load? During heavy rain and snow melt. A stream channel is most effective when it is in bankfull condition.

RIVER EROSION:Four main process of river erosion:

1. Abrasion/Corrasion:What: When fragments of rock are rolled and dragged along river flow grinding and wearing away river bed and banks of a river.When active: River at bankfull/floodHow it looks like: The river appears brown, charged with sand and silt.

2. Hydraulic action: What: impact of moving water and its frictional drag on particles lying on river bed.

When & where:

5

(i) Hydraulic power is high below waterfall and rapids where it will cause rocks to fragments especially when joints and bedding planes, and lines of weakness are present.

(ii) Outside bend of river Hydraulic action lead to undercutting and collapse of river banks to form cliffs.

3. Corrosion/solution :What? Mineral in rocks dissolves in water that is slightly acidic and carried in solution.

When and where occur? Most active on rocks that contain carbonates such as limestone and chalk and silicate such as quartz.

4. Attrition: What? Rocks and pebbles strike one another as well as river bed thus reduce particle size of load.In many rivers, upstream area the loads are larger and has more angular boulders while in downstream area the loads are smaller and more rounded.

When river flows over bedrock the erosion of bedrock most effective by corrasion and result in pot holes.

When a river flows over alluvial channel erosion is effective by hydraulic action.

Attrition: material is moved along the bed of a river, collides with other material, and breaks up into smaller pieces.

Corrasion: fine material rubs against the river bank. The bank is worn away by a sand-papering action called abrasion, and collapses.

Corrosion/solution: rocks forming the banks and bed of a river are dissolved by acids in the water.

Hydraulic Action: the sheer force of water hitting the banks of the river.

Transportation:There are two main source of a river load;

1. 90% from weathering and mass movement.2. 10% from erosion of river bed and banks.

6

Amount of load transported by a river depends on its capacity and competence.

Both capacity and competence increases downstream as river discharge and velocity increases.

Ability of a river to transport depends on:1. Location or supply of materials2. Amount of material available3. Character of material e.g. boulder, gravel etc.4. Volume of water5. Amount of energy of a river.(A river has the greatest energy when it has maximum discharge/maximum flow and outside bend of meander)

The load carried by a river is of three types:1. Bedload: coarse fragment moved only slowly and under condition

of very high energy.

2. Suspension load : particles such as sand, silt and clay capable of being moved along flow of water.

3. Solution load : dissolved material carried along water.

Competence: maximum size or weight of material that a river can transport.

Capacity: total load measured in volume, mass or weight that a river is able to transport at a particular discharge or energy level.

7

Rivers move (transport) the eroded material downstream in four main ways:

(i) Traction: large rocks and boulders are rolled along the bed of the river.

(ii) Saltation: smaller stones are bounced along the bed of a river in a leap- frogging motion.

(iii) Suspension: fine material, light enough in weight to be carried by the river. It is this material which discolours the water.

(iv) Solution: dissolved material transported by the river.

Hjulstrom graph

What? The Hjulstrom graph is a graph used to determine whether a river will erode, transport or deposit of sediments depending on flow velocity.

Interpretation of graph:X-axis: size of particles in mm.

8

Y-axis: Velocity of a river in cm/s

The Hjulstrøm 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 which require a higher velocity to produce the energy required to split the small clay particles which have coagulated.

Larger particles such as pebbles are eroded at higher velocities and very large objects such as 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.

* Look at the graph and state at which velocity can a load such as sand, clay and boulder ‘deposited’ or ‘eroded’

Deposition:Velocity begins to fall as competence and capacity are reduced and load of largest material will be deposited.

When will deposition occur?When the discharge of a river is low (low flow) energy of the river will be reduced thus loss capacity to erode and transport result in deposition of load. This will make the channel to become narrower and shallower, split into separate channel as braided channel.

Condition for deposition to occur?1. Discharge is reduced, i.e. during dry spell.2. Velocity decreases as a river enters lake or sea.3. In shallow water such as inside bend of a meander.4. Load suddenly increase e.g. due to a landslide into river.5. River floods and overflow the river bank thus lower velocity and

deposition occur.

Load in long river profile.In upland areas river has large boulders thus increase wetted perimeter and turbulence.

Sand, clay and sand will be transported downstream and deposited in middle and lower course.

9

In downstream, during flood sand and silt are deposited in flat floodplain area. The finest clay and silt will be deposited at river mouth forming mud flats and delta.

Landforms formed by erosion.(A) V- shaped valley and interlocking spurs.(B) Pothole(C) Waterfall and rapid(D) Meanders and ox-bow lake(E) Pool and riffles.

(A) V-shaped valley and interlocking spurs. The channel of a river in its upper course is often chocked with large, angular boulders.

This bedload produces a large wetted perimeter which used up much of the river’s energy.

Erosion is minimal because less energy is left to pick up and transport material.

But during periods of heavy rainfall or after rapid snowmelt, the discharge of a river may rise rapidly.

As water flows between boulders, turbulence increases and may result either in bedload being transported by rolling or bouncing along river bed, or suspension.

The result is intensive vertical erosion which enables river to create a steep-sided valley called V-shaped valley.

10

The steepness of the valley sides depends upon several factors:(i) Climate -Valley is steeper when there is sufficient rainfall; for

mass movement to occur, create enough discharge to transport bedload and erode vertically or river cross desert areas to wash down valley sides e.g. Grand Canyon.

(ii) Rock structure - Resistant, permeable rocks such as limestone produce vertical sides in contrast impermeable rocks such as clay are likely to produce more gentle slopes.

(iii) Vegetation - Vegetation help to bind soil together and thus keep hillslope more stable.

Interlocking spur.Interlocking spur is formed when the river is forced to follow a winding course around the protrusions of the surrounding highland. As a result spur interlock.

(B) Pothole Active corrasion along stream bed create pot holes, especially in fast flowing water with strong eddying.

Potholes are cylindrical holes drilled into bedrock by turbulence high high velocity floe.

Eddying create shallow bowl which become occupied with small stones and pebbles.

Constant swirling of pebbles deepens depression into a pothole in a process called pothole drilling.

11

Adjacent potholes join together that deepen channel.

(C ) Waterfall and rapid.Waterfall result when outcrop more resistance rock, called cap rock, overlying softer rock.

Erosion especially hydraulic power in water concentrated in plunge pool at base of waterfall.

Waterfall become undercut and hard cap rock above collapse result in headward erosion of waterfall and formation of gorge of recession.

Rapids:

12

Rapids develop where the gradient of the river bed increases without a sudden break of slope (as in waterfall) or where the stream flows over a series of gently dipping bands of harder rock.

Rapid increase the turbulence of a river and hence its erosive power.(D) Meanders and ox-bow lake.

Meanders are bends in the course of a river channel.

It begins when a river approaches middle course and gradient become less steep.

Meander characterised with river cliff on outside of bend and a gentle slip-off slope, called point bar on inside of bend.

It is result from helicoidal flow in which fastest current spiral downstream in corkscrew fashion.

This result in erosion in outside of bend to form cliff and deposition in inside of bend to form slip-off slope.

The material eroded outer bank spiral downstream and deposited on inner bank build up or aggrades to form point bar deposits. This produces asymmetrical shape cross-section meander.

13

(F) Pool and riffle. The riffles are areas of deposition coarse material create shallow water.

Pools are areas of deeper water between riffles.

Pools and riffles developed along river channel that creates different gradient in channel.

Coarse pebbles create steep gradient than eroded pool.

Pool and riffles are equally space in natural and artificial channel.

Water velocity increases as it passed over riffles but flow more sluggishly out of deep pools.

Sequence pools and riffles developed with average spacing of 5 and 7 channel widths and entire channel meanders.

14

Meander migration:Meander change location overtime (migrate)

Move in to direction:1. Sideways-lateral erosion broadens floodplain and erode away

ending interlocking spurs.

2. Downstream-due to pattern of erosion in relation to thalweg (zone of fastest velocity, in channel flow. Greatest erosion downstream of midpoint in meander bend flow is strongest.

Formation of ox-bow lake:

15

As meander move downstream, one side of meander catch up with river downstream.

Eventually river breaks through beck of meander causing major river diversion.

River abandoned original meander channel in favour of shorterm steeper new route.

Cut-off form at time of high energy of river i.e. peak discharge during flood/bankfull condition.

Velocity reduce at entrance to former meander especially when floodwater subside result in deposition which seal off meander to leave an ox-bow lake.

Water in ox-bow lake is calm result in deposition of any sediment and overtime water in lake disappear through infiltration and evaporation leaving a meander scar.

Depositional landforms:(A) Levees(B) Floodplain(C) Delta(D) Braided channel

(A) Levee:High banks of silt close to river channel.

Formed by repeated river flooding most common in lower course of a river where there is floodplain.

Where river overflow banks increase friction between water leaves river channel and floodplain.

Water on river bank and valley floor shallow and velocity falls lead to deposition of load.

Coarser material deposited first build up natural embankment along channel called levees.

Time of low flow during dry season river also deposited sand and silt aggrading (build up) river bed.

16

This raises river and lead to increase river level above floodplain.

(B) Floodplain:Flat area of land either side of river form valley floor.

It develops in lower course of a river near the sea.

It composed of alluvium river deposited material and form most fertile soil for agriculture.

Width of floodplain of meander migrate and lateral erosion

Depth of alluvium on floodplain result in flooding.

(D) Delta:Areas of land at mouth of river jutting out into sea.

It is a flat areas of land crossed by many stream channel called distributaries.

Distributaries flanked by levees.

Levees joined together by spits and bars seal off shallow area water to formed lagoon.

17

Lagoon infilled by silt and sand to form marshes and eventually dry land colonised by vegetation.

Fertile delta such as Nile and Ganges Delta occupied by people for farming and settlement area.

Condition for Delta to form:1. River carry large load e.g. Mississippi River carries 450 million

tonnes of sediment into delta distributaries every year.

2. Material deposited faster than removed action by tides, waves and current.

3. Delta forms in calm sea with gently sloping sea bed.

4. River meets sea acts as a break slowing velocity and encourage deposition.

5. Salt in sea water on meeting river water generate electrical change cause particle to coagulate or stick together increase weight and encourage deposition called fluocculation.

6. River floods frequently in lower course depositing alluvium in delta, build up levees and create new distributaries.

Classification:1. Bird foot Delta2. Arcuate Delta3. Alluvial fans

18

(1) Bird foot Delta: Finger of deposited material extend out into sea along line distribution e.g. Mississippi in Gulf of Mexico.

(2) Arcuate Delta: Triangular shape with smooth coastline e.g. Nile Delta in Egypt.

(3) Alluvial fans: Fan-shaped landform similar to delta but deposited on land.Where:(i) Stream comes from steep mountain course which enters

flatter plain with low gradient e.g. Spitzbergen in Norway

(ii) Glacial, temperate and semi-arid environment where rapid changing stream discharge e.g. Rhone in Switzerland.

(iii) Stream flow across fan it often split into series of distributaries. Channel is wider and shallower reduce velocity and encourages deposition.

(iv) Fan-cone shaped and become broader and shallower away from apex. Gradient decreases due to variation in

19

size of material deposited. Coarse grain deposited close to apex, finer sand and gravel transported further to edge of fan.

(E) Braided channel:

Braided channel has islands or eyots of deposited material within channel.

Channel is straight although eyots and smaller channel rapidly and frequently change position.

It occurs in stream with load contain high proportion of coarser sand and gravel.

Braiding characteristics stream and river with very variable discharge in semi-arid environment, or glacier fedstream.

In semi-arid torrential downpour lead to overland flow create stream with high velocity and large load.

Rapid infiltration and evaporation lead to fall in stream volume and velocity result in deposition.

Stream fed with glacier has high discharge due to rapid melting ice in summer but low discharge in winter.

At time of high discharge stream capable of transporting large load.

But velocity fall stream capacity and competence is reduced.

Large load deposited to form eyots and cause stream to divide into a series of smaller channel.

20

Flood:When occur?When capacity of a river channel exceeded and water overflows river banks onto surrounding land.

Amount of water available more than infiltration capacity.

Flood is caused by both human and physical factors.

Physical causes:Overland flow result from:

(i) Intense precipitation Convectional thunderstorm rainfall intensity more than infiltration capacity, common in semi arid area. Huge convectional storm cause intense downpour.

(ii) Prolonged rainfall This lead to saturation of soil and overland flow.Infiltration rate depends on rocks and soil type.

21

Clay soil are prone to overland flow since it has smaller pores. Soil already saturated less infiltration occurs but high risk to flooding prolonged rainfall.

(iii) Sudden rise in temperature This cause rapid snowmelt, made worse if ground is frozen thus less infiltrate into soil.

Human causes:Human activities lead to high risk flooding.(i) Dam burst:(ii) Land management(iii) Building bridge and embankment(iv) Urbanisation(v) Deforestation

Things to do:

Can you find one case study of flooding in LEDCs and one from MEDCs. Flooding in Bangladesh in 1998 Flooding in Mississippi I 1993.

Include in these case studies:1. Causes-physical and human 2. Impact of flooding3. Respond to flooding-local people, local and national government

and aid from organisation.

22