Drainage Basin
Part II
Geomorphological Process
Contents
Weathering Subsystem
Slope Subsystem
Channel Subsystem
Weathering Subsystem
What is weathering
Physical weathering
Chemical weathering
Biological / Organic weathering
Case study – weathering in desert
What is weathering
Weathering refers to the process of
disintegration (physical break down) or
decomposition (chemical decay) of solid
rocks in situ at or near the earth’s
surface.
It is a very slow process.
Main condition affecting the scale and
intensity of weathering
Characteristics or rocks
Mineral composition, joints, hardness of the rocks
Climatic conditions
Changes of temp. intensity of precipitation,
frequencies of freezing,….
Vegetation cover
Types and density of vegetation cover….
Topographical conditions
Gradients, south-facing slope…., windward an
leeward slopes…..
Physical weathering
It is the disintegration of solid rocks into smaller
fragments without involving any change in the
chemical composition of rocks
Changes in size and shape
It encourages chemical weathering by increasing
rock’s surface area.
It is the most predominant in areas with great
diurnal range of temperature. (desert regions and
high mountains)
Conditions for mechanical weathering
Alternate heating and cooling.
Different degree of expansion.
• Bare rock surface are highly heated by sunshine, the outer
layer expand , but the inner layer expand little.
• The different degree of expansion develops a series of joints
on the exposed surface, rock tends to break into blocks
• Different minerals have different coefficient of expansion.
• The alterante heating and cooling produces alternate
expansion and contraction lead to the cracking and granular
disintegration.
• During the night time, cooling by raditation causes water
freezes in joints, which exert an enormous expansive force.
Freeze and Thaw action:
• The volume of water increase 8% when it freezes, and exerts a
pressure force of about 200kg per m2.
Conditions for mechanical weathering
Alternate of dry and wet:
Water absorption causes rocks to swell but when
they dry out they contract. Alternate wetting and
drying results alternate expansion and contraction.
Pressure release / unloading:
All intrusive rock are formed under great pressure.
When they exposed to the air through erosion
process, they will break-up for pressure release.
Plants and animals:
They constitutes another important mechanical
force to disrupt rocks.
Types of Physical weathering
Exfoliation / Onion Scaling
Rocks with homogeneous structure.
Repeated heating and cooling by daily temperature
changes.
Repeated expansion and contraction create stress in rock
and produce radial and concentric cracks.
The outer layers eventually peel off to form exfoliation.
Types of Physical weathering
Granular Disintegration
Rocks composed by rather coarse grains or
heterogeneous structure.
Repeated heating and cooling by temperature changes,
which causes alternate expansion and contraction.
Minerals disintegrate grain by grain.
Types of Physical weathering
Block disintegration
Well-jointed rock (eg. Granite)
Great diurnal range of temperature with more than 10oC
between freezing point (0oC)
Repeated heating and cooling by temperature changes,
which causes alternate expansion and contraction.
Cracks and joints are widened or breaks down into small
blocks or fragments.
It may be accompanied by frost action and chemical
weathering.
Block disintegration
Types of Physical weathering
Frost action / Freeze and Thaw Action
Diurnal range of temperature fluctuating above 0oC.
During daytime, water seeps into cracks or joints of rocks.
Night time, temperature drops to below freezing point,
water freezes in joints or cracks, which expands and
widen the cracks.
Alternate freez and thaw, the rocks break down into
smaller, angular fragments
Freeze thaw action
Freeze thaw action
Chemical Weathering
Introduction
Processes of Chemical Weathering
Types of Chemical Weathering
Chemical weathering
Introduction
Chemical weathering is the decomposition or decay
of solid rocks as a result of chemical reactions
between the rock minerals and moisture, rain water,
sea water and organic acids produced by plants and
animals,
Chemical weathering can be accelerated by high
temperature.
It also can accelerated by physical weathering which
breaks rocks up and increase the surface exposed to
possible chemical weathering.
Processes of Chemical weathering
Oxidation
It is the process of the combination of oxygen.
Hydrolysis
Free hydrogen ions in water enter into the mineral structure and
create a new compound.
Eg. Feldspar in Granite to Kaolinite.
Hydration
The whole water molecule combines with the mineral.
Carbonation
Carbon dioxide (rain water pH=5.7) is capable of reaction with
certain minerals
It is particularly effective in limestone with humid climate.
Solution
Soluble minerals (rock salt,…)are dissolved directly in water.
Types of Chemical weathering
Spheroidal weathering
Chemical reaction is affected by penetrating water.
A well-jointed rock allows this to go on reading.
Honeycomb weathering
Rocks with
heterogeneous structure
and containing soluble
minerals.
By the processes of
oxidation, hydrolysis,
hydration, carbonation
and solution.
Very common in coastal
area.
Honeycomb weathering
Features Produced by Chemical
Weathering
Weathering Profile of Granite
Ground surface is weathered for a longer period.
Further inwards, the rock remains more fresh and
stable. And It is possible to see a graduation of four
weathered zones.
Weathering Profile of Granite
Weathering Profile of Granite
Tors
Tors
Tors may be the result of the surface rotting of
granite through the action of acidulation
rainwater penetrating along joints into the body
of the granitic mass.
The pattern of tors is controlled by the joints.
When the overlying weathered materials were
moved away (eroded) and the corestones
exposed to the air, tors are formed.
Tors
Biological / Organic weathering
Biological weathering is the physical
disintegration or chemical decomposition of
rocks in situ by organic agents – plants and
animals.
It is effective in regions with a continuous
vegetation cover and burrowing animals.
In desert regions and polar regions, it is
insignificant for limited plants and animals.
Biological weathering - plants
Biological weathering - Plants
The growth of plants roots and their
penetration into rocks are sufficiently
effective to widen cracks and joints.
Rocks may be weathered by orgranic
acids secreted by roots of plants and
from decayed plants.
Biological weathering - animals
Animals
Burrowing animals may dig or turn up and loosen the
joints of rocks.
Earthworms and termites also loosen and expose the
surface materials for weathering.
Wastes secreted by animals or derived from dead
animals (organic acids) help chemical weathering
Human
Human activities often cause large scale disintegration
of rocks (mining, quarrying, excavation for building….)
Careless removal of vegetation by man exposes large
surface area to weathering processes. (deforestation
for farming, for lumbering, for firewood; abandon
farmland, overgrazing, hill fires,…..)
Case study – weathering in desert
Traditional concept:
Little rainfall, strong winds and large daily range of
temperature.
Mechanical weathering (block disintegration and
granular disintegration ) is dominant.
Exfoliation is very common in deserts.
Chemical weathering only takes place by the drawing
of strong solutions to the surface by capillary and
forms duricrust (a hard compact layer) on the land
surface.
Case study – weathering in desert
New founding
Barton:
• He found that weathering of the stonework was in general more
pronounced in the Delta than higher up the Nile Valley where has
the maximum heating and cooling effects.
Griggs’s experiment:
• He gave the granite to 90000 fifteen-minutes cycles of alternate
heating and cooling over a temperature range of nearly 90oC.
• He found the rock totally undamaged at the end.
• When he gave some water in the experiment, the whole block
very quickly disintegrated.
Chemical weathering involving water was the real
destroyer of rocks in the desert regions.
Weathering regions - 1
Weathering regions -2
Slope Sub-system
Contents
Slope as a system
Slope Profiles
Slope development / Slope evolution
Geomorphic processes on a slope
Slope as a system
All landforms are made up of slopes.
They originate by a combination of
tectonic (endogenetic) and erosional
activities (exogenetic).
Slope as a system
Slope can be divided into 3 types:
Tectonic slopes:
• Formed by earth movement: folding and faulting.
Erosional slopes:
• Primary slopes:
– the slope reduced by agents of erosion. Eg. Slip-off slope in
river, U-shaped valleys in glaciated area, cliffs in coastal region.
• Secondary slopes:
– modified by weathering or mass wasting.
Depositional slopes:
• It is formed by aggregation and may be either convex or
concave. Eg. barchan, sand dunes in desert areas
Slope as a system
Inputs of slope
Energy:
• Solar radiation, falling raindrops, winds
Mass:
• All forms of water
– Rainfall, snow-melt, springs and seepages
• Inorganic minerals from bedrock
• Organic materials from vegetation and animals
Outputs of slope
Energy:
• Loss of heat
Mass:
• Water, weathered debris, solutes and organic waste, which
leave the system by streams or other transporting media (eg.
Wind) at the slope base.
Slope as a system
Slope systems are sustained by inputs
of energy and mass, which may be
balanced by outputs, giving a steady
state or equilibrium condition.
Slopes are controlled by increased or
decreased inputs and outputs, so as to
maintain its equilibrium.
Slope as a system
Slopes reflect the interaction of 3 factors.
Earth movement (tectonic activities)
• Rate of uplift or subsidence
Rock types
• Resistance to weathering and erosion
Weathering and transport processes operating
on the slope
• Vegetation cover, animals and human activities
Effects of agents (erosion & deposition):
• Running water, glaciers, winds, sea waves….
Slope elements
Slope profiles may be divided into a
series of slope units for analysis
Four units model
A. Wood divided a slope into 4 elements
Waxing slope:
• It is the convex curve of the hill crest.
Free Face / Cliff
• It is a vertical or very steep rock-face.
Constant slope:
• It maintains a constant angle of rest.
• It is formed by the debris fall from free face and gradually
accumulates to building up a heap of scree (talus).
Waning slope:
• It is below the constant slope, which is formed by fine materials.
• It is also the washing slope because it is derived from the
material washed down from constant slope.
• The low- angle wash slopes will gradually coalesce to form a
depositional pediment.
Slope units
Talus or Scree
Nine units model
Slope development / evolution
The forms of slopes develop through time
and the factors (rock structure, lithology,
soil, climate, vegetation and human
activities).
There are few models of slope
development or slope evolution.
Slope Decline
Slope replacement
Slope parallel retreat
Slope decline
American geographer, W.M. Davis (1899)
From NW Europe and NE USA
Normal (Humid) climates
Concept of the “cycle of erosion”.
Steepest slopes at beginning of process with a
progressively decreasing angle in time to give a
convex upper slope and a concave lower slope.
Slopes will continuous to decline and develop or
evolve from youthful stage, maturity stage, old
age stage and finally to become a low relief
peneplain (almost a plain).
Slope decline
Slope replacement
By W. Penck (1924)
Evidence from the Alps and Andes (tectonic
areas)
The slope of maximum angle decreases as the
gentler lower slopes.
Waxing slope will be replaced by free face
Free face will be replaced by constant slope
Constant slope will be replaced by waning slope.
Slope replacement
Slope replacement
Slope Parallel Retreat
By L.C. King (1948, 1957)
From South Africa
Semi-arid regions and sea cliffs with wave-cut
platforms
Sedimentary rocks structure
The slope units retreat by the same amount
(proportion) so that the whole profile retains but
leaves an extending concave unit (pediment) at its
foot.
This sequence is controlled by the rate of retreat of
free face which is controlled by geology and
climate (weathering and transport processes)
Slope Parallel Retreat
Slope Parallel Retreat
Slope Parallel Retreat
Geomorphic Processes on Slope
Weathering, mass movement and erosion
are the major processes in shaping slopes.
On hard rocks which weather very
slowly – weathering limited slope
High potential weathering but outputs
from slope are restricted – transport
limited.
Slope as a process-response system
Mass movement - landslide
Mass movement involve the transport of debris
under the influence of gravity.
It depends on the instability (stresses and strength)
of slope.
Loss strength (frictional resistance and cohesion)
Heavy rainfall
Severe weathering
Oversteepening of hillslope (excavation)
Increase stress
Heavy rainfall (increase weight of slope)
Earthquakes or volcanic activities
Building on slope increase loading
Vibration from passing heavy vehicle
Landslides
Landslides
Slope processes(non-cohesive materials)
Gravels and sands
Landforms: alluvial fans, screes (talus),
sand dunes and glacial outwash features.
The movement occurs largely through the
sliding or rolling of individual particles.
It may be triggered by minor events such as
rainfall or vibration.
Alluvial fan
Slope processes (cohesive materials)
Soil and clay
The cohesion is derived from electrochemical bonds between fine particles and
the surface tension effects of water films
in the pore spaces.
Main features
Rapid movement
• Slumping and mudflows
Slow movement
• Soil creep and solifluction
Slumping / Rotational slips
It occurs along clearly-defined concave
(curved) sliding plane.
Very common in humid climate.
Mudflows
It may operate on very low angles slope.
High moisture content of the material reduces the strength
(frictional resistance and the cohesion almost to zero.)
Very common in desert after heavy rain and volcanic
eruption with heavy rainfall.
Soil creep
It can be found on all slopes
It was formed by the effects of gravity,
temperature fluctuations (Freeze-thaw) and
variations in moisture (wet-dry periods) content
within the soil may all act to cause displacement
of particles.
Soil creep
Slope processes in hard-rock slopes
Rockfalls
It builds up a constant slope (talus or
scree)
Soil erosion
Water plays a significant role to soil erosion
in humid regions
Rain splash can redistribution the materials
without any transport on horizontal surface.
It can move the materials downslide on a
slope
Rain splash
rills
gullies
badland
Rills
Gullies
Badland
Other forms of slide
Channel Sub-system
Channel as a sub-system
Stream Velocity
Channel Processes (Geomorphological
work of stream)
Channel Form
Rivers work in the two landscapes
Channel as a sub-system
Stream channels are systems
Inputs:
• Water
– Direct precipitation
– From tributaries
– From seepage from the river banks
• Solid materials
– Debris from stream banks and bed
Outputs:
• Water
– Losses of water throughout the length of the channel by seepage
– By evaporation
– To sea
• Solid materials
– To sea
• Channel forms
Channel as a sub-system
Stream Velocity
It is the most important factor affecting
the channel / stream processes (Erosion,
Transportation and Deposition)
Mean velocity of a river increases
downstream.
Reasons – Along upper course
Channel gradient is great but river
velocity is low
Low discharge (Q=VA)
River course is irregular
Turbulence flow
• Channel floor is uneven and broken by potholes
• Much energy is needed to overcome the
roughness of channel floor.
Reasons – lower course
Channel gradient is small but river
velocity is increased
Rise in river discharge
• Joining of major tributaries
Straighter course in spite of the presence
of marked meanders
Laminar Flow
• Reduction in friction along the channel floor for
the deposition of fine sediments
Stream velocity
Erosion, transportation and deposition
Erosion, transportation and deposition
Critical tractive force
Minimum force to entrainment of grains from bedrock on
the sides or floor of channel.
Critical (erosion) velocity
Lowest velocity to obtain the critical tractive force (to move
grains from channel bed)
Erosion
The easiest eroded material is of diameter of about 0.5mm.
Coarse materials (medium sand to boulders) require greater
velocity.
Very fine materials are also difficult to erode for strong
binding by chemical bonds.
Smooth channel bed formed by fine materials is more
resistant to erosion.
Erosion, transportation and deposition
Transportation velocity
Between the critical erosion velocity and the curve
of deposition of particles
Fall /Settling velocity
Velocity at which materials in transport are
dropped and deposited on the channel bed.
It is high for the larger and heavier particles but
extremely low for clay (transported in suspension)
Channel Processes
(Geomorphological work of river)
The morphology of natural river channel
is determined by the interaction of
flowing water and solid materials.
Channel Processes
Erosion
Transportation
Deposition
Erosion – along the river courses
Stream erosion is the progressive removal of
mineral material from the floor and sides of the
channel, whether bedrock or regolith.
Upper Course
Erosion is the dominant process for the steep
channel gradient.
Middle Course
Erosion is a bit reduced as some depositions occur
where channel beds are flat.
Lower Course
Erosion becomes far less important than deposition.
Erosion – erosion processes
Hydraulic action:
Removal of loose materials by direct force of impact
of running / flowing water.
Abrasion (Corrasion):
Mechanical wearing and tearing of rock particles at
or being dragged along the channel bed.
Corrosion (Solution):
Rocks minerals are dissolved by water.
Attrition:
Reduction in size of loads in transport as they strike
at each other or the channel bed.
Erosion – erosion direction 1
Headward erosion:
Erosion – erosion direction 1
Erosion – erosion direction 2
Lateral erosion
Erosion of the sides of a river channel
More lateral erosion is found along the
concave banks than along the convex slipoff banks.
More active along lower course for the
gentle slope.
The kinetic energy transfers to lateral
erosion.
Erosion – erosion direction 2
Lateral erosion
Erosion – erosion direction -3
Vertical erosion /
downcutting
It is the erosion and
subsequent deepening
of the floor.
It is more active along
upper course for steep
slope.
A narrow V-shape
valley is develop.
Erosion – erosion direction 3
Erosion – incised meanders
Erosion – incised meanders
Transportation
Weathering on channel side slopes and plains
produces loosened masses of materials that
can be washed into the channel.
Such moving materials are called loads
There are four types of load
Dissolved load / Soluble load
Suspended load
Saltation load
Traction load / Bed load
Transportation
Dissolved load:
They are enters the water current by corrosion, and is
transported in solution by the river water.
Chemical composition of river water depends on
• Topography
– Steep bank-side slopes is likely to be richer in dissolved minerals.
• Climate
– High temperature can increase the rate of chemical reaction.
• Geology
– Rocks minerals are dissolvable or not.
• Vegetation
– Supplier of organic matter.
Transportation
Suspended load
They are carried downstream by the irregular
turbulent in suspension.
Saltation load
They are moved forward by the water current in a
series of leaps and bounds.
Traction load
The larger fragment are moved by the water current
in rolling and sliding on the river bed.
Transportation
Stream Competence
The largest transported particles that a stream
is able to move in traction as bed load.
Normally, stream channel is less competent in
removeing coarse bedload materials because
coarser materials are heavier and more irregular
in shape such that much energy is needed to
overcome their friction.
Stream channels are more competent in
removing bedload at high water flow level for
increasing in river discharge.
Stream Capacity
It is maximum amount of load materials
that a stream can transport.
It varies according to velocity which
depends on channel gradient, stream
discharge and weight of load.
Loads
Suspended sediment:
It tends to increase with discharge levels at any
point on a stream.
Solute:
It is the greatest concentrated at low flows, or
channel water is derived entirely from
groundwater seepage.
During periods of higher discharge, solute will
be diluted by throughflow and overland flow.
Bedload:
It increases at higher levels of discharge.
Loads
Three types of load varies according to
the nature of load available, the
discharge and courses of the river.
Solute load being more important at low
flows.
Suspended sediment is transported in
greater at flood time.
Bedload moves only once a threshold level
of discharge has been attained.
Deposition
The deposition of sediments occurs
when a stream is no longer to carry
loads, loss of competence or
transporting ability.
It may be the result of
Decrease in channel gradient
Decrease in discharge volume
Increase loads supply in the channel
Deposition
Decrease in channel gradient
Stream water enter to its lower course at flat plain
Stream water enter into a pre-existing depression
(lake, lagoon, or any type of still water…)
Decrease in discharge volume
Discharge will decrease in dry season
It will decrease by river capture.
It will decrease by seepage which is very common
along exotic rivers (River Nile), along a permeable
channel floor (desert) or flowing into limestone areas.
Increase loads supply in the channel
Excessive increase in the supply of load materials
(serious mass movement)
Channel Form
The channel form can be regarded as
the response by the channel inputs.
It can be considered in terms of
Cross profile
Long profile
Plan
Plan – Channel patterns
There are three major patterns:
Straight channel pattern
Meandering channel pattern
Braided channel pattern
Straight channel pattern
It refers to the channel characterized with straight
banks.
In fact, it is impossible of natural rivers with straight
banks for irregularities of river channels.
The stream line of flow is usually in a winding path
or sinuous pattern.
Sinuosity
Sinuosity = actual channel length / straight line distance.
Sinuosity
Meandering channel pattern
If forms when pronounced bends or loops
develop in the course of river.
It may be the result of
Helical flow
• When stream water moves in a winding pattern, it produces a
strong centrifugal force which causes a helical flow in concave
outside bank.
• It results in effective erosion in concave bank.
Slope gradient is reduced in lower course.
Large proportion of loads is carried in suspension form
There is significant local bank erosion and deposition.
Braided channel pattern
River channel is subdivided into two or more
bifurcated channel which are separated by bars of
alluvial materials (usually visible in dry seasons).
It occurs when
Too many bedloads in the river
The bedloads are too coarse to move.
The discharge is variable
Long Profile
The graded river which is capable of
existing in a state of balance, or
dynamic equilibrium, with the rate of
erosion being equal to the rate of
deposition.
Long Profile
Cross Profile
Discharge increases in a downstream direction.
The channel adjusts to the change by deepening and
widening (cross-section form)
The width / depth ratio also changes (increases)
downstream as width increases more rapidly than depth.
Work done by rivers in the two
landscape
Tropical Rainforest
Tropical Desert
Tropical Rainforest
Intense chemical weathering under the hot humid
condition.
Erosion:
Small streams of steep gradient have bouldery channels
but the boulders experience little downstream
movement.
Lateral erosion is very serious in middle and lower
course for gentle slope and large discharge.
Loads and transportation
Greater portion is in the form of dissolved solids.
Little coarse sediments move as bed load, because the
cover of vegetation holds back all
Finest soil particles from transport by overland flow.
Tropical Rainforest
Deposition:
Deposition is very common in the lower
course, meandering channels, flood plains
and deltas can be found in lower lower
course.
Tropical Desert
Introduction
Streamflow characteristics in the arid
lands
Streamflood
Introduction
River work is very effectiveness because of
the meagreness of vegetation in dry desert.
Without a thick vegetation , large quantities
of coarse rock debris are swept into streams
to transform a dry channel into a flood
stream.
Sediment concentration can be extremely
high up to 50%.
Stream flow in deserts usually accounts for
most of the water from precipitation.
Introduction
Much of the precipitation is held as surface
runoff rather than infiltrating deeply into the
ground:
Reasons
Lack of vegetation
Lack of water-absorbent organic layer in desert soil
Presence of hardpans (salt cover) in the topsoil.
Clayey surface of some deserts.
Rainfall is too intense for a large amount of water to
percolate.
Streamflow characteristics
Precipitation tends to run off into wadis which
are normally dry out occasionally subjected to
large flows of water and sediment.
Rock-cut gorges may developed on pediments
and fans at the foot of mountains.
Floods are rather occasional and even fewer
floods in the almost totally arid parts.
Streamflood
Rainfall in deserts in usually of high intensity,
most of the rainfall is available as runoff and
enters the dry stream courses.
Where streams flow across plains of gravel and
sand, water is lost from the channels by
seepages to water table as underground water.
The role of water in erosion, transportation,
deposition should not be ignored or made in
secondary to the action of wind, once though to
be the most important geomorphological agent
in desert .
Geomorphological work of
stream-floods
Large amounts of debris due to weathering, slow
mass movements and wind action.
Debris of all sizes from clay particles to boulders
are moved until eventually they reach the upland
edges.
Stream floods can also cause erosion of stream
channels themselves.
Streamfloods in deserts are more important than
the floods of humid regions in erosion. It is
because weathered materials are not cohesive in
dry environment and few plants to hold the soil
together.
Geomorphological work of
stream-floods
Lateral erosion of wadis channels is also due to
streamfloods.
Drainage density is very high (eg. 350 km/km2 in
parts of arid North America), but in sandy deserts
with high infiltration rate and hence little runoff,
drainage intensity will be low.
Deposition occurs and braided channels are
conspicuous because of the heavy load of sediments
carried by the streams.