Geomorphological Processes in a Drainage Basin:
A. Denudation:
Forces which lower the level of the land by wearing it away are called external forces.
The process is called DENUDATION.
Denudation represents all the processes of breaking up the surface rocks and removing
them.
Denudation consists of three components:
a. Weathering,
b. Mass wasting or mass-movement, and
c. Erosion.
B. The Weathering Subsystem:
1. What is weathering?
Weathering refers to the process of disintegration (physical break down) or
decomposition (chemical decay) of solid rocks in situ (in place) at or near the earth's surface.
It is a static process and little or no movement of material is involved. In the course of time,
solid rocks which are exposed to the atmosphere or near the surface are turned into rock
wastes.
Weathering is a very slow process. It marks the beginning of denudation. Loosened
rock fragments are prepared by weathering and are ready for subsequent movement under the
influence of gravity, or for removal by the agents of erosion (agents of gradation) - running
water, moving ice, ground water, waves and wind.
The main conditions affecting the scale and intensity of weathering include:
1. Characteristics of rocks such as the mineral composition, joints, and hardness of
the rocks, etc.
2. Climatic conditions such as the diurnal changes of temperature, intensity of
precipitation, frequencies of freezing, etc.
3. Vegetation cover such as the types and density of vegetation, etc.
4. Topographical conditions such as the gradients of slopes, exposure of the ground
to sunshine, windward and leeward slopes of the mountains, etc.
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There are three kinds of weathering according to the action of the agencies involved
and the resulting effects: physical, chemical and biological.
2. Physical weathering:
a. Introduction:
Physical/ Mechanical weathering is the disintegration of solid rocks into smaller
fragments without involving any change in the chemical composition of the rocks.
Physical weathering results from insolation, the growth of frost crystals and biological
agents such as burrowing animals and plant roots.
There are mainly physical changes in the size and shape of the rocks. The broken
rock fragments are usually angular debris. Physical weathering encourages chemical
weathering by increasing the rock's surface area and thus exposing more of the rock to
possible chemical weathering. Physical weathering is most predominant in areas with
great diurnal range of temperature, notably in desert regions and high mountains.
b. Conditions for Mechanical weathering
Mechanical weathering takes place under the following circumstances:
-
When bare rock surface are highly heated by sunshine, the outer layer expand
considerately, but the inner layers expand little. The different degree of expansion
develops a series of joints on the exposed surface, and the rock tends to break into
blocks. The boulders produced from the disintegrated granite in the hills of Hong
Kong is one of the examples of this process.
-
The heat of the sun causes rocks to expand, and the subsequent cooling causes them to
contract. The alternate expansion and contraction lead to the cracking and breaking of
the rocks near the surface. from experiments, when temperature is 50oC the granites
expand about 2 cm per 30 metres in length.
-
Rocks may become soaked with water during the day, while during the night this
interstitial water freezes, exerting an enormous expansive force. Thus block
disintegration results. The volume of water increases about 8% when it freezes, which
exerts a pressure force of about 200 kg per square metre of water. Thus, the chance of
disintegration of rocks increases with the increasing frequency of freezing at a place.
-
The mineral constituents of the rock are complex, each of the constituents has a
specific Coefficient of Expansion, so that complex internal strains are set up, which
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cause the rock to fall apart into its constituent mineral grains. Generally speaking,
igneous rocks comprise minerals with different sizes crystals, so that internal strains
are high. In comparison, the sizes of various mineral crystals in sedimentary rocks are
rather similar, and the internal strains are comparatively low.
-
Alternate of dry & wet: All rocks can absorb water - the amount absorbed
depends on the mineral composition of the rocks. Water absorption causes rocks to
swell but when they dry out they contract. Alternate wetting and drying results in
alternate expansion and contraction, rocks weaken and begin to disintegrate.
-
Pressure release: All intrusive rocks are formed under the overlying rocks (in the high
pressure condition). They are exposed through the erosion of the overlying rocks, The
intrusive rocks break-up is the result of pressure release or unloading.
-
The slow prying action of plants and animals constitutes another important
mechanical force which tends to disrupt rocks.
c. Types of Physical weathering:
- Exfoliation / Onion Scaling:
Exfoliation is a process of physical weathering due to repeated heating and cooling as
a result of temperature changes. It is common in rocks composed of rather fine grains,
or in rocks with homogeneous structure. Intense solar heating in the daytime causes
the outer layers of a rock or boulder to expand outward more freely than the interior.
At night the temperature is lower. The outer layers contract, but do not sink back to
their original position. Repeated expansion and contraction create stress in the rock
and produce radial and concentric cracks. This leads to the formation of curved rock
shells which separate in succession from the original rock mass. The outer layers
eventually peel off, exposing the inner layers to the surface for further exfoliation.
Solid rock mass becomes disintegrated by this peeling process.
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-
Granular Disintegration:
Granular disintegration is a process of physical weathering due to repeated heating
and cooling as a result of temperature changes. It is common in rocks composed of
rather coarse grains, or in rocks with heterogeneous structure. Rocks fall apart or
disintegrate grain by grain. This produces a gravel or sand in which each grain
consists of a single mineral.
-
Block Disintegration:
Block disintegration is a process of
physical weathering as a result of
temperature changes
Rocks are intensely heated up in the
daytime and they expand. At night, in
the absence of cloud cover to act as a
protective blanket, rapid radiation
results in great loss of heat and the rocks
cool and contract. Due to repeating
daytime heating and night time cooling,
cracks and joints develop. Eventually the solid rock break down into smaller blocks.
This process is common in well-jointed rocks, eg. granite. It is particularly effective in
areas with great diurnal range of temperature of 10oC to 15oC, and barren rocks
without a protective vegetation cover. Continuous stress or pressure is set up within
the rocks as a result of repeated daytime expansion and night time contraction of the
rocks. Joints and cracks are widened and the solid rock mass shatters or breaks down
into small blocks or fragments. Block disintegration may be accompanied by frost
action and chemical weathering.
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-
Frost Action/ Freeze and Thaw Action:
Frost action is predominant in areas with daily temperature fluctuating above and
below 0oC, notably in high mountains in temperate latitudes.
During the daytime, ice thaws (melts) when the temperature rises above freezing point.
The water seeps into the cracks or joints of the rocks. At night, the temperature drops
to below freezing point. The water which has collected in the cracks or joints freezes
and expands, thereby widening the cracks. The repeated growth and melting of ice
crystals in the joints or fractures of rocks, or repeated and alternate freeze-and-thaw,
cause the rocks to loosen and break down into smaller, angular fragments.
3. Chemical weathering:
a. 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
sometimes organic acids produced by plants and animals. Such chemical processes are
accelerated by high temperatures.
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The original chemical composition and structure of the rocks are changed, and
new substances result. Such changes cause solid rocks to crumple in situ. Chemical
weathering is often accelerated by physical weathering which breaks rocks up and thereby
increase the surface exposed to possible chemical weathering.
b. Processes of Chemical Weathering:
- Oxidation: It is the process of the combination of oxygen ions in air, water or
moisture with metallic ions, such as iron, calcium and magnesium in rock minerals. (It
is a process by which compounds lose an electron) The presence of dissolved oxygen
in water in contact with minerals in the rocks leads to oxidation.
-
Hydrolysis: It is possibly one of the most important chemical reactions involved in
weathering. It occurs when free hydrogen ions in the water enter into the mineral
structure and create a new compound. The products of hydrolysis are not readily
reversible under atmospheric conditions.
eg.
Feldspar in Granite to Kaolinite
KAlSiO8 + H2O ---> HAlSi3O8 + KOH
2HAlSi3O8 +5H2O --> Al2(OH)+ SiO5 + 4H2SiO3
-
Hydration: It is a related process in which not just the hydrogen ion, but the whole
water molecule combines with the mineral. In the process volumetric changes take
place, setting up physical stresses and causing physical disaggregation.
-
Carbonation: Rain water or moisture contains carbon dioxide from the atmosphere.
Carbon dioxide in solution forms a weak carbonic acid (pH = 5.7) which is capable of
reaction with certain minerals. Carbonation is particularly effective in limestone
regions with a humid climate.
-
Solution: Some minerals, eg. rock salt, dissolve directly in water. By solution, soluble
minerals are removed from the rocks, causing the rocks to crumple.
eg. Limestone:
H2CO3 + CaCO3 --> Ca(HCO3)2
c. Types of Chemical Weathering:
- Spheroidal Weathering:
One result of chemical activity similar to physical weathering is known as spheroidal
weathering, and its effects are very similar to exfoliation. The outer shell of such
rocks as basalt is affected by penetrating water, and the chemical reactions is affected
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by penetrating water, and the chemical reactions cause this to swell or expand, and so
pull away from the solid core, presenting a fresh surface to the atmosphere. A
well-jointed rock allows this to go on readily, so that the blocks become more and
more rounded as each shell of decayed rock breaks away.
-
Honeycomb Weathering:
It is common in rocks with a heterogeneous structure and containing soluble minerals.
Water itself may dissolve certain minerals which are readily soluble in water - by the
process of solution. Some minerals may react with water by the processes of oxidation,
hydration and hydrolysis. Rain water or moisture contains dissolved carbon dioxide
and may weather some minerals by carbonation. Sea water may act like a salt spray
and weather some minerals. Ultimately weathered minerals are removed, leaving pits
or cavities on the rock surface.
d. Features Produced by Chemical Weathering:
- Weathering Profile of Granite:
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Berry and Ruxton have put forth the idea of a weathering profile of granite in Hong
Kong, the cause being that weathering penetrates inwards from the ground surface.
Therefore the ground surface is weathered for a longer period. Further inwards, the
rock remains more fresh and stable. Thus it is possible to see a graduation of four
weathered zones:
i. Zone I - It is structureless and consists of clay or clayey sand. Weathering of the
rocks is in the senile stage.
ii. Zone II - Corestones subordinate, rounded and free.
iii Zone III - Corestones dominate, rectangular and locked. Both zone II and III are
regarded as the mature stage of weathering or rock.
iv Zone IV - Minor residual debris along major structural planes, but may be
considerably iron-stained. Weathering of the rocks is in the early stage.
-
Tors:
Some recent work indicates that the 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. Thus the pattern of the tor is controlled by the joints, which
will leave between them broadly rectangular "corestones", this may well have taken
place in pre-glacial or interglacial times.
Where the jointing is
widely spaced, massive
corestones remain; where
the jointing is close, there
is more shattering and
rapid removal of debris,
thus forming depressions
between the tors. Then
followed a post-glacial
period of 'exhumation',
when the overlying
weathered material and the fine-grained products of rock-decay were removed by
solifluxion or by melt-water, thus revealing the tors.
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4. Biological/ Organic weathering:
Biological weathering is the physical disintegration or chemical decomposition of
solid 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,
where vegetation is scarce due to aridity and extreme coldness respectively, animals are also
limited. Biological weathering is insignificant.
a. By Plants:
The growth of plants roots, especially tree roots, and their penetration into rocks
are sufficiently effective to widen cracks and joints or rocks. Rocks penetrated by plant
roots may split apart.
Rocks may be weathered by organic acids secreted by roots of plants and from
plant decay. Mosses and lichens help decay the rock by maintaining moisture on rock
surfaces.
b. By Animals:
Rodents, eg. rabbits, moles, may dig or turn up and loosen the joints of rocks.
They tend to expose the underlying materials to weathering. Earthworms and termites
also help loosen and expose the surface material.
Wastes secreted by animals or derived from dead animals help weather the rocks
chemically. Organic acids secreted by micro-organisms also help decay the rocks.
b. By Human Beings:
Human activities often cause relatively large scale disintegration of rocks, eg.
mining, quarrying, excavation for building purposes, etc.
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The careless removal of the vegetation from the land by man often exposes
relatively large surface area to weathering processes, eg. clearing the forests for farming,
for lumbering, or for firewood; abandonment of farmland; overgrazing; hill fires, etc.
5. Relative importance of mechanical and chemical weathering in deserts:
In desert areas there is little weathering by ordinary leaching, but considerably
mechanical weathering, while chemical weathering takes place by the drawing of strong
solutions to the surface by capillary. Sometimes this process leads to a concentration near the
surface of various minerals, which form a hard compact layer known as duricrust. This is
especially common in semi-arid areas with a brief rainy season.
The relative absence of vegetation and soil exposes a large fraction of bedrock to
direct atmospheric weathering in many desert location, and in hilly ground the bare rock is
typically littered with rock fragments and residues. From these the wind constantly sifts out
the accessible finer particles as they form, carry much of the dust right out of the desert zone
and concentrating the slower-moving sand in particular localities.
Impressed by the enormous quantities of rock fragments, often marked angular, the
apparent total absence of water, and the temperature differences between day and night,
earlier observers concluded that the dominant agency at work must be thermal shattering or
block disintegration or granular disintegration. It was thought that such forces would tend
rapidly to break up and detach the surface layers of bedrock, and would continue to attack
loosened boulders and fragments to the extent of reducing them to granular form by breaking
the bonds between crystals or particles of minerals with different coefficients of thermal
expansion. Livingstone, with other early travellers, reported hearing rocks burst with
pistol-shot reports under this action and strains involved were quite sufficient to produce the
observed results. The same kind of process was invoked to explain the onion-skin exfoliation
so commonly to be seen on massive crystalline rocks in the deserts.
However, doubts began to cast on such action basing evidences in both laboratory test
and field studies. Barton, studying ancient monuments in Egypt, found that weathering of the
stonework was in general more pronounced in the Delta than higher up the Nile Valley, and
that on those parts which remained constantly in shadow, and least of all on surfaces directly
exposed to the sun and to the maximum heating and cooling. Blackwelder's field observation
in America produced a similar result.
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A further blow to the traditional theory was given by various laboratory experiments.
Griggs exposed a polished face of granite to some 90,000 fifteen-minute cycles of alternate
heating and cooling over a temperature range of nearly 90oC (far greater than anything
experienced naturally in the deserts) and found the rock totally undamaged at the end. Griggs
found that the whole block very quickly disintegrated. All these point out one thing, that
chemical weathering involving water was the real destroyer of rocks even in the desert
region.
At present, increasing volumes of evidences have grown up pointing concluding to
the great importance of chemical processes in the desert region. Moisture indeed plays a
major part in the slow development of larger-scale features such as honey-comb weathering
often seen on steep sandstone faces in wadis, and the deep tafoni cavities often found on
granite masses. Such hollows, which could not have conceivably have been cut out by sand
blast, or produced purely by mechanical weathering, most commonly occur where moisture is
likely to have lingered (stayed at a place).
Actually, evidences of chemical weathering has been found in the Sahara by Peel.
When climbing Gebel Kissu, a 1,600m granite peak in the southern Libya Desert, Peel found
that the granite was so rotted in places that they could be crumbled into sand immediately.
Deep cavities were also found on the undersides of granite boulders. The emergence of
incipient (beginning) rounded boulders from bedrock granite, and the rounding of those
boulders when detached all pointed to chemical processes of feldspar decomposition.
Moreover, subsurface weathering was also found to have taken place in the desert region.
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B. The Slope Subsystem:
1. Slope as a system
All landforms are made up of slopes which include all land surfaces, ranging from the
horizontal to the vertical. They originate by a combination of tectonic and erosional activity:
thus uplift or faulting will provide slopes, whilst valley sides, cliffs or depositional areas will
have slopes of subaerial process origin.
According to their origin, slopes can be divided into three types:
a. Tectonic slopes: This kind of slope is formed by earth movement, eg. folding, faulting.
b. Erosional Slopes:
i. Primary: It is the slope reduced by agents of erosion, eg. bluff or slip-off slope in river;
U-shaped valleys in glaciated area; cliffs in coastal region.
ii. Secondary: It is the slope further modified by weathering or mass wasting.
c. Depositional Slopes: This is due to aggregation. It may be either convex or concave, eg.
barchan, sand dunes in desert.
A slope receives inputs of energy ( from solar radiation, falling raindrops, wind).
Moreover, the very existence of the slope (with its difference of height between crest and
base) means that it possesses a 'store' of potential energy which is converted into kinetic
energy by way of rainwash and mass movements. The slope also receives inputs of mass, in
the form of water from rainfall, snow-melt, springs and seepages, inorganic minerals from the
weathering of bedrock, organic material from vegetation, and so on.
Just as it gains energy and mass, so the slope system loses energy and mass. The
principal output of energy is loss of heat. Outputs of mass include water, weathered debris,
solutes and organic waste, all of which most frequently leave the system by way of streams or
other transporting media at the slope base.
Slope systems are sustained by inputs of energy and mass, which may be balanced by
outputs, giving a steady state or equilibrium condition. Where the factors controlling slopes
undergo change, resulting in increased or decreased inputs and outputs, the slope system will
necessarily adjust to the new conditions to maintain equilibrium.
Thus, slopes reflect the interaction of:
- the rate of uplift (or subsidence);
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- the resistance to weathering of the underlying rock-type;
- the weathering and transport processes operating on the slope;
- the effect of running water, glaciers or the sea wearing away or depositing at the slope foot.
2. Slope profiles:
a. Slope elements:
Slope profiles are surveyed lines across the ground following the maximum slope,
and are usually carried out at points selected by sampling techniques. A slope profiles
may be divided into a series of slope units for analysis
A. Wood divided a slope into four elements
- Waxing Slope: It is the convex curve of the hill crest above the free face.
- Free Face: It is a vertical or very steep rock-face. As it weathers, the debris fails to its
foot and builds up a heap of scree which gradually accumulates to form the constant
slope.
- Constant Slope: It is so called because it maintains a constant angle of rest. It extends
upwards to the rock-cut slope of the free face. Thus the areas of the free face decreases
as the accumulation of scree grows.
- Waning Slope: It is below the constant slope. It is a slope of fine materials which
becomes progressively less steep and merges into the valley floor. It is also the
washing/ debris slope because it is derived from the material washed down as the scree
of the constant slope is weathered and broken. These low-angle wash slopes may
become more and more dominant and will gradually coalesce to form a depositional
pediment.
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Nine Unit Model
b. Slope Development:
If we look at the landscape around us, we can detect many different types of slope.
Their angles vary; so does their curvature. Some are straight (rectilinear), some are
concave, some convex. Many are composite in form. The manner in which slopes develop
through time, and the factors (rock structure and lithology, soil, climate, vegetation and
human activity) which influence the changes, are clearly complex problems to unravel.
Slope decline or parallel retreat may not be due to a systematic series of events in terms
of a rigidly followed cyclic sequence, but may be more closely related to a dominant local
factor, which may change with time.
c. Slope evolution
The history of slope studies has dwelt less on the measurement of form and
process followed by an analysis of their relationships, and more on a geometric view of
the evolution of a slope profile from its initial state to the present. Theories of this type
are related more clearly to a preconceived view of the way in which a landscape is worn
down, than to a foundation of observation.
A number of major approaches have emerged.
i) Slope Decline:
W.M. Davis, an American geographer, put forward the concept of the 'cycle of
erosion at the end of the 19th century. In this he envisaged an initial landscape
following uplift increasingly attacked by surface processes. This resulted in early,
'youthful features, followed by maximum relief in 'maturity' and an overall lowering
by 'old age' to a low relief landscape known as a peneplain ('almost a plain').
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Accordingly his ideas on slope evolution led to the smoothing of slope profiles and a
lowering of slope angle with time.
ii) Slope Replacement:
Penck suggested that the retreat of a free face above an extending debris slope would
be ultimately replaced by a lower angle debris slope, though during the period of
retreat it maintains its angle so that in this very limited sense there is an element of
localized 'parallel' retreat. The debris slope or talus will itself ultimately be replaced
by a still gentler slope produced when the transport of increasingly weathered debris
begins to extend a new depositional unit from the slope-foot. In principle, this
sequence can be further repeated, with each slope unit retreating until it is replaced by
a lower-angle unit growing up from below. Thus although the ultimate end-product is
a gentler profile, this has not been produced by the 'hinged' decline covered in the
previous model. Evidence of a slope replacement sequence would be that within a
given area, any specific unit such as the free face will occupy markedly different
proportions of the profile, indicating its consumption by gentler units.
iii) Parallel Slope Retreat
L.C. King, a South African,
favoured a different style of
landscape evolution, whereby
landscapes are reduced to
'pediplains' formed by
back-wearing and parallel
retreat of slopes. In these
cases, the free face appears to
occupy much the same
proportion of each profile
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whether it is associated with a major cliff line or a small erosional remnant (butte).
This suggests that each of the upper slope units retreats by the same amount so that
the whole profile retains its form but leaves an extending concave unit ( a pediment)
at its foot. Only this lower unit tends to reduce in angle with time. The most likely
explanation is that the development of this sequence is controlled by the rate of retreat
of the free face, which in turn can be attributed to the presence of a particularly
resistant rock stratum. Both geology and climate (through weathering and transport
processes) thus contribute to the sequence, which reaches its optimum development in
sedimentary rocks under semi-arid climate.
3. Geomorphic processes on a slope:
Weathering, mass movement and erosion are the major processes in shaping slopes.
Where inputs are the controlling factor - as on hard rocks which weather very slowly - the
slope is said to be weathering limited. Conversely, where the potential for weathering is high
but outputs from the hillslope are restricted, the system is said to be transport limited.
Mass movement involves the transport of debris for the most part not as individual
particles but en masses, under the direct influence of gravity. The operation of mass
movement processes depends on the development of instability (stresses exerted on the slope
materials exceed their strength) in the hillslope. Both stress and strength tend to fluctuate
over time, and it is these fluctuations which often trigger off slope failures.
Many factors may act as trigger mechanisms. One of the most common is prolonged
or heavy rainfall, for this results in saturation of the hillslope materials and a consequent loss
in strength. In the longer term, weathering also reduces the strength of materials. Changes in
the magnitude of hillslope stresses similarly occur. Earthquakes or volcanic activity may
cause tremors which give rise to short-lived but large increase in stress. Even the passage of a
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heavy vehicle may generate minor shock waves which trigger off slope failure. In addition,
loading of the slope by building may increase the stress, while oversteepening of the hillslope
- by natural erosion or by excavation - increases the gravitational force operating on the
material and reduces slope stability. Strength is derived largely from frictional resistance
and is therefore low failure often occurs at relatively low slope angles. Cohesion provides an
additional strength and thus even steep slopes may be relatively stable.
i) Slope processes in non-cohesive materials (gravels and sands):
Slopes formed from non-cohesive materials are characteristic of many landforms
constructed of coarse-grained sediments, such as alluvial fans, screes (talus) sand dunes
and glacial outwash features.
On slopes of this nature movement occurs largely through the sliding or rolling of
individual particles or through a process of shallow sliding. In the latter, planes of
weakness develop near the surface which reduce the resistance of the material and create
instability. Failure of the material along this plane may be triggered by minor events such
as rainfall or vibration, and the surface material rapidly slides en-masse downslope. The
shear plane tends to be relatively planar and shallow. Debris flow is a good example.
ii) Slope processes in cohesive materials (soil and clay):
In these cases, the materials have a degree of cohesion which makes them
potentially more stable than non-cohesive material. This cohesion is derived mainly from
the electro-chemical bonds which operate between the fine particles and the surface
tension effects of water films contained in the pore spaces. Both these sources of cohesion
are dependent on moisture content. This has considerable significance, for it provides a
means by which stability may be reduced in the short term, without any change in slope
form.
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Rotational slips (slumping) and mudflows are the forms of rapid movement. Soil
creep and Solifluction are the forms of slow movement.
Rotational slump occur along clearly-defined planes of weakness which develop
concave to the surface. It is very common in regions of humid climate. Some steplike
terraces may be caused by slumping. If the material is so saturated that it acts almost like
a fluid, mudflows may occur. The saturated material flows like a thick slurry downslope
until, as the water is lost through seepage, the flow solidifies and comes to a halt.
Mudflows may operate on very low slope angles, for the high moisture content of the
material reduces the frictional resistance and the cohesion almost to zero. Mudflow is
common after heavy rainstorm in desert. Mudflows also occur on the slope of erupting
volcanoes. Freshly fallen volcanic ash and dust are turned into mud by heavy rains and
flow down the slopes of the volcano.
Soil creep almost can be found on all slopes. Material in the weathered surface
layers moves gradually downslope in a series of jerky steps. The processes may be
powered by a variety of mechanisms. The effect 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 can be classified into two types,
discontinuous and continuous. In the discontinuous type, the two most important
mechanisms are the heave and settlement of particles due to freeze-thaw activity and the
shrinkage and expansion of material due to wetting and dry. Continuous soil creep is
largely independent of the moisture or temperature fluctuations, and is due , instead, to
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the gravitational force. Slow movement caused by freeze-thaw processes is often referred
to as solifluction.
iii) Slope processes in hard-rock slopes:
Hard rocks derive their strength almost entirely from the strong inter-granular
bonds, but weaknesses occur along bedding planes and joints. In relatively massive rocks,
small fragments may be loose by gravitational stresses, aided perhaps by freeze-thaw
processes, to give rockfalls or granular disintegration. The material then builds up as a
scree slope (talus) at the foot of the slope.
Water plays a significant role (Soil erosion) in the development of many slopes,
especially in humid temperate regions. Rain splash is a particularly widespread and effective
process. On a horizontal surface, the effect is to redistribution the material without any net
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transport, but on a slope there is a tendency for more material to be splashed downslide than
upslope. On relatively smooth surfaces, runoff occurs in the form of sheetwash. The erosive
power of sheetwash is limited because the sheet of water is shallow. By local topographic
irregularities, stimulates scour and leads to the development of small depressions. These
rapidly extend into rills which further concentrate flow and grow to form gullies. Over time,
much of this material is disgorged at the slope floor, where the gullies open out on to the
valley floor, and here the debris may build up to form an alluvial fan.
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Channel Sub-system
A. Channel as a sub-system:
Stream channels are systems. They receive, transport, modify and lose material. Water
enters the channel by direct precipitation, by inflow from tributaries and by seepage from the
banks. it brings with it dissolved and solid materials. Additional debris is eroded from the stream
banks and bed. These sediments are carried down the channel towards the sea; on the way, they
are occasionally deposited and stored. Losses of water occur throughout the length of the channel,
by seepage into the bedrock and by evaporation to the atmosphere.
The Stream System
B. Stream Velocity:
Stream velocity is the most important factor affecting the channel/ stream processes.
Generally, the mean velocity of a river increases downstream although channel gradient is the
greater along the upper course of a river. The reasons are as follows:
i) Along the upper course:
Channel gradient is greater but river velocity is not very great because:
- low discharge (Q=VA) along the upper course since tributaries are lacking and the
river level is shallow.
- the river course is irregular and not straight because of the presence of
interlocking spurs.
- the channel floor is uneven and is often broken by potholes. Much energy is
needed to overcome the roughness of the channel floor. (Turbulence Flow)
AL Notes (Drainage Basin System)
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ii) Along the lower course:
Channel gradient is small but river velocity is increased by:
- the joining of major
tributaries and the
subsequent rise in river
discharge.
- the possibility for the
river to flow along a
straighter course in
-
spite of the presence of
marked meanders.
a reduction in friction
along the channel floor
because of the
deposition of fine
sediments. (Laminar
Flow)
Stream velocity is an important determinant of which channel/ stream processes (Erosion,
Transportation and Deposition) will occur.
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i) Critical tractive force and critical velocity:
The entrainment of grains from bedrock on the sides or floor of a channel is
achieved by the critical tractive force of the stream.
The critical velocity is the lowest velocity at which grains of a given size lying
loose on the channel bed will move.
The easiest material to erode is of diameter of about 0.5 mm. Coarse materials
(medium sand to boulders) require greater velocity to move because of their greater
weight. Very fine materials especially clay are also difficult to erode because they are
bound together by chemical bonds that are difficult to overcome, and the smooth
channel bed these fine materials form which is resistant to erosion.
ii) Transportation velocity:
Between the critical erosion velocity and the curve of deposition is the zone where
particles are transported.
Much higher velocity is needed to initiate movement than to maintain
transportation of a grain of any given size. One velocity falls below a certain point
(settling velocity) the particles are dropped or deposited.
iii) Fall/ Settling velocity:
This is the velocity at which materials in transport are dropped and deposited on
the channel bed. The settling velocity is high for the larger and heavier particles but
for clay it is extremely low (transported in suspension).
C. Channel Processes (Geomorphological work of stream):
The morphology of natural river channel is determined by the interaction of the fluid flow
with erodible materials within the channel boundary. It is therefore necessary to have an
understanding in the processes of erosion, transportation and deposition.
a. Erosion:
Stream erosion is the progressive removal of mineral material from the floor and sides of
the channel, whether bedrock or regolith.
Along the upper course of the river, erosion is the dominant process because of the steep
channel gradient, widespread occurrence of exposed bedrock and low velocity of river flow.
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Along the middle course, the importance of erosion is a bit reduced as some depositions
occur where channel beds are flat. Erosion becomes far less importance than deposition along
the lower course although it still occurs along the concave banks of river meanders.
River erosion at any one point of the river course can involve one or more of the
processes of hydraulic action, abrasion, solution and attrition.
a) Hydraulic action: It refers to the removal of loose materials by the force of impact of
water alone.
b) Abrasion (Corrasion): It refers to the mechanical wearing and tearing of rock particles
forced to strike at or being dragged along the channel bed.
c) Corrosion (Solution): It refers to any chemical process from the reaction of water and
rocks on the surface of the land. Effects of corrosion are most marked in
limestone, which is a hard rock not easily carved by erosion, but yielding
readily to the action of carbonic acid in solution in the stream water.
d) Attrition: It refers to the reduction in size of the particles in transport as they strike at
one another or the channel bed.
These three processes enable the river cut into the land in three directions or ways:
a) Headward erosion:
It is the undercutting of the base of the slope immediately above the river's initial
origin. Undercutting can be caused by :
- Groundwater percolation which removes the underlying fine material and causes the
top soil to sink and slide downslope.
- Surface wash and slump on the slope immediately above the initial origin of the
river.
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b) Lateral erosion:
This is the erosion of the sides of a river channel. The two sides of a channel do
not experience the same amount of erosion.
More lateral erosion is found along the concave banks (where deeper water is
found and having greater velocity) than along the convex slip-off banks.
Along the upper course, channel gradient is great and lateral erosion is checked by
the fast removal of river water. Lateral erosion becomes more active along the lower
course because the gentle slope is not favoured for downcutting and the kinetic energy
transfers to lateral erosion. On the other hand, the river banks in lower course are made
of softer alluvium that can be easily eroded.
c) Vertical erosion / downcutting:
Vertical erosion is the erosion and subsequent deepening of the floor. Where it is
more active than lateral erosion, as along the upper course for steep slope, a narrow
V-shape valley is developed. V-shaped valleys are common in humid regions where
thick vegetation cover keeps lateral erosion in check. In arid regions, vertical erosion is
checked while slope processes are aided on the bare slopes, as a result stream valley are
often steep-walled and flat-bottomed.
b. Transportation:
Within drainage basin, weathering on channel side slopes and plains produces
loosened masses of materials that can be washed into the channel. Upon reaching the channel,
such materials from the load of the moving stream or river.
There are four types of load being transported by a stream as it flows. Each of kinds is
transported in different way and requires different minimum velocity to get started.
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-
Dissolved load/ Soluble load:
Dissolved load enters the water current by means of corrosion, and is transported
in solution by the river water. Water rich in acids (such as the carbonic acid derived from
atmospheric carbonic dioxide and precipitation or the organic acids supplied by the decay
of vegetation) are especially potent in corrosion and contributing to the dissolved load of
the river.
The chemical composition of river water depends on topography, climate, geology
and vegetation characteristics of the drainage basin in which it is found.
i) A river flowing through an area with steep bank-side slopes is likely to be richer
in dissolved minerals because erosion and surface wash from the two sides of the
channel are facilitated.
ii) Corrosion and solution are enhanced by an increase in temperature and water
supply. Both increase the rate of chemical reaction.
iii) Rocks consisting of minerals with large amounts of soluble ions are more easily
affected by corrosion and solution. Acidic ions derived from precipitation (eg.
HCO ion from rainwater) and the decomposition of organic matter (eg. Sulphate
SO4 ions and nitrate NO3 ions) that has entered the river channel can dissolve
metallic ions (eg. potassium K, sodium Na and Calcium ions Ca).
iv) Vegetation is a large and constant supplier of organic matter which is carried to
river channels by overland flow. Its main effect is to produce acidic ions which
can dissolve metallic ions from the surface of minerals.
-
Suspended load:
Fine materials are sustained in the river because of turbulence generated by water
movement and hence cannot sink down to the river bed. They are carried downstream by
the irregular turbulent in suspension and deposited only where are major obstacles in the
river bed or where the channel gradient is drastically reduced.
-
Saltation load:
Sand particles and other materials of similar size remain close to the river bed
because of their greater weight. They are, however, moved forward by the water current
in a series of leaps and bounds. Materials on saltation thus seldom come to rest for any
long period.
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-
Traction load/ Bed load:
Traction is composed of a large variety of coarser fragments ranging in size from
gravels to boulders. The larger the fragment, the lesser is the distance over which it has
been moved by the water current. Because of their greater weight, bed load materials can
only be moved by rolling and sliding at times of very high water level.
It is because the larger the particle size, the more stream energy and velocity are
required to overcome friction and entrain particles for erosion and transportation.
Stream Competence:
It refers to the largest particles that can be moved at a given discharge. The higher
velocity makes the river competent to carry large particles. The largest transported particles
that a stream is able to move in traction as bed load is the stream competence.
Stream channels are less competent in removing coarse bedload materials because
coarser materials are heavier to move and more irregular in shape such that much energy is
needed to overcome their friction with the channel bed. Stream channels are more competent
in removing bedload materials at times of high water flow levels because an increase in river
discharge, associated with higher water flow levels, can remove the coarser load as well as
the finer load at the same time.
Stream capacity:
Stream capacity is the maximum amount of bedload materials that a stream can
transport. It varies according to the velocity and is dependent upon channel gradient, stream
AL Notes (Drainage Basin System)
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discharge and calibre/ weight of load. Increased capacity results from increasing channel
gradient, increasing discharge and decreasing grain size.
Suspended sediment:
The concentration of suspended sediment tend to increase with discharge levels at any
point on a stream. On the other hand, solute concentration is greatest at low flows, when the
water in the channel is derived entirely from groundwater seepage. It is because percolating
water has been in contact with rock and soil for an extended period and has thereby picked up
ions released by chemical weathering. During periods of higher discharge, the water in the
channel is diluted by the throughflow and overland flow contributing to the channel flow and
hence the overall solute concentration decreases.
However total quantity of dissolved material transported per unit time increases since
the dilution effect is more than offset by the increased discharge.
Tractive force increases with velocity, and the bedload accordingly increases at higher
levels of discharge once a critical threshold of discharge is attained.
The relative importance of these three modes of transport varies according to the
nature of load available, the discharge and stage of the river. The solute load being more
important at low flows, while suspended sediment is transported in greater abundance at
times of flood, and bedload moves only once a threshold level of discharge has been attained.
c. Deposition:
The deposition of sediments in transport occurs when a stream is no longer competent
to carry them. loss of competence or transporting ability of a stream is often a result of a
decrease in channel gradient or discharge volume or of the damming of the channel.
i) Decrease in channel gradient
- Entry of a stream from a steep middle course to its lower course at flat plain.
- Entry of a stream into a pre-existing depression which is now occupied by a lake, lagoon,
pool or any other type of still water.
ii) Decrease in discharge volume
- The discharge volume of a river usually decreases in the dry season but increases during
the wet season.
- Moreover, a reduction in the discharge volume of a river may be resulted from river
capture or piracy.
AL Notes (Drainage Basin System)
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- By seepage which is common along exotic rivers (eg. River Nile) and wadis which flow
along a permeable channel floor and surface (eg. sandy desert). River flowing into
limestone areas also lose a lot of water by solution and seepage.
- Excessive increase in the supply of load materials. This may occur after a serious mass
movement during which an excessive amount of weathered material is washed down to
a major channel from tributary channel and adjacent slope faces. It may also occur
when a stream encounters materials of much larger grain size. In such cases, stream
capacity and stream competence are respectively exceeded, transportation can no longer
continue and deposition occurs.
D. Channel Form:
The form of the river channel can be regarded as the response by the channel to its inputs,
which can be considered in terms of adjustments to the cross profile, long profile and to the form
of the channel in plan.
a. Plan:
There are three major channel patterns, namely the straight channel pattern,
meandering channel pattern and braided channel pattern.
i) Straight channel pattern: It refers to the channel characterized with straight banks.
Although the bank of the channel is straight, it does not mean that the line joining
the deepest points on the stream channel, will also be straight. In fact, due to the
irregularities of the river channel, the stream line of flow is usually in a winding
path or sinuous pattern. (Sinuosity = actual channel length / straight line distance)
ii) Meandering channel pattern: It forms when pronounced bends or loops develop in the
course of the river. Meandering channel pattern will probably be resulted when
- helical flow is intensified at the bend; (when water moves in a winding pattern, it
produces a strong centrifugal force which causes a superelevation of water level on
the outside bend. The excess weight of water produces tremendous pressure which
AL Notes (Drainage Basin System)
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intensifies the helical flow at the bend. The strong downward movement of the river
flow probably results in effective erosion.)
- the slope gradient is reduced;
- a large proportion of the load is carried in suspension form ; and
- there is significant local bank erosion and deposition.
Meander wavelength and radius of curative are closely and directly related to
channel width and thus to discharge. The effect of meanders is to lengthen the stream
channel through increased sinuosity, and hence promoting evenly distributed energy loss
and thereby decreasing its gradient.
iii) Braided channel pattern:
When the river channel is subdivided into two or more bifurcated channel which
are separated by bars of alluvial materials (usually visible in dry seasons), braided
channel pattern forms. It occurs when
- the materials of the river bed are
too coarse to move;
- the channel can transport neither
the amount nor the size of debris
that is supplied to it as bed load;
-
and
the discharge of the channel is
variable.
b. Long Profile:
The long profiles of streams tend to be concave, as gradient gradually decreases
downstream. The grade river
is that a river is capable of
existing in a state of balance,
or dynamic equilibrium, with
the rate of erosion being equal
to the rate of deposition.
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c. Cross Profile:
Discharge increases in a downstream direction according to the increase in drainage
area. The channel thus adjust to the change by deepening and widening its form so as
possessing and greater cross-section area. In this way it develops a greater hydraulic radius
and becomes more efficient.
The width/ depth ratio also changes (increases) downstream as width increases more
rapidly than depth. Thus the channel becomes proportionately wider, accommodating the
increasing discharge and energy.
E. Work done by rivers in the three landscapes:
a. Tropical Rainforest:
- Of the total mineral load carried by streams, by far the greater portion is in the form of
dissolved solids because of intense chemical weathering under the hot humid condition.
- Little coarse sediments move as bed load, because the cover of vegetation holds back all
but the finest soil particles from transport by overland flow.
- Small streams of steep gradient have bouldery channels but the boulders experience little
downstream movement.
b. Tropical Desert:
- Although running water is a rather rare phenomenon in dry deserts, it works with more
spectacular effectiveness on the fewer occasions when it does act because of the
meagerness of vegetation in dry deserts. Without a thick vegetation cover to protect the
ground and hold back the swift downslope flow of water, large quantities of coarse rock
debris are swept into the streams to transform a dry channel into a flood stream, heavily
charge with rock fragment. Sediment concentration can be extremely high up to 50%.
-
Stream flow in deserts usually accounts for most of the water from precipitation except in
the case of water falling onto sandy deserts (ergs). Much of the precipitation is held at the
surface rather than infiltrating deeply into the ground because of the following factors:
AL Notes (Drainage Basin System)
Page 31
*
*
*
*
*
-
-
lack of vegetation
lack of a water-absorbent organic layer in the desert soils.
presence of hardpans (usually salt accumulation) in the topsoil.
clayey surface of some deserts.
shower nature of the rain which falls at too rapid a rate for a large amount of water to
percolate before it has to start as runoff.
Streamflow characteristics in the arid lands
* Precipitation falling onto mountainous desert areas tends to run off into wadis which
are normally dry out occasionally subjected to large flows of water and sediment.
Wadis may be deep, flat bottomed, rock-cut gorges or else braided and frequently
shifting 'washes' (sediments) developed on pediments and fans at the foot of
mountains.
* Floods are rather occasional, eg. once a year in the semi-arid edges of Sahara: 6 to 7
floods in 10 years in Tademait (in America) and even fewer floods in the almost
totally arid parts.
Streamflood:
* Rainfall in deserts in usually of high intensity, and is brief in occurrences and highly
*
localized in distribution. As pervious sand accumulations account only for a relatively
small portion of the desert surface and percolation is also checked by steep slopes or
encrusted surfaces, most of the rainfall is available as runoff. It readily enters the dry
stream courses.
Geomorphological work of stream-floods:
i) Large amounts of debris due to weathering, slow mass movements (colluvial
deposits) and wind action accumulate in the valleys during the long dry periods.
Yet they do not build up to high levels as to choke the valleys, because of the
streamfloods. Debris of all sizes from clay particles to boulders are moved until
eventually they reach the upland edges. Since then the work of stream is mainly
depositional in the form of alluvial fans, cones and spread of material on the lower
ground.
ii) Stream floods can also cause erosion of the stream channels themselves, and
thereby carve out deep branching wadis (by downcutting) which could be made
possible by streams with normal flow patterns.
iii) Streamfloods in deserts are more important than the floods of humid regions in
eroding materials. It is because weathered materials are not cohesive in these dry
environment and few roots to hold the soil together.
iv) Lateral erosion of wadi channels is also due to streamfloods. This process
eventually widens up the wadi floors.
AL Notes (Drainage Basin System)
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*
*
v) On erodible rocks and sediments badland may develop that have drainage density
as high as 350 km/km2 in parts of arid North America, however in sandy deserts
with high infiltration rate and hence little runoff, drainage intensity will be low.
vi) Deposition occurs and braided channels are conspicuous because of the heavy
load of sediments carried by the streams.
Where streams flow across plains of gravel and sand, water is lost from the channels
by seepages to a water table lies below the level of the channel floor.
The role of running water in the desert landscape:
Most major landform of desert are not attributed to the action of running water either
during past fluvial periods or under existing condition. 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 the desert. In
fact many of the characteristic landforms of desert bear the clear imprint of water
action.
c. Tundra:
- Streams are not well developed due to low precipitation and the presence of permafrost,
-
-
so material delivered to the valleys is not removed very effectively by streams and
consequently valleys are often filled with weathered debris.
In Arctic areas today about 80% of the year's runoff occurs in only 3% of the time. Peak
discharge mainly occurs in summer partly because of the summer rain and partly owing to
the snow-melt.
An impermeable barrier of permafrost beneath the surface could speed runoff responses
from the catchments. This further accentuates the peaky hydrographs of the streams.
Peak discharges following snowmelt can be very effective in both erosion and transport.
Flowing over tills, outwash and solifluction deposits the streams are liberally provided
with rock material.
As discharge fall during the autumn the streams' velocities and competence are reduced
and the load is dumped in and around the shrinking stream where it can be acted on by
frost-shattering and the wind.
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