GEOLOGICAL WORKS OF RIVER - FLUVIAL CLASSIFICATION SYSTEM - ENGINEERING GEOLOGY (StudyCivilEngg.com)

 GEOLOGICAL WORK OF RIVER

FLUVIAL CLASSIFICATION SYSTEM

• Fluvial geomorphology is very important in major Civil Engineering operations. The most fundamental landforms seen on the earth's surface are developed by fluvial action, though other processes also substantially contribute to their modification and development.
• An understanding of applied fluvial geomorphology, including quantitative study of sediment load, is essential where a dam is to be constructed across a river valley as a part of hydroelectric and irrigation projects. The study is useful in calculating the life of a reservoir. In rare cases, the river may flood, overflow their banks and an abnormal sediment load may be deposited over their flood plains.
• Here, an understanding of artificial channelization may be beneficial which helps to reduce the problem by speeding up the escape of the flood water. The required information is derived from quantitative estimations of discharge variations in rivers, the nature and amount of sediment load etc.
• The capacity of a river to carry sediment load and to erode its channels, laterally as well as vertically, is evaluated by the amount of energy it possesses.
• A river has a potential energy controlled by the amount of water present (volume) and the head of the water (vertical distance above sea level). The kinetic energy (an energy of movement) generated by the river is evaluated by the flow of water and its velocity i.e. discharge. Thus, an increase in the discharge of water and its velocity leads to an increase in river energy. Rivers may show substantial spatial as well as temporal variations.

FLUVIAL CLASSIFICATION SYSTEM

• Rivers normally flowing into channels cut either into bedrock or in alluvial (recent) sediments. Rivers in bedrock tend to follow a stable course while those in alluvium have a strong tendency to change their position and behaviour. Failure to understand the natural behaviour of alluvial river systems can lead to damaging effects particularly if artificial changes are introduced in the system.
• Bedrock channels are likely to be much more irregular than alluvial channels. Whether in alluvium or bedrock, the Thalweg, or line connecting points of maximum water depth in a general downstream direction along the channel is seldom straight.
• In humid regions, rivers are called Effluent as they receive water also from groundwater. Rivers in arid regions generally lose water to ground in addition to losing it by evaporation and they often dry up completely without reaching the sea. They are called Influent streams.
• The shape of river and channel segment is used to describe different terms. Some segments are straight in plan view. Sinuous channels develop in bedrock and alluvium either by rounding the corners of zigzag channels or alternately eroding and depositing sediment as pools and riffles along former straight reaches. The sinuous channels are called meanders. Rivers that carry large amount of coarse sediment construct midstream bars at frequent intervals and divide into numerous intersecting and shifting channels, called as braided or anastomosing.

FLUVIAL EROSION

• Erosion by a river or a stream is the process of continuous removal of weathered material from in-situ.
• The most common process of stream erosion is mechanical abrasion or corrosion. The rate of corrosion depends on the volume of water, the gradient, hardness and softness of bedrock. The coarse disintegrated particles of hard rock are rolled and dragged along the channel floor, slowly removing away exposed rock outcrops.
• Pot holes are often created in the beds of the fast flowing rivers due to strong eddy motions by swirling action of pebbles. The sheer hydraulic power exerted by rapid river flow, referred as pot-hole drilling may also be responsible for shattering of bedrock in the channel. The process becomes easier if joints and bedding planes are widened by localised corrosion or chemical attack.
• Erosion by a river or a stream is influential in three interactions; viz vertical downcutting, erosion and headward erosion.

Vertical Downcutting

Vertical downcutting is peculiar of the fast flowing rivers that transport a large bed load. The bed load is used to abrade and pot hole the channel floor, which leads to the formation of deep narrow gorges. However, river water cannot continue to flow when there is no slope and there is also a limit below which a river cannot excavate its own bed. Thus, no further conversion of potential energy to river work is possible. The ultimate base level is the sea level into which the rivers must flow. But for smaller streams, temporary base levels are created by lower limiting planar surfaces produced by lakes, resistant rock masses etc.

Lateral Erosion

Once the temporary base level is attained, the kinetic energy of the river is used for lateral excavation causing bank erosion. However, the role played by weathering and slope transport in the process of valley widening is substantial. As a result of these interactions meanders or sinusoidal bents are developed.

Headward Erosion

Headward erosion is prominent where river profile is steep. It is associated with waterfalls where an alternate sequence of hard and soft rock is observed. Thus, on steep end valley sections, the rapidly flowing water increases the rate of erosion with the result that steepened section migrates upstream.

In the case of waterfalls, erosion is concentrated at the base of the fall and may involve sudden burst of trapped air bubbles. This process is known as cavitation. This leads to retreating of the fall and periodic collapse of hard rocks so that in course of time, the fall may be abandoned.

FLUVIAL TRANSPORT

The theoretical maximum load that a stream can transport is  labelled as its capacity; while, competence is the measure of a  river's ability to transport a maximum grain size of disintegrated  particles. It depends on the factors like velocity, the shape and  size of particles, temperature of water etc.  The weathered material (disintegrated and decomposed) is  transported in three forms

Bed Load: Bed load is often a permanent load situated in the  river channel. It includes the gravel and large size fragments,  transported close to the channel floor by rolling or sliding.

Suspension Load: Clay, silt and sand particles transported  during moderate/high flow, are called as suspension load.

Solution Load: The decomposed material available in  solution or colloidal mixtures is called dissolved or solution  bed. 

FLUVIAL GRADATION

• The concept of stream gradation is controlled and influenced  by many factors including discharge, sediment load, base  level, channel width, depth, bed roughness etc. The fluvial  system is said to be graded if the stream channel system  adjusts its gradient and attains an average steady state of  operation for a measurable period.
• The system is then said to have achieved an equilibrium state  of operation. The equilibrium may be achieved in a process  of a gradational step. Thus, the concept of a river in  graded system may be visualised by first imagining an  ungraded river that flows over a tectonic landscape, newly  raised from the other normal land or from the floor of the  sea (Fig (a)). Rainfall varies over the landscape, water flows down-slope  along easy water carrying paths. Drainage networks are  eroded and valleys are broadened by mass wasting on slopes  along the deeper channels (Fig (c))

(a) Initial stage, stream is flowing on a  pre-existing topography
(b) Youthful stage, vertical downcutting
(c) Maturity stage, meandering channel, lateral cutting
(d) Widening of channel, flood-plain development
(e) Old stage, near peneplain, ox-bow lakes 

• In this way, the initial channel originates and begins its travel  towards the sea. The gradation processes may be marked in  the smoothness of river profile. Irregularities in profile are related to the presence of lakes, rapids, falls or beds of hard rock which provide temporary  base levels so that profile flattens off on approaching these  features, and then steepens again below them. 
• Here, the velocity of a river or a stream rapidly increases and  thus abrasion of bedrock becomes intense. As a result, falls  are cut back and the rapids are trenched, while the lakes are  filled by sediments. As a continuous process, the lakes  disappear and falls are transformed into rapids while the  rapids are reduced to a gradient of minimum irregularities.
• Simultaneously, tributaries are also being  introduced into the land mass, excavating the drainage area  and converting the original landscape into a fluvial land form  system.
• In the initial stages, the stream's energy is utilised in  excavating the floor (downcutting) so that very little or no  debris accumulates in the channel. However, continuous  excavation contributes to an increasing supply of rock debris  to the channels. Thus, there comes a stage when the supply  of bed exactly matches the stream's capacity for transport.  This stage is called the graded condition.
• After attaining this stage, the stream continues to excavate  on the outsides of the bank (lateral erosion) and the first  indication of having attained a graded condition may be  visualised in the development of a flood plain valley.
• On the outer side of a bend, the channel shifts laterally  into a curve of a large diameter and erodes the outer bank.
• On the inside of the bend, the alluvium accumulates in the  form of point bar deposits
• As a continuous process, the flood-plain valley is widened  and meanders or sinusoidal bends are developed
• Occasionally they produce cut-offs, leaving crescentic mort or ox-bow lakes
• Because of flood plain  development, the deeper valley gradually disappears and  forms an open valley with soil covered slopes.

Evolution of a Stream

• The profile, if plotted, of such a stream, is referred as a  graded profile. However, the profile may  not be very smooth, if considered the effects of major  tributaries which cause abrupt increases in the discharge and  load from point to point along the main stream. 
• Thus, at each confluence point, the profile of a graded  stream is segmented. Irregularities of profile may often be  due to a distinct change in bedrock from place to place, for example, a sandstone shale sequence. Theoretically, the goal  of the erosion process is to reduce land mass to penultimate  base level. After a time span of millions of years, the land  surface is reduced to an undulating surface of minimum  elevation known as Peneplain.
• Thus, the history of the fluvial system from the initial period  to peneplain surface, is considered, in three stages.
• The youth stage of a river system is characterised by steep  gradients giving rise to gorges or narrow, steep sided valleys water falls and rapids are common  and the river profiles are not in equilibrium. The rapidly  flowing streams consume their energy in the process of  excavation and thus there is no flood-plain.

Stage of valley erosion
(a) Extreme youth: Vertical downcutting characterised by canyons, rapids, waterfalls and seldom lakes
(b) Youth: Characterised by V shaped valleys, falls, rapids and also lakes
(c) Maturity: Lateral cutting, valley flood is broader than the river channel
(d) Old: Development of a flood plain valley is very broad with gentle slopes.

• In the maturity stage, the gradient is moderate and the valley  is deeper and wider. The stream is in equilibrium and said to  possess a flood-plain along with meandering curves
• In the old stage, the gradient is still lowered; the valley is  widened and lateral cutting by the migrating meander belt  becomes the main erosional process. A broad flood plain,  characterised by large meanders, braided or anastomosing  stream, is formed
• Irregularities are  minimum and thus the condition of base levelling or peneplanation is almost achieved. 

FLUVIAL MORPHOLOGY

Every river is seen as a three dimensional body. In order to  understand the concept of river evolution it is necessary to explain the elements of channel geometry i.e. plan view, cross  section and longitudinal profiles; and to understand their mutual  dependence. They are also part of an 'interconnected system'  which is further controlled by the external factors (natural base  factors, tectonics, lithology, climate), discharge and sediment  load.

Plan View: Three channel patterns are recognised: straight,  meandering and braided.  In nature, geometrically straight channels are rare. Channels  are thus called straight if they exhibit a very small river  evolution over a certain distance. Two main factors govern  the straight nature of a channel: a steep gradient and a  narrow course caused by geological and morphological  influences. Elongation due to steep gradients is typical in  geologically young mountain ranges and tectonically active  regions. In small scale maps minor morphological features  are suppressed to an extent that the whole river gives the  impression of a straight course.

River Channel Patterns

Meandering River Channels

• Sinuous river channels are usually referred as meanders.  More precisely a meandering stream is the one whose  channel alignment consists essentially of pronounced bends  with deep pools in the outer side and shallow crossing in the  short, straight reach connecting the bends. The  thalweg flows from a pool through a crossing to the next  pool, forming the sinuous curve of a single meander loop.
• The pool tend to be triangular in  section with point bars located on the inside of the bend. In  the crossings, the channel is more rectangular with greater  width and shallower depth. At low flows the local slope is  steeper and velocities are greater in the crossing than in the  pool. At low stages the thalweg is located very close to the  outside of the bend. At higher stages the thalweg tends to  straighten. It moves away from the outside of the bend and  encroaches on the point bar partially. With increase in time  meanders widen their loop at various rates and move  downstream.
• The latter process is described as migration. The phenomena  of widening and migration can be developed separately but  usually they are interlinked. Migration to a large extent, is  controlled by erosional resistance of the bedrock.
• When the meander loops approach each other due to their  lateral erosion, a cut-off is established at the contacts that  leads to the establishment of ox-bow lakes

Development of ox bow lake

• Each cut-off shortens the course and thus leads to an  increase of the gradient in the area which proportionally  increases the energy potential of the river and accelerates  the widening of the loops
• Meandering pattern is very common in alluvial streams. In tortuous meandering channels, bends are  deformed and smoothness which is typical of meander  bends is absent. Irregular meanders consist of a  pattern of low amplitude and wavelength superimposed on a larger pattern. It is suggested that  smaller meanders may be related to the periods of low  perennial flow, while the larger owe their origin to the mean  annual flood. 

Rivers in Alluvium, classification based on their Geometry

The regular pattern shows regular waveform in plan. The transitional pattern is characterised by very flat  curves and the straight pattern has minor bends without any irregularity.

Braided River Channels: 

This are commonly encountered in areas with strong bed load transport. The river is split in numerous diverging channels  which reunite again and change their appearance with every  major flood. While one channel is being filled up,  the water scours a new path until the next flood leads to its  starvation and widens another. The two primary factors that  may be responsible for the braided condition are  overloading and steep slopes. Disturbances of a braided  reach can be caused by variation in one of these basic  parameters.

If changes in the supply of bed load are reduced for any  reason, braided river quickly lose their characteristic features.  Degradation starts to prevail and the network of braided  sections is replaced by a stretched channel that cuts itself  into the bottom and causes the former channel to fall dry.

Longitudinal Sections

• The longitudinal profile of a river is  one of the most important morphological element, to mark  the earth's surface. Their development controls the  morphological character of a landscape. The longitudinal  profile represents its gradient of the bottom at each point as  a reaction to tectonic, lithological and climatic factors. The  gradient in the upper reaches is steep, it decreases gradually  downstream and is very low at the base for the erosion or at  sea level. Thus, every longitudinal profile is divided into upper, middle and lower reaches

Stages of Denundation

• Erosion is predominant in upper reaches, transportation in  the middle and accumulation in the lower reaches. It is seen  from the below figure that slope and distance of longitudinal  profile varies consistently with stages of denudation. This roughly corresponds with the youthful, maturity and old  stage of a river system. 
• In case of a river in  youthful stage where erosion is predominant, its slope is  steep and distance of upper reaches is much more than that  of middle and lower reaches. In the maturity  stage, slope is gentle and distance of middle and lower  reaches is more than upper reaches. In the old stage elevation difference between source and month is not  significantly large. Lower reaches cover about 80% of the  profile compared to middle and upper reaches.

Cross-sectional View

Cross-section on rivers is a direct  expression of the controlling forces of erosion and  transportation. In every phase of development, a river will  show the appropriate cross-sections. Direct or close  relationship to the river is developed only for the gorge, the  canyon or the V shaped valley. Lateral erosion gradually  becoming predominant. This gives rise to valley widening. 

In a gorge or V shaped valley of narrow width, the river  occupies the entire width of the valley, the bed is rocky and  covered by large boulders. While U shaped broad valley  indicates that the river has developed its flood plain and  except during the flood-time, the river occupies deepest  portion of the valley. A characteristic of many low land river  is development of levees.

FAQs COVERED IN THIS POST

What are the Geological Works of River?

What is Fluvial Erosion?

What is Fluvial Gradation?

What is Fluvial Transport?

What is Fluvial Morphology?

What is Fluvial Classification System?

What is Meandering River Channel?

What is Braided River Channel?

Describe Evolution of the Stream

Classification of rivers in Alluvium based on their geometry

Development of ox bow lake

What are the river channel patterns?

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