river erosion and its associated fetures
TRANSCRIPT
R i v e r E r o s i o n & i t s
A s s o c i a t e d F e a t u r e s
Dr. V. R Ghodake
Department of Civil Engineering.
Sinhgad College Of Engineering, Vadgaon (Bk), Pune.Email Id- [email protected]
Mobile- +919764484757
Longitudinal River Profile
Ele
vation (
feet)
0
3,000
Headwaters
UltimateBase level
River Length (Miles)100 0
This diagram outlines the longitudinal profile of ariver basin from the headwaters to the ultimatebase level, or sea level. A rivers gradient issteepest near the headwaters and gentlest nearthe base level. In South Carolina, the steepestsections are often found in the blue Ridge andPiedmont, while the gentler gradient occurs in theCoastal Plain. River length, or distance, ismeasured from the rivers mouth to itsheadwaters, seemingly reversed from what onemight expect.
Longitudinal Profile and WatershedsHeadwaters:
multiple low-order streams near
drainage divide
Base level: single outflow and
highest order stream
This figure is a hypothetical river basin. The black dotted linerepresents the drainage divide and the numbers refer to streamorder. Any rainfall that falls within the black dotted line willeventually flow into the main strem river and out at the mouth.Stream order increases from the headwaters to the base level.In this example, the river is a5th-order river.
Stream Order:
1st order and 1st order = 2nd order
2nd order and 2nd order = 3rd order
3rd order and 3rd order = 4th order
4th order and 4th order = 5th order
And so on, 5 and 5 =6, 6 and 6 =7th order…
5
1
1
1
2 2
11
3
4
3
4
5
3
Table of Contents
Rivers and Basins
Radial type Drainage Pattern
Radial Drainage pattern
Dendritic type Drainage Pattern
–
Trellis type Drainage Pattern
Rectangular type Drainage Pattern
Common River Features
A river can erode material from its bed and
banks in 4 main ways
Chemical Action
Abrasion
Hydraulic Action
Attrition
The Work of Rivers
The erosional work of streams/rivers and shapes
the landscape through which they flow.
functions of rivers
a. Erosion
b. Transportation
c. Deposition
The Work of Rivers
Erosion
•A river may erode in 4 ways
1.Abrasion/corrasion
Load carried by a river will grind against its
bed and sides.
This process slowly wears the bed and sides
away.
The Work of Rivers
2. Attrition
When thrown against the sides and bed of
rivers, the load gets broken into smaller
pieces.
The Work of Rivers
3. Hydraulic action
The work of turbulence in the water.
Running water causes friction in the joints of
rocks in a stream channel
Joints may be enlarged
The Work of Rivers
4. Solution/Corrosion
Certain minerals in rocks like limestone can
be dissolved in water.
Rocks are then eroded.
Methods of Transportation
Rivers transport material in 3 main ways
Solutio- Some minerals (particularly in limestone areas) dissolve easily in
water and are not visible to the naked eye
Suspension- As the speed or velocity of a river increases, it is able to pick up
and carry larger and larger particles in its flow. Where particles are carried
along in the flow and are not in contact with the river bed, they are said to be
travelling in suspension.
Bed load- heavy bigger rock fragments moves along with the flow of water is
known as bed load.
Methods of Transportation
Siltation - Heavier particles may not be held in the flow all the
time but may be bounced along the bed
Traction - The heaviest particles are rolled along the bed. Such
particles may only be moved when the river has a large volume of
water in it
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Boulders are large and semi-rounded, due to attrition within the load and abrasion with the stream bed and banks
Why are they rounded?
River load in upper course
River Transportation
BouncingRolling
SuspensionSolution
• A kettle-like small depressions in the rocky beds of the river valleys are called pot holes which are usually cylindrical in shape.
• Pot holes are generally formed in coarse-grained rocks e.g. sandstone and granites.
• The diameter of pot holes ranges from a few centimeters to several meters.
• They are found in the upper course of a river where it has enough potential energy to erode vertically.
• The pot holes go on increasing in both diameter and depth.
Pot Hole
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Pot holes,human scale!!
Waterfalls• Waterfalls occur where there is resistant bedrock, abrupt changes in bedrock resistance, or
along fractures or faults in the bedrock.
• Less resistant materials are weathered more quickly than resistant rocks, creating stair-
stepped ledges or drop offs where waterfalls occur. Less resistant rocks may also form pools
between resistant rocks that form waterfalls.
• Faults and fractures often provide natural pathways for the downslope movement of water.
• The location of the waterfalls origin may be referred to as a “knick-point”, continued
weathering by the stream flow causes the knick-point to slowly migrate upstream.
• Most waterfalls in South Carolina occur along streams in the Blue Ridge, Piedmont, and the
along the Regional Fall Line where there are rock layers of varying resistance.
This waterfall was formed by differential weathering
between the softer shale and harder more resistant
limestone.
Photo: SCGS
Lower White Water
Falls in the Jocassee
Gorges area of South
Carolina drops nearly
200 ft. Here, the
Toxaway Gneiss forms
a resistant bedrock that
the Lower White Water
River flows over before
draining into Lake
Jocasse.
Waterfalls
• Erosion:
• In this picture, which layers are being eroded to cause undermining?
• Kaaterskill Falls…
What is a meander?
38
Floodplain
Pebble deposits on the inner meander bend where there is low energy
River CliffSlip-Off Slope
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Meandering Rivers
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Lower Severn Valley
Well developed meanders with barsin the channel indicating high sediment load
Very gentle valley side gradients
Very wide
floodplain
Formation of Oxbow lake
Ox Bow lake on Mississippi
Can you annotate this photo graph ?
A flood occurs whenever a river overflows its banks (over ‘bank full discharge’). However, a flood becomes a problem when the water rises to a level where it Destructs property and life.
bank full
dischargeMeander cross
section
What is a Flood?
Flood plains• Flood plains are
constructive, depositional landforms created by stream flow and sediment deposition.
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This aerial view of the Mississippi RiverValley contains many typical floodplainfeatures. The darker, green areas arefloodplain forest and they likely flood themost frequently and thus are notdeveloped with agriculture or housing.The surrounding patchwork representsagricultural fields and other developedlands that are probably at a higherelevation formed by natural or artificiallevees.
Pointbar
Cutbank
Oxbow
Lakes
Infilled
Channel
• The narrow flat surface on either side of the valley floor are called river terrace.
• Some times, the river valleys are frequented by several terraces on either side wherein they are arranged in step-like forms.
• River terraces are generally formed due to dissection of fluvial sediments of flood plains deposited along a valley floor.
• Terrace represents the level of former valley floors and the remnants of former flood plain.
River Terrace
Floods and Flood Control
Floods
A flood occurs when the discharge of a
stream becomes so great that it exceeds
the capacity of its channel and overflows its
banks.
Measures to control flooding include
artificial levees, flood control dams, and
placing limits on floodplain development.
NaturalLevees
River
Stages in the Development of a
River Valley
The River’s Journey
Rivers usually begin in the mountains. They flow downhill onto flat land and into the sea
Characteristics of Streams
• Water always flows downhill, even on theslightest of slopes.
• The speed of water flow depends upon theslope. As slope increases, the velocity of thewater increases.
Running Water is the Predominant Agent of Erosion on the Earth
• Every stream and its branches make up asingle system that collects all the runoffwithin a definite area called the drainagebasin of the system.
• A stream consists of running water, the landsurface it drains, the sediment it transports,and the potential energy used to drive it.
What determines a streams characteristics
• Stream Discharge: the volume of water that passes a point in the stream during a given amount of time.
• The average velocity of a stream increases as its discharge increases.
Life History of a Stream
• YOUTHFUL STREAM
• Steep gradients
• V-shaped cross sections
• Rough sediments flowing rapidly down stream.
• Due to greater water velocity larger sediment can be moved.
• Sediments cut through bedrock as they are moved along
• Common to find rapids and waterfalls due to differences in resistance of the bedrock to weathering.
V- Shape Valley
Mature Streams
• Potential energy for cutting and removing rock becomes less.
• The average gradient is decreased.
• Velocity near the bed becomes less.
• The size of sediment that can be moved decreases.
• Bed becomes covered with loose material, thus protecting it from further erosion.
• Cutting action of the stream becomes very slow.
• The base of the V has been widened due to weathering and the action of tributaries the valley now has gentle slopes.
• Meanders begin to form
• There is an increase in the volume of water that is carried.
• A greater mass of sediment can be carried, but most of it is silt and clay.
Old Streams
• Gradient becomes extremely small and onlythe finest of sediments can be moved.
• During times of peak flow the banks willoverflow and flood the nearby portions of itsvalley.
• When the flow subsides a layer of silt and clayis left behind on the valley surface, this is theflood plain.
• Meanders are strongly looped and from time to time they become cut off to form oxbow lakes.
Locations of Stages
• It is unlikely that any stream is at the same stage of development throughout it entire length.
• Most streams tend to have the characteristics of youth near their source and of old age near their mouths and to be in the mature stage somewhere in-between.
• A stream can be rejuvenated through crustal uplift.
The river cuts downward to
form a ‘V’shaped
valley. The river starts
to meander
River uses its meanders to cut from side
to side eating into
the valley
Floodplain
starts to
form1
2
1
2
Formatioin of Valleys
Alluvium = silt deposited by a river
Alluvial fan Caused when a stream falling from a side valley reaches flatter ground on the valley floor.
Material is dropped at the ‘break of slope’ to form this fan shape.
(This is really a depositional feature.)
Erosion/Deposition
Alluvial fan
Deltas
Deltas form where the mouth of a river meets its ultimate base level at the ocean or
sea. As the river’s velocity decreases, it looses the capacity to carry its sediment load
and the resulting deposits form a delta. Delta shapes and forms vary depending on
tidal influences, waves, currents, sediment type and quantity, river discharge, and the
stream gradient near the outlet. The most common types of deltas include bird-foot,
estuarine, and arcuate.
Not all rivers form deltas, for example the Amazon deposits its sediment load directly
into the ocean onto an underwater seaward sloping continental shelf. The Columbia
River in the northwest United States, lacks a delta altogether, because the currents
are too strong and erosive for the sediments to deposit.
Mississippi River Delta: Bird-Foot Delta
A bird-foot delta contains a large channel with multiplesmaller distributary channels draining off from the mainchannel and depositing sediments. They generally formwith rivers that have a high sediment load and flowinto an area with minimal tidal influences. This false-color infrared image provides a satellite view of theMississippi River delta. This delta has shifted positionsseveral times over the last 5000 years in relation tochanges in the Mississippi River. Scientist recognizeatleast 7 distinct deltas. The most recent beganforming 500 years ago and forms a classic bird-footdelta.
Deltas
Nile River and Arcuate Delta ACE Basin: Estuarine Delta
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River Nile
The Nile Delta, satellite view
Formation of Delta
That’s all aboutRiver & Its Associated Landforms