Transcript The Work of Rivers - Se civil engineering

Geomorphology
• Study of surface features of the Earth, curved by
river; wind or glacial action.
• Evolution and structure of various landforms
related to mountains, plains, plateaus, valleys and
basins are specialized field of study within
geomorphology.
• Fluvial Geomorphology
River
• Running water is the most important agent of
erosion on the continents and the stream valleys
are the most common landforms.
• Rivers flowing to the oceans drain about 68 % of
the Earth's land surface. The remainder of the land
either is covered by ice or drains to closed basins.
• River gradually mould the land by eroding away the
material in some place and depositing it in other
The Work of Rivers
The erosional work of streams/rivers carves
and shapes the landscape through which
they flow.
Functions of rivers
a. Erosion
b. Transportation
c. Deposition
The Work of Rivers
a. Erosion
River may erode in 4 ways
1. Abrasion / Corrosion
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.
Attrition is when bed load collides into each
other with the current flow and breaks down
into smaller particles.
The Work of Rivers
3. Hydraulic action
Ability of flowing water to pick up and move
rock and sediment
The work of turbulence in the water.
Running water causes friction in the joints of rocks
in a stream channel
Joints may be enlarged
Loosened fragments of rocks get swept away.
The Work of Rivers
Erosion
4. Solution
where some material is dissolved (like sugar
in a cup of tea) and is carried downstream.
Occurs often in limestone landscapes where
the water if very acidic.
Certain minerals in rocks like limestone can be
dissolved in water.
Rocks are then eroded.
The Work of Rivers
B. Transportation (4 ways)
1.Traction
Larger and heavier rocks/gravels are dragged or
rolled along the bed.
The Work of Rivers
B. Transportation (4 ways)
2. Saltation (saltim: by leaps/jumps)
Smaller and lighter rock fragments and sand
hop and bounce along the river bed.
At times, the distinction between traction and
saltation may be difficult to determine.
The Work of Rivers
B. Transportation (4 ways)
3. Suspension
Some of the load like silt and clay (finegrained) will float along.
They may only be deposited when stream
velocity reaches near 0.
Turbulence in the water is crucial in holding a
load of sediments.
The Work of Rivers
B. Transportation (4 ways)
4. Solution
Some minerals are transported in dissolved
form.
Especially chemical solution derived from
minerals like limestone or dolomite.
Animation #75: Sediment Transport
by Streams
The Work of Rivers
B. Transportation (4 ways)
Methods of transportation.
Shows the rate of flow needed
Traction
Saltation
Suspension
Solution
The Work of Rivers
C. Deposition
A river will drop its load when:
a. Volume decreases
b. Speed decreases
The Work of Rivers
C. Deposition
A river’s volume decreases when
• Dry season
• Dry region with high evaporation
• Presence of permeable rocks
• Receding flood waters
The Work of Rivers
C. Deposition
A river’s speed decreases when
• It enters a lake
• It enters a calm sea
• It enters a gently sloping plain
The Work of Rivers
The work of a river depends on its energy
Energy a function of
a. Volume of water
b. Speed of water flow (dependent on gradient)
Stages
of
Stream/River
Development
Upper Course
• Stage 1, the Upper Course, is a youthful river. This
when the river is a narrower stream. It starts as a
trickle then becomes fast flowing because it flows
down through steep land. There are lots of stones
and boulders for the water to flow over.
• They have high levels of energy and erode the
stream bottom more than the sides.
Middle Course
• Stage 2, the Middle Course, is the mature river. This
is the river in its middle age where it is beginning to
flow more slowly. Other rivers join as tributaries.
The middle course is where the river starts to
become wider and deeper. The land which the river
flows over is becoming flatter. The river starts to
meander or bend in the middle course.
Mature Rivers
– Mature rivers flows down a
more gradual slope with curves
called meanders.
The floodplain
Lower Course
• Stage 3, the Lower Course, is the river in old age.
This is where it flows slowly towards its mouth at the
sea. It is very wide and deep. The lower course is
where the river becomes its widest and deepest. This
course is found closest to the sea where the river has
its mouth.
• The lower course is where the river slows down as it
nears the sea
• Land is flat and little erosion happens
• Lots of deposition occurs, particular at the mouth
Old-stage rivers flow very slowly through a
very broad, flat floodplain that is curved.
• A river in this stage mostly erodes
its sides causing changes in its
meanders.
• Meanders can eventually be cut
off forming Oxbow lakes.
Stages of a River
Upper Stage
Middle Stage
Lower Stage
Erosion
Active vertical mostly
Active vertical and horizontal
erosion
Some horizontal but not
much erosion
Gradient
Steep, steep valley sides
Becoming flat
Flat
Deposition
Some boulders too heavy to be
transported in small stream
Deposition inside meander
bends
Lots of deposition
especially near mouth
where river slows
Depth
Deep
Deep in places
Shallow (because of
deposition)
Velocity
Fast
Medium
Slow
Width
Narrow
Wide in parts
Very wide
Volume
Very low
Medium
Large
Landforms
Waterfalls, V-shaped valley, gorges,
interlocking spurs
Small meanders, small
floodplains
Floodplains, mouth, delta,
estuary, large meanders,
oxbows