1. Formation of island in
a natural stream and
scour around the
formed island
By: Manish Kumar Gupta
05/CE/24
2. Processes Involved:
►Bedrock Weathering
►Sediment Transport
►Sedimentation
►Formation of Riverine Island
►Erosion and Scouring
3. Bedrock Weathering
- weathering is the decomposition of earth rocks
►Soil production function
Soil production function refers to the rate of
bedrock weathering into soil as a function of soil
thickness.
where
h is soil thickness [m],
P0 [mm/year] is the potential (or maximum)
weathering rate of bedrock and
k [m-1] is an empirical constant.
4. A number of landscape evolution models have
adopted the so-called humped model
5. Sediment Transport
Initiation of motion
- shear stress exerted by the fluid must
exceed the critical shear stress for the
motion of grains at the bed.
Type of entrainment
- Bed Load
- Suspended Load
- Wash Load
6. Erosion and Sedimentation
►available energy of water > sediment
load, water will erode soil to obtain
sediment.
►sediment load > available energy,
deposition of transported material will
occur.
7. Formation of Riverine Island
►Deposition of Sediments on the river
Bed results into formation of sand
dunes.
►Continuous deposition over a long
period results in formation of island.
►These islands may grew in size if the
process continues or may erode again
due to high flow.
10. Stream Channel Erosion
►Stream channel erosion occurs
as the volume and velocity of
flow increase sufficiently to
cause movement of the stream
bed and bank materials.
13. Formation Of Majuli
► The Majuli island was formed by the
diversion of the Kherkutia channel from the
main stream of the Brahmaputra.
► From Geomorphologic point of view, the
landform of Majuli is as old as the landform
of the upper Brahmaputra valley.
► The phenomenon of conversion of Majuli
into Riverine Island is not more than 300
years old.
14. SHRINKING OF MAJULI ISLAND
Shrinking Majuli Island
1200
1000
800
m
A
q
k
s
a
e
r
)
(
.
600
400
1880 1905 1930 1955 1980
Year
17. Scour Occurs:
• The part of the island facing the flow directly
gets eroded very fast in comparison with the
other parts.
• Flowage failure occurs predominantly during
falling water levels after floods.
• The rate of erosion increases as any of the
factor discharge or velocity of the flow
increases.
