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.

8. EROSION TYPES
 Overland Erosion
• Raindrop Erosion (Splash Erosion)
• Sheet Erosion
• Rill Erosion
• Gully Erosion
 Stream Channel Erosion

9. Erosion Patterns

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.

11. Example
MAJULI ISLAND IN
RIVER
BRAHMAPUTRA

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

15. Erosion Pattern

16. Erosion and its Dynamics in Majuli

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.

18. Thank You