Formation of island ina natural stream and scour around the formed island By: Manish Kumar Gupta 05/CE/24
Processes Involved: ►Bedrock Weathering ►Sediment Transport ►Sedimentation ►Formation of Riverine Island ►Erosion and Scouring
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.whereh is soil thickness [m],P0 [mm/year] is the potential (or maximum)weathering rate of bedrock andk [m-1] is an empirical constant.
A number of landscape evolution models haveadopted the so-called humped model
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
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.
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.
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.
ExampleMAJULI ISLAND IN RIVER BRAHMAPUTRA
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.
SHRINKING OF MAJULI ISLAND Shrinking Majuli Island 1200 1000 800mAqksaer)(. 600 400 1880 1905 1930 1955 1980 Year
Erosion and its Dynamics in Majuli
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.