Post on 10-Jun-2015
description
Meander Formation
Learning OutcomesYou will be able to explain the sequence
involved in the formation of a meander.
You will be able to explain the sequence using the key terminology.
KEY WORDS – helicoidal flowsinousitythalwegcentripetal force
Meander FormationRiffles and Pools
Riffle – a shallow part of the channel
Pool – a deep part of the channel
On average pools occur 5-6 times the width of the river bed
Meander FormationThe riffle reduces the hydraulic radius of
that section of the channel as the water flows inefficiently over it.
Asymettrical shape
The meandering ThalwegThalweg – the line of fastest flow in a river
Centripetal ForceAt times of high flow the water is thrown around the riffles and is thrown to the edge by centripetal force eroding and undercutting it. (MAINLY ABRASION AND HYDRAULIC ACTION)
DepositionSlower flow on the inside of the bend leads to depostion there.
It is slower because the reduction in the rivers efficiency being reduced due to the frictional contact with the river bed.
Helicoidal FlowThe movement of water in a meander has been found to move from the outside to the inside.
This helicoidal flow allows material eroded from the outer bank to be moved to the point bar on the inside of the next meander downstream.
Helicoidal Flow•Corkscrew motionCorkscrew motion
•In bends, water forced to outside of bend hits bank In bends, water forced to outside of bend hits bank with forcewith force
•Water piles-up on outside of bendWater piles-up on outside of bend
•Forced downward along river cliff, back towards point Forced downward along river cliff, back towards point barbar
•Transfers and reverses motion around next Transfers and reverses motion around next alternating bendalternating bend
Exiting the meanderAs the water exits the meander the channel becomes shallower. This reduces the hydraulic radius thus increasing deposition.
As such new riffles are created.
SinuosityActual channel lengthStraight Line Distance
A sinuosity of 1.5 is the dividing line between a straight and meandering channel.
= sinousity
Point Bar
River Cliff
Thalweg
Thalweg
Deposition
Point Bar
Helicoidal Flow
River Cliff
Abrasion
Hydraulic Action
Riffle
Pool
Sinuosity
Meander
Centripetal Force
Explain the formation of a meander
(8 marks)
Meander Formation
Explain the formation of a meander using
helicoidal flowsinousitythalwegcentripetal force