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