Meander.ppt

MEANDERING RIVER SYSTEM

CONTENTS• INTRODUCTION

• MEANDERING RIVER SYSTEM FORMATION MORPHOLOGY PROCESSES FEATURES FACIES CONCLUSION

INTRODUCTION

MEANDERING STREAMA meander in general is a bend in a sinuous

watercourse

FORMATION

• The thalweg in a river (which is the line of the deepest part of the channel where there is the highest velocity flow) is not straight even if the channel banks are straight and parallel. It will follow a sinuous path moving from side to side along the length of the channel. In any part of the river, the bank closest to the thalweg has relatively fast flowing water against it while the opposite bank has slower flowing water alongside

Meanders develop by the erosion of the bank closest to the thalweg, accompanied by deposition on the opposite side of the channel where the flow is sluggish and the bed load can no longer be carried.

With continued erosion of the outer bank and deposition of bedload on the inner bank, the channel develops a bend and meander loops are formed

MORPHOLOGY

They are most commonly formed in coastal plain regions.

They are characterized by single channel in contrast to multichannel braided streams

Its morphology results from disruption in uniform flow across the channel, caused by variation in;

Sediments Slope or Gradient Bed roughness The channel profile comprises of a steep

side & a gently sloping side relative to stream bed.

The steep side experiences lateral erosion and gently sloping side is characterised by sedimentation

PROCESSES

They are characterized by turbulent flow; velocity varies both horizontally & vertically across the channel.

It transport the material both as bed-load and suspended load.

Unlike braided streams, meandering streams provides a regular pattern of flow.

There is a consensus about the flow in meanders which may be ellaborated as;

Helical flow

It is the major flow in the meander bends.

This flow causes an elevation of water level on the outside of meander.

This helical flow produces a component of flow which is normal to stream bank; towards the eroding bank near the surface and towards the accreting bank near the bottom

This has the net effect of producing a circulation cell which interacts with the bed to carry sediments upslope along the accretion surface

HELICAL FLOW

FEATURES

There are numerous environment & subenvironments related to meandering streams; each having characteristics deposits

These includes; Channel Lag Point Bar Overbank Deposits Natural Levees Crevasse-splays Oxbow Lakes

• CHANNEL LAG

Below the channel floor or thalweg, coarsest material is sorted out and left behind on the stream bed. Lying just above the basal erosional surface (or scour base) is the channel lag which consists of mudclasts and blocks from bank erosion, plant debris, boulders and bed-load sand and gravel

• POINT BARThe sediments accumulating on the convex side of the meandering loop as the channel is migrating and the outside bank is eroding results in the formation of point bar. Most of the sedimentation in meandering streams occur in form of point bar

Sediment deposited by a river on a valley flow outside the stream channel. Such waters usually contain much sediment in suspension resulting in fine layers of silt/sand deposition known as overbank deposits

• OVERBANK DEPOSITS

Sheets of sand and silt deposited during floods are thickest near to the channel bank because coarser suspended load is dumped quickly by the floodwaters as soon as they start flowing away from the channel. Repeated deposition of sand close to the channel edge leads to the formation of a levee’, a bank of sediment at the channel edge which is higher than the level of the floodplain

• NATURAL LEVEES

In times of flooding, the river breaches its banks. It may temporarily cut through the outer levee and spill large quantities of water and sediments. This is known as a crevasse splay which is typically in a lobe shape with a mixture of fine and coarse grained sediments.

• CREVASSE SPLAY

Oxbow lakes are created when growing

meanders intersect each other and cut off a meander loop. These occurs when meanders grow laterally through erosion (outside bend) and sediment deposition (inside bend, point bar). When the loops get too large and consume too much energy (friction), the river will eventually find a less energetically “taxing” shortcut and a part of the old channel will be abandoned and becomes an oxbow lake. Over a period of time, these oxbow lakes tend to dry out or fill in with sediments.

OXBOW LAKE

MEANDERING RIVER FACIES

A facies is a body of rock characterized by a particular combination of lithology, texture, suite of sedimentary structures, fossil content, colour, geometry, paleocurrent pattern, etc. A facies is produced by one or several processes operating in a depositional environment.

Walther's Law of Facies named after the geologist Johannes Walther(1860-1937), states that: the vertical succession of facies reflects lateral changes in environment. Conversely, it states that when a depositional environment "migrates" laterally, sediments of one depositional environment come to lie on top of another.

In meandering river channels, the facies always fines upward starting with an erosive base

Meandering river channels facies: Scoured base of flow Channel Lag deposits Fining upward sand with trough cross

stratification Rippled sands Cross stratification from migrating point bars

CONCLUSION

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