Lesson 4 Sediment Dynamics.pdf

8/10/2019 Lesson 4 Sediment Dynamics.pdf

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LESSON 4 SEDIMENT DYNAMICS


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SUMMARY

LESSON 4 Sediment Dynamics

The particles move due to the average bed shear stress exceeding

the critical shear stress of the particle and display different ways ofmovement depending on the flow condition, sediment size, fluid and

sediment densities, and the channel conditions.

AT RELATIVELY SLOW SHEAR STRESSparticles roll or slide along the bedparticles remain in continuous contact with the bed and the

movement is generally intermittent (CONTACT LOAD)

ON INCREASING THE SHEAR STRESSsome particles loose contact with the bed for some time,

and hop or bounce from one point to another in the

direction of the flow (SALTATION LOAD)


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SUMMARY

CONTACT LOAD

SALTATION LOAD

BED LOAD



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SUMMARY

FURTHER INCREASE IN SHEAR STRESS

particles may go in suspension and remain thus due to theturbulent fluctuations and get carried downstream by stream

flowthese sediment particles are termed as SUSPENDED LOAD

TWO PARTS DEPENDING ON THE SIZES OF MATERIAL IN

SUSPENSION

one part is composed of the sizes of sediment found inabundance in the bedsecond part is composed of those fine sizes not available

in appreciable quantities in the bed (WASH LOAD)



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SUMMARY

BED LOAD

SUSPENDED LOAD

TOTAL LOAD



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REGIMES OF FLOW

The regimes of flow can be divided into the following

categories:

1.Plane bed with no motion of sediment particles

2.Ripples and dunes

3.Transition

4.Antidunes



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1. PLANE BED WITH NO MOTION OF SEDIMENT PARTICLES

flow characteristics are such that the average shear stress on

the bed is less than the critical shear stress, therefore, the

sediment particle on the bed does not move

water surface remains fairly smooth if the Froude number is low

Mannings equation can be used for prediction of the mean

velocity of flow

REGIMES OF FLOW



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1. RIPPLES AND DUNES

sediment particles on the bed start moving when the averageshear stress of the flow exceeds the critical shear stress which

results in small triangular undulations on the channel bed called

RIPPLES

REGIMES OF FLOW



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REGIMES OF FLOW

Do NOT occur for sediment particles coarser than 0.6 mmDistance between the successive crests of the ripples is usually

less than 0.4mHeight from crest to trough is usually less than 0.04mSediment movement is confined in the region near the channel

bed



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2. RIPPLES AND DUNES

with increase in discharge, and consequently the average shearstress, the ripples grow in sizes which are then termed as DUNES

REGIMES OF FLOW



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REGIMES OF FLOW

Triangular in shape but larger than ripplesTriangular sections are not symmetric and the upstream face is

inclined at about 10- 20

and downstream face at and angleof about 30- 40with the horizontal

In rivers, dunes may be quite long and also the height may be

great



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3. TRANSITION

with further increase in discharge over the dune bed, the ripplesand dunes are washed away, and only very small inundations are

leftwith slight increase in discharge, the bed and water surfaces

attain a shape of sinusoidal wave form called STANDING WAVES

REGIMES OF FLOW



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REGIMES OF FLOW

These waves form and disappear and their size doesnt increaseRapid changes in bed and water configuration occur with

relatively small changes in flow conditionsFroude number is relatively high but the flow conditions are sub-

critical



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4. ANTIDUNES

when the discharge is increased further and the Froude numberincreases to more than one, indicating super critical flow, the

standing waves, which are symmetrical sand and water waves

in the phase, move slowly upstream and break intermittently

the movement of the direction of dunes is backward compared tothe direction of the flow

since supercritical flow is rare in case of natural streams and

channels, this type of bed forms do not occur generally in nature

REGIMES OF FLOW



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LET LEARNING CONTINUE.

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Lesson 4 Sediment Dynamics.pdf