8/10/2019 Lesson 4 Sediment Dynamics.pdf
1/15
LESSON 4 SEDIMENT DYNAMICS
8/10/2019 Lesson 4 Sediment Dynamics.pdf
2/15
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)
8/10/2019 Lesson 4 Sediment Dynamics.pdf
3/15
SUMMARY
CONTACT LOAD
SALTATION LOAD
BED LOAD
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
4/15
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)
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
5/15
SUMMARY
BED LOAD
SUSPENDED LOAD
TOTAL LOAD
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
6/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
7/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
8/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
9/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
10/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
11/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
12/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
13/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
14/15
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
LESSON 4 Sediment Dynamics
8/10/2019 Lesson 4 Sediment Dynamics.pdf
15/15
LET LEARNING CONTINUE.
Top Related