School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Lecture 10: Threshold Motion of Sediments CEM001 Hydraulic

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering

Lecture 10: Threshold Motion of Sediments

CEM001 Hydraulic Structures, Coastal and River Engineering

River Engineering Section

Dr Md Rowshon Kamal

[email protected]

H/P: 0126627589

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The topics we cover under ‘River Sediment Transport’ are:

1. Threshold motion of sediment

2. Design of channels in erodible material

3. Modes of sediment transport

Content

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• Deposition and filling of reservoir in hydropower plants.

• Blockage of water inlets (river navigation) in irrigation schemes.

• Wear and tear of water turbines.• Attach organic and toxic to sediment particles

hence effect water quality.• Problems from old polluted sediments on

ecosystems of the water course.

Why Do We Study ‘River Sediment Transport’?

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Bed load

- Particles roll and slide along bed with occasional jumps into the main stream.

Sediment transport mechanisms

Depend on U, s and d50

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Saltation load

- Particles bounce or hog along the bed due to the impact of bouncing particles.

Sediment Transport Mechanisms (con’t)


Suspended load

- Particles suspend in water due to the turbulent velocity fluctuations.

- Wash load can be considered as a part of suspended load.

Sediment Transport Mechanisms (con’t)

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Initiation of Particle Motion & Erosion

Low discharge (or velocity)

No particle movement i.e. flow condition is similar to a fixed bed

Discharge ≈ Certain value

Random motion of individual particles i.e. initiation of sediment transport. Condition is known as incipient motion/ threshold of motion or critical motion

Discharge > Certain value

Appreciable sediment transport

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Initiation of Particle Motion & Erosion (con’t)

Threshold of Motion

- Movement of single particle,- Movement of few particles,- Movement of bed,- Sediment transport rate tends to be zero.

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Average Boundary Shear Stress (τo)

W

x

FLOW

W sin

y1y1

y2

A

τ

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Average Boundary Shear Stress (τo)

Force acting in the direction of flow

xP

xAg

xP

W

Area

Forceo

sin)(sin

sinWF

gRSP

gASo

Hydraulic radius, R = A/P

SsinSometimes S is denoted by So

P=2y1+y2

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Shear Velocity (u*)

gRSu o

*

It has the units of velocity (m/s), however it is not a physical velocity

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Shields Analysis (1936)

Particle will begin to move when the combined drag (FD) and lift (FL) moment equals the weight (G) moment.

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Shields Analysis (1936) con’t

Taking moment about ‘Point of Contact’,

cos)( 211 bba

sin22 ba

sin33 ba

From geometry

231 GaaFaF LD

Combining and multiplying by cos/G

tan)(

32

21 G

GbF

b

bbF

L

D

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tan)(

2

21 Gb

bbFD For simplicity, we

can assume 1/ GFL

tan1GFD Where

At high Reynolds numbers pressure forces >> viscous/skin friction force.

FD will act through the centre of the particle (b1=0, α1)

tan

tanGFD

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21 bb

b

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Let G = Submerged weight (N.B. G/ in lecture notes),

gd

G s )(6

3

6/4

Let FD = Drag force,24

22B

wDD

UdCF

*2UU B

dUCD

*

223 42

1 DC

gdG s3

4 )(

2*

23 UdFD

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gdG s3

4 )( 2*

23 UdFD

SCF

tanGFD

tan)1( 5

2*

gds

UCritical value = 0.056

)(Re)1( *

2* f

gds

U

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Shields Diagram

)1(

2*

sgd

uFs

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

1 10 100 1000

Particle Reynolds Number, Re*

Sh

ield

s E

ntr

ain

me

nt

Fu

nc

tio

n,

F S

du*

0.056

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Shields Parameter for Larger Particles

056.0)1(

2*

sgd

uFs

400*

duGoverning conditions

smg /81.9 sm /1014.1 26 65.2s

906.02*

d

U 6* 104.456 dU

mmd 619


Manning’s and Strickler’s Formulae

n

SARQ o

2/13/2

6/1041.0 dn

Manning’s formula

Strickler’s formula d is in metres

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Example 01

An irrigation canal is to be constructed to pass 3.0m3/s along a line having a slope of 0.01. The sides are to be banked and protected with grass (this tells us that we don’t need to worry about the stability of the banks). Calculate the minimum width of canal if the bed material consists predominantly of:

i) Gravel d75 = 50.0mmii) Gravel d75 = 4.0mm

(N.B. This is the d50 size i.e. 75% of the particles by weight are smaller. The bed tends to become ARMOURED with this size of particle)

Smaller particles at top of bed are transported away – leaving the larger particles which then protect the smaller particles beneath d75 is used because it takes into account armouring)

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Thank You

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Documents

School of Civil Engineering/Linton School of Computing, Information Technology & Engineering Lecture 10: Threshold Motion of Sediments CEM001 Hydraulic