Kennedy’s and Lacey’s theories.

RG KENNEDY AN EXECUTIVE ENGINEER OF PUNJAB PWD CARRIED OUT EXTENSIVE INVESTIGATIONS ON SOME OF THE CANAL REACHES IN THE UPPER BARI DOAB CANAL SYSTEM HE SELECTED SOME STRAIGHT REACHES OF THE CANAL SECTION WHICH HAD NOT POSED ANY SILTING AND SCOURING PROBLEMS DURING THE PREVIOUS 30 YEARS.

FROM THE OBSERVATION HE CONCLUDED THAT THE SILT SUPPORTING POWER IN A CHANNEL CROSS SECTION WAS MAINLY DEPENDENT UPON THE GENERATION OF THE EDDIES RISING TO THE SURFACE.THESE EDDIES ARE GENERATED DUE TO THE FRICTION OF THE FLOWING WATWR WITH THE CHANNEL SURFACE.THE VERTICAL COMPONENT OF THESE EDDIES TRY TO MOVE THE SEDIMENT UP WHILE WEIGHT OF THE SEDIMENT TRIES TO BRING IT DOWN.SO IF THE VEL IS SUFFICIENT TO GENERATE EDDIES SO AS TO KEEP THE SEDIMENT JUST IN SUSPENSION SILTING WILL BE AVOIDED BASED ON THIS COCEPT HE DEFINED CRITICAL VELOCITY.

According to Kennedy the critical velocity ratio Vc in a channel may be defined as the mean velocity of flow which will just keep the channel free from silting or scouring.

His investigations pertain to Upper bari Doab canal in UP.

64.0..55.0 dmVc m = Critical velocity ratio

= 1.1 to 1.2 for coarse sand

= 0.8 to 0.9 for fine sand

KENNEDY’S METHOD OF CHANNEL DESIGN PROCEDURE

Q = A x V

V

Rn

S

SnC

00155.0231

00155.0231

RSCV

64.0.. dmCV vc

Assume a depth of flow = d, m Compute the critical velocity from kennady’s formula Compute are of c/s of flow = Q/Vc Assuming a side slope of channel, say 0.5:1 compute

the bed width Compute the wetted perimeter for the assumed depth

abd computed bed width p=b*(1+1/4)^(1/2)*(y) Calculate C from Kutter’s formula and then the

velocity of flow by Chezy’s equation If the Velocity computed now is same as found by

kennady’s method the design depth is correct Otherwise repeat the above steps by assuming

different depth of flow

CWPC PRACTICE FOR “n”Type of soil Canal discharge (cumecs) Value of n1. Soil other than rock

Up to 0.0140.14 to 1.41.4 to 14Above 14

0.030.0250.02250.020

2. Rocky cuts 1. When rock portion at least 15 cm above the excavated bed level is left out in working out cross sectional area.

0.035 to 0.05

2. When no portion above bed level is left out

0.05 to 0.080

Channel of condition

Value of n

1. Very good 0.02252. Good 0.0253. Indifferent 0.02754. Poor 0.03

LACEY’S REGIME THEORY

LACEY,S AN EMINENT ENGINEER OF UP IRRIGATION DEPARTMENT CARRIED OUT EXTENSIVE INVESTIGATIONS ON THE DESIGN OF STABLE CHANNEL IN ALLUVIUMS. ON THE BASICS OF HIS RESEARCH WORK HE FOUND MANY DRAWBACKS IN KENNEDY THEORY AND HE PUT FORWARD ITS NEW THEORY .HE DIFFRENTIATED BETWEEN THREE REGIME CONDITION. 1.TRUE REGIME 2.INITIAL REGIME 3. FINAL REGIME

THREE REGIMES – TRUE INITIAL AND FINAL

A channel will bi in true rigime if these condition are satisfied1.Dishcharge is constant2.Flow is uniform3slit charge is constant4.Slit grade is constant

When only the bed slope of a channel varies due to dropping of slit its cross section or wetted parmeter un affected even then the channel can excebit no silting no scoring properties called initial regime.

But if there is no resistance from the sides ad all rhe variables such as perimeter,depth,slope,e.t.c are equally free to vary and finally get adjusted acc to discharge and slit grade then the channel is said to have permanent stability called final regime.

Silt factor = mf 76.1

Where,

m = mean particle size, mm

Lacey’s theory

6/12

140

QFV

VQA

QP 75.4

fVR2

25

6/1

3/5

.3340QfS

Kennedy theory Lacey’s theory

1.It states that the silt carried by the following water is kept in suspension by the vertical component of eddies which are generated from the bed of the channel.

1.It states that the silt carried by the following water is kept in suspension by the vertical component of eddies which are generated from the entire wetted perimeter of the channel.

2. Relation between ‘V’ & ‘D’. 2. Relation between ‘V’ & ‘R’.

3. Critical velocity ratio ‘m’ is introduced to make the equation applicable to diff. channels with diff. silt grades.

3. Silt factor ‘f’ is introduced to make the equation applicable to diff. channels with diff. silt grades.

4., kutter’s equation is used for finding the mean velocity.

4. This theory given an equation for finding the mean velocity.

5. This theory gives no equation for bed slope.

5. This theory gives an equation for bed slope.

6.In this theory, the design is based on trial and error method.

6. This theory does not in valve trial and error method.

Design procedure Q and m are initially known Calculate the silt factor “f” Compute V from Lazey’s equation Compute A from continuity equation Compute P & S from Lazey’s

equations