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# Measurement of Discharge

Discharge

Methods for determining discharge can be classified as:

• Velocity Area Method
• Permanent Structures Method
• Chemical Gaging Method

## Velocity Area Method

This is the most commonly used procedure to find Discharge.

Q= AV

Where,

Q = Discharge
A= Area of flow
V= Velocity of flow

The mean velocity of flow is found by

• Velocity Measurement by Current meter

V= a+bN

Where,

N= No. of revolutions per second
a= Rating constant
b= Rating contant

• Velocity Measurement by Pitot Tube

V = (2gh)^0.5

Where,

g= Gravitational acceleration

• Velocity Measurement by Floats:

V=S/T

Where,

S= Distance
T= Time

### Chemical Gaging Method

Q= Vol*Co/∫C2dt

Where,

Q = Discharge
Vol=Volume of injected solution
Co =  Conc. of chemical in dozing    solution
C2  =  Conc. Of chemical in water at sampling point
C2dt  = Represents the area under the time conc. Curve.
Constant Rate Injection
Q = Q1(C1-C2)
(C2-Co)

### Parament Structure Method

Q = CdxBxhxVc

Where ,

Q = Discharge
Vc = 2g(H-h)^0.5
Cd = Coefficient of discharge
H   = Head over the weir at upstream
h    =  Critical head over the weir

## Extension of Rating Curves

Special methods have to be used to extend the original rating curve to extrapolate the discharge corresponding to the measured high stage.

• THE LOGRITHMIC METHOD
• A√D METHOD
• SLOPE AREA METHOD

### Logarithmic Method

Used if cross section of stream at gaging station is of regular shape
Used if there is no possibility of getting number of lines of varying slopes.

Formula:

Q = C (g-a)n

Wheres,

Q = Discharge
C = Constant
n  =  Slope
a  =  Vertical distance b/w zero flow EL. & Datum
(g-a) =  Actual water depth

### A√D Method

• This method is based on the Chezy’s formula

Q = AC (RS)1/2

Where,

C = Roughness coefficient
S = Slope of Energy Line
A = Area of flow
D” The mean depth is substituted for “R”
R=A/P=By/(B+2y)     (P=Wetted Perimeter)

For a very wide channel “2y” is neglected,

So R = D
Q = K A√D

### Slope Area Method

This is based upon Hydraulic Principles.

Manning formula

Q = 1/n A R2/3 S1/2    (In SI units)

Q = 1.49/n A R2/3 S1/2    (In FPS units)

Wheres,

A= Area of crossection
S= Frictional slope