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Discharge

Measurement of Discharge

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,

h= Velocity head
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
R = Hydraulic radius
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
R= Hyrdaulic radius
n= Manning’s constant

These are the methods of finding discharge.

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My name is Mustafa Awais, I am basically from Pakistan & Currently living in Saudia Arabia, I am Civil Engineer as well as Web Master.

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