Measuring flow in open channels (weirs)

Measuring flow in open channels (weirs)
Broad-Crested and Sharp-Crested Weirs
Weirs are overflow structures that alter the flow so that:
1. Volumetric flow rate can be calculated,
2. Flooding can be prevented, or
3. Make a body of water more navigable
Types of Weirs:
Main Types of Weirs
Sharp-Crested
a. Rectangular
b. Triangular
c. Trapezoidal
2. Broad-Crested
a. Rectangular
Sharp- vs. Broad-Crested Weirs
SHARP-CRESTED WEIR
BROAD-CRESTED WEIR

Critical depth (yc) occurs off the crest of the weir Critical depth (yc) occurs at the crest of the weir
Usually used to:
Measure the discharge of smaller rivers and canals
Change water elevation of
smaller rivers and canals Usually used to:
Measure the discharge of larger rivers and canals
Change water elevation of larger rivers and canals



Sharp-Crested Weir
There are three main types of sharp-crested weirs:

1.Rectangular—Measure Discharge and Change Water Elevations
2.Triangular—Measure Discharge
3.Trapezoidal—Measure Discharge and Change Water Elevations with
Large Head

Sharp-crested weirs are usually used for smaller rivers and canals.

Sharp crested weirs differ from broad crested weirs due to the detached water surface falling away from the downstream edge of the structure, known as a free-falling nappe

?The flow surfaces at the top and bottom of the nappe are exposed to the air and at atmospheric pressure

?A nappe that clings to the weir must be avoided in order to improve the accuracy of the weir discharge calculation


Sharp-Crested Weir Rectangular/Suppressed Discharge
?Rectangular and suppressed weirs have the same general discharge equation (below), but differing weir lengths that the water flows over
Q=2/3 C_D ?2g BH^(3/2)
?????=0.602+0.083??/??

Where:

Q (m³/s) is the volumetric flow rate over the weir

????is the discharge coefficient usually ranging from 0.60 to 0.62

H (m) is the head over the weir(from the weir crest to the upstream water surface )

P (m) is the height of the weir plate

? B (m) is the width of the contracted notch (rectangular), or the width of
the channel (suppressed)
?g is the acceleration of gravity (9.81 m/s²)