Contents preface (VII) introduction 1—37

Fig. 5.20 Comparison of discharge characteristics of different weirs under submerged conditions (8)
0.08


0.06
C₁
0.04

0.02

0.1 0.3 0.5 0.7 0.9 1.0
b/b₁
Fig. 5.21 Variation of C₁ with b/b₁ for contracted thin-plate weirs

202 IRRIGATION AND WATER RESOURCES ENGINEERING
The advantages of weirs for discharge measurement are as follows: (i) Simplicity and ease in construction,

(ii) Durability, and (iii) Accuracy.

However, the main requirement of considerable fall of water surface makes their use in areas of level ground impracticable. Besides, deposition of sand, gravel, and silt upstream of the weir prevents accurate measurements.
5.12.2. Flumes
A flume is a flow measuring device formed by a constriction in an open channel. The constriction can be either a narrowing of the channel or a narrowing in combination with a hump in the invert. By providing sufficient amount of constriction, it is possible to produce critical flow conditions there. When this happens, there exists a unique stage-discharge relationship independent of the downstream conditions. The use of critical-depth flumes for discharge measurement is based on this principle.
The main advantage of a critical-depth flume over a weir is in situations when material (sediment or sewage) is being transported by the flow. This material gets deposited upstream of the weir and affects the discharge relation and results in a foul-smelling site in case of sewage flow. The critical-depth flumes consisting only of horizontal contraction would easily carry the material through the flume. Critical-depth fumes can be grouped into two main categories.