CHAPTER 9 - SUBMERGED ORIFICES

3. Fully Contracted Submerged Orifice

The principal type of submerged orifice used for measuring irrigation water in an open channel is the vertical, sharp-edged, fully contracted, rectangular orifice. This orifice is accurate and is the principal type for which the discharge coefficient has been carefully determined. The longer dimension of the rectangle is horizontal to help meet the approach flow boundary conditions discussed in the next section of this chapter. Such an orifice is illustrated on figure 9-1 with recommended box dimensions on figure 9-1 given in table 9-1.

figure
Figure 9-1 -- Submerged-orifice measurement structure viewed from upstream (refer to table 9-1 for dimensions)..

Table 9-1. Recommended box sizes and dimensions for a submerged orifice

Size of orifice

Height of structure, B
(ft)

Width of head wall, A
(ft)

Length, E
(ft)

Width, W
(ft)

Length of downstream wing wall, C
(ft)

Height, D

(in)

Length, L

(in)





3
3
6
6
6
9
9

12
24
12
18
24
16
24

4
4
5
5
5
6
6

10
12
12
14
14
14
16

3.0
3.0
3.5
3.5
3.5
3.5
3.5

2.5
3.5
2.5
3.0
3.5
3.0
3.5

2
2
3
3
3
3
3