This is a standard orifice defined by area. The orifice can be sharp or rounded, circular, rectangular or any other configuration. It is up to the user to specify the coefficient to apply to the orifice. No provision is made to compute weir flows for the weir at low head conditions. It is assumed that the orifice opening is submerged and that the orifice equation applies. This is typically not a problem for small orifice; there would be a problem for large openings.
Figure 14: Vertical Orifice
For circular sharp edged orifice, the following practical design limitsCasey_Ref are recommended:
-
edge distance >= d/2
-
upstream channel cross-sectional area >= 10 times orifice area
-
upstream submergence of top of orifice >= d
-
Dh >= 0.03 m
Where
-
Dh is the difference between upstream and downstream water surface elevations
-
d is the orifice diameter
Under fully contracted, submerged conditions, the discharge coefficient cd may be taken as 0.61 for rectangular sharp crested orifice shapes.
Uses the orifice formula:
Where
-
c is the orifice coefficient
-
A is the area of the opening in square feet (sf)
-
h is the driving head acting on the orifice
Typically 0.61 for a sharp crested orifice, but can be anything that is reasonable. Some typically coefficients for sharp crested weirs:
|
Orifice Diameter (m) |
Cd - Free flow |
Cd - Submerged Flow |
|
0.02 |
0.61 |
0.57 |
|
0.025 |
0.62 |
0.58 |
|
0.035 |
0.64 |
0.61 |
|
0.045 |
0.63 |
0.61 |
|
0.05 |
0.62 |
0.61 |
|
0.065 |
0.61 |
0.60 |
|
>=0.075 |
0.60 |
0.60 |
Table 15: Typical Orifice Coefficients for Sharp Crested Orifice