Broad
This is the standard weir formulation. All that is required is the weir elevation
and length. The max elevation is required for report printing only. The program will
compute the discharge for any elevation greater than the crest elevation.
Figure 7: Broad Crested Weir
The following practical design recommendationsCasey_Ref have been proposed for broad crested weirs.
- h1 >= 0.06 m or >= 0.05L, whichever is greater
- radius of cill nose r=0.2H1max;
- p1 >= 0.15 m or >= 0.67H1, whichever is greater;
- 20H1 >= L >= 2H1, to ensure parallel flow while avoiding undulation over the cill;
- b = 0.3 m or >= H1 or >= L/5, whichever is greater;
- to ensure modular flow conditions, the downstream depth and step height p2 should comply with modular limit values set out in the following table
| H1/p2 |
H2/H1 |
|---|
| Vertical back face | Sloping back face(1:4) |
|---|
| 0.1 | 0.71 | 0.74 |
| 0.2 | 0.74 | 0.79 |
| 0.4 | 0.78 | 0.85 |
| 0.6 | 0.82 | 0.88 |
| 0.8 | 0.84 | 0.91 |
| 1.0 | 0.86 | 0.92 |
| 2.0 | 0.90 | 0.96 |
| 4.0 | 0.94 | 0.97 |
| 7.0 | 0.96 | 0.98 |
| 10.0 | 0.98 | 0.99 |
Where
- h1 is the approach HGL.
- H1 is the approach EGL
- L is the length of the weir
- p1 is the depth from the weir cill to the approach bottom of channel.
- p2 is the depth from the weir cill to the bottom of the departing channel.
The broad crested weir as implemented by this program simplifies the input
requirement for the following equation:
Where Q is the discharge in cfs; Cd = 3.216;
L is the weir length;
and H is the stage above the weir.
Roadway Overtopping
In the case of roadway overtopping, the coefficient is an overtopping coefficient defined as:
Cd=kt * Cr
Where kt is a submergence factor and Cr is a discharge coefficient.
The coefficent differs from a weir with a well rounded upstream edge because
the upstream edge is really the roadway sagging curve.
The Federal Highway Administration (1985) characterizes the coefficents in Figure III - Discharge coefficients for roadway overtopping.