BasinFlow: Weir Structure
Input Parameters:
name anything can go in here
L length of the weir in feet
D an option to use a diameter in inches for a circular opening, instead of the weir length
Cw weir coefficient of discharge: as a general rule, most engineers use a value of 3.3. This is a reasonable assumption for sharp crested weirs, such as might be found in riser structures. A value closer to 3.0 should be used for broad crested weirs, such as emergency spillways in dams. Some more detailed examples follow;
weir depth Cw for crest widths 0.5/0.75/1.0
0.2 2.8/2.75/2.69
0.8 3.3/3.04/2.85
1.0 3.32/3.14/2.98
Inv invert, or low point from which controlling head is computed
in riser option for flow into a riser structure
out option for flow out of a riser
dam option for flow directly through a dam
top option for transition to orifice flow, in the case of an opening on a horizontal plane
Co used in the "top" option; coefficient of orifice discharge
Area used in the "top" option: area of orifice opening. If the diameter option is selected for the weir length, and this field is zero, the area value is computed from the given diameter.

The formula used for weirs is given below, where "h" is the head, or water elevation in the basin computed from the weir invert.(Roberson, Cassidy, and Chaudhry) This formula is applicable to rectangular and trapezoidal weirs in general. It is not applicable to triangular, or irregularly shaped weirs.

The "top" option computes an elevation at which orifice flow will begin to control.  Orifice behavior is assumed to occur when orifice flow would be less than weir flow at the same elevation.  This elevation is derived by setting the two equations equal to eachother, and yields the following:

(return)