Weir flow is automatically computed for Inline Weir sections. The program employs the standard weir equation as described below. If the weir structure width (b) is less than the one-half the flow depth (H), it is considered a sharp crested weir. Otherwise, it’s treated as a broad crested weir.

Where:

Q = Weir flow

Cw = Broad-crested weir coefficient = 2.66 (1.5)

Cw = Sharp-crested weir coefficient = 3.33 (1.84)

L = Crest width

H = Water surface – crest elevation

### Adjustment for Submerged Weirs

Weirs can be affected by submergence, i.e., when the tailwater or downstream water surface rises above the weirâ€™s crest, thus reducing the discharge. The weir flow Q computed from the above equation is reduced by the following:

The equation for the reduction in flow is:

Where:

Qs = submerged flow (cfs)

Qr = unsubmerged flow from standard weir equations

H1 = upstream head above crest (ft)

H2 = downstream head above crest (ft)