Frequently, weirs are installed together with a tide gate at points of overflow into the receiving waters.
Flow across the weir is restricted by the tide gate, which may be partially closed at times. This is accounted for by reducing the effective driving head across the weir according to an empirical factor published by Armco (undated):
Equation 46: h’ = h - (4/g)·V2·exp(-1.15·V/h) (46)
where h is the previously computed head before correction for flap gate, and V is the mean velocity of flow in the upstream conduit.