One takes into account the fact that the undershot gate has a certain height and if the water level rises upstream of the gate, water can flow over the gate. The flow overtopping the gate is then added to the flow resulting from the previous pipe flow computations

To activate overflow on a gate:

• check the box "Overflow" at the bottom of the screen,
• define height of gate
• choose a sill equation (from rectangular sill equations)
• define discharge coefficient

Overflow starts when upstream water elevation exceeds the sum of the sill elevation, the opening of the gate, and the gate height.

The width taken into account for overflow is:

• the width of a rectangular gate;
• the width of a trapezoidal gate at bank elevation.

If Overflow is not checked, gate’s height is considered as infinite and no overflow is possible.