When cross structures exist on the canal (singular section) the water surface profile equation cannot be used locally to calculate the water surface elevation upstream of the structure.

The hydraulic laws of the different devices present in the section must be applied.

The modeling of these devices is a delicate problem to solve when developing open channel mathematical models. The equations used to represent the hydraulic devices are numerous and do not cover all the possible operating conditions.

In particular, it is rather difficult to maintain the continuity between the different formulations, for example, at the instant of transition between free-flow conditions and submerged conditions, or between open-channel conditions and pipe-flow conditions.

A distinction has been made between devices with a high sill elevation (called hereafter Weir / Orifice) and devices with a low sill elevation (called hereafter Weir / Undershot gates).