# Choice of reaches

The choice of reaches by the model user is subject to some constraints which are detailed below:

- constraints due to network topology.
- constraints associated with points of inflows or outflows (they can occur only at model nodes).

The user may however divide any part of the canal into several reaches in order to take into account some particularity, even if such a division is not imposed by the constraints described above.

For instance, one can create a different reach for a lined canal zone (low roughness), and an unlined canal zone (high roughness). One can also create reaches for administrative reasons, etc.

The division into reaches for the user’s convenience does not influence the results of the hydraulic calculation. Generally, when one divides a reach artificially, the lowermost cross-section of the upstream reach is the same as the uppermost cross-section of the reach immediately downstream.

If different regulating/control devices exist across the canal, they can be integrated within a reach and do not need any special division. This approach facilitates the modeling of the hydraulic transition from free flow conditions to submerged conditions at such devices.