Specification of a Reach¶
A reach requires specification of where the reach connects and specification of its digi points.
A reach can connect upstream and downstream in two ways:
- By specifying an upstream/downstream node.
- By specifying a reach location in another reach.
In the latter case, if there is not already a node at that reach location, a junction node will be added at the specified reach location, and the other reach will be split into two smaller reaches, which are connected to the newly created node.
Digi points define the georeferencing of the reach. They are also used to define the vertical profile of the reach, in case the cross section data for the reach does not include any vertical specification. If no vertical profile is specified, it is assumed that the reach has a constant slope from the upstream node bottom level to the downstream node bottom level.
The length of the reach is defined by the difference in the chainage value of the first and the last digi points.
The direction of natural flow can also be specified. The default is positive flow, and defines that positive flow is in the direction of increasing chainages. By setting the flow direction to negative, the positive flow is in the direction of decreasing chainages. The flow direction has impact on the definition of upstream and downstream direction and the specification of all flow parameter on the reach, as e.g. discharge initial conditions, open inflow boundary conditions (inflow and QH boundary), structure and control parameters, etc.
In a MIKE+ (Collection System) setup, two digi points, one at each end, are automatically added as the first/last digi point in the list, with the (x,y) coordinate of the upstream and downstream node and the invert level respectively. If more digi points are defined in MIKE+ without a z-coordinate, then it is assumed that the reach has a constant bottom slope from the upstream node to the downstream node.
The cross section definition, friction parameters and other reach parameters are specified on the reach. Each reach has a cross section reference and a material reference, and specifies amongst other things the length of the link and the upstream and downstream invert level (the level where the reach connects to the nodes.)
In a MIKE+ (Collection System) setup, a reach can be specified as a non-return reach. A non-return reach will gradually decrease the discharge across the first q-grid point in the reach is downstream water level becomes larger then upstream water level.
If the upstream water level becomes larger then downstream water level, the reach will gradually re-open to allow free flow across the first q-grid point. The gradual closing and re-opening of the flow is implemented to suppress artificial wave formation in the network.
The time period used for re-opening the flow can be controlled in the dhiapp.ini file with the NON_RETURN_VALVE_STARTUP_PERIOD parameter.n