Boundaries¶
A boundary defines the interaction between a model and its external surroundings.
There are two conceptually different types of boundaries: Open boundary conditions and additional boundary sources.
An open boundary condition describes the interaction of a model boundary with its surrounding. An open boundary can be identified by a volume free node with only one connecting reach. An example is the end or beginning of a river, or an outlet from a sewer. Such a node must be provided with an open boundary condition in order to solve the flow equations.
Additional boundary sources are used to interact with a specified part of the network. An example is inflow from a factory to the sewer. Additional boundaries are optional, i.e. they can be added and removed from the model, and the model will still be valid.
Additional boundary sources come in several variations:
- Point source boundaries
- Distributed source boundaries
- Global source boundaries.
They differ in the way they connect to the model.
A boundary provides a value during the time when it is active. Its value can change over time. For most types of boundaries the following time variations are supported
- Constant value.
- Time variable, defined in a time series file (dfs0 or similar).
- Periodic daily variation.
These “value providers” for a boundary are called boundary items. One boundary can have several boundary items, that are active simultaneously or at different times. See Section 5.3 for details.
An open boundary condition must be defined over the entire simulation period.
Most boundary conditions specified in MIKE+ are additional boundary conditions or based on a structure like a weir or similar. The only open boundary condition in MIKE+ is the outlet/water level boundary. Usually in MIKE+ a network end is a reach ending in a manhole, which has an additional boundary source attached.