Boundary Items¶
A boundary item is a “value provider” for a boundary. Every boundary has at least one boundary item. Typical boundary items that are supported by most boundaries are:
- Constant value boundary item
- Time varying boundary item, defined in time series (dfs0 or similar)
- Daily periodic pattern
A boundary can have several boundary items.
Some boundary types allow for only one item to be active at any time, while other boundary types allow for several items to be active at the same time. For example, an inflow boundary can have several active items, which will combine their contributions, while a water level boundary only allows for one active item at a time.
Each item can have a defined application time, which is a time interval where the item is active.
Application time¶
The application time defines the start time and the end time when the boundary item is active.
The use of the application time is by default disabled, and can be enabled for each boundary item individually.
Some boundary items have a pre-defined application time. For example, a time series has a pre-defined application time equal to the interval over which data is available. The application time for such boundary items can only be set to limit the application time, not extend it beyond the pre-defined application time.
When application time is disabled, the item is applicable for the entire simulation period or within the pre-defined application time of the item.
Soft start¶
Soft start is used to modify the value of a boundary item such that it smoothly changes from a user-defined value to the value of the boundary item over a period of time.
To define a soft start, you must specify:
- Type of soft start
- Soft start value
- Soft start duration
The type of soft start can be one of:
- None; no soft start
- Linear; a linear weighting of the soft start value and the actual boundary value within the soft start period
- Sinusoidal; a smooth, non-linear weighting of the soft start value and the actual boundary value within the soft start period. The weighting is based on a sinusoidal function.
The start time of the soft start is the boundary item application start time. If no start time is defined, then it is the start of the simulation, or the predefined start time of the item.
Soft start is mostly used for stability reasons. For example: if the initial conditions at a water level boundary define the water level to 5 metres, and the water level boundary condition defines a constant water level of 10 m, then in the first time step the water is raised from 5 to 10 m, thus creating an unnatural wave through the model, and in the worst case destabilising the model and causing it to fail. In such a case it is wise to set soft start parameters that gently increase the value from 5 to 10 metres over the first 10 minutes of the simulation.
Resolution of a time varying boundary item¶
For a time-varying boundary item, the times defined in the item do not usually match the times in the simulation. To ensure that values match the time in the simulation, linear interpolation is performed.
If the temporal resolution of a boundary item is not very good, for example if it describes only the extremes of a tidal cycle, some preprocessing of the data may be necessary in order to describe a more correct and smoother variation of the tidal cycle.