### Defining model area / coverage

After deciding what map co-ordinate system is most practical to apply (see TB1), depending on the system that the available data is given in, the first step in generating a grid is to determine the required extent of the model area. The location of the boundaries is in many cases arbitrary, and a poor choice of the boundary locations may affect the reliability of the model results.

The first consideration in choosing the model domain is to pinpoint the area of interest. Boundaries must be sufficient far away from this area, so that changes in the area of interest do not affect the hydrodynamics / sediment dynamics near the open boundaries, and equally, that boundary disturbances do not affect the local conditions in the area of interest. A second consideration is to determine which processes are of interest. The model area must cover the length scale of these processes.

In case of a simple coastal model, the model extent might be chosen based on the following considerations:

- Longshore distance of model: decision based on hydrodynamic processes. As a rule of thumb, the longshore tidal excursion of a water particle should be smaller than the length of the model, so that the conditions in the area of interest are based on the upstream boundary conditions only.
- Cross-shore distance of model: a specific depth contour might be chosen as a suitable offshore boundary. This contour should be chosen such that relevant surf zone processes do not act outside the model domain. The offshore boundary condition should be beyond the closure depth (definition closure depth; HdV)
- The boundaries of the model might be dictated by the available data to define the boundary conditions, e.g. location of tidal stations.
- The extent of the model on dry land needs to account for the rise of water level above MSL due to tide, waves, wave set-up and wind set-up if this is to be included in the model.
- Model boundaries should be orientated perpendicular or parallel to the main flow direction, but also to the coastal orientation.