Water Model Limits

The water model is based on this theory.

The way this is calculated also has impact on practical use-cases. Below are some basics rules that need to be adhered to get a proper result.

Surface Waterway Rules

Waterways are also calculated as 2D surface flow (not a 1D-line). This has several advantages, like interaction with the shoreline and easy loading of geo-data but is also requires a proper setup of the grid and base data to get flow through waterways.

• The smallest waterway width needs to be at least 6-8x the cell width. So for example a 3m wide waterway channel (shoreline exclusive) needs to have 3/6 (or even 3/8) = ~0,4m grid cells.

-This is because water must be able to flow from cell to cell especially also when the watery runs in a 45 degree angle to the square grid cells. - When this rule is not adhered the surface theory does not work and a buildup or to lees water can occur.

2 When working with small grid cells (e.g. 0,5m cell width) the underlying elevation model (DEM) also needs to have this resolution.

• For example when you have a 3m wide waterway channel and 0,5m grid cell, but the elevation model uses 10m cell accuracy all grid cells have the same (or interpolated height) and in case the waterway channel the bathymetry is averaged out.
• When this rule is not adhered the bathymetry becomes to shallow and water cannot flow properly. It can also result in overflow around shorelines because small levees are ignored.

Impulse & Depth Rules

The calculation model is based on typical use cases and is therefor limited to the min/max values variables may take. Allow even larger min/max values is possible but has a drastic impact on performance and memory usage.

1 Water depth (distance between bathymetry and water datum) is limited to max 100m. The water depth (h) is an important variable in the surface theory and having larger values increases the UV vector out of its accuracy and making the simulation unstable.

2 Water speed (m/s) is also limited to a maximum of 10m/s or 36kmph which is faster then a high speed river in mountainous terrain.

3 Water depth also has a minimal value of 0,5 millimeter, a water depth less then 0,5 millimeter is ignored for the surface flow but is still counted in the overall water balance.