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.

1 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.

Grid cell to big for proper flow

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.

Breaches and Weirs

Breaches and weirs are 1D objects that connect to the 2D grid

1 A breach that grows over time to 100m width also needs a breach area of at least 100m width. As the breach grows more 2D cells are used to flush the water from the 1D object onto the 2D grid. For proper flow the grid cells also need to be small enough, e.g. a breach of 20m width on a 100m grid cell cannot create stable breach growth. Typically at least 10 cells are needed for a breach thus 100m width needs at least 10m cell width.

• This may result in less flow through the weir or shock-waves as the breach increases in large steps.

2 Same as the breach a Weir or inlet can also flush the water on multiple cells, thus rule 1 must be adhered too. Furthermore a 100m wide Weir must also be in a waterway channel of at least 100m wide.

• Creating a Weir that is 100m wide on a 3m wide channel may cause water to flow over the shorelines or no flow at all due to DEM averaging (shoreline + bathymetry).