Water Model Limits
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The water model is based on this theory. This calculation method impacts practical use cases. Follow these six basic rules to ensure accurate results.
Surface Waterways
Waterways are also calculated as 2D surface flow (not a 1D-line). This has several advantages, like interaction with the shoreline and creating detailed bathymetries. However, it also requires a proper setup of the grid raster to get flow through the waterway.
1 The smallest waterway channel needs to be at least 8 cells wide
- This ensures water can flow between cells, especially when the waterway runs diagonally across the square grid.
- So for example a 4m wide waterway channel needs to have 4m / 8 cells = 0.5m grid cells.
- If this rule is not followed, the surface theory may result in invalid water flow, such as zero or reduced throughput (m3/s).
- It may also result in a double shoreline effect if opposing shorelines are too close, preventing proper water level reconstruction (see surface theory). This creates a minuscule impulse that leads to water buildup over time in stationary models (Dutch "scheeftrekker" effect).
- Note: These same rules apply when ensuring water flows between buildings in dense urban areas.
- See the Grid Overlay page on how to change the grid cell size. When you have reached the smallest grid value (0.25m) you can also activate Increased DEM Resolution under advanced options, when activated only 4 cells are needed.
2 For the best results the elevation model (DEM) needs to be of the same resolution
- For example, if you have a 3m wide waterway channel with 0.5m grid cells, but the imported elevation model has 10m accuracy, all grid cells will share the same (or interpolated) height. Small elevation changes (like a waterway bathymetry) are lost in the average cell value.
- When this rule is not adhered the bathymetry becomes too shallow and water cannot flow properly. It can also result in overflow around shorelines because small levees are ignored.
- See the Advanced options (New Project Wizard) page on how to change the project DEM resolution when creating a new project and optionally uploading your own bathymetry DEM using the Geo Data tutorial.
- Note: to improve waterway bathymetry the model automatically lowers cells inside a water polygon, by using the lowest value instead of average. This can improve results but can never compensate for a low resolution DEM. You can test this behavior via Min/Max elevation under advanced options and comparing the original DEM Overlay with the water child result Surface Elevation
Impulse & Depth
The calculation model is based on typical use cases and is therefore limited to the min/max values variables may take. Allowing even larger min/max values is possible but has a drastic impact on performance and memory usage. If you have a use case that requires different limits, please let us know.
3 Water Depth
- Water depth (distance between bathymetry and water datum) is limited to max 100m. Water depth (h) is a key variable in the surface theory; excessively large values can reduce UV vector accuracy, making the simulation unstable.
- Water depth has a minimum value of 0.5 millimeters; depths below this threshold are ignored for surface flow but remain part of the overall water balance.
4 Water Speed
- Water speed (m/s) is also limited to a maximum of 10m/s or 36kmph, which is faster than a fast flowing river in mountainous terrain.
Breaches & Hydraulic structures
Breaches and hydraulic structures are 1D objects that connect to the 2D grid.
5 Breach growth
- 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 required for a breach; therefore, a 100m wide breach requires a maximum cell width of 10m.
- This may result in reduced flow through the breach or shock-waves as the breach width increases in large steps.
6 Weir width
- Like breaches, weirs or inlets can flush water across multiple cells; therefore, Rule 1 must be followed. 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).