Friction Maps¶
Friction controls how much the ground retards the flow, as a Manning's n value. Anywhere you do not specify it, a single default of 0.04 is used.
There are two ways to vary it: draw friction polygons, or upload a friction raster.
Friction polygons¶
Each polygon carries one value for the area it covers.
| Property | Type | Description |
|---|---|---|
mannings |
number | Manning's roughness coefficient for this zone |
The drawing form starts at 0.035, which suits dense grass or crops; change it to whatever your surface is.
Friction rasters¶
When your roughness varies per cell rather than per zone, upload it instead: a GeoTIFF of Manning's n values, using the upload icon for friction rasters. This suits roughness derived from a land-cover classification, where drawing a polygon per class would be busywork.
A scenario references friction polygons and a friction raster through separate fields, so you can supply either.
Common Manning's n values¶
| Surface | Manning's n |
|---|---|
| Smooth concrete | 0.012 |
| Short grass | 0.025 |
| Dense grass / crops | 0.035 |
| Light brush | 0.050 |
| Dense brush | 0.100 |
| Buildings, Mannings method | 10.0 |
How it works¶
Friction polygons are combined into a composite friction function that is evaluated in order, with a catchall applying the 0.04 default to anything no polygon covers.
Structures using the Mannings method are added to that same set with a Manning's n of 10.0, which is high enough to stop flow through a building without removing it from the mesh. Structures using the default Reflective method do not appear here at all, because they are cut out of the mesh instead. See Structures.