I’m working on the development of a 2D model using a coarse DEM (20m) for a reach which has an average river width of 20m, whereas the floodplain is over 1 km at places. When i construct a mesh of 50m X 50m, it seems to run with no errors. However, reducing the mesh resolution to 25m or 20m increases instabilities. Is 50m a reasonable grid size? is there a particular guideline to fix the mesh resolution? I seem most works using 2D models are used on very small reaches and on very fine resolution DTMs.