prevent div by zero in Froude computation (#213)

Author: lrntct

docs/conf_file.rst

@@ -220,6 +220,8 @@ This does not apply to the map of maximum values.
 
 If an exported map is totally empty, it is deleted at the end of the simulation when registered in the STRDS.
 
+When the water depth is zero, the Froude number is set to zero to prevent undefined values.
+
 [statistics]
 ------------
 

src/itzi/flow.pyx

@@ -511,8 +511,8 @@ def solve_h(
             vdir = vdir + 360. * (vdir < 0)
             arr_vdir[r, c] = vdir
 
-            # Froude number
-            arr_fr[r, c] = v / c_sqrt(g * h_new)
+            # Froude number - use epsilon to avoid division by zero
+            arr_fr[r, c] = v / c_sqrt(g * fmax(h_new, eps)) * (h_new > 0.)
 
 
 @cython.wraparound(False)  # Disable negative index check