Dear Jozef
these strange borders are perhaps due to some holes in your DEM. The way to solve this is to adapt your calculation domain (green line in the input mode). When creating a new project, the calculation domain is always rectangular and enclosing your whole project region.
It's
recommended to adapt the calculation domain to the specific avalanche track you are simulating. Choose "Track -> Input -> Calculation Domain... -> Draw New Domain" and draw a new calculation domain, making sure that the avalanche can not flow out of the domain. By limiting your calculation domain to a smaller region, you save a lot of calculation time! See pictures below!
Default calculation domain:
Adapted calculation domain: