I suggest that you look through the options here on OSM Wiki. There is 
https://wiki.openstreetmap.org/wiki/Key:obstacle . There is some usage of obstacle = 'landslide', which seems most relevant, to me anyway. But you need to decide, since this key is little used, especially outside Europe. The wiki suggests obstacle = 'heap', which is likely to confuse  English speakers, especially in this settings. 