The only reason that this particular river flood has taken any traction outside of Nepal is because of the foreigners who are presently unaccounted for who may have lost their lives in this particular flood. This flood was caused by a possible earthquake in the mountains of Tibet which cause a landslide of rocks, ice and snow to slide down into the riverbed below and temporarily dammed the river and when this dam let go a much higher wall of water flowed down the river and was much higher and therefore more devastating than the usual yearly floods.
Below is a snip pf the region in Nepal where the devasting flood occurred as noted in the copied map.
The river that flooded is west of Kathmandu and I have travelled along the river that flooded from the main Kathmandu Pokhara Road to the road just south of Bidur which goes through Thansing back into Kathmandu last time I was in Nepal.
The main rain season in Tibet and Napal occurs during the northern Summer such that either the Spring or Autumn seasons is the best time to travel in Nepal and Tibet.
Yes, flooding happens yearly in Nepal and because of Nepal's mountains in the northern region of the country the floods can be quite bad.
Shalom