The heavy rain accompanying Baiu front overtook Iwami district of Shimane prefecture in July 23th 1983. In this district, rain continuously began on the morning of the 20th and lasted until the morning of the 23th of July for a period of four days. Heavy rainfall areas with greater than 650 mm of precipitation are found from Yasaka to Misumi areas. Maximum value was observed to have been 722.5mm at Yasaka. A most remarkable heavy rainfall areas with greater than 60 mm of hourly precipitation was observed at Misumi from 5 a.m. to 6 a.m. and then shifted for Masuda from 6 a.m. to 7 a.m. in 23th. Maximum value was observed to have been 91 mm for one hour from 6 a.m. to 7 a.m. at Masuda.
Numerous slop collapses, which was triggered by a localized downpour at stated above, attacked Iwami district from midnight to early morning of july 23th. These slope collapses let to frequent downvalley massmovements in thh form of mud and debris flows, flowed out from the valleys and spread out th valley plains and alluvial lowland, damaging croplands and settlements.
In this paper, the writer will to explain about these slope disasters from geomorphological and geological view points.