Abstract
On July 3, 2021, a debris flow caused a total of 27 deaths and missing persons in the Aizome
River in Atami City, Shizuoka Prefecture. This damage was the largest number of victims from
a sediment disaster in one location since 1982. When compared to five sediment disasters that
caused similar levels of victims, the slope gradient of the Aizome River was similar to the other
cases. The amount of sediment spilled was also not particularly large compared to other cases.
The Aizome River has a longer steep channel section than other cases, and houses were densely
built along it. This feature may have been a contributing factor to the large number of victims.
On the other hand, the number of victims relative to the number of lost or collapsed houses in
the Aizome River was low. This could be due to the following reasons: 1) calls for evacuation
had already begun, 2) it was daytime and people were more likely to act, 3) some people were
out, and 4) some houses were uninhabited.
