Until 1960, many people died by floods in Japan. The table below shows 7 worst flood disaster after Word War II.
|Flood name or cause||Flood name or cause||Number of Deaths||Number of Missings||Year||from MM/DD||to MM/DD|
|3||Toya Maru typhoon||洞爺丸台風||1361||400||1954||9/24||9/27|
|6||1953 North Kyushu Flood||昭和28年西日本水害||759||242||1953||6/25||6/29|
|7||1953 Wakayama Flood||紀州大水害||615||431||1953||7/17||7/18|
As you can see major floods were caused by typhoons. September was the dangerous month in terms of floods.
Source is Wikipedia.