ELUCIDATION OF THE ACTUAL STATE ON THE FLOOD IN THE UPSTREAM PART OF THE KUMA RIVER DURING THE HEAVY RAIN IN JULY 2020

Abstract

 The heavy rain in July 2020 caused the record-breaking flow in the Kuma River, which is a first-class river in Kyushu region. This flood flow caused 1) human damages at the special nursing home named Senjuen and 2) the dike break at two places in the Kuma River. In this study, we conducted the field survey to measure the detailed topography and flood mark depths. We also conducted the flood analyses for 1) the entire upstream part of the Kuma River where the inundation damage was concentrated, 2) the river confluence near Senjuen, and 3) the region around the broken points of the embankment. Our results indicated that the backwater of the Kuma River caused a significant water level rise near Senjuen. In addition, the strong return flows from the floodplain to the river occurred for a long time at the low land area where the flow tends to gather in the downstream of the Hitoyoshi City. This strong return flow may have caused the dike break at the two places.