Research Trend & Results

Investigation of flooding streams caused by heavy
rain disaster in the Northern Kyusyu
ONAMI Hiroyuki, Researcher
ITO Hiroyuki, Head of Flood Disaster Prevention Division
Research Center for Disaster Management(Keywords)

Heavy rain disaster in the Northern Kyusyu, flooding stream, field survey

1.

Introduction
In July 2011, torrential rain disasters occurred in theNorthern Kyusyu area of Japan, hitting the wide areafrom Fukuoka Prefecture to Kumamoto Prefecture. In
this disaster, the highest water levels ever observed wererecorded in some rivers, causing overtopping andflooding in many places. Although investigations onflood marks of the past inundations that occurred in
the areas protected by levees were conducted so far,behaviors of flooding streams had been neverinvestigated. However, it has been pointed out that taking
appropriate actions corresponding to the behaviors offlooding stream is important in a flood disaster. Therefore,
it is important to grasp the actual behaviors of flooding
stream in conjunction with improving inundation analysis
techniques. In this study, observations and inquiring
surveys were conducted after the flood disaster, and tograsp the behaviors of flooding stream qualitatively.
Flooding courses at basins of Kagetsu-gawa Riverand Yamakuni-gawa River
Heavy rains and flooding hit Hita City of Oita
Prefecture where the Kagetsu-gawa River stretch of theChikugo-gawa River system flows and Nakatsu City ofOita where the Yamakuni-gawa River flows twice on Jul
3rd and 14th. The following shows the results of thesituation of flooding that occurred on July 3rd.
2.1 Kagetsu-gawa River (Miyuki-bashi Bridge ~Kagetsu-gawa Bridge downstream)
This area is a district widely damaged by inundation .
According to the inquiring surveys, we have heard
residents in this area saying “overflowing water poured
into the residential street from the street alongembankments and the waterways inside the levees”.
Based on which, it is suggested that not only
configuration of the ground but also arrangement of
spaces or roads and waterways will greatly impact on
flooding streams (Figure-1). What is more, concrete
block walls of Showa Gakuen High School werecollapsed on a large scale by driftwood (Photo 1). This
was an extremely large damage compared to otherfacilities around the area, leading to the assumption thatfluid dynamic force increased substantially by flotsam. At
the same time, there were a large number of lumber millsin the upper river and dam-up and facilities damage
caused by drifting woods were seen in many places.
2.2 Yamakuni-gawa River
Although the Yamakuni-gawa River is an entrenchedchannel, overflow on levees were found here and there

due to dam-up caused by rubbles adhering to bridges.
Figure-2 shows the situation of inundation in the Hida
district. Controlled bridges of the Yabakei Bridge and theArase dam were blocked by driftwood and there wasoverflowing in the scarp. Additionally, flooding water
passed through the road in a community as its maincourse and the water level inundating houses at themountain side was higher than that at the river side.
Destroyed walls

Showa Gakuen High School

Direction of flooding stream

Photo 1 Destroyed block walls of
Showa Gakuen High School

Kagetsu-gawa R.

Kagetsu-gawa R.

2.

Figure-1 Study results of flooding stream ofKagetsu-gawa River

Yabakei Bridge

Yamakuni-gawaRiverArase Dam

LegendPhoto 2 Evidence of drifting woods in the Arase dam

Figure-2 Study results of flooding stream of
Yamakuni-gawa River
3.

Conclusion
We showed one example of investigating results ofbehavior of flooding stream causing flood disasters. In
the future, we will conduct a swift field-survey after
occurring flood disaster and accumulate obtained datasystematically for the purpose of contributing toconsideration of risk-management measures andverification of inundation calculation accuracy based on
the characteristics of flooding stream.