TOPICS

Technical Assistance for Sediment Disasters
Caused by Heavy Rainstorms in theNorthern Kyushu District

OKAMOTO Atsushi, HeadMIZUNO Masaki, Senior Researcher
NOMURA Yasuhiro, Researcher
SATO Takumi, Guest Research Engineer
NIWA Satoshi, Guest Research Engineer
Research Center for Disaster Management, Erosion and Sediment Control Division(Key words) Northern Kyushu District Heavy Rainstorm, debris flow, slope failure, occurring all-at-once
1. Introduction
From July 11th to 14th, 2012, an active seasonal storm
front delivered a record rainfall in the Northern Kyushu
district. A total of 268 sediment disasters resulted in 23 dead(or missing) persons and the full or partial destruction of 152
houses in Fukuoka, Oita and Kumamoto prefectures,according to Sabo Department, Ministry of Land,Infrastructure, Transport and Tourism on December 31. Byrequest of Kyushu Regional Development Bureau andKumamoto prefecture, etc., Erosion and Sediment Control
Division, NILIM, along with Erosion and Sediment Control
Research Group, PWRI, investigated the Aso District andKawabe River Basin in Kumamoto prefecture and provided
technical guidance on warning and evacuation, restoration
measures, and direction for investigations to be made.
The Japan Self-Defense Forces and Fire Departments incharge of searching for missing persons in the Aso District
sought advice for secondary disaster prevention. The
researchers explained that considerations should be madeduring searches, amid occasional heavy rains immediately
after the occurrence of the disasters.

Photo 1 Technical assistance for missing person
searches by the Japan Self-Defense Forces(Shinsho Area in Minami Aso Village on July 13th）
2. Summary of investigation / technical assistance
2. 1 Aso District
Debris flow and slope failure occurred simultaneously in
the Aso District mainly at the caldera walls of the somma ofMt. Aso. Since the number of the sediment disasters thatcaused human and house damage was extensive, NILIMand PWRI organized 3 groups (a total of 9 members) andperformed investigations. At the locations where missing
person searches were being conducted, the groups explained
to the Japan Self-Defense Forces about observation points
such as sediment movements in the streams, rock falls at thehead of the failed slopes, and the tilts of the trees, etc., about
directions to evacuate such as higher areas in the crossdirections instead of downstream areas, and about the speedof debris flow and window time to evacuation, etc. The sitemanager of the Japan Self-Defense Forces said, “We wereconcerned about the secondary disasters looking at the debris
such as boulders, but now we can make decisions calmly
with their advice.”
One of the characteristics of sediment disasters in the Aso
District is that many surface failures occurred on the caldera
walls at the relative height difference of a few hundred
meters, and they transform into debris flow, dragging
driftwoods into the flow. They include those occurring in thezero-order basins, which do not represent distinct
geographical features of valleys. There are cases wherecollapsed sediment reached areas 50m or more off the toesof the slopes due to fine volcanic soil saturated by a large
amount of rainfall and ground water, or other factors.
Moreover, there were reoccurrence of slope failure anddebris flow in the same torrents as the Aso disaster in 1990;
however, there were many cases that the sediment-control
dams and other structures prepared by the prefectural
government after the earlier disaster prevented or reduced
the damage.
2. 2 Kawabe River Basin
Many debris flow and slope failure occurred in Itsuki
Village under the jurisdiction of Kawabe River Dam Sabo
Office. Though there was no reported human or housedamage, unstable sediment remained in the torrents and on
the slopes upstream the roads, so the groups provided
technical guidance to the office and the village government
for the future monitoring systems.
3. Conclusions
The Aso disaster taught us a number of lessons. In thefuture, we are going to conduct investigations and research
into the mechanisms of slope failure and debris flow, effects
of sediment control facilities, and warning and evacuation,
etc.
【Reference】
NILIM HP: http://www.nilim.go.jp/lab/rbg/index.htm