Failure Knowledge Database / 100 Selected Cases

The debris flow in Kanbarasawa【December 6th, 1996, Otani village, Kitaazumi district, Nagano prefecture】
Kunishima Masahiko・Hasegwa Tomoaki (note1)
The large-scale debris flow arose in Kanbarasawa , Otani village, Nagano Prefect ure at ar ound
10:40 a.m. on December 6th, 1996. The destruction at an altitude of around 1300m became a trigger
for this debris flow, and it flowed over five waves (or eight waves (when small waves are included).
one of two valley closing constructions which did not have deposited silt right after the concrete cast
was completely destroyed and the other was half destroyed by the first wave which was the biggest.
After the d ebris flow passed t he 2 ch eck dam s dow nstream w ith no de posited si lt, t he check d amunder constructi on a nd the watercourse c onstruction, i t reached t he Himekawa Ri ver. This debris
flow h it th e construction sites of disaster related project etc, w hich Ministry of C onstruction (n ow
Ministry of Land, Infrastructure and Transport), Forestry Agency and Nagano Prefecture ordered. In
this d isaster, th ere w ere fourt een fa talities, a nd nine people were in jured. Th e position of
Kanbarasawa in Honshu is shown in Figure 1.

Figure1

The position of Kanbarasawa

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Failure Knowledge Database / 100 Selected Cases

1．Event
The debris flow was generated from thedestruction at around 1300m altitude in
Kanbarasawa on December 6th, 1996. It
flowed over five waves, and one valley
closing co nstruction w as com pletely
destroyed an d one w as ha lf destroy ed.
The construction fie ld w as hit dir ectly.
There were fourteen fatalities, and ninepeople we re i njured. Figure 2 is an
expanded map of the accident scene.

Valley closing constructions
Check dam

Check dam

Figure2 (Source: Nikkei construction)
2．Course（１） The climate before the accident occurred
Around K anbarasawa th e cold w aves attacked t he shore o n D ecember 1 ～ 2, 19 96. The
temperature became less than 0℃ and the 35 cm snow depth was recorded. The precipitation was 32
mm. Then, t he cycl one passed the Sea of Ja pan on December 5 th. W hen t he debris fl ow was
generated, t he preci pitation in the O tani o bservatory over 24 hours w as 49 mm. Then, t heprecipitation was not r ecorded from the 5th, 21 o’clock, to the time the debris flow was generated.
And, because of the passage of the cyclone, the temperature increased to a bout 10℃, and the snow
depth lowered from 18cm to 6cm over 5～6 day during which temperature exceeded 0℃. There wasa 109mm amount of rainfall over a 24 hour period, when in the Otari observatory 49mm of rain fall
and the calculation of snowmelt quantity by the degree-hour method were calculated. This was notan especially bi g th an a mount of ra infall (lar gest 2 4 hours w as 3 60mm am ounts of ra infall:
Destruction generated in heavy rain which was generated in Kanbarasawa on July 11th, 1995).
（２）The situation in the disaster2

Failure Knowledge Database / 100 Selected Cases

The debris flow arose from the landslide on the right side slop near the 1300m altitude. The
39000m3 gravel which was destroyed in the upper section destroyed the valley closing constructionsof afforestation (one was completely destroyed, one was half destroyed) and it got over 2 checkdams, and it became the debris flow which were over 5 waves (or 8 waves (when small waves areincluded)), and attacked the lower section. It is said that the first wave was a scale of 3m height and9.1m speed per second. Figure 3 is a photograph of the destruction field.
(３) Safety management of main contractor and subcontractor.
1) Installation such as the alarm facility.
The alarm facility such as siren etc. for carrying out the urgent communication to the workers in thecase of detecting the generation of debris flow etc. was not in place. And, the way of signing was notdetermined either.
2) On the standard for the work stop.
In "Katsuba-Kanbara liaison council", installed for communication adjustment between
businessmen who constructs what the Ministry of Construction orders, caution and shunt standard byamount of rainfall had been set. When the amount of rainfall exceeded the standard, contact could bemade to the member workplace. In the meantime, there were some workplaces which had not
adjusted communication network depending on the amount of rainfall between other workplaces.
3) Arrangement of installation of the machinery monitoring equipment and monitoring person
The arrangement of installation of the machinery monitoring equipment and monitoring person for
detecting generated debris flow was not carried out.
4) Measures for the refuge
The following were not carried out: Improvement of the emergency communication system on theassumption of the case in which the debris flow is flowing under the work, execution of the fire drill
and installation of the escape facility
Though the four rainfall which exceeds shunt communication standard was generated in the day,the refuge was not carried out, since it was not under work.

Figure 3(Source: Nikkei construction)

3. Cause
The primary causes which the debris flow aro se are di vided in to l andform conditi on andgeotechnical con dition. A s a lan dform condition, th e reason was “the ex tension destru ction” (thatmore and m ore large-scale destruction occurs as a trigger of the former de struction which occurred
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Failure Knowledge Database / 100 Selected Cases

previously) of the slope which became unstable because of the destruction which occurred on July
11th, 1995 and that the landslide was the erosion front, the steep slope. As a geotechnical condition,
the Ka nbarasawa watershe d is l ocated i n t he we stern edg e of Fos sa Ma gna (right o n t he upper
Itoigawa-Shizuoka tectonic line). It is a boundary between weak nature of andesite lava and strong
sandstone bed. As a trigger, there was the fact that snowfall and snowmelt flowed and were supplied
to the landslide as underground water through the boundary between weak nature of andesite lava inwhich the water permeability is high and strong sand bed in which the water permeability is low.
4. Immediate Action(1) The beginning of the accident occurrence
Hashimoto Ryutaro, the Prime Minister, asked the Self-Defense Forces for the urgent dispatch to
the disaster rescue area. The field disaster countermeasure headquarters were stationed, and thesearch for the missing persons was undertaken by 1600 persons. Afterwards, 3 sensors which detect
generation of the debris flow were installed in the swamp from the confluence with the HimekawaRiver about 800 m upstream.
(2) The restart of the disaster recovery construction.
In the disaster recovery construction, the full-automation construction was adopted by utilizing
lessons of the debris flow. After the large-scale fire drill on August 19th, 1997 was carried out, on
the 22nd, the construction was resumed after an interval of about 8 months. The construction isintended to ensure the safety of the workers and the maintenance of the check dam. For example, in
the Kanbarasawa check dam, to begin with, the gravel which was piled up in the mountain stream
was removed by using the heavy machinery with the remote control operation. Besides, theconstruction of the stopped left bank division was carried out. In the subordinate layer on the leftbank, the sandbags piled by the crane instead of the mould were used and the high workable concrete
was injected. Not sandbags but concrete blocks were piled up when the dam went above the fixedheight, and by pouring high workable concrete, mould and compaction of the concrete wereprevented. People are completely prevented from entering the mountain stream by this method. And,large change was also carried out on the caution and refuge system. In the new system, not only
standard amount of rainfall when people refuge was lowered, but also the monitoring by men was
done thoroughly. Two monitoring huts were established, and the slope- judging people andauxiliaries (two people each) were stationed. When the fog descended and the field of view from themonitoring hut was bad, the construction was stopped. The safety management was done thoroughly.
Based on this thorough safety management, Kanbarasawa check dam was completed.
5. Countermeasure.
There are three main countermeasures. Firstly, grasping the watershed situation and informing theconstruction party: On construction object watershed and nearby watershed, it is necessary toobtain information such as meteorological phenomena, landform and geologic feature, distributionof the dangerous place in landslide disaster, and landslide disaster generation which arose in thepast before beginning construction. It is required that temperature and snowfall which were notincluded in the survey items are also grasped, based on this lesson. It is necessary to estimate theway of monitoring on the basis of the information. Secondly, the development of forecast anddetection technique of the debris flows: Though the forecast of debris flow generation by amount of
rainfall has been carried out until now, it is necessary to forecast temperature and snowmelt. Also,
it is necessary to examine the system which detects the debris flow generation. Thirdly, the ideal
4Failure Knowledge Database / 100 Selected Cases

way of caution and refuge system: It is necessary to lower the current standard amount of rainfall
level for caution and refuge, when the unstable slope exists. It is desirable that the standard is set by
the divisions based on the landform and the range of construction when lowering the standard of
the caution and when refuge is carried out.
6. Generalization.
The recognition of conventional debris flow was changed completely by this debris flow. Firstly, it
was the debris flow that arose in winter. Secondly, there was no indication phenomenon such as roar,
muddy strea m or redu ced dis charge. T hirdly, as a l andform con dition, t he reas on w as t he sl ope
which became unstable by the past destruction and that the landslide was the erosion front, the steep
slope. As a geotechnical condition, it is said to be a boundary between weak nature of andesite lava
and stro ng sa ndstone bed. The de bris f low i n K anbarasawa c an b e sai d to b e a rare e xample ofbelonging to “Unknown Phenomenon”.
7. Knowledge.
1.

2.

3.

4.

The standard for caution and refuge is established in proportion to the construction field withthe possibility nat ural d isaster arises. The lan dform and g eotechnical co ndition v ary if theconstruction place differs a little.
After the ra infall wh ich e xceeds t he st andard of amount of ra infall, w hen s nowmelt is
anticipated or the work is carried out after earthquake, the watchman should be stationed or
people should move to the place where there is no possibility of the disaster by debris flow.
The developm ent of synthet ic m ethod of debr is fl ow forecast and de tection s ystem (notforecasting the current downpour, but groundwater supply by t he snowmelt of tem peratures,
snow depth etc are also considered.)In the disast er recovery c onstruction i n the em ergency, t he f ull-automation i nstallation
method is introduced.

8. Background.
Otani village in N agano prefecture is o n t he w estern edg e of Fos sa Ma gna (th e bi g fa ult inJapanese Isla nd) and is rig ht u pper It oigawa-Shizuoka te ctonic li ne, a nd i t is known as a freq uentzone of the l andslide. In Decem ber, 1988, i n prefectural roa d in Ot ani vi llage, t he workers underroad construction were buried in the destroyed gravel. Five workers died and one person received theserious w ound. O n J uly 11th, 19 95, by lo cal heav y rain fall, dest royed g ravel etc. occurred an d theaccess to the village was cut. Over 1000 homes and 3000 persons (integrating the nearby area) were
evacuated. By the helicopter of Self-D efense Forces and prefectural police, about 150 tourists who
had come to t he hot spring in the village and villagers were evacuated. In that downpour, the slope
where this disaster occurred was destroyed. On June 25th, 1996, in Otani hot spring in the village, 5inns became isolated since the gravel of the prefectural road was destroyed and closed t he bri dge.
Since there is the fear that the debris flow arises afterwards, the construction work of the check dam
was started b y Minis try of Cons truction (now Mi nistry of Land , In frastructure a nd T ransport) in
February, 1996, and it would be completed in January, 1997. Debris flow attacked the construction
field where the near completion area.

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Failure Knowledge Database / 100 Selected Cases

9. Sequel
Three persons fro m Ministry of Constr uction River Bureau length, Director General of Forestry
Agency, Nagano prefectural governor in those days asked Erosion -Control Engineering Society to
examine the c ause of t his debris fl ow an d t he preparation (cau tion and refu ge) to understand thedebris flow in construction fields in the future.
10. On the Side
After the disaster, the search for people which were buried in the gravel was carried out by policeand fire fi ghting and Self-Defense Forces. The search was hindere d by much gravel and t he dangerof secondary disaster, so it made slow progress. As the principle of searching in far-reaching area,
two principles sev erely conflicted. One was searc hing et ernally until a ll t he m issing p eople we rediscovered. The other was searching in some range by the constant members. Finally, the day when
the rem ains of the fina l missing p erson were disco vered w as May, 1997 aft er the snowm elt. The
national tax covered the search cost. The Kanbarasawa memorial was erecte d in order to r emember
14 persons who died in the disaster by the debris flow and to convey the memory of this disaster tofuture generations.
<Reference>”Construction Accident” (Nikkei Construction, Nikkei BP)(Note1) Institute of Environmental Science, Graduate school of Frontier Science, the University of Tokyo

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