Meteorological characteristics of debris flow disaster in Otari village on 6th December 1996.

Motyuki USHIYAMA*

　

*Tokyo metropolitan Univ., Tokyo, Japan.

Keywords: debris flow, The high temperature condition of the mountain area during winter, snowmelting,

　

　

1. Introduction

On 6th December 1996, a debris flow occurred in the Gamahara Torrent in Otari village at central district of Japan(Fig. 1). Fourteen peoples died due to this debris flow. This mass disaster occurred in the winter although the possibility of occurrence of such debris flow during winter is rare in Japan.

　

　

2. The use data

Daily rainfall, temperature, and snow depth data for 7 stations are compiled from the record available at the JMA(Japan Meteorological Agency), the Nagano prefecture office and a company(Fig. 1.).

　

3. Result

3.1 Recent debris disasters occurred at the Otari village

Recently, Otari village was affected by the heavy rainfall in July 1995(Case A), and June 1996(Case B). In Case A, total precipitation was 389 mm and it was 193 mm for Case B. The highest daily precipitation at the Otari was recorded for Case A, and the second highest was recorded for Case B(Fig. 2). In both case, many debris flow and slope's landslides occurred. For Case A, some debris flow also occurred at Gamahara torrent basin, which also a primary cause in occurring the debris flow over this area on 6th Dec 1996.

　


Fig. 2. Highest dairy precipitation in a year(Otari, 1951-1996)

　

3.2 The existence of the high temperature condition of the mountain area

It was snowing when the debris flow occurred. But, many precipitation was recorded on previous day, 5th Dec. On this day, the central area of Otari village temperature was 2 to 5 . The temperature near the area(1560m) was also recorded of 2 to 5 (Fig. 3). Therefore, the mountain area recorded rain. I call this weather condition "The high temperature condition of the mountain area during the winter". It is the phenomenon which is sometimes observed in the central mountain area in Japan. As a result, high amount of rain comes under this area during the winter.

Fig. 3. The temperature of Otari village preceding the debris flow.

　

3.3 Precipitation

On Dec 5, the total amount of precipitation near Otari village was 35 to 77 mm. This day, the depth of snow cover decreased by about 30 cm(Fig. 4). Therefore we may argue say that it was about 30 mm of snowmelting. As a result, water overthis area accumulated about 60 to 110 mm. Such accumulated rainfall along may not be the provoking cause and futher detail analysis are needed.


Fig. 4. Depth of snow cover and precipitation of Otari village preceding the debris flow