Debris Flows and Landslides - River Dynamics and Integrated River Management - page 205

movement in debris flows was about 6.2 million m 3 . Moreover, debris flow will continue to occur in
future. Thus, the attenuation factor would be about 0.08. In chain 3, the attenuation factor may be
calculated with the area of rocks on which avalanches and grain erosion occurred. On the Minjiang River
from Caopo to Wenchuan, grain erosion occurred on about 30% of the bare rock surface. In other words,
the attenuation factor for avalanches and grain erosion was about 0.3. In chain 4, the new avalanches
occurred only at several places. The ratio of areas with new avalanches was very small. In conclusion the
intensity of disasters attenuates from one episode to the next episode. If the attenuation factor is defined
as the ratio of the volume of mass movement in one episode to that of the previous episode, the
attenuation factor varied in the range of 0.08-0.3.
4.1.4 Debris Flow Disasters
4.1.4.1 Burying Cities and Towns
Debris flows have brought disasters to cities, towns, and villages. Records show that about 100 cities and
towns have been hit by debris flows, such as Hanyuan, Luding, Derong, Xichang, Nanping, Luhuo, and
Jinchuan in Sichuan Province; Dongchuan, Qiaojia, Nanjian, Deqin, and Yangbi in Yunnan Province;
Jiangzi, Yadong, Basu, Dingri, Suoxian, and Dingqing in Tibet Autonomous Region; and Lanzhou, Wudu,
Wenxian, and Lixian in Gansu Province. A huge scale of debris flow occurred in the suburbs of the Xichang
in Sichuan Province in 1891, it damaged 5 streets and caused more than 1,000 casualties.
On July 8, 1984, a debris flow was triggered in the Guanmiao Ravine, Nanping County in Sichuan
Province. The debris flow carried 60 huge stones of diameter 5-10 m and 430 stones of diameter 2-5 m
and rushed down to the county town with a velocity of 9.2 m/s. It cut away half of a three story building
and destroyed a 1 m thick concrete wall of a prison. Boulders, rocks, gravel, and silt buried the street.
Rain continued from August 18 to 20, 1978, in the Pingshan County of Hebei Province, which is located
on the east slope of Taihang Mountain. The soil was saturated with water. Finally, a strong rainstorm with
an intensity of 400 mm in 4 hours on August 20 initiated a debris flow in the evening. The debris flow
carried 12 million m 3 of solid material and rushed into the Niujuangou at high speed. The head of the
debris flow was 13 m high. The whole village was flattened and more than 20 people were killed.
In the watershed of the Daying River in Yunnan Province, there are 116 debris flow gullies. The
Lianghe is located in the downstream fan of two debris flow gullies. Two debris flows occurred in 1975
and buried the long distance bus terminal and the Lianghe High School. A debris flow in 1977 rushed
into the town of Jiubao, and destroyed 100 houses and killed 1 person. In 1968 a terrible debris flow
occurred in the Nanhuai River (a tributary of the Daying River) and washed 3 villages away killing 97
people. More than 200 ha of farmland and several sections of the highway were buried.
In Xide County in Sichuan Province, the town of Hongmo was flattened by a debris flow in 1967. All
buildings and houses were destroyed and 80 people were killed. The town residents had to resettle in
neighboring areas. On July 10, 1988, a debris flow from Huaying Mountain buried the town of Xikou,
Ma-an-ping village, the Xikou cement factory and the No.12 coal mine. Gravel and silt buried many
buildings, transportation facilities, and 221 people. On May 27, 1984, a debris flow from the Heishan
Gully buried the Yinmin copper mine in Dongchuan, Yunnan Province. The mine stopped production for
half a month and lost a million dollars.
4.1.4.2 Cutting Railways and Highways
Debris flows have brought disasters to railways by burying railway tracks and stations, damaging railway
structures, derailing trains, killing passengers, and cutting off transportation. There are 1,368 debris flow
gullies along the railways in China. Two hundred and ninety-two debris flow disasters have happened in
the past years and 41 railway stations were buried. Railway transportation was stopped for 7,500 hours
between 1950 and 1990 (Shen et al., 1991).