Debris Flows and Landslides - River Dynamics and Integrated River Management - page 194

Debris flow occurs mainly in Sichuan, Yunnan, and Tibet. More than 800 counties in these provinces
(about 40% of the total) have recorded debris flows and more than 60 towns were damaged by debris
flows. According to a preliminary investigation, there are more than 10,000 debris flow gullies throughout
the country. Rainfall debris flow frequently occurs in Yunnan, Sichuan, and Gansu provinces. The Xiaojiang
watershed in Yunnan Province has a drainage area of 3,220 km 2 . There are 107 debris flow gullies in the
area. More than 100, sometimes more than 2,000 events of rainfall debris flows take place in the area and
more than 2×10 7 tons of solid material is carried into the Xiaojiang River annually. Among these debris
flow gullies the Jiangjia Ravine is the most notorious. More than 10 events of debris flow take place in
the gully every year and 28 events of debris flow were recorded in 1965. The upstream gully bed is cut
down 2-3 m per year and the incision causes debris flows every year, whereas the mouth of the gully
aggrades 1.36 m annually due to deposition of the debris. It is no wonder that the Xiaojiang watershed is
called a museum of debris flow. In the middle reach of the Bailong River in Gansu Province almost all gullies
are debris flow gullies. There are 10 debris flow gullies per kilometer along the river on average.
Glacial debris flow takes place mainly on the Qinghai-Tibet Plateau. The Guxiang gully in the plateau
is a large glacial debris flow gully. Several tens of glacial debris flows occur there annually. A glacial
debris flow of extremely large scale occurred in the gully in 1953. The maximum depth of the debris flow
was estimated at 40-95 m and the maximum discharge was estimated as 28,600 m 3 /s. About 10 million m 3
of solid material was transported by the debris flow.
The Loess Plateau is located in the middle reaches of the Yellow River, which is covered by a layer of
Quaternary loess deposits with a maximum depth of 400 m. The loess soil is liable to be eroded and the
plateau is cut by an uncountable number of gullies. Mudflows with silt concentrations up to 1,600 kg/m 3
occur in the gullies every year. The mudflows are very viscous and non-Newtonian. In many cases the
mudflows were laminar.
Debris flow shows obvious periodicity. The occurrence of very active debris flow lasts 50-70 years.
Overlapping on the long period are 6-year, 11-year, and 22-year short periods. The 1960s and 1980s were
two active periods of debris flow. Debris flows took place simultaneously in Tibet, Sichuan, Gansu,
Sha-anxi, Liaoning, and Jilin provinces in 1981. In Sichuan Province alone 61 counties were hit by debris
flows. Figure 4.5 shows the 6-year and 11-year periods of occurrence of debris flow events in the Jiangjia
Ravine near Kunming in Yunnan Province in southwest China. It also shows that the annual amount of
sediment transportation is proportional to the number of debris flow events.
Fig. 4.5 Periodic occurrence of debris flow events and annual volume of sediment transportation in the Jiangjia
Ravine near Kunming in Yunnan Province in southwest China (He et al., 2003)