Debris Flows and Landslides - River Dynamics and Integrated River Management - page 207

along the highway, and damaged or blocked 200 culverts. Highway transportation was stopped for more
than 1,500 days. Annual cost resulting from debris flow was more than 1 million dollars. In 1985, a huge scale
debris flow occurred in the Peilong Gully, which is located in the Bomi-Dongjiu section of the highway.
The debris flow destroyed several hundred meters of the highway and overturned more than 80 vehicles
including trucks, buses, and cars. The highway was then blocked for more than 6 months (Wu et al., 1993).
4.1.4.3 Damming Rivers
Many debris flow gullies are perpendicular to main river channels. Debris flow carries a huge amount of
solid materials to the confluence with the main river and often dams the main river channel. Sometimes
the debris dam is high and a barrier lake is formed. The retained water inundates farmland, roads, highways,
and villages upstream from the debris dam. When the debris bar is broken, a more serious catastrophe
can be caused to the downstream area.
It is recorded that debris flows from the Baishui Gully in Yunnan Province dammed the Yangtze River
two times. Debris flows from the Hailuo Gully dammed the Longchuan River, a tributary of the Yangtze
River three times. On July 18, 1984, a debris flow from the Guanmiao Ravine of Nanping County, Sichuan
Province, dammed the Bailong River. The debris dam was broken after 30 minutes and a torrential flood
was generated. The flood carried the sediment downstream and destroyed villages, many houses, farmland,
and highways. The flood also resulted in aggradation and incision of the downstream channel.
Debris flows from the Jiangjia Gully dammed the Xiaojiang River for 48 days in 1919, 40 days in 1937,
30 days in 1949, 19 days in 1954, 78 days in 1961, 98 days in 1964, and 90 days in 1968. The dammed
river water inundated 600 ha of farmland, highway, and railway. Figure 4.19(a) shows the debris flow
deposit, which dammed the Xiaojiang River for 2 weeks. Figure 4.19(b) shows the Yangtze River, which
is almost dammed by the debris flow deposit from the Xiaojiang River.
(a)
Fig. 4.19 (a) A debris flow deposit dammed the Xiaojiang River; (b) The upper Yangtze River (Jinsha River) is almost
dammed by a debris flow deposit from the Xiaojiang River