Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 60

the highway along the Minjiang River. The highway managers have hired many people to monitor the flying
stones and issue warning signals. The highways are occasionally closed because of these flying stones.
Grain erosion provides plenty of solid materials for mass movements. The deposit fans consist of
uniform and loose solid materials and have high slope. Rainfall with an intensity of more than 20 mm/day
triggers mass movements of the grains. These mass movements behave like debris flows but the distance
of movement is, however, much shorter than normal debris flows, and in general travel distance is only
several tens to one hundred meters. With water in the interstices of particles, which plays the role of
lubrication, the grains move down the slope to streams or highways. Such a mass movement is called slope
debris flow. The slope debris flow carries a lot of grains into rivers or deposits the grains on highways,
causing blockage of highway transportation or local sedimentation on the riverbed. Figure 2.12(a) shows
slope debris flow sediment deposited on the highway along the Mianyuan River. A rainfall of intensity of
47 mm/day initiated the slope debris flow, which carried sediment from the grain erosion deposit fan for
a short distance from the high slope to the highway. The highway transportation was cut off for a few
months by numerous slope debris flows. The angle of the debris flow deposit was about 15 degrees,
much higher than normal debris flow deposit. A lot of grain erosion occurred along the Minjiang River.
Figure 2.12(b) shows that a grain erosion deposit caused local sedimentation in the Minjiang River and
changed the flow regime. The aquatic ecosystem was impacted by the sedimentation. Fish and benthic
invertebrates lost their habitat due to the sedimentation.
(a) (b)
(c) (d)
Fig. 2.11 (a) Grain flow in the Xiaobaini Ravine scoured the bank slope to form a 2 m deep 42 o slope channel; (b)
Several grain erosion sites along the Minjiang River near Wenchuan; (c) Grain erosion on the north bank of the
upper Jiangjia Ravine, in Yunnan, China; (d) Grain flow on the slope of Jiuzhai Creek, which has killed many trees
(See color figure at the end of this topic)