Vegetation-Erosion Dynamics - River Dynamics and Integrated River Management - page 64

for 40 minutes. At the same wind speed the amount of grains blown down by wind per time reduced from
1,000 g/m 2 min in the first 10 minutes to only 100 g/m 2 ·min in the fourth 10 minutes because after the
grains on the top surface were removed the remaining grains were not readily detached from the rock. No
general laws for the size of largest grains blown down by wind were found. It seems that the grain size
increased from 20 mm to 45 mm, and, then, it reduced to about 25 mm as shown in Fig. 2.16(d).
Fig. 2.16 (a) Amount of grains blown by wind from 1 m 2 of rock surface per time as a function of the wind speed;
(b) The largest grain size blown by wind as a function of wind speed; (c) Reduction with time of the amount of grains
blown down by wind from 1 m 2 of rock surface per time in consecutive experiments; (d) Variation of the largest size
of blown grains in consecutive experiments (Wang et al., 2010)
The rate of grain erosion blown by wind per area per time in Fig 2.16 represents a high instantaneous
rate of grain erosion. The annual rate of grain erosion, however, depends on the frequency of high speed
winds, rate of sun weathering, and the action of temperature change. In general, the bare rock may be
eroded by several to several tens of centimeters per year, depending on the lithology, location, local
weather, and winds. As shown in Fig. 2.13(b) the depth of the grain erosion deposit on the avalanche
deposit fan, which could be easily identified by its uniform size, was measured with a scale at two or
three places. The average depth of the grain erosion deposit multiplied by the surface area of the fan was
the volume of grains eroded from each grain erosion site in the past year. The area of grain erosion of the
bare rock surface was measured with laser range meters, which have a maximum error of 1m. The bare
rock surfaces were generally larger than 100 m in length and width, therefore, the maximum relative error