3.1 The quantity of alluvial fans
The quantity of alluvial fans with different size is provided in Fig.3. As a whole, there are 826 alluvial fans in LRB. The number of 2nd size (0.1 km²<area≤1 km²) is the most in all the sizes with a number of 517, and it’s proportion is 62.59%. And then, the number of 3rd size (1 km²<area≤10 km²)is the second with a number of 205. The number of 4th (area>10 km²) is 17, just only accounted for 2.06% in the total alluvial fans. That means the alluvial fans with area less than 1 km²are dominated, and with area more than 10 km²are in LRB.
Furthermore, the alluvial fans are ranked by area from small to large, and there is an exponential relationship (the equation: y=3.93e0.067x, R2=0.92) between accumulating area with quantity of alluvial fans, which is showed in Fig4. It is found that the accumulating area of alluvial fans is 83.39 km2, when the quantity accounting for 50% of total quantity of alluvial fans. Furthermore, the quantity of alluvial fans is 746.27, when the area accounting for 50% of total area with 1166.03 km2, which indicates that the area is occupied by 26 large-size alluvial fans (4th).