3.2.1 Quantity and area distribution
The quantity of alluvial fans is concentratedly distributed in the center and east of LRB, especially in the Lhundup and Medro Gongkar and south of Chali county (Fig5.a). The number of alluvial fans in those three counties is 452, accounting for 54.72% in all alluvial fans. However, the distributing trend of area is contrary with quantity (Fig5.b). Compared with the east, the large area is in the west, especially in Damshung county, and the area proportion of all alluvial fans is 41.61% of total area. Besides, there are a great number of alluvial fans in Medro Gongkar and Chali county (156 and 122), however, the area is small (89.95 km²and 60.42 km²),and their proportion in all alluvial fans’ area are 7.71% and 5.18%, respectively.