3.2.2 The spatial distribution of different size alluvial fans
The spatial distribution of different size alluvial fans is shown in Fig.6. The 1st to 3rd size of alluvial fans almost exist in all counties in LRB, and the 4th size exists 4 counties. The quantity of 1st alluvial fans mainly distributes in the east (Medro Gongkar and Chali county), and are rare in the west and south (Fig. 6a). The quantity of 2nd alluvial fans is largely distributed in Lhundup, Modero Gongkar, and Chali (Fig. 6b). Besides, the 2nd ones relatively have more quantity in those three counties than other sizes. The 3rd mainly distributes in the Damshung and Lhundup county (Fig.6c). The quantity of 4th is a few than other sizes, and are sporadically distributed in the Damshung and Lhundup couty (Fig.6d). Especially, the 4th alluvial fans existed in the Damshung county just are 10 with the area of 361.61 km², which is account for 31.01% of total area of alluvial fans.