4.1 The quantity, area and distribution of alluvial fans
There are many sizes of alluvial fans in LRB, and the distribution of quantity and area is uniformed. The formation and development of alluvial fans base the catchment characteristics, tectonics and climate (Sarp, 2015). The size of alluvial fans is major determined by the characteristics of catchment (Mather et al., 2016). Typically, the larger alluvial fans have larger and gender catchment with greater possibility to accumulate to sediment flows. Moreover, the catchments of smaller alluvial fans are smaller and steeper, which the ability of storing slope deposits is limited (Mather et al., 2016). And this regulation needs to be verified in LRB. As above, the sediment flow is a direct factor for alluvial fan. The mountain including many catchments of alluvial fan is covered by ice and glaciers throughout the year in the west of LRB. Especially, the area of ice and glaciers of Damshung is largest in LRB with 535.95 km2 in 2010 (Xu, 2013), so the water (meltwater) carrying sediment is enough. The ice or glaciers in LRB except Daumshung are shortage because the majority of elevation of those regions is not more than 4000m (Wei et al., 2012). Therefore, the size of alluvial fans in the west is more than other regions in LRB. Furthermore, the exist of ice or glaciers should be an important factor for the sizes of alluvial fans.
Meanwhile, the size and uniformed distribution of alluvial fans may be caused by tectonic activity. The topographic uplift of Damshung area is earlier than other area in LRB ((Wang et al., 2017), which induce a difference of accommodation space of alluvial fan and fan catchment due to tectonic force (Sarp, 2015). The size, distribution, formation and development of alluvial fan should be studied in next step.