1 Introduction
Alluvial fan is a fan landform which surface forms a cone deposit that radiates downslope (Bull, 1977). It is consisted of solid material eroded from an upland catchment (Ashworth, 2006). Alluvial fans can be developed in many types of terrestrial settings, such as alpine, humid tropical, humid mid-latitude, periglacial and different paraglacial settings (Dorn, 1994). There are many factors for alluvial fans development. The area, slope gradient, bedrock types, geology characteristics of catchment effect the landform material of alluvial fan (Blair, 2002; Stock et al., 2008). For example, there are two adjoining alluvial fans in the Death Valley of California, and one is formed through flooding processed and another is through debris flow processed just because of the different of bedrocks with the other almost similarity. At the same time, the average slope of flooding-processed fan is lower than debris-processed fan, but the area, radius is opposite. Besides, the material of former is mainly composed by Andesite and Granite, but the latter is mainly Precambrian to Cambrian sedimentary rocks (Blair, 2002). Regional tectonic and climate effect the deposit process and geomorphology of alluvial fans (Bull, 1977). Tectonic forces effect the morphology by controlling the scale of accommodation space and organizational types of deposit material (Sarp, 2015). The increasing aridity in the Late Pleniglacial result that the alluvial fan is small-scale in central Europe, and the scale is more in the humid times (Sarp, 2015). Even though existing many factors for alluvial fan’s formation, the more directly factor for alluvial fans are runoff flow, including river, flood and debris flow. Accordingly, there are mainly three types about forming process of alluvial fans, including fluvial process, flooding process and debris flow process (Birch et al., 2016; Dickerson et al., 2013; Santangelo et al., 2012).
The land that is suitable for settlement or cultivation is shortage due to harsh climate and natural hazards in many mountain regions, but some area, like alluvial fans, also have possibilities about better hydrological conditions, fertile soil, relatively flat terrain and so on (Khan et al., 2013; Mazzorana et al., 2020; Rahaman, 2016; Telbisz et al., 2016). Therefore, alluvial fans are priority among settlement areas or cultivation in many mountain regions (Ma et al., 2004; Mazzorana et al., 2020). Even some large-scale alluvial fans are used to develop cities or towns (Chen et al., 2017; Maghsoudi et al., 2014; Santangelo et al., 2012). Besides, alluvial fans include kinds of solid materials from clay, slit to sand, gravel and large boulders, so some are regarded as construction resources (Bahrami et al., 2015). Hence, alluvial fan is an important land resource although few researches could focus on this point. Moreover, alluvial fans are threatened by some types of natural hazards because of special forming processes and environments. And natural hazards vary among different-processed alluvial fans. The fluvial-processed alluvial fans can easily develop some cities due to relatively plenty water resource including groundwater. So, the land subsidence, water shortage and pollution are commonly natural hazards in those alluvial fans because of long-term overutilization and contamination of water resource (Alkinani et al., 2019; Bull, 1973; Chen et al., 2017). The surface of flooding-processed and debris-flow-processed alluvial fans are more taken in the risk of floods (Dickerson et al., 2013; Santangelo et al., 2012), debris flows (Khan et al., 2013; Okunishi & Suwa, 2001), or gully erosion (Deng et al., 2019). That is to say, those 2 types of alluvial fan are formed by debris flow or flood processing, meanwhile the surfaces of alluvial fans are threatened by hazards due to those processings. Indeed, alluvial fans exist above natural hazards, but they are still used in kinds of ways. The contradiction also reflects the fact the available land is shortage in some mountain regions. Therefore, alluvial fan is an important land resource although few researches focused on this point. The studies about survey, protection, plan and reasonable utilization of alluvial fans need to be down more.
Tibet plateau is the highest region in the earth, and the majority of land in Tibet isn’t suitable for local human to use and live, because of the extreme and vulnerable environment, which means the land is limited for development (Liang & Hui, 2019). A good habitability of mountain regions is based on hydrological, geomorphological, biogeographical properties and so on ((Telbisz et al., 2016). Only terraces, alluvial fans and gentle-slope maintain areas can basically satisfy those properties in Tibet, but the terraces have been well utilized due to better gentle terrain, fertile soil and hydrological conditions. For this reason, the area that suitable to local human to live is more finite in Tibet. Meanwhile, the population is dramatically increasing, and Tibet has been in an accelerated urbanization stage in recent years (Fan et al., 2010). Therefore, the human-environment interaction is gradually becoming to uncoordinated state due to shortage of the land for cultivation and settlement (Liang & Hui, 2019). Many farmlands and villages are distributed in alluvial fans, although they are threatened by floods and gully erosion (Ma et al., 2004). In order to relieve this problem, alluvial fans located in Tibet need to be focused. However, researches about alluvial fan is rarely, even though alluvial fan is an important land resource in Tibet. The quantity, distribution and land use of alluvial fans in this region are insufficiently understood, but it is important to know, use and protect this land resource. Lhasa city is the capital of Tibet in China and is located in Lhasa River Basin (LRB), where the need for land is urgent. So, we chose this basin to investigate the amount, distribution and land use of alluvial fans. Meanwhile, the distance from alluvial fans to different scale of roads, places (town and village) and rivers are analyzed to illustrate the value, function and utilization potential of this land resource in LRB.