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.