# Effects of Glacier and Geomorphology on the Mechanism Difference of Glacier-Related Debris Flow on the South and North Banks of Parlung Zangbo River, Southeastern Tibetan Plateau

Authors: Jiajia Zhang; Jiankang Liu; Yuanling Li; Junchao Wang; Long Chen; Bo Gao
Journal: Advances in Civil Engineering
 (2022)

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## Abstract

Alpine glaciers are vulnerable to climate changes, and their recession due to warming has already induced a large number of geohazards closely related to the glacial motion, such as debris flow. Strong coupling of the geology, geomorphology, climate, and glacial action controls the type, size, development, and frequency of debris flow along the Parlung Zangbo River, Southeastern (SE) Tibetan Plateau. Field investigation in recent years indicates that the north bank is prone to much more frequenter debris flows than the south bank. The sharp contrast between the two river banks is due to the different formation conditions of debris flows, especially glaciers and geomorphology. The present paper examines the differences in the glacier and geomorphology conditions for glacier-related debris flows to occur, through a combination of field investigation, interviews with local residents, and geomorphological parameter, and glacier distribution analysis in the ArcGIS platform. The result indicates that, compared to the south bank of the Parlung River, the north bank has more favorable conditions in the glacier and geomorphology south, which is more conducive to the occurrence of debris flow. In conclusion, three kinds of mechanisms due to different formation conditions, especially glacier and geomorphology, are analyzed. The glacier-related debris flows on the south and north banks occur through different mechanisms due to different formation conditions. The debris flows on the north bank primarily occur in modes I and III, while the south bank is dominated by modes II and III debris flows. Based on the result, effective measures are proposed for mitigating damage to roads and railways within this area.


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## Body

## 1. Introduction

Glacier-related debris flow is a natural geological phenomenon closely related to the glacial motion, usually triggered by glacial meltwater or intense rainfall. Large glacier-related debris flows could also be caused by icefalls, avalanches, or failure of dams across gullies [1–6]. Glacier-related debris flows are sudden, violent, and difficult to forecast, and tend to travel long distances at high speed. This kind of geohazard, which typically occurs in alpine regions, can cause severe damage to villages, towns, factories, mines, roads, and railways along gullies or at their mouths and thereby terrible loss of life and property [1, 7–10].Running across the Tibetan Plateau, the Parlung Zangbo River (hereafter “Parlung River”) is among the world’s rivers with the highest elevation drops and greatest erosive forces. Due to this as well as the frequent earthquakes and abundant precipitation there, this river is located in one of the regions subjected to the largest-scale, most violent, and most hazardous debris flows in the world [11–13]. The Parlung River is home to the largest temperate glaciers in China [14, 15]. The river basin has undergone active endogenic and exogenic geological processes and has an extremely complex tectonic setting and strong neotectonic activity [16]. This, combined with the alpine-gorge terrain, high precipitation, and modern glacial activity, contributes to landslides, debris flow, and other major geohazards, among which glacial debris flow is the most destructive [13, 16, 17].Many large debris flows have taken place along the Parlung River throughout history. The Sichuan–Tibet Highway, a section of China National Highway 318 (G318) on the north bank of the Parlung River, has been blocked by many debris flows since it was built in the 1950s. For this reason, researchers have conducted a systematic regional survey. Glacier-related debris flows in this area were intensively studied during the comprehensive scientific investigation of the Tibetan Plateau in the 1970s [16, 18]. Recent studies have looked at deglaciation resulting from global warming in detail. Existing research has provided plenty of valuable data and insights into the characteristics of geohazards along the Sichuan–Tibet Highway and the conditions and mechanisms for their occurrence and also proposed preventive measures [17, 19, 20].Large glacier-related debris flows are considered a serious obstacle to regional economic development and a threat to major construction projects, such as the Sichuan–Tibet Railway (from Chengdu to Lhasa, Figure1), which is planned to span the Parlung River region. In order to ensure normal traffic along the Sichuan–Tibet Highway and the successful implementation of China’s Western Development Program, there is currently an urgent need to address glacier-related debris flows and ensuing environmental problems in the study area in glacier cover and geomorphological settings.Figure 1
Geological sketch map of the eastern Himalayan syntaxis (after [21, 22]); (Abbrs.: towns, BM-Bomi, ST-Suotong, TM-Tongmai, LY-Layue, LZ-Linzhi, ML-Milin, MT-Motuo. Faults, TZF-Tongmai–Zhongkang fault; TJF-Tongmai–Jinzhula fault; DMF-Dongjiu–Milin fault; MTF-Motuo fault. Numbers: 1-bitong debris flow; 2-dada rock avalanche-debris flow; 3-jiaolong debris flow; 4-cangguo debris flow; 5-zhataduo debris flow; 6-naha debris flow; 7-guxiang rock avalanche-debris flow; 8-suotong debris flow; 9-qiuzhu debris flow; 10-chidan debris flow; 11-songrao debris flow; 12-michong debris flow; 13-danka debris flow).The present paper examines the differences in the Glacier and geomorphology conditions for glacier-related debris flows to occur between the south and north banks of the Parlung River, analyzes the primary mechanisms due to different formation conditions, through a combination of field investigation, and interviews with local residents, geomorphology parameters analysis and glacier distribution analysis in ArcGIS platform. Based on the result, effective measures are proposed for mitigating damages to roads and railways in the study area.Geohazard history and size scale are from field investigation, interviews with local residents, and previous research. Geomorphology parameters like catchment morphological index, HI which are extracted from ArcGIS software, and high-resolution DEM which is from unmanned aerial vehicle mapping, describe the feature of gullies on both banks. Glacier data from the Second Glacier Inventory Dataset of China [15] are used to analyze the glacier distribution in every gully in GIS.


## 2. Materials and Methods

### 2.1. Geological Conditions, Climate, and Precipitation

The Parlung River is located in the northeastern part of the Eastern Himalayan Syntaxis, at the eastern end of the Gangdise–Nyainqentangla fault-fold belt. The area is mainly composed of gneiss of the Nyainqentanglha group and the Gangdise granite and metamorphic Paleozoic rocks (Figure1; [21, 23–25]). Neotectonics is characterized by fast regional crustal uplift with multiple fault activities. The Tongmai–Zhongkxing fault (TZF) and the Tongmai–Jingzhou (TJF) are two important active faults with an average dextral strike-slip rate of 4 mm/a (Figure 1; [26, 27]). The syntaxis region is highly seismic with 100 earthquakes of M 5.0 or greater from 1950 to 2018, including the Ms 8.6 Assam–Tibet earthquake in 1950 [28].Located in the southeastern Tibetan Plateau, the Parlung River is more sensitive to climate change than any other parts of Western China. It has a subtropical alpine climate affected by the Indian monsoon. The Indian monsoon carries moisture to the Tibetan Plateau, primarily along the valley of the Yarlung Zangbo River (hereafter “Yarlung River”) west of the Parlung River and the Dandong and Chayu Rivers to the south, which all flow southward (Figure1). This leads to high precipitation on the plateau. For example, the average annual precipitation is as high as 1100–1400 mm/a in Tongmai and Bomi (Figure 1). Due to the humid air and high elevations, the Parlung River has the largest expanses of temperate glaciers in China. A total of 1320 modern temperate glaciers are distributed in the Kangri Karpo Mountains on the river’s south bank, covering an area of 2655 km2 [29–31]. The high precipitation and temperature attributed to the warm moist air from the Bay of Bengal, plus global warming, keep the surface temperatures of the temperate glaciers in this area at around 0°C. Therefore, the glaciers undergo faster ablation than an accumulation of ice and move actively. As a result, deglaciation occurs extensively and provides a lot of materials and water needed for the formation of debris flows.


### 2.2. Geomorphology

The Parlung River extends to the northeast of the Eastern Himalayan Syntaxis. It is characterized by alpine gorges and deep-incised river valleys with extremely high erosion rates of >5 mm/a [24], and its elevation tends to decrease from east to west. The river flows through wide valleys in its upper reaches and through narrow gorges in its lower reaches. It is bounded to the north by the southeastern branch of the Nyainqentangla Mountains and to the south by the Kangri Karpo Mountains (Figure 2). The extensive development of glaciers and intense tectonic processes has created a wide variety of geomorphic features, such as glacial landforms, alpine gorges, and fluvial depositional landforms (Figure 2).Figure 2
Glacier distribution and overall form of gullies along two banks between Bomi and Suotong in Figure1 (glacier data are from [15]).In the paper, Ke and HI parameters are used to describe the geomorphology features of each gully on the two banks of the Palung River, Ke is the plane shape index of the catchment, and HI is the hypsometric integral. The parameters are calculated in ArcGIS software based on the digital elevation model (DEM), whose resolution is 8 m. The larger the Ke value is, the closer the plane shape of the catchment is to the circle, and the better the catchment capacity is [32]. Ke can be obtained by using equation (1) [33].(1)Ke=WL.where W is the largest width of the catchment and L is the length of the valley.HI is widely used to quantitatively determine the evolution stage of landforms. The smaller the value is, the more mature the landform is, and the higher the erosion degree is. HI can be obtained by equation (2) [34].(2)E=Hmean−HminHmax−Hmin=HI.Hmean, Hmax, and Hmin are mean value, maximum value, and minimum value of elevation in one catchment.


### 2.3. Geohazard History

Strong coupling of the above geology, climate, geomorphology, and glacial actions control the type, size, development, and frequency of debris flows in the Parlung River region. Since the 1950s, the study area has been frequently hit by debris flows of different sizes, resulting in huge loss of life and properties (e.g. 1–13 in Figure1; [16]) and has a tendency for concentrated outbreaks (Figure 3). In particular, some debris flows were extremely disastrous, such as that occurring in the Gu Xiang area on September 29, 1953 [14].Figure 3
Debris flow events from investigation in the field. (a) Bitong debris flow in 2016; (b) Dada debris flow in 2017. (c) Guxiang debris flow in 2020. (d) Jiaolong debris flow in 2016; (e) Chidan debris flow in 2016. (f) Songrao debris flow in 2018).
(a)(b)(c)(d)(e)(f)According to the size classification, which is based on the total volume, peak discharge, and flooded area in the field, the size classes of the debris flows are given in Table1, coinciding with the debris flow frequency based on interviews with locals and previous studies. The result shows that in the region between Bomi and Suotong (Figure 2), the south and north banks of the Parlung River differ greatly in terms of the occurrence of debris flows. Compared to the north bank, the south bank shows much smaller sizes and a lower frequency of debris flows (Table 1).Table 1
Typical debris flows along the Parlung River from Bomi to Suotong (e.g. 1–13 in Figure1).
No.NameDateSize classData sourceLocation1Bitong gully5 Sep. 2007MiddleInterviews with local residentsNorth bank5 Sep. 2016LargeField investigation2Dada gully4 Aug. 2013SmallInterviews with local residents20 Jul. 2017MiddleField investigation3Jiaolong gullySep. 1988Middle[14]15 Aug. 1989Large[14]5 Sep. 2016MiddleField investigation4Cangguo gully1975Large[14]1996Large[14]5Zhataduo gullyJul. 1987Large[14]6Naha gully1968Super-largeInterviews with local residents1978Large[14]Sep. 2018MiddleField investigation7Guxiang gully29 Sep. 1953Super-large[28]1954Large[14]1963Large[14]30 Jul. 2005LargeInterviews with local residentsJul. 2020LargeField investigation8Suotong gully26 Jul. 1991Middle[14]9Qiuzhu gully2014MiddleInterviews with local residentsSouth bank28 Aug. 2016MiddleField investigation10Chidan gully5 Sep. 2016MiddleField investigation11Songrao gully4 Sep. 2007Large[32]25 Jul. 2010Large[32]11 Jul. 2018LargeField investigation12Michong gullyAug. 2015SmallInterviews with local residents13Danka gullyAug. 2015SmallInterviews with local residents


## 2.1. Geological Conditions, Climate, and Precipitation

The Parlung River is located in the northeastern part of the Eastern Himalayan Syntaxis, at the eastern end of the Gangdise–Nyainqentangla fault-fold belt. The area is mainly composed of gneiss of the Nyainqentanglha group and the Gangdise granite and metamorphic Paleozoic rocks (Figure1; [21, 23–25]). Neotectonics is characterized by fast regional crustal uplift with multiple fault activities. The Tongmai–Zhongkxing fault (TZF) and the Tongmai–Jingzhou (TJF) are two important active faults with an average dextral strike-slip rate of 4 mm/a (Figure 1; [26, 27]). The syntaxis region is highly seismic with 100 earthquakes of M 5.0 or greater from 1950 to 2018, including the Ms 8.6 Assam–Tibet earthquake in 1950 [28].Located in the southeastern Tibetan Plateau, the Parlung River is more sensitive to climate change than any other parts of Western China. It has a subtropical alpine climate affected by the Indian monsoon. The Indian monsoon carries moisture to the Tibetan Plateau, primarily along the valley of the Yarlung Zangbo River (hereafter “Yarlung River”) west of the Parlung River and the Dandong and Chayu Rivers to the south, which all flow southward (Figure1). This leads to high precipitation on the plateau. For example, the average annual precipitation is as high as 1100–1400 mm/a in Tongmai and Bomi (Figure 1). Due to the humid air and high elevations, the Parlung River has the largest expanses of temperate glaciers in China. A total of 1320 modern temperate glaciers are distributed in the Kangri Karpo Mountains on the river’s south bank, covering an area of 2655 km2 [29–31]. The high precipitation and temperature attributed to the warm moist air from the Bay of Bengal, plus global warming, keep the surface temperatures of the temperate glaciers in this area at around 0°C. Therefore, the glaciers undergo faster ablation than an accumulation of ice and move actively. As a result, deglaciation occurs extensively and provides a lot of materials and water needed for the formation of debris flows.


## 2.2. Geomorphology

The Parlung River extends to the northeast of the Eastern Himalayan Syntaxis. It is characterized by alpine gorges and deep-incised river valleys with extremely high erosion rates of >5 mm/a [24], and its elevation tends to decrease from east to west. The river flows through wide valleys in its upper reaches and through narrow gorges in its lower reaches. It is bounded to the north by the southeastern branch of the Nyainqentangla Mountains and to the south by the Kangri Karpo Mountains (Figure 2). The extensive development of glaciers and intense tectonic processes has created a wide variety of geomorphic features, such as glacial landforms, alpine gorges, and fluvial depositional landforms (Figure 2).Figure 2
Glacier distribution and overall form of gullies along two banks between Bomi and Suotong in Figure1 (glacier data are from [15]).In the paper, Ke and HI parameters are used to describe the geomorphology features of each gully on the two banks of the Palung River, Ke is the plane shape index of the catchment, and HI is the hypsometric integral. The parameters are calculated in ArcGIS software based on the digital elevation model (DEM), whose resolution is 8 m. The larger the Ke value is, the closer the plane shape of the catchment is to the circle, and the better the catchment capacity is [32]. Ke can be obtained by using equation (1) [33].(1)Ke=WL.where W is the largest width of the catchment and L is the length of the valley.HI is widely used to quantitatively determine the evolution stage of landforms. The smaller the value is, the more mature the landform is, and the higher the erosion degree is. HI can be obtained by equation (2) [34].(2)E=Hmean−HminHmax−Hmin=HI.Hmean, Hmax, and Hmin are mean value, maximum value, and minimum value of elevation in one catchment.


## 2.3. Geohazard History

Strong coupling of the above geology, climate, geomorphology, and glacial actions control the type, size, development, and frequency of debris flows in the Parlung River region. Since the 1950s, the study area has been frequently hit by debris flows of different sizes, resulting in huge loss of life and properties (e.g. 1–13 in Figure1; [16]) and has a tendency for concentrated outbreaks (Figure 3). In particular, some debris flows were extremely disastrous, such as that occurring in the Gu Xiang area on September 29, 1953 [14].Figure 3
Debris flow events from investigation in the field. (a) Bitong debris flow in 2016; (b) Dada debris flow in 2017. (c) Guxiang debris flow in 2020. (d) Jiaolong debris flow in 2016; (e) Chidan debris flow in 2016. (f) Songrao debris flow in 2018).
(a)(b)(c)(d)(e)(f)According to the size classification, which is based on the total volume, peak discharge, and flooded area in the field, the size classes of the debris flows are given in Table1, coinciding with the debris flow frequency based on interviews with locals and previous studies. The result shows that in the region between Bomi and Suotong (Figure 2), the south and north banks of the Parlung River differ greatly in terms of the occurrence of debris flows. Compared to the north bank, the south bank shows much smaller sizes and a lower frequency of debris flows (Table 1).Table 1
Typical debris flows along the Parlung River from Bomi to Suotong (e.g. 1–13 in Figure1).
No.NameDateSize classData sourceLocation1Bitong gully5 Sep. 2007MiddleInterviews with local residentsNorth bank5 Sep. 2016LargeField investigation2Dada gully4 Aug. 2013SmallInterviews with local residents20 Jul. 2017MiddleField investigation3Jiaolong gullySep. 1988Middle[14]15 Aug. 1989Large[14]5 Sep. 2016MiddleField investigation4Cangguo gully1975Large[14]1996Large[14]5Zhataduo gullyJul. 1987Large[14]6Naha gully1968Super-largeInterviews with local residents1978Large[14]Sep. 2018MiddleField investigation7Guxiang gully29 Sep. 1953Super-large[28]1954Large[14]1963Large[14]30 Jul. 2005LargeInterviews with local residentsJul. 2020LargeField investigation8Suotong gully26 Jul. 1991Middle[14]9Qiuzhu gully2014MiddleInterviews with local residentsSouth bank28 Aug. 2016MiddleField investigation10Chidan gully5 Sep. 2016MiddleField investigation11Songrao gully4 Sep. 2007Large[32]25 Jul. 2010Large[32]11 Jul. 2018LargeField investigation12Michong gullyAug. 2015SmallInterviews with local residents13Danka gullyAug. 2015SmallInterviews with local residents


## 3. Analysis of the Glacier and Geomorphology Conditions of Debris Flow

Due to its unique geographic location and climate, debris flows along the Parlung River are closely associated with moisture distribution and glacial movement. Zheng et al., [35] found that they are a direct consequence of glacier’s recession.


### 3.1. Comparison of Hydrology and Glacier Distribution between the Two River Banks

The data from field investigation and the Second Glacier Inventory Dataset of China [15] suggest significant differences in glacier distribution between the north and south banks of the Parlung River. In terms of the glacier type, valley glaciers are distributed only along the south bank, while the north bank is covered primarily by cirque glaciers and hanging glaciers. The snow line is generally higher in the east than in the west (Table 2).Table 2
Glaciers along the Parlung river from Bomi to Suotong.
LocationTypes of glacierNumber of glaciers and percentageArea of glaciers (km2) and percentageVolume of glaciers (106× m3) and percentageAverage snow line (m)North bankHanging glacier1820.93%41.1230.84%2867.4533.934040Cirque glacier1011.63%4.963.72%142.571.69%Valley glacier00%00%00%South bankHanging glacier2326.74%31.1823.38%2058.0124.35%3429Cirque glacier1517.44%5.824.36%194.732.30%Valley glacier2023.26%50.2737.70%3188.5437.73%Sum total—86100%133.35100%8451.3100%—In the region between Bomi and Suotong, the north bank of the river has 28 glaciers, which account for 32.56% of the region’s total number of glaciers, 34.56% of glacier coverage, and 35.62% of ice reserves. The south bank has 58 glaciers, which make up 67.44% of the region’s total number of glaciers, 65.44% of glacier coverage, and 64.39% of ice reserves (Table2). It is clear that the south bank has more glaciers than the north bank.It is clear that glaciers are more developed along the south bank of the Parlung River than along its north bank. The north bank shows a snow line higher than that along the south bank and has no valley glacier, indicating that snow and ice along the north bank are more lacking. Numerous large glaciers have formed on the Kangri Karpo Mountains because the mountains just stand in the way of the warm moist air brought by the southwest monsoon blowing from the great bend of the Yarlung River (Figure1) and thus become the wettest part of the Tibetan Plateau. After the moist air climbs over the Kangri Karpo Mountains and passes the Parlung River, it decreases in moisture content and thus supplies less moisture to the river’s north bank (the Nyainqentangla Mountains). Therefore, the north bank is dominated by cirques and hanging glaciers.Moreover, glaciers along the south bank (ubac) melt at slow rates as they have lower surface temperatures due to low solar irradiation received. After a certain amount of snow has accumulated on a glacier, the glacier tends to move down the valley under gravity or pressure. This contributes to the higher probability of the formation of valley glaciers along this bank. In contrast, glaciers along the north bank (adret) show higher surface temperatures due to greater solar irradiation and thus melt faster. For this reason, glaciers are unlikely to fill valleys, and the dominant glacier types are cirque glaciers and hanging glaciers. Due to their differences in moisture supply and solar insolation, the north bank has a snow line higher than that of the south bank. The more active glacial motions and faster rate of melting along the north bank are regarded as preconditions for the formation of glacial debris flow. Glacier melting provides enough water and glacier transforms into solid matter in the glacial debris flow.


### 3.2. Differences in Geomorphology between North and South Banks

Ke and HI values of every valley are obtained to analyze the difference of geomorphology between north and south banks. Figure4 just shows that Ke and HI values of valleys along the north bank are both larger than valleys along the south bank, especially the Ke index, which means closer the plane shape of catchment is to the circle, and higher the erosion degree is.Figure 4
Comparison of Ke index and HI between valleys along two banks ((a) Ke index; (b) HI index).
(a)(b)Therefore, under the effect of long-term structural erosion and glacier erosion, valleys along the two banks of Parlung Zangbo reflect sharply different geomorphological features (Figure4), see Table S1 in the Supplementary Material for comprehensive analysis. As shown in Figure 5, a generalized model is used to describe the geomorphological differences of the valleys between the two banks. The overall form of the valley along the north bank is prone to taking the shape of a funnel and the cross section takes V-shape. While for most valleys along the south bank, its overall form is prone to take the shape of oak leaf and cross section takes U-shape. The probable reason may be that due to intense sun exposure, it is hard to form a large-scale glacier running through the whole valley along the north bank, and the glacier remains mostly at the edge of the valley at high altitude. Under the effect of freeze-thaw weathering and glacier erosion over the years, only mountains at high altitudes suffer repeated erosion, thus forming wide tops and narrow bottoms. Its overall form takes the shape of a funnel, and its cross section takes the shape of “v” at the lower part, which is favorable for debris flow events (Figure 5).Figure 5
The generalized model to describe the comparison of gully geography along the two banks ((a) cross section of most gullies in the south bank; (b) cross section of most gullies in the north bank).


## 3.1. Comparison of Hydrology and Glacier Distribution between the Two River Banks

The data from field investigation and the Second Glacier Inventory Dataset of China [15] suggest significant differences in glacier distribution between the north and south banks of the Parlung River. In terms of the glacier type, valley glaciers are distributed only along the south bank, while the north bank is covered primarily by cirque glaciers and hanging glaciers. The snow line is generally higher in the east than in the west (Table 2).Table 2
Glaciers along the Parlung river from Bomi to Suotong.
LocationTypes of glacierNumber of glaciers and percentageArea of glaciers (km2) and percentageVolume of glaciers (106× m3) and percentageAverage snow line (m)North bankHanging glacier1820.93%41.1230.84%2867.4533.934040Cirque glacier1011.63%4.963.72%142.571.69%Valley glacier00%00%00%South bankHanging glacier2326.74%31.1823.38%2058.0124.35%3429Cirque glacier1517.44%5.824.36%194.732.30%Valley glacier2023.26%50.2737.70%3188.5437.73%Sum total—86100%133.35100%8451.3100%—In the region between Bomi and Suotong, the north bank of the river has 28 glaciers, which account for 32.56% of the region’s total number of glaciers, 34.56% of glacier coverage, and 35.62% of ice reserves. The south bank has 58 glaciers, which make up 67.44% of the region’s total number of glaciers, 65.44% of glacier coverage, and 64.39% of ice reserves (Table2). It is clear that the south bank has more glaciers than the north bank.It is clear that glaciers are more developed along the south bank of the Parlung River than along its north bank. The north bank shows a snow line higher than that along the south bank and has no valley glacier, indicating that snow and ice along the north bank are more lacking. Numerous large glaciers have formed on the Kangri Karpo Mountains because the mountains just stand in the way of the warm moist air brought by the southwest monsoon blowing from the great bend of the Yarlung River (Figure1) and thus become the wettest part of the Tibetan Plateau. After the moist air climbs over the Kangri Karpo Mountains and passes the Parlung River, it decreases in moisture content and thus supplies less moisture to the river’s north bank (the Nyainqentangla Mountains). Therefore, the north bank is dominated by cirques and hanging glaciers.Moreover, glaciers along the south bank (ubac) melt at slow rates as they have lower surface temperatures due to low solar irradiation received. After a certain amount of snow has accumulated on a glacier, the glacier tends to move down the valley under gravity or pressure. This contributes to the higher probability of the formation of valley glaciers along this bank. In contrast, glaciers along the north bank (adret) show higher surface temperatures due to greater solar irradiation and thus melt faster. For this reason, glaciers are unlikely to fill valleys, and the dominant glacier types are cirque glaciers and hanging glaciers. Due to their differences in moisture supply and solar insolation, the north bank has a snow line higher than that of the south bank. The more active glacial motions and faster rate of melting along the north bank are regarded as preconditions for the formation of glacial debris flow. Glacier melting provides enough water and glacier transforms into solid matter in the glacial debris flow.


## 3.2. Differences in Geomorphology between North and South Banks

Ke and HI values of every valley are obtained to analyze the difference of geomorphology between north and south banks. Figure4 just shows that Ke and HI values of valleys along the north bank are both larger than valleys along the south bank, especially the Ke index, which means closer the plane shape of catchment is to the circle, and higher the erosion degree is.Figure 4
Comparison of Ke index and HI between valleys along two banks ((a) Ke index; (b) HI index).
(a)(b)Therefore, under the effect of long-term structural erosion and glacier erosion, valleys along the two banks of Parlung Zangbo reflect sharply different geomorphological features (Figure4), see Table S1 in the Supplementary Material for comprehensive analysis. As shown in Figure 5, a generalized model is used to describe the geomorphological differences of the valleys between the two banks. The overall form of the valley along the north bank is prone to taking the shape of a funnel and the cross section takes V-shape. While for most valleys along the south bank, its overall form is prone to take the shape of oak leaf and cross section takes U-shape. The probable reason may be that due to intense sun exposure, it is hard to form a large-scale glacier running through the whole valley along the north bank, and the glacier remains mostly at the edge of the valley at high altitude. Under the effect of freeze-thaw weathering and glacier erosion over the years, only mountains at high altitudes suffer repeated erosion, thus forming wide tops and narrow bottoms. Its overall form takes the shape of a funnel, and its cross section takes the shape of “v” at the lower part, which is favorable for debris flow events (Figure 5).Figure 5
The generalized model to describe the comparison of gully geography along the two banks ((a) cross section of most gullies in the south bank; (b) cross section of most gullies in the north bank).


## 4. Formation Mechanism of Debris Flow along the Parlung River

The occurrence of landslides, debris flows, and other geohazards depends on a combination of internal and external conditions, including active tectonics, topography, climate, and hydrology. Field investigation and the abovementioned analysis show that glacier-related debris flows vary between the south and north banks despite their overall wide distribution across the region. According to field investigation and previous research [14, 16, 22, 36], the formation of debris flows on the south and north banks of the Parlung River can be explained by three modes:Mode I: During movement, a glacier tends to fracture during intense melt at its snout. The glacial meltwater then moves into the glacial body along the resulting fractures and accumulates into runoff at the glacier base. Cavities form at the glacier snout due to erosion by runoff. As the fractures continue to grow the glacial till deposited by the glacier tends to collapse, motivating the slow-moving glacier to move downward faster. During fall and breakage, ice will melt and the huge amounts of energy contained in meltwater runoff will destabilize the loose gully deposits, which then develop into a debris flow (Figure6(a)).Figure 6
Formation mechanisms of debris flow along the Parlung River. (a) Mode I, (b) mode II, and (c) mode III.Mode II: In the early stage, the small quantities of glacial meltwater and surface runoff generated by rainfall also provide solid materials for the formation of a debris flow. When the ablation of a glacier accelerates or heavy rainfall occurs, the resulting water forms runoff, which will erode the glacial till and debris from landslides previously deposited in the gully and entrain the underlying sediments. The sediments will then be fluidized and move, forming a debris flow (Figure6(b)).In mode III, upstream rainfall and glacial meltwater will be blocked by the dam formed by a landslide in the narrow part and moraine in the gully. After the dam breaks the water will be discharged rapidly. The discharged water then entrains the underlying sediments, resulting in a glacial debris flow (Figure6(c)).


## 5. Discussion

As mentioned above, the north bank has more favorable conditions in glacier and geomorphology than the south bank, which is more conducive to the occurrence of debris flows and this is consistent with the present situation of debris flow events. The dominance of cirque and hanging glaciers on the upper part of gullies in the north bank indicates greater potential energy carried by icefalls and avalanches. As the north bank is the adret, the glaciers there are more active and melt more intensely, increasing the probability of icefalls, avalanches, and rockfalls. Therefore, the mode of debris flow changes to mode I. The Guxiang debris flow on September 29, 1953, is the typical event [14]. Active glaciers will produce more meltwater and modern glacial till. The unique funnel shape of gullies not only facilitates the confluence of water from different sources but also increases the likelihood of blockage and dam breakage at the narrow part of a gully. Then, mode III debris flow will occur. The Bitong debris flow in 2016 (Figure 3(a)) and Dada debris flow in 2017 (Figure 3(b)) are the typical events on the north bank. On the south bank (ubac), the gullies are oak leaf-shaped and typically covered with valley glaciers. As the speeds of glacier motion and water catchment are slow, it will take a longer time for the materials produced by repeated freezing and thawing in the early stages and from a gully to develop into a debris flow. In this situation, a mode II debris flow is more likely to occur. In fact, as the statistics in Table 1, most of the debris flows along the south bank are Mode II. However, in extreme conditions, there is also the possibility of a model I or III debris flow. The debris flow event in 2018 in Songrao gully is just the typical one which outbreak for the reason of rockfall/moraine-initiating (Figure 3(f)) and dam-breaking debris flow in 2010, respectively [22, 36]. For the reason that the initiating course is hard to observe, the formation mechanisms of several debris flows could not be obtained; however, according to the field investigation in recent years, the result is clear that the debris flows on the north banks primarily occur in modes I and III, while the south banks are dominated by mode II debris flows (Table 3).Table 3
Formation mechanisms of debris flows along the Parlung River from Bomi to Suotong (e.g., 1–13 in Figure1).
NO.12345678910111213NameBitong gullyDada gullyJiaolong gullyCangguo gullyZhataduo gullyNaha gullyGuxiang gullySuotong gullyQiuzhu gullyChidan gullySongrao gullyMichong gullyDanka gullyFormation mechanismModes I and IIIMode IIIMode IIIMode II—Modes I and IIIModes I and IIIMode IIIMode IIMode IIModes I and IIIMode IIMode IIData sourceThis studyThis study[16]This study—This study[16][16]This studyThis study[22]This studyThis studyLocationNorth bankSouth bankThe Parlung glacier is the controlling dynamic factor of debris flow in the Parlung River. Warming is the main cause of glacial recession. Mass meteorological data show that climate warming has been significant in the Parlung River since the 1970s, the warming rate in Bomi is 0.23°C/10 years [36, 37]. It is speculated that the melting rate of glaciers will accelerate in the future, and that the north bank will be more affected than the south bank, for smaller glaciers are more sensitive than larger ones and are easier to lose material.The disappearance and recession of glaciers brings enough water and impact solid matter for glacier-related debris flows. No doubt, debris flows in the region will increase gradually in the future [16, 17, 22, 38, 39]. Strong earthquakes are bound to be the center of the centralized breaking out of glacier-related debris flows. In addition, the melting of the glacier will form moraine lakes, which pose a hidden danger from the subsequent debris flows caused by glacial-lake outburst floods.The paper suggests that the major transport projects to be built on the Parlung River, such as the Sichuan–Tibet Expressway and the Sichuan–Tibet Railway, should be located on its southern bank, in order to mitigate the influence of debris flows. To spare gullies prone to debris flows, bridges and tunnels are proper solutions.


## 6. Conclusion

The present paper indicates that compared to the south bank of the Parlung River, the north bank shows much larger and higher frequency debris flows in recent years. In fact, the north bank has more favorable conditions in glacier and geomorphology than the south bank, which is more conducive to the occurrence of debris flow. There are more active and distributed glaciers on the north bank of the Parlung River than on the south bank, which means enough water and material for the debris flow. The Ke index and HI show that valleys along the north bank reflect higher circle and erosion degree, the overall shape of gullies in the north bank is mainly funnel shaped and the cross section shows the shape of “v” at the lower part, which is favorable for debris flow formation. Relevant results provide a way to study the relationship between environmental conditions and the glacier-related debris flow, not just in SE Tibet Plateau but also in the marine alpine area.Three formation mechanisms are concluded: mode I: icefalls, avalanches, and rockfalls mix with water in a gully and transform into a debris flow; mode II: gully bed deposits are destabilized by surface runoff and then develop into a debris flow; and mode III: the break of a debris dam suddenly causes a debris flow. The glacier-related debris flows on the south and north banks occur through different mechanisms due to different formation conditions, the debris flows on the north banks primarily occur in modes I and III, while the south banks are dominated by modes II debris flows. The understanding of the regional debris flow could be promoted in the initiation mechanism.The disappearance and recession of glaciers bring enough water and impact solid matter for glacier debris flows, no doubt debris flow in the region will increase gradually in the future, and the north bank is more prone than the south bank. This paper suggests that the major transport projects to be built on the Parlung River should be located on its south bank, in order to mitigate the influence of debris flows. To spare gullies prone to debris flows, bridges and tunnels are proper solutions.



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*Source: 3510944-2022-09-30.xml*