PHYSICAL FEATURES

The Himalaya-China region are situated on the south of the Tibetan Autonomous Region (TAR) of the Peoples’ Republic of China. The research regions include Pumqu, Poiqu, Rongxer, Jilongcangbu, Zangbuqin, Daoliqu, Jiazhagangge, Majiacangbu (see figure 2.1). The total area of the Himalaya-China research regions within China is 38325 km2. The Himalaya-China regions have average altitude above 4500 m and receive abundant precipitation of monsoon form Indian Ocean, both of them providing favorable topography and climate condition to develop glaciers.
 
The Pumqu (Arun) River Basin is situated in the southwest of Tibet Autonomous Region of P. R. China and between 27°49΄N to 29°05΄N latitude and 85°38΄E to 88°57΄E longitude. It is bounded to the north by the Mimanjinzhu Range of the Gandiseshan border on the Yarlungzangbo (Brahmaputra) river, and to the south by the world’s highest-Himalayan Range neighbouring Nepal and Sikkim. The basin extends into the Biakuco continental lake in the west. The Yap Mountains seated in the southwest of the basin separate the Pumqu (Arun) and Poiqu (Bhote-Sun Kosi) River Basins. The eastern part of the basin extends into Mts. Qumo, Xaya and Joding boarding on Nyangqu River, a tributary of the Yarlungzangbo (Brahmaputra) River. The total drainage area of the Pumqu (Arun) River Basin within Tibet is 25307 km2.The length from east to west is about 320km and the width from south to north is about 120 km.

The total drainage area of the Poiqu basin (Bhote-Sun Koshi) and Rongxer basin (Tama Koshi) within Tibet, China, is 3430 sq. km. The length from east to west is about 84 km and the width from south to north is about 69 km.

Both Pumqu river and Poiqu river originates from the northern slope of Xixiapama mountain, flows through Nepal and into the Ganga through the Kosi.


Fig 2.1 The position of Himalaya-China research regions

CLIMATE

Being located on the leeward side of the Himalayan range of China receive considerably less precipitation than the southern Himalayan range. Generally, the precipitation in the Tibet basin decreases from west to east and also from south to north.
 
The major source of precipitation is the warm-moist air from the southwest monsoon. The precipitation decreases from south to north gradually. For example, the annual precipitation is 2817mm at Zham (Khasa) District in the southern edge of the Poiqu River valley, and reduces to 666 mm at Nyalam. It is estimated that there may be an annual precipitation of 300-400 mm at the northeast edge of the basin. Precipitation in the Poiqu River Basin from June to September constitutes 51-53% of annual precipitation. On the other hand, due to the presence of the Himalayan Range, the warm-moist air generally follows the river valleys and so does the precipitation. Hence precipitation at the lower reaches of the River is comparatively higher. In general, due to the barrier effect of the mountains, the annual mean precipitation decrease with the increase of altitude.
 
 

Meteorological data such as temperature, rainfall, and evaporation of the Himalaya-China region are available from the meteorological stations at Tingri from 1960 to 1987. The annual distribution of rainfall in the region is not uniform. About 96% of the annual precipitation occurs only in the summer season from June to September. There are two distinct wet and dry seasons in the basins, but in its southern part, the seasonal distribution of rainfall is reported to be even and the precipitation from June through September is approximately 50% of the annual precipitation.
 

The mean elevation of the Himalaya-China region is above 4,500 masl. Therefore, the annual mean air temperature is low because of the high altitude. The annual mean temperature at Tingri is 2.7 degrees centigrade and the extreme mean monthly temperature from 1970 to1999 ranged from -9.9 to 13.2°C. From November to March, the temperature falls gradually below zero. The variation of air temperature from year to year is small, but the diurnal variation of the temperature is very large.
 

Evaporation is extremely high due to strong wind, high solar radiation, and low humidity. The annual mean evaporation (1971-1980) observed at Tingri is 2,553mm. The highest evaporation rate occurs in the months of May and June and the lowest in December and January. The annual evaporation in Chentang, the lower reaches of the Pumqu River basin near the Nepal/China boarder, is estimated at about 1,000mm.
 

RIVER SYSTEMS

The river systems in the Himalaya-China region are well developed (see Figure 2.2). The main tributaries of the Pumqu River are Rongpuqu, Yairuzangbo, Natangqu and Ganmazangbo. The left affluents of the Pumqu (Arun) River are Pamjuqu, Lopu, Loloqu, Yairuzangbo and Ganmazangbo are the right tributaries. Drainage areas and observed discharges of the main tributaries are presented in Table 2.1.
 

Table 2.1 The key elements of the main tributaries in the Pumqu (Arun) River basin

Name of Tributaries

Drainage

Length (km)

Area (km2)

Ratio to total area within China (%)

Langlongqu

Jialaqu

Zongboxan

Raquzangbo

Rongpuqu

Kadaqu

Ganmazangbo

The

 

right

 

side

36

38

38

51

91

34

50

581

563

451

858

2360

396

665

2.3

2.2

1.3

3.4

9.3

1.6

2.6

Pamjuqu

Lopu

Loloqu

Yairuzangbo

Natangqu

The

left

side

41

34

53

198

66

1187

480

2046

8342

988

4.7

1.9

8.1

33.1

3.9

Pumqu

Mean steam

376

25307

100.0


The Poiqu basin in China quoted as 5O191 basin number and the downstream in Nepal is named Bhote-Sun Koshi. Rongxer basin in China quoted as 5O192 basin number and the downstream in Nepal is named as Tama Koshi. Both the basins are sub-basin of Koshi basin of Ganges basin.

 The nine main affluents of Poiqu (Bhote-Sun Koshi) River are Lazapu, Tongpu, Gyaiyipu. Koryagpu, Targyailing, Karrup, Congduipu, Zhangzanbo and Pumqu. It is about 80 km in length and the total catchments area is 1987 km2.
 

Rongxer river (Tama Koshi) originates from the DuokaPula Mountain which elevation is about 5611m. There are so many glaciers at the source of the river. The total catchments area is 1484 km2. The main river course is about 45 km in length.

Figure 2.2 The River System in Himalaya-China Region

GEOLOGY AND GEOMORPHOLOGY

The Himalaya-China region is located in the middle of the Great Himalayan Range. The geological and geomorphological features have mainly depended on the upward motion of the Himalayas since the end of the Tertiary Period.
 
The basin lies in High Himalayan structural zone in-between the Central Fault Zone of the Himalayas and the Gyirongtogda-Dinggyenyela fault zone. This zone is the basement rock system of the northern edge ocean of the Indian Continent. It passes through the central reverse fault and covers the low Himalayan constructural zone southwards. The strata are mainly Nyalam Group of Pre-Sinian (Pre-Cambrian) System. The rocks are mostly kyanite-garnet-mica schist, kyanite-green landsite-biotite schist, mica-quartz schist etc. In the investigated region, there are complicated geological structures, evidence of intense earthquakes, older rock formations, and frequent neotectonic movements. These provide favorable geological conditions for the development of various landforms.
 

 GLACIERS

A glacier is a huge flowing ice mass. The flow is an essential property in defining a glacier. Usually a glacier develops under conditions of low temperature caused by the cold climate, which in itself is not sufficient to create a glacier. There are regions in which the amount of the total deposited mass of snow exceeds the total mass of snow melt during a year in both the polar and high mountain regions. A stretch of such an area is defined as an accumulation area. Thus, snow layers are piled up year after year in the accumulation area because of the fact that the annual net mass balance is positive. As a result of the overburden pressure due to their own weight, compression occurs in the deeper snow layers. As a consequence, the density of the snow layers increases whereby snow finally changes to ice below a certain depth. At the critical density of approximately 0.83g cm-3, snow becomes impermeable to air. The impermeable snow is called ice. Its density ranges from 0.83 to a pure ice density of 0.917g cm-3. Snow has a density range from 0.01g cm-3 for fresh snow layers just after snowfall to ice at a density of 0.83g cm-3. Perennial snow with high density is called firn. When the thickness of ice exceeds a certain critical depth, the ice mass starts to flow down along the slope by a plastic deformation and slides along the ground driven by its own weight. The lower the altitude, the warmer the climate. Below a critical altitude, the annual mass of deposited snow melts completely. Snow disappears during the hot season and may not accumulate year after year. Such an area in terms of negative annual mass balance is defined as an ablation area. A glacier is divided into two such areas, the accumulation area in the upper part of the glacier and the ablation area in the lower part. The boundary line between them is defined as the equilibrium line where the deposited snow mass is equal to the melting mass in a year. Ice mass in the accumulation area flows down into the ablation area and melts away. Such a dynamic mass circulation system is defined as a glacier.
 

A glacier sometimes changes in size and shape due to the influence of climatic change. A glacier advances when the climate changes to a cool summer and a heavy snowfall in winter and the monsoon season. As the glacier advances, it expands and the terminus shifts down to a lower altitude. On the contrary, a glacier retreats when the climate changes to a warm summer and less snowfall. As the glacier retreats, it shrinks and the terminus climbs up to a higher altitude. Thus, climatic change results in a glacier shifting to another equilibrium size and shape.
 

According to the glacier inventory of 1990, there are 1578 glaciers in the Pumqu, Poiqu, Rongxer, Jilongcangbu, Zangbuqin, Daoliqu, Jiazhagangge and Majiacangbu basins in China, with an area of 2906.017 km2. The present study shows 1578 glaciers covering area of 2864.33 km2.

Figure 2.3 The Distribution of Glaciers in the Himalaya-China regions

GLACIAL LAKES

The study of glacial lakes is very important for the planning and implementation of any water resource development project. Past records show that glacial lakes have produced devastating floods and damage to major constructions and infrastructure. In 1987, a glacial lake inventory was made for the Poiqu and Pumqu river basin with large-scale topographical maps and aerial photographs.

According to the statistics of 1990s, there are 782 glacial lakes with an area of 74 km2 in the Pumqu, Poiqu, Rongxer, Jilongcangbu, Zangbuqin, Daoliqu, Jiazhagangge and Majiacangbu basins in China. The present study shows 824 lakes with 85.2 km2. According to contributing factors, the glacial lake can be divided into four types: cirque lakes, end moraine-dammed lakes, valley trough lakes, and blocked lakes. Of these, the most common are the end moraine-dammed lakes. Because the end moraine-dammed lakes mostly consist of end moraines formed in the Little Ice Age and close to their source glaciers, or connect directly with the glaciers, changes in the glaciers directly influence the water level of the glacier lake and the stability of the dam. At the same time, owing to the fact that the end moraine dams are composed of new and loose till, these are un-compacted and therefore unstable. This type of glacial lake burst easily and cause floods and debris flows.

 


The figure2.4 The Distribution of Glacial Lakes in Himalaya-China Region

 
GLACIAL LAKE OUTBURST FLOOD EVENTS

Several GLOF events have occurred over the past few decades in the Himalaya-China region, causing extensive damage to roads, bridges, trekking trials, villages, as well as loss of human life and other infrastructures.
 
The GLOFs have caused extensive damage to major infrastructures. The main processes and the degree of hazard and destruction from the glacial lake outburst cases, based on literature and field investigations, are presented in the Chapter 10.