An outline of avalanches in China

Cold Regions Science and Technology, 13 (1986) 11-18

11

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

AN OUTLINE OF A V A L A N C H E S IN CHINA Wang Yanlong and Huang Maohuan Lanzhou Institute o f Glaciology and Geocryology, Academia Sinica, Lanzhou (China)

(Received October 17, 1985; accepted in revised form January 17, 1986)

ABSTRACT

in some places from late autumn to early summer. They are very harmful to national construction.

The distribution and features o f avalanches in China are briefly introduced. Seasonal avalanches are regionalized into maritime, transitional and continental. A sketch map showing avalanche regionalization in western China is drawn. The maritime avalanche region is characterized by plenty o f solid precipitation and the continental avalanche region by severe coM and sparse solid precipitation. In China there are many mountainous regions with much solid precipitation during winter and spring. There, avalanches which endanger economic construction and human life occur frequently on mountain slopes covered by thick snow. With improvements in mountain areas there is an increasing need to prevent and control avalanches. Therefore, research on regional avalanches is one o f the important tasks in China. For the last ten years and more, in cooperation with some o f the department concerned, we have carried out a general investigation and local observations on avalanche damage, causes and types. The paper discusses the distribution, regionalization and some o f the characteristics o f avalanches in China.

1. AVALANCHE DISTRIBUTION IN CHINA Avalanches in China can be classified as perennial or seasonal. In the alpine zone with perennial snow cover, avalanches occur all the year round. They are one of the important feed sources for glaciers. In the intermediate zone of mountains with low air temperature and plenty of solid precipitation, avalanches occur seasonally mainly from winter to spring, and 0165-232X/86/$03.50

© 1986 Elsevier Science Publishers B.V.

1.1 Perennial avalanches In China, perennial avalanches occur mainly in the alpine zone, which is rich in solid precipitation. The distribution of glaciers is always consistent with geography as has been expounded by Shi and others (1980). The lowest limit of avalanches is obviously affected by the distinct circulation and climate of the Qinghai-Xizang (Tibet) Plateau. There are plenty of data to show that the lowest altitude of perennial avalanches is in the peripheral regions of the plateau. The highest region of perennial avalanches is in the northern and central parts of the plateau, especially in the Ali region of the western Xizang and the Aksayqin region of southern Xingjiang. In the southeast of Xizang at 25°-30°N, the altitude of perennial avalanche occurrence is similar to that in the Tian Shan at 40°-45°N, but remarkably drops in the bend of Yalung Zangbo Jiang. Perennial avalanches are one of the important feed sources for glaciers. At the initial stage of an ice age some glaciers might originate from avalanche deposition. Although it is not a constant input to a glacier, on some glaciers feeds from avalanches are no less than precipitation and influence the mass balance and large-scale structures in the glacier. On the biggest glacier in Gongga Shan, for example, avalanche deposition covers two-thirds of the firn basin, being a main source of accumulation. It is not an exaggeration to say that it is impossible for some glaciers to exist without avalanche nourishment (Wang and Shao, 1984). In the Guxiang region of southeastern Xizang, avalanche deposition accounts for 54% of

12 the annual accumulation of a glacier, as much as 4.6% of the water equivalent of the glacier (Deng, 1980). On the Batura Glacier in the Karakoram mountains, the apparent snow line due to extreme occurrence of avalanches is much lower than the normal snow line (the former fluctuates at an altitude of 4000-4500 m, the latter at 4900-5350 m.) (Lanzhou Institute of Glaciology and Cryopedology, Academia Sinica, 1980). In the perennial avalanche region, there is a possibility of avalanches occurring all the year round, mainly wet avalanches in summer and dry avalanches in winter. In some mountainous regions, avalanches start from perennial avalanche areas, and reach seasonal avalanche areas, travelling perhaps 10 km.

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1.2 Seasonal avalanches

Qinghai-Xizang Plateau exerts an obvious influence upon the distribution of avalanches in China. The plateau has a split which obstructs and strengthens zonal and meridianai air currents below 500 mb (Luo et al., 1980). Moreover, convective cloud cells on the plateau can concentrate on a cloud mass under proper conditions, thus affecting precipitation in the avalanche regions of the eastern periphery of the plateau (Ye and Gao, 1979). There are many mountains along the plateau periphery above a mean altitude of 4000 m where uplift movement is strong and relative height difference is great. Since these face vapour currents from the Indian, Atlantic, Arctic and Pacific oceans, they provide advantageous topography and abundant material for avalanches, and obviously influence the distribution of avalanches. Seasonal avalanches in China occur mainly in the plateau periphery and its neighbouring regions as Fig. 1 shows. The northern part of the plateau is one of the great avalanche damage regions, for the west wind current bringing abundant precipitation is subjected to the shield effect of the Tian Shan and the Karakoram Mountains, resulting in heavy solid precipitation. Another great avalanche damage region, characterized by massive scale and widespread damage, is south and southeast of the Plateau, where the relative height difference is great and the precipitation plentiful due to the southwestern monsoon from the Indian Ocean. In this region the maximum depth of snow cover is over 2 m. There is an avalanche dam-

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Fig. 1. The potential avalanche areas and avalanche disaster areas in western China. (1) Yigong of Xizang; (2) Bomi of Xizang; (3) Baimangxue Shan of Yunnan; (4) Yulongxue Shan of Yunnan; (5) Gongga Shah of Sichuan; (6) Erlang Shan of Siehuan; (7) Queer Shan of Siehuan; (8) Min Shan of Gansu; (9) Qinling of Shanxi. age region in northern Yunnan and Western Sichuan centered on Gongga Shan, a main peak in the Hengduan Shan, which is the eastern periphery of the plateau. Water vapour from the Indian or the Pacific Ocean contributes to this region. Furthermore, water vapour from the Arctic Ocean brings about favourable conditions for avalanche occurrence in the Altay Shan and the northern slope of the Tian Shan. In the north and northeast of China, only small scale avalanches occur because of the gentle terrain and the relatively small height differences. Although snow cover is rather deep in the Da Hingganling, Xiao Hingganling and Changbai Shah of the northeast of China, there is no heavy avalanche activity. On the inner plateau and its neighbouring mountains, besides the water vapour supply mentioned above, the supply from local circulation also contributes to avalanches, especially in some mountains around lake basins of the inner plateau, where local circulation makes up for the deficiency of advective precipitation (Wu and Liu, 1979; Li, 1982). As a result, in some basins the solid precipitation is short,

13

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Fig. 2. The avalanche distribution section from Bao Shan to Shiqu. Western Tlan Shan Queer Shan Balmangxue Shan Gongga Shah Yigong District Bomi District

1700 4500 3700 3700 3800 3800

2.0 -2.2 1.5 1.6 1.5 1.6

135 230 190 150 148 145

350 300 300 330 600 600

aaccumulation days with daily mean temperature _~O°C

but it increases with elevation in the mountains so that it is greater than that in the basins by a factor of 6 - 1 1 . Therefore in the inner plateau seasonal avalanches are rare, while in mountains higher than 5000 m perennial avalanches would be found. The lowest altitude of seasonal avalanche occurrence varies as Table 1 shows. The highest, about 4500 m above sea level (a.s.1.), is in northwest Sichuan and northern Xizang, where 7.~D-is 2 2 0 - 2 4 0 days and the mean annual solid precipitation is about 300 mm, E D - being a cold index consisting of the number of days with daily mean temperature ~0°C. In northern Yunnan and southeastern Xizang, the lowest altitude of avalanches is about 3700 m a.s.l., T_Jg- is 145-150 days and the mean annual solid precipitation 3 0 0 - 6 0 0 mm. The lowest altitude, about 1700 m a.s.l., is in Tian Shan and Altay Shan, where E D - is 130 days and the mean annual solid precipitation is more than 300 mm (Wang and Shao, 1983a). Terrain profile, mean annual solid precipitation and E D - can be drawn together as in Fig. 2 which shows a profile located in the eastern periphery of the plateau. From Fig. 2 we can make a distinction between two avalanche regions, a less-snow.type and a snowy type, the boundary between the two regions being the intersection of two curves. One curve is of the altitude at which the mean annual solid precipitation = 300 mm and the other at which T_,D-=

150 days. The former type of region is chiefly in the northern Hengduan Shan, Qilian Shan, Tanggula Shan, Tian Shah and Altay Shan, etc., where avalanche occurrence is dominated mainly by solid precipitation. Even though in some mountains E D - i s over 150 days, there is no avalanche occurrence because solid precipitation is less than 300 mm. The latter is the case mainly on the southern slope of the Himalaya Shan, the eastern part of NyainqSngtangglha Shan, Gaoligong Shan and Gongga Shan, forming an arc along the south-eastern periphery of the plateau, with a limit being consistent with the northern limit of maritime avalanche regions (see the section 3 below). In snowy-type avalanche regions, T_,D- is a leading factor in avalanche occurrence. Generally avalanches occur when Z D - i s over 150 days. In some mountains, although annual solid precipitation is more than 300 mm, it is difficult for avalanches to occur because E D - i s less than 150 days.

2. R E G I O N A L I Z A T I O N OF SEASONAL AVALANCHES

Because snow is a mass condition for avalanches, the distribution and variations of snow cover have to be studied in order to regionalize avalanches. Many researchers (Hu, 1982; Li et al., 1983; Central Meteorological Bureau, 1978) have pointed out that snow cover depth increases progressively from south to north. The isopleth of snow cover depth in eastern China is obviously different from that in western China, because of the difference of the dominating synoptic regime. The zero isopleth is roughly along latitude 25°N. In the middle and lower reaches of

14 Changjiang, maximum snow depth reaches 500 mm. The deepest snow cover in China appears in Xinjiang, southeastern Xizang, Himalaya Shan, Hengduan Shah, Da Hingganling and Xiao Hingganling, etc. However, avalanches do not occur in all of the above places. They occur only in places where the factor of dissection force K is over 150 and snow cover is deeper (Wang and Shao, 1983a). K = where H is the difference between the erosion datum plane and the highest point of the basin (m), and p is the basin area in hectares. The critical depth of snow cover is closely related to the slope angle (Lanzhou Institute of Glaciology and Cryopedology, Academia Sinica, 1979). It is possible for avalanches to occur on slopes of 350-40 ° when the depth of new snow is over 300 mm. But in the case of solid-type depth hoar, avalanches do not occur on a slope of 35 °, even if snow depth reaches 1 m. From observations on the stability of snow cover, we have found that in moutainous regions where K = 150 avalanches may occur if newly deposited snow is deeper than 300 mm, and disastrous avalanches if new snow is deeper than 700 mm. Therefore, the regions where the 10-year maximum snow depth is deeper than 300 mm or 700 mm can be classified as potential avalanche regions or avalanche disaster regions respectively (Fig. 1).

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In the seasonal avalanche region, air temperature, the annual range of temperature and the annual precipitation directly influence the mechanical properties and metamorphic processes of snow cover (Wang, 1982). Mechanical properties, metamorphism and depth of snow cover determine the type and movement of avalanches. For the same landform conditions depth of snow cover directly determines avalanche scale and degree of damage. Therefore we suggest some main and referring indices in Table 2 to classify seasonal avalanches, occurring mainly over western China, into maritime type, continental type and transitional type (Fig. 3). In eastern China there are no avalanches except for some small ones in Changbai Shan, Da Hingganling and Xiao Hingganling. We suggest two boundary lines. One starts at Erlang Shan in Sichuan and runs to the southern slope of the Himalayas through the northern slope of Gongga Shan, Baimangxue Shan, the eastern Nyainq~ntanglha Shan and the north of Yigong District. The other goes through Qinling, Min Shan in the southern Gansu, Nyainqfintanglha Shan and the southern slope of Gangdise Shan. To the south of the former line is the maritime-type avalanche region, such as the southern Sichuan, the northern Yunnan, the southern Xizang and the southern slope of Himalaya. To the

TABLE 2 The indices of seasonal avalanche regionalization Index

Main Indices

Maritime-type

Transitional-type

Continental-type

Occurrence altitude (m) Occurrence period Mean temperature in January (°c)a Annual range of air temperature (°c)a Precipitation days in year

3700-4600 Dec.-Mareh

4500-5000 Nov.-May

1700-3900 Dec.-April

Over ~4

-4--10

below -10

Metamorphic typeb

Mainly ET, secondary TG & MF 0.28 (average) 0.50 (maximum) Mainly wet full-depth avalanche, secondary depth-hoar and new dry-snow avalanche.

Snow density (g/cm3) Referring Indices

Type of avalanche causing heavy damage

20 140

20-24 140-120 Mainly ET & TG, secondary MF 0.24 (average) 0.40 (maximum) Mainly new dry-snow and wet full-depth avalanche, secondary depth-hoar avalanche.

aCentral MeteorologicalBureau (1978). bMetamorphic type: ET, Equi-temperature; MF, Melt-freeze;TG, Temperature gradient.

24 120-160 Mainly TG, secondary ET & MF 0.19 (average) 0.31 (maximum) Mainly new dry-snow avalanche, secondary wet full-depth and solar-radiation avalanche.

15

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Fig. 3. Avalanche regionalization in western China. (1) Baimangxue Shan of Yunnan; (2) Yulongxue Shan of Yunnan; (3) Erlang Shah of Sichuan; (4) Queer Shan of Sichuan; (5) Min Shah of Gansu; (6) Qinling of Shanxi. north of the latter is the continental-type avalanche region. The transitional-type avalanche region is between the two lines. 3. REGIONAL CHARACTERISTICS OF SEASONAL A V A L A N C H E S

Regional investigations of avalanches (Qiu, 1982; Qiu and Deng, 1983) and observations of the physical properties of snow (Wang, 1986) indicate obvious regional differences in avalanche characteristics. 3.1. Regional characteristics of the continental-type avalanche

In China, the continental-type avalanche occurs mainly over Tian Shan, Altay Shan, etc. The altitude of this avalanche region is generally between 1700 m and 3400 m, the lower limits is 1200 m and the alpine zone, over 4000 m, is a perennial avalanche region. The intermediate and the alpine zones of Tian Shan are cut apart by a planar surface that slopes to the west between 2200 m and 2600 m a.s.l., with obviously different landscapes. Valleys in the inter-

mediate mountain zone are relatively well-developed, but the relative height different is not large; branch valleys are short and narrow, generally 500-700 m long, with few longer than 2000 m. The maximum volume deposits of avalanche debris is 3 × l0 s m 3 and more. Generally the distance moved coincides with length of the valley. Avalanche damage is relatively heavy in new, drysnow avalanches, but secondary in wet full-depth avalanches solar radiation avalanches. New, dry-snow avalanches on a large scale are related to continuous snow fall, snow depth and air temperature (Fig. 4). Air temperature determines the occurrence period of avalanches. New, dry-snow avalanches with heavy damage mostly occur in the months when the air temperature is below -5°C: for instance, December and January in Tian Shan and Altay Shan. The total snow depth determines the scale and the harmfulness of dry-snow avalanches. In general, heavy dry-snow may occur if a continuous snowfall deposits a snow layer over 500 mm. Moreover, a deposition by a continuous snowfall of less than 500 mm will excite drysnow avalanches, with relatively heavy damage if deposited upon a snow cover deeper than 600 mm. In the maritime avalanche region, the presence of depth hoar can be usually regarded as a sign of largescale full-depth avalanches. But in the continental region, even if depth hoar is 500-600 mm deep, two thirds of the total depth, large-scale, full-depth avalanches may not take place; only a potential danger of full layer avalanches exists. The geographical characteristics in this region are specific. In an ordinary year, the snow cover in continental climates is only about 700 mm and of low density. As a result, the overburden snow layer is so light that the gravity driving component cannot overcome the resistance of depth hoar to release a full-depth avalanche, even though continental climate also brings about a severe cold condition. The latter produces large temperature gradients within the snow cover, and thus causes depth hoar to develop abundantly. 3.2. Regional characteristics of the maritime-type avalanche

The southeastern Xizang is a typical region for the maritime-type avalanche. Where new tectogenesis is intense, K is more than 150 (Wang and Shao,

16

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1983b) and the divides are generally 4 0 0 0 - 5 0 0 0 m high, i.e. 800-2500 m higher than the valley. The altitude of the seasonal avalanche region is usually between 3700 m and 4600 m. Most of the starting zones of seasonal avalanches extend into the perennial avalanche region. Some perennial aval/mches fall through the seasonal avalanche track from peaks higher than 6000 m a.s.1, into valleys about 2000 m a.s.l., with a total path of about 8 km. Under the effect of moist air from the Indian Ocean the precipitation is quite abundant. In southeastern Xizang, for example, mean annual precipitation in the valleys amounts to 874 mm, and 60 percent of it occurs in spring, autumn and winter. With an increase of elevation, precipitation increases at a rate of 90 mm per 100 m. From this rate the annual precipitation at an elevation of 3500-4500 m is estimated to be 1500-2000 mm, with about 1 m of it in a solid

state. This is three times as much as in the serious avalanche region of Tian Shan so, with favourable terrain, avalanche deposition may reach 1.2 X 101° m 3 . In the maritime avalanche region heavy avalanche damage comes mainly from wet-snow avalanches and secondarily from new, dry.snow and full.depth, depth-hoar avalanches. Here depth-hoar is really an important cause of full-depth, depth-hoar avalanches, though it does not develop so well as in the continental-type avalanche region (Fig. 5). The reasons are as follows. (1) Snow density in the maritime avalanche region is rather high, with an average value over 0.3 g/cm 3. In case of snow cover thicker than 500 mm on a slope of 35 °, full-depth, depth-hoar avalanches will occur. (2) The depth-hoar layer is situated at the bottom of the snow cover and bears most of its weight, so it is very sensitive to increases in the depth of the overburden layer. (3) The depth-

17 IA ) lSaimongxue Shah ( Jan 1983)

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lanches mostly occur in November and December, following a continuous snow fall (Fig. 6). Wet-snow avalanches will occur in late spring, when the air temperature is going up. The transitional-type avalanche has the highest occurrence altitude, generally 4 5 0 0 - 5 0 0 0 m a.si., a short track, usually 1 0 0 - 5 0 0 m, but not more than 1500 m, and a small amount of debris, typically 3 0 0 - 5 0 0 m a with the maximum 10 s m a .

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hoar layer is loose in texture and lower in density and strength, making it easy to form a gliding surface and therefore to release a full-depth, depth-hoar avalanche. In the maritime avalanche region, the temperature and precipitation state is mainly as follows. The mean annual temperature is about 0°C at an elevation of 4200 m. Stable snow cover starts in October at elevations over 4000 m; meanwhile small-scale avalanches will occur in the case of continuous snow fall. Once the winter begins, gully avalanches commence as the snow depth gradually increases. From February or March the temperature goes up again and snow fall increases sharply; large-scale wet-snow avalanches occur; bringing huge amounts of debris and terrific destruction that is rarely seen in other regions in China. With the rise of the 0°C isotherm, the starting point of wet, full-depth avalanche increases in elevation, so that more than one avalanche may run in the same gully or on the same slope each year in this region. 3.3. Regional characteristics of the transitional-type avalanche

The transitional-type avalanche occurs mainly over the northern slope of Daxue Shan in Sichuan, the western Nyainq6ntanglha Shan and Gandise Shan in Xizang. It has some characteristics of both the maritime-type and the continental-type avalanche. Relatively heavy damage results from new, dry-snow avalanches and wet-snow avalanches. Fulldepth, depth-hoar and solar-radiation avalanches can be seen once in a while. New, dry-snow ava-

1980 year

Fig. 6. Maximum continuous precipitation, compared with heavy, dry-snow avalanches, at a point in Queer Shah from 1951 to 1980. The asterisks indicate heavy dry-snow avalanches. ACKNOWLEDGEMENTS

We are grateful to Prof. Xie Zichu and Prof. Zhang Xiangsong, who gave us great help in completing this paper.

REFERENCES

Central Meterological Bureau (1978). A collection of climatical maps of People's Republic of China. Map Press, Beijing (Chinese). Deng Yangxin (1980). Snow avalanche in Guxiang region of Xizang and its role in the physical geographic processes of high mountains. Acta Geographica Siniea, 5(3): 242-250. (Chinese, English summary). Hu Ruji (1982). Preliminary discussion on the distribution of snow cover in China. Proceedings of the symposium on glaciology and cryopedology, Geographical Society of China (Glaciology), Science Press, Beijing (Chinese). Lanzhou Institute of Glaciology and Cryopedology, Academia Sinica (1979). Avalanche and its prevention. Science Press, Beijing (Chinese). Lanzhou Institute of Glaciology and Cryopedology, Academia Sinica (1980). Professional papers on the Batura Glacier, Karakoram Mountains. Science Press, Beijing (Chinese, English summary).

18

Li Bingyuan (1982). General description on the existing glacier of the northern part of Qiangtang Plateau, Northern Xizang. Proceedings of the Symposium on Glaciology and Cryopedology held by Geographical Society of China (Glaciology), Science Press, Beijing (chinese, English summary). Li Peiji et al. (1983). Distribution of snow cover in China. Journal of Glaciology and Cryopedology, 5(4): 9 - 1 2 (Chinese, English summary). Luo Hanmin et al. (1980). Meteorology, Meteorological Press, Beijing (Chinese). Qiu Jiaqi (1982). One typical avalanche occurring in warm season in the high mountain zone, Tian Shan Mts., China, Proceedings of the Symposium on Glaciology and Cryopedology, Geographical Society of China (Glaciology), Science Press, Beijing (Chinese, English summary). Qiu Jiaqi and Deng Yangxin (1983). Snow avalanche in Bogda Region, Tian Shan. Journal of Glaciology and Cryopedology, 5(3): 225-234 (Chinese, English summary). Shi Ya-feng, Hsieh Tze-chu, Cheng Pen-hsing and Li Chichun (1980). Distribution, features and variations of glaciers in China. IAHS-AISG Publ. No. 126: 111-116. Wang Yanlong (1982). Metamorphism of seasonal snow cover in the upper reaches of yilihe in Tian Shan. Journal of Glaciology and Cryopedology, 4(2): 63-72 (Chinese, English summary).

Wang Yanlong (1986). Some physical properties of seasonal snow cover in northern Yunnan and western Tianshan Mts. Proceedings of the National Conference on Glaciology, Geographical Society of China (Selection), The People's Publishing House of Gansu (Chinese, English summary). Wang Yanlong and Shao Wenzhang (1983a). Distribution and features of snow disaster in west Sichuan, north Yunnan and southeast Xizang. Professional papers on the investigation in Hengduan Shan, 1: 154-156, The People's Publishing House of Yunnan (Chinese, English summary). Wang Yanlong and Shao Wenzhang (1983b). A preliminary study on snow calamities in west Sichuan, north Yunnan and southeast Xizang. Mountain Research, 1(4): 17-21 (Chinese, English summary). Wang Yanlong and Shao Wenzhang (1984). Avalanche and glacier at Hailuogou in the Gongga Mts. Journal of Glaciology and Cryopedology, 6(2): 38-44 (chinese, English summary). Wu Guanglie and Liu Chaohai (1979). Glacial process characteristics and utilization of water resources in the Hat Hu Valley. Journal of Glaciology and Cryopedology, 2(4): 27-32 (Chinese, English summary). Ye Duzheng and Gao Youxi (1979). The meteorology of Qinghai-Xizang (Tibet) Plateau. Science Press (chinese).