Mud-Stone Flow at Yiong: Reflections

YANG YICHOU

An extraordinarily large mud-stone flow that hit Yiong in southeast Tibet on April 9, 2000 shocked the entire country. The Zhongba Resource Satellite No.1, the French Spot Satellite and the US Land Resource Satellite No.5 detected the flow, and produced color photographs of it.

Geological Environment of the Yiong Zangbo River

The flow blocked the main course of the Yiong Zangbo River, a major tributary of the Parlung Zangbo River, which it meets at Tongmei, before making a sharp turn to empty into the Yarlung Zangbo River.Originating in the snow-covered mountains of Jiali, south of the Nyainqentanglha Mountains, the Yiong Zangbo River flows westward. Its geological structure is almost identical with the middle and upper reaches of the Parlung Zangbo River. Geological data reveals a fault line in the southern part of the Nyainqentanglha Mountain, extending from Jiali to Raog, which controls the pattern of the Yiong and Parlung Zangbo River systems. The lower reaches of both rivers are located north of the U-turn of the Yarlung Zangbo River, where the modern crust changes vehemently, leading to frequent landslides, snow avalanches and mud-stone flows. The lower reaches of the Yarlung Zangbo River separates the Himalayas from the plateau, serving as the main channel for the warm and humid air currents of the southwestern monsoon. In the meantime, Nyainqentanglha faces the grand canyon in the southeast. The lands high elevation leads to greater rainfall than other parts of the plateau area. As a result, snow and ice avalanches are frequent. The Sichuan-Tibet Highway has become notorious in terms of natural disasters. For example, it suffered from an extremely large-scale mud-stone flow in 1952, a cave-in in 1967, a mud-stone flow in 1983, and a snow avalanche in 1996. So the latest mud-stone flow was not that unexpected.The bed of Yiong Lake lies at an elevation of 2,100-2,200 meters. In this part of the world, the annual precipitation amounts to 1,000mm. Mountains in the area have a clay layer of up to 70-80 cm. The land flanking the lake has been developed into farmland growing wheat, maize and qingke barley twice a year. It is also one of the major tea producing areas in Tibet.

Yiong Lake:

Evolution The Yiong Co (Lake), located in Bome, is tucked away in the valley drained by the lower reaches of the Yiong Zangbo River. It is 16 km long and 2 km wide, covering an area of 23 square km, and a maximum depth of 25 meters. It was formed as a result of an extra-large mud-stone flow that blocked the Yiong Zangbo River Valley in 1900. To the north of the lake is Nyainqentanglha Mountain. It is home to some famous glaciers, including the 33.5-km-long Karqen Glacier. Lake water, plus pleasant weather, has turned the area into a land of forests, orchards, crops and tea plantations. Tibets first tea garden is located here. Over the past century, however, alongside with the warming of the global climate, the mountain glaciers have shrunk, as has Yiong Lake. In the upper reaches, mud and sand brought down to the lake have formed chains of islets. A survey of the lake area shows that, with the construction of the Sichuan-Tibet Highway, there has been a sharp increase in the areas population. From the 1960s to the 1980s, the area suffered from random tree felling. Soil erosion was serious, and landslides and mud-stone flows occurred more frequently.

Reflections Over the Disaster

A survey of the satellite pictures shows the mud-stone flow that hit the area in April 2000 began to gain momentum in November 1998 or earlier, due to large-scale surface movement in the Himalayas, especially in the upper reaches of the Yiong Zangbo River. Root causes include:First, strong crust movement. The Himalayas and Nyainqentanglha Mountains meet the Henduan Mountains here to produce many precipices. Geologically, the Indian Plate in the south collides with the Eurasian Plate, and the two then grind against the Pacific Plate in the east. The area features earthquakes and thermal energy resources, which give out accumulated power to cause landslides.Second, the valley at the U-turn of the Yarlung Zangbo River opens a channel for the eastern Himalayas and highlands in eastern Tibet, making it possible for the warm and humid air currents from the Indian Ocean to invade the plateau. When the air current reaches the top of the U-turn it finds it is under the control of the geographical location, and is forced to branch out into three sub-air currents: one westward towards Shannan; the second northward along the Yiong Zangbo River to the inland plateau; and the third traveling along the Parlung Zangbo River to reach Raog. Of the three, the northward sub-current has an air passage of 30 percent to reach 500-700g.cm.s. The invasion of rainfall and heat to Tongmei may be good for the local environment, but, given the local geological fragility, they also bring landslides.Increased human activities also undermine the local environment. For example, the opening of the Sichuan-Tibet Highway worsens the instability of the natural slopes flanking the road.Poor geological structures, the negative aspect of the water and heat resources, and human activities will combine to encourage more earthquakes, high temperatures (leading to the melting of glaciers and, thus, to ice avalanches), and concentrated rainfall; and these will invite even more landslides, ice and snow avalanches, and mud-stone flows.Given the above facts, a monitoring station should be set up in that area so as to provide accurate data for preventive decisions and measures.