Ling Yuquan, Qu Jianjun, Fan Jinshi, and Li Yunhe
Windblown sand has long posed a severe problem at the Mogao grottoes. Carried by prevailing seasonal winds from the extensive dunes on the plateau above the grottoes, the sand erodes the cliff slopes and accumulates at the base of the grottoes. Some 2,000 m3 of sand are swept up and removed annually Moreover, sand and accompanying dust infiltrate the grottoes themselves, where they obscure the ancient sculptures and wall paintings.
Previously, experimental windbreaks of brush and reed have failed to control this problem. In 1989, the Lanzhou Desert Research Institute of the Chinese Academy of Sciences, the Dunhuang Academy, and the Getty Conservation Institute initiated a study of the problem to develop preventive measures. The Desert Research Institute designed a 3.7 km wind fence, in an A-shaped configuration. The design was based on seasonal and diurnal wind velocity and direction data gathered from the Getty Conservation Institute's solar-powered meteorological station on the cliff top and from sand traps used to determine quantity and particle size. The stability of the dunes with respect to growth and movement was also investigated. Data indicated that seasonal wind changes would disperse the accumulation of sand at the fence. For construction of the fence, the Getty Conservation Institute provided an ultraviolet-stabilized, knitted aerotextile, which reduced wind velocity by approximately 50%, and the Dunhuang Academy erected the fence in 1990. Although some reconfiguration of the fence has been necessary where heavy sand accumulation has not been subsequently dispersed, the fence has reduced sand at the cliff base by about 62%. In time, this efficiency is expected to increase, as the present accumulation is believed to derive from residual sand between the fence and the cliff edge. The fence is expected to have a life of at least fifteen years, by which time the vegetation windbreak now being planted, with a drip irrigation system, should be well established.
The Mogao grottoes are a national protected site of the first rank and are also a world famous treasure-house of ancient Chinese art. It has been
Called the “art gallery of the world” and “a museum on walls.” The murals and polychrome statues inside this immense group of caves, and especially the information content of the murals, embrace a broad range of complicated themes. They occupy an extremely important position in Chinese and world art history.
It has been sixteen hundred years since the initial excavations were made at Mogao. During this period, the effects of natural and human factors have caused damage of differing degrees to the caves, murals, and statues. Windblown sand is one of the most damaging factors. Erosion or denudation by windblown sand has left many of the caves with thin roofs and poses a direct threat to the conditions of preservation and to the environment of the murals. Accumulation of sand has resulted in pressure on the roofs. Sand accumulation on the walkways blocks traffic. Sand and dust have also abraded the murals.
In the early 1960s, a plan to control sand was formulated and small-scale sand-control experiments were initiated. However, there was insufficient recognition of the severity of the damage brought about by windblown sand, and not enough serious consideration was given to the importance of controlling windblown sand. Furthermore, because the methods taken for controlling sand were inappropriate, and because of lack of funding, the sand control experiments could not be continued. For example, the local dry brush fence built on the edge of the cliff top led to accumulation of sand on top of the cliff, creating a new source of sand near the cliff face and a latent danger. Another instance was the digging of sand-control ditches on the top of the cliff. The ditches were quickly filled by sand carried by the westerly wind. The principal reasons for these failures were that there was insufficient understanding of the patterns of wind and sand movement and poor awareness of the severity of the damage that they bring about. Therefore, for many years a passive approach was taken, and sand that had accumulated in front of the caves was removed by hand. After the World Heritage Committee of Unesco listed the Mogao grottoes as a World Heritage Site in 1987, research on damage to the Mogao grottoes by windblown sand was given serious consideration and support by leaders in various departments and at various levels. In July 1989, the Dunhuang Academy and the Lanzhou Desert Research Institute of the Chinese Academy of Sciences with the Getty Conservation Institute initiated experimental research on sand control using knitted polyethylene textile as the windbreak fence material.
The research was carried out in two stages. The first stage, from October 1989 to September 1990, involved making a topographic map (scale 1:1,000) of a 2 km2 test area, then monitoring and studying the patterns of wind and sand movement using meteorological data and information collected from sand traps. The second stage began in October 1990 with the construction of the windbreak fence and continued until the end of 1992. The main objectives during this stage were to monitor the effectiveness of the fence on the basis of patterns of windblown sand activity and calculations of the intensity of the activity.
World History
