Relationship Between Cultural Relic Diseases And The Precipitation At Mogao Grottoes,Dunhuang

Dunhuang Mogao Grottoes is one of the most important historical and cultural sites on the ancient Silk Road in China,which has a history of more than 1,650 years.There are 735 caves of different sizes from sixteen kingdoms to the Yuan dynasty.More than 45,000 square meters of murals and more than 2,000 painted sculptures are preserved.Due to the outstanding universal value of Mogao Grottoes,it was included in the World Cultural Heritage by UNESCO in 1987.The caves are made of shale conglomerates with weak clay and clay and the materials for the murals and statues in the caves are mainly clay.Due to the long-term changes in natural environment and man-made destruction activities,cliffs and murals have produced a variety of diseases and the occurrence and development of these diseases seriously threaten the safety of the heritage.Among the various types of cultural relics,the weathering of the cliff body and the efflorescence of murals are the most serious and most difficult to treat.A large number of studies have shown that water-driven migration of soluble salts is the main reason inducing such diseases.The Mogao Grottoes is located in the arid northwestern region,with an average annual rainfall of only 39 mm.Therefore,people have neglected the potential hazards of precipitation for a long time.In recent years,with the increase of the frequency of heavy rainfall in the heritage sites,the potential harm of rainfall on the preservation of cultural relics has become increasingly prominent,and aroused a great attention of the conservation personnel.In order to formulate reasonable measures to effectively prevent the destructive impact of precipitation on cultural relics,this study is based on the analysis of rainfall evolution characteristics in Mogao Grottoes in recent 29 years.The mechanisms of infiltration,runoff,weathering and erosion of the cultural relics induced by the change of environmental humidity during rainfall were studied by conducting field artificial rainfall simulation tests and environmental monitoring of caves.And combined with the investigation results of the cultural relic diseases,the vulnerable areas of weathering and erosion diseases,and rainfall thresholds of cultural relics in different forms were determined.The results of rainfall monitoring from the meteorological station on the top of the cave indicate that the rainfall types in the Mogao Grottoes are mainly light rain and moderate rain.The probability of heavy rainfall is very low.But due to the short duration and strong intensity of heavy rainfall,the potential threat to the long-term preservation of cultural relics is also relatively large.The artificial rainfall simulation test shows that rainwater along the large pores,fissures or pores and other channels preferentially infiltrated and slope exceed-infiltration runoff transported surface particles and salt,which is the main reason for inducing the mural salt damage and the cliff weathering erosion.It can be seen from the change of the environmental humidity of the cave with the change of rainfall that the relative humidity exceeds the critical value of thenardite phase transition in the salty ground layer,which is the main reason for the development of salt damage in the mural.According to the mechanism of the weathering erosion disease of cultural relics induced by precipitation,combined with the results of the investigation of cultural relics over the years,it can be seen that the top of the cave is relatively thin,and the caves with cracks in the surrounding rock or the caves with externally constructed wooden eaves are most vulnerable to rainfall infiltration;cliffs with gentle slopes between 20°and 35°are most susceptible to runoff erosion;the murals in the large and poor sealing caves on the lowest level are most vulnerable to change in environment humidity.The rainfall thresholds of rainfall infiltration,slope runoff and the change of environmental humidity which are the main reasons of causing weathering and erosion diseases of the cultural relics was 30 mm rainfall and the maximum rain intensity in 30 min was 0.2 mm·min^-1,respectively.This study has important practical significance for promoting the risk monitoring and early warning system of the deterioration of cultural relics in Mogao Grottoes,which is aimed at developing measures to prevent the occurrence and development of weathering and erosion diseases of the cultural relics,and finally achieving the preventive protection of heritage.The non-destructive testing and comprehensive analysis methods used in the research process provide a new idea for determining rainfall thresholds in areas where rainfall monitoring data is insufficient or missing.