RESEARCH ON "7·20" MOUNTAIN TORRENTS AND GEOLOGICAL DISASTERS IN ZHENGZHOU CITY, HENAN PROVINCE OF CHINA

On July 20,2021,an extreme rainstorm in Henan province triggered mountain torrents and geological disasters in four cities(Xingyang,Gongyi,Xinmi,and Dengfeng) in the western area of Zhengzhou city,Henan province of China,which caused 251 deaths and missing people. The victims are distributed in 140 villages and communities of 44 towns. The disasters have the characteristics of group-occurring,dispersion,and relatively agglomeration. The mountain torrents and geological disasters were scattered and complex,and the watershed disaster chain and regional disaster groups coexisted. The disaster chain of mountain torrents showed the effects of spatial relevancy,time continuity,dynamic conversion,and disaster amplification. In order to reveal the cause of the mountain torrents and geological disasters,we summarized its spatio-temporal distribution characteristics and analyzed its triggering factors. We discussed the cause of a typical mountain torrent disaster chain,which is an outburst of subgrade blocking water in waterlogging from Xingmentangnao embankment in the upper reach of Suohe river,flash flood of Wangzongdian Village,and damming water of Haigouzhai in Cuimiao Village. We also investigated the geomechanical model of the translational landslide of Nantou Community in Wangzongdian Village,and the relationship between its stability and mechanical parameters. We suggest an early warning and response criterion of mountain torrent and geological hazards,i. e.,if the current process or daily rainfall reaches 200 mm,and the rainfall forecast exceeds 40 mm in the next one hour or 100 mm in three hours,a red alert response must be initiated. The research results can provide decision-making support for prevention and emergency response of mountain torrent and geological hazards in the western area of Zhengzhou city in the future,and can also provide a reference for disaster risk reduction of urban and rural communities in similar regions.