The landscape pattern of the whole country can be roughly divided into three steps: the third step is the eastern Daxinganling-Yanshan-Taihang Mountain-eastern Sichuan-Miaoling, which is dominated by hills and plains, and the altitude is generally lower than1000 m; The first level is the Qinghai-Tibet Plateau, south of Altun Mountain-Qilian Mountain, west of Minshan Mountain-Hengduan Mountain, with an altitude of more than 4000m; The second steps between them are Inner Mongolia Plateau, Loess Plateau, Yunnan-Guizhou Plateau, Sichuan Basin, Xinjiang Mountain and two inland basins, with an altitude of 1000 ~ 2000m. Because of the height difference between the two steps, the material at the edge of the step has great potential energy. Therefore, the slope zones with the transition from the first step to the second step, such as the northern foot of Altun Mountain, the northern foot of Qilian Mountain, western Sichuan and Hengduan Mountain, have become the hardest hit areas for gravity and geomorphology disasters such as collapse, landslide and debris flow, and western Sichuan has become the birthplace of floods in Sichuan Basin. The foothills of Taihang Mountain, the eastern mountains of Sichuan and the western mountains of Guangxi in the transition from Yunnan-Guizhou Plateau also have obvious height differences, and the potential energy of surface materials is large, which has become the hardest hit area of gravity landform disasters and the birthplace of floods. At the same time, the two terrace slopes incline eastward, which is opposite to the southeast monsoon. The warm and humid air flow in the Pacific Ocean is forced to rise on the windward slope, forming heavy rain on the slope and in front of the mountain, and then gathering in the plain area, causing floods.
Another geomorphic feature is the alternating distribution of basins and mountains. The western part of China is dominated by inland basins, such as Tarim Basin, Junggar Basin, Qaidam Basin, Sichuan Basin and Hexi Corridor. These basins and valleys are surrounded by tall mountains. Because the local climate is dry and cold, physical weathering is strong, there are many mudslides on the slope, and the relative height difference between the mountain and the basin is very large, and the collapse, landslide, debris flow, rainstorm and flood are very serious. The southeastern part of China is dominated by small mountain basins, and the relative height difference with the surrounding low hills is more than 200m m m. Many mountains are composed of Mesozoic and Tertiary red beds, and the slope rocks are easily weathered and broken, and the disasters such as collapse, landslide and soil erosion are also very serious. The basin is usually located in densely populated and economically developed towns, and it is also an area flooded by heavy rains and floods, which often causes heavy flood losses.
Karst areas are widely distributed in southwest areas such as Yunnan, Guizhou and Guangxi, and are prone to drought, flood and collapse.
(b) Neotectonic movement is active.
Chinese mainland is located at the intersection of the Pacific plate, the Eurasian plate and the Indian Ocean plate, and the China block itself is composed of several small blocks. The suture structural zones between plates are all active zones of neotectonic movement, which are mainly characterized by earthquakes, volcanoes, folds and faults. The famous earthquakes in China include Taitung seismic belt, Himalayan seismic belt, Kunlun seismic belt, Tianshan north-south seismic belt, Qilian seismic belt, western Sichuan seismic belt, Central China north-south seismic belt, Wei Fen seismic belt, Hebei (ancient rift valley) seismic belt, Tanlu seismic belt and southeast coastal seismic belt.
Neotectonic movement continues to cause differential tectonic movements in various blocks in China, resulting in the gradual decline of the Qinghai-Tibet Plateau to the Pacific Ocean. First, it determines the change of China's natural environment. The northwest region has become an arid and semi-arid climate zone, forming desert and loess accumulation, and the eastern region has formed a typical monsoon climate. Summer monsoon and winter monsoon advance and retreat alternately every year, which is influenced by the east-west mountain system, so it has the characteristics of forward jump (cold and warm front movement). The second is to increase the relative height difference of the ground. Since the Himalayan Movement, the Qinghai-Tibet Plateau has risen by about 2,000 m, the Tianshan Mountains by about 2,000 m and the Qinling Mountains by about 2,000 m. Therefore, the relative height difference and topographic slope in most areas have increased, and the surface materials have a strong downward trend. At the same time, strong freezing weathering and strong tectonic movement break the materials on the slope, providing a large number of solid material sources for collapse, landslide, debris flow and running water. The third is to form the basic pattern of China water system. The main rivers originating from the Qinghai-Tibet Plateau flow into the sea to the east, with large catchment area, large drop and large sediment transport, which causes serious floods and soil erosion in the basin.
(c) Monsoon is the main source of climate disasters in China.
Summer monsoon brings abundant precipitation to the east of China, but it also causes typhoons and other disasters. Winter monsoon brings low temperature and strong wind to most parts of China.
Under the influence of seasonal migration of westerly belt and subtropical high, Siberian-Mongolian cold high is stronger in winter, and the thermal difference between East Asia continent and Pacific Ocean, summer monsoon and winter monsoon prevail and confront each other alternately in eastern China. So summer is hot and rainy, winter is cold and dry, and the monthly difference between temperature and precipitation is very large. Coupled with the influence of topographic factors, the regional differences between temperature and precipitation are even greater. Monsoon climate is particularly sensitive to factors such as El Nino, volcanic eruption and sunspot caused by global change, which leads to the increase of interannual difference between temperature and precipitation. The change of monsoon climate directly affects the change of thermal conditions of surface water, and then causes natural disasters, including: ① freezing injury and low temperature in winter. In winter, Siberian cold air masses frequently invade south under the guidance of the northwest airflow behind the high altitude trough (high altitude East Asia trough), making China the coldest country in the same latitude in the world. The average temperature in China in June 5438+ 10 is 5 ~ 18℃ lower than that in other areas of the same latitude, which causes huge heat loss and shortens the growth period of crops. There are early rice rotten seedlings with low temperature in spring and cold dew wind in autumn in the south of the Yangtze River. There are obstacle chilling injury (cold dew wind) and delayed chilling injury (low temperature delays crop development in summer) in the northeast and outside the Great Wall. ② Drought and flood. Because the summer monsoon is very unstable, the length of rainy season, rainfall and rain area change greatly. The interannual difference of annual precipitation is greater in the north than in the south, and the average variability of annual precipitation is as high as 20% ~ 30%. For example, the precipitation in Beijing 1959 is 8.5 times that in 189 1 year; During the period of 1994, severe drought occurred in Central China, and severe floods occurred in South China and North China, resulting in economic losses of 70 billion to 80 billion yuan. Because the summer monsoon and rainy season only reach North China and Northeast China in July, these areas are called "ten years and nine spring droughts". ③ Wind disaster. First, the cold wave and strong wind caused the white hair wind (storm) in the northern pastoral areas, which would freeze the livestock to death. Yellow hair wind often carries a lot of sand and dust, which makes the pasture degenerate and desertification; Causing black wind disasters (sandstorms) in arid areas. For example,1May 5-6, 1993, black winds swept across Xinjiang, Gansu, Ningxia and other provinces (regions), 5.6 million mu of farmland was affected, 245,000 mu of orchards were affected, 85 people were killed, 3 1 person was missing, and 240,000 livestock died. Cold waves and strong winds can also subvert trains and ships. The second is typhoon. Typhoons usually move to the west or northwest under the guidance of the southeast airflow at the southern edge of the subtropical high, and land in the coastal areas of South China, Jiangsu and Zhejiang. Although the typhoon brought abundant precipitation to the southeast of China, it also caused disasters such as strong winds, storm surges and floods. For example, Typhoon 94 17 landed in Ruian, Wenzhou. Although hundreds of thousands of residents were moved ahead of schedule, 1 126 people were still killed, more than 300 people were missing, and the direct economic loss was17.8 billion yuan.
(D) One of the leading industries has a low ability to resist disasters in agriculture.
China has a large population, less per capita resources, low level of agricultural production, vulnerability to weather, weak economic strength and poor ability to resist natural disasters. Once the weather system changes, agriculture will be affected in a large area and the recovery rate will be slow.
China's population and economically developed areas are mainly concentrated in rivers, lakes and coastal plains, which are vulnerable to floods, storm surges and other disasters. With the rapid growth of population, in order to obtain local short-term benefits, human society continues to develop rivers, coasts and piedmont areas that are vulnerable to natural disasters, forcing more human resources to invest in disaster relief and falling into a vicious circle. At present, the predicament that the floodplain in the middle and lower reaches of the Yellow River is higher than the plain behind the dike is caused by the vicious circle of "flood-dike building-siltation-flood level rising-dike heightening again …". Therefore, the threat of flood has not been eliminated, but the flood disaster is getting worse every day. Similarly, blind reclamation and development of beaches in coastal areas will also lead to a passive situation where the loss outweighs the gain.
(e) China City is an affected area.
Most cities in China have a long history, so it is easy to find the historical records of disasters. Of the 666 cities in China, about 45% are located in high-intensity areas with earthquake intensity above seven degrees, and more than 360 large and medium-sized cities have obvious land subsidence; There are 300 cities short of water, including more than 0/00 cities with severe water shortage/kloc; Most cities in history have suffered from floods. Some cities in North China have ancient cities buried under them, and there are also bridges under them. For example, Xuzhou has a complete Ming Dynasty ancient city buried 5 ~ 6m underground, and Luoyang and Kaifeng have underground ancient cities. At least 70 counties and cities in China are directly threatened by collapses, landslides and mudslides. For example, Zhouqu County, Gansu Province suffered a catastrophic mudslide disaster in August 20 10, and the county seat was rebuilt next to the debris flow valley after the disaster. In addition, there are typhoon storm surge, seawater intrusion, sandstorm, sandstorm invasion, ground fissures, collapses (such as Liuzhou and Guilin), heavy rain, acid rain, etc., which make the city suffer losses and restrict its development. According to preliminary estimates, about half of the total disaster losses in China are caused by direct disasters in cities or to reduce disaster losses in big cities (such as flood discharge and flood diversion).
Serious natural disasters in cities are mainly due to serious soil erosion, increased flood flow, higher and earlier flood peaks, land subsidence, salt water intrusion (such as towns in the Pearl River Delta region) and increased impervious surface caused by human activities, in addition to being often affected by floods, earthquakes, typhoons, rainstorms and storm surges.
Urban disasters are also determined by the topography of the city. In a modern city with relatively low productivity, it is a place where things are bartered, and then it gradually develops and grows. So its location must be "near the mountain and near the water". The mountain is close to the hillside or the valley mouth, which is convenient for the exchange of goods between people in mountainous areas and plains, and the water is along the river, lake and shore, which is convenient for land and water transportation and goods trade. This landform is vulnerable to all kinds of disasters. Because of the complex geological structure, active neotectonic movement, large relative height difference and unstable slope materials, geological disasters and gravity landform disasters are prone to occur, and disasters are also prone to occur due to the confluence of heavy rains. Along the river, along the lake and along the coast, it is easy to be threatened by floods and storm surges, especially the development of Hongyi Economic Development Zone to flood-prone areas, which will inevitably lead to a lot of money and manpower invested in flood control system construction. For example, the lowest ground points in Hankou and Hanyang are only 18m and 20m above sea level, while the average water level in Wuhan section of the Yangtze River has reached 19. 15m, 1954 flood level reached 29.73m, and 1998 flood level reached 29.43m (Hankou Hydrological Station).
The reason why the city is easy to become the hardest hit area is also because of the large population, high-tech goods, fragile property concentration, and wide range of life and production activities.