Earlier, the press conference of Gansu provincial government pointed out that the Zhouqu disaster area mainly faced three difficulties and problems. One is that it is difficult to confirm the statistical data of missing persons among migrant workers and migrant workers, and it is difficult to ensure the accuracy of names, places of origin, gender, nationality, occupation and quantity.
On August 15, the whole country mourned, and the national flag was lowered again for the dignity of life. Sudden disasters bring people not only pain and sadness, but also reflection and warning behind the disasters.
Learn from a painful experience, in order to comfort the deceased and let more people alive know the reasons behind the disaster, China Economic Times reporter has been conducting comprehensive interviews and in-depth investigations since he entered Zhouqu the next morning after the disaster. The reporter tried to follow the trajectory of the disaster and pursue various reasons that induced and led to the disaster.
first part
According to academic data, debris flow refers to a special torrent that loose debris on a hillside or valley is saturated by heavy rain or snow and glacier melt water and flows along the hillside or valley under the action of gravity. It is characterized by sudden outbreak, short duration, fierce momentum and great harm.
The formation of debris flow must meet three conditions at the same time: first, there are rich and loose solid substances in the basin; Secondly, the valley slope in the basin is steep and the slope of the gully bed is large; Third, there are storms, floods or melted snow and ice in the middle and upper reaches of the valley, and lakes and reservoirs break to provide sufficient water.
It should be said that the "August 7" Zhouqu catastrophic mudslide disaster has the above three conditions.
Terrain formed by debris flow
Let's talk about Zhouqu County first. The basic topographic structure of Zhouqu County is that the valley is deep, two mountains are sandwiched by a river, and the town is built on the floodplain. Its two mountains are composed of two east-west peaks in the Xuanshan Mountain System. Strictly speaking, the county seat is located on the Bailong River at the foot of a steep mountain.
Debris flow is poured into Zhouqu County from the towering Beishan Canyon.
Catastrophic debris flow enters the county seat in two ways, both located in the northeast of the county seat. The entrances to the city are Sanyanyugou and Luojiayugou respectively. After entering the county seat, two alluvial fans with an average width of about 300 meters and 100 meters were formed, which were 2.3 kilometers and 2.5 kilometers respectively. Pile up a lot of stones on the long alluvial fan. Debris flow finally rushed into the Bailong River, burying the Bailong River valley for two or three kilometers, causing the riverbed to rise by 10 meter, and the Bailong River water was blocked into the county seat, flooding one third of the urban area.
Among them, the biggest debris flow comes from Sanyanyu. Go north along Sanyanyu and enter the valley, which is Sanyanyugou River. Sanyan Yugou River originates from the east Cuifeng Mountain Valley of Gulei Mountain (elevation 4 154m) in the north of Zhouqu County, with a flow of 0.75m3 seconds. Belonging to the first-class tributary of the left bank of Bailong River, it is a disastrous sparse debris flow ditch.
Sanyanyugou is a "gourd ladle" with an area of 25.75 square kilometers, which is more than twice that of the whole Zhouqu county (12 square kilometers) and more than six times that of the county (4 square kilometers).
At the same time, Sanyanyugou, as the main water source of the county, is 7.8 kilometers from the water source to the reservoir at the mouth of Sanyanyugou.
In addition, the highest point of Sanyanyugou is 3828 meters above sea level, and the lowest point is only 1340 meters above sea level, with a drop of 2488 meters. The slopes on both sides are mostly above 50 degrees.
Zhouqu County is located in the alpine valley, just at the confluence of Sanyanyugou, Luojiayugou and Xiaoshuigou.
From the analysis of topographic structure, Sanyanyugou has the conditions of high mountain and deep ditch, steep terrain, large longitudinal slope of ditch bed and convenient confluence.
From the landform analysis, debris flow landform can be generally divided into three parts: formation area, circulation area and accumulation area.
The reporter walked into the valley and found that the upper reaches of Sanyanyugou were surrounded by mountains on three sides, and there was a spoon-shaped exit on one side. The terrain is relatively open, the surrounding mountains are steep, the mountains are broken and the vegetation grows poorly. This terrain is conducive to the accumulation of water and debris. The circulation area in the middle reaches is a narrow and steep canyon with a large longitudinal slope at the bottom, which enables debris flow to flow quickly and directly. The downstream accumulation area is an open and flat valley terrace, which happens to be the seat of Zhouqu County.
Loose material of debris flow origin
Xu, Minister of Land and Resources, analyzed the five major reasons for this catastrophic mudslide, two of which are as follows:
First, the "5. 12" Wenchuan earthquake affected Zhouqu, and the strong earthquake caused the mountain to loosen and the rock to break. Experts say it will take at least three to five years for rocks to stabilize. Second, meteorological reasons. In the first half of this year, Zhouqu suffered from drought; The drought has shrunk the mountains in this area, widened the gap between rocks and mountains, and made the already loose rocks and mountains more loose and not adhered to each other.
Gulei Mountain belongs to Minshan-Dieshan Mountain System, bordering Tanchang County in the north, and Zhouqu, south of the ridge, is a geological fault zone, covered with deep loess and weathered sandstone at rock peaks.
Ma, a professor at Chengdu Institute of Mountain Hazards and Environment, Ministry of Water Resources, Chinese Academy of Sciences, was a researcher at Lanzhou Institute of Glacier and Frozen Soil, Chinese Academy of Sciences 0/4 years ago, and specially led a team to Sanyanyugou for a comprehensive investigation.
Ma pointed out in a previously published paper that there were many sediments in the ditch due to several major earthquakes in history. What is particularly rare is that the collapsed mountain formed four huge stone dams with a height of 80 meters to 283 meters in the ditch, which blocked most of the sediment in the ditch. There are more than 50 million cubic meters of deposits in Sanyanyugou gully, of which 20 million cubic meters are unstable, which can be directly used as the supply of debris flow.
After the "August 7" Zhouqu mudslide broke out, experts from mountain torrents and mudslides went to the disaster area to carry out disaster investigation. It is preliminarily determined that super rainfall and fragile geological environment are the main factors for the "8.7" Zhouqu debris flow outbreak.
First, there are a large number of loose solid materials such as landslides and collapses in Sanyanyugou, which provide sufficient material conditions for the occurrence of debris flows, most of which were induced by the July 1879 Wenxian earthquake with M = 8 in Gansu. At the same time, Zhouqu is located in the northern edge of the Longmenshan seismic activity zone, close to the Tianshui seismic activity zone. Affected by the Wenchuan earthquake, the soil was relatively loose, and it was easy to form mudslides when encountering heavy rainfall.
Second, the vegetation in the upper reaches of Sanyanyugou is dominated by young forests, with a high proportion of shrub and grass, partially exposed and weak water storage capacity. After a long and severe drought this summer, the topsoil has become drier and looser.
Third, under the recent heavy rainfall, the strength of soil is greatly reduced, forming a slope debris flow, which gradually drives the landslide rock and soil on the gully slope and forms a debris flow with great impact. In the process of convergence from the middle and upper reaches to the middle and lower reaches, the natural rockfill dam formed by the earthquake will gradually burst, eventually leading to an increase in debris flow and enhanced destructive power.
Short-term heavy rainfall caused by debris flow
According to the data of the local meteorological observatory, there was heavy rainfall in the north of Zhouqu County on August 7, when the rainfall in Zhouqu County was relatively small.
"The rainfall recorded by Zhouqu County Meteorological Station started at 22: 57 on the 7th, and the rainfall at 0: 00 was 2.4 mm.. The maximum rainfall from 0: 00 to 1 hour is 6.8 mm, and the whole rainfall process lasting for 6 hours is 12.8 mm. "
Wei Xingong, director of the Meteorological Bureau of Zhouqu County, said in an interview with China Economic Times: "That night, the Dongshan Township Regional Meteorological Station, which is also located in the northern part of the county, monitored the hourly rainfall of 77.3mm from 22: 00 to 23: 00 on the 7th. The whole rainfall process lasted 9 hours and the rainfall was 96.3 mm. "
Because there are no meteorological records in Sanyanyugou and Luojiayugou in the heavy rainfall area, the meteorological department can only express heavy rainfall according to the meteorological records of Dongshan District Station nearest to the heavy rainfall area.
Therefore, there is no clear answer to the rainfall in Sanyanyugou, which caused the catastrophic mudslide disaster in Zhouqu.
The second part of a two-part program/book.
On the evening of August 7th, a catastrophic flash flood and debris flow occurred in Sanyanyugou, north of Zhouqu County. Sanyanyu Village and Yuan Yue Village in Chengguan Town of Zhouqu County were the first to bear the brunt, and Yuan Yue Village in the middle of the river was devastated.
The debris flow roared down the hillside and attacked Beijie Village, Dongjie Village, Beiguan Village, Nanmen Village and Henan New Village on the south bank of Bailong River along the way. Few villagers and residents survived.
According to the investigation, the maximum flow of debris flow in Sanyanyu reaches 1.394 m3/s, the instantaneous speed of debris flow rushing out of the mountain pass reaches 9.8 m/s, the amount of debris flow rushing out is1.5000m3, and the process of debris flow takes about 20 minutes.
In addition to the above natural reasons, there are also some social reasons that have to arouse people's vigilance.
Urbanization construction
An indisputable fact is that no matter how big a mudslide is, if it happens in an uninhabited place, it is only a mudslide, and there is no way to talk about disaster.
Since Zhouqu was built, the county has been located on the debris flow accumulation fan in Sanyanyugou. In the past, due to the small population, the urban area was concentrated on both sides of Bailong River, and the debris flow was less harmful.
With the population growth and the rapid expansion of the county, Sanyan Yugou River, that is, the debris flow danger zone, has been developed into urban areas or residential areas.
Zhouqu county covers an area of less than 4 square kilometers, but it accommodates more than 40 thousand people. With the pace of urbanization, more and more people are pouring into this town.
From Sanyanyugoukou to Yuan Yue Village where the disaster occurred, all the rivers were developed into cultivated land; From Yuan Yue Village to Bailong River, the river was completely replaced by houses, leaving only a spillway less than 5 meters in the middle. All the buildings in the river were washed away in this disaster.
As early as five years ago, Gansu Lanzhou Morning Post described the hidden danger of Zhouqu debris flow disaster with "ubiquitous threat". The article points out that "Zhaizigou, Nishuigou, Sanyanyugou, Luojiayugou and other high-frequency debris flow ditches directly threaten the safety of the county".
According to reports, in Zhouqu, roads and bridges are washed away by mudslides every year. Three times, only 1.978, 1.989 and 1.992, 842 houses were destroyed by the debris flow in Sanyan Yugou, with two people killed and 1.94 injured.
The frequent occurrence of debris flow in Zhouqu not only failed to attract enough attention from city managers and planners, but was a predatory development of Sanyanyugou, the last "geomantic treasure land" in the city.
At the site of the "August 7" Zhouqu catastrophic mudslide disaster, the remaining half buildings in Luojiayugou were built on a dam less than 5 meters wide, and there were three completed buildings less than 30 meters away from the dam, waiting for the final decoration.
Some people describe that "the area destroyed by debris flow in Sanyanyugou is less than one-fifth of the county, but it has lost half of the county's wealth". The reason is that "every inch of land is worth every inch of gold" along the river. "Those who can buy a house and build a house here are either the elite of Zhouqu or the rich of Zhouqu".
Li Aihui, deputy director of the Urban and Rural Construction Bureau of Zhouqu County, said that Zhouqu City is less than 2 kilometers from east to west and less than 1.5 kilometers from north to south. In this narrow space, the population is close to 50 thousand, while the total population of Zhouqu County is only136,900. The urban population density ranks first among the county-level cities in the province, and it is close to Lanzhou, the capital of Gansu Province.
With the continuous increase of urban population and the continuous development of rural economy, in the process of urbanization, the phenomenon of land competition between people and rivers is not unique to Zhouqu County. Many mountainous towns and villages in China are facing such a dilemma.
Ecological destruction
Historically, Sanyanyugou has dense forests and high vegetation coverage. In the impression of the older generation of Zhouqu, Zhouqu, which is densely forested, is often called "Yangzhou" and "Little Jiangnan". The Sanyanyu of this disaster was a dense forest decades ago, and it took several people to hold the biggest tree.
Feng Changyi, a 7 1 year-old man from Sanyanyu Village, told the China Economic Times reporter that he "when he was a teenager, almost all the trees in Sanyanyugou were big trees held by one person, and the trees were very dense"; However, "a large number of trees were cut down in the sixties and seventies", "the most serious deforestation was in the eighties and nineties" and "it is difficult to see a mature tree within 5 kilometers of the ditch now".
According to the records of Zhouqu County, after more than 30 years of logging, the forest resources of many forest farms have been exhausted and there is no wood to cut. In the 1970s, 80,000 cubic meters of wood were harvested and produced annually, with a consumption of 250,000 cubic meters.
Relevant data show that the original forestland area in Zhouqu County is 29 1 10,000 mu, accounting for 65% of the county's land area, including forest area 1.23 million mu and standing trees 1.77 million cubic meters. The forest coverage rate in Zhouqu is 44.7%, which is much higher than the average level in Gansu Province (7.48%) and the national average level (22%).
According to the survey, before the implementation of 1998 national natural forest protection project, local villagers used the shrub in Sanyangou as the fuel for heating and other living.
And the villagers also have the habit of digging roots. In the words of Liu Qingquan, a villager in Sanyan Yugou, every autumn and winter, 60% people chop wood and 40% people dig roots. "People burn firewood and consume nearly 654.38+10,000 cubic meters of forest resources every year."
"Even though the country implemented the natural forest protection project more than ten years ago, even though the 923 forest farm has a protection station in Sanyangou, in the past two years, many villagers have seen roots dug from Sanyangou for firewood. Five or six female forest rangers only stay at the protection station during working hours, and no one is in charge at 6 o'clock. "
Pei Juanju, deputy editor-in-chief of local chronicles in Zhouqu County and deputy editor-in-chief of local chronicles sequel, told the China Economic Times reporter that "the editor-in-chief and another deputy editor-in-chief of local chronicles in Zhouqu County were also killed in this catastrophic disaster", and he was the only one who "narrowly escaped".
Pei Juanju believes that long-term deforestation at the expense of ecology and overdraft resources has not only failed to attract the attention of the local party and government departments, but has been "fully affirmed": "We have contributed tens of billions of cubic meters of wood to the country in the past and made great contributions to economic development."
According to statistics, from the establishment of Zhouqu Forestry Bureau in August 1952 to June, 1990, 189750 mu of forest was cut down, and forests in many places became dilapidated secondary forests.
Coupled with civil timber, deforestation, reselling and misappropriation, the forest area of the county is reduced at a rate of 654.38+10,000 cubic meters every year, and the vegetation is seriously damaged, and the ecological environment is destroyed beyond the limit. Excessive logging and illegal exploitation not only destroyed the vegetation in Sanyanyugou, but also destroyed the ecology of Zhouqu. What is even more frightening is that it has left a huge hidden danger for foreseeable disasters.
The decrease of vegetation not only reduces the surface water storage capacity, but also increases the slope runoff, which is more likely to induce the occurrence of debris flow disasters.
198 1 year, a huge mudslide hit Zhouqu hard. At that time, 50 million cubic meters of debris flow poured down, blocking the Bailong River and flooding the main city. Fortunately, there were not many casualties except farmland houses.
Jia Wenping, director of the Gansu Provincial Water Resources Department Office, was blunt in an interview with the China Economic Times reporter: "This flood and debris flow disaster has a lot to do with soil erosion."
Dam break of flood retaining dam
On August 12, Li Zhiheng, president of Gansu Geological Environment Monitoring Institute and director of Gansu Geological Disaster Emergency Center, said many times that after investigation, "there are no water conservancy facilities and reservoirs above Sanyanyugou and Luojiayugou where mudslides occurred".
"There are two debris flow retaining dams on the mountain where the debris flow occurred, which started construction on 1997, and have played a certain role in retaining debris flow after completion". "A * * * has seven dams, and this mudslide destroyed three," Li Zhiheng said.
On the morning of August 14, China Economic Times reporter, led by Feng Changyi, a 7/kloc-0-year-old man in Sanyanyu Village, and Yang Jinchao, a branch secretary of Sanyanyu Village who just left his post after the "8.7" flood, walked into Sanyanyugou, the main source of this mudslide.
From the mouth of Sanyanyugou to the ditch, the reporter clearly saw six flood retaining dams destroyed by water under the identification of villagers, of which three were completed on 1998, and the other three were basically completed this year but not yet delivered.
Our reporter saw on the dams of two flood dams built this year and destroyed by floods: the two sides of the dams are less than one meter, lined with cement mortar, and the middle part is about five meters, filled with stones and sand; But without concrete, the sand is dry, and the stone will fall out with a little hand.
"The flood dam is purely a bean curd residue project. Instead of playing a role in flood control, it has become a reservoir that breaks its banks at any time. " The two daughters of the former director of the Zhouqu County Public Security Bureau said in an interview with the China Economic Times reporter, "Without the flood control dam, the flood can only flow along the river to Bailong River, and there will not be such a large area and so many people will die."
"Our sisters grew up here, and the place 3 meters below our home is the vegetable field of our neighbor's house. The width of debris flow in this place is 500 meters. If the flood control dam collapses, the mudslide will not get here at all. " The two sisters cried and said, "Even if my brother and mother were killed, at least so many innocent people downstream would not be submerged by mudslides."
Yang Bingcheng, a member of the Zhouqu County Political Consultative Conference, is even more blunt to the China Economic Times reporter: "The dam built this year is an out-and-out bean curd residue project, with a flood control project of nearly/kloc-0.0 million yuan and four flood control dams, only using 400 tons of cement!" Yang Bingcheng said that he was willing to "take legal responsibility for what he said."
Yang is an engineer. He said, "It takes at least 500 tons of cement to paint an 8,000-square-meter wall, and this flood dam is 120 meters long, 8 meters high, 6 meters wide at the bottom and 2 meters wide at the top ..."
He also pointed out that the Sanyanyugou flood dam project not only cut corners seriously, but also had repeated contracting. In this regard, Yang Bingcheng specifically reported to the Zhouqu County Environmental Protection Bureau, but the problem has not been solved.
China Economic Times reporter saw on the website of Gansu Provincial Department of Housing and Urban-Rural Development that the winning bidder of Sanyan Yugou Project of Zhouqu County Comprehensive Flood Control Project was Gansu Guang Lin Construction and Installation Co., Ltd., with a cost of 58,795,438+05.5438+0 yuan.
Regarding the Sanyanyugou flood control dam project, China Economic Times reporter repeatedly contacted Zhao, member of the Standing Committee of Gannan Tibetan Autonomous Prefecture and Minister of Propaganda, and the member of the Standing Committee of Zhouqu County Committee and Minister of Propaganda, hoping to interview the relevant personnel of Zhouqu County Environmental Protection Bureau, and was told that "the director of environmental protection needs surgery urgently because of cancer, and other relevant personnel do not know about the project".
In addition, the mining, quarrying and slag disposal in Sanyan Yugou mountain, and six dams destroyed by water also provided a large number of loose materials for this debris flow.
Early warning loophole
An investigation and evaluation report of the working group in front of the emergency rescue and disaster relief in Gansu Province shows that there are more than 20 cubic meters of boulders 100 in Sanyanyu debris flow disaster, and the largest stone that rushes out of the gully is 7.5 meters in diameter, 290 cubic meters in volume and 770 tons in weight; The impact zone of debris flow is about 2.3 kilometers long from the gully mouth.
On the morning of August 14, our reporter went to Sanyanyu for an on-the-spot interview, and happened to meet Dr. Chen Ningsheng, director of the flash flood research office, and his party for investigation. They measured that the largest boulder at the mouth of Dayugou in Sanyanyu is 800 cubic meters.
Old buddy Feng Changyi said that this boulder was originally "lying flat" 300 meters upstream and was washed upside down by the flood. The old man said that there was a natural stone pit in the shape of a fish. When there is a lot of rain, the pit is full of water; The drier the weather, the shallower the water. Over the years, local people have often used this to judge the quality of their years.
The upper reaches of Bailong River where Zhouqu is located is one of the four major landslide and debris flow development areas in China. Zhouqu County is located on the accumulation fan of five debris flow gullies, such as Xiaoshuigou, Longmiaogou, Sanyanyugou, Luojiayugou and Nanyugou, which are tributaries on the left bank of Bailong River, and it is prone to debris flow.
From 1823 to 187, there have been 12 large-scale mudslides in Sanyanyugou, which have caused harm to the county, and small-scale mudslides are more frequent, about once every 3-4 years.
1On June 4, 1992, the torrential rain in Zhouqu plummeted, which triggered a catastrophic mudslide once in 50 years. 344 houses were destroyed, 87.73 hectares of farmland were destroyed, and 87 people were killed or injured.
The "August 7" catastrophic debris flow disaster in Zhouqu is the biggest debris flow disaster event in the history of this area. The maximum flow rate of this debris flow is 1394 cubic meters per second, and the velocity of debris flow at the exit of the mountain pass reaches 9.6 meters per second.
On August 1 1, another mudslide occurred in Zhouqu, which was called "an example of successful risk avoidance" by the local land department.
China Economic Times reporter asked Zhang Guohua, deputy director of Gansu Provincial Department of Land and Resources: "It seems that our early warning system was very successful in this recurring mudslide incident, ensuring that no one died. Then, did our early warning system play a role before the' 8 7' mudslide that just happened a few days ago? "
Zhang Guohua replied that the mudslide of 5438+0 1 in June was within our monitoring range; Before the "August 7" mudslide, there were 56 monitoring points in the county, and 56 monitors were responsible for monitoring; Each monitor is handed over to 600 yuan, who is responsible for a monitoring point, watching the daily changes and making reports regularly. After discovering the danger, the squad leader is responsible for reporting it, or formally calling the police to organize the evacuation.
Zhang Guohua said: "The' 8 7' debris flow is a mountain flood disaster at the mouth of Sanyanyugou and Luojiayugou, which happened outside our early warning and monitoring points. So our monitor can't play any role. "
Our reporter saw in the Plan of Prevention and Control of Key Landslides and Debris Flows in Zhouqu County that there are two original 56 monitoring points involving Chengguan Town: one is Suoertou Village and the other is Zhenyatou Village. The monitoring personnel are divided into He Zhang and the monitoring type is landslide and unstable slope. Sanyanyugou and Luojiayugou in Chengguan Town still do not appear on the newly added three monitoring lists.
The biggest feature of debris flow is seasonality. The outbreak of debris flow in China is mainly triggered by continuous rainfall, heavy rain, especially the concentrated rainfall of heavy rain.
Therefore, the time law of debris flow is consistent with that of concentrated rainfall, which has obvious seasonality. It usually occurs in rainy summer and autumn, which is different due to the time difference of concentrated rainfall.
Rainfall in southwest areas such as Sichuan and Yunnan is mostly concentrated in June-September, so mudslides in southwest areas mostly occur in June-September. However, the rainfall in northwest China is mostly concentrated in June, July and August, especially in July and August, when the rainstorm intensity is high; Therefore, the debris flow in northwest China mostly occurs in July and August.