"6·" Debris flow in Dashu Village, Suichang County, Zhejiang Province

1 Introduction

The disaster point is located in the mountain depression on the northwest side of Dashuhui Natural Village, Dashu Village, Huangshayao Town, Suichang County. There is a village road from Dashu Village to Suichang County. , the direct distance is about 50km, and the traffic conditions are average (Figure 1). The geographical coordinates of the center of the debris flow ditch mouth are: 118°48′19″ east longitude, 28°24′36″ north latitude.

Figure 1 Geographic location of the disaster site

From 6 to 10 o'clock on June 27, 2014, due to the impact of continuous rainfall, Dashuhui, Dashu Village, Huangshayao Town, Suichang County A debris flow geological disaster occurred in the gully on the north and west side of the natural village. The main flow direction is 147° and the volume is initially estimated to be about 35,000m3. The director and inspectors of Dashu Village discovered a dangerous situation during the inspection, and immediately organized personnel to actively respond and evacuated and moved 18 households with 75 people and valuable property as quickly as possible. A debris flow occurred at 9:40 on June 27, mainly due to the debris flow disaster caused by concentrated heavy rainfall (more than 100mm in 2 hours) on the morning of the 27th. It caused the collapse of 79 villagers' houses in 17 households and flooded 123 villagers' houses in 35 households. About 100 acres of farmland, 750m of flood control embankment, 600m of mechanical plowing road and 3 bridges were destroyed. The Damshu Power Station was seriously damaged, with 186 people directly affected and a direct economic loss of about 12.2 million yuan (Photo 1).

Photo 1 Disaster point perspective

2 Overview of geological disasters

2.1 Geological environmental conditions

(1) Topography

This area is located in a low mountainous area with structural erosion and denudation. The altitude is between 380 and 1300m. The terrain is steep and the mountain slope is between 25° and 45°. There are developed valleys in the area, with cutting depths ranging from 200 to 300m. The ditches are narrow, and a large amount of loose soil accumulates on the slopes on both sides. The vegetation is relatively developed, mainly shrubs. In the sloping floodplain area around Dashu Village, the terrain is gentle, with an elevation between 400 and 450m, and a terrain slope between 5° and 10°. The terraces built by the villagers are important agricultural production land for the local villagers.

The planar shape of the debris flow basin is funnel-shaped, wide at the top and narrow at the bottom. The upstream catchment area is a medium-low mountain with an altitude between 650 and 1300m, with the highest being 1307.0m. The terrain slope is between 30° and 45°. Around °, the vegetation is well developed, mainly pine, shrubs, etc.

The elevation of the debris flow gully area is between 450 and 650m. The channel is long and narrow, and the shape of the channel changes greatly. The average longitudinal slope is about 250‰, and the cross section is "V" shaped. The slope of the mountain slope is about 30° to 45°, and the vegetation is relatively developed, with pine trees, shrubs, etc. distributed there.

The elevation of the ditch mouth accumulation area is between 400 and 450m. It is a diluvial mountain front slope area. The terrain slope is about 5° to 15°. The terrain is relatively open and is mainly terraced dry land and paddy fields. The downstream is To the east of Dashu Village is the Zhougongyuan Riverbed (Photo 2).

Photo 2 Overview of the Mizokou area (looking down through the mirror to the east)

(2) Stratum lithology

The exposed strata in this area and surrounding areas are mainly lower Cretaceous Chawan Formation (K1c), Xishantou Formation (K1x), Gaowu Formation (K1g), as well as Quaternary residual slope accumulation (Q), Upper Pleistocene flood slope accumulation ( ) and Holocene alluvium ( ) .

A brief description of the lithological characteristics of each formation is as follows:

Lower Cretaceous Chawan Formation (K1c): The lithology is dark gray dacitic, rhyolitic crystal vitreous fusion Condensed tuff, with a massive structure, is mainly exposed to weathering, with moderately developed joints and fissures, and good structural integrity of the rock mass. It is distributed near Aotou and Huangqimen in the area and in the Shangbanling area in the east.

Lower Cretaceous Xishantou Formation (K1x): The lithology is gray-purple massive rhyolitic breccia-bearing vitreous tuff, rhyolitic vitreous crystal tuff and gray-black massive dacite. It is composed of high-quality vitrified crystal tuff, interspersed with thick layers of rhyolite tuff containing volcanic mud balls. It has a massive structure and is mainly weathered when exposed. The joints and fissures are generally developed. The structural integrity of the rock mass is good and it is hard. Rocks. Mainly distributed in the middle and low mountainous areas of the north.

Lower Cretaceous Gaowu Formation (K1g): widely distributed in the mapped area, lithology is dark gray, gray-black crystal glass tuff, tuff structure, massive structure, part of the left bank of the channel can be seen The bedrock is exposed, with good integrity, and is weathered. The bedrock on the right bank is poorly exposed, buried deep, rock joints and fissures are developed, and the integrity is poor. The thickness of the fully-strongly weathered layer on both sides of the mountain is generally 0.5 ~1.5m, and the thickness is larger on the right bank, reaching more than 3.0m.

Quaternary residual slope accumulation (Qel-dl): widely distributed on the surface of the mountain. The lithology is gravel-containing silty clay, gray-brown, with a loose structure. The gravel is composed of weathered bedrock. Mainly, the content is 10-20, the particle size is generally 5-15cm, and the local large ones can reach about 25cm. The thickness of the left bank is generally 1.0-1.5m, the thickness of the right bank can reach 2.0-3.0m, and the local area is larger than 3.0m. The structure loose.

Upper Pleistocene flood slope accumulation layer ( ): mainly distributed in the relatively gentle area at the mouth of the gully and the slope of the right bank of the gully. The lithology is fragmented stone soil mixed with sand. It can be seen that the thickness is about 5.0m. The low and gentle section downstream is thicker, generally 8.0 to 10.0m, grey-brown, with a loose structure, a fragmented stone content of about 50, a block diameter of 5 to 50cm, chaotic distribution, sub-angular shape, and a lithology of crystalline tuff.

Holocene alluvium ( ): Locally distributed in the Zhougongyuan river bed on the east side of the village, the lithology is block rocks, pebbles and medium-coarse sand, with a particle size of 0.2 to 0.5m and a content of about 30; pebbles The content is about 40, the particle size is 2-20cm, the roundness is good, the sorting property is poor, and the rest is medium-coarse sand and a small amount of clay.

(3) Geological structure

The geotectonic location of this area belongs to the South China fold system (Ⅰ), the southeastern Zhejiang fold belt (Ⅱ), the Lishui-Ningbo uplift (Ⅲ), and the Longquan- In the middle part of the Suichang fault uplift (IV), the Suichang-Pingyang regional fault passes through the northeastern periphery of the site. The fault structures in this area are mainly northeast-trending faults, followed by north-south faults. The northeast-trending faults belong to the Cathaysian tectonic system and are thrust faults. The north-south faults are small in scale and are also thrust faults, often cutting the northeast direction. The fractures were generated later, distributed sporadicly, and less developed. According to regional geological data, folds are not very developed in this area. The structural traces in the bedrock area are mainly joints and faults. The overall development level is average, and the right bank of the gully is more developed.

(4) Engineering geological characteristics of rock and soil bodies

Based on field surveys, combined with the collection and analysis of collected data, according to the origin, age, lithological characteristics of rock and soil bodies, etc. The exposed rock and soil mass is divided into four engineering geological rock groups, whose characteristics are shown in Table 1.

(5) Hydrogeological conditions

Loose rock pore water: The loose rock pore water in the area is mainly Holocene alluvial, Upper Pleistocene slope alluvial and residual slope accumulation. Diving in pores. Holocene alluvial gravel and pebbles have good water permeability and good water volume. They are distributed along the Zhougongyuan stream on the east side. The thickness of the aquifer is generally more than 3.0-5.0m, and the water depth of Zhougongyuan is generally 0.5-1.5m. During the flood season The water level is relatively high, with a depth of about 2.0-4.0m. It is mainly supplied by atmospheric precipitation and surface water. The water from the upstream flows along the terrain and flows toward the downstream town.

In the alluvial slope area of ??the mountain front slope, the water-bearing medium is mainly gravel-containing soil and gravel-containing clay soil. The gravel content is about 50%, and the diameter of the fragments is 5 to 50 cm. They are scattered randomly on the slope. The distribution of the alluvial layer is uneven, with good water permeability and good water richness. The thickness is generally greater than 5.0m, ranging from 8.0 to 10.0m. The water level depth is generally between 2.0 and 2.5m, and the water quality is average.

The gravel-containing silty clay layer accumulated on the residual slope is mainly distributed on the surface of the mountain. It is generally small in thickness and has poor water-richness. It only has groundwater seepage after rainfall, and is recharged by atmospheric precipitation and surface runoff. , permeate and excrete from high places to low places.

Bedrock fissure water: Bedrock fissure water mainly occurs in the weathering fissures and structural fissures of the Upper Cretaceous Chawan Formation ignimbrite, Xishantou Formation, and Gaowu Formation vitreous tuff. Its water richness is controlled by the degree of fissure development.

The bedrock in the area is mainly weathered, with a dense structure. The joints and fissures are generally developed and mainly closed. The connectivity is poor, the water richness is poor, and the water is poor. It is mainly supplied by atmospheric precipitation and surface runoff. Along the joints and The fissures seep from high to low areas of the terrain, and are finally discharged in the form of recharge of Quaternary groundwater. According to regional data, groundwater is weakly corrosive to reinforced concrete.

Table 1 List of geotechnical engineering geological characteristics

2.2 Disaster scale and scope of impact

At around 10 a.m. on June 27, 2014, a short-duration disaster Affected by heavy rainfall (the rainfall in this area reached 107.0mm from 6 to 10 o'clock, and accounted for more than 70% between 8 and 10 o'clock), a gully occurred on the north and west side of Dashuhui Natural Village, Dashu Village, Huangshayao Town, Suichang County, Zhejiang Province Debris flow, debris flow materials accumulate along the gully exit to the Zhougongyuan river bed in the east of the village. The maximum width of the accumulation is about 150m, and the longest point is about 600m. The accumulation thickness is generally about 0.5-1.5m, and in some areas it is 2.0-2.5m. The square volume is about 35000m3 (Photo 3).

This mudslide caused the collapse of 79 villagers' houses in 17 households, and flooding of 123 villagers' houses in 35 households. About 100 acres of farmland, 750m of flood control embankments, 600m of mechanical plowing roads and 3 bridges were destroyed. The Dashu Power Station was severely damaged, with 186 people directly affected and a direct economic loss of approximately 12.2 million yuan. When this mudslide occurred, due to timely warning and successful avoidance, no casualties were caused. At present, a large amount of debris flow material has accumulated in the Zhougongyuan River area from the foot of Hongtouling to the east side of Dashuhui Natural Village. It is in a messy manner and affects the normal production, life and other activities of the villagers (Photo 4).

Photo 3 The accumulation of loose materials in the debris flow (mirror to the upstream)

Photo 4 The village and related facilities destroyed (mirror to the downstream)

2.3 Causes of the debris flow

(1) The longitudinal slope of the gully where the debris flow is located is steep, and the channel is narrow and curved. There were originally a lot of rocks accumulated in the gully, and the channel was blocked to a certain extent, laying the groundwork for the occurrence of the debris flow. hidden dangers.

(2) The gravel soil on the slope on the right bank of the channel is thick, loose in structure, and has poor engineering geological conditions, which provides a rich source of materials for debris flow disasters.

(3) Heavy rainfall has continued recently, especially before the disaster. The rainfall reached 107mm, and the gully water catchment area reached 2.3km2. It is difficult to drain the large amount of water in a timely and smooth manner, and the channel blockage caused the water level to plummet. As the water rises, a steady stream of water washes away the loose soil on the slope, causing the soil to slide down and mix with the original debris under the action of hydrodynamic forces to form a debris flow disaster.

3 Inspection and Monitoring of Geological Disasters

3.1 Daily Inspection and Monitoring

Water conservancy, land, meteorology and other departments provide timely warnings, and the town government, land office, and village committee arrange Monitors are on duty 24 hours a day to monitor, keep information flowing, and keep on-duty and dynamic monitoring records to ensure the safety of people's lives and property. Propaganda and education are carried out to the masses and disaster prevention awareness cards are issued, so that farmers have a strong awareness of self-prevention.

3.2 Disaster emergency warning mechanism

On June 27, 2014, Suichang County launched a heavy rainfall warning, and the water conservancy, land and meteorological departments and other departments issued timely warnings. Suichang County Huangsha The director and inspectors of Dashu Village in Yao Town discovered a dangerous situation during the investigation, and immediately organized personnel to actively respond and evacuated and moved 18 households with 75 people and valuable property as quickly as possible. After the disaster occurred, the county government immediately launched a geological disaster emergency plan, established an emergency rescue leading group, took active and powerful measures to carry out emergency rescue and rescue, organized the emergency evacuation and avoidance of threatened personnel, ensured the safety of people's lives, and arranged disaster relief funds. Assist disaster-stricken villagers and ensure their basic livelihood, while organizing professional and technical personnel to carry out emergency geological disaster relief surveys. Moreover, the disaster has attracted great attention from relevant leaders and experts such as the Provincial Department of Land and Resources and the Municipal Land and Resources Bureau, and they have visited the site many times to guide disaster prevention and reduction work.

4 Geological disaster avoidance and resettlement

After the disaster occurred, staff from the Suichang County Land and Resources Bureau, the Land and Resources Office, and the town government rushed to the scene in time and followed the geological disaster emergency plan. , based on the principles of people-oriented and avoidance; unified leadership, hierarchical responsibility; departmental cooperation, division of labor and collaboration, an on-site command center is established, cordons and warning signs are set up, and no other personnel are allowed to enter the dangerous area except rescue and disaster relief team members; use In the form of phone calls and text messages, production personnel near the disaster area are urgently notified to safely move to disaster evacuation and resettlement sites.

5 Experience and Enlightenment

Before the occurrence of this geological disaster, the county government and relevant departments such as land, land, water conservancy and meteorology attached great importance to organizing personnel to actively respond, and the public's awareness of disaster prevention was With strong and active cooperation, 18 households with 75 people and valuable property were evacuated and moved as quickly as possible, ensuring the safety of lives and property of residents and pedestrians to the greatest extent. This successful avoidance not only improved Suichang County's emergency response ability to sudden geological disasters, but also enhanced the people's safety awareness and ability to respond to sudden geological disasters. The main experiences are as follows:

(1) The government attaches great importance to the prevention and control of geological disasters, placing it as a "life project" in a prominent position in the overall work, and has repeatedly deployed and refined various disaster prevention measures. The land and resources system organized learning, communication and implementation to ensure that the threatened people could evacuate safely in an organized manner within a very short time after the warning was issued.

(2) Before the disaster occurs, monitoring personnel detect dangers in time and issue early warnings in a timely manner. This is the prerequisite for successful avoidance this time.

(3) The county land and resources department lists geological disaster prevention and control knowledge publicity training and emergency drills as one of its key tasks, highlighting the key points of training drills, focusing on innovation in work methods, and enhancing the effectiveness of disaster prevention work. The training laid the foundation for disaster recognition and identification, and the drill bought time for the people to avoid disasters in time.

(4) Further implement the responsibilities for geological disaster prevention, bring various disaster prevention measures down to the grassroots level, effectively strengthen dynamic inspections and investigations of geological disasters, strengthen monitoring personnel to perform their duties, and pay close attention to geological disaster monitoring Early warning and proactive prevention and avoidance should be carried out, and efforts should be made to improve disaster prevention capabilities and awareness at the grassroots operational level to ensure the safety of life and property of the people.