| Debris flow is a mixture of loose soil, water and gas, flowing along natural slope or pressure slope. It is a kind of gas-liquid-solid three phase fluid which contains a large amount of sediment block and boulders. Debris flow is a kind of special phenomenon of sudden natural disasters in mountains. It is widely distributed in our country with numerous types, frequent outbreaks and serious harm. The debris flow disaster has characteristics of suddenly happening and easy causing casualties, and accurate prediction of debris flow can reduce or even avoid loss of life, is one of the important means of disaster mitigation, therefore, debris flow forecasting has been one of the focus in the study of debris flow. In current researches of debris flow, the study of debris flow deposit as source formation restarting debris flow is lacking. Under the condition of heavy rainfall, restarting of channel debris flow deposit (which is called revival of channel debris flow deposit in this paper) can cause recurrence of debris flow, and the scale and the ability to cause disasters of debris flow is higher than ever before. Therefore, revival of channel debris flow deposit is a significant research topic.Luanchuan County is located in the west of Henan province, is adjacent to Chong County in the east, Lushi County in the west, Xixia County in the south and Luoning County in the north. It is one of the famous tourist attractions in our country. Due to mountainous territory, concentrated rainfall, coupled with environmental damages cause by production activities of human life, the debris flow disaster is development in Luchuan County. In addition, the agricultural economy is important economic pillar of Luanchuan County, and is sensitive to the debris flow disasters. In Luanchuan County, residents are always living in the valleys, thereby are vulnerable to debris flows and mountain torrents. On July24,2010, affected by the heveay rains, Luanchuan County suffered unprecedented floods which triggered several debris flow disasters in Luanchuan County.14townships encountered a total of29times debris flows which killed68people, made21people missing, and cause direct economic loss of about1.98billion. These have been a serious threat to the local residents’ life and property safety. In addition, investigations conducted by Geological Survey Institute of Henan Province, the Luchuan Department of Geology and Mineral Industries and other units show that there are many ancient or old debris flow deposit. These suggest that luanchuan county belongs to prone areas of debris flow.Due to a lack of knowledge of debris flow disasters in the past, it is failed to realize the possibility of the channel debris flow deposit as source formation to trigger debris flow on the condition of heavy rainfall. After debris flow occurring, effective measures are not taken to clean up debris flow deposit or build interception project. In the7.24debris flow, the channel debris flow deposit served as the main source to start debris flows in several gullies and caused huge disasters. Among these gullies, Shishu Gully is the most typical one. According to field survey, it is found that Shishu Gully exists a large amount of debris flow deposits, especially in the flat area in the middle and lower reaches of the gully. On July24,2010, debris flow occurred in the gully. The debris flow washed out all downstream thefarmland, silted up the channel nearly300m. At present, researches of debris flow are less involved in the effect of channel debris flow deposit on the debris flow and related prediction study. Therefore, it is necessary to analyze the revival mechanism of channel debris flow deposit and its contribution to the debris flow, so as to make an accurate forecast of debris flow, help the government make relief decisions, reduce loss of the residents’life and property, and provide the reference for debris flow disaster prevention system in other similar area.To study the problem of the revival of the channel debris flow deposit in Luanchuan County, the research methods of the data collection and sorting, experimental analysis and theoretical analysis are utilized in this paper. The research status of debris flow prediciton at home and abroad was summarized based on great literatures. The formation condition of debris flows in Shishu Gully and characteristics of7.24debris flow was analyzed based on field survey and related data, on this basis, the revival mechanism of debris flow deposit in Shishu Gully was analyzed in detail. The hydraulic-driven model of channel debris flow deposit was established by using the theory of hydraulics and rock and soil mechanics, on this basis, the prediction model of revival of channel debris flow deposit was build. In this paper, the main work and achievements are as follows:1. The geological environmental characteristics, formation conditions and the history of debris flow in Luanchuan County are investigated. Luanchuan County is located in the transition zone of low-middle mountain area and middle mountain area. The terrain condition is complex with strongly dissected topography. The geologic structure is developed well; the rock mass are mainly schist and granite which belong to weathered strata, and human activities such as mining, road construction are strong. These conditions caused rich sources for debris flow occurring in Luanchuan County. Luanchuan County has rich rainfall conditions, and the rainfall is always large and concentrated. The average annual rainfall is870.6mm. All these factors lead to a poor quality of geological environment in the study area in which debris flow always occur. In Luanchuan County, the frequency of a wide range of debris flow activities is low with an average of once ten year. Because of the difference of antecedent rainfall characteristics, rainfall intensity, total rainfall, landform, vegetation and sources, debris flow in the county does not features large areas of damage but local area of damage. In2010,7.24debris flows caused the most damage. Debris flows occurred many times in Luanchuan County. Field investigations show that the large amounts of debris flow deposit exist in several gullies. These channel debris flow deposit can serve as source formations to restart debris flow activities which has huge destructive power. So it should be strengthened to evaluate the possibility of revival of channel debris flow deposit.2. The developmental characteristics of debris flows in Shishu Gully were analyzed in three aspects of formation condition of debris flows, characteristics of7.24debris flow and its characteristic parameters. The rainfall is rich in Shishu Gully’s area, the basin area and the average longitudinal slope drop fall within the prone scope of debris flow, and source is rich. These formation conditions are very favorable for debris flow. The main source of7.24debris flow is channel debris flow deposit which mainly distributes in broad-gentle zone inside the gully and was formed by the past debris flows whose scale is less than7.24debris flow. Under the condition of sufficient antecedent rainfall, channel debris flow deposit quickly saturated, and its shear strength significantly lower; this provides a big advantage for debris flow under condition of the heavy rainfall. Extremely heavy rainfall and rich antecedent rainfall is the key to the occurrence of7.24debris flow. Its formation has a characteristic of raising channel deposit by water flow; this indicates7.24debris flow is the type of gully bed starting. The artificial reforming is relatively serious in Shishu Gully, which contains reclamation, planting and building, takeing up the channel space. These activities greatly weakened the discharge capacity of the gully during the flood period, which resulted in increased losses when7.24debris flow occured. Through the calculation of the characteristic parameters, we obtained that the fluid density of7.24debris flow is1.775g/cm3, the velocity is4.07-5.94m/s, the quantity of flow is about240209m3.3. Based on view of rainfall dividing, the revival process of channel debris flow deposit can be divided into two stages:saturation of channel debris flow deposit during rainfall infiltration and scouring of channel debris flow deposit by surface runoff. The revival mechanism of channel debris flow deposit in Shishu Gully is describe as follows:under the condition of sufficient antecedent rainfall, the channel debris flow deposit quickly saturated, which significantly lowered its shear strength; under the condition of continuous high intensity rainfall, surface runoff with great kinetic energy was formed and scoured the channel debris flow deposit to start debris flow. According to the theory of hydraulics, it is pointed out that the drag force of runoff to debris flow deposit deposit is derived from the friction resistance of the deposit deposit to the fluid flow. Analysis results show that the runoff can gradually started the channel debris flow deposit under the condition of7.24rainfalls. The stress pattern of the sediment particle movement is used for reference to describe the interaction between solid particles and debris flow, on this basis, stress pattern of big particles in debris flows and its starting flow velocity condition is established. Analysis results show that with an average flow velocity of1.74m/s, the debris flow can carry big stone in diameter of2m, which indicates that7.24debris flow in Shishu Gully hasa strong ability to carry large particles.4. The influence factors of the revival of debris flow deposit were concluded. These factors are the position of debris flow deposit, the physical and mechanical characteristics of debris flow deposit, strength weakened condition (antecedent rainfall) and hydrodynamic conditions (landform conditions and short duration rainfall). How to evaluate the possibility of revival of debris flow deposit was analyzed in three aspects:debris flow deposit evaluation, terrain conditions evaluation and rainfall condition evaluation, on this basis, the qualitative evaluation scale was put forward. The favorable condition of the revival of debris flow deposit was pointed out:debris flow deposit are widely distributed, the quantity is numerous, forming time is late and the intensity is low; debris flow deposit is located in broad-gentle zone inside the middle and lower reaches of gully, and gully bed width and slope is moderate with no good runoff channels; catchment area is small in the upstream of debris flow deposit with large gully bed gradient, or catchment area is large in the upstream of debris flow deposit with small gully bed gradient; antecedent rainfall is adequate, and the current rainfall intensity meets or exceeds the heaviest rain in50to100years. Debris flow deposit in Shishu Gully distribut widely, is located in the middle and lower reaches of the gully with no good runoff channels. The gully bed width and slope is moderate. Catchment area is moderate in the upstream of debris flow deposit with large gully bed gradient. On24July, antecedent rainfall is adequate, and the current rainfall intensity met or exceeded the heaviest rain in100years. Therefore, the channel debris flow deposit in Shishu Gully revived and leaded to debris flows.5. The present debris flow forecast models were summarized and divided into three categories:forecast model based on the statistical data, forecast model based on the starting mechanism and forecast model based on comprehensive evaluation. It was pointed out that the key of the forecast model based on the statistical data lies in the quantity and quality of the statistical data. Its applicability is limited to a ditch or an area. However, the Shishu Gully is lack of related monitoring data, so it is difficult to establish the forecast model based on statistical data. For the forecast model based on the starting mechanism, the key to its forecast effect is whether the mechanical model is in line with the actual situation and the mechanical parameters are representative and accuracy. These restrict the application effect of the model. In this paper, we have established the mechanical model of the revival of channel debris flow deposit. On this basis, we selected the forecast model based on the starting mechanism to forecast the revival of channel debris flow deposit. The forecast model based on comprehensive evaluation overall considers the influence of each factors on the occurring of debris flow and has quantifide these factors. Therefore, this model is generally useful. However, in this model, the non-linear relationship of factors is not considered. For the revival of channel debris flow deposit, the alternative of evaluation factors and quantitative has no statistical basis, so it is difficult to establish the prediction model based on the comprehensive evaluation.6. The forecast model of the revival of debris flow deposit based on the critical condition was established by using hydraulics theory and the mechanical model of the revival of debris flow deposit. From three aspects including the runoff formation, the revival of debris flow deposit and velocity attenuation of surface flow above debris flow deposit, the critical flow velocity, flow depth and flow quantity for revival and completely revival of debris flow deposit was proposed. In the case of statistical data and empirical formula, the critical rainfall intensity can determine according to the critical flow quantity condition. Analysis results show that on24July the debris flow velocity condition has met the critical condition of revival, but not completely revival, which was in line with the actual result. When runoff arrived at the debris flow deposit, it began to spread wide and its velocity lowered, so it was unable to revive the debris flow deposit on both sides of the channel. The runoff velocity on the main stream line is high, and the runoff incised the channel debris flow deposit to form run-off channel on the basis of the original narrow water channel, and carried the debris flow deposit to the outlet. After7.24debris flow, a good run-off channel was formated, so the remaining debris flow deposit has a lower possibility to revive.The innovations of this paper are shown as follows.1. The related concepts of revival of channel debris flow deposit were put forward. A systematic research on the the formation mechanism of debris flow deposit in Shishu Gully was conducted. The hydraulics starting model of channel debris flow deposit was established based on hydraulics and rock and soil mechanics theory. It is pointed out that the drag force of runoff to channel debris flow deposit is the key to revival. The drag force of runoff is determined based on the hydraulics theory. It is derived from the friction resistance of channel debris flow deposit to fluid flow, and related to fluid density, fluid velocity, flow deep and the roughness of channel debris flow deposit. The stress pattern of the sediment particle movement is used for reference to describe the interaction between solid particles and debris flow, on this basis, stress pattern of big particles in debris flows and its starting flow velocity condition is established.2. The present debris flow forecast models were summarized and divided into three categories:forecast model based on the statistical data, forecast model based on the starting mechanism and forecast model based on comprehensive evaluation. The advantages and disadvantages of three kinds of models were compared, and the forecast model based on the starting mechanism is chosen as the basic model to forecast the revival of channel debris flow deposit. Combined with the mechanical model of channel debris flow deposit resurrection and manning formula of velocity calculation, risen from the runoff, the formation of debris flow deposit and deposit debris flow and surface flow velocity attenuation analysis of debris flow deposit is established in three aspects:the resurrection and the resurrection of the critical flow velocity, flow depth completely and traffic conditions. The forecast model of the revival of debris flow deposit based on the critical condition was established by using manning formula and the mechanical model of the revival of debris flow deposit. From three aspects including the runoff formation, the revival of debris flow deposit and velocity attenuation of surface flow above debris flow deposit, the critical flow velocity, flow depth and flow quantity for revival and completely revival of debris flow deposit was proposed.In conclusion, this paper revealed the formation mechanism of the revival of channel debris flow deposit in Shishu Gully and established mechanism model of the revival of channel debris flow deposit and on this basis built forecast model of the revival of channel debris flow deposit. The work in this paper has some scientific reference for disaster prevention and mitigation. |
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