Typhoon Rainstorm-triggered Debris Flow Risk

Typhoon rainstorm-triggered debris flow refers to the debris flow that is induced directly by typhoon rainstorm.It is a special kind of debris flow,and is widely spread in the southeast coastal areas affecting by typhoon in China.This kind of debris flow differs significantly from other kinds of debris flows,as they have special characteristics of low frequency,clustering outbreaks,and so on,bringing about multi-outbreaks synchronously as well as causing casualties and loss of property.It is a main type of geological hazard in southeast mountain areas.Wenzhou is located in the southeast coastal area of Zhejiang,where the meteorological hazard,typhoon,occurs frequently.Induced by the typhoon rainstorm,the debris-flow hazards occurred in Whenzhou in the recent 30 years accumulated up to 258.Especially in the year one or more strong typhoons passes over,clustering debris-flow hazards usually happened.There have been 4 clustering debris-flow hazards,every of which covered more than 10 gullies in one day.Undergoing the severe disasters caused by typhoon rainstorm-triggered debris flows,and lacking of the adequate research,the existing debris flow risk evaluating methods can't meet the needs of the prevention and control of debris flows.Studies on the characteristic of this kind of debris flows as well as the triggering mechanism and risk assessment technique is urgently needed,to provide scientific basis for the prevention and control of such debris flow.Based on the introduction of the social and economic situation,the geological environment background and the characteristics of the typhoon rainstorm in the study area,this paper makes an in-depth analysis on the characteristics of typhoon rainstorm-triggered debris flows as well as their triggering mechanisms,and does research on the hazard,vulnerability and risk assessment of this type of debris flows in different scales(in region or a single gully),and brings up an idea of forcasting and early warning of this kind of debris flows.The main research work and achievements of this paper are as follows:(1)The special regularities of temporal-spatial distribution and the developmental characteristics of typhoon rainstorm-triggered debris flows.In terms of the spatial distribution regularities,the density of debris-flow hazards inthe middle and low hilly areas of the north-central Wenzhou is significantly higher than that in the southern and coastal hilly areas.In lithology,the lithological assemblages mainly containing tuff clastic rock(Hs)are most widely distributed in the region,covering the highest number of debris flows as well.Yet the granite assemblages(Qg)exist the largest distribution density of the debris flows in the region.In terms of the temporal distribution regularities,the occurring time of a hazard is extremely uneven.The debris flows occurred in the 5 years that significantly influenced by typhoon took up 65.1% of the total count in the resent 30 years.The monthly distribution shows that the hazards mainly concentrated in August and September.The debris flows outbreaking in these two months took up to 85.3% of the total count.There are 4 days that more than 10 debris-flow hazards occurred in one day.The rate could be up to 65.0% of the total count.Generally,the debris flows in the study area occurred at low or extramly low frequency.In spite of this,the study also found that under the excitations of the heavy rainfall with short duration,there is a high risk of clustering debris flows,and the starting time of their outbreaks are concentrated in the maximum period within 1H rainfall.(2)Bringing up with an initiating mechanism of the typhoon rainstorm-triggered debris flow.A two-dimension limit equilibrium method is used to analyze the destruction process from the point of view of slope stability.It is considered that the slope is in the stage of groundwater seepage under the influence of typhoon in the beginning.With the precipitation permeating into the slope continuously,the matrix suction decrease gradually resulting from the reduction of the effective stress.A super heavy rainfall within a short duration bringing forth significant surface runoff on the slope that enhances the sliding force of the slope,speeding up the destruction of the slope.Above-mentioned initiating mechanisms of a debris flow is a good answer to the question why the typhoon rainstorm-triggered debris flows outbreak synchronously,which the traditional debris flow triggering theory is difficult to explain.At the same time,it also explains that most of this type of debris flows start from a micro gully or a negative slope,while a normal slope is rarely to trigger a debris flow.(3)Risk zoning of the debris-flow regions basing on the information entropy model.First,based on the information entropy model,the hazard zoning model of the typhoon rainstorm-triggered debris flows is constructed,using the mean of the heaviest rainfall amounts and the number of times when the typhoon passing by as the influencefactors of the typhoon.Through the hazard assessment of the regional debris flows,the hazard degree of debris flows in Wenzhou hilly area could be divided into three grades,which is concentrated in between low and high hazard degree,without extramly low or extramly high,which is related to the randomness of the formation and development of typhoon and the distribution law of typhoon rainstorm-triggered debris flows.Secondly,according to the assessment model proposed by Xilin Liu and so on,the vulnerability zoning model of typhoon rainstorm-triggered debris flows is constructed,and the land use uniformity index and the land use breakage index are incorporated into the evaluation model for the first time.The level of social and economic development in Wenzhou hilly area is high,human engineering is active,and the vulnerability degree of debris flow is high as well.The vulnerability degree of debris flow in the study area is both medium or above,and the vulnerability index of different regions is distinct.Finally,using the method proposed by the International Union of Geological Sciences(IUGS),debris flow risk is calculated by the product of hazard and vulnerability.Combining the hazard and the vulnerability of debris flows in the region,the debris flow risk in Wenzhou mountain area is generally at medium-low levels.(4)Typhoon rainstorm-triggered debris flow risk assessment model of a single gully basing on multiple combination factors.First,the hazard assessment model of typhoon rainstorm-triggered debris flows basing on multiple combination factors is constructed and the weights of the combination factors are obtained by information entropy model.The model is applied to 14 typical typhoon rainstorm-triggered debris flows,and the hazard degree of each gully is obtained.The evaluation results conform to the fact that the debris flows in the region outbreak at low frequency,and the gully with high hazard degree has the typical geological and geomorphic conditions which are the most likely to form a debris flow in the region.Secondly,a comprehensive evaluation model combining property indexes and population indexes is used to evaluate single gully debris-flow vulnerability.The debris flow gully(WZ001)in Xianrentan Village of Longxi Township of Yueqing City was selected as the evaluation object.The calculation shows that the gully is a high vulnerability gully of debris flow.Combined with hazard and vulnerability degree,the debris flow risk of the gully is moderate.(5)Risk zoning of a single gully basing on numerical simulation.FLO-2D software is used to carry out debris flow risk zoning of a single gully basedon numerical simulation.The parameters of numerical simulation are obtained by sampling test and remote sensing interpretation.The velocity and depth of debris flow under different conditions are obtained by numerical simulation,and the hazard zoning of a single debris flow gully is carried out with the coupling indexes of depth and velocity of the flow by in the simulated conditions of once-in-50 years or once-in-a century.On the basis of remote sensing image,the risk area of debris flow is interpreted and classified.According to the vulnerability degree of various disarster bearing subjects such as their actual economic values and costs,their economic values are evaluated.The vulnerability zoning of a single debris flow gully is carried out in combination with the calculation and analysis of Geographic information system.Finally,the hazard zoning and vulnerability zoning of a single debris flow gully with different rainfall frequencies are coupled to get the risk zoning of a single gully.The result shows as the typhoon rainstorm increases,the risk increases correspondingly.(6)The procedure of early-warning and forecasting of typhoon rainstorm-triggered debris flows,from sky to the earth.In terms of temporal warning and forecasting,the early warning of debris flow should be issued when the short duration rainfall intensity(1h rainfall intensity)is greater than60mm/h on the basis of a certain amount of early rainfall.In terms of spatial early warning and forcasting,preliminary prediction and detailed prediction are carried out respectively according to the degree of investigation and research.In the preliminary prediction,the Melton Ratio(R value)is used as a hazard index for debris flow.In detailed prediction,debris flow hazard assessment model based on the combination factors is used in debris flow hazard assessment.Through the verification of 14 typical debris flow gullies,the two stage prediction results are basically the same,which is also consistent with the fact.Based on the above two aspects,the early warning and forecasting steps of typhoon-rainstorm triggered debris flow are put forward according to the real-time rainfall of typhoon.