| Landslide dams are usually formed by river blockages with massive amounts of material from avalanches,landslides,or debris flows,and has the characteristics of sudden occurrence,poor stability,short longevity,large breach discharge.In the history,the sudden breach of landslide dams seriously threatened the safety of people and property downstream.The rapid assessment of landslide dams hazard is of great significance for emergency risks management.However,most of landslide dams occur in mountain valley,and there are large differences in dam shape and material,as well as the wide range of particle size distribution.Some existing models are difficult to be applied to rapid assessment or have low accuracy in prediction due to the difficulty in obtaining model parameters quickly or unconsideration of dam material properties.Therefore,it is of great significance to study the influencing factors and mechanism of the stability,longevity and breach parameters of landslide dams,and establish a risk assessment method based on the parameters that can be quickly obtained or estimated.In this paper,the morphological and hydrological parameters,and debris characteristics of landslide dams are studied.The efficient and rapid assessment models for dam stability,longevity,and breach flood are proposed.Firstly,a database of landslide dams is established,and rapid evaluation models of dam stability and longevity are presented.Secondly,the correlation between flow velocity and shear stress on the erosion rate of dam material is analyzed through flume test,and a evaluation formula for erosion parameters of dam material is established to improve the DABA physical model.Thirdly,the influence of dam material on the failure mechanism and breach parameters of landslide dams is studied based on the laboratory experiments,and the evaluation formula of the breach development process is investigated to establish a semi-empirical and semi-physical dam breach model.Forthly,the rapid assessment method of flood routing is proposed,and the attenuation and overlapping effects of flood are comprehensively analyzed based on the Yarlung Tsangpo River basin.Finally,the proposed rapid assessment model is applied to the Baige landslide dam for verification.The main achievements of this paper are as follows:(1)A rapid assessment model of dam stability is proposed based on the analysis of influencing factors.The database of 1737 landslide dams is established.The influences of trigger,dam morphology,dam material,and hydrological parameters on the stability of landslide dams are studied.The rainfall induced or earth material landslide dams usually have poorer stability.The increase of dam volume and decrease of annual average discharge is helpful to improve the dam stability.A rapid assessment model of dam stability is established based on eight dimensionless parameters.The absolute accuracy of the model is 83.30%.(2)A rapid assessment model of dam longevity is established based on the analysis of three stages and influencing factors.The longevity of rainfall induced or soil-rock mixed landslide dams is shorter than others.The most important factors affecting the duration of the stage of infilling,overtopping,and breach are annual average discharge coefficient,lake shape coefficient and trigger,respectively.The rapid assessment models of whole longevity,as well as three-stage(infilling,overtopping,and breaching stage)duration are proposed.The validation of the field landslide dams shows that the accuracy of the proposed models are better than other existing models.(3)A quick evaluation formula of erosion parameters is established and the DABA physical model is improved based on the analysis of erosion mechanism.With the increase of fines content,the erosion process of dam material changes from bedload movement to suspended movement.The erosion rate is approximately linear with flow velocity and shear stress.The erodibility coefficient decreases with the increase of median diameter and angle of reposing,but increases with the increase of fine particle content.The critical shear stress increases with the increase of median diameter.A quick evaluation formula for the erodibility coefficient and critical shear stress is established based on the dam material parameters that can be quickly obtained or predicted,and the DABA physical model is improved.(4)A quick evaluation formula for the development of dam breach and a semiempirical and semi-physical dam breach model is proposed based on the study of dam breach mechanism.The failure mode is regulated by the composition of dam material:dams remain stable with seepage for a coarse-dominated dam,fail by overtopping along with piping for a fines-dominated dam,and fail by overtopping alone for a balancedcomposition dam.The discharge hydrograph changes from a single peak to multiple peaks with increasing fines content,and the peak discharge is sensitive to the median diameter and fines content of dam material.The breach degree decreases with increasing gravel content.With the increase of fines content,the width-depth ratio of the breach changes from first decrease and then increase to first increase and then decrease.A quick evaluation formula for the development of the breach is established,and a semi-empirical and semi-physical dam breach model is proposed by combining the empirical formula with the HEC-RAS hydrodynamic model.(5)A rapid assessment method for flood routing that combines the DABA and HEC-RAS model is established to study the breach flood routing along river channel.The river basin model is established in advance,the DABA model is used to analyze the dam breach discharge,and the breach discharge is input into HEC-RAS model as boundary conditions to conduct flood routing analysis.The method is applied to the dam breach flood in the Yarlung Tsangbo River basin.The flood discharge along the river increases and inundates a larger area downstream when the breach flood from a mainstream dam overlaps with the breach flood from a tributary dam or when cascading dam breaches occur.Flood attenuation in downstream channel is mainly controlled by channel width,channel slope and the shape of discharge hydrograph.The influence of dam breach flood on the downstream can be controlled and reduced by engineering measures at the upstream,dam site and downstream of the dam.(6)The Baige landslide dam is studied based on the rapid hazard assessment method.Both the two Baige landslide dams are unstable.The longevity are 7.87 days and 1.95 days for the “10·10” Baige landslide dam,and 11.81 days and 4.03 days for the “11·03” Baige landslide dam based on the full-stage and three-stage longevity assessment models.The error between measured and predicted peak diacharge values of two dams is within 7.5%,and the error of arrival time is within 10.5% based on the improved DABA physical model,and semi-empirical and semi-physical model.The flood routing results of the downstream channel are close to the measured peak discharge and peak arrival time,which proves the validity of the model proposed in this paper.The research results of this paper are helpful to predict the dam stability,longevity,and breach flood,which can be effectively applied to the rapid assessment of landslide dam disaster,and provide a scientific basis for the disaster prevention. |
PDF Full Text Request