Risk Analysis Of Moraine-dammed Lakes On Southern Tibet Plateau And Mitigation Strategies For Major Routes

The southern Tibet Plateau is the connecting area between China and the South Asian Subcontinent,where China has designed a growing amount of main-line railways.Among them,the China-Pakistan railway travels along the Hongza-Indus river corridor;the China-Nepal railway crosses the Himalayas to Nepal along the tributary of Ganges River.In addition,the Sichuan-Tibet railway,as China's century project,travels through the mountains and valleys in Southeast Tibet,and then along the Yarlung Zangbo River to Lhasa,Tibet.For these main-line railways(i.e.,Sichuan-Tibet,China-Nepal,and China-Pakistan railways),the moraine lake outburst flood has become a major disaster that plays a controlling role for the railway schemes.However,there are no relevant provisions on moraine lake outburst in domestic and foreign railway technical specifications at present,and the railway department has no relevent experience to deal with the risk.Therefore,the study foucused on moraine-dammed lakes on the southern Tibet Plateau and aimed at:(a)characteristics and quantification of earthquake induced resonant water waves(ERWWs)in moraine-dammed lakes;(b)the critical conditions of overtopping failure for moraine dams under water wave actions;(c)the routing characteristics of glacial lake outburst floods(GLOFs)in complex river valleys;(d)risk assessment of moraine lakes along the major routes;(e)route selection strategy including corridor selections and spatial alignments for railway location in the moraine-dammed lake outburst risk area.The main conclusions are as follows:1.Based on the remote sensing interpretation,literature and field investigations,the paper statistically studied the distributions of glaciers,glacial lakes and 74 outburst events in the study area,and analyzed the characteristics of glacier related disasters in the basins where the main-line railways across.The study provides disaster reduction basis for the construction of main-line railways across the Himalayas.2.We compared the movements of earthquake-triggered water waves under resonant and non-resonant conditions based on shaking table experiments.The results show that,compared with non-resonant water waves,the resonant surges have not only greater amplitudes but also much slower attenuation with time,and a steady state with continuous large-amplitude wave motion exists in the time history of the resonant surge.With the experiment data we examined the main influencing factors of maximum height of earthquake-induced resonant surges and proposed a prediction equation for the maximum height through dimensional analysis.Finally,based on the critical water head condition for coarse particle initiation and critical water volume condition for sediment transportation in dam break,we established the critical conditions of overtopping failure for barrier dams under water surge actions and assessed the risk of 34 moraine-dammed lakes located in the Parlung Zangbo river basin.This study has obtained understandings of the characteristics of earthquake-induced surges under resonant conditions,and provides a scientific basis for seismic risk assessment of barrier lakes along the Sichuan-Tibet Railway.3.Aiming at the calculation of the GLOFs evolution from branching valleys into the main river along the mountainous complex river,we designed a large-scale flume experiments and analyzes the blocking effects of the cross section and channel contraction during the evolution of the outburst floods.Based on literature investigations,we selected eight representative empirical formulas to calculate the peak discharge of the outburst floods,and then verified these empirical formulas in the calculation of moraine-dammed lakes in the southern Tibet Plateau with real examples of the outburst events.Based on the experimental results,we modified the formula for calculating the peak discharge of floods in idealized river valleys.The calculation method provided by the study can quickly calculate the peak discharge and peak water level along the flood routing,so as to provide basis for formulating disaster prevention measures for railways and highways.4.This study aimed at risk assessments of railway schemes located in the moraine-dammed lake outburst risk areas.Based on the mechanism analysis of the74 outburst events on the southern Tibet Plateau,we established a multi-state B-Ns prediction model considering the complex relationships among the influence factors and high uncertainties of the outburst events.Based on the risk assessment of moraine-dammed lakes and geomorphological characteristics of the river valley,we proposed a calculation program of the railway scheme length in various risk areas and an evaluation system for railway schemes considering moraine-dammed lake outburst risk.Finally,taking the Zhangmu and Gyirong schemes of the China-Nepal railway as examples,we demonstrated the operation procedure of the proposed method.The study provides a universal technical evaluation system for risk assessments of railway schemes in moraine-dammed lake outburst risk areas.5.Firstly,we gathered the traditional design concepts of railway along rivers,and quantitatively analyzed the influence of line elevations on project investment in the broad valleys with advantaged terrace by the example of Dongchuan branch line.The example shows that,in wide valleys where low river terrace abundantly developed,when the elevation of the railway line along the river is raised by35?40 m,the ratio of bridges and tunnels in the line increases by 12.2% and the project investment increases by 38%.Secondly,by taking the Dasu section of China-Pakistan railway as the supporting project,we analyzed the influence of the elevation of the line along the river on the project investment in V-shaped river valley where disaster effects of GLOFs are more serious.The case study suggests that there are generally few terraces scattered in V-shaped river valley,which are hardly used by railway location.In this kind of valleys,there is little difference in the engineering quantity of bridges and tunnels between the high and low line schemes.When elevations of the line are raised,the project investment only increases by 2.5%,but the improvement of the plane alignment leads to the decreasing of the operating cost.As a result,the annual conversion operating cost of the high and low line scheme is basically the same.Therefore,in V-shaped river valleys with risk of GLOFs,it is operable to raise elevations of the railway along the river to deal with risk of moraine-dammed lake outburst floods.