Landslide Hazard Assessment In Hagiang City And The Surroundings,hagiang Province,vietnam

The general objective of the thesis is to assess the current situation of landslide hazard based on the landslide inventory,the rainfall data that related to the landslide events and evaluating the role of landslide conditioning factors of the study area.The ultimate objective is to produce a landslide susceptibility map with a high precision in order to help local population and authorities in landslide mitigation and preparedness plans and may be used for land use planning purposes.Firstly,based on the landslide inventory map which was generated from the landslide database of the previous projects and the rainfall data,a rainfall threshold and a Bayesian probability model are presented for the landslide occurrence of shallow landslides in Ha Giang city and the surroundings,Vietnam.The model requires the data on daily rainfall combined with the actual dates of landslide occurrences.Careful study on the database is a prerequisite for the paper.For this reason,selecting the input data was carried out carefully to ensure the reliable results of the study.The daily rainfall data covering a time span of 57 years was collected from a unique rain gauge station of National Centre for Hydro-meteorological Forecasting of Vietnam(from 1957 to 2013)and a landslide database with some landslides(37 of total of 245 landslides)that containing dates of occurrence,was prepared from historical records for the period 1989 to 2013.Rainfall thresholds were generated for the study area based on the relationship between daily and antecedent rainfall of the landslide events.The results shows that3-day antecedent rainfall(with the rainfall threshold was established: RT = 40.8-0.201R3ad)gives the best fit for the existing landslides in the landslide database.The Bayesian probability model for one-dimensional case was established based on 26 landslides for the period 1989 to 2009,daily rainfall data with the same time and the values of probability varies from 0.03 to 0.44.Next,the Bayesian probability model for two-dimensional case was generated based on 11 landslides,rainfall intensity and duration in three months(May,June and July)of 2013 and the values of probability ranges from 0.08 to 0.67,and computed values of conditional landslide probability P(A|B)from two-dimensional case of Bayesian approach are clearly controlled by rainfall intensity > 40 mm with rainfall duration > 0.3 day.Secondly,in order to establish the landslide susceptibility map for the study area,the different zonation techniques were applied including the expert knowledge-based models such as analytical hierarchy process-AHP and spatial multi-criteria evaluation-SMCE,the statistical methods such as weights of evidence-Wo E and logistic regression-LR.The main purpose of the section is to compare the results of the integrated models,the weights of evidence(Wo E),analytical hierarchical process(AHP)and logistic regression(LR)models combine with a Flow-R model for landslide susceptibility assessment in Ha Giang city,Vietnam and the surrounding areas.First,three landslide susceptibility index(LSI)maps were calculated using thirteen landslide conditioning factor maps.Then,a runout map was generated by Flow-R model using the digital elevation model and the slope map of the study area.Secondly,the success rate curve,prediction rate curve and area under the curves(AUC)were calculated to assess prediction capability.The final landslide susceptibility maps were produced by the integration of the LSI maps with the runout map that was extracted from Flow R model.The validation results showed that the AUC of LSI maps in the integrated models,AHP-Flow R,Wo E-Flow R and LR-Flow R,were 82.89%,88.15% and 86.53%with prediction accuracies of 80.41%,84.94% and 85.71%,respectively.The combined models of Wo E-Flow R(88.15%)and LR-Flow R(86.53%)have shown the highest prediction capability.In general,the integrated models provide reasonable results that may be aided in prevention planning and land use planning purposes.After that,in order to estimate the element at risk in the study area,some important information that relate to the exposure analysis based on the relationship between the landslide susceptibility maps and elements at risk such as: the buildings,transportation lines,cultivated lands will also be presented in this chapter.Thirdly,to investigate the mechanism of triggering slope failures during the extreme rainfall events in the study area,some soil samples were taken and analyzed geotechnical properties.The main parameters that relate to engineering geology such as unit weight,cohesion,phi,and the hydraulic functions were assigned in coupled SEEP/W-SLOPE/W model to calculate the factor of safety(FS)under the influence of rainfall intensity and duration on cut slopes of the study area.Finally,based on the results of this research,the author has made conclusions and proposed the next research directions for landslide management in the study area.Recommendation of a methodology for landslide risk assessment,applying the structural measures to reinforce slope stability for safety of residential areas and transportation routes and constructing an early warning and monitoring system that suitable with local socio-economic conditions are very urgent tasks for sustainable development goals in the mountainous areas like this region.