Flood susceptibility modelling using advanced ensemble machine learning models

Floods are one of nature's most destructive disasters because of the immense damage to land, buildings, and human fatalities.It is difficult to forecast the areas that are vulnerable to flash flooding due to the dynamic and complex nature of the flash floods.Therefore, earlier identification of flash flood susceptible sites can be performed using advanced machine learning models for managing flood disasters.In this study, we applied and assessed two new hybrid ensemble models, namely Dagging and Random Subspace(RS) coupled with Artificial Neural Network(ANN), Random Forest(RF), and Support Vector Machine(SVM) which are the other three state-of-the-art machine learning models for modelling flood susceptibility maps at the Teesta River basin, the northern region of Bangladesh.The application of these models includes twelve flood influencing factors with 413 current and former flooding points, which were transferred in a GIS environment.The information gain ratio, the multicollinearity diagnostics tests were employed to determine the association between the occurrences and flood influential factors.For the validation and the comparison of these models, for the ability to predict the statistical appraisal measures such as Freidman, Wilcoxon signed-rank, and t-paired tests and Receiver Operating Characteristic Curve(ROC) were employed.The value of the Area Under the Curve(AUC) of ROC was above 0.80 for all models.For flood susceptibility modelling, the Dagging model performs superior, followed by RF,the ANN, the SVM, and the RS, then the several benchmark models.The approach and solution-oriented outcomes outlined in this paper will assist state and local authorities as well as policy makers in reducing flood-related threats and will also assist in the implementation of effective mitigation strategies to mitigate future damage.     

A two-phase numerical model for suspended-sediment transport in estuaries

This paper presents a full 2-D X/Z numerical model for sediment transport in open channels and estuaries using a two-phase (fluid–solid particle) approach. The physical concept and the mathematical background of the model are given and test-cases have been carried out to validate the proposed model. In order to illustrate its feasibility for a real estuary, the model has been applied to simulate the suspended-sediment transport and the formation of turbidity maximum in the Seine estuary. The numerical results show that the main characteristics of estuarine hydro-sediment dynamics in the Seine estuary are in fact reproduced by the proposed model. A qualitative agreement between the numerical results and the actual observations has been obtained and is presented in this paper.     

Calculation of seismic bearing capacity of shallow strip foundations using the cell-based smoothed finite element method

This paper adopts an upper bound procedure using the cell-based smoothed finite element method (CS-FEM) to estimate the seismic bearing capacity of shallow strip footings, focussing on seismic soil-structure interactions. In simulations, soil behaviour is assumed as the Mohr–Coulomb material, and increment of plasticity deformation obeys the associated flow rule. The first step of the numerical procedure involves approximating the kinematically admissible displacement fields using the cell-based smoothed finite element method, while the second relates to the establishments of the optimization problem as the conic programming. The inclusion of seismic conditions in the simulations was made using the pseudo-static approach. Initially, three seismic bearing capacity factors were resolved for both smooth and rough foundations by including horizontal and vertical inertia forces caused by the soil weight, the superstructure and the surcharge in the analyses. All seismic bearing capacity components obtained are in excellent agreement with those obtained using the method of characteristics and other finite element analyses. Subsequently, the reduction coefficients that correlate static and seismic bearing capacity factors were computed to facilitate the seismic design of the foundation.

     

Assessing the contribution of electrical resistivity tomography (ERT) and self-potential (SP) methods for a water well drilling program in fractured/karstified limestones

ERT and SP investigations were conducted in carbonate rocks of the Dinant Synclinorium (Walloon Region of Belgium) to find suitable locations for new water wells in zones with little hydrogeological data. Since boreholes information needed to be representative of the area, large fractured zones were searched for the drillings. Large ERT profiles (320 to 640 m) allowed us to image the resistivity distribution of the first 60 m of the subsurface and to detect and characterize (in terms of direction, width and depth) fractured zones expected to be less resistive. Data errors, depth of investigation (DOI) indexes and sensitivity models were analyzed in order to avoid a misinterpretation of the resulting images. Self-potential measurements were performed along electrical profiles to complement our electrical results. Some negative anomalies possibly related to preferential flow pathways were detected. A drilling campaign was conducted according to geophysical results. ‘Ground truth’ geological data as well as pumping tests information gave us a way to assess the contribution of geophysics to a drilling program. We noticed that all the wells placed in low resistivity zones associated with SP anomalies provide very high yields and inversely, wells drilled in resistive zones or outside SP anomalies are limited in terms of capacity. An apparent coupling coefficient between SP signals and differences in hydraulic heads was also estimated in order to image the water table.     

东南亚断裂构造的演化

本研究项目是采用诸如地质学解释、影像判读和地球物理探测等多种方法相结合进行的。在所得结果的基础上参考了越南国内外同行的一些资料编制了东南亚、越南及邻区的断裂构造图．其比例尺分别为1：4百万和1：1百万。分析所得结果显示出东南亚断裂构造演化的下列情况：1)在现今地质结构方面东南亚是欧亚岩石圈板块的东南部分．由一条消减带围绕．这条消减带的伸展从Myanmer开始,通过Nicobar,Java Timor直到东菲律宾。东南亚被Song Hong(即红河)断裂,Three Pagodas断裂和Hainam-Natuna断裂等2级断裂系统分成3个微板块。2)在早新生代．东南亚是分为5个微板块的。它们的分界断裂中有2个一级断裂(中央东海扩张带和Lupar-Kuching消减带)和3个二级断裂(即上述3个)。3)上述绝大多数二和三级断裂从晚新生代起活化且继承了从早新生代即已发生和发展了的二、三级断裂,但在某些条件下．运动方向却完全变成了相反,尤其是走滑运动的方向。我们的研究结果表明：在这一地区内,盆地、隆起、岩浆侵入、褶皱和局部断裂等构造的形成都取决于这些沿着一、二级走滑断裂的微板块运动。     

Correcting the distortion of apparent resistivity pseudosection produced by 3D effect of the embankment geometry

Two-dimensional electrical resistivity tomography data acquired on the crest of the embankment are distorted by the 3D effect of the embankment geometry, reservoir water level and abutment. The distortion affects seriously the final solution of the 2D inverse problem. By comparing the apparent resistivity pseudosections from a 3D and 2D electrical resistivity model of the embankment, the distortion degrees of the apparent resistivity pseudosections along the axis on the crest were estimated for the cases of reservoir and which does not contain water. The obtained results indicate that the distortion degree acquired in the case of a reservoir that contains water was much less than that in the case of the reservoir that does not contain water. In the case of reservoir that contains water, the apparent resistivity pseudosections of the P–P and ED–ED arrays had the largest distortion degree and of D–D, W–S and P–D arrays had the smallest distortion degree. In the case of the reservoir that does not contain water, the apparent resistivity pseudosection of P–P array had the smallest distortion among all arrays. Through modeling investigation, a correction process to reduce the distortion of the apparent resistivity pseudosection was proposed. The correction process was tested in the embankment model, and two field works were carried out in the To Lich River in Hanoi and Khuan Cat embankment in Lang Son province, Vietnam. It is possible to bring the distortion of the apparent resistivity pseudosection down to 2.8–13.9%, depending on the type of electrode arrays and the type of reservoirs, containing or does not contain water. The distortion correction of the apparent resistivity pseudosection is strongly recommended before doing the 2D inverse interpretation.     

Assessing the copula selection for bivariate frequency analysis based on the tail dependence test

The flood characteristics, namely, peak, duration and volume provide important information for the design of hydraulic structures, water resources planning, reservoir management and flood hazard mapping. Flood is a complex phenomenon defined by strongly correlated characteristics such as peak, duration and volume. Therefore, it is necessary to study the simultaneous, multivariate, probabilistic behaviour of flood characteristics. Traditional multivariate parametric distributions have widely been applied for hydrological applications. However, this approach has some drawbacks such as the dependence structure between the variables, which depends on the marginal distributions or the flood variables that have the same type of marginal distributions. Copulas are applied to overcome the restriction of traditional bivariate frequency analysis by choosing the marginals from different families of the probability distribution for flood variables. The most important step in the modelling process using copula is the selection of copula function which is the best fit for the data sample. The choice of copula may significantly impact the bivariate quantiles. Indeed, this study indicates that there is a huge difference in the joint return period estimation using the families of extreme value copulas and no upper tail copulas (Frank, Clayton and Gaussian) if there exists asymptotic dependence in the flood characteristics. This study suggests that the copula function should be selected based on the dependence structure of the variables. From the results, it is observed that the result from tail dependence test is very useful in selecting the appropriate copula for modelling the joint dependence structure of flood variables. The extreme value copulas with upper tail dependence have proved that they are appropriate models for the dependence structure of the flood characteristics and Frank, Clayton and Gaussian copulas are the appropriate copula models in case of variables which are diagnosed as asymptotic independence.     

Spatio-temporal changes of underground coal fires during 2008–2016 in Khanh Hoa coal field (North-east of Viet Nam) using Landsat time-series data

Underground coal fires are one of the most common and serious geohazards in most coal producing countries in the world. Monitoring their spatio-temporal changes plays an important role in controlling and preventing the effects of coal fires, and their environmental impact. In this study, the spatio-temporal changes of underground coal fires in Khanh Hoa coal field(North-East of Viet Nam) were analyzed using Landsat time-series data during the 2008-2016 period. Based on land surface temperatures retrieved from Landsat thermal data, underground coal fires related to thermal anomalies were identified using the MEDIAN+1.5×IQR(IQR: Interquartile range) threshold technique. The locations of underground coal fires were validated using a coal fire map produced by the field survey data and cross-validated using the daytime ASTER thermal infrared imagery. Based on the fires extracted from seven Landsat thermal imageries, the spatiotemporal changes of underground coal fire areas were analyzed. The results showed that the thermalanomalous zones have been correlated with known coal fires. Cross-validation of coal fires using ASTER TIR data showed a high consistency of 79.3%. The largest coal fire area of 184.6 hectares was detected in 2010, followed by 2014(181.1 hectares) and 2016(178.5 hectares). The smaller coal fire areas were extracted with areas of 133.6 and 152.5 hectares in 2011 and 2009 respectively. Underground coal fires were mainly detected in the northern and southern part, and tend to spread to north-west of the coal field.