Influence of Tropical Cyclones of the South China Sea on the Variability of the Vietnamese Coastal Current Structure

The state of the natural environment of the marginal seas of the Northwest Pacific is largely controlled by the interaction of the atmospheric and hydrophysical processes. Tropical cyclones (typhoons), originating in the tropical zone of the Northwest Pacific and over the South China Sea basin, occupy a special place among atmospheric processes. The main destructive impact of typhoons falls on Southeast Asia. However, a significant number of are moving to the Russian Far East. The region of the South China Sea plays a significant role in the formation of tropical cyclones. This determines the importance of studying hydrometeorological processes not only in the Far East, but also in the South China Sea, and the need for cooperation between Vietnamese and Russian scientists. The main hydrodynamic structure of the western South China Sea is the Vietnamese Coastal Current (Western Boundary current), which depends not only on the seasonal monsoons but also on typhoons. The paper presents the results of joint Russian–Vietnamese studies of the dependence of the vertical structure of the Vietnamese Coastal Current on the Pacific tropical cyclones that form in the South China Sea. The study is done with numerical modeling. The period from April to June 1999 was used for modeling, provided with the necessary field data. The simulation results showed that, in general, the structure of water masses depends on the trajectories of tropical cyclones. In all cases considered, the Vietnamese Coastal Current is not a single flow, but represents a zone of eddy structures of different directions. An exception is the only situation in the condition of a tropical cyclone in the central region of the South China Sea when this current acquired the form of a single continuous flow directed from north to south only in the 200-m layer. The general patterns of changes in the dynamic structure of the Vietnamese Coastal Current for all the considered tropical cyclone trajectories include the following: areas with water transport in the northern direction prevail on the surface, while the rest of the water mass continues to flow generally in the southern direction. This transport of surface waters may be due to the influence of the emerging summer monsoon, and the rest of the water mass, which is less exposed to the still weak atmospheric processes of the monsoon type, continues to flow in the winter regime.

     

Geoheritage values in the Cat Ba islands, Vietnam

Geological wonders have been generally known as natural wonderful products. Resulted from geological processes, geological wonders are diverse in size that have geoheritage values that should be protected from damaging of substance, form and natural development. In a large scale, geological wonders can be geoheritage areas, containing several geodiversity elements that are geologically important or in a smaller scale, they can be geosites of heritage values (or geoheritage sites). In the delimitation of areas, having geoheritage values and the establishment of geoparks, the first thing is to recognise them as geosites and geoheritage areas that indicate great geological values. Besides the Ha Long bay, the world natural heritage with its outstanding aesthetic and geological values, the Cat Ba islands are typical and grandeur karst landscapes formed in tropical condition. Based on the geodiversity elements with their own geoheritage values on aesthetics, uniqueness and grandeur in the Cat Ba islands, the authors have recognised three geoheritage areas: the south cape of the Cat Ba embayment, Tung Gau (shelter), and the Lan Ha bay. Sites where Brachiopods, Crinoids and Tetracorals are exposed on the way through the island are considered as palaeontological geosites. The folds of limestone layers in the northern part of Cat Co 3 beach, with typical turbidite structures in carbonate formations are considered as a lithological geosite. The Devonian-Carboniferous boundary near the Cat Co 3 beach is regarded as a stratigraphical geosite while Que Kem and Turtle islands, etc. are considered as geomorphological geosites.     

Multi-hazard assessment using machine learning and remote sensing in the North Central region of Vietnam

Natural hazards constitute a diverse category and are unevenly distributed in time and space. This hinders predictive efforts, leading to significant impacts on human life and economies. Multi-hazard prediction is vital for any natural hazard risk management plan. The main objective of this study was the development of a multi-hazard susceptibility mapping framework, by combining two natural hazards—flooding and landslides—in the North Central region of Vietnam. This was accomplished using support vector machines, random forest, and AdaBoost. The input data consisted of 4591 flood points, 1315 landslide points, and 13 conditioning factors, split into training (70%), and testing (30%) datasets. The accuracy of the models' predictions was evaluated using the statistical indices root mean square error, area under curve (AUC), mean absolute error, and coefficient of determination. All proposed models were good at predicting multi-hazard susceptibility, with AUC values over 0.95. Among them, the AUC value for the support vector machine model was 0.98 and 0.99 for landslide and flood, respectively. For the random forest model, these values were 0.98 and 0.98, and for AdaBoost, they were 0.99 and 0.99. The multi-hazard maps were built by combining the landslide and flood susceptibility maps. The results showed that approximately 60% of the study area was affected by landslides, 30% by flood, and 8% by both hazards. These results illustrate how North Central is one of the regions of Vietnam that is most severely affected by natural hazards, particularly flooding, and landslides. The proposed models adapt to evaluate multi-hazard susceptibility at different scales, although expert intervention is also required, to optimize the algorithms. Multi-hazard maps can provide a valuable point of reference for decision makers in sustainable land-use planning and infrastructure development in regions faced with multiple hazards, and to prevent and reduce more effectively the frequency of floods and landslides and their damage to human life and property.     

Numerical study of the adiabatic index effect for the Vishniac instability in supernova remnants

The Vishniac instability is supposed to explain the fragmentation of the thin shell of shocked matter in the radiative phase of supernova remnants. However its implication and its consequence on the morphological evolution of stellar systems is not fully demonstrated. The present paper tackles this subject by numerical simulations and focus on the role of the adiabatic index in the instability growth. The HYDRO-MUSCL 2D hydrodynamics code has been used to simulate the evolution of a supernova remnant thin shell and the triggering of the Vishniac instability in this thin shell. We have studied the temporal behavior of the perturbation. The first result of the numerical study is the existence of the Vishniac instability in the simulations. This result is proved by the overstability process observed in the simulations as predicted by the theoretical analysis. The second important result is the damping of the perturbation at late evolution and for all the set of parameters. Indeed the accretion of matter onto the shock damps the instability when theoretical analysis predicts its occurrence.     

Understanding spatial variations of malaria in Vietnam using remotely sensed data integrated into GIS and machine learning classifiers

Abstract

This study examines the potentials of remotely sensed data, GIS and some machine learning classifiers and ensemble techniques in the investigation of the non-linear relationship between malaria occurrences and socio-physical conditions in the Dak Nong province of Viet Nam. Accuracy assessment was determined with Receiver Operating Characteristic (ROC) curve and pair t-test. The results showed that the area under ROC of Random Subspace ensemble model performed better than the other models based on statistical indicators. Comparing pair t-test with Area Under Curve values showed a slight difference of about 1%. Therefore ensemble techniques had significantly improved the performance of the base classifier. However, the performances might vary according to geographic locations. It is concluded that the machine learning classifiers combined with remotely sensed data and GIS is promising for malaria vulnerability mapping, and the derived maps can be used as a fundamental basis for programmes on spatial disease control.     

Application of an analytical hierarchical process approach for landslide susceptibility mapping in A Luoi district,Thua Thien Hue Province,Vietnam

A Luoi is a Vietnamese–Laotian border district situated in the western part of Thua Thien Hue province, central Vietnam, where landslides occur frequently and seriously affect local living conditions. This study focuses on the spatial analysis of landslide susceptibility in this 263-km² area. To analyze landslide manifestation in the study area, causative factor maps are derived of slope angle, weathering, land use, geomorphology, fault density, geology, drainage distance, elevation, and precipitation. The analytical hierarchical process approach is used to combine these maps for landslide susceptibility mapping. A landslide susceptibility zonation map with four landslide susceptibility classes, i.e. low, moderate, high, and very high susceptibility for landsliding, is derived based on the correspondence with an inventory of observed landslides. The final map indicates that about 37% of the area is very highly susceptible for landsliding and about 22% is highly susceptible, which means that more than half of the area should be considered prone to landsliding.     

Modern climate of the Cát Tiên National Park (Southern Vietnam): Climatological data for ecological studies

A method of using the standard network weather station data for local ecosystem research is considered on the example of the modern climate of the Cát Tiên National Park (Southern Vietnam) and local climate change in 1980–2010. Special attention is focused on the environmental parameters, which play a role of the limiting factors. It is shown that the climate of Southern Vietnam responds with statistical significance to global climate change. Suggestions about the possible reactions of tropical monsoon forest ecosystem to climate change are given.     

Correlating mass physical properties with ALOS reflectance spectra for intertidal sediments from the Ba Lat Estuary (northern Vietnam): an exploratory laboratory study

Characterization of the sediment composition of tidal flats and monitoring of their spatiotemporal changes has become an important part of the sustainable management of coastal environments. To accurately classify sediments through remote sensing, a comprehensive understanding of sediment reflectance spectra is indispensable. The present laboratory-based study explores the performance of the high spatial resolution (10?×?10 m) Advanced Land Observing Satellite (ALOS) launched in 2006. Relationships between reflectance spectra (bands 1 to 4) and four typical mass physical properties were investigated under wet and dry experimental conditions for intertidal sediments sampled near the Ba Lat Estuary in northern Vietnam. Reflectance in the near-infrared region corresponding to ALOS band 4 (0.76–0.89 μm) was found (1) to have a strong negative correlation with sand content (dry wt%) under both wet and dry conditions (linear correlation coefficient r?=?–0.7859 and –0.8094, respectively), (2) to increase with decreasing relative water content (%) in a given sediment type (r?=?–0.7748 to –0.9367 for mud, sandy mud, muddy sand, and sand), (3) to have a positive correlation with organic matter content (r?=?0.7610 and 0.6460 under wet and dry conditions for contents >0.20 dry wt%), and (4) to be insignificantly correlated with mineral composition assessed in terms of contents (wt%) of quartz, clay minerals, and mica group minerals. Positive relationships between reflectance and water content for the pooled data of all sediment types (r?=?0.6395) or organic matter content contrast with previous findings, and can be attributed to close interrelationships between these properties and the predominance of sand content as controlling factor of reflectance. This study clarifies that ALOS band 4 provides the most useful imagery for intertidal monitoring because its reflectance, as simulated using the laboratory data, shows the strongest correlation with sand content. In a next step, these experimental findings should be verified by identifying the reflectance relationships at satellite image scales, and also considering the effects of other tidal flat features on reflectance, such as microtopography and biological surface characteristics.     

Effects of perforated baffle on reducing sloshing in rectangular tank: Experimental and numerical study

A liquid sloshing experimental rig driven by a wave-maker is designed and built to study liquid sloshing problems in a rectangular liquid tank with perforated baffle. A series of experiments are conducted in this experimental rig to estimate the free surface fluctuation and pressure distribution by changing external excitation frequency of the shaking table. An in-house CFD code is also used in this study to simulate the liquid sloshing in three-dimensional （3D） rectangular tank with perforated baffle. Good agreements of free surface elevation and pressure between the numerical results and the experimental data are obtained and presented. Spectral analysis of the time history of free surface elevation is conducted by using the fast Fourier transformation.     

Inversion of Scholte wave dispersion and waveform modeling for shallow structure of the Ninetyeast Ridge

The construction of S-wave velocity models of marine sediments down to hundreds of meters below the seafloor is important in a number of disciplines. One of the most significant trends in marine geophysics is to use interface waves to estimate shallow shear velocities which play an important role in determining the shallow crustal structure. In marine settings, the waves trapped near the fluid–solid interface are called Scholte waves, and this is the subject of the study. In 1998, there were experiments on the Ninetyeast Ridge (Central Indian Ocean) to study the shallow seismic structure at the drilled site. The data were acquired by both ocean bottom seismometer and ocean bottom hydrophone. A new type of seafloor implosion sources has been used in this experiment, which successfully excited fast and high frequency (>500 Hz) body waves and slow, intermediate frequency (<20 Hz) Scholte waves. The fundamental and first higher mode Scholte waves have both been excited by the implosion source. Here, the Scholte waves are investigated with a full waveform modeling and a group velocity inversion approach. Shear wave velocities for the uppermost layers of the region are inferred and results from the different methods are compared. We find that the full waveform modeling is important to understand the intrinsic attenuation of the Scholte waves between 1 and 20 Hz. The modeling shows that the S-wave velocity varies from 195 to 350 m/s in the first 16 m of the uppermost layer. Depths levels of high S-wave impedance contrasts compare well to the layer depth derived from a P-wave analysis as well as from drilling data. As expected, the P- to S-wave velocity ratio is very high in the uppermost 16 m of the seafloor and the Poisson ratio is nearly 0.5. Depth levels of high S-wave impedance contrasts are comparable to the layer depth derived from drilling data.