Joo-Heon Lee

Effect of Powder Size of Mg-Zn-Y Alloy on the Consolidation

MgZn4.3Y0.7 alloy powders were prepared using an industrial scale gas atomizer, followed by warm extrusion. The powders were almost spherical in shape. The microstructure of powders as atomized and bars as extruded was examined as a function of initial powder size distribution using Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscope (EDS) and X-ray Diffractometer (XRD). The grain sizes were decreased with extruding as well as decreasing the initial powder sizes. Both the ultimate strength and elongation were enhanced as the initial powder sizes were decreased.

Determining the Relationship between Average Concentration of Saturated Aquifer and Groundwater Outflow subject to Nonpoint Source Contaminants

In the stream-aquifer setting entered by nonpoint source contaminants, numerical simulation analyses for the coupled advection-dispersion and Richards equations were performed to evaluate and quantify the impacts of transport controlling factors on the relationship between average concentration of saturated aquifer and groundwater outflow concentration , and on the dynamic responses of and variables. As compared to the previous studies, the unique features of this study were to include unsaturated flow effect and to provide the physical basis and quantification for the — relationship. Numerical simulation results showed that — relations exhibited nonlinear, looping behaviour and were sensitive to the changes of dispersivity and aquifer thickness parameters. Furthermore, and responses were significantly affected by these two factors of dispersivity and aquifer thickness. For unsaturated-saturated flow condition, the significant impact of dispersivity values on the groundwater outflow concentration response might have an important implication for modeling the responses.

Assessment of Probabilistic Multi-Index Drought Using a Dynamic Naive Bayesian Classifier

The proper consideration of all plausible feature spaces of the hydrological cycle and inherent uncertainty in preceding developed drought indices is inevitable for comprehensive drought assessment. Therefore, this study employed the Dynamic Naive Bayesian Classifier (DNBC) for multi-index probabilistic drought assessment by integrating various drought indices (i.e., Standardized Precipitation Index (SPI), Streamflow Drought Index (SDI), and Normalized Vegetation Supply Water Index (NVSWI)) as indicators of different feature spaces (i.e., meteorological, hydrological, and agricultural) contributing to drought occurrence. The overall results showed that the proposed model was able to account for various physical forms of drought in probabilistic drought assessment, to accurately detect a drought event better than (or occasionally equal to) any single drought index, to provide useful information for assessing potential drought risk, and to precisely capture drought persistence in terms of drought state transition probability in drought monitoring. This easily produced an alternative method for comprehensive drought assessment with combined use of different drought indices.

Evaluation of multi-sensor satellite data for monitoring different drought impacts

Drought is a natural disaster that significantly affects human life; therefore, precise monitoring and prediction is necessary to minimize drought damage. Conventional drought monitoring is based predominantly on ground observation stations; however, satellite imagery can be used to overcome the disadvantages of existing monitoring methods and has the advantage of monitoring wide areas. In this research, we assess the applicability of drought monitoring based on satellite imagery, focusing on historic droughts in 2001 and 2014, which caused major agricultural and hydrological issues in South Korea. To assess the applicability and accuracy of the drought index, drought impact areas in the study years were investigated, and spatiotemporal comparative analyses between the calculated drought index and previously affected areas were conducted. For drought monitoring based on satellite imagery, we used hydro-meteorological factors such as precipitation, land surface temperature, vegetation, and evapotranspiration, and applied remote sensing data from various sensors. We verified the effectiveness of using precipitation data for meteorological drought monitoring, vegetation and land surface temperature data for agricultural drought monitoring, and evapotranspiration data for hydrological drought monitoring. Moreover, we confirmed that the Standard Precipitation Index (SPI) can be indirectly applied to agricultural or hydrological drought monitoring by determining the temporal correlation between SPI, calculated for various time scales, and satellite-based drought indices.

Hydrologic Risk Analysis Based on Extreme Drought Over the Korean Peninsula Under Climate Change

This study analyzed the regional characteristics of extreme drought events in each of the medium-scale basins in the Korean Peninsula by using the Standardized Precipitation Index(SPI), one of the typical drought indexes, and analyzed hydrologic risk by season and basin in consideration of the exceedance probability of all the observational data. According to the results of estimating SPI with the observational rainfall data (1976-2010) and analyzing severe droughts` time and space characteristics as well as tendencies, spring droughts are more serious in the Korean Peninsula. In addition, according to the results of analyzing average hydrologic risk by using 4 GCMs for five major rivers` basins in the Korean Peninsula, about short-term mid-term droughts, basin regions weak for droughts are expected to increase in the Korean Peninsula. It is expected that the method for analyzing basins` hydrologic risk in consideration of extreme droughts suggested here in this study will show high applicability in predicting droughts in the Korean Peninsula according to the climatic change and establishing practical coping strategies.

Hydrologic Safety Evaluation of Small Scale Reservoir by Simplified Assesment Method

Based on the statistical annual report, there are 17,649 reservoirs are operating for the purpose of agricultural water supply in Korea. 58 % of entire agricultural reservoirs had been constructed before 1948 which indicate the termination of required service life and rest of those reservoirs have also exposed to the dam break risk by extreme flood event caused by current ongoing climate change. To prevent damages from dam failure accident of these risky small size dams, it is necessary to evaluate and manage the structural and hydrological safety of the reservoirs. In this study, a simplified evaluation method for hydrologic safety of dam is suggested by using Rational and Creager formula. Hydrologic safety of small scale dams has evaluated by calculating flood discharge capacity of the spillway and compares the results with design frequency of each reservoir. Applicability and stability of suggested simplified method have examined and reviewd by comparing the results from rainfall-runoff modeling with dam break simulation using HEC-HMS. Application results of developed methodology for three sample reservoirs show that simplified assessment method tends to calculate greater inflow to the reservoirs then HEC-HMS model which lead lowered hydrologic safety of reservoirs. Based on the results of application, it is expected that the developed methodology can be adapted as useful tool for small scale reservoir`s hydrologic safety evaluation.