Seung Oh Lee

Effect of Sediment Size Scaling on Physical Modeling of Bridge Pier Scour

Local pier scour experiments were performed in the laboratory to investigate the effect of relative sediment size on pier scour depth using three uniform sediment sizes and three bridge pier designs at different geometric model scales. When the data from a large number of experimental and field investigations are filtered according to a Froude number criterion, the effect of relative sediment size on dimensionless pier scour depth is brought into focus. The choice of sediment size in the laboratory model distorts the value of the ratio of pier width to sediment size in comparison with the prototype which in turn causes larger values of scour depth in the laboratory than in the field. This model distortion due to sediment size is shown to be related to the scaling of the large-scale unsteadiness of the horseshoe vortex by studying the relevant time scales of its coherent structure upstream of a bridge pier using acoustic Doppler velocimeter measurements. Observations of sediment movement, probability distributions of velocity components, and phase-averaging of velocity measured upstream of a bridge pier reveal properties of coherent motions that are discussed in terms of their contribution to the relationship between dimensionless pier scour depth and the ratio of pier width to sediment size over a large range of physical scales.

Estimation of instantaneous and daily net radiation from MODIS data under clear sky conditions: a case study in East Asia

The Moderate Resolution Imaging Spectroradiometer-based net radiation (RN) model was built and applied in East Asia in 2005. Because there have hardly been simple parameterization schemes developed over a large area using remote-sensing technology, the model was aimed to present physical simplicity in complex topography at multiple spatiotemporal scales. The model successfully reproduced the instantaneous RN values obtained at four flux tower sites having individually different ecohydrology. The diurnal cycle of RN was contextually simulated using a simple sine curve to determine the daily and monthly average net radiation. The diurnal RN estimation method was proven to be a reliable model as long as accurate boundary conditions, sunrise and sunset times, for example, were obtained. The monthly average net radiation (MANR) was estimated using the diurnal patterns of the instantaneous RN. Distribution of the monthly RN demonstrated that elevation and latitude were the primary factors affecting the MANR. The proposed RN algorithm turned out to be a promising method for valuable applications in various fields due to systematic simplicity and fewer input parameters.

Transport analysis in reverse electrodialysis with pulsatile flows for enhanced power generation

Time-dependent velocity profile and concentration distributions formed in a single reverse electrodialysis (RED) unit have been successfully pursued using simulation framework for evaluating performance of the unit, i.e., open circuit voltage and short circuit current. The single RED unit consists of two adjunct fluid channels, separated by the semi-permeable membrane. Through one of the channels, sea water flows, and the other is occupied by fresh water, flowing in the opposite direction (countercurrent operation). The diffusion-convection transport of the rate-limiting ion, Na+ in this study, for both channels is treated. The diffusive transport of cation across the membrane is expressed as boundary conditions for the bi-mechanism model. Our simulations conducted using an orthogonal collocation on finite element scheme show that the concentration difference of 35 g/L between sea water and fresh water results in the open circuit voltage of 63 mV and the short circuit current density of 11.5 A/m2. These values are close to ones that were obtained from the experiments.

Best-fit distribution and log-normality for tsunami heights along coastal lines

The best-fit distribution of the tsunami height was investigated along the Eastern Coast of Korean Peninsula. Firstly, the tsunami heights corresponding to the nine probable undersea earthquakes were obtained along the coastline using the numerical simulation. The method of L-moment ratio diagram was used to identify the best-fit probability density function of the tsunami heights caused by each undersea earthquake. The result indicates the generalized Pareto distribution is the best-fit distribution representing the tsunami heights regardless of the characteristics of the undersea earthquakes. This is particularly because the area of high tsunami heights and its relative magnitude to the adjacent locations were similar for the most simulations cases. In addition, this study further investigated the reason why the tsunami height distribution is not represented by the log-normal (LN) distribution as suggested by the previous studies. Result of the investigation indicates that the log-normality of the tsunami heights can be preserved when the length of a coastal line is not long such that the homogeneity of the length of the wave propagation paths reaching at different locations of the coastal line is preserved. This subsequently secures the central limit theorem making the distribution of the tsunami heights have the LN distribution. As the length of the coastal line increases, the deviation of the tsunami height distribution from the log-normality increases.

Numerical studies on the pressure-retarded osmosis (PRO) system with the spiral wound module for power generation

The pressure-retarded osmosis system with the spiral wound module for power generation has been studied numerically. The system includes draw channel, membrane, and feed-channel. The water flux and the solute flux across membrane were calculated. In addition, changes in concentration, flow rate, and pressure of channel-fluids were obtained. Water flux across membrane decreases about 10% along the direction of draw-fluid in our system and increases slightly along the direction of feed-fluid. The concentration of draw-fluid decreases along the direction of draw-fluid. Power density is almost proportional to the inlet concentration difference, and increases at first and then decreases as the difference between inlet pressures of feed- and draw-fluids increases.

Estimating Quantiles of Extreme Rainfall Using a Mixed Gumbel Distribution Model

Recently, due to various climate variabilities, extreme rainfall events have been occurring all over the world. Extreme rainfall events in Korea mainly result from the summer typhoon storms and the localized convective storms. In order to estimate appropriate quantiles for extreme rainfall, this study considered the probability behavior of daily rainfall from the typhoons and the convective storms which compose the annual maximum rainfalls (AMRs). The conventional rainfall frequency analysis estimates rainfall quantiles based on the assumption that the AMRs are extracted from an identified single population, whereas this study employed a mixed distribution function to incorporate the different statistical characteristics of two types of rainfalls into the hydrologic frequency analysis. Selecting 15 rainfall gauge stations where contain comparatively large number of measurements of daily rainfall, for various return periods, quantiles of daily rainfalls were estimated and analyzed in this study. The results indicate that the mixed Gumbel distribution locally results in significant gains and losses in quantiles. This would provide useful information in designing flood protection systems.

Stable Numerical Model for Transcritical Flow and Sediment Transport on Uneven Bathymetry

A stable and well-balanced finite volume method was proposed to solve the shallow water equations coupled with a sediment transport equation. An approximate Riemann solver with a monotone upstream-centered schemes for conservation laws (MUSCL) reconstruction was used for the computation of the flux terms. A special but relatively simple treatment of source terms made it possible to ensure numerically and physically balanced computational results on very steep sloped beds. Various numerical simulations conducted on rigid beds have demonstrated that the proposed numerical scheme could provide stable computational results in the cases of flat and nonflat bottoms. From the comparisons of experiments on erodible beds in one- and two-dimensional channels, good agreement was obtained when proper parameters for turbulent flow and sediment transport were provided. Lastly, the effects of the parameters in relation to hydraulic characteristics included in the flow and the sediment transport models were investigated ...

Hydraulic behavior and removal efficiency of settling tanks

The performance of a rectangular settling tank with two effluent system is described by hydraulic laboratory experiments. The velocity profiles of neutral current by dye tracer method and LDA describe the effect of the two effluent system. The proper location of a baffle may be determined by using flow-through curves from dye tests. The experiments using the clay turbid water as settling suspension show that the overflow rate, the installation of baffle and the location of effluent are crucial influential factors on removal efficiency.

Estimation of flood risk index considering the regional flood characteristics: a case of South Korea

Global warming is increasing the variability of climate change and intensifying hydrologic cycle components including precipitation, infiltration, evapotranspiration, and runoff. These changes increase the chance of more severe and frequent natural conditions, and limit ecosystem function and human activities. Adaptation to climate change requires assessment of the potential disaster risk. The objectives of this study were to estimate the flood risk index (FRI) considering regional flood characteristics at the national level and to prioritize the factors affecting flood risk through principal component analysis. FRI was estimated based on the Delphi survey results from 50 water resources experts in South Korea. The potential risk analysis was conducted for 229 local governments in South Korea. The results showed that natural and social factors were more influential flood risk factors to South Korea than administrative and economic and facility factors. Specifically, natural, social, administrative and economic, and facility factors were, respectively, highest at Jindo-Gun in Jennam-Do, Gumi-Si in Kyongsanbuk-Do, Dong-Gu in Incheon-Si, and Suwon-Si, Kyonggi-Do. Overall, the highest FRI is shown in Anyang-Si, Kyongggi-Do. The spatial distribution of the FRI was high in the southeastern coastal region and basins of the two biggest rivers in South Korea, and normalized flood frequency followed spatial patterns similar to FRIs. This study provided information on the relative flood risk index among administrative units for investment prioritization in flood risk management. In this regard, the suggested FRI is expected to significantly contribute to methodical and economic improvements in budget allocations for flood risk management.

Assessment of the Effects of Urbanization on the Watershed Streamflow

ABSTRACT 도시화는 불투수면적의 증가로 인해 수문학적 변화를 초래하며 또한 지하수량의 감소와 지표유출의 증가와 같은 현상이 발생한다. 따라서 본 연구의 목적은 일유량을 기본으로 한 토지이용 변화 현상에 대한 영향을 정량적이고 공간적으로 다른 토지이 용도를 이용하여 산정하는 것이다. 이에 따라 본 연구에서는 ARCSWAT의 SWAT2005 버전을 이용하여 갑천 유역을 일유량으로 모의 하였다 . 모형의 검보정 후, 1997년부터 2001년 일유량으로 모의된 결과를 통계학적으로 분석하였고 , 모의된 유량은 Mann-Whiteny method로 테스트 하였다. 도시 면적이 28 % 차이가 있는 경우 SWAT의 유량 모의 결과는 5 % 유의수준에서 도시화의 영향이 모의된 유량에 크게 영향을 주는 것으로 나타났다 . 그러나 도시화의 공간적 변화는 5 % 유의수준에서 유량의 변화에 미치는 영향이 거의 없는 것으로 나타났다. Keywords: ARCSWAT, Flood, Impervious area, Mann-Whitey method, Urbanization I. INTRODUCTION * As nature is affected by human activities, the hydrologic cycle cannot avoid the new circumstance. Climate change, groundwater use, and land cover change are typical factors affecting change of hydrologic response. Urban area, in particular, has such hydrologic characteristics as the decrease of infiltration rate and the reduced amount of evaporation. As impervious areas like buildings, roads, compacted soil, and underground structures block the infiltration of rainfall, more surface runoff or overland flow results. There are a variety of sources influencing hydrologic response in urban area, but it is not an easy matter to show the effect of urbanization quantitatively. The changes of population and