Broken-Line Model of Dam Storage and Evaluation of Water Supply Safety Using a Bivariate Frequency Analysis

Yoo, Chulsang,Jun, Changhyun American Society of Civil Engineers 2017 Journal of water resources planning and management Vol.143 No.10

<P>In this study, a new evaluation method for the water supply safety of a dam is proposed. This method considers both climatic conditions and storage characteristics of a dam. The annual variation of dam storage is represented by a broken-line model, the lines of which are determined by only three-factors: the monthly design water supply during the dry period; dam storage at the end of the dry period; and changes in dam storage during the wet period. From the results of bivariate frequency analysis, a unique broken-line model is derived under multiple scenarios with the given return periods for the evaluation of water supply. The proposed method is applied to a total of five major dams in Korea. It is shown that the water supply safety of a dam can be quantified by using the concept of the return period, and five major dams have water supply safety corresponding to return periods of 10-50 years. Among them, the Soyanggang Dam is found to be the most stable with respect to water supply safety in Korea. (c) 2017 American Society of Civil Engineers.</P>

Theoretical evaluation of concentration time and storage coefficient with their application to major dam basins in Korea

Yoo, Chulsang,Lee, Jiho,Cho, Eunsaem IWA Publishing 2019 Water science & technology--water supply Vol.19 No.2

<P>This study theoretically evaluated the basin concentration time and storage coefficient with their empirical formulas available worldwide. The evaluation results were also validated in the application to major dam basins in Korea. The findings are summarized as follows. As a result of analytical analysis, the concentration time was found to be proportional to the main channel length under laminar flow conditions and to the square of it under turbulent flow conditions, but inversely proportional to the channel slope. It was also found that the storage coefficient and the concentration time are linearly but loosely related. Most empirical formulas for the concentration time concurred with the basic equation form, but just a few for the storage coefficient. Applications to major dam basins in Korea also showed that the concentration time agrees well with the result of theoretical analysis. However, the behavior of the storage coefficient varied much, basin by basin, indicating that additional factors may be needed to explain it.</P>

Evaluation of the gap filler radar as an implementation of the 1.5 km CAPPI data in Korea

Yoo, Chulsang,Yoon, Jungsoo,Kim, Jungho,Ro, Yonghun John Wiley Sons, Ltd 2016 Meteorological applications Vol.23 No.1

<P><B>ABSTRACT</B></P><P>This study evaluated the gap filler radar as an implementation of the 1.5 km CAPPI (constant altitude plan position indicator) data in Korea. The use of the 1.5 km CAPPI data was an inevitable choice, given the topography of the Korean Peninsula and the location of the radar. However, there still exists a significant portion of beam blockage, and thus there has been debate about the need to introduce the gap filler radar (or, the gap‐filler). This study evaluated the possible benefits of introducing gap‐fillers over the Korean Peninsula. As a first step, the radar data error was quantified by <I>G</I>/<I>R</I> ratio and root mean square error (RMSE), and the radar data over the Korean Peninsula were evaluated. The gap‐fillers were then located where the error was high, and their effect was then evaluated by the decrease in the <I>G</I>/<I>R</I> ratio and RMSE. The mean values of the <I>G</I>/<I>R</I> ratio and RMSE of the 1.5 km CAPPI data over the Korean Peninsula were estimated to be about 2.5 and 4.5 mm h<SUP>–1</SUP>, respectively. Even after the mean‐field bias correction, the RMSE of the 1.5 km CAPPI data was not decreased and mostly remained very high, around 4.4 mm h<SUP>–1</SUP>. Unfortunately, the effect of the gap‐filler on the 1.5 km CAPPI data was also found to be very small, just 1–2%. However, the gap‐filler could be beneficial, if the lowest elevation angle data were used instead of the 1.5 km CAPPI data. The effect of five gap‐fillers could be up to 7% decrease in RMSE.</P>

Markov Chain Decomposition of Monthly Rainfall into Daily Rainfall: Evaluation of Climate Change Impact

Yoo, Chulsang,Lee, Jinwook,Ro, Yonghun Hindawi Limited 2016 Advances in meteorology Vol.2016 No.-

<P>This study evaluates the effect of climate change on daily rainfall, especially on the mean number of wet days and the mean rainfall intensity. Assuming that the mechanism of daily rainfall occurrences follows the first-order Markov chain model, the possible changes in the transition probabilities are estimated by considering the climate change scenarios. Also, the change of the stationary probabilities of wet and dry day occurrences and finally the change in the number of wet days are derived for the comparison of current (1x CO2) and 2x CO2conditions. As a result of this study, the increase or decrease in the mean number of wet days was found to be not enough to explain all of the change in monthly rainfall amounts, so rainfall intensity should also be modified. The application to the Seoul weather station in Korea shows that about 30% of the total change in monthly rainfall amount can be explained by the change in the number of wet days and the remaining 70% by the change in the rainfall intensity. That is, as an effect of climate change, the increase in the rainfall intensity could be more significant than the increase in the wet days and, thus, the risk of flood will be much highly increased.</P>

Sampling Error Variation due to Rainfall Seasonality

Yoo Chulsang 한국수자원학회 2001 한국수자원학회 학술대회 Vol.2001 No.1

Vertical Variation of Z-R Relationship at Hallasan Mountain during Typhoon Nakri in 2014

Yoo, Chulsang,Ku, Jung Mo Hindawi Limited 2017 Advances in meteorology Vol.2017 No.-

<P>Hallasan Mountain is located at the center of Jeju Island, Korea. Even though the height of the mountain is just 1,950 m, the orographic effect is strong enough to cause heavy rainfall. In this study, a rainfall event, due to Typhoon Nakri in 2014, observed in Jeju Island was analyzed fully using the radar and rain gauge data. First, the Z-R relationship Z=A<SUP>Rb</SUP> was derived for every 250 m interval from the sea level to the mountain top. The resulting Z-R relationships showed that the exponent b could be assumed as constant but that the parameter A showed a significant decreasing trend up to an altitude around 1,000 m before it increased again. The orographic effect was found to be most significant at this altitude of 1,000 m. Second, the derived Z-R relationships were applied to the corresponding altitude radar reflectivity data to generate the rain rate field over Jeju Island. This rain rate field was then used to derive the areal-average rain rate data. These data were found to be very similar to the rain gauge estimates but were significantly different from those derived from the application of the Marshall-Palmer equation to the 1.5 km CAPPI data, which is the data type that is generally used by the Korea Meteorological Administration (KMA).</P>

Detection of mean-field bias of the radar rain rate using rain gauges available within a small portion of radar umbrella: a case study of the Donghae (East Sea) radar in Korea

Yoo, Chulsang,Yoon, Jungsoo,Ha, Eunho Springer-Verlag 2013 Stochastic environmental research and risk assessm Vol.27 No.2

Effect of Rainfall Temporal Distribution on the Conversion Factor to Convert the Fixed-Interval into True-Interval Rainfall

Yoo, Chulsang,Jun, Changhyun,Park, Changyeol American Society of Civil Engineers 2015 Journal of hydrologic engineering Vol.20 No.10

<P>In this study, the Weiss approach to derive the CF (the conversion factor to convert the fixed-interval annual maximum rainfall into the true-interval one) was examined and revised to consider the rainfall temporal distribution. As examples, several rainfall temporal distribution models currently being used in the rainfall-runoff analysis, along with several simple distributions such as triangular or pentagonal, were considered to derive the CF. The resulting CFs were then compared with the CFs estimated by analyzing the observed rainfall data, both in Korea and in several other countries, such as the United States, the United Kingdom, Australia, and New Zealand. The findings from this study can be summarized as follows. First, the effect of the temporal distribution of rainfall is very significant on the estimation of the CF. The CF for the impulse rainfall was the smallest at 1.0, and that for the uniformly-distributed rainfall was the highest at 1.333. Second, the CFs derived for the temporal distribution models considered in this study were higher than the empirical CFs used worldwide. Finally, it was found that, among simple distributions and temporal distribution models analyzed in this study, the quadratic functional form and the Keifer and Chu method provide the most similar CF value to the empirical CF values used in many countries mentioned above.</P>

Rainfall-runoff analysis based on competing linear impulse responses: decomposition of rainfall-runoff processes

Yoo, Chulsang,Park, Jooyoung John Wiley Sons, Ltd. 2008 Hydrological processes Vol.22 No.5

<P>Many recent studies have successfully used neural networks for non-linear rainfall-runoff modelling. Due to fundamental limitation of linear structures, approaches employing linear models have been generally considered inferior to the neural network approaches in this area. However, the authors believe that with an appropriate extension, the concept of linear impulse responses can be a viable tool since it enables one to understand underlying dynamics of rainfall-runoff processes. In this paper, the use of competing impulse responses for rainfall-runoff analysis is proposed. The proposed method is based on the switch over of competing linear impulse-responses, each of which satisfies the constraints of non-negativity and uni-modality. The computational analyses performed for the rainfall-runoff data in the Seolma-Chun experimental basin, Korea showed that the proposed method can yield promising results. Considering the basin characteristics as well as the results from this study, it may be concluded that three impulse responses are enough for rainfall-runoff analysis. Copyright © 2007 John Wiley & Sons, Ltd.</P>

Evaluation of Error Indices of Radar Rain Rate Targeting Rainfall-Runoff Analysis

Yoo, Chulsang,Ku, JungMo,Yoon, Jungsoo,Kim, Jungho American Society of Civil Engineers 2016 Journal of hydrologic engineering Vol.21 No.9

<P>This study evaluates several error indices of the radar rain rate from the viewpoint of the accuracy of rainfall-runoff analysis. The error indices considered in this study are the mean error (ME), mean absolute error (MAE), normalized standard deviation (NSD), correlation coefficient (CC), bias (BS), and radar rain rate quality criterion (RRQC). These indices were analyzed using the concept of sum of squares (SS) and mean squares (MS) in the analysis of variance (ANOVA), which explains how the bias and random error of the radar rain rate are combined in an error index. Also, these error indices were linked to the errors in the runoff result, such as the total runoff volume, peak flow, and peak time. As an application, two dam basins in Korea were selected, the Chungju Dam basin and the Namgang Dam basin. The radar rain rates from Gwangdeoksan Radar and Gudeoksan Radar were used as input for the rainfall-runoff analysis. Six different rainfall events were applied to secure various rainfall types. As a result, it was found that ME, BS, and RRQC were linearly proportional to the errors in the runoff result. This result indicates that the bias plays a dominant role in the evaluation of the radar rain rate, targeting the rainfall-runoff analysis.</P>