• KSCE Journal of Civil and Environmental Engineering Research
• /
• v.36 no.5
• /
• pp.791-803
• /
• 2016

Recently, frequency of extreme rainfall events in South Korea has been substantially increased due to the enhanced climate variability. Korea is prone to flooding due to being surrounded by mountains, along with high rainfall intensity during a short period. In the past three decades, an increase in the frequency of heavy rainfall events has been observed due to enhanced climate variability and climate change. This study aimed to analyze extreme rainfalls informed by their frequency of occurrences using a long-term rainfall data. In this respect, we developed a Poisson-Generalized Pareto Distribution (Poisson-GPD) based rainfall frequency method which allows us to simultaneously explore changes in the amount and exceedance probability of the extreme rainfall events defined by different thresholds. Additionally, this study utilized a Bayesian approach to better estimate both parameters and their uncertainties. We also investigated the synoptic patterns associated with the extreme events considered in this study. The results showed that the Poisson-GPD based design rainfalls were rather larger than those of based on the Gumbel distribution. It seems that the Poisson-GPD model offers a more reasonable explanation in the context of flood safety issue, by explicitly considering the changes in the frequency. Also, this study confirmed that low and high pressure system in the East China Sea and the central North Pacific, respectively, plays crucial roles in the development of the extreme rainfall in South Korea.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2018.05a
• /
• pp.312-312
• /
• 2018

우리나라의 기후 지형적 특성에 따라 연강수량의 50% 이상이 여름철에 내리며 이러한 짧은 기간에 집중적으로 내리는 강수패턴 조건하에서 수공구조물 설계시 대부분 극치빈도분석을 활용한다. 우리나라의 경우 단일 Gumbel 분포를 활용한 극치빈도분석을 많이 이용한다. 하지만, 최근 이상기후로 인하여 전세계적으로 강수패턴의 특징이 급격히 변하고 있으며, 우리나라의 강수패턴 또한 바뀌어가고 있다. 연강수량의 대부분은 태풍과 장마로 인한 강수량으로 이루어져 있고, 일반적으로 두 개의 모집단으로 이루어진 형태를 보인다. 앞선 연구에서 두 개 이상의 첨두를 가지는 형태의 연최대강수량 자료에 대해 8개의 지속시간별(1, 2, 3, 6, 9, 12, 18, 24hr)로 Bayesian 기법의 단일 Gumbel 분포형과 혼합 Gumbel분포형 기반의 극치빈도분석 결과를 비교하였고, 혼합 Gumbel 분포형이 이중첨두 부분의 거동을 효과적으로 모의하는 것을 확인하였다. 본 연구에서는 이상기후로 인한 강수량의 특징의 급격한 변화에 일정한 패턴이 있음을 가정하고 이중첨두의 연 최대일강수량 자료에 대해 혼합 Gumbel 분포형 기반 비정상성 빈도분석을 실시하였다. 정상성 빈도분석과의 비교를 위해 확률분포의 매개변수 산정시 우도함수를 Bayesian 기법을 통해 산정하여 각 분포형의 Bayesian information criterion(BIC) 값을 비교하였다. 비정상성일 경우의 BIC 값이 정상성일 경우 보다 작게 산정되었고, 강수패턴이 경향성을 가지는 것으로 판단할 수 있었다. 비정상성 혼합 Gumbel 분포형 모델은 최근 급격한 강수패턴의 변화에 대한 대응책으로서 활용성이 높을 것으로 기대된다.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.28 no.1
• /
• pp.34-43
• /
• 2016

Urban development and population increases are continuously progressed in the coastal areas in Korea, thus it is expected that vulnerability towards coastal disasters by sea level rise (SLR) would be accelerated. This study investigated trend of the sea level data using Mann-Kendall (MK) test, and the results showed that the increasing trends of annual average sea level at 17 locations were statistically significant. For annual maximum extremes, seven locations exhibited statistically significant trends. In this study, non-stationary frequency analysis for the annual extreme data together with average sea level data as a covariate was performed. Non-stationary frequency analysis results showed that sea level at the coastal areas of Korean Peninsula would be increased from a minimum of 60.33 mm to a maximum of 214.90 mm by 2100.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2015.05a
• /
• pp.466-466
• /
• 2015

지역빈도해석은 수문학에서 오랜 역사를 갖고 있으며, 수년에 걸쳐 수문학적 변량의 정량적 추정을 위해 다양한 접근방법들이 제안되어 왔다. 그러나 제안된 방법들의 가설설정 수준이 높기 때문에 실제 적용에 제약이 많고, 적용 시에도 예측에 대한 불확실성이 높은 문제점이 있다. 본 연구에서는 이러한 문제점을 개선하기 위한 방법으로 계층적 베이지안 모델을 이용한 지역빈도해석 모형을 제안하고자 한다. 본 모형은 2개의 계층적 구조로 구성된다. 첫번째 계층은 재현기간별 GEV 분포의 매개변수를 정규화하여 주변분포로 설정하고, Kriging 기법을 이용하여 지형학적, 기상학적 정보들과 극치강수량 효과를 적합시켜 공간적 이질성과 미계측 유역에 대한 효과적인 보간을 가능하게 한다. 두번째 계층은 지점의 특성을 나타내는 매개변수들간의 공분산을 Bayesian 모델에 연계하여 매개변수들의 공간적 변동성을 나타낸다. 2개 계층의 결합확률분포는 MCMC 기법을 이용하여 예측값에 대한 불확실성을 정량적으로 분석하게 된다. 본 모형을 통해 홍수량 추정 시 필요한 시간 단위 극치강수량의 공간적 분포를 효과적으로 추정할 수 있을 것으로 판단된다.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2001.05a
• /
• pp.447-452
• /
• 2001

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.28 no.5
• /
• pp.292-302
• /
• 2016

In this study, shallow-water design waves are estimated for various return periods based on statistical analysis of extreme waves observed 13 years at four stations on the Korean east coast (Sokcho, Mukho, Hupo, Jinha). These values are compared with the results from SWAN simulation by using the deep water design waves conventionally used in Korea (KORDI, 2005). It was found that the simulated values of the shallow-water design waves are comparatively smaller than the values from the extreme value analysis, expecially below 30 years frequency, which implies possible under-estimation of the deep-water design waves on the Korean east coast.

• Journal of the Korean Society of Marine Environment & Safety
• /
• v.28 no.5
• /
• pp.700-711
• /
• 2022

In this study, deepwater design waves were estimated for 16 wave directions and various return periods based on statistical analysis of extreme waves observed for more than 20 years at three stations (Chilbal-do, Geomun-do, Donghae). These values were compared with design waves estimated based on the omni-directional wave data. The Weibull distribution was used as the probability distribution function whose parameters were determined by the least square method. The Kolmogorov-Smirnov test was applied for the goodness of fit test. Notably, the directional design waves were smaller than the omni-directional design wave for every wave direction. The maximum 50-year wave heights for directional sectors were 7.46 m (NNE), 12.05 m (S), and 9,59 m (SSW) at Chilbal-do, Geomun-do and Donghae whereas those for uni-directional wave data were 7.91 m, 13.82 m and 10.38 m, respectively. This implied possible under-estimation of the deepwater design waves for 16 wave directions being currently used in the design of offshore and coastal structures.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
• /
• v.16 no.4
• /
• pp.223-237
• /
• 2011

The intermittent freshwater discharge has an critical influence upon the biophysical environments and the ecosystems of the Yeongsan Estuary where the estuary dam altered the continuous mixing of saltwater and freshwater. Though freshwater discharge is controlled by human, the extreme events are mainly driven by the heavy rainfall in the river basin, and provide various impacts, depending on its magnitude and frequency. This research aims to evaluate the magnitude and frequency of extreme freshwater discharges, and to establish the magnitude-frequency relationships between basin-wide rainfall and freshwater inflow. Daily discharge and daily basin-averaged rainfall from Jan 1, 1997 to Aug 31, 2010 were used to determine the relations between discharge and rainfall. Consecutive daily discharges were grouped into independent events using well-defined event-separation algorithm. Partial duration series were extracted to obtain the proper probability distribution function for extreme discharges and corresponding rainfall events. Extreme discharge events over the threshold 133,656,000 $m^3$ count up to 46 for 13.7y years, following the Weibull distribution with k=1.4. The 3-day accumulated rain-falls which occurred one day before peak discharges (1day-before-3day -sum rainfall), are determined as a control variable for discharge, because their magnitude is best correlated with that of the extreme discharge events. The minimum value of the corresponding 1day-before-3day-sum rainfall, 50.98mm is initially set to a threshold for the selection of discharge-inducing rainfall cases. The number of 1day-before-3day-sum rainfall groups after selection, however, exceeds that of the extreme discharge events. The canonical discriminant analysis indicates that water level over target level (-1.35 m EL.) can be useful to divide the 1day-before-3day-sum rainfall groups into discharge-induced and non-discharge ones. It also shows that the newly-set threshold, 104mm, can just separate these two cases without errors. The magnitude-frequency relationships between rainfall and discharge are established with the newly-selected lday-before-3day-sum rainfalls: $D=1.111{\times}10^8+1.677{\times}10^6{\overline{r_{3day}}$, (${\overline{r_{3day}}{\geqq}104$, $R^2=0.459$), $T_d=1.326T^{0.683}_{r3}$, $T_d=0.117{\exp}[0.0155{\overline{r_{3day}}]$, where D is the quantity of discharge, ${\overline{r_{3day}}$ the 1day-before-3day-sum rainfall, $T_{r3}$ and $T_d$, are respectively return periods of 1day-before-3day-sum rainfall and freshwater discharge. These relations provide the framework to evaluate the effect of freshwater discharge on estuarine flow structure, water quality, responses of ecosystems from the perspective of magnitude and frequency.

• Proceedings of the Korea Water Resources Association Conference
• /
• 2012.05a
• /
• pp.981-981
• /
• 2012

최근 지구환경 변화에 따른 기후변화의 영향으로 자연재해의 형태는 점차 대형화, 다양화되고 있으며 극치사상의 발생 빈도가 계속해서 증가하고 있는 추세이다. 특히 도시하천의 경우 인구와 재산이 밀집해 있어 기후변화에 따른 홍수위험 및 취약성이 클 것으로 사료된다. 본 연구에서는 기후 변화에 따른 홍수위험 및 취약성 분석을 위하여 위험도 기반 불확실성을 다루는 수단으로 UQR-MCS (Upper Quartile Range-Monte Carlo Simulation)을 적용하였으며, 다양한 형태의 확률 분포로부터 특정변량(variable)의 확률분포 Quartile을 모의하였다. 또한 기후변화에 따른 도시하천의 홍수위험 및 취약성 평가를 위하여 도시하천에 적합한 홍수위험 및 취약성평가 지수(FVI: flood vulnerability index)를 산정하였으며, 홍수취약성지수는 기후변화(Climate change)와 도시화(Urbanization), 제방월류위험(Overtopping risk) 및 홍수범람 면적(Flood area) 등의 지표를 사용하였다. 각각의 지표는 엔트로피(Entropy) 기법을 적용하여 가중치를 부여하였으며, 표준화과정을 통한 일반화된 지표 값을 산정하였다. 우이천 유역의 기후변화에 따른 홍수위험 및 취약성 지표값은 KMA RCM A1B 시나리오자료를 바탕으로 추정한 미래 확률강수량과 각 인자별 재현기간에 따른 수문변량의 변화를 통하여 산정하였다. 본 연구의 결과는 향후 도시하천의 기후변화에 따른 홍수위험도분석 및 취약성 평가, 극치 수문사상에 대한 신뢰성 있는 분석과 더불어 예상치 못할 이상홍수에 대비한 하천방재 연구에 도움이 되리라 사료된다.

• Journal of Korean Society of Coastal and Ocean Engineers
• /
• v.19 no.4
• /
• pp.313-319
• /
• 2007

Long term wave climate of both extreme and operational wave height is essential for planning and designing coastal structures. Since the availability of the field wave data for the waters around Korean peninsula is limited to provide a reliable wave statistics, the wave climate information has been generated by means of long-term wave hindcasting using available meteorological data. In this paper, a set of deep water wave data obtained from KORDI(2003) were analyzed for extreme wave heights. These wave data at 67 stations off the Korean coast from 1979 to 1998 were arranged in the 16 directions. The probability distributions considered in this research were the FT-I and Weibull distribution. For each of these distributions, the method proposed by Goda(2004) was applied to estimate the parameters. For judgment of best fitting, MIR criterion proposed by Goda and Gobune(1990) was used. FT-I distribution which best fits to the 886 data, while Weibull(k=0.75) 81 data, Weibull(k=1.00) 105 data.