• Journal of Wetlands Research
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• v.18 no.1
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• pp.76-83
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• 2016

In recent, the series of extreme storm events were occurred by those continuous typhoons and the severe flood damages due to the loss of life and the destruction of property were involved. In this study, we call Mega flood for the Extreme flood occurred by these successive storm events and so we can have a hypothetical Mega flood by assuming that a extreme event can be successively occurred with a certain time interval. Inter Event Time Definition (IETD) method was used to determine the time interval between continuous events in order to simulate Mega flood. Therefore, the continuous extreme rainfall events are determined with IETD then Mega flood is simulated by the consecutive events : (1) consecutive occurrence of two historical extreme events, (2) consecutive occurrence of two design events obtained by the frequency analysis based on the historical data. We have shown that Mega floods by continuous extreme rainfall events were increased by 6-17% when we compared to typical flood by a single event. We can expect that flood damage caused by Mega flood leads to much greater than damage driven by a single rainfall event. The second increase in the flood caused by heavy rain is not much compared to the first flood caused by heavy rain. But Continuous heavy rain brings the two times of flood damage. Therefore, flood damage caused by the virtual Mega flood of is judged to be very large. Here we used the hypothetical rainfall events which can occur Mega floods and this could be used for preparing for unexpected flood disaster by simulating Mega floods defined in this study.

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.235-235
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• 2015

This study investigates the impact of event characteristics on runoff dynamics during extreme flood events observed in a $8.5km^2$ experimental watershed located in South Korea. The 37 most extreme flood events with event rainfall in excess of 50 mm were analysed using an event-based rainfall-runoff model; the Revitalised Flood Hydrograph (ReFH) routinely used for design flood estimation in the United Kingdom. The ReFH model was fitted to each event in turn, and links were investigated between each of the two model parameters controlling runoff production and response time, respectively, and event characteristics such as rainfall depth, duration, intensity and also antecedent soil moisture. The results show that the structure of the ReFH model can effectively accommodate any nonlinearity in runoff production, but that the linear unit hydrograph fails to adequately represent a reduction in watershed response time observed for the more extreme events. By linking the unit hydrograph shape directly to rainfall depth, the consequence of the observed nonlinearity in response time is to increase design peak flow by between 50% for a 10 year return period, and up to 80% when considering the probable maximum flood (PMF).

• Proceedings of the Korea Water Resources Association Conference
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• 2021.06a
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• pp.141-141
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• 2021

This study proposed an equation for Rainfall Threshold for Flood Warning (RTFW) for urban areas based on computer simulations. First, a coupled 1D-2D dual-drainage model was developed for nine watersheds in Seoul, Korea. Next, the model simulation was repeated for a total of 540 combinations of the synthetic rainfall events and watershed imperviousness (9 watersheds × 4 NRCS Curve Number (CN) values × 15 rainfall events). Then, the results of the 101 simulations with the critical flooded depth (0.25m-0.35m) were used to develop the equation that relates the value of RTFW to the rainfall event temporal variability (represented as coefficient of variation) and the watershed Curve Number. The results suggest that 1) the rainfall with greater temporal variability causes critical floods with less amount of total rainfall; and that 2) the greater imperviousness requires less rainfall to have critical floods. For validation, the proposed equation was applied for the flood warning system with two storm events occurred in 2010 and 2011 over 239 watersheds in Seoul. The results of the application showed high performance of the warning system in issuing the flood warning, with the hit, false and missed alarm rates at 68%, 32% and 7.4% respectively for the 2010 event and 49%, 51% and 10.7% for the event in 2011.

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.173-173
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• 2015

The flood water level in tidal river is determined by the joint effects of flood discharge and tidal water levels at downstream boundary. Due to the variable tidal boundary conditions, the evaluated design water levels associated with a certain flood event can be significantly different. To avoid determining of design water levels just by a certain tidal boundary condition and remove the influence of variability in boundary condition from the evaluation of design water levels, a probabilistic approach is considered in this study. This study focuses on the development of a method to evaluate the realistic design water levels in tidal river with taking into account the combined effects of river discharge and tidal level. The flood water levels are described by the joint probability of two driving forces, river discharge and tidal water levels. The developed method is applied to determine design water levels for the tidal reach of the Han River. An unsteady flow model is used to simulate the flow in the reach. To determine design water levels associated with a certain flood event, first, possible boundary conditions are obtained by sampling starting times of tidal level time series; then for each tidal boundary condition, corresponding peak water levels along the channel are computed; and finally, design water levels are determined by computing the expectations of the peak water levels. Two types of tides which are composed by different constituents are assumed (one is composed by $M_2$, and the other one is composed by $M_2$ and $M_2$) at downstream boundary, and two flood events with different maximum flood discharges are considered in this study. It is found that (a) the computed design water levels with two assumed tides have no significant difference for a certain flood event, though variability of peak water levels due to the tidal effect is considerably different; (b) tidal effect can reach to the Jamsil submerged weir and the effect is obvious in the downstream reach of the Singok submerged weir; (c) in the tidally affected reach, the variability of peak water levels due to the tidal effect is greater if the maximum flood discharge is smaller.

• Proceedings of the Korea Water Resources Association Conference
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• 2022.05a
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• pp.147-147
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• 2022

In January 2021 heavy flood affected South Kalimantan with causing many casualties. The heavy rainfall is predicted to be generated due to the ENSO (El Nino-Southern Oscillation). The weak La-Nina mode appeared to generate more convective cloud above the warmed ocean and result in extreme rainfall with high anomaly compared to past historical rainfall event. Subsequently, the antecedent soil moisture distribution showed to have an important role in generating the flood response. Saturated flow and infiltration excess mainly contributed to the runoff generation due to the high moisture capacity. The hydro-meteorological processes in this event were deeply analyzed using the coupled atmospheric model of Weather Research and Forecasting (WRF) and the hydrological model extension (WRF-Hydro). The sensitivity analysis of the flood response to the SST anomaly and the soil moisture capacity also compared. Result showed that although SST and soil moisture are the main contributors, soil moisture have more significant contribution to the runoff generation despite of anomaly rainfall occurred. Model performance was validated using the Global Precipitation Measurement (GPM) and Soil Moisture Operational Products System (SMOPS) and performed reasonably well. The model was able to capture the hydro-meteorological process of atmosphere and hydrological feedbacks in the extreme weather event.

• Proceedings of the Korea Water Resources Association Conference
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• 2015.05a
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• pp.236-236
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• 2015

Flood is one of the major natural disasters affecting millions of people. Thailand also, frequently faces with this type of disaster. Especially, 2011 mega flood in Central Thailand, inundated highway severely attributed to the failure of national economic and risk to life. Lesson learned from such an extreme event caused flood monitoring and warning becomes one of the sound mitigations. The highway flood hazard mapping accomplished in this research is one of the strategies. This is due to highway flood is the potential risk to life and limb, and potential damage to property. Monitoring and warning therefore help reducing live and property losses. In this study, degree of highway flood hazard was assessed by weighting factors for each cause of the highway flood using Multi Criteria Analysis (MCA) based Analytic Hierarchy Process (AHP). These weighting factors are the essential information to classify the degree of highway flood hazard to enable pinpoint on flood monitoring and flood warning in hazard areas. The highway flood causes were then investigated. It was found that three major factors influence to the highway flood are namely the highway characteristics, the hydrological characteristics and the land topography characteristics. The weight of importance for each cause of the highway flood in the whole country was assessed by weighting 3 major factors influence to the highway flood. According to the result of MCA analysis, the highway, the hydrological and the land topography characteristics were respectively weighted as 35, 35 and 30 percent influence to the cause of highway flood. These weighting factors were further utilized to classify the degree of highway flood hazard. The Weight Linear Combination (WLC) method was used to compute the total score of all highways according to each factor. This score was later used to categorize highway flood as high, moderate and low degree of hazard levels. Highway flood hazard map accomplished in this research study is applicable to serve as the handy tool for highway flood warning. However, to complete the whole warning process, flood water level monitoring system for example the camera gauge should be installed in the hazard highway. This is expected to serve as a simple flood monitor as part of the warning system during such extreme or critical event.

• Journal of Korea Water Resources Association
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• v.41 no.2
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• pp.185-196
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• 2008

The flash flood has been studied in the climatological aspect which considers temporal and spatial characteristics of rainfall. However, we have not interested in runoff hydrograph for flash flood study. Therefore, our objectives of this study are to apply a work of Bhaskar et. al (2000) which studied runoff hydrograph to represent the flash flood to Korea and also to distinguish flash flood event from general flood event. That is, we quantified the severity of flash flood by estimation of flash flood index using runoff hydrograph. This study estimated the flash flood index for investigating the relative severity of flash flood in Han river basin with 101 flood events. Also we quantified the flash flood severity for flood event by heavy rainfall occurred in July of 2006. As a result, Kangwon-do province showed more severe flash flood than other areas in Han river basin and urban area such as Jungrang cheon stream also showed severe flash flood. We analyzed a flash flood of July of 2006 by dividing July into 1st to 3rd terms. From the analysis we knew that the 1st term of July showed the severe flash flood was occurred in Seoul area and the 2nd term showed it was occurred in Kangwon-do province.

• Animal cells and systems
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• v.15 no.2
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• pp.169-177
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• 2011

In August 2002, a heavy rainfall (445 mm in total for 5 consecutive days) resulted in a catastrophic flood, and it completely washed away the benthic fauna from the mainstream channel of the Gapyeong stream, a typical mid-sized stream in the central Korean peninsula. This study was to investigate the recovery patterns of aquatic insect communities that were damaged by the flood. Aquatic insects were sampled quantitatively using a Surber sampler ($50{\times}50$ cm, 1 riffle and 1 pool/run habitats per site) from three sites (4th-6th order) of the Gapyeong stream prior to 2000 and seasonally after the flood event from 2003 to 2006. Before the flood in the reference year (2000), a total of 77 species of aquatic insects were collected, whereas after the flood 47 species (2003), 51 species (2004), 64 species (2005) and 55 species (2006) were collected from the whole sampling sites. The aquatic insect density decreased to 26.85% (2003), 90.25% (2004), 52.53% (2005) and 54.95% (2006) of that recorded in the reference year. Although approximately 70% of the aquatic insect fauna has recovered since the flood event, the species composition in the most recent year differed substantially (similarity ca. 50%). On the other hand, the compositions of functional groups have not significantly changed. Aquatic insect communities at the riffle sites were affected more profoundly than those at the pool/run sites. The aquatic insect communities at the upstream site recovered more rapidly than those at the downstream sites.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.3B
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• pp.257-267
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• 2010

A flood event can be characterized by three attributes such as peak discharge, total flood volume, and flood duration, which are correlated each other. However, the amount of peak discharge is only used to evaluate the flood events for the hydrological plan and design. The univariate analysis has a limitation in describing the complex probability behavior of flood events. Thus, the univariate analysis cannot derive satisfying results in flood frequency analysis. This study proposed bivariate flood frequency analysis methods for evaluating flood events considering correlations among attributes of flood events. Parametric distributions such as Gumbel mixed model and bivariate gamma distribution, and a non-parametric model using a bivariate kernel function were introduced in this study. A time series of annual flood events were extracted from observations of inflow to the Soyang River Dam and the Daechung Dam, respectively. The joint probability distributions and return periods were derived from the relationship between the amount of peak discharge and the total volume of flood runoff. Applicabilities of bivariate flood frequency analysis were examined by comparing the return period acquired from the proposed bivariate analyses and the conventional univariate analysis.

• Journal of Korea Water Resources Association
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• v.54 no.11
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• pp.925-932
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• 2021

A drought-flood abrupt alternation event is an overlapping extreme event that is harder to cope with than a single event of drought and flood. It is also expected to have a significant adverse impact on ecosystems as well as industries and agriculture. However, there has not yet been a comprehensive study that characterizes the drought-flood abrupt alternation events in Korea. Therefore, this study employed a standard weighted average precipitation (SWAP) index, which is efficient to analyze not only individual events of drought and flood but also the drought-flood abrupt alternation events considering various time scales. The SWAP standardized the weighted average precipitation (WAP) by adding temporal weights to the precipitation. The SWAP indices were calculated for middle-sized watersheds of the Han River basin using the area average precipitation during 1966 and 2018. The severity K was calculated to represent the relative regional severity considering normal rainfalls, and used to characterize the drought-flood abrupt alternation in the study areas. The results indicated that 20 of the 30 middle-sized watersheds in the Han River basin were confirmed to increase the severity of drought-flood abrupt alternation over time. Considering the frequency and severity of drought-flood abrupt alternation events in each watershed, vulnerable areas and dangerous areas due to drought-flood abrupt alternation were identified, for example, the Upstream Namhan River (#1001).