Woo-Yul Jung

Comparison of Flood Discharge and Velocity Measurements in a Mountain Stream Using Electromagnetic Wave and Surface Image

Due to the difficulties for measuring flood discharge in the dangerous field conditions, conventional instruments with relatively low accuracy such as float still have been widely utilized for the field survey. It is also limited to use simple stage-discharge relationship for assessment of the flood discharge, since the stage-discharge relationship during the flood becomes complicated loop shape. In recent years, various non-intrusive velocity measurement techniques such as electromagnetic wave or surface images have been developed, which is quite adequate for the flood discharge measurements. However, these new non-intrusive techniques have little tested in the flood condition, though they promised efficiency and accuracy. Throughout the field observations, we evaluated the validity of these techniques by comparing discharge and velocity measurements acquired concurrently during the flood in a mountain stream. As a result, the flood discharge measurements between electromagnetic wave and surface image processing techniques showed high positive relationship, but velocities did not matched very well particularly for the high current speed more 3 m/s. Therefore, it should be noted here that special cares are required when the velocity measurements by those two different techniques are used, for instance, for the validation of the numerical models. In addition, authors assured that, for the more accurate flood discharge measurements, velocity observation as well as stage height is strongly necessary owing that the unsteady flow occurs during the flood.

Analysis and Comparison of Stream Discharge Measurements in Jeju Island Using Various Recent Monitoring Techniques

Different from the main land of South Korea, Jeju Island has been in difficulties for measuring discharge. Due to high infiltration rate, most of streams in Jeju Island are usually in the dried state except six streams with the steady base flow, and the unique geological characteristics such as steep slope and short traveling distance of runoff have forced rainfall runoff usually to occur during very short period of time like one or two days. While discharge observations in Jeju Island have been conducted only for 16 sites with fixed electromagnetic surface velocimetry, effective analysis and validation of observed discharge data and operation of the monitoring sites still have been limited due to very few professions to maintain such jobs. This research is sponsored by Ministry of Land, Transport and Maritime Affairs to build water cycle monitoring and management system of Jeju Island. Specifically, the research focuses on optimizing discharge measurement techniques adjusted for Jeju Island, expanding the monitoring sites, and validating the existing discharge data. First of all, we attempted to conduct discharge measurements in streams with steady base flow, by utilizing various recent discharge monitoring techniques, such as ADCP, LSPIV, Magnetic Velocimetry, and Electromagnetic Wave Surface Velocimetry. ADCP has been known to be the most accurate in terms of discharge measurement compared with other techniques, thus that the discharge measurement taken by ADCP could be used as a benchmark data for validation of others. However, there are still concerns of using ADCP in flood seasons; thereby LSPIV would be able to be applied for replacing ADCP in such flooded situation in the stream. In addition, sort of practical approaches such as Magnetic Velocimetry, and Electromagnetic Wave Surface Velocimetry would also be validated, which usually measure velocity in the designated parts of stream and assume the measured velocity to be representative for whole cross-section or profile at any specified location. The result of the comparison and analysis will be used for correcting existing discharge measurement by Electromagnetic Wave Surface Velocimetry and finding the most optimized discharge techniques in the future.

Simulation on Runoff of Rivers in Jeju Island Using SWAT Model

The discharge within the basin in Jeju Island was calculated by using SWAT model, which a Semi-distributed rainfall-runoff model to the important rivers. The basin of Chunmi river of the eastern region of Jeju Island, as the result of correcting as utilizing direct runoff data of 2 surveys, appeared the similar value to the existing basin average runoff rate as 22% of average direct runoff rate for the applied period. The basin of Oaedo river of the northern region showed of 0.93, RMSE of 14.92 and ME of 0.70 as the result of correcting as utilizing runoff data in the occurrence of 7 rainfalls. The basin of Ongpo river of the western region showed of 0.86, RMSE of 0.62 and ME of 0.56 as the result of correcting as utilizing runoff data except for the period of flood in . Yeonoae river of the southern region showed of 0.85, RMSE of 0.99 and ME of 0.83 as the result of correcting as utilizing runoff data of 2003. As the result of calculating runoff for the long term about 4 basins of Jeju Island from the above results, SWAT model wholly appears the excellent results about the long-term daily runoff simulation.

Flood Discharge to Decision of Parameters in Han Stream Watershed

Abstract The streams in Jeju Island have very distinctive hydrological and geological properties and there are a lot of limits in applying the general flood estimation method. This study presented parameters dominant in the Hancheon stream of Jeju Island by analyzing the sensitivity of parameters of HEC-HMS model regarding rainfall events in the target basin, and extracted the optimal parameter(Time of Concentration of Clark Unit Hydrograph: Kraven Ⅱ method, Storage Coefficient: Sabol method) by analyzing and comparing it with the flood runoff data observed in the site and Jeju Islands observation data. Key words : HEC-HMS, Jeju island, Time of concentration, Storage coefficient 1. 서 론 1) 해양성기후와 한라산에 의한 지형성 강우의 영향을 받는 제주도는 연 평균 2,061 mm의 강우량을 나타내는 국내 최다우 지역이지만 투수성이 좋은 다공질 화산암류 및 화산회토로 이루어져 있어 총 강우량의 48.5%에 이르는 강우량이 자하로 침투하여 평상시 대부분의 하천들은 건천을 이루고 있다(Jung과 Yang, 2009). 제주도 하천은 한라산 정상부를 중심으로 남 ․ 북사면으로 발달해 있어 경사가 매우 급하고, 각각의 하천이 독립적으로 해안으로 유출되어 유로 연장이 짧은 특성을 보인다(Yang, 2007). 제주도는 투수성이 높은 지질학적 특성과 급한 하상 경사, 짧은 유하거리 등의 수문학적 특성으로 인하여 특정 강우이상에서만 하천의 유출이 발생한 후 1

Rainfall-Runoff Characteristics in a Jeju Stream considering Antecedent Precipitation

Abstract The rainfall-runoff characteristics in Jeju Island significantly differ from those in inland, due to highly permeable geologic features driven by volcanic island. Streams are usually sustained in the dry conditions and thereby the rainfall-runoff characteristics changes in terms of initiating stream discharge and its types, depending highly on the antecedent precipitation. Among various the rainfall-runoff characteristics, lag time mainly used for flood warning system in river and direct runoff ratio for determining water budget to estimate groundwater recharge quantity are practically crucial. They are expected to vary accordingly with the given antecedent precipitation. This study assessed the lag time in the measured hydrograph and direct runoff ratio, which are especially in the upstream watershed having the outlet as 2 nd Dongsan bridge of Han stream, Jeju, based upon several typhoon events such as Khanun, Bolaven, Tembin, Sanba as well as a specific heavy rainfall event in August 23, 2012. As results, considering that the lag time changed a bit over the rainfall events, the averaged lag time without antecedent precipitation was around 1.5 hour, but it became increased with antecedent precipitation. Though the direct run-off ratio showed similar percentages (i.e., 23%)without antecedent precipitation, it was substantially increased up to around 45% when antecedent precipitation existed. In addition, the direct run-off ration without antecedent precipitation was also very high (43.8%), especially when there was extremely heavy rainfall event in the more than five hundreds return period such as typhoon Sanba.

Characteristics of Runoff on Urban Watershed in Jeju island, Korea

Jeju Island, the heaviest raining area in Korea, is a volcanic Island located at the southernmost of Korea, but most streams are of the dry due to its hydrological/geological characteristics different from those of inland areas. Therefore, there are limitations in applying the results from the mainland to the studies on stream run-off characteristics analysis and water resource analysis of Jeju Island. In this study, the SWAT(soil & water assessment tool) model is used for the Hwabuk stream watershed located east of the downtown to calculate the long-term stream run-off rate, and WMS(watershed modeling system) and HEC-HMS(hydrologic modeling system) models are used to figure out the stream run-off characteristics due to short-term heavy rainfall. As the result of SWAT modelling for the long-term rainfall-runoff model for Hwabuk stream watershed in 2008, 5.66% of the average precipitation of the entire basin was run off, with 3.47% in 2009, 8.12% in 2010, and root mean square error(RMSE) and determination coefficient(  ) was 496.9 and 0.87, respectively, with model efficient(ME) of 0.72. From the results of WMS and HEC-HMS models which are short-term rainfall-runoff models, unless there was a preceding rainfall, the runoff occurred only for rainfall of 40mm or greater, and the run-off duration averaged 10~14 hours.

Application of SWAT Model on Rivers in Jeju Island

The SWAT model developed by the USDA-Agricultural Research service for the prediction of rainfall run-off, sediment, and chemical yields in a basin was applied to Jeju Island watershed to estimate the amount of runoff. The research outcomes revealed that the estimated amount of runoff for the long term on 2 water-sheds showed fairly good performance by the long-term daily runoff simulation. The watershed of Chunmi river located the eastern region in Jeju Island, after calibrations of direct runoff data of 2 surveys, showed the similar values to the existing watershed average runoff rate as 22% of average direct runoff rate for the applied period. The watershed of Oaedo river located the northern region showed of 0.93, RMSE of 14.92 and ME of 0.70 as the result of calibrations by runoff data in the occurrence of 7 rainfalls.

Error Analysis for Electromagnetic Surface Velocity and Discharge Measurement in Rapid Mountain Stream Flow

Abstract Fixed Electromagnetic Wave Surface Velocimetry (Fixed EWSV) has been started to be used to measure flood discharge in the mountain stream, since it has various advantages such that it works well to continuously measure stream discharge even in the night time as well as very strong weather. On the contrary, the Fixed EWSV only measures single point surface velocity, thus it does not consider varying feature of the transverse velocity profile in the given stream cross-section. In addition, a conventional value of 0.85 was generally used as the ratio for converting the measured surface velocity into the depth-averaged velocity. These aspects could bring in error for accurately measuring the stream discharge. The capacity of the EWSV for capturing rapid flow velocity was also not properly validated. This study aims at conducting error analysis of using the EWSV by: 1) measuring transverse velocity at multiple points along the cross-section to assess an error driven by the single point measurement; 2) figuring out ratio between surface velocity and the depth-averaged velocity based on the concurrent ADCP measurements; 3) validating the capacity of the EWSV for capturing rapid flow velocity. As results, the velocity measured near the center by the fixed EWSV overestimated about 15% of the cross-sectional mean velocity. The converting ratio from the surface velocity to the depth-averaged velocity was 0.8 rather than 0.85 of a conventional ratio. Finally, the EWSV revealed unstable velocity output when the flow velocity was higher than 2 m/s.

Estimation of Design Rainfall Based on Climate Change Scenario in Jeju Island

【As occurrence of gradually increasing extreme temperature events in Jeju Island, a hybrid downscaling technique that simultaneously applies by dynamical method and statistical method has implemented on design rainfall in order to reduce flood damages from severe storms and typhoons.As a result of computation, Case 1 shows a strong tendency to excessively compute rainfall, which is continuously increasing. While Case 2 showed similar trend as Case 1, low design rainfall has computed by rainfall in A1B scenario. Based on the design rainfall computation method mainly used in Preventive Disaster System through Pre-disaster Effect Examination System and Basic Plan for River of Jeju Island which are considering climatic change for selecting 50-year and 100-year frequencies. Case 3 selecting for Jeju rain gage station and Case 1 for Seogwipo rain gage station. The results were different for each rain gage station because of difference in rainfall characteristics according to recent climatic change, and the risk of currently known design rainfall can be increased in near future.】

A Proposal of Unit Hydrograph Using Statistical Analysis in Oedo Stream, Jeju

Abstract Rainfall-runoff model of Jeju Oedo Stream was used to compute the optimal unit hydrograph by HEC-HMS model that reflecting on watershed characteristics. Each rainfall event was comparatively analyzed with the actual flow measurement using Clark, Snyder and SCS synthetic methods for derived unit hydrograph. Subsequently, the null hypothesis was established as p-value for peak flow and peak time of each unit hydrograph by one-way ANOVA(Analysis of variance) was larger than significance level of 0.05. There was no significant difference in peak flow and peak time between different methods of unit hydrograph. As a result of comparing error rate with actual flow measurement data, Clark synthetic unit graph best reflected in Oedo Stream as compared to other methods, and error rate of Clark unit hydrograph was 0.02~1.93% and error rate at peak time was 0∼2.74%. Key words : HEC-HMS, Clark, Snyder, SCS, One-way ANOVA, Unit hydrograph 1. 서 론 1) 전 세계적인 기후변화로 인해 기상이변 현상 중 하나인 돌발홍수가 빈번하게 발생하고 있으며, 우리나라도 돌발홍수로 인한 수해의 규모도 점차 증가하고 있다. 기존의 강우-유출 해석 방법은 대부분 외국에서 개발된 방법으로서 국내 지형특성을 반영하기 위해 부분적으로 수정되어 이용되고 있으나, 현장 수문관측 자료의 부족으로 인해 대상유역별 대표 단위유량도를 결정할 수가 없는 한계점이 있다(Jung, 2002). 이러한 문제를 해결하기 위해 수문관측 자료는 지속적인 유량관측 자료가 매우 중요하지만 제주도의 지역특성상 현장관측 자료가 매우 부족하여 분석하는데 어려움이 있다(Yang 등, 2012). 수자원분야에서 GIS를 이용한 경우는 미국공병단 연구소(Hydrologic Engineering Center; HEC)의 연구진에 의해 수자원분야에서 GIS 적용이 처음 시도된 이래, Jung 등(2013)은 제주도 도심하천유역에 SWAT 모형과 HEC-HMS 모형을 이용하여 유출특성을 분석하였으며, Yang 등(2013)은 SMS-RMA2의 2차원 해석을 시행하고 1차원 하천흐름의 결과를 비교 하였다. 또한 Kim 등(2013)은 표면영상유속계(SIV)를 적용하여 유속 및 유량을 산정하였으며, Kang 등(2013)은 SWAT모형을