Chang Geun Song

Flow and dispersion analysis of shallow water problems with Froude number variation

A shallow water flow model and a pollutant transport model were developed, and this study highlighted the practical applications of the coupled flow and dispersion models in terms of general properties. The flow model used the Petrov–Galerkin method in which the test functions are deformed by the current direction to introduce balancing diffusion only in the flow direction. The developed flow model was coupled to the pollutant transport model with a spatially varied dispersion tensor to demonstrate soluble constituent mixing. Three applications were considered to test the model performance under various Fr numbers. In the first application, the flow passing through a flume with an abrupt sidewall expansion was considered, and the model prediction was verified by experimental values, focusing on recirculation and the pollutant cloud trap. In the second and the third applications, the trans-critical and super-critical flow regimes evolved in non-uniform channels with an expanding or converging sidewall were reproduced, and pollutant mixing by a spatially varied dispersion tensor was investigated.

Stability Analysis of Riverfront Facility on Inundated Floodplain Based on Flow Characteristic

Recent years have seen an increase in the intensity of extreme rainfalls and the frequency of floods caused by climate change. As a result, South Korea is facing an ever-increasing risk of flooding due to typhoons in the summer season. In South Korea, since the implementation of the ‘Four Major Rivers Restoration Project’, a variety of eco-friendly riverfront facilities, such as ecological parks as well as sports and recreation areas, have been constructed on floodplains. These riverfront facilities have a high risk of damage due to inundations during the rainy season. To perform hydraulic analysis and stability assessment of these facilities, accurate numerical modelling is necessary. First and foremost, it is crucial to address the dry/wet (DW) phenomena caused by rising and falling water levels. In this study, a two-dimensional (2D) finite element model was developed and its applicability was checked against the results of various DW analyses. The developed 2D model was then applied to a natural river undergoing DW phenomena. In the validation test, the applied 2D model demonstrated a high correlation coefficient (R = 0.954). Subsequently, using the results of the model, we calculated four types of flood risk indexes and analysed their correlations to evaluate the stability of the riverside facilities.

Risk Analysis Considering the Topography Characteristics of Debris Flow Occurrence Area

Recently, debris flow has occurred so frequently due to abnormal heavy rainfall. Although various studies have been conducted tosimulate the topographic behavior of debris flow, geometric effect such as the shapes of slope side has been insufficient. In this study,simulations were conducted with FLO-2D, the commercial software to be recommended by FEMA to simulate debris flow. And tar-get geometries are selected as flatbed, square shape, trapezoidal shape and V-shape. Firstly FLO-2D was verified by the comparisonwith USGS debris flow experiments (Iverson, 2010). The comparisons showed good agreements in term of the velocity range and thepropagation length of debris flows. Furthermore, the concept of HAZUS-HM risk analysis was used to assess the vulnerabilities of thestructures by the impact of the debris flow. Simulated results were confirmed debris flow behavior to be changed due to the geometrytype. Behavior of debris flows were analyzed according to the maximum depth, velocity, propagation length and vulnerabilities of thestructures by the debris flow. This study would be practical to provide quantitatively the spatial distribution of hazard caused by debrisflow.Key words : Debris flow, FLO-2D, HAZUS-HM, Topographic feature

Hydraulic Characteristics of Dam Break Flow by Flow Resistance Stresses and Initial Depths

The flood wave generated due to dam break is affected by initial depth upstream since it is related with hydraulic characteristics propagating downstream, and flow resistance stress has influence on the celerity, travel distance, and approaching depth of shock wave in implementing numerical simulation. In this study, a shallow water flow model employing SU/PG scheme was developed and verified by analytic solutions; propagation characteristics of dam break according to flow resistance and initial depth were analyzed. When bottom frictional stress was applied, the flow depth was relatively higher while the travel distance of shock wave was shorter. In the case of Coulomb stress, the flow velocity behind the location of dam break became lower compared with other cases, and showed values between no stress and turbulent stress at the reach of shock wave. The value of Froude number obtained by no frictional stress at the discontinuous boundary was the closest to 1.0 regardless of initial depth. The adaption of Coulomb stress gave more appropriate results compared with turbulent stress at low initial depth. However, as the initial depth became increased, the dominance of flow resistance terms was weakened and the opposite result was observed. keywords : dam break, initial depth, flow resistance stress, bottom frictional stress, Coulomb stress .............................................................................................................................................................................................. 요 지 댐붕괴에 의해 발생하는 홍수파는 초기수심의 깊이에 따라서 하류부로 전달되는 수리학적 특성이 다르게 나타나며, 수치모의시흐름저항응력은충격파의전파속도, 도달거리및접근수심등에영향을미친다. 본연구에서는천수방정식을 SU/PG 기법으로 이산화한 모형을 개발하고 해석해를 이용하여 모형을 검증한 후, 초기수심 및 흐름저항응력에 따른 댐붕괴류의전파특성을분석하였다. 바닥마찰력을적용한경우수심은상대적으로컸으나충격파의도달거리는짧게나타 났다. Coulomb 응력을적용한경우댐붕괴후면에서의유속이상대적으로작게나타났으나, 충격파가도달하는영역에서는 바닥마찰력을적용한값과흐름저항응력을고려하지않은값사이의유속을보였다. 또한초기수심에관계없이흐름저항응 력을 고려하지 않은 경우의 불연속면에서의  수가 1.0에가장 근사하였다. 초기수심이 얕은 경우Coulomb 응력에의한 모의결과가 난류응력을 적용한 경우에 비해 우수한 모의결과를 도출하였으나, 초기수심이 깊어지는 경우 흐름저항항의 영향력이 소멸되므로 반대의 양상이 나타났다. 핵심용어 : 댐붕괴, 초기수심, 흐름저항응력, 바닥마찰력, Coulomb 응력 .............................................................................................................................................................................................. * 인천대학교 안전공학과 조교수 (e-mail: baybreeze119@incheon.ac.kr) Assistant professor, Dept. of. Safety Engrg, Incheon National Univ., Incheon 406-772, Korea ** 교신저자, 홍익대학교 토목공학과 부교수 (e-mail: seungoh.lee@hongik.ac.kr, Tel: 82-2-320-3046) Corresponding Author, Associate professor, Dept. of Civil Engrg., Hongik Univ., Seoul 121-791, Korea J. Korea Water Resour. Assoc. Vol. 47, No. 11:1077-1086, November 2014 http://dx.doi.org/10.3741/JKWRA.2014.47.11.1077 pISSN 1226-6280 • eISSN 2287-6138

Influence of flow resistance stresses on debris flow runout

The hydrodynamic modeling of debris flows is challenged by the complicated mechanism of debris flow and the large variability of material composition. Therefore, in the dynamic continuum modeling of debris flows it is essential to select suitable rheological and associated friction parameters. The quadratic rheological model is the most comprehensive approach because it considers the effects of viscous behavior, solid-particle resistance, turbulent properties, and friction characteristics of debris flow. Therefore, this study adopted the quadratic model as the governing flow resistance stress and analyzed the influence of various flow resistance stresses to evaluate the runout performance. The shallow-water equations with modified bottom friction were discretized and the developed model was validated against a dam break flow and a ‘spreading of circular cone’ problem. Then, to investigate the relationship between the flow resistance relations and the bottom shear stresses exerted by the cones and dry bed, the model was applied to runout in a rectangular domain with three cone-shaped obstacles. An actual field runout event that occurred in Korea in 2011 was replicated and the runout paths and flow patterns were analyzed according to the various resistance stresses. By considering both the extremely rapid runout velocity and the striking run-up height, the results obtained by the quadratic stress relation achieved the best performance. However, for the successful application of the quadratic approach, three parameters had to be adjusted with consideration to locality.

Analysis of Characteristic of Debris Flow with Angle of Slope

In Korea, there exist many mountains, and sudden storms occur during the summer season. When severe rainstorm events occur in steep slope topography, risk of debris flow is increased. Once debris flow occurs in urban area, it may cause casualties and physical damages due to rapid debris flow velocity along a steep slope. Accordingly, preventing method of sediment-related disaster for demage mitigation are essential. Recently, various studies on debris flow have been conducted. However, the prediction of the physical propagation of debris flow along the steep slope was not thoroughly investigated. Debris flow is characterized by various factors such as topography, properties of debris flow, amount of debris flow. In the study the numerical simulation was focused on the topographic factor. Fundamental analysis of the risk area was implemented with emphasis on the propagation length, thickness, and the development of maximum velocity. The proposed results and the methodology of estimating the structural vulnerability would be helpful in predicting the behavior and the risk assessment of debris flow in urban area. These results will be able to estimate the vulnerability of urban areas affected the most damage by debris flow.

Analysis of Initial Mass Distribution and Facility Shape to Determine Structural Alternative for Hazardous Zone Vulnerable to Debris Flow Disaster

A 2-D hydrodynamic model for predicting the movement of debris flow was developed. The developed model was validated against a dam break flow problem conducted in EU CADAM project, and the performance of the model was shown to be satisfactory. In order to suggest structural alternative for hazardous zone vulnerable to debris flow disaster, two types of initial mass distribution and two shapes of defensive structure were considered. It was found that 1) the collapse of debris mass initiated with square pyramid shape induced more damage compared with that of cubic shape; and 2) a defensive structure with semi-circular shape was vulnerable to debris flow disaster in terms of debris control or primary defense compared with that of rectangular-shaped structure.