Won-Cheol Cho

A comparative study of dual-porosity model and discrete fracture network model

The dual-porosity model and the discrete fracture network model are being used for the analysis of groundwater flow in a fractured aquifer. The dual-porosity model assumes that the porous medium consists of two continua at the macroscopic level, namely, fracture and matrix block. Saturated water flow in the matrix as well as in the fracture pore system is described by using two flow equations which are coupled by water transfer term. The discrete fracture network model assumes that only fracture forms paths for seeping water and that the conductivity in matrix block is neglected. The cubic law describing water flow through fractures, yields system of equations at each node. In this paper, it is attempted to investigate the applicability of the dual-porosity model and the discrete fracture network model and to establish a relationship between the dual-porosity approach and the discrete fracture network approach with a view to bridging the gap between two models.

Stochastic hydraulic safety factor for gas containment in underground storage caverns

Since the previous hydraulic safety factor for gas containment in gas storage caverns did not consider the spatial hydraulic conductivity variation, which directly affects the variation of hydraulic head and gradient, it is insufficient to fulfill the hydraulic safety from gas leakage. Therefore, based on the stochastic simulation considering the heterogeneity of hydraulic conductivity, a method for determining the hydraulic safety factor for gas containment in underground storage cavern is suggested. Instead of a single hydraulic gradient value obtained by using deterministic modeling, a possible range of a hydraulic gradient under a given probability was examined by means of stochastic simulation. The term ‘stochastic safety factor’ is newly defined as a head value at the water curtains, which is needed to make the critical hydraulic gradient of the cavern larger than a proposed critical gradient. By using this stochastic safety factor, the shortage of hydraulic gradient can be replenished and the risk of gas leakage due to heterogeneity of hydraulic conductivity can be reduced. q 2003 Elsevier B.V. All rights reserved.

On the stochastic simulation procedure of estimating critical hydraulic gradient for gas storage in unlined rock caverns

We investigated some aspects of spatial variability and their effect on critical hydraulic gradient which is essential for gas containment of underground storage caverns. Monte Carlo technique can be effectively applied to obtain an approximate solution to the two-dimensional steady flow of a stochastically defined non-uniform medium. For the stochastic simulation we generated hydraulic conductivity field on the selected grid resolution using HYDRO_GEN with estimated (based on actual data) In-K statistics with mean, variance, anisotropic integral scales. In this study, among various covariance functions, a Gaussian covariance function (GCF) was used. To find the critical value of the hydraulic gradient, probability density functions (PDFs) using 1000 outputs at an interested cell were developed. The results obtained in this study were compared with previous results for an exponential covariance function (ECF). It was found that in a stationary In K field the uncertainty of hydraulic head and gradient depend not only on the variance and integral scale of the In K field but also on the shape of its covariance function. From these results we can conclude that the critical range of hydraulic gradient is significantly affected by the type of covariance function. Thus, when critical hydraulic gradient is to be determined one should consider shape of covariance function as well as statistical parameters such as mean, variance and correlation scale.

Effects of Mean Wave Drift Force on Mooring Tension and Performance of a Moored Floating Breakwater

This paper is a numerical investigation of incident wave interactions with a moored, pontoon-type floating breakwater. We employ a boundary element method to solve the diffraction and radiation boundary value problems describing the situation of a moored floating breakwater in waves. The numerical model includes the hydrodynamic and mooring analyses, verified by the previous numerical and experimental results. Using the numerical model, we are able to assess the hydrodynamic performance of a moored single and dual pontoon floating breakwaters in regular waves. In particular, the effects of the mean wave drift force on the mooring system and wave attenuation characteristics are examined. The numerical results show that mooring tensions are significantly influenced by the mean wave drift force. However, the effect of the drift force on the performance of the floating breakwater in regular waves is not as significant.

Derivation of a Tank Model with a Conceptual Rainfall-Infiltration Process

This study derives an event-based tank model with a conceptual rainfall-infiltration process, modifying conventional tank models. The model comprises two serial tanks, one parallel tank and an infiltration regulating element. The infiltration process within the element is not represented as a function of only time, but as a function of soil moisture content for three possible cases owing to the relationship between rainfall intensity and infiltration capacity. This study considers the previous soil moisture condition of a watershed by using antecedent precipitation index. Six parameters of the model are identified by using the real coded genetic algorithm. The applicability and validity of the proposed model are assessed for the observed stormwater data from the research basin of the International Hydrological Program, the Pyeongchanggang River basin, Republic of Korea. The results computed streamflows show relatively good agreement with observed ones.

A Study on Flooding Characteristic Value for the Decision Method of an Urban Basin Design Magnitude

This paper is on the decision of design magnitude for flood control of urban basin, based on flooding characteristic values. In Korea, a design magnitude for flood control is established based on peak discharge of the outlet of basin. However, this method is inappropriate in an urban basin because sewerage only can flow out as much as it could and other discharge overflow to basin. In order to calculate a design magnitude for flood control of an urban basin, flooding characteristic values (peak discharge of pipe, average flooded depth, maximum flooded depths of an important point, flooded area, flooded volume, flooded time) were used as a tool. Using the Gwanghwamun Square as an example, a methodology was proposed that used XP-SWMM 2010 model as a platform to predict urban flood disaster. It can help other local government and residents to better understand, prepare for and manage a flood in urban environments.