Ching-Min Chang

Sensitivity and uncertainty analysis of a sediment transport model: a global approach

Computerized sediment transport models are frequently employed to quantitatively simulate the movement of sediment materials in rivers. In spite of the deterministic nature of the models, the outputs are subject to uncertainty due to the inherent variability of many input parameters in time and in space, along with the lack of complete understanding of the involved processes. The commonly used first-order method for sensitivity and uncertainty analyses is to approximate a model by linear expansion at a selected point. Conclusions from the first-order method could be of limited use if the model responses drastically vary at different points in parameter space. To obtain the global sensitivity and uncertainty features of a sediment transport model over a larger input parameter space, the Latin hypercubic sampling technique along with regression procedures were employed. For the purpose of illustrating the methodologies, the computer model HEC2-SR was selected in this study. Through an example application, the results about the parameters sensitivity and uncertainty of water surface, bed elevation and sediment discharge were discussed.

Large-time behavior of macrodispersion in heterogeneous trending aquifers

[1] This paper presents a stochastic analysis of the large-time behavior of macrodispersion in a three-dimensional heterogeneous aquifer with a linear trend in the mean log hydraulic conductivity. To solve the problem analytically, focus is placed on the particular case where the linear trend is aligned in the direction of mean hydraulic head gradient. A spectral approach based on Fourier-Stieltjes representations for the perturbed quantities is used to develop closed-form expressions that describe variability of flow velocity, the second-order mean flow, and asymptotic macrodispersion. The impact of the mean log hydraulic conductivity gradient on these results is examined. It is found that the asymptotic longitudinal and transverse macrodispersion coefficients decrease with the increasing trend gradient of mean log hydraulic conductivity in the case of finite Peclet numbers. This feature is a consequence of the reduction in variability of flow velocity with the trend gradient.

Stochastic modeling of variations in stream flow discharge induced by random spatiotemporal fluctuations in lateral inflow rate

Due to natural heterogeneity in runoff processes, the analysis of response of stream channels to the variation of lateral inflow is therefore viewed in terms of stochastic spatiotemporal processes. Based on the representation theorem, a closed-form expression is derived to describe the spectral response characteristic of stream subject to spatiotemporal fluctuations in lateral inflow. It provides a basis for evaluating the induced discharge variability in stream channels. It is found that the evolutionary power spectrum of the stream flow discharge process and therefore the variance is increased with the distance from the upstream boundary and the characteristic length scale of the lateral inflow process. Flow discharge prediction in the downstream region has a high degree of uncertainty by solving the deterministic partial differential equation.

Induced groundwater flux by increases in the aquifer's total stress.

Fluid-filled granular soils experience changes in total stress because of earth and oceanic tides, earthquakes, erosion, sedimentation, and changes in atmospheric pressure. The pore volume may deform in response to the changes in stress and this may lead to changes in pore fluid pressure. The transient fluid flow can therefore be induced by the gradient in excess pressure in a fluid-saturated porous medium. This work demonstrates the use of stochastic methodology in prediction of induced one-dimensional field-scale groundwater flow through a heterogeneous aquifer. A closed-form of mean groundwater flux is developed to quantify the induced field-scale mean behavior of groundwater flow and analyze the impacts of the spatial correlation length scale of log hydraulic conductivity and the pore compressibility. The findings provided here could be useful for the rational planning and management of groundwater resources in aquifers that contain lenses with large vertical aquifer matrix compressibility values.

Quantification of Lagrangian travel time statistics under nonstationary random groundwater flow conditions

Hydrological Processes. 2018;32:1561–1570. Abstract This study presents a Lagrangian description of advection dominated field‐scale transport of inert solutes, namely, the statistics of advective solute travel time and trajectory, in nonstationary random groundwater flow fields. The vertical flow field is generated by the effects of changes in total stress applied in fluid‐saturated heterogeneous deformable porous media. The Lagrangian statistics are developed directly from the velocity statistics based on the application of stochastic methodology such as the representation theorem and Fourier–Stieltjes integral representation. The focus of our numerical evaluation is placed on the influence of the soil compressibility on the Lagrangian statistics. The prediction on the statistics of solute travel time and trajectory may serve as the basic input to the environmental risk assessment.

Stochastic Analysis of Unsaturated Flow Subject to Temporally Correlated Infiltration

In this article, we attempt to quantify the variability of water pressure head in response to temporally correlated fluctuations in infiltration rate in partially saturated heterogeneous formations. This study concentrates on the unsaturated case of predominantly vertical movement of moisture. A closed-form solution is developed to analyze the impacts of the temporal correlation scale of infiltration fluctuations and the spatial correlation scale of log-saturated hydraulic conductivity (

Stochastic analysis of bounded unsaturated flow in heterogeneous aquifers: Spectral/perturbation approach

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Variability quantification of excess pressure head in heterogeneous deformable aquifers

lnKS) field on the variability in water pressure head. Our result indicates that the temporal correlation scale of infiltration process or the spatial correlation scale of