Browsing by Author "Chang, Ya-Chi"

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A new analytical solution solved by triple series equations method for constant-head tests in confined aquifers
(2010-06-01)
Chang, Ya-Chi
;
Yeh, Hund-Der
;
環境工程研究所
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Institute of Environmental Engineering
The constant-head pumping tests are usually employed to determine the aquifer parameters and they can be performed in fully or partially penetrating wells. Generally, the Dirichlet condition is prescribed along the well screen and the Neumann type no-flow condition is specified over the unscreened part of the test well. The mathematical model describing the aquifer response to a constant-head test performed in a fully penetrating well can be easily solved by the conventional integral transform technique under the uniform Dirichlet-type condition along the rim of wellbore. However, the boundary condition for a test well with partial penetration should be considered as a mixed-type condition. This mixed boundary value problem in a confined aquifer system of infinite radial extent and finite vertical extent is solved by the Laplace and finite Fourier transforms in conjunction with the triple series equations method. This approach provides analytical results for the drawdown in a partially penetrating well for arbitrary location of the well screen in a finite thickness aquifer. The semi-analytical solutions are particularly useful for the practical applications from the computational point of view. (C) 2010 Elsevier Ltd. All rights reserved.
A new approximate solution for chlorine concentration decay in pipes
(2008-05-01)
Yeh, Hund-Der
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Wen, Shi-Bin
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Chang, Ya-Chi
;
Lu, Chung-Sying
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環境工程研究所
Biswas et al. (1993. A model for chlorine concentration decay in pipes. Water Res. 27(12), 1715-1724) presented an analytical solution of a two-dimensional (2-D) steady-state chlorine transport equation in a pipe under the turbulent condition and employed fractional error function and regression technique to develop an approximate solution. However, their approximate solution may not give a good result if the wall decay parameter is large. This paper provides a more accurate approximate solution of the 2-D steady-state chlorine transport equation under the turbulent condition. This new approximate solution has advantages of easy evaluation and good accuracy when compared with the approximate solution given by Biswas et al. (1993). In addition, this paper also develops a methodology that combines simulated annealing (SA) with this new approximate solution to determine the wall decay parameter. Two cases are chosen to demonstrate the application of the present approximate solution and methodology. The first case is to use this new approximate solution in simulating chlorine decay in pipes with the experiment-observed data given by Rossman (2006. The effect of advanced treatment on chlorine decay in metallic pipes. Water Res. 40(13), 2493-2502), while the second case presents the determination of the wall consumption at the end of the pipe network. (c) 2008 Elsevier Ltd. All rights reserved.
Analysis of groundwater flow and stream depletion in L-shaped fluvial aquifers
(2018-04-19)
Lin, Chao-Chih
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Chang, Ya-Chi
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Yeh, Hund-Der
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環境工程研究所
;
Institute of Environmental Engineering
Understanding the head distribution in aquifers is crucial for the evaluation of groundwater resources. This article develops a model for describing flow induced by pumping in an L-shaped fluvial aquifer bounded by impermeable bedrocks and two nearly fully penetrating streams. A similar scenario for numerical studies was reported in Kihm et al. (2007). The water level of the streams is assumed to be linearly varying with distance. The aquifer is divided into two subregions and the continuity conditions of the hydraulic head and flux are imposed at the interface of the subregions. The steady-state solution describing the head distribution for the model without pumping is first developed by the method of separation of variables. The transient solution for the head distribution induced by pumping is then derived based on the steady-state solution as initial condition and the methods of finite Fourier transform and Laplace transform. Moreover, the solution for stream depletion rate (SDR) from each of the two streams is also developed based on the head solution and Darcy's law. Both head and SDR solutions in the real time domain are obtained by a numerical inversion scheme called the Stehfest algorithm. The software MODFLOW is chosen to compare with the proposed head solution for the L-shaped aquifer. The steady-state and transient head distributions within the L-shaped aquifer predicted by the present solution are compared with the numerical simulations and measurement data presented in Kihm et al. (2007).
Analytical solution for groundwater flow in an anisotropic sloping aquifer with arbitrarily located multiwells
(2007-12-15)
Chang, Ya-Chi
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Yeh, Hund-Der
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環境工程研究所
;
Institute of Environmental Engineering
This paper presents a new analytical solution to describe the hydraulic head distribution and flow system in an anisotropic unconfined aquifer with a sloping bed and arbitrarily located multiwells under transient recharge. Unlike the existing analytical solutions for delineating the capture zones, the solution presented in this study is easy and convenient to calculate flow field in some complicated systems, and it can provide useful information for the pump-and-treat design. In addition, the presented solution may have the possible application in evaluating the sensitivity of hydrogeological parameters, or estimating the hydraulic parameters when coupled with an optimization approach to analyze the aquifer data. (c) 2007 Elsevier B.V. All rights reserved.
Determination of the parameter pattern and values for a one-dimensional multi-zone unconfined aquifer
(2008-03-01)
Chang, Ya-Chi
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Yeh, Hund-Der
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Huang, Yen-Chen
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環境工程研究所
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Institute of Environmental Engineering
In an aquifer, heterogeneity plays an important role in governing groundwater flow. Hence, aquifer characterization should involve both the pattern and values of the hydrogeological parameters. A new analytical solution describing the one-dimensional groundwater flow in a multi-zone unconfined aquifer is presented, and a methodology developed from the analytical solution and a heuristic approach for determining the pattern and values of the aquifer parameters are proposed. The analytical solution demonstrates that the hydraulic head varies spatially and is influenced by aquifer heterogeneity. Simulated annealing, a heuristic approach, is incorporated with the solution to simultaneously identify the pattern and values of the hydraulic conductivity for a horizontal multi-zone unconfined aquifer. This approach may be used to give an approximate result for a two-dimensional problem by dividing the model area into a number of transects along the transverse direction, identifying the parameter values along the longitudinal direction for each transect, and then smoothing the identified results.
Development of a Hydrological Ensemble Prediction System to Assist with Decision-Making for Floods during Typhoons
(2020-05-01)
Yang, Sheng-Chi
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Yang, Tsun-Hua
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Chang, Ya-Chi
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Chen, Cheng-Hsin
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Lin, Mei-Ying
Hydrological ensemble prediction systems (HEPSs) can provide decision makers with early warning information, such as peak stage and peak time, with enough lead time to take the necessary measures to mitigate disasters. This study develops a HEPS that integrates meteorological, hydrological, storm surge, and global tidal models. It is established to understand information about the uncertainty of numerical weather predictions and then to provide probabilistic flood forecasts instead of commonly adopted deterministic forecasts. The accuracy of flood forecasting is increased. However, the spatiotemporal uncertainty associated with these numerical models in the HEPS and the difficulty in interpreting the model results hinder effective decision-making during emergency response situations. As a result, the efficiency of decision-making is not always increased. Thus, this study also presents a visualization method to interpret the ensemble results to enhance the understanding of probabilistic runoff forecasts for operational purposes. A small watershed with area of 100 km(2) and four historical typhoon events were selected to evaluate the performance of the method. The results showed that the proposed HEPS along with the visualization approach improved the intelligibility of forecasts of the peak stages and peak times compared to that of approaches previously described in the literature. The capture rate is greater than 50%, which is considered practical for decision makers. The proposed HEPS with the visualization method has potential for both decreasing the uncertainty of numerical rainfall forecasts and improving the efficiency of decision-making for flood forecasts.
Optimum allocation for soil contamination investigations in Hsinchu, Taiwan, by double sampling
(2007-09-01)
Chang, Ya-Chi
;
Yeh, Hund-Der
;
交大名義發表
;
National Chiao Tung University
The double sampling (DS) scheme is a cost-effective sampling method that combines an expensive measurement procedure with an inexpensive but less accurate one. Double sampling works when the true correlation of determination (rho(2)) between two techniques is known in advance, but that is hardly ever the case. By assuming a rho(2) < 0.9, a three-stage procedure (TSP) in DS selects a preliminary pool of samples and estimates rho(2), from which the optimal allocation of samples is determined. There may be excessive and unnecessary sampling during a TSP when rho(2) > 0.9. The main objective of this study was to extend the TSP and determine the optimum allocation of the samples under a fixed budget condition for the case rho(2) > 0.9. A soil from Hsinchu, Taiwan, contaminated with heavy metals Zn, Cu, Pb, Ni, Cr, and Cd was sampled at 0.15, 0.3, 0.45, and 0.6 to deep at 36 sites (144 samples). All samples were analyzed with field-portable x-ray fluorescence and a subset of 40 samples was selected for analyses of Zn, Cu, and Pb with standard methods in the laboratory. Results from both measurements were linearly correlated with estimated rho(2) values of 0.96, 0.95, and 0.97 for Zn, Cu, and Pb, respectively. Considering a rho(2) = 0.99, the optimum subsample and sample sizes were 5 out of 167, 4 out of 173, and 5 out of 167 for Zn, Cu, and Pb, respectively. The extended TSP analyses reduced the number of superfluous samples to only two or three, which was less than obtained by TSP (9-16).
Recent advances in modeling of well hydraulics
(2013-01-01)
Yeh, Hund-Der
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Chang, Ya-Chi
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環境工程研究所
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Institute of Environmental Engineering
Well hydraulics is a discipline to understand the process of flow to the well in an aquifer which is regarded as a source of groundwater. A variety of analytical and numerical models have been developed over the last few decades to provide a framework for understanding and quantifying the flow behavior in aquifer systems. In this review, we first briefly introduce the background of the theory of well hydraulics and the concepts, methodologies, and applications of analytical, semi-analytical, numerical and approximate methods in solving the well-hydraulic problems. We then address the subjects of current interests such as the incorporation of effects of finite well radius, wellbore storage, well partial penetration, and the presence of skin into various practical problems of groundwater flow. Furthermore, we also summarize recent developments of flow modeling such as the flow in aquifers with horizontal wells or collector wells, the capture zone delineation, and the non-Darcian flow in porous media and fractured formations. Finally, we present a comprehensive review on the numerical calculations for five well functions frequently appearing in well-hydraulic literature and suggest some topics in groundwater flow for future research. (C) 2012 Elsevier Ltd. All rights reserved.
Skin effect in generalized radial flow model in fractured media
(2011-04-01)
Chang, Ya-Chi
;
Yeh, Hund-Der
;
環境工程研究所
;
Institute of Environmental Engineering
A generalized radial flow model, treating the flow dimension as an arbitrary number and taking into account the well skin effect, is presented to describe the transient head distribution in a fractured medium under the Robin condition during constant-head tests. The Laplace-domain solution for the head distribution is first developed via the Laplace transforms; and the corresponding time-domain solution in terms of the hydraulic head is then obtained using the Bromwich integral method. In addition, based on the head solution and Darcy's law, the solution for the wellbore flux is further developed to investigate the skin effect. It is found that the skin effect on the wellbore flux is significant at early times. Under this circumstance, it would not be appropriate to neglect the skin effect in the model in determining the hydraulic parameters from the analysis of field data. Our solutions will be useful for the predictions of the spatial and temporal head distribution and wellbore flux or in investigating the skin effect on the head distribution and wellbore flux for flow with various dimensions in fractured media.
Stream depletion rate and volume from groundwater pumping in wedge-shape aquifers
(2008-02-01)
Yeh, Hund-Der
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Chang, Ya-Chi
;
Mow, Vitaly A.
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環境工程研究所
;
Institute of Environmental Engineering
In river basins with several tributaries, wells in alluvial valleys are often located near the wedge-shaped confluence of two tributaries. Therefore, stream depletion characteristics, namely stream depletion rate (SDR) and stream depletion volume (SDV), induced by the pumping wells are distributed between two tributaries upstream from the confluence. Existing methods for the evaluation of stream depletion characteristics are applicable only for a wedge-shaped aquifer with special or uncommon wedge angles or for steady-state conditions. In water resources management, temporal stream depletion characteristics are important for the adjudication of water rights. A new and practically important method for analytical evaluation of transient SDR and SDV is developed for a wedge-shaped aquifer with arbitrary wedge angles. Results are obtained for each of two tributaries or the tributary segment lengths. A simple expression for the SDR fraction that originates from each tributary is presented for long period of pumping after which 100% of the pumping rate is supplied by stream depletion. The sensitivity analyses for different parameters are performed to examine their affects on the influence period of the total SDR and the results indicated that the SDR is sensitive to the angles of wedge-shaped aquifer and the pumping well. The results and sensitivity analysis presented in this study may provide useful information in the assessment of water balance or exploitation of river basins. (C) 2007 Elsevier B.V. All rights reserved.
Transient Flow into a Partially Penetrating Well during the Constant-Head Test in Unconfined Aquifers
(2011-09-01)
Chang, Ya-Chi
;
Chen, Geng-Yuan
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Yeh, Hund-Der
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環境工程研究所
;
Institute of Environmental Engineering
This study derives a semianalytical solution for drawdown distribution during a constant-head test at a partially penetrating well in an unconfined aquifer. The constant-head condition is used to describe the boundary along the screen. In addition, a free-surface condition is used to delineate the upper boundary of the unconfined aquifer. The Laplace-domain solution is then derived using separation of variables and Laplace transform. This solution can be used to identify the aquifer parameters from the data of the constant-head test when integrated with an optimization scheme or to investigate the effects of vertical flow caused by the partially penetrating well and free-surface boundary in an unconfined aquifer. DOI: 10.1061/(ASCE)HY.1943-7900.0000392. (C) 2011 American Society of Civil Engineers.