Browsing by Author "Chang, Yin-Lung"

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ASSESSING VULNERABILITY OF RIVERBANK REVETMENTS IN A REGIONAL DRAINAGE SYSTEM DURING STORM OR TYPHOON EVENTS
(2015-01-01)
Lin, Ying-Tien
;
Yang, Jinn-Chuang
;
Hung, Sheng-Hsiang
;
Chiang, Shih-Wei
;
Chang, Yin-Lung
In this study, a quasi three-dimensional hydrodynamic model and field measurements were combined to assess the vulnerable segments of the De-Shang Creek drainage system during the 610 flood and Typhoon Kong-Ray. An automatic monitoring system was installed to record channel water levels, and the displacement and pore water pressure variations inside the riverbank revetments. The results show that the variations of water levels and pore water pressure were almost at the same pace, which indicates the drainage system inside the revetments work well. The flow-induced lateral shear stress and safety factor (S.F.) of river embankments from numerical simulations are the two indexes used to assess the vulnerability of the riverbank revetments. By using the 610 flood and Typhoon Kong-Ray as test cases, the combined hydrodynamic and revetment stability model is able to indicate if riverbank revetments are in danger during the storm or typhoon events. In the observational site, the damages of the revetments in the 610 flood may be due to the exceedance of the flow-induced shear stress over the critical resistance shear stress of the revetments. Overall, this study show the combined hydrodynamic and revetment stability model is a useful non-engineering method on assessing stability of riverbank revetments during storm or typhoon events.
Flood hazard mitigation in land subsidence prone coastal areas by optimal groundwater pumping
(2019-11-01)
Chang, Yin-Lung
;
Tsai, Tung-Lin
;
Yang, Jinn-Chuang
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土木工程學系
;
防災與水環境研究中心
Continuing land subsidence can aggravate flood hazard and diminish the effectiveness of existing flood mitigation systems. This paper presents a study of groundwater- pumping management for mitigating flood hazard in a subsidence infested area in the southwest coast of Taiwan. An optimal groundwater-pumping model is developed for preventing flood hazard from worsening and for satisfying the groundwater demand. In an area where the land is susceptible to pumping-induced subsidence, the study shows that proper long-term pumping strategy to manage spatial-temporal variation of future land subsidence should be an integral part of flood risk management as it can sustain the effectiveness of the flood mitigation measures.
Stochastically optimal considering groundwater management land subsidence
(2007-11-01)
Chang, Yin-Lung
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Tsai, Tung-Lin
;
Yang, Jinn-Chuang
;
Tung, Yeou-Koung
;
土木工程學系
This paper presents a stochastic groundwater management model explicitly considering land subsidence. Through the use of response matrix technique and one-dimensional consolidation equation, a deterministic management model is first developed. By Latin hypercube sampling technique, along with numerical subsurface flow simulation, statistical features of unit response coefficients due to random hydrogeologic parameters, including hydraulic conductivity (K) and Lame constants (mu and lambda), are quantified. The first-ordervariance-estimation method is adopted to analyze the uncertainties of drawdown and land subsidence based on which the concept of chance-con strained programming is applied to transfer the original deterministic management model into its stochastic form. The stochastic management model enables the determination of optimal total pumpage subject to the constraints that drawdown and land subsidence do not exceed the allowable values with a specified reliability. A hypothetical example is utilized to demonstrate the applicability of the stochastic model to five cases in which various levels of parameter uncertainty are considered. The results indicate that joint consideration of drawdown and land subsidence is essential, and the proposed stochastic management model can be generally applied for regional groundwater resources management in conjunction with controlling land subsidence.
Two-Dimensional Numerical Investigation for Short- and Long-Term Effects of Spur Dikes on Weighted Usable Area of Rhinogobius candidianus (Goby)
(2013-12-01)
Chang, Yin-Lung
;
Hsieh, Te-Yung
;
Chen, Chien-Hua
;
Yang, Jinn-Chuang
;
土木工程學系
The weighted usable area (WUA) is one of the popular metrics to quantify the stream physical habitat and the effects of instream structures. However, most habitat studies only considered existing or proposed channel geometries in their WUA computations and not how the channel will evolve over time. This study reports a numerical investigation for the effects of spur dikes on WUA along the Juanchiao levee, Touchien River, Taiwan. Seven virtual dikes were placed along the outer bend in the study reach and the Rhinogobius candidianus was selected for target species. The horizontal two-dimensional (2D) flow fields at various flows and the corresponding WUAs for R. candidianus were computed under four channel bathymetry configurations, including: (1)existing; (2)immediately after dikes are implemented; (3)after 10-year channel migration without dikes; and (4)10years after dikes are implemented. The WUA obtained from the first two bathymetry configurations aimed to investigate the short-term spur dikes effect, whereas the last two examined the long-term effect. The channel geometry after 10years channel migration under the conditions of both with and without spur dikes were simulated through a 2D mobile bed model. The analyses of short-term effect showed the WUA of R. candidianus, shortly after the virtual dikes were placed, increases due to increased water depth near the dikes, especially in low-flow conditions. As for long-term effects, however, the WUA in the case with spur dikes are lower than the case without spur dikes, especially in relatively high-flow conditions. The adverse effects of spur dikes on WUA appeared in the long-term investigation and resulted from the ever-increasing water depth and flow velocity around the tip of spur dikes and the main channel. This study showed that the habitat quality and availability might be overestimated if the channel bed changes over time. When using the WUA approach to evaluate the effects of instream structure on physical habitat prior to actually placing them in the river, quantitative predictions of the long-term variation of channel geometry and the associated physical habitat conditions are important. (C) 2013 American Society of Civil Engineers.