| Summary: | 博士 === 國立成功大學 === 資源工程學系碩博士班 === 98 === Taiwan is hit by typhoons along with torrential rains in recent years, the raw water for the water system would usually rise to a level of high turbidity, far beyond the capacity of the water purification station. Therefore, it is of great urgency to establish a steady and sustainable water supply backup system. The shallow aquifer is sourced by rains, rivers, and groundwater side recharge. By pumping near the riverbank, it is able to obtain clean water in wetting season and also intercept part of the groundwater during the drying season. Furthermore, the seepage on the soil stratum as well as the seepage is able to decrease the contamination and bacterial concentration of the water from rivers. Thus, it can be considered as a developable backup water supply program.
To evaluate the applicability of MODFLOW being used for all hydrological variations on the site, the study makes use of analytical models to verify its applicability. The verification consists of two steps. The first step involves research on the effect of water level rise to the river seepage, by mainly using the analytical mode formulated by Ferris et al. (1962). This step likewise involves the comparison of the results. The second step is to assess the effect of the pumping activity to the river seepage, using four analytical models that were developed by Grigoryev (1957), Hantush (1965), Wilson (1993), and Hunt (1999) to evaluate the applicability of MODFLOW being used for simulating pumping near riverbank. Furthermore, the second step utilizes the analytical mode of Joshi (1988) to verify the horizontal collection well mode. According to the research results, the analytical solution and the numerical solution have shown good verification results. This indicates that the MODFLOW can simulate the on-site situation under all kinds of hydrological conditions.
The study makes use of rubber dam installations to evaluate the influence of water level rise on river seepage. River flow estimation, experience analytical, and numerical simulation are respectively used for analysis. By combining on-site materials, analytical solution, and numerical solution, as well as other research approaches, a quantification study on river seepage increase for the unique hydrological geology of the region has been conducted. According to the research results, the river seepage amounts estimated from the stream flow data and numerical solution are 2,642 thousand m3/year and 1,709 thousand m3/year respectively. In relation to this, the increased river seepage amounts estimated from the analytical solution and numerical solution are 316 thousand m3/year and 264 thousand m3/year respectively. Thus, it is estimated that, at a 2.5 m of rise of the river water level, the amount of river seepage will be increased from 10.0% to 18.5%.
The study proposes large wells to be built for water source development in the Kuaiguan area of North Zhanghua and the area along the Maoluo River. To undergo sensitivity analysis for hydrological geology parameters that can help to verify the mode of the site, the corollary process of sensitivity analysis for the pumping well form can be applied in the water supplying program design. Based on the results of the research and design, the project is building 8 pumping wells along the Maoluo River, each with a water yield of 7,500 CMD, cross-distributed in two lines with 4 m of well diameter. The simulation results show that the groundwater drawdown accords with the lower limit standard and the drawdown does not involve residential areas located 700 m away.
The water source development in South Zhanghua involves building large-radial wells along the north bank of the Zhuoshui River in the Zhutang Area. Moreover, the sensitivity analysis for the horizontal collection well form is undertaken for the water supplying program design. Based on the analysis of the research, the proposal is to build 6 large-radial wells, each with a water yield of 7,400 CMD, 4 m well diameter, with horizontal collection wells 2 m in length reaching 10 m from the wells. The simulation results indicate that the groundwater drawdown is in accordance with the lower limit standard. Furthermore, the drawdown affects the residential areas situated 600 m away between the degrees of 0.2 to 0.3 m.
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