Effect of Chi-Chi earthquake on the hydrogeological properties in the Choushi River alluvial fan
Date Issued
2005
Date
2005
Author(s)
Chen, You-Ching
DOI
zh-TW
Abstract
Hydraulic conductivity (K) is an important parameter for the investigations of the hydrogeology, groundwater flow and contaminant transport. Soil structures and hydraulic conductivities maybe vary due to the 1999 Chi-Chi earthquake in the Choushui River alluvial fan. In this study, the changes of groundwater level induced by the Chi-Chi earthquake are regarded as the water-level fluctuations of a hydraulic test made by nature. We respect to understand the effect of groundwater hydrogeological properties in Chi-Chi earthquake by analyze the water level changes. First, this study analyzed the relationship of coseismic water-level change versus the distance from epicenter and transmisivity of wells in unconfined aquifers, confined aquifers and mountain region. The recovery of water levels in specified observation wells and hydrogeological properties was analyzed. The analyzed results indicate that the change of coseismic groundwater levels does not correlate well with the distance from epicenter and transmisivity. The numerical model, MODFLOW, was used to simulate the recovery of coseismic water-level changes and the automatically calibrated model, UCODE, was used to determine the changes of K/S on the boundary conditions after the Chi-Chi earthquake. This work performed the in-situ pumping test and the slug test in the liquefaction area of Shi-Hu, Yan-Lin and Shi-Chou monitoring wells after Chi-Chi earthquake occurrence. The transmissivity and storage coefficients were determined and compared with those measured before the Chi-Chi earthquake in 1993-1994.
The results of pumping test show that the transmissivity increases with the highest of 80.52% at the Shi-Chou 2 observation well and increases 61.02% at the Yan-Lin 2 observation well. However, the transmissivity decreases to original one of 4.2% at the Shi- Hu 2 observation well. Furthermore, the storage coefficients decrease with the highest of 83.64% at the Shi-Chou 2 observation well, and decrease 60.07% at the Shi-Hu 2 observation well. The analyzed results of the seismic acceleration and soil liquefaction reveal that the changes of the porosity and soil consolidation coefficients agree with the changes of the transmissivity and storage coefficients, respectively, after the earthquake. At the observation well of Yan-Lin 2 where the soil liquefaction occurred, the soil porosity and the transmissivity increase due to the large seismic acceleration. At the observation well of Shi-Chou 2 where the soil liquefaction does not occur remarkably, the soil porosity and the transmissivity also increase because of the large seismic acceleration. Meanwhile, at the observation wells of Shi-Chou 2 and Shi- Hu 2, the storage coefficients decrease owing to the decreased soil consolidation coefficients. The simulation of the coseismic water-level recovery at the monitoring wells of Yan-Lin and Shi-Chou performs better than that at the monitoring well of Shi-Hu when the simulated conditions of groundwater inflow into rivers and groundwater loss in mountains are imposed on the model. Some factors, such as the transient changes of porosity and undrain loading, do not be considered in this model. A robust model containing these factors should be considered to develop and obtain on improved simulation and observation result.
Subjects
同震地下水位
地下水流模擬
水力傳導係數
抽水試驗
參數優選
Coseismic water-level change
groundwater model
hydraulic conductivity
pumping test
parameter determines
Type
thesis
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