地下水位標準對地層下陷模式建構之影響

The Effect of Groundwater Standard on Building the Land Subsidence Model

雲林縣因地下水超抽而為地層下陷嚴重區，且高鐵將由北至南經由西螺、虎尾、土庫與元長等鄉鎮，地層持續下陷將對結構安全造成威脅，因此雲林地區之地層下陷現象為目前重要環境議題。為探討不同水位標準對地層下陷模式建構之影響，本研究首先利用地下水流數值模擬軟體MODFLOW建構濁水溪沖積扇地下水流數值模型，由現地觀測水井資料律定建構現地地下水流數值模型，藉由層間貯水套件（storage coefficient）建立雲林地區地層下陷模型。由參數敏感性分析結果顯示，水文地質模式影響地層下陷主要參數分別為：垂向滲漏係數、彈性與非彈性比儲水係數、預壓密水頭。本文將地下水流數值模型之觀測水井資料分成月平均水位（模式一）、當月最高水位（模式二）與當月最低水位（模式三），分別進行地層下陷模擬，並將其結果與雲林地區10口地層下陷監測井比較，發現下陷趨勢大致相符，且主要下陷區域為雲林縣之崙背鄉、二崙鄉、褒忠鄉、元長鄉、口湖鄉、水林鄉。模式一以元長國小、虎尾國小與客厝國小等靠內陸地區之監測站較為準確；模式二以建陽國小與金湖國小等沿海偏南之監測站較佳；模式三以崙豐國小、新興國小、龍岩國小與峰安國小等沿海偏北之監測站較符合。

Over-pumping groundwater has been leading to serious problems in the western coast of Taiwan, especially for Yunlin area. Yunlin has the most extensive land subsidence area, and the subsidence rate ranks the second place in Taiwan. Land subsidence has not only caused the supply and demand of groundwater unbalanced but also affected geo-engineering constructions of the country.The purpose of this thesis is to study the amount of land subsidence in Yunlin area. The numerical simulation groundwater flow software MODFLOW is applied to construct numerical models of groundwater flow at Chou-Shui alluvial fan. By setting hydraulic parameters, the combined hydrological model with the code （Interbed Storage Package） can be built the model of land subsidence in Yunlin. Then, we evaluate the difference in amount of land subsidence under three types of water level observation data: average water level of the month （model 1）, the highest water level of the month （model 2）, and the lowest water level of the month （model 3）. Finally, the difference between collected data and the simulation results of the land subsidence is compared.The comparison between field measurement data and simulation results with three models indicates that the trend of simulation results of the land subsidence corresponds to the filed measurement of land subsidence. First, at subsidence stations in inland area of Yunlin, such as Yuan-Chang elementary school, Hu-Wei elementary school, and Ke-Tsuo elementary school, the simulation results of subsidence in model I had more accurate results of subsidence compared with field measurement. Next, the subsidence stations, like Jian-Yang elementary school and Jin-Hu elementary school of southern coast, the simulation results of subsidence in model 2 had better correction results of subsidence. Last, the subsidence stations of northern coast area, for example Lun-Feng elementary school, Shin-Shing elementary school, Lung-Yan elementary school, and Feng-An elementary school, the simulation results of subsidence in model 3 also showed better correction of subsidence.