利用MODFLOW配合SUB套件推估雲林地區垂向平均長期地層下陷趨勢

Using MODFLOW Associated with SUB Package to Predict Vertical Average of Long-Term Land Subsidence in Yunlin.

洪秋香(Chiou-Shiang Hung)

指導教授 ： 倪春發

國立中央大學/地球科學學院/應用地質研究所/碩士(2011年)

摘要

根據經濟部水利署統計資料，雲林地區地下抽水井口數超過十萬口以上，近幾十年來，由於養殖漁業、工業、民生、灌溉用水量的大幅增加，雲林沿海地區的大量抽取地下水行為導致沿海地區地下水位持續下降，加上雲林地區雨量豐枯差異極大，以致地層下陷情形日益嚴重，最近的觀測結果顯示：持續的抽水行為亦造成地層下陷中心有往內陸移動的趨勢，並危及到地面結構物及公共設施結構使用上之安全。為了解雲林地區之地層下陷趨勢，本研究利用A modular three-dimensional finite-difference ground-water flow model(MODFLOW)數值模式配合Subsidence and Aquifer-System Compaction (SUB)套件推估雲林地區長期地層下陷趨勢，主要研究重點包括：(1) 地下水流模式建立與水流參數率定；以MODFLOW建立三維地下水流模式，配合經濟部水利署的觀測水位，率定雲林地區地下水流模式，使用1998年至2003年及2005至2009年地下水觀測站網水位，進行含水層參數率定；(2) 地層下陷模式建立與壓密參數敏感度分析；配合SUB套件建立地層下陷模式，以初始壓密參數的0.1、0.2及2倍等倍數進行敏感度分析；(3) 壓密參數率定與三種長期地層下陷趨勢情境分析；以經濟部水利署提供19口雲林地區地層下陷監測井資料，率定雲林地區相關的含水層壓密係數，將研究區域分為5區，依據岩心資料取得黏土分佈之垂向平均厚度以地層下陷監測井2002年到2009年地層下陷資料率定壓密參數，由驗證後模式再討論抽水條件下的三種情境分析。模擬結果顯示：(1) 觀測年平均水位與計算水位的平均絕對百分比誤差率(MAPE)是3.28%，而均方根誤差(RMSE)則是4.84。(2) 壓密參數中垂直水力傳導係數(K_v^'')與彈性儲蓄係數(Ske)對於地層下陷曲線變化與地層下陷量敏感度較高；參數率定選用的6口監測井與模擬結果相近。(3) 以率定後參數保持現況持續抽水，地層下陷曲線會趨緩，但後期可能有0.1至0.2公尺地層下陷量；空間分佈上沿海地區地層下陷量隨時間增加，較大地層下陷區發生在低水頭區，雖然20年暫態應力期後水頭無明顯變化，但地層下陷量仍持續增加，甚至已影響至雲林內陸地區。若考慮抽水條件變異情境，亦即，於5與10年後全區停止抽水的兩種測試情境，停抽10年後地層下陷量明顯減少(或回彈)至0.1公尺，顯示此時期有效應力仍低於前期預壓密應力，模式主要以彈性壓密行為進行計算。其中土庫國中與客厝國小地層下陷量降低約0.25公尺；海豐分校、豐安國小及金湖國小降低約0.15公尺，而元長國小反應幅度最大，降低約0.4公尺。第三類情境是加大抽水量持續抽水，分別選用有效應力小於及大於預壓密應力時停抽。當停抽時位彈性壓密階段，25年後地層下陷量可回復至0.8公尺；若停抽時已位於地層過壓密階段，停抽15年後地層下陷量雖會降低，仍有約1.5公尺的下陷量。

關鍵字

MODFLOW ； 地層下陷 ； 預壓密應力 ； 數值模式 ； SUB套件 ； 含水層壓密係數 ； 有效應力

並列摘要

According to the Water Resources Agency, there are more than one hundred thousand groundwater pumping wells in Yunlin area, in central Taiwan. In recent decades, the increasing pumping of groundwater for fish farming, industry, household and agriculture causes the decline of groundwater level in coastal area here. Moreover, the great difference of available water resources in wet and dry seasons lead to highly demand of groundwater resource and result in serious land subsidence. Recent observation also indicate that the center of land subsidence moves inland from the coast area due to the continuous pumping of groundwater. Such land subsidence may harm the sustainability of public constructions. To predict the long-term variations of subsidence in Yunlin area, this study employs a modular three-dimensional finite-difference ground-water flow model(MODFLOW) associated with Subsidence and Aquifer-System Compaction (SUB) package to build physical-based numerical model for groundwater flow and land subsidence. This study divides into three major terms: (1) Using MODFLOW to build the three-dimensional groundwater flow model and calibrate the model based on the water level observations from Water Resources Agency. Such long-term groundwater levels are from 1998 to 2003 and from 2005 to 2009. (2) Building a land subsidence model with SUB modulus, and investigates the sensitivity of the consolidation parameters by increasing or decreasing 0.1, 0.2, and 2 times of selected parameters in baseline case, (3) calibrating the aquifer consolidation parameters based on nineteenth multi-level compaction monitoring wells from Water Resources Agency, and analyzing the impacts of land subsidence on three different pumping scenarios in Yunlin area. Results show that: (1) the calibrated model obtains the root mean square error (RMSE) of 4.84 and average absolute percentage error rate (MAPE) of 3.28%. (2) The vertical hydraulic conductivity(K_v^'') and elastic skeletal storage coefficient(Ske) in the clay layer are the high sensitivity parameters for the accumulated land subsidence. The simulated land subsidence show solutions similar with the selected multi-level compaction monitoring wells. (3) Continuous pumping will result in 0.1 and 0.2 meters of subsidence in amount in the later period. Although the groundwater head has no obvious change in the stress period after 20 years, the values of land subsidence increase gradually and continuously. Three pumping scenarios are employed to assess the long-term effects of pumping events on the land subsidence in Yunlin area. The results of first two scenarios show that the values of land subsidence will recover 0.1 meters after 10 years if all the pumping events are stopped at the fifth and tenth years. Such result also indicate that the effective stress caused by pumping is still below the preconsolidation stress in the aquifer and the recovering procedures are mainly controlled elastic compaction. The recovery of land subsidence are about 0.25 meters at Tu Ku junior high school and Keh Tsuoh elementary school stations, and about 0.15 meters at Haifong District school station, Fengan and Chinhu elementary school stations. The Yuan Chang elementary school has greatest recovery about 0.4 meters. The result of third scenario show that the pumping induced effective stress less than the preconsolidation stress will result in 0.8 meters of difference from initial ground levels, while the pumping induced effective stress greater than the preconsolidation stress will lead to 1.5 meters of difference from initial ground levels.

並列關鍵字

SUB package ； MODFLOW ； land subsidence ； aquifer compact coefficient ； effective stress ； numerical model ； preconsolidation stress

參考文獻

〔53〕 洪偉嘉，「應用多重感應器監測雲林地區三維變形」，國立交通大學土木工程所博士論文，2009。

〔54〕 侯伊浩，「應用地層下陷模式探討地下水位與地層下陷量相關性之研究」，國立成功大學資源工程所碩士論文，2010。

〔25〕 陳建銘，「地層下陷模擬程序之建立與應用—以大城鄉西港地區為例」，國立成功大學土木工程所碩士論文，2005。

〔2〕 Bear, J. and Corapcioglu, M.Y., “Mathematical Model for Regional Land Subsidence Due to Pumping, 1. Intergrated Aquifer Subsidence Equations Based on Vertical Displacement Only” , Water Resources Research, Vol. 17, no. 4, pp. 937-946, 1981a.

〔3〕 Bear, J., and Corapcioglu, M.Y., “Mathematical Model for Regional Land Subsidence Due to Pumping, 2. Intergrated Aquifer Subsidence Equations for Vertical and Horizontal Displacements” , Water Resources Research, Vol. 17, no. 4, pp. 947-958, 1981b.