宜蘭近年地層下陷水準觀測：地下水抽取與地陷關係分析

Contemporary land subsidence in Ilan from leveling observations: correlation analysis of groundwater pumping and subsidence

宜蘭精密水準觀測（往返閉合差小於2.5mmk）自1992年前開始約每隔2～3年進行一次全區水準檢測，近十年分別於2000年、2007年、2009年、2012年與2014年實施全區水準檢測。而本文主要詳述2012-2014年宜蘭水準觀測及其成果。該觀測計畫由水利署資助，目的在偵測因抽取地下水而引起的地層下陷。在宜蘭東部沿岸地區，1992～2014年之累積地層下陷量最高達30公分。2012年後，由於地下水管制，大部分地區的下陷速率在1公分／年內，顯著下陷面積縮小到0.01平方公里。自從國道5號通車之後，蘇澳地區的一大型觀光飯店周邊之下陷速率達3.3公分／年，應與大量抽取地下水有關。宜蘭地區的地層壓縮大部分發生在深度0到38公尺處，主因為抽取地下水。壓縮呈現出年變化：四月時，地下水位為最低而壓縮量亦為最大值；反之，九月時，地下水位為最高而地表些微反彈。蘭陽平原西部含不可壓縮之粗礫石，而東部含易壓縮之細砂及黏土。本文建議應持續觀測宜蘭的地層下陷以檢驗地下水管制之成效。 Precision leveling observations (allowable misclosure 2.5 mm k ) of Ilan began in 1992 and continue to date. Since 1992, precision leveling with a tolerable double-run misclosure of 2.5 mm k (k is distance between two neighboring benchmarks) has been carried out in Ilan at intervals of 2-3 years. Specifically, leveling campaigns were conducted in 2000, 2007, 2009, 2012 and 2014. This paper reports the leveling result from the observations in 2012 and 2014, sponsored by Water Resource Agency. Leveling is used to detect subsidence in connection to ground water withdrawals in Ilan. The cumulative land subsidence from 1992 to 2014 in Ilan reaches a maximum of 30 cm in coastal zones. After 2012, the subsidence rates and areas turn smaller, at the levels of 1 cm/year (rate) and 0.01 km2 (area), due to a measure to restrict groundwater use. Since the opening of Highway Number 5, a maximum subsidence rate of 3.3 cm/year in 2014 was detected around a hotel in Suao, where considerable groundwater was extracted for tourism use. The depths of the major soil compaction range from 0 to 38 m, where groundwater pumping takes place. The compaction exhibits an annual variation, with April being the largest (lowest groundwater level) and September the least (highest groundwater level). The western part of the Ilan alluvial plain contains incompressible gravels, while its eastern part is composed of fine sands and clays susceptible to compaction by increased stress due to groundwater reduction. Continual monitoring of subsidence in Ilan by leveling is encouraged to examine the effect of the restricted groundwater use.

Precision leveling observations (allowable misclosure 2.5 mm k ) of Ilan began in 1992 and continue to date. Since 1992, precision leveling with a tolerable double-run misclosure of 2.5 mm k (k is distance between two neighboring benchmarks) has been carried out in Ilan at intervals of 2-3 years. Specifically, leveling campaigns were conducted in 2000, 2007, 2009, 2012 and 2014. This paper reports the leveling result from the observations in 2012 and 2014, sponsored by Water Resource Agency. Leveling is used to detect subsidence in connection to ground water withdrawals in Ilan. The cumulative land subsidence from 1992 to 2014 in Ilan reaches a maximum of 30 cm in coastal zones. After 2012, the subsidence rates and areas turn smaller, at the levels of 1 cm/year (rate) and 0.01 km2 (area), due to a measure to restrict groundwater use. Since the opening of Highway Number 5, a maximum subsidence rate of 3.3 cm/year in 2014 was detected around a hotel in Suao, where considerable groundwater was extracted for tourism use. The depths of the major soil compaction range from 0 to 38 m, where groundwater pumping takes place. The compaction exhibits an annual variation, with April being the largest (lowest groundwater level) and September the least (highest groundwater level). The western part of the Ilan alluvial plain contains incompressible gravels, while its eastern part is composed of fine sands and clays susceptible to compaction by increased stress due to groundwater reduction. Continual monitoring of subsidence in Ilan by leveling is encouraged to examine the effect of the restricted groundwater use.