Browsing by Author "Hung, Wei-Chia"

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Land Subsidence in Chiayi, Taiwan, from Compaction Well, Leveling and ALOS/PALSAR: Aquaculture-Induced Relative Sea Level Rise
(2018-01-01)
Hung, Wei-Chia
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Hwang, Cheinway
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Chen, Yi-An
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Zhang, Lei
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Chen, Kuan-Hung
Chiayi County is located in the largest alluvial plain of Taiwan with extensive aquaculture and rice farming sustained by water extracted from groundwater wells. Chiayi is a typical aquaculture area affected by land subsidence, yet such lands worldwide combine to provide nearly 90% of global aquaculture products, greatly reducing oceanic overfishing problems. This study uses precision leveling, multi-layer compaction monitoring well (MLCW) and spaceborne SAR interferometry (InSAR) to examine the cause and effect of land subsidence in Chiayi associated with groundwater extractions and changes. Heights at benchmarks in a leveling network are measured annually and soil compactions at 24-26 layers up to 300-m depths at 7 MLCWs are collected at one-month intervals. Over 2007-2011, 15 ALOS/PALSAR images are processed by the method of TCPInSAR to produce subsidence rates. All sensors show that land subsidence occur in most parts of Chiayi, with rates reaching 4.5 cm/year around its coast, a result of groundwater pumping from shallow to deep aquifers. MLCWs detect mm-accuracy seasonal soil compactions coinciding with groundwater level fluctuations and causing dynamic compactions. Compactions near Taiwan High Speed Rail may reduce the strength of the rail's supporting columns to degrade its safety. The SAR images yield subsidence rates consistent with those from leveling and compaction wells after corrections for systematic errors by the leveling result. Subsidence in Chiayi's coastal area leads to relative sea level rises at rates up to 15 times larger than the global eustatic sea level rising rate, a risk typical for world's aquaculture-rich regions. At the fish pond-covered Budai Township, InSAR identifies subsidence spots not detected by leveling, providing crucial geo-information for a sustainable land management for aquaculture industry.
Modeling aquifer-system compaction and predicting land subsidence in central Taiwan
(2012-10-12)
Hung, Wei-Chia
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Hwang, Cheinway
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Liou, Jyh-Chau
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Lin, Yan-Syun
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Yang, Hsiu-Lung
Changhua is a county in central Taiwan that will house several major economic development projects. Extracting groundwater has caused large-scale land subsidence in Changhua, with the largest cumulative subsidence being 210 cm over 1992-2010. A multi-sensor monitoring system consisting of continuous GPS stations, a leveling network, multi-layer compaction monitoring wells and groundwater wells is deployed to monitor land subsidence and its mechanism in Changhua. Data from the monitoring well CGSG in Dacheng Township of Changhua show a cumulative compaction of 110.6 cm over 1997-2010, occurring mainly at two aquifers and one aquitard. A novel combination of GPS and monitoring well data was used to determine the stress-strain relations. The stratum compaction turns from plasticity to elastoplasticity after a long-term compaction at the second aquifer below the surface. Four hydrogeological parameters of the three sediment layers in Dacheng, vertical hydraulic conductivity, elastic skeletal specific storage, inelastic skeletal specific storage, and the initial maximum preconsolidation stress, in the one-dimensional compaction model COMPAC, are estimated using the genetic algorithm. With the parameters. COMPAC predicts compactions to an accuracy consistent with in situ measurements, and the mean absolute percentage errors of prediction are below 10%. The result provides a key reference for water management in central Taiwan. (C) 2012 Elsevier B.V. All rights reserved.
Monitoring severe aquifer-system compaction and land subsidence in Taiwan using multiple sensors: Yunlin, the southern Choushui River Alluvial Fan
(2010-02-01)
Hung, Wei-Chia
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Hwang, Cheinway
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Chang, Chung-Pai
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Yen, Jiun-Yee
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Liu, Chih-Hsi
During 1992-2007, excessive pumping of ground-water caused large-scale aquifer-system compaction and land subsidence in the Choshui River Alluvial Fan, especially in the area of Yunlin county. The subsidence impedes surface-water runoff and endangers the operation of Taiwan High Speed Rail. Leveling, Global Positioning System (GPS), multi-level compaction monitoring well, and Differential Interferometric Synthetic Aperture Radar (DInSAR) are used to study the extent of subsidence in Yunlin and its mechanism. These sensors complement each other in spatial and temporal resolutions. A leveling network totaling 434 km in length was deployed to derive subsidence at every 1.5 km along the routes, and the result is accurate to few mm and shows a basin-like subsidence pattern centering at Tuku Township. Four multi-level compaction monitoring wells, co-located with GPS pillars, detect compactions at different depths, showing that the aquifer-system compaction (the cause of subsidence) occurs mostly below depths > 200 m, where reduction of ground-water pumping is most needed. The vertical displacements from GPS and leveling agree to within 1 cm, and are larger than the cumulative compaction detected by the compaction-monitoring wells, suggesting that compaction also occurs below 300 m (the depth of the wells). The vertical displacements derived using DInSAR and 8 ENVISAT SAR images agree with the leveling result to 1-2 cm.
Results of geodetic and geotechnical monitoring of subsidence for Taiwan High Speed Rail operation
(2008-10-01)
Hwang, Cheinway
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Hung, Wei-Chia
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Liu, Chih-Hsi
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土木工程學系
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Department of Civil Engineering
Taiwan High Speed Rail (THSR), which began operations in January 2007, passes through an area in Yunlin County where the largest cumulative subsidence measured during 1992-2006 exceeds 100 cm. Leveling benchmarks, GPS pillars and multi-level monitoring wells were deployed in this area to collect detailed subsidence data from October 2003 to 2006. Leveling is carried out on both ground benchmarks and survey bolts attached to THSR columns. Minimum constraint solutions of leveling networks produce estimated heights accurate to a few mm. Special attention is paid to code smoothing, ionospheric, tropospheric and ocean tidal loading (OTL) effects, so that height estimates from GPS are optimal. Leveling and GPS-derived height changes are consistent to 1 cm, and show that from Stations 210 to 240K of TSHR, the subsidence is bowl shaped. Measurements of sediment compaction in specific depth intervals at three monitoring wells indicate that most of the subsidence is caused by sediment compaction at depths from 50 to 300 m. The major compaction occurs in the interval 220-300 m and is attributed to ground water withdrawal. Large angular deflections as determined from subsidence measurements are detected at some columns, but are below the upper bound (1/1,000) of tolerance specified in the safety code. With the current subsidence and sediment compaction, no significantly reduced loading capacity of the columns is expected to occur. For a safe THSR operation, subsidence and sediment-compaction monitoring should be continued, and current ground water withdrawal in Yunlin must be reduced or stopped.
Surface deformation from persistent scatterers SAR interferometry and fusion with leveling data: A case study over the Choushui River Alluvial Fan, Taiwan
(2011-04-15)
Hung, Wei-Chia
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Hwang, Cheinway
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Chen, Yi-An
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Chang, Chung-Pai
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Yen, Jiun-Yee
The Choushui River Alluvial Fan (CRAF), one of the most important agricultural areas in Taiwan, suffers severe subsidence caused by groundwater withdrawal. We use 20 ENVISAT images from 2006 to 2008 to derive vertical deformation over CRAF by persistent scatterer interferometry (PSI), which effectively reduces errors affecting conventional DInSAR techniques. The pixel density over CRAF is 107.6 pixels/km(2), compared to 0.19 points/km(2) of the leveling benchmarks. PSI yields vertical displacements matching the leveling result to 0.6 cm/year (RMS), and provides a higher spatial resolution of subsidence than the latter. We also develop a data fusion method that considers the high-precision, low point-density leveling result as a smoothed correction to the PSI result. The combined field is more representative of overall deformation characteristics than the PSI-only or leveling-only field, and it is better suited for the assessment of the impact of subsidence over CRAF. (C) 2010 Elsevier Inc. All rights reserved.
Surface Deformation from Sentinel-1A InSAR: Relation to Seasonal Groundwater Extraction and Rainfall in Central Taiwan
(2019-12-01)
Yang, Yi-Jie
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Hwangl, Cheinway
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Hung, Wei-Chia
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Fuhrmann, Thomas
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Chen, Yi-An
Extracting groundwater for agricultural, aquacultural, and industrial use in central Taiwan has caused large-scale land subsidence that poses a threat to the operation of the Taiwan High Speed Railway near Yunlin County. We detected Yunlin subsidence using the Sentinel-1A Synthetic Aperture Radar (SAR) by the Small BAseline Subset (SBAS) method from April 2016 to April 2017. We calibrated the initial InSAR-derived displacement rates using GPS measurements and reduced the velocity difference between the two sensors from 15.0 to 8.5 mm/a. In Yunlin's severe subsidence regions, cumulative displacements from InSAR and GPS showed that the dry-season subsidence contributed 60%-74% of the annual subsidence. The InSAR-derived vertical velocities matched the velocities from leveling to better than 10 mm/a. In regions with few leveling measurements, InSAR increased the spatial resolution of the vertical velocity field and identified two previously unknown subsidence spots over an industrial zone and steel factory. Annual significant subsidence areas (subsidence rate > 30 mm/a) from leveling from 2011 to 2017 increased with the declining dry-season rainfalls, suggesting that the dry-season rainfall was the deciding factor for land subsidence. A severe drought in 2015 (an El Nino year) dramatically increased the significant subsidence area to 659 km(2). Both InSAR and leveling detected similarly significant subsidence areas in 2017, showing that InSAR was an effective technique for assessing whether a subsidence mitigation measure worked. The newly opened Hushan Reservoir can supply surface water during dry seasons and droughts to counter rain shortage and can thereby potentially reduce land subsidence caused by groundwater extraction.
Time-Variant Adjustment for a Level Network
(2014-05-01)
Han, Jen-Yu
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Hwang, Cheinway
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Chou, Jun-Yun
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Hung, Wei-Chia
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土木工程學系
It has been routine work to determine the elevations of points on Earth's surface for a wide range of purposes. However, surveyors are now facing new challenges because of the increasing demand for high-quality measurements despite potential errors caused by Earth's dynamics, which introduce time-dependent signals in the points being investigated. Classical level network adjustment works well when benchmark elevations are known precisely and remain constant over time. However, the traditional approach fails to consider possible ground motions in a single process of network adjustment. This study proposes a time-variant model and unified least-squares technique for level network adjustment. The main emphasis of this approach is to accommodate the time-dependent behaviors of benchmarks caused by ground motions and other unknown sources. The numerical results from a simulated test and a real case study show that the proposed approach can capture the time-dependent signals of benchmarks. Consequently, significant perturbations in benchmarks can be identified, and an improved analysis for level network adjustment thus can be achieved.
Time-varying land subsidence detected by radar altimetry: California, Taiwan and north China
(2016-06-21)
Hwang, Cheinway
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Yang, Yuande
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Kao, Ricky
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Han, Jiancheng
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Shum, C. K.
Contemporary applications of radar altimetry include sea-level rise, ocean circulation, marine gravity, and icesheet elevation change. Unlike InSAR and GNSS, which are widely used to map surface deformation, altimetry is neither reliant on highly temporally-correlated ground features nor as limited by the available spatial coverage, and can provide long-term temporal subsidence monitoring capability. Here we use multi-mission radar altimetry with an approximately 23 year data-span to quantify land subsidence in cropland areas. Subsidence rates from TOPEX/POSEIDON, JASON-1, ENVISAT, and JASON-2 during 1992-2015 show time-varying trends with respect to displacement over time in California's San Joaquin Valley and central Taiwan, possibly related to changes in land use, climatic conditions (drought) and regulatory measures affecting groundwater use. Near Hanford, California, subsidence rates reach 18 cm yr(-1) with a cumulative subsidence of 206 cm, which potentially could adversely affect operations of the planned California High-Speed Rail. The maximum subsidence rate in central Taiwan is 8 cm yr(-1). Radar altimetry also reveals time-varying subsidence in the North China Plain consistent with the declines of groundwater storage and existing water infrastructure detected by the Gravity Recovery And Climate Experiment (GRACE) satellites, with rates reaching 20 cm yr(-1) and cumulative subsidence as much as 155 cm.