In 2020, the maximum annual subsidence rate in Chiayi area is 2.9 cm/year (located in Xikou Township), and the significant subsidence area is 0.0 square kilometers. According to the results of the 109-year leveling inspection, the maximum annual subsidence rate in Chiayi area was lower than that in 2019. The main sag detection points in Chiayi area in 2020 are shown in Table 2; the average sag rate from 2019 to 2020 is shown in Fig. 2.

The subsidence area in Chiayi area can be divided into coastal and inland areas. The coastal areas are Dongshi Township, Budai Township and Yizhu Township. The main pumping industry is aquaculture and fishery. According to the data of subsidence monitoring wells, recent strata compression in coastal areas The depth is mainly deep compression, and the main compression range is between 125 and 300 meters; while the inland areas are Xingang Township, Liujiao Township, Xikou Township and Puzi City, and the stratum compression depth is shallow compression. Mainly, the main compression range is between the surface and the depth of about 150 meters. The relevant analysis of each major subsidence area is described below.

1.Dongshi Township:

(1)The internal G056 of the main inspection points and the calendar chart of the water Jia 18 show that the area has the most serious subsidence in 1993. The subsidence rate of internal G056 reached 9.0 cm/year, and the subsidence rate slowed down in 2013 and 2014, and in 2015 and 2018 Because of the poor water conditions that year, the subsidence increased significantly to more than 3 cm/year, and gradually slowed down in 2019 and 2020. In 2020, the subsidence rates of internal G056 and Shuizijia 18 were 1.0 and 1.3 cm/year, respectively.

(2)Analyzing the monitoring well data of the small sag in Dongshi Country, Dongshi Township, the main compression range in this area is between the depth of 0-37 and 126-274 meters. The analysis of the relationship between the groundwater level and the amount of compression shows that there is plastic deformation in the amount of compression from aquifer 2 to aquifer 4, indicating that the soil is still being compacted.

2.Budai Town:

(1)The main subsidence points in the analysis area, Jia 45 and Shuili Jia 56A, show that the detection point Jia 45 has a significant subsidence in 2015 and 2018, with a maximum subsidence rate of 4.4 cm/year, which will gradually slow down after 2019. In 2020, the rate of subsidence will be reduced to 1.1 cm/year; in recent years, Shuilijia 56A has experienced significant subsidence (3.8 cm/year) in 2018, and gradually slowed down in 2019 and 2020. The rate of subsidence in 2020 is 2.1 cm/year.

(2)Analyzing the data from the monitoring wells of the Budai Elementary School in Budai Town, the main compression areas in this area are concentrated in the mud and fine sand layers between the depths of 7 to 54 meters, 152 to 194 meters, and 257 to 284 meters. The analysis of the relationship between the groundwater level, the amount of compression and the time shows that the formation changes with the groundwater level, and when the local water level drops, the formation still produces plastic deformation.

3.Yizhu Township:

(1)The main subsidence points in the analysis area, Shuizijia 09 and CYWRA13, show that Shuizijia 09 had the most serious subsidence in 2004, with an annual subsidence rate of 5.5 cm/year, and the subsidence rate slowed down year by year from 2010 to 2014. In 2015, the rate of subsidence increased to 3.0 cm/year. From 2016 to 2019, the rate of subsidence was approximately 1.1-3.5 cm/year. In 2020, the rate of subsidence decreased to 1.0 cm/year; CYWRA13 has been significantly more recent in 2018 The subsidence (3.8 cm/year) will gradually slow down in 2019 and 2020, and the subsidence rate in 2020 will be 2.1 cm/year.

(2)Analyze the data from the monitoring wells of the Nanxing National Small Depression in Yizhu Township. The main compression ranges are concentrated in the mud and sand layers with a depth of 202-286 meters. The analysis of the relationship between the groundwater level, the amount of compression and the time shows that the formation changes with the groundwater level, and when the local water level drops, the formation still has plastic deformation, which means that the formation will continue to be compressed.

4.Xingang Township and Liujiao Township:

(1)Analysis of the main subsidence points of Xingang Township, Shuilijia 49 and Liujiao Township, the main subsidence points of Shuilijia 42. The calendar chart shows that the detection point of Shuilijia 49 has a significant subsidence in 2015, 2017 and 2018, in the past 3 years. The subsidence rate is mostly 2 to 3 cm/year, and the subsidence rate in 2020 is 1.9 cm/year; Shuilijia 42 has a significant subsidence in 2010, 2011 and 2017, and the subsidence rate has decreased year by year in the past 3 years, and it will sink in 2020. The speed is 1.6 cm/year.

(2)According to the data from the sag monitoring wells of the Anhe Elementary School in Xingang Township, compression occurs from the surface to the depth of 186 meters, and the main compression range occurs in the sand and mud layers with a depth of 39 to 156 meters. The Anhe Groundwater Level Observation Station mainly has 4 deep groundwater level observation wells, covering 3 aquifers (aquifer 1, 2 and 4), with well depths of 59, 96, 164, and 285 meters, respectively. Among them, the groundwater level of 96 and 164 meters has a significant downward trend year by year, indicating that there is concentrated pumping in aquifer 2, which leads to stratum subsidence. Since the main water industry in this area is agriculture, it is necessary to pay attention to the use of agricultural water in this area in the future and adjust The area of ​​arable land and the amount of underground water taken to further slow down the subsidence of the ground.

5.Puzi City:

The internal I042 and Shuilijia 64 calendar diagrams of the main subsidence points in the analysis area show that the subsidence rate decreased year by year from 2007 to 2014. In 2015, the subsidence rate exceeded 3 cm/year due to poor water conditions, and the subsidence rate was approximately approximately from 2016 to 2019. Between 1.2-2.4 cm/year and 0.2-2.6 cm/year, the sinking rate in 2020 will be 1.7 and 1.5 cm/year respectively.

1.Calculate the leveling results of Chiayi area in the past 29 years. According to the cumulative subsidence statistics drawn every 10 years, as shown in Table 5, it shows that the cumulative change in the three different stages of the Chiayi area has gradually decreased, indicating that the stratum subsidence in the Chiayi area has significantly improved .

2.Analysis of the leveling results from May 2019 to May 2020 shows that the maximum annual subsidence rate in Chiayi area is 2.9 cm/year. It is located in Xikou Township with a significant subsidence area of ​​0 square kilometers; the main subsidence locations are divided into coastal and internal areas. In land areas, the coastal areas are mainly in Dongshi Township, Budai Township and Yizhu Township, and the inland areas are mainly subsided in Xikou Township, Xingang Township, Liujiao Township and Puzi City.

3.According to the data of sag monitoring wells, the main compression depths of Chiayi inland areas (Liujiao and Xingang) are within 180 meters of the stratum, while the main compression depths of coastal areas (Dongshi, Budai and Yizhu) are between 125 and 300. Within meters, after field surveys, the main land use in Chiayi's inland areas is agricultural areas, and the main use types in coastal areas are fish farming areas.

4.According to the data from the sag monitoring well of the Anhe Elementary School in Xingang Township, compression occurs in the stratum from the surface to a depth of 186 meters, and the main compression range occurs in the sand and mud layers at a depth of 39 to 156 meters. The groundwater level at the depth of 96 and 164 meters at the Anhe Groundwater Level Observation Station has a significant downward trend year by year, indicating that there may be concentrated pumping in the second aquifer, which may lead to stratum subsidence. Since the main industry in this area is an agricultural area, attention needs to be paid in the future The use of agricultural water in this area, the regulation of the area of ​​arable land and the amount of underground water withdrawal, to further slow down the subsidence of the ground.

The formation compression behavior in the Chiayi area continues to occur, and the long-term average compression rate of the monitoring wells in the other sags in the area is greater than 1 cm except for the long-term average compression rate of the monitoring wells in the Xinpi National Subsidence is less than 1 cm/year /Year (compression rate ranged from 1.3 to 3.1 cm/year), of which the Wangliao small sag monitoring well in the coastal area is the most significant, with a long-term average compression rate of 3.1 cm/year; and the recent period from December 2014 to 2020 The stratum compression behavior in October of 2010 is most obvious in the Anhe Elementary School adjacent to the high-speed rail, with an average annual compression rate of 2.0 cm/year, followed by Budai Elementary School and Nanxing Elementary School in the coastal area. , The annual average compression rate is between 1.5 and 1.9 cm/year, and the annual average compression rate of monitoring wells in other regions is between 0.8 and 1.4 cm/year.

 

An analysis and comparison of the elevation results of the inspection points in Chiayi County with the results of the level inspections over the years are as follows:

1.Analysis of the subsidence of Chiayi area from May 2019 to May 2020

Analysis of the average subsidence rate in Chiayi area from May 2019 to May 2020 is shown in Figure 3-44. The maximum annual subsidence rate is 2.9 cm/year (located in Xikou Township), and the area of significant subsidence (sagging more than 3 cm/year) is 0.0 square kilometers. The map of the average subsidence rate from 2018 to 2019 and 2019 to 20202 is shown in Figure 3-45, which shows that the main subsidence locations in Chiayi in the past two years have occurred in Dongshi Township, Budai Township, Yizhu Township, Xikou Township, Xingang Township, and Liuzhou Township. In Jiaoxiang and Puzi City, the subsidence rate is about 1 to 3 cm/year. Table 3-28 is the analysis table of the main detection points of the main subsidence towns in the Chiayi level survey line in 2020.

The following is an analysis of the subsidence of the aforementioned coastal towns and towns such as Dongshi Township, Budai Township and Yizhu Township, Puzi City, Xingang Township and Liujiao Township in the inland high-speed rail neighboring areas, and Tainan North Gate:

2.Analysis of cumulative subsidence in Chiayi area from 1991 to 2000, 2001 to 2010 and 2011 to 2020

Applying the leveling results of Chiayi area from 1991 to 2010, analyzing the changes of subsidence every 10 years, the explanation is as follows:

(1)Cumulative change in Chiayi area from 1991 to 2000

Analyze the cumulative change of Chiayi area from 1991 to 2000 as shown in Figure 3-59. During the above period, Dongshi Township and Budai Town were the main subsidence centers in Chiayi area, and the subsidence area was mainly west of Provincial Highway No. 19 (west of Puzi City). ). Before 1991, the area of ​​significant subsidence in Chiayi remained above 190 square kilometers, and the maximum cumulative subsidence reached 93.4 cm, mainly in Dongshi Township.

(2)Cumulative changes in Chiayi area from 1991 to 2010

Analyze the cumulative changes in Chiayi area from 1991 to 2010 as shown in Figure 3-60. During the above period, Dongshi Township, Budai Township, and Yizhu Township were the main subsidence centers in Chiayi area. At the same time, it can be seen that Xingang Township also subsided along the high-speed railway. . During this period, it can be seen that the prevention and control of stratum subsidence began to take effect gradually. The area of ​​significant subsidence dropped rapidly to less than 30 square kilometers from 1996 to 2011. In 2010, due to the obvious drought in that year, the area of ​​significant subsidence increased to 198 square kilometers. During this period, the maximum accumulated subsidence reached 63.8 cm, which mainly occurred at the junction of Dongshi Township and Budai Town.

(3)Cumulative changes in Chiayi area from 2011 to 2020

Analyzing the cumulative change of Chiayi region from 2011 to 2020 is shown in Figure 3-61. During the above period, the Chiayi region was mainly subsided by Dongshi Township, Budai Township, Yizhu Township in the coastal areas, and Xingang Township and Liujiao Township inland. During this period, concrete results have been achieved in the prevention and control of stratum subsidence. During the period from 2012 to 2020, the area of ​​significant subsidence was reduced to 0 square kilometers most of the time. Only in 2015 and 2018, because of the poor water conditions in that year, Therefore, the area of ​​significant subsidence increased to 90.9 and 114.2 square kilometers. The maximum accumulated subsidence during this period was 31.6 cm, which mainly occurred in Dongshi Township and Budai Township.

3.Analysis of cumulative subsidence in Chiayi area from 1991 to 2020

Based on the comparative analysis of measurement data from 1991 to 2020, the total subsidence in Chiayi area in the past 29 years has been above 60 cm. The subsidence area covers Dongshi Township, Budai Township, Puzi City, Yizhu Township and Liujiao Township; The main subsidence centers are Dongshi Township and Budai Township, and the largest cumulative subsidence area occurs in Dongshi Township. The analysis of the area of subsidence in Chiayi area from 1991 to 2020 is shown in Figure 3-62, and the equivalent distribution of accumulated subsidence is shown in Table 3-35 and Table 3-36.