Past area

Overview of Changhua Subsidence

1.Level network detection
In 2020, the maximum annual subsidence rate in Changhua area occurred in the inspection point 1109 and Shuilizhang 59 in Xizhou Township and C078 in Xihu Town. The subsidence rate is 3.2 cm/year, and the significant subsidence area is 0.9 square kilometers; the subsidence rate Townships exceeding 3 cm/year include Xizhou Township and Xihu Township.
2. Data analysis of landslide monitoring wells
Statistics of the leveling results of the Changhua area in the past 29 years, according to the cumulative subsidence statistics drawn every 10 years. For example, Table 2 shows that the cumulative change of the Changhua area in the three different stages has gradually decreased, indicating that the Changhua area has a significant improvement in the subsidence . Based on the analysis of the data from the sag monitoring wells, the recent stratum compression trend within 300 meters is more obvious in the inland areas than in the coastal areas. Among them, the Xinsheng Elementary School, Hunan Elementary School, Zhutang Workstation, Qiaoyi Elementary School and Xinghua Elementary School have been used for sag monitoring. The compression rate of the well is relatively large, between 1.6 and 2.0 cm/year, so it is necessary to continue to pay attention to the changes in the formation compression behavior of these areas in the future.
3.Leveling results
(1) Calculate the leveling results of the Changhua region in the past 29 years, and draw a cumulative subsidence statistics table every 10 years, as shown in Table 2. The table shows that the cumulative change in the Changhua region at three different stages is gradually decreasing, showing the Changhua region’s strata. The sag has been significantly improved.
(2) In 2020, the maximum annual subsidence rate in Changhua area is 3.2 cm/year (calculated from April 2019 to April 2020), the area of ​​significant subsidence is 0.9 square kilometers, and the subsidence rate is more than 3 cm/year for townships Xihu Town and Xizhou Township.
(3) In the past, the maximum subsidence rate of Xihu Town was 14.2 cm/year, and it has dropped to about 3 cm/year. In the long-term trend, the stratum subsidence rate has slowed down significantly, but the short-term trend will still be affected by climate and underground pumping. influences.
(4)This year, the maximum subsidence rate in Xizhou Township reached 3.2 cm/year, and the subsidence range covered some sections of the high-speed railway. According to the regression analysis of the results of leveling surveys and stratum subsidence monitoring wells in Changhua area in recent years, the subsidence rate measured by leveling measurement was greater than that of subsidence monitoring. The sinking rate measured by the well, therefore, it is speculated that there may still be compression below 300 meters in the near future. In the future, it is necessary to continue to track the local industrial water use and groundwater use conditions to further clarify the cause of the deep subsidence.
(5) The current stratum subsidence in Changhua area has been gradually controlled, and the main subsidence locations are Xihu Town and Xizhou Township. According to the monitoring data of the subsidence monitoring wells, the compression depth in Xihu Township is about 169-254 meters, while in Xizhou Township it is about 109-300 meters and below 300 meters.
4. Achievements over the years
A comprehensive review of the time course and regions for stratum subsidence detection over the past years (from 1991 to 2020). Until 2020, the annual maximum subsidence rate, maximum cumulative subsidence and significant subsidence area statistics in Changhua and Yunlin areas are listed in Table 1- 2 to Table 1-4.
4.Analysis of the subsidence of Changhua area from May 2019 to May 2020
The inspection results of the Changhua regional level in 2020 show that the townships in Changhua with a subsidence rate of more than 3 cm/year are Xihu Town and Xizhou Township. The maximum subsidence rate is about 3.2 cm/year, and the significant subsidence area is 0.9 square kilometers (as shown in the figure). As shown in 3-93); The map of the average subsidence rate from 2018 to 2019 and 2019 to 20209 (as shown in Figure 3-94) shows that the main subsidence centers in recent years are located in Xihu Town and Xizhou Township. The following will analyze the results of each inspection point in the main subsidence areas and key areas this year (as shown in Table 3-15), and explain the inspection results in each area.
5.Analysis of cumulative subsidence in Changhua area from 1992 to 2001, 2001 to 2011 and 2011 to 2020
Use the leveling results from 1992 to 2020 to analyze the rate of subsidence, the area of ​​significant subsidence, and the cumulative amount of subsidence. The results are shown in Figure 3-107 and Table 3-19. As shown in Figure 3-107, the total subsidence in Changhua has been more than 30 cm in the past two decades. The subsidence areas include Dacheng Township, Fangyuan Township, Erlin Township, Zhutang Township, Puyan Township, Xihu Township, and Pitou. Townships, Xizhou Townships and Puxin Townships, among which the largest accumulated subsidence area is Dacheng and Rural Areas.
Dacheng and rural areas were the most severely subsided towns in Changhua in the past. Although the rate of subsidence has slowed down significantly in recent years, the long-term cumulative subsidence has exceeded 210 cm. In the future, seawalls near Dacheng and rural areas should be the focus of monitoring. At the same time, attention should be paid. In typhoon seasons and during high and low tides, seawater intrusion may occur. In addition, from 1992 to 2020, the cumulative subsidence in Xizhou area has exceeded 80 cm. The long-term subsidence in this area may affect the operational safety of high-speed railways. Therefore, long-term monitoring and mitigation policies for this area Recommendations must still be continued.