Overview of subsidence and drawdown
Fig. 3 displays the correlation between subsidence and variations in
groundwater level. The subsidence–drawdown relation is nonlinear
(correlation coefficient = 0.27, 0.54, 0.57, and 0.54, respectively, in
each of the four aquifer layers). Groundwater drawdowns in Layers 2–4
are similar (correlation coefficient = 0.84 in Layers 2 and 3; 0.78 in
Layers 2 and 4). Drawdowns in Layer 1 (unconfined aquifer) vary
spatially over shorter distances compared to those of the confined
aquifer (Layers 2–4). Figs. 1b and 4 show the spatial distribution of
annual observed subsidence and accumulated drawdown, respectively, for
2015, when the maximum annual
permanent subsidence is approximately 7 cm (Fig. 1b). The spatial
distribution of subsidence reveals a bowl in the central area of Yunlin
(Hwang et al., 2008). Within the study area, the maximum accumulated
drawdown in Layers 1–4 ranges from 15–20 m (Fig. 4). The cumulative
drawdowns implies that similar cones of depression are located in Layers
2–4 in the southwest distal fan area, near the coast. Drawdown cones
are located along the distal fan and mountain areas in Layer 1. Compared
with the four-layer drawdown pattern, the drawdown is greatest in Layer
2 in the study area. In addition, drawdowns in Layers 2–4 are highly
correlated (Fig. 3), while Layer 3 is most closely correlated to the
subsidence of the four-layer drawdown model (Fig. 3).
The compression behavior of clay agrees with Terzaghi’s consolidation
theory (Liu et al., 2004). Land subsidence with inelastic compression
continues as water levels continuously decline. The inelastic
compression of fine-grained aquifer interbeds is likely responsible for
the vast majority of subsidence problems, based on the compressibility
and total thickness of fine-grained deposits throughout the region
(Galloway et al., 1998b; Galloway and Sneed, 2013). The cone of maximum
drawdown occurs in the distal fan area, whereas the current major
subsidence bowl occurs inland.
Consequently, the cone of maximum
drawdown fails to fully coincide with the location of the subsidence
bowl (Erban et al., 2014). In the
coastal area or distal fan area, the rate of pumping is proposed to
decrease to stabilize or reduce water level declines. This subsequently
reduces land subsidence, given that the largest occurrence in the inland
area induced by heavy withdrawal of groundwater. This is a sensitive
environmental concern because the Taiwan High Speed Rail is constructed
through the central subsidence area, which might pose a serious threat
to its operation (Hwang et al., 2008; Tung and Hu, 2012).