Abstract
Abstract. Since the 1990s the land subsidence due to the rapid urbanization has been considered a severely destructive hazard in the center of Hanoi City. Although previous studies and measurements have quantified the subsiding deformation in Hanoi center, no data exist for the newly established districts in the south and the west, where construction development has been most significant and where groundwater pumping has been very intensive over the last decade. With a multi-temporal InSAR approach, we quantify the spatial distribution of the land subsidence in the entire Hanoi urban region using ALOS images over the 2007–2011 period. The map of the mean subsidence velocity reveals that the northern bank of the Red River appears stable, whereas some areas in southern bank are subsiding with a mean vertical rate up to 68.0 mm yr−1, especially within the three new urban districts of Hoang Mai, Ha Dong – Thanh Xuan and Hoai Duc – Tu Liem. We interpret the spatial distribution of the surface deformation as the combination of the nature of the unsaturated layer, the lowering of groundwater in the aquifers due to pumping withdrawal capacity, the increase of built-up surfaces and the type of building foundation. The piezometric level in Qp aquifer lowers particularly after 2008, whereas the groundwater level in Qh aquifer remains steady, even if it loses its seasonal fluctuation in urban areas and drawdowns in neighboring water production plants. The time evolution deduced from the InSAR time series is consistent with previous leveling data and shows that the lowering rate of the surface slightly decreases till 2008. The analysis of groundwater levels in instrumented wells shows a correlation between the behavior of groundwater with the urban development and the acceleration of groundwater withdrawal. Also, the time variations suggest that the deformation became non-stationary, with upward and downward transient displacements related to the charge and discharge of the aquifers.
Highlights
The phenomenon of land subsidence due to the underground exploitation has accelerated over the last decades because of the increasing demand of natural resources (Johnson, 1995)
Assuming that most of the measured deformation corresponds to vertical displacement of the surface due to the subsidence, we convert the velocity obtained along the satellite line of sight into vertical velocity using the incident angle (θ) at each measurement point
The northern part of the southern bank, corresponding to the urban districts of Tay Ho, Ba Dinh, Cau Giay, and Tu Liem suburban district, is clearly distinguishable with very little deformation, except along the Red River, where a surface subsidence occurs at a slow rate, lower than 10.0 mm yr−1
Summary
The phenomenon of land subsidence due to the underground exploitation has accelerated over the last decades because of the increasing demand of natural resources (Johnson, 1995). The ground deformation is unsettling when it occurs in large cities, where the recent intense urbanization implies growing water consumption with a significant pumping of the aquifers. This problem has been observed in many large cities such as Mexico City (Mays, 2009), Bangkok (Aobpaet et al, 2010), Shanghai (Lin et al, 2011), Venice (Brambati et al, 2003) and Las Vegas (Bell et al, 2002). Since the 1960s, a large number of studies on land subsidence have been carried out using several methods of investigation (Poland, 1984; Galloway et al, 2000; Gourmelen et al, 2007) for a better understanding of the involved mechanisms (Holzer, 1984; ZhenDong and Ya-Jie, 2012) and its environmental consequences. The measurement techniques quantifying the ground deformation related to ground subsidence vary from in situ techniques such as leveling (Bitelli and Russo, 1991) and extensometry (Ireland et al, 1984; Poland, 1984) to spatial techniques such as static global positioning system (Ikehara, 1994), radar interferometry (Massonnet and Feigl, 1998) and airborne lidar (Froese and Mei, 2008)
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