Processes governing natural land subsidence in the shallow coastal aquifer of the Ravenna coast, Italy
Introduction
Subsidence is one of the most diverse forms of downward settling of the ground with little horizontal movement, ranging from small or local collapses to broad regional lowering of the earth surface. It is a global problem and the principal causes are aquifer-system compaction (Sneed and Galloway, 2000), dewatering (oxidation) of peat or organic soils (Grzywna, 2017; Zanello et al., 2011), underground mining (Dong et al., 2015; Ishwar et al., 2017), hydrocompaction and sinkholes (Psimoulis et al., 2007; Yechieli et al., 2016), crustal deformation, withdrawal of fluids (groundwater, hydrocarbons, geothermal) (Donaldson et al., 1995), and thawing permafrost (Liang et al., 2006).
The relationships between land subsidence and fluid withdrawal from deep confined aquifers is extensively described in the literature (Corapcioglu, 1984; Domenico and Mifflin, 1965; Galloway et al., 1999; Poland and Davis, 1969; Schmidt and Bürgmann, 2003; Sun et al., 1999) and many analytical (Calderhead et al., 2011; Liu and Helm, 2008; Tarn and Lu, 1991) and numerical modelling (Baú et al., 2004; Galloway and Burbey, 2011; Gambolati et al., 1996) studies were done to identify the physical processes involved. Natural subsidence processes connected to the compaction of sediments or tectonic phenomena were also extensively treated in the literature (Amelung et al., 1999; Gambolati and Teatini, 1998; Gambolati et al., 1999; Carminati et al., 2003). On the other hand, very few studies consider the relationships between water table fluctuations and natural land subsidence (Chang et al., 2004; Strack et al., 2008). To the best of our knowledge, no high-resolution continuous time series of land subsidence and water table levels were ever presented and interpreted in the literature.
By using data from a high-resolution settlement gauge, our present work aims to define the processes governing shallow ground settlement (magnitude and development over time since the installation of the instrument) and verify land subsidence and water table fluctuations interactions in the shallow coastal aquifer of Ravenna. Our work highlights the contribution of natural processes such as primary consolidation (compaction of sediments under their own weight via expulsion of interstitial pore water) and water table fluctuations in the Holocene shallow coastal aquifer of Ravenna to the cumulative land subsidence rate observed in the area (Bertoni et al., 1995; Teatini et al., 2005; Baldi et al., 2009). The processes in the shallow coastal aquifer are uncoupled from other processes contributing to the cumulative land subsidence rate and that are primary and secondary consolidation of deep aquifers (Teatini et al., 2011), fluids extraction from reservoirs (Teatini et al., 2006), tectonism, and isostasy (Carminati et al., 2003). The methodology we used includes the decomposition of data time series (subsidence-settlement, water table level, precipitation, drainage, sea level) into the trend, seasonality and noise components and find correlation coefficients between each analyzed component, especially between settlement and changes in water levels. Analytical solutions are also applied to model the irrecoverable and elastic components of land subsidence.
Section snippets
Study area
The area addressed by the present study includes the coastal area of the Ravenna city, in the Emilia-Romagna coastland, south of the Po River Delta (Northeastern Italy, Fig. 1). It is a lowland coastal area not exceeding 2 m above sea level (a.s.l.), with a large portion below mean sea level, because of the combined effects of natural and anthropogenic land subsidence, land reclamation, and sea level rise. In the study area, the trend in sea level rise during the time period 1990–2011 was
Time series analysis
Daily and monthly correlation coefficients calculated for time series dataset are listed in Table 2 and Table 3 as supporting information. The time series data for the most relevant (and correlated) parameters recorded (settlement [mm], water table level [m], drainage [mm], sea level [m]) are shown in Fig. 4. The results of the decomposition of the settlement daily time series are presented in Fig. 5a, b, and Fig. 6. Fig. 5a represents the seasonal component of the settlement, water table level
Discussion
The data we present and the methodology used help to constrain the processes contributing to natural land subsidence in the shallow coastal aquifer of Ravenna, which suffered extreme anthropogenic land subsidence in the past century because of gas and water exploitation (Bertoni et al., 1995; Teatini et al., 2005). This is important, because it allows to separate and quantify the effects of natural processes (primary consolidation, water table fluctuations, etc.) in the recently deposited
Conclusions
The natural land subsidence rate in the Holocene sediments of the shallow coastal aquifer of Ravenna (North eastern Italy) measured in this study accounts for 10–20% of the total land subsidence rate observed in the Ravenna area (10–20 mm/year).
Modelling and time series analysis of natural land subsidence and water table fluctuations (as well as parameters such as rainfall, drainage, and sea level) highlights three deformation components connected to the vertical ground motion: an elastic, a
Acknowledgments
The authors would like to thanks the IGRG Lab people (Integrated Geosciences Research Group) of the University of Bologna for the help during the field work activities and the Carabinieri for Biodiversity, Punta Marina Office, who allowed the access to the protected natural areas where the instruments were installed.
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