Multi-component and multi-source approach to model subsidence in deltas. Application to Po Delta Area

This thesis focused on the definition of a study approach able to deal with the complexity of the land subsidence phenomenon in deltas. In the framework of the most up- to-date multi-methodological and multi-disciplinary studies concerning land subsidence and targeting to predict and prevent flooding risk, the thesis introduces a procedure based on two main innovations: the multi-component study and the multi-source analysis. The proposed approach is a “multi-component” procedure as it investigates, in the available geodetic datasets, the permanent component apart from the periodic one, and, at the same time, it is a “multi-source” approach because it attempts to identify the relevant processes causing subsidence (sources) by a modelling based on multi-source data analysis. The latter task is accomplished first through multi-disciplinary and multi-methodological comparative analyses, then through modelling of the selected processes. With respect to past and current approaches for studying subsidence phenomena, the developed procedure allows one to: i. overcome the one-component investigation, improving the accuracy in the geodetic velocity estimate; ii. fix the “analyses to modelling” procedure, enhancing qualitative or semi-quantitative procedures that often characterize the “data to source” and the “residual to source” approaches; iii. quicken the source validation phase, accrediting the relevance of the source on the basis of the analysis results and before the modelling phase, differently from the “peering approach”, which validates the source on the basis of the model findings. The proposed procedure has been tested on the Po Delta (northern Italy), an area historically affected by land subsidence and recently interested by accurate continuous geodetic monitoring through GNSS stations. Daily-CGPS time series (three stations), weekly- CGPS time series (two stations) and seven sites of DInSAR-derived time series spanning over the time interval 2009 – 2017 constituted the used geodetic datasets. Several meteo/hydro parameters collected from fifty-seven stations and wide stratigraphic-geological information formed the base for the performed comparative analyses. From the application of the proposed procedure, it turns out that the periodic annual component highlighted in the continuous GPS stations is explained by two water mass-dependent processes: soil moisture mass change, which seems to control the ground level up-or-down lift in the southern part of the Delta, and the river water mass change, which influences the ground displacement in the central part of the Delta. As it concerns the permanent component, the lower rate found over 2012 - 2016 period in the central part of the Delta with respect to the eastern part is interpreted as due to the sediment compaction process of the Holocene prograding sequences and to the increase of rich-clay deposits

Importance of longevity, growth, and diapause in the evolution of asellus aquaticus

In Asellus aquaticus it was shown that: (1) populations of South Europe have embryos/larvae resistant to 4 °C, a life span of 9 months, small body size, and lack of reproductive diapause; (2) populations of North Europe have a life span of 20 months, large body size, long reproductive diapause, and embryos/larvae not resistant to 4 °C; (3) life span and reproductive diapause are genetic characters; and (4) body size is determined by indeterminate growt

Isostatic rebound between the riverbanks and the discharge of the Po River (northern Italy) by wavelet coherence analysis of high-resolution remote sensing and discharge data

Land movements cause changes in topographic gradients which could favor flooding of urban, agricultural or coastal areas. Moreover, sea level rise linked to recent climate changes has increased awareness that the ground deformation, although slow over time, can become hazardous both for humans and environment. The understanding of the causes of land movements is a not easy task due to the complex relation between natural and anthropic mechanisms. In order to improve the knowledge of natural versus anthropogenic variability of vertical land movements of the Po River Delta (northern Italy), in this work advanced data analysis techniques were used for finding possible correlations between high-resolution remote sensing data and river discharge records. In particular, focus was placed on the isostatic rebound between the river banks and the discharge of the Po River within its Delta area. Daily ground deformation measurements obtained with Continuous Global Positioning System (CGPS) were used, referring to the stations of Porto Tolle (44°57'07.2"N - 12°20'02.4"E) and Taglio di Po (45°00'10.8"N - 12°13'40.8"E) between 2012 and 2015, and the daily Po River discharge record at Pontelagoscuro (44°53'19.68"N - 11°36'29.52"E). It has been found that the vertical displacement functions between the two river bank stations, which are located at about 10 km apart, are strongly correlated. This suggests that the Po riverbanks in this area could be characterized by similar dynamics. On average, CGPS measurements indicate a downward movement of about 4 mm/yr. The use of cross wavelet and wavelet coherence analysis between the CGPS data and the Po River discharge data put in evidence that these records present strong negative correlation between 3- to 12-month scales. In particular, three main correlated frequency ranges with periods of 3 months, 4 - 6 months and 8 - 14 months were identified. These ranges correspond to the seasonal, semiannual and annual natural meteorological variations observed in the entire Po River area. The found negative correlations indicate that the increase/decrease of the river discharge induces the simultaneous decrease/increase of the near riverbank. The observed oscillations have an amplitude of a few millimeters (at most 5 mm). A possible cause of this regular seasonal oscillating movement could be an isostatic rebound of the riverbank in response to the change of water pressure on the river bed, which is directly related to the river discharge volume

Multi-Component and Multi-Source Approach for Studying Land Subsidence in Deltas

International audienceThe coupled effects of climate change and land sinking make deltas and coastal areas prone to inundation and flooding, meaning that reliable estimation of land subsidence is becoming crucial. Commonly, land subsidence is monitored by accurate continuous and discrete measurements collected by terrestrial and space geodetic techniques, such as Global Navigation Satellite System (GNSS), Interferometry Synthetic Aperture Radar (InSAR), and high precision leveling. In particular, GNSS, which includes the Global Positioning System (GPS), provides geospatial positioning with global coverage, then used for deriving local displacements through time. These site-positioning time series usually exhibit a linear trend plus seasonal oscillations of annual and semi-annual periods. Although the periodic components observed in the geodetic signal affect the velocity estimate, studies dealing with the prediction and prevention of risks associated with subsidence focus mainly on the permanent component. Periodic components are simply removed from the original dataset by statistical analyses not based on the underlying physical mechanisms. Here, we propose a systematic approach for detecting the physical mechanisms that better explain the permanent and periodic components of subsidence observed in the geodetic time series. It consists of three steps involving a component recognition phase, based on statistical and spectral analyses of geodetic time series, a source selection phase, based on their comparison with data of different nature (e.g., geological, hydro-meteorological, hydrogeological records), and a source validation step, where the selected sources are validated through physically-based models. The application of the proposed procedure to the Codigoro area (Po River Delta, Northern Italy), historically affected by land subsidence, allowed for an accurate estimation of the subsidence rate over the period 2009-2017. Significant differences turn out in the retrieved subsidence velocities by using or not periodic trends obtained by physically based models

Quantitative interpretation ofmultiple self-potential anomaly sources by a global optimization approach

Multiple self-potential (SP) anomalies are analyzed by using a Genetic-Price Algorithm (GPA), which has been recently introduced for the inversion of SP data. The proposed approach is tested on multiple synthetic anomalies, which are modeled by horizontal cylinders. First, a forward modeling is used to analyze the resolution of such anomalies by varying all model parameters. Then, GPA is applied to invert synthetic multiple SP anomalies. The numerical analyses show that the proposed approach is able to fully characterize the anomaly sources by providing the correct values of the model parameters as well as the number of sources, even if Gaussian random noise is added to the synthetic data. Furthermore, to show the computational efficiency of GPA, the results of a comparative analysis with the Very Fast Simulated Annealing algorithm are given. The validity of the GPA approach is confirmed by its application to three examples of self-potential field data from mineral exploration and groundwater investigations, which are presented and discussed in relation to other inversion approaches. Finally, the quantitative interpretation of multiple anomalies along a SP profile crossing the Mt. Somma-Vesuvius volcano caldera (southern Italy) is provided

Analysis of the Periodic Component of Vertical Land Motion in the Po Delta (Northern Italy) by GNSS and Hydrological Data

Nowadays, several methodologies, implemented for satellite or terrestrial surveys, reveal that daily and weekly site-positioning time series can exhibit linear trends plus seasonal oscillations. Such periodic components affect the evaluation of subsidence rates and, thus, they must be recognized and properly modelled. In this work, the periodic component of vertical land motion in Po Delta (Northern Italy) is estimated by a multi-component and multi-source procedure recently proposed by some of the authors for studying land subsidence in delta areas. First, land vertical motion data, acquired in the central part of the Po Delta over a six-year time interval, were compared with hydro-meteorological and climate datasets collected from nineteen stations distributed over the entire Delta. Then, four physically based models of the test site were implemented to verify the water pressure- and water mass-dependent processes inferred from the analytical phase. Modelling results show that the annual ground oscillation is better explained by soil moisture change, although river water mass variation gives a relevant contribution to land deformation, especially in the wet periods. Finally, to account for intra-annual processes, the joint contributions of all the inferred sources were treated as a nonlinear problem and solved applying the generalized reduced gradient method. The obtained combination is well supported by statistical parameters and provides the best agreement with the geodetic observations

Wavelet analysis of remote sensing and discharge data for understanding vertical ground movements in sandy and clayey terrains of the Po Delta area (Northern Italy)

Subsidence phenomena change the proneness of urban and coastal areas to huge flooding, particularly, in river delta regions. Many natural and/or anthropic processes can induce vertical ground movements; identifying the causes of the observed phenomena is a useful, even if not easy, task for flood forecasting. In order to improve the knowledge of natural versus anthropic contributions to vertical ground movements, in the present work data analysis techniques (polynomial regression analysis, frequency analysis and cross wavelet and wavelet coherence analysis) are used to analyze both the long- and short-time scale dependencies between Continuous Global Positioning System (CGPS) and variations of meteorological, hydrogeological and hydrological data collected in the Po River Delta area (Northern Italy) from January 2009 to December 2015. The main findings are as follows: (i) occurrence of positive correlations and periodic oscillations of about 2–3 years for all the analyzed meteorological, hydrogeological and hydrological records; (ii) a general decrease of the ground level with a faster soil lowering in clayey terrain with respect to sandy terrain; and, (iii) negative correlations between CGPS and Po River discharge records at the intra-annual scale (2–6 months), which are attributed to an isostatic rebound induced by the change of the river discharge due to precipitation changes over northern Italy