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

Abstract

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

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