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Land subsidence severely threatens most of the coastal plains around the world where high productive industrial and agricultural activities and urban centers are concentrated. Coastal subsidence damages infrastructures and exacerbates the effect of the sea-level rise at regional scale. Although it is a well-known process, there is still much more to be improved on the monitoring, mapping and modeling of ground movements, as well as the understanding of controlling mechanisms. The International Geoscience Programme recently approved an international project (IGCP 663) aiming to bring together worldwide researchers to share expertise on subsidence processes typically occurring in coastal areas and cities, including basic research, monitoring and observation, modelling and management. In this paper, we provide the research communities and potential stakeholders with the basic information to join the participating teams in developing this project. Specifically, major advances on coastal subsidence studies and information on well-known and new case studies of land subsidence in China, Italy, The Netherlands, Indonesia, Vietnam and Thailand are highlighted and summarized. Meanwhile, the networking, dissemination, annual meeting and field trip are briefly introduced.

Coastal lagoons are highly valued ephemeral habitats that have experienced in many cases the pressure of human activities since the development of urbanisation and economic activities within or around their boundaries. One typical intervention is dredging of canals to increase the exchange of water with the sea or for navigation purposes. In order to divert the route of large cruise liners from the historic center of Venice, Italy, the Venice Port Authority has recently proposed a project for the dredging of a new 3-km long and 10-m deep navigation canal (called Marghera-Venice Canal, MVC, in the sequel) through the shallows of the Venice Lagoon. The MVC will connect the passenger terminal located in the southwestern part of the historic center to a main channel that reaches the industrial area on the western lagoon margin. Can the new MVC facilitate saltwater intrusion below the lagoon bottom? Can the release into the lagoon of the chemicals detected in the groundwater around the industrial site be favoured by the MVC excavation? Can the depression waves generated by the ship transit (known as ship-wakes) along the MVC affect the flow and contaminant exchange between the subsurface and surficial systems? A response to these questions has been provided by the use of uncoupled and coupled density-dependent groundwater flow and transport simulators. The hydrogeological modelling has been supported by an in-depth characterization of the Venice lagoon subsurface along the MVC. Geophysical surveys, laboratory analyses on groundwater and sediment samples, in-situ measurements through piezometers and pressure sensors, and the outcome of 3D hydrodynamic and computational fluid dynamic (CFD) models have been used to set-up and calibrate the subsurface multi-model approach. The modelling results can be summarized as follows: i) the MVC has a negligible effect in relation to the propagation of the tidal regime into the subsoil; ii) the depression caused by the ship transit along the MVC is responsible of a groundwater efflux from the canal bottom into the lagoon ranging up to 100 m3 per ship; iii) ship-wakes enhance the release of anthropogenic contaminants (As, Se, Cd, Hg, Pb, Cu, Cr) from the subsoil into the lagoon over few years after the canal excavation; iv) the MVC excavation cut the over-consolidated clay layer marking the Holocene-Pleistocene limit, thus favouring a localized salt contamination of the almost fresh-water aquifers located in the uppermost part of the Pleistocene series.

We present the result of a test aimed at evaluating the capability of RADARSAT-2 and COSMO-SkyMed to map the natural subsidence and ground movements induced by anthropogenic activities in the historical center of Venice. Firstly, ground movements have been retrieved at quite long- and short-term by the Persistent Scattered Interferometry (PSI) on 2008-2015 RADARSAT-2 and 2013-2015 COSMO-SkyMed image stacks, respectively. Secondly, PSI has been calibrated at regional scale using the records of permanent GPS stations. Thirdly, considering that over the last two decades "in the historical center of Venice" natural land movements are primarily ascribed to long-term processes, and those induced by human activities act at short-term, we have properly resampled 83-month RADARSAT-2 C-band and 27-month COSMO-SkyMed X-band interferometric products by a common grid and processed the outcome to estimate the two components of the displacements. Results show that the average natural subsidence is generally in the range of 0.9 - 1.1 mm/yr and the anthropogenic ground movements are up to 2 mm/yr.

From leveling to SAR-based interferometry, the monitoring of land subsidence has improved significantly. However, the accurate assessment of the ground movements by SAR in deltaic regions, where marshlands, coastal infrastructures, river embankments, farmlands, and urbanized areas coexist, is still a challenge. Usually, relatively small built-up zones and infrastructures are scattered within large natural and rural lands. In this contribution we present a multi-band SAR methodology to integrate COSMO-SkyMed and ALOS-PALSAR images. The method consists in a proper combination of the very high-resolution X-band Persistent Scatterer Interferometry (PSI), which achieves high-density and precise measurements on single structures and constructed areas, with the L-band Short-Baseline SAR Interferometry (SBAS), properly implemented to rise its effectiveness in retrieving information in vegetated and wet zones. The combined methodology is applied on the Po River Delta, northern Italy, using 16 ALOS-PALSAR and 31 COSMO-SkyMed images covering the period between 2007 and 2011. After a proper calibration of the single PSI and SBAS solution using available GPS records, the dataset have been combined at both the regional and local scales. The measured displacements range from ~0 mm/yr to -35 mm/yr. The results reveal the variable pattern of the subsidence characterizing the more natural and rural environments without losing the accuracy in quantifying the sinking of urban areas and infrastructures. Moreover, they allow improving the interpretation of the natural and anthropogenic processes responsible for the ongoing subsidence, helping to delineate the relative importance of the various drivers contributing to the cumulative subsidence.

In times of natural and anthropogenic climate change, tidal bio-geomorphic systems are most exposed to possibly irreversible transformations with far-reaching ecological and socio-economic implications. It is thus of critical importance to develop models for predicting the evolution of such systems under varying forcings and, if present, their dynamically-accessible stable states. The notion that freshwater and terrestrial ecosystems may switch abruptly to alternative stable states as a result of feedbacks between consumers and limiting resources is widely acknowledged. On the contrary, theoretical or observational proofs of the existence of alternative equilibrium states in intertidal ecosystems has until recently proven to be elusive. This is due to a prevalent reductionist approach, which has until recently mostly produced either purely ecological or purely geomorphological models, while the coupled dynamics of landforms and biota in the intertidal zone has remained largely unexplored. The presence and continued existence of tidal morphologies, and in particular of salt marshes, is intimately connected with biological activity, especially with the presence of halophytic vegetation. In fact, observations and models coupling geomorphological and biological processes indicate that vegetation crucially affects marsh equilibrium configurations through the production of organic soil, the capture of sediment, and the stabilization against erosion produced by wind waves. Often, different vegetation species live within very narrow elevation intervals, associated with similarly narrow ranges of environmental pressures, thus leading to the emerge of the zonation phenomenon. Here we present modeling analysis on the spatial distribution of geomorphological and vegetational spatial patterns in tidal landscapes arising as a result of two-way feedbacks between physical and biological processes. We challenge the traditional interpretation of zonation as a one--way relation between dominant processes in the intertidal frame (i.e. competition vs. edaphic controls), which fails to capture the active role played by vegetation in engineering its own environment. We use a point model of the coupled elevation-vegetation dynamics, which retains the description of the chief processes shaping these systems, to show how competing stable states are responsible for the formation of characteristic large-scale bio-geomorphic features in tidal landscapes worldwide. Our analyses extended to a one-dimensional context allows us to explore the mechanism that leads to the formation of well-known, smaller-scale patterns associated with marsh vegetation species distributions. We develop and present a model that for the first time incorporates species competition, species mutations, sediment transport and soil accretion in a spatially-extended setting, emphasizing that the formation of smaller-scale intertwined topographic and vegetation patterns are driven by bio-geomorphic feedbacks. We finally analyze the robustness of large-scale and marsh-scale bio-geomorphic features to changes in the forcings, with implications for marsh ecosystem resilience to climate change and anthropogenic pressures

As is the case with many deltas in the world, the Po Delta (Italy) and its barrier systems have experienced severe modifications over the last century as a consequence of anthropogenic subsidence and river input decrease. The interruption of subsurface fluid withdrawals in the early 1960s created an opportunity to evaluate the effects of the subsequent subsidence decrease along the 11 km of beaches and 45 km of barrier islands of the Po Delta, and improve the understanding of coastal system response to relative sea level rise. To this aim, shoreline and bathymetric multi-decadal dataset were coupled with the local rate of subsidence and classified with a geomorphic - response approach. From 1954-78, a mean shoreline variation of −224 m (−9.34 m y−1) occurred under high rates of subsidence (14–43 mm y−1) with a diffuse process of landward barrier rollover (up to 1124 m), frequently associated with break-up. This phase is evidenced by the diffuse sinking of the shoreface (up to 4 m), the negative upper shoreface sediment budget and the parallel drowning of the cross-shore profiles highlighted by the 1968–2008 bathymetric dataset. Despite the loss of altimetry and shoreline retreat being almost irrecoverable, during the period from 1978 to 1996 and 1996 to 2014 barrier islands experienced a stabilisation in response to a decrease in the subsidence rates down to 6 mm y−1. The mean rates of shoreline changes (−0.25 and + 1.51 m y−1 respectively) are the results of different local behaviour with progressively dominant longshore processes and progradation at the most active fluvial branches. The most recent bathymetric data (2008–2014) confirm this reversal trend, both in the altimetry differences and in the upper shoreface sediment budget. This trend is at odds with signals from many of the world's deltas and represents an important issue for the present and future coastal management and adaptation plans.

For the first time a comprehensive investigation has been carried out to quantify the possible effects of dredging a navigable canal on the hydrogeological system underlying a coastal lagoon. The study is focused on the Venice Lagoon, Italy, where the port authority is planning to open a new 10m deep and 3km long canal to connect the city passenger terminal to the central lagoon inlet, thus avoiding the passage of large cruise ships through the historic center of Venice. A modeling study has been developed to evaluate the short (minutes), medium (months), and long (decades) term processes of water and pollutant exchange between the shallow aquifer system and the lagoon, possibly enhanced by the canal excavation, and ship wakes. An in-depth characterization of the lagoon subsurface along the channel has supported the numerical modeling. Piezometer and sea level records, geophysical acquisitions, laboratory analyses of groundwater and sediment samples (chemical analyses and ecotoxicity testing), and the outcome of 3-D hydrodynamic and computational fluid dynamic (CFD) models have been used to set up and calibrate the subsurface multi-model approach. The numerical outcomes allow us to quantify the groundwater volume and estimate the mass of anthropogenic contaminants (As, Cd, Cu, Cr, Hg, Pb, Se) likely leaked from the nearby industrial area over the past decades, and released into the lagoon from the canal bed by the action of depression waves generated by ships. Moreover, the model outcomes help to understand the effect of the hydrogeological layering on the propagation of the tidal fluctuation and salt concentration into the shallow brackish aquifers underlying the lagoon bottom., Facultad de Ciencias Naturales y Museo, Centro de Investigaciones Geológicas