Saltwater intrusion is a growing threat for coastal aquifers and agricultural practices in low-lying plains. Most of the farmlands located between the margin of the Southern Venice lagoon and the Northern Po delta, Italy, lie a few meters below mean sea level and are drained by a large network of artificial channels and hydraulic infrastructures to avoid frequent flooding and allow agricultural practices. This work proposes an assessment of the vulnerability to saltwater intrusion, following a new concept of the hazard status, resulting in combining the depth of the freshwater/saltwater interface and the electrical resistivity of the shallow subsoil. The sensitivity of the farmland system was assessed by using ground elevation, distance from freshwater and saltwater sources, permeability, potential runoff, land subsidence, and sea-level rise indicators. Relative weights were assigned by a pairwise comparison following the Analytic Hierarchy Process approach. The computed vulnerability map highlights that about 30% of the farmlands is under strong and extreme conditions, 28% between marginal and moderate, and 40% under negligible conditions. Results from previous vulnerability assessments are discussed in order to explain their differences in terms of hazard status conceptualization and sensitivity characterization of farmland system.

Over the past decades coastal marshes around the world have declined dramatically. Their deterioration is controlled by scarcity of sediments, erosion and accelerated rise of relative sea-level. The feedbacks between these processes control marsh evolution and determine their long-term survivability. Aggradation of a marsh to keep pace with relative sea-level rise mainly depends on the interplay between sedimentation and autocompaction, but their interactions are severely understudied. Here we present an in-situ loading experiment applied in the Venice Lagoon, Italy, to assess long-term autocompaction, with subsurface displacements and pressure monitored during loading cycles, up to ∼40 kN applied on a ∼4 m² surface. Two identical experiments carried out in inorganic and organic soil-dominated marshes provided unique insights on the spatio-temporal subsurface dynamics. The large differences in behavior and maximum compaction (6 vs 32 mm) underscore the crucial role of autocompaction and soil heterogeneity when predicting the fate of coastal marshes worldwide.

Intertidal meanders developed on salt marshes are known to expand and produce inclined heterolithic stratification rich in fine‐grained sediments and to bear evidence for rhythmic deposition in the upper part of the inner meander bend (i.e. the upper part of the point bar). This occurs because the lower point‐bar deposits are washed by strong currents, which remove mud drapes and develop discontinuous record of tidal cycles. Although these criteria are widely accepted, facies models for tidal point bars still lack a three‐dimensional perspective and overlook the along‐bend variability of sediment distribution. The present study focuses on a hypertidal point bar belonging to the upper‐intertidal domain of the Mont‐Saint‐Michel Bay (France), and it analyses the sedimentology of a 3D time‐framed accretionary package formed between 28 March 2012 and 29 November 2012 by means of lidar topographic data, geomorphological field surveys and sedimentological core data. To define the 3D time‐framed accretionary package, data from thirteen lidar (light detection and ranging) topographic surveys were used to create the point‐bar synthetic stratigraphy. Data show that over the study period the point bar expanded alternating deposition along its seaward and landward sides, pointing out the occurrence of depositional patterns more complex than a simple progressive expansion of the bend. The thickest deposits were accumulated in the point‐bar‐apex zone, where the largest amount of mud was also stored. High sediment accretion in the bend‐apex zone is ascribed to the development of low‐energy conditions due to flow and bed configuration. High accretion rate of the point‐bar‐apex zone promoted also a better preservation of rhythmites, which are almost missing from deposits accumulated along the point‐bar sides. This study remarks that preservation of mud and tidal rhythmites within intertidal‐point‐bar deposits is controlled, not only by their elevation with respect to the tidal range, but also by their location along the point bar.