Federica Rizzetto

Morphostratigraphic framework of the Venice Lagoon (Italy) by very shallow water VHRS surveys: Evidence of radical changes triggered by human‐induced river diversions

[1] This study is mainly based on a wide Very High Resolution Seismic (VHRS) survey that utilized an ad hoc technique designed for investigations in very shallow waters (about 1 m depth). This method allowed the acquisition of excellent images of the subsurface down to 15-20 m b.s.1. with a resolution of about 10 cm. Buried geomorphological features, such as fluvial channel-levee systems and tidal channels, were imaged for the first time in the shallows and provided new insight into the Holocene evolution of the southern lagoon basin. Furthermore, the new seismic data were used to reconstruct the morphostratigraphic framework of the Venice Lagoon. We provide an Upper Quaternary morphostratigraphic model of the Venice Lagoon and present some evidence of radical changes resulting from human-induced river diversion in the sedimentary regime and in the morphological setting of the southern basin that has occurred over the last millennium.

Peat land oxidation enhances subsidence in the Venice watershed

The southernmost part of the Venice Lagoon catchment was progressively reclaimed from marshland starting from the end of the 19th century and finishing in the late 1930s (Figure 1). As a major result, the area was turned into a fertile farmland. At present, the area is kept dry by a distributed drainage system that collects the water from a capillary network of ditches, and pumps it into the lagoon or the sea. By its very origin, this area lies below sea level and progressively sinks mainly because of bio-oxidation of the histosols (soils with high organic content) that represent a large fraction of the outcropping soil in the area. The bio-oxidation process occurs in close connection with the agricultural practices and is currently responsible for a subsidence rate of between 1.5 and 2 cm/yr.

Aptitude of modern salt marshes to counteract relative sea-level rise, Venice Lagoon (Italy)

In the past 100 yr, variations in relative sea-level rise (RSLR), the increase of frequency of very high tides, and a decrease of sediment availability caused progressive morphological changes of the Venice Lagoon tidal flats. In contrast with the general erosional trend, some salt marshes in the northern lagoon preserved their main original characteristics and showed accretion and development of the tidal creek network. The evolution of one of them was sketched by the interpretation of ultrahigh-resolution aerial photographs taken from A.D. 1938 to 2006, and results were compared with RSLR rates and storm tide frequency. The long-term investigation pointed out the most significant morphological changes that occurred over the entire period (i.e., erosion of the margin, modifications of the drainage network), whereas the short-term analysis showed in detail the subsequent phases of salt marsh evolution and their relations with sea-level variations. Margin shift was mainly in agreement with RSLR trend, whereas changes of the tidal creek network also reflected frequency of very high tides. Moreover, in the past 70 yr the salt marsh demonstrated a self-renewal aptitude to counteract RSLR: even if it underwent low accretion rates, it did not disappear, probably because the remobilization of sediments eroded from the marsh front and the lagoon bottom by tides and other local hydrodynamic processes, and their accumulation on the marsh surface favored by vegetation were sufficient to offset RSLR.

Soil contamination and land subsidence raise concern in the Venice watershed, Italy

The southern catchment of the Venice watershed is threatened by shallow aquifer salinization and anthropogenic land subsidence due primarily to the microbial oxidation of organic soils that outcrop in the coastal farmland reclaimed from the Adriatic Sea over the last century. Recent hydrogeological and geophysical surveys provide documentary evidence that saltwater intrusion may extend inshore up to 20 km away from the Adriatic coastline with the contaminant plume from near ground surface down to 100 m depth in some areas. The actual salt distribution is the outcome of a number of factors, including the ground elevation markedly below the mean sea level (down to -4 m locally), the seawater encroachment along the final 10-15 km of the regional watercourses (Brenta, Bacchiglione, Adige, Gorzone), and the drainage practices implemented in the reclaimed area. The fresh-salt water interface is generally between 2 and 30 m deep and exhibits a pronounced seasonal variation. At the same time an ongoing settlement due to peat oxidation promoted by farming activities is observed in most of the area south of the Venice Lagoon that was reclaimed from 1892 to 1967 and is rich in organic matter. Overall land subsidence over the last 70 years ranges between 1.5-2 m and is still in progress at a rate of 1.5-2 cm/y. As a major result a large fraction of the reclaimed land lies below the mean sea level with an increasing exposure to flooding during severe winter storms and saltwater intrusion from the Adriatic Sea, the nearby Venice Lagoon, and the river beds that locally lie above the surrounding ground surface. To mitigate both hazards the implementation of a drainage strategy of the reclaimed area intended to maintain the water table as high as possible would be required. This could decelerate the peat oxidation (hence the related land subsidence) and oppose the inland subsurface salt convection and dispersion. Moreover the design of mobile gates at a few river mouths (e.g. the Brenta river) could create an effective barrier against the seawater migration upstream the watercourses in the hottest and driest summers.

Use of remote sensing for the delineation of surface peat deposits south of the Venice Lagoon (Italy)

The aim of the present study is to map the regional extent of the peat soils using remote sensing. This is a first step of a wider project focused on the simulation of land subsidence due to the oxidation of organic soils in an area, the Zennare Basin, located south of the Venice Lagoon, where this process is responsible for a settlement rates of the order of 2-3 cm/year. Landsat 7 ETM+, Aster and Ikonos scenes have been analyzed using the Principal Component Analyses (PCA), the Soil Brightness Index (SBI) calculated through a Tasseled Cap Rotation, and supervised classifications based on the Maximum Likelihood and Spectral Angle Mapper (SAM) algorithms. The results, calibrated on field investigations (geomorphological surveys and sediment sampling), allowed the classification of the outcropping sediments and the production of an accurate geomorphological map.

High resolution multibeam and hydrodynamic datasets of tidal channels and inlets of the Venice Lagoon

Tidal channels are crucial for the functioning of wetlands, though their morphological properties, which are relevant for seafloor habitats and flow, have been understudied so far. Here, we release a dataset composed of Digital Terrain Models (DTMs) extracted from a total of 2,500 linear kilometres of high-resolution multibeam echosounder (MBES) data collected in 2013 covering the entire network of tidal channels and inlets of the Venice Lagoon, Italy. The dataset comprises also the backscatter (BS) data, which reflect the acoustic properties of the seafloor, and the tidal current fields simulated by means of a high-resolution three-dimensional unstructured hydrodynamic model. The DTMs and the current fields help define how morphological and benthic properties of tidal channels are affected by the action of currents. These data are of potential broad interest not only to geomorphologists, oceanographers and ecologists studying the morphology, hydrodynamics, sediment transport and benthic habitats of tidal environments, but also to coastal engineers and stakeholders for cost-effective monitoring and sustainable management of this peculiar shallow coastal system.

Shallow Coastal Landforms

Shallow coastal landforms are often highly dynamic environments due to natural and anthropogenic pressure. The action of waves, tidal currents, rivers inputs, sea-level rise, climate, geology and coastal engineering shapes their morphology at different temporal and spatial scales. The recent technological development of the multibeam echosounder, LiDAR and satellite systems now permits the mapping of shallow coastal landforms at very high resolution, even for depths shallower than 10 m, providing improved understanding of these morphological features. In this chapter, we provide a review of the main shallow coastal submarine landforms and of the newest methods to map them and measure their changes over time.