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

Abstract

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.