Soil Accretionary Dynamics, Sea-Level Rise and the Survival of Wetlands in Venice Lagoon: A Field and Modelling Approach

Abstract

Over the past century, Venice Lagoon (Italy) has experienced a high rate of wetland loss. To gain an understanding of the factors leading to this loss, from March 1993 until May 1996 the soil accretionary dynamics of these wetlands were studied. Vertical accretion, short term sedimentation, soil vertical elevation change and horizontal shoreline change were measured at several sites with varying sediment availability and wave energy. Short term sedimentation averaged 3–7g drym−2day−1per site with a maximum of 76gm−2day−1. The highest values were measured during strong pulsing events, such as storms and river floods, that mobilized and transported suspended sediments. Accretion ranged from 2–23mmyr−1and soil elevation change ranged from −32 to 13·8mmyr−1. The sites with highest accretion were near a river mouth and in an area where strong wave energy resuspended bottom sediments that were deposited on the marsh surface. A marsh created with dredged spoil had a high rate of elevation loss, probably due mainly to compaction. Shoreline retreat and expansion of tidal channels also occurred at several sites due to high wave energy and a greater tidal prism. The current rate of elevation gain at some sites was not sufficient to offset relative sea-level rise. The results suggest that reduction of wave energy and increasing sediment availability are needed to offset wetland loss in different areas of the lagoon. Using the data collected as part of this project, we developed a wetland elevation model designed to predict the effect of increasing rates of eustatic sea-level rise on wetland sustainability. The advantage of this model, in conjunction with measured short-term rates of soil elevation change, to determine sustainability is that the model integrates the effects of long term processes (e.g. compaction and decomposition) and takes into account feedback mechanisms that affect elevation. Specifically, changes in elevation can result in changes in allogenic sediment deposition, decomposition and autogenic primary production. Model results revealed that, given the Intergovernmental Panel on Climate Change (IPCC) ‘best estimate’ eustatic sea-level rise scenario of 48cm in the next 100 years, only one site could maintain its elevation relative to sea level over the next century. Under the IPCC ‘current conditions’ scenario of 15cm in the next 100 years, four of seven sites remained stable. This work demonstrates that more accurate predictions of the future of coastal wetlands with rising sea level will be obtained with a combination of short-term measurements of accretion and soil elevation change and long-term modelling.