Thirty-year changes (1970 to 2000) in bathymetry and sediment texture recorded in the Lagoon of Venice sub-basins, Italy

摘要

A detailed comparison was made of two bathymetric charts of the Lagoon of Venice (LV) from 1970 and 2000, in tandem with a comparison of sediment grain size data. Analysis of the data revealed marked changes in both morphology and sedimentation, with more than 50 percent of the 400 km~2 assessed in this study 15-20 cm shallower in 1970 than in 2000. The four sub-basins into which the LV is subdivided saw different patterns of change. The Northern basin A was identified as "pristine", i.e. still in quasi-natural condition, with slight clay enrichment and a small degree of deepening (4-5 cm), essentially due to sea level rise (SLR). The bathymetry and sedimentology of the northern-central sub-basin B (identified as "urban") and the southern-central basin C (identified as "open") were affected by infill activities and excavation of industrial navigation channels in the 1970s, causing the loss of approx 60 km~2 of mudflats, and creating an "open" lagoon. The southernmost basin D (identified as "exploited-subsiding") of the LV was found to be relatively unchanged and still in semi-natural condition. Comparison of sediment types showed depletion of fine-grained fractions (<22 mu m) in all sub-basins except the northernmost one. Consequent enrichment in sand (>63 mu m) was recorded, except in the southern-central sub-basin C where medium and coarse sand fractions declined. The results suggest that climate-related SLR accounts for a small fraction of bathymetric change, whilst variations in hydrodynamics (currents and wind patterns) and sediment supply are likely causal factors for the different evolution of the four sub-basins. Definition of the attributes of each sub-basin provided data that was essential not only for the LV but also for predicting the fate of transitional environments facing both anthropic (fishing, navigation, land use changes) as well as natural (SLR, eustatism) forcing factors.