final results (phase 1,2 and 2b)
The data that is gathered constitutes an important collection of information on the currents that form within the system and between the system and the surrounding areas, and on the chemical and physical characteristics of the column of water. This knowledge provides a reference framework for the interpretation of analytical data regarding the movement of masses of water and the transport of matter and polluting substances inside the network of canals.
The expansion of the system under observation has also made it possible to validate the two-dimensional model of the finite elements for the lagoon – available to Insula – that simulates the propagation of a wave from the inlets at the outer margins of the city (technical report “D2”) and that provides the surrounding conditions for the two-dimensional link-node model of the inner canals.
With the uninterrupted acquisition of time-based series of chemical and physical parameters, the periodicity of some processes was also investigated. In particular, the series of turbidity values made it possible to assess the phenomenon of the re-suspension of sediment from the bottom of the canals over a more extended span of time.
The study highlighted how the primary factor in determining the turbidity of the waters inside the system is the circulation of motor boats and depends on the intensity of the traffic, of the water head and the tide conditions. An estimate of the flow of sedimentation made by placing five traps across a more extended area has made it possible to compare different canal systems. The data relative to the entire monitoring period highlights a distinct differentiation of the flow of sediment, in relation to the different hydrodynamic conditions in the five canals under observation. However, the evolution over time of the flow in the different sites is similar, with relatively low values in winter, followed by a progressive increase in the flow in the springtime.
The results confirm the strict relationship between the hydrodynamic characteristics of each single canal, the quality of the material that collects on the bottom, and the intensity of the metabolic processes. As seen in a preliminary manner during phase 1, the sedimentation traps, jointly with the constant measuring of the turbidity, are an effective system for studying the variations in the phenomena of sediment re-suspension and transport within the network of canals in the historic city centre of Venice.
The investigation makes it possible to explore the mechanisms of sedimentation, an aspect that is particularly interesting to Insula: the objective is to predict the rate of sedimentation of the materials in the canals, and hence to program dredging operations.
The results of the analyses on the water samples taken during the measuring campaigns, confirm the importance of determining the concentration of suspended particles for the evaluation of the transport of solids and heavy metals in the canals of Venice.
The dissolved species of nitrogen and phosphorus, in particular ammonia and orthophosphate, constitute a very precise tracer of the qualitative variations of a column of water, that are introduced as a result of the draining of waste water, or the route of the water and the length of time they stay in the system. These results further highlight the fundamental role of circulation in establishing the behaviour of contaminants from the urban waste inside the system.
At the close of phase 2, on the basis of the results acquired during the study, a methodological protocol (or specification) was drafted, describing appropriate monitoring methodologies for assessing possible variations, over a relatively short span of time, in the quality of the water in the canal networks of the historic city centre (Icaro Project, phases 1 and 2 – results and methodologies, January 2004).