Icaro, the hydrodynamic model
The Icaro 2 project, unlike its predecessor Icaro 1, also contained a numerical modeling research study. The goal of this sub-project was to build a model of circulation and sedimentation for the canals that could be a valid tool for managing canal dredging. To create this model, it was decided to start with an existing hydrodynamic model, developed for a project coordinated by Unesco, and to proceed to the necessary phases of re-elaboration, updating and adjustment.
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Because of the close connection between the lagoon and the canals, two different models were combined. For the lagoon, we used the Shyfem model, a two-dimensional model with finite elements that provides a precise representation of zones with a complex bathymetry, varying the size and shape of the elements. It was conceived by Isdgm-Cnr as part of the Unesco project “Development of a Sediment Transport Model of the Inner Canals of Venice”.
For the network of inner canals, a two-dimensional (x-z directions) link-node type of model was used, with surrounding conditions provided by Shyfem. The values of the tide-level, calculated from the model with finite elements in the geographic nodes that were closest to the contour points, were established as externally applied forces to the model of inner canals for the entire duration of the simulation. In fact, the circulation of water through the inner network is caused essentially by the change in phase created during the propagation of the tidal wave from the southern to the northern edge of the city. A level gradient is therefore produced around the city that regulates circulation in the inner canals.
Having resolved other software problems afflicting the model and redrawn the grid using Gis tools and the bathymetric data surveyed by Insula, we proceeded to make the grid of the model variable, in relation to Insula’s construction sites, to allow the use of the Manning coefficient, constant or variable, to correct existing problems of stability, to conceive a graphic interface for the model and to make a series of simulations based on the 2003 data. The data from Icaro served in part to calibrate the model and in part to validate it.
Once the model was perfected, it was used to simulate the behaviour of the network in the hypothetical case that a canal was closed off with steel pilings, to dredge the sediment without water. This type of simulation is important for management purposes, to guarantee that the location of the pilings decided by Insula creates as little change in the dynamics of the system as possible. When a canal is closed the dynamics of the system are influenced locally, producing effects on the speed and direction of the water flow. A decisive factor is not the number of canals that are closed, but their relative location. Closing them “by insulae” has less impact than closing canals on a more widespread basis throughout the entire system. The experience of those who deal daily with this system has ensured that planning the works not only reduces costs (more canals can be closed at the same time with a smaller number of pilings and moving around less) but also reduces the impact on the system itself. Once the dredging is completed and the natural morphology of the system is established, it becomes clear that the speed of the current changes considerably when the number of canals dredged by the end of the project is increased. Obviously dredging one single canal does not bring appreciable results on a global level, but you can see the difference when 50-100 canals are dredged (out of 505). An increase in the speed of the current means greater circulation and air flow through the water and the sludge on the bottom.
The model was also applied for the analysis of the hydro-dynamic behaviour of the Grand Canal. The simulations also served to assess the motion of the water during the various phases of the tides and to highlight the phases in which the flow of water reverses direction in some sections of the canal, even assessing the role of smaller canals. A necessary study, especially considering the important role played by the Grand Canal in determining water circulation in the network of canals in the city and its significant influence on the motion of the tidal currents in the entire center-west sector.