Valutazione del rischio alluvionale: integrazione tra GIS e modelli numerici

Large-scale modifications of terrain topography – like the construction of a new motorway – may affect the surficial drainage and the flood-propagation characteristics in case of a flooding event. Especially in flood-prone environments, human artefacts can increase or decrease the risk of a given event by compartmentalising the floodplain. The effects of changes in topography on flood propagation characteristics can be simulated by using a two-dimensional finite difference model. Such simulations, made during the planning and designing stage of a project, may show if and when flood characteristics are changed by the new construction and if these changes indicate a decrease or an increase of flood risk for those living and working in the affected area. This approach is illustrated by a real case study in the valley of the Adige river northward of Trento (Italy) where a new motorway is planned. On the alluvial plain the scarce space is claimed for urban areas, industrial sites, infrastructure and agricultural activities. Even though constrained in it’s activity (and space) by dikes on either side, the highly dynamic Adige still poses a threat for those living on the plain. For the simulations, the flood propagation model Sobek Delft-FLS was applied to various terrain models, that included the proposed scenarios for the lay-out of the new motorway. The recent development of powerful flood-propagation models that can be applied to complex digital elevation models (DEM), allow to forecast the effects of new elevated structures on the flooding processes. Starting from the proposition that human land-cover alterations can be translated in changes of the DEM and of the surface friction coefficients, the new flood propagation characteristics can be calculated, such as: the maximum height of the flood at a specific location at a certain time, water level rising and the flow-velocity. Together with the season in which the flood occurs and the duration of the event, these flood characteristics will have different effects on the various types of landuse. With these models it is now possible to compare pre-project and the post-project impact to show which areas benefit (where impact decreases) and where impact increases. The method can also be used to aid designing mitigation measures