A GIS tool to create fluvial flooding maps. Interaction of 1D hydrodynamic model and GIS

The present work proposes an automatic procedure implemented in GIS environment to create perifluvial flooding maps. The potentially fluvial inundated areas are obtained knowing (i) the conformation of the floodplain surrounding the river by an high resolution Digital Terrain Model (DTM) and (ii) a water surface profile along the river axis calculated for given discharge through a generic one-dimensional hydraulic model (HEC-RAS, Basement, MIKE 11, etc). The interest for the interaction between a GIS and common hydrodynamic models is shown by many works in literature and by the implementation of several software to this purpose (HEC-GeoRAS®, Mike 11 GIS and Surface water Modeling System). The implemented procedure has innovating characteristics: in fact, even if it remains substantially one-dimensional, it takes into account the two-dimensionality of floodplain and inundation phenomena, adducing hypotheses that let to correct many errors typical of one-dimensional usually employed procedures. Hence, with respect to the use of an one-dimensional model without the GIS support, this procedure allows to obtain more realistic perifluvial flooding map. Moreover, with respect to a two-dimensional model, it needs a lower computational effort, that allows to apply it to river reaches very long (of the order of 100 km). A first validation of the procedure was performed (Federici et al., 2007) on a reach of the Tanaro River (about 120 Km, Italy): the historical inundated area and the potentially inundated area caused by the alluvial event in 1994 that interested the river through Alessandria city were compared with satisfactory results. Recently, the tool has been applied also to a little stream (Roggia Scairolo) in the Ticino Canton, for the flooding hazard valuation (Pozzoni et al., 2009) obtaining reliable results.