Fully-hydrodinamic modelling supportingflood hazarda assessment snd communication: a reference framework

Abstract

From a hydraulic point of view, flood propagation in natural channels or in floodplains should be simulated using the fully-dynamic shallow water equations (SWEs), although their kinematic or diffusive approximations are still commonly used in practical applications. Indeed, only the use of the fully dynamics equations, formulated in a conservative form, allows the correct treatment of localized hydraulic phenomena and regime changes, factors that are very important for risk assessment. Currents originated by dam break phenomena are a field of study where these localized phenomena arise in a preponderant way. Until a few decades ago, the SWE were essentially developed following a one-dimensional approach, using various schematization and numerical tricks for describing the propagation in floodplain areas. The reasons why the 1-D approach was the preferred one were twofold: the lack of high-resolution topographic data for the detailed description of the hydraulic processes across the section and the limited computational efforts. Nowadays, the importance of these aspects are dramatically reduced and the use of the two-dimensional SWEs starts to be considered as the reference approach for flood propagation studies. Although the use of the fully dynamic wave equations may generate accurate results, the complexity of the associated numerical solvers and their computational times favored the development of simplified approach whose reliability is questionable especially for urban flood simulation. LiDAR data availability and the development of high performance computing technology allow the researchers to develop also flood simulations at the basin scale based on the 2D fully dynamic shallow water equations. In simulations like these, it is not very simple to achieve stable computations in presence of very shallow depths over abrupt changes of the bottom slopes and dry/wet interfaces. However, the significant improvements made in the river flows modeling, flood propagation and dam breaking flows allowed one to obtain stable results also in these complex situations. So a lot of work has been carried out in the context of the hydraulic numerical simulations for flood mapping in order to fulfill the European Floods Directive. However, it should be born in mind that the Directive itself requires to take care of risk communication with the people involved, encouraging the active involvement of the interested parties in the development of flood management plans. The integration of the classic 2-D flood maps, obtained using the models mentioned before, with 3-D representations of flood inundations using virtual reality techniques might allow non-expert public an adequate perception of the flooding impact. Following all these considerations, the paper reviews the current state-of-the-art for hydraulic modelling of floods, focusing on the above-mentioned topics and providing practical suggestions for flood hazard assessment and communications.