Design hydrograph and routing scheme for flood mapping in a dense urban area

Definition of flood risk maps is a task to which modern surface hydrology devotes substantial research effort. Their impact on the management of flood-prone, dense, urban areas has increased the need for better investigation of inundation dynamics. The problems associated with the aforementioned topics range from the definition of the design hydrograph and the identification of the surface boundary conditions for the flood routing over the inundation plan, to the choice of the hydrodynamic model to simulate urban flooding. Most of academic and commercial mathematical models, solving the De Saint Venant equations, fail on complex topography. Frequently encountered difficulties concern steep slopes, geometric discontinuities, mixed flow regimes, and initially dry areas. In the present paper, flood routing modelling approaches in urban areas and principles for the definition of the design flood events are outlined. The paper shows how urban flooding can be simulated by a quasi-2D hydrodynamic model that makes use of a network of connected channels and storages to simulate flow, respectively, on the streets and into the building blocks. Furthermore, the paper shows that, when flood hazard is assessed by considering flood extent, water depth and flow velocity, an in-depth analysis of the use of design hydrographs that maximise peak flow or inundation volume is needed.