Assessing fluvial flooding hazard with a DEM-based Hierarchical Filling-&-Spilling Algorithm: a case study in Northern Italy

The steady increase of economic losses and social consequences caused by flood events in Europe is triggering the development of updated and efficient technologies for assessing flood hazard over large areas, where detailed hydrologic-hydrodynamic numerical models are resource intensive and therefore scarcely suitable. In this context, the EIT-Climate KIC SaferPLACES (https://saferplaces.co) project aims at exploring and developing innovative and simplified modelling techniques to assess and map pluvial, fluvial and coastal flood hazard and risk under current and future climates, mainly based on LiDAR (Light Detection And Ranging) high-resolution DEMs (Digital Elevation Models) raster-based analysis. Within the SaferPLACES activities, a fast-processing Hierarchical Filling-&-Spilling Algorithm (HFSA), named Safer_RAIN (see Samela et al., 2020; https://www.mdpi.com/2073-4441/12/6/1514/htm), has been recently developed for mapping pluvial flooding in large urban areas by accounting for spatially distributed rainfall inputs and infiltration processes. Although it does not incorporate any detailed description of the dynamics of overland flow and water-depth routing, previous applications have shown Safer_RAIN to be an effective tool for a rapid and consistent identification of pluvial-hazard hotspots under different rainfall and land-use scenarios.

Although Safer_RAIN has been conceived for pluvial flooding hazard assessment, its structure suggests its suitability for delineating flooded areas and computing water depth in the aftermath of fluvial inundation (i.e. once the dynamic components of the inundation process become negligible) in predominantly flat floodplains. To this aim, a given flood volume can be assigned to the pixels coinciding with the fluvial flooding point-sources (e.g. simulated levee breach or overtopping) as the input to Safer_RAIN, which is then used for flooding the downstream floodplain portion according to a HSFA approach. We present a first test of the fluvial-application of Safer_RAIN for the case study of the Pisciatello river (Northern Italy, floodplain area of approximately 1300 hectares). Results for different flood scenarios obtained with Safer_RAIN at 1m resolution are compared with the corresponding flooding scenarios simulated with the fully two-dimensional numerical model HEC-RAS at 1m and 5m resolutions. The outcomes of both models are compared in terms of flooded area extent and water depth distribution, highlighting potential and limitations of Safer_RAIN for identifying fluvial flooding hazard.