GIS-based methodology for pluvial flood risk analysis: a case study in the urban area of Rome

The increasing climate variability is responsible for the rising of pluvial flooding risk in urban areas. Pluvial flooding occurs when rainfall remains on impermeable surfaces and flows overland or into local depressions and topographic lows to create temporary ponds. Pluvial flooding only occurs when the rainfall rate exceeds the capacity of the sewer drainage as well as the capacity of the ground to absorb water. This is usually associated with short-duration storms (of up to three hours) and with rainfalls > 20-30 mm/hour. It can also occur following lower intensity rainfalls (~ 15 mm/hour) over longer periods. In the city of Rome, the pluvial trend analysis shows that storms with 30-year return period occurred several times in the last 4 years, causing loss of human lives as well as huge economic damages and threating the artistic and cultural heritage. The present study conducted by CNR-IGAG in collaboration with Civil Protection Department and Rome Municipal Civil Protection (Urban Georisk project), is aimed to provide a fast tool for managing pluvial flood risk, allowing the detection of susceptible areas during a storm event and, thus, planning suitable interventions. The proposed methodology consists of two steps: Rainfall Data Analysis (RDA) and Topographic Analysis (TA). The RDA used storm rainfall records occurred between 2001 and 2014 to obtain hourly rainfall maps, maps of the influence areas for each rain gauge by using Voronoi polygons, and maps of observed flooded areas by using GIS tools. The associated table is filled with floodings vs hourly rain height, and then plotted to check the possible relationships. The calculated minimum rain height, which produced floodings in the studied period, is considered critical, and assigned to each rain gauge influence area. The TA involves the analysis of detailed (2x2m grid) DTM built corrected by filling small sinks in order to obtain a hydrologically corrected surface. The ArcMap Depression Evaluation tool was used to detect topographic depressions, considered as highly susceptible areas. The depression map was overlaid to the map of critical rain height for each of the influence areas in order to assign a critical rain threshold to each depression. Finally, the map of buildings and infrastructures of social importance and/or civil protection use was overlaid to the map of depressions to obtain a ranking of the susceptibility areas. These areas should be of primarily interest in the case of intervention. This study allows detecting points of chronical flooding and, therefore, identifying highest susceptibility areas and ranking hazardous areas on the base of predicted rainfall over the city.