Susceptibility of Flood-Prone Areas to Remain Inundated for Shorter or Longer Periods During a Flooding Event: An Assessment Method Applied in the Friuli Venezia Giulia Region (Italy)

River and coastal floods are triggered by meteorological events such as heavy rainfall and storm surges, melting snow in upstream areas, tidal-related influences, or a combination of them. They occur when the river runoff volume exceeds local flow capacities or as a consequence of river bank/coastal defense system breakage/failure. Depending on land use and cover conditions, soil capacity to drain surface water, terrain morphology, and amount of exceeding flood water volume, the vulnerability of flood-prone areas in relation to their susceptibility to remain inundated for longer or shorter periods can be higher or lower even in areas affected by similar conditions of flood hazard. Especially in areas with flat slopes and in deltaic areas the fall of water can be particularly long, possibly exacerbating the flooding impacts and consequent damages. These events are also expected to increase in the future as a consequence of climate change, with Italy being particularly exposed due to its geographical location and geomorphological conditions. However, attempts to assess the temporal dimension of flooding are very rare, despite its recognized importance in influencing flood damages and losses. This work presents a first attempt to assess the vulnerability of flood-prone areas in relation to their susceptibility to remain inundated for shorter or longer periods during a flooding event by proposing a rather simple and quick method based on spatial multi-criteria decision analysis. The assessment is tested in the plain and coastal zone of the Friuli Venezia Giulia region, one of the Italian areas with the largest flood hazard potential. It combines the expected flood water depths during a major flooding event and the main geophysical characteristics that may potentially influence the duration of inundations to spatially determine vulnerability patterns in flood-prone areas. Results can be further refined by expert involvement and used to complement existing flood hazard/risk data to support decisions during both the emergency response and the flood risk mitigation planning phases.