Urban micro-scale flood risk estimation with parsimonious hydraulic modelling and census data

The adoption of 2007/60/EC Directive requires European countries to implement �ood hazard and �ood risk maps by the end of 2013. Flood risk is the product of �ood hazard, vulnerability and exposure, all three to be estimated with comparable level of accuracy. The route to �ood risk assessment is consequently much more than hydraulic modelling of inundation, that is hazard mapping. While hazard maps have already been implemented in many countries, quantitative damage and risk maps are still at a preliminary level. A parsimonious quasi-2-D hydraulic model is here adopted, having many advantages in terms of easy set-up. It is here evaluated as being accurate in �ood depth estimation in urban areas with a high-resolution and up-todate Digital Surface Model (DSM). The accuracy, estimated by comparison with marble-plate records of a historic �ood in the city of Florence, is characterized in the downtown�s most �ooded area by a bias of a very few centimetres and a determination coef�cient of 0.73. The average risk is found to be about 14 E m�2 yr�1, corresponding to about 8.3 % of residents� income. The spatial distribution of estimated risk highlights a complex interaction between the �ood pattern and the building characteristics. As a �nal example application, the estimated risk values have been used to compare different retro�tting measures. Proceeding through the risk estimation steps, a new micro-scale potential damage assessment method is proposed. This is based on the georeferenced census system as the optimal compromise between spatial detail and open availability of socio-economic data. The results of �ood risk assessment at the census section scale resolve most of the risk spatial variability, and they can be easily aggregated to whatever upper scale is needed given that they are geographically de�ned as contiguous polygons. Damage is calculated through stage�damage curves, starting from census data on building type and function, for the main categories in the study area: structures, household contents and commercial contents. This method is tested in the area of the St. Croce district in Florence, one of the most seriously affected in the famous 1966 �ood.